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prof. Ing. Tomáš Polcar, Ph.D.

All publications

A new reactive Mo-S-C force field to explore structure and properties of MoS2-C 2D materials and films

  • DOI: 10.1016/j.commatsci.2024.113034
  • Link: https://doi.org/10.1016/j.commatsci.2024.113034
  • Department: Department of Control Engineering
  • Annotation:
    We employed a Monte-Carlo optimization scheme to successfully create a novel ReaxFF parameter set tailored for the exploration of the structures and energetics of Mo-S-C materials. This force field underwent rigorous testing against Density Functional Theory (DFT) data and exhibited its efficacy in modeling a broad spectrum of structures, ranging from composite heterolayer 2D structures to fully amorphous coatings. Subsequently, we applied this force field to get insights into the structural formation of amorphous MoS2-C films. Our simulations yielded promising results, revealing, in agreement with the experimental findings, preferential formation of Mo-S and C-C bonds along with rapid phase segregation.

Charge-induced ultralow friction between graphite and atomically flat surfaces

  • DOI: 10.1016/j.carbon.2024.119036
  • Link: https://doi.org/10.1016/j.carbon.2024.119036
  • Department: Department of Control Engineering
  • Annotation:
    Reaching near-zero friction is one of the jewels on the crown of tribology, and structural superlubricity is a crucial mechanism to achieve it. Previous works focus mainly on the structural superlubricity at incommensurate crystalline interfaces. However, realizing such interfaces on a large scale without defects and contaminations is a formidable challenge. Here, we report a charge-induced robust macroscale superlubricity between graphite and atomically flat surfaces in the ambient condition. We transferred graphite flakes on Si3N4 balls and used them to measure the friction properties on pristine and charged atomically flat surfaces such as 300 nm SiO2/Si and sapphire. We found that the surface charge can dramatically reduce the coefficient of friction between graphite and substrates by two orders of magnitude to 10−4, and the sliding is wearless even under harsh contact conditions (∼1.1 GPa center pressure and >100 m). We demonstrate that the surface charge is critical in achieving superlubricity possibly because it can reduce adhesion between graphite and substrate surfaces and make the substrate surfaces resistant to contaminations. Our method offers a ready-to-use solution to superlubricity alternative to achieve incommensurate crystalline interfaces. Thus, it can reduce the difficulty of realizing macroscale superlubricity for applications.

Corrosion behavior of Cr coating on ferritic/martensitic steels in liquid lead-bismuth eutectic at 600 °C and 700 °C

  • DOI: 10.1016/j.jmrt.2024.02.116
  • Link: https://doi.org/10.1016/j.jmrt.2024.02.116
  • Department: Department of Control Engineering
  • Annotation:
    The surface coating technology, encompassing ceramics, refractory materials, metallic alloys containing Al or Si, and multicomponent composites, presents a viable approach to improve the corrosion resistance of ferritic/martensitic (F/M) steels with (9–12) wt.% Cr in liquid lead-bismuth eutectic (LBE) environment. Among these coating materials, chromium (Cr) coating emerges as a particularly noteworthy option. This study specifically focused on depositing a 3 μm thick Cr coating on on T91 and SIMP steels using magnetron sputtering. Subsequently, the corrosion behavior of the Cr coating was investigated in LBE at temperatures of 600 °C and 700 °C. The results revealed that, after 300 h at 600 °C, T91 and SIMP steels formed oxide scales with approximately 32.6 μm and 19.3 μm thicknesses, respectively. At 700 °C for 140 h, these oxide scales increased to about 82.4 μm and 73.1 μm for T91 and SIMP steels, respectively. However, the application of a Cr coating resulted in the formation of a dense layer of chromium oxide with a thickness of 4–5 μm. This layer effectively impeded oxygen diffusion and Fe migration leading to a significant reduction in the corrosion rate of the steel. Notably, the Cr coating maintained secure attachment to the steel even after exposure to high-temperature LBE corrosion. These findings underscore the capacity of coating to markedly enhance the corrosion resistance of T91 and SIMP steels in high-temperature LBE environments, providing robust protection against the detrimental effects of challenging conditions. Consequently, Cr coating emerges as a promising solution for future fission nuclear reactors.

High throughput selection of organic cathode materials

  • Authors: López-Carballeira, D., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Journal of Computational Chemistry. 2024, 45(5), 264-273. ISSN 0192-8651.
  • Year: 2024
  • DOI: 10.1002/jcc.27236
  • Link: https://doi.org/10.1002/jcc.27236
  • Department: Department of Control Engineering
  • Annotation:
    Efficient and affordable batteries require the design of novel organic electrode materials to overcome the drawbacks of the traditionally used inorganic materials, and the computational screening of potential candidates is a very efficient way to identify prospective solutions and minimize experimental testing. Here we present a DFT high-throughput computational screening where 86 million molecules contained in the PUBCHEM database have been analyzed and classified according to their estimated electrochemical features. The 5445 top-performing candidates were identified, and among them, 2306 are expected to have a one-electron reduction potential higher than 4 V versus (Li/Li+). Analogously, one-electron energy densities higher than 800 Whkg−1 have been predicted for 626 molecules. Explicit calculations performed for certain materials show that at least 69 candidates with a two-electron energy density higher than 1300 Whkg−1. Successful molecules were sorted into several families, some of them already commonly used electrode materials, and others still experimentally untested. Most of them are small systems containing conjugated C=O, N=N, or N=C functional groups. Our selected molecules form a valuable starting point for experimentalists exploring new materials for organic electrodes.

High-velocity dust impacts in plasma facing materials: Insights from molecular dynamics simulations

  • DOI: 10.1016/j.jnucmat.2024.155042
  • Link: https://doi.org/10.1016/j.jnucmat.2024.155042
  • Department: Department of Control Engineering
  • Annotation:
    This research investigates the interaction between high-speed tungsten (W) dust and plasma-facing components (PFCs) in fusion reactors, particularly focusing on W walls. Through molecular dynamics (MD) simulations, the study covers a broad spectrum of W dust velocities to evaluate their effect on wall materials with various crystal orientations. We found that high-speed impacts cause considerable damage, including sputtering, degradation, and deformation. The study introduces a damage model derived from experimental and simulation data that reveals the patterns and mechanisms of damage caused by dust impacts. The proposed model significantly improves our understanding of dust-wall interactions and underscores the importance of MD simulations as a reliable technique for exploring such phenomena in the challenging conditions of fusion devices. These insights are crucial to predict and mitigate damage to PFCs, helping to develop more resilient and efficient components. Overall, the research offers valuable knowledge on the atomic-level dynamics of dust impacts and represents a notable advancement in the durability and efficiency of materials used in fusion energy technologies.

Study on the corrosion behavior of laser surface remelted and laser cladding of ferritic/martensitic steels after exposure to lead-bismuth eutectic at 700 °C

  • DOI: 10.1016/j.jnucmat.2023.154888
  • Link: https://doi.org/10.1016/j.jnucmat.2023.154888
  • Department: Department of Control Engineering
  • Annotation:
    This study explores two vital structural materials, T91 (Fe-9Cr) and SIMP (Fe-11Cr) steels, in the context of lead-cooled fast reactors and accelerator-driven sub-critical systems (ADS). Lead-bismuth eutectic (LBE) functions as a key coolant and spallation target material due to its impressive thermal conductivity, neutron yield, and chemical properties. Unfortunately, materials in contact with LBE are prone to severe corrosion at elevated temperatures (T>500 °C), compromising their integrity. To bolster corrosion resistance, we utilized laser remelting and laser cladding to apply FeCrAl/TiN coatings on the steel surfaces. Our study scrutinizes the corrosion behavior of steel in LBE saturated with oxygen at 700 °C and investigates the underlying causes. Following 240 h of exposure to corrosion, T91 and SIMP steels subjected to laser remelting displayed substantial oxide scale formation. Lead-bismuth atoms infiltrated the outer oxide layer (Fe3O4), diminishing adhesion between the inner and outer oxide layers, leading to the detachment of the outer oxide layer. The inner oxide layers, composed of Fe-Cr spinel, were approximately 123 μm thick for T91 steel and 77 μm for SIMP steel, underscoring SIMP steel's superior corrosion resistance. For T91 steel treated with laser cladding FeCrAl/TiN coating, a characteristic duplex oxide layer with a total thickness of around 83 μm was formed, with noticeable deposition of Pb-Bi atoms at the interface between the outer and inner oxide layers. Conversely, only a protective alumina layer safeguarded SIMP steel from LBE corrosion. This outcome emphasizes the efficacy of laser cladding FeCrAl/TiN coating in providing superior protection for SIMP steel over T91 steel. Our research significantly contributes to the development of anti-corrosion coatings for high-temperature LBE environments.

Wear and friction of self-lubricating coatings applied to spur gears in fluidfree aerospace actuation gearboxes

  • DOI: 10.1177/13506501231197486
  • Link: https://doi.org/10.1177/13506501231197486
  • Department: Department of Control Engineering
  • Annotation:
    Aerospace actuation gearboxes operate in low-temperature environments where increased lubricant viscosity leads to significant no-load power losses. Replacing fluid lubricants with coatings applied to the gear teeth is one potential approach to improving gearbox efficiency. Here we develop an approach to determining average wear rates of coated gears using a power-recirculating test stand, profile measurements and a model of the tooth contact. Worn gears are inspected using scanning electron imagery, and energy dispersive X-ray and Raman spectroscopy to understand the wear mechanisms and failure modes. Average coefficients of friction are determined at 20°C and −40°C using a power-absorbing test stand and isolation of tooth friction losses by calculation. These methods are then demonstrated on a promising C/Cr composite coating.

Al-based composites reinforced with ceramic particles formed by in situ reactions between Al and amorphous SiNxOy

  • Authors: Kutzhanov, Magzhan K., Matveev, Andrei T., Bondarev, A., prof. Ing. Tomáš Polcar, Ph.D., Duchon, J., Shtansky, D.V.
  • Publication: MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING. 2022, 842 ISSN 0921-5093.
  • Year: 2022
  • DOI: 10.1016/j.msea.2022.143105
  • Link: https://doi.org/10.1016/j.msea.2022.143105
  • Department: Department of Control Engineering
  • Annotation:
    This work combines two approaches to the creation of high-strength heat-resistant Al-based materials: dispersion hardening and stabilization of nanosized Al grains by grain-boundary precipitates. For this, the amorphous SiNxOy phase (1, 2, 3, 4, 5, and 10 wt%) was used as a reactive additive to the Al nanopowder. The Al-SiNxOy composites were obtained by a combination of high-energy ball milling and spark plasma sintering. Chemical reactions between a-SiNxOy and Al led to the formation of nanoscale AlN, Al2O3, and SiO2 phases located at the Al grain boundaries and inside the metal matrix with a bimodal size distribution: approximately 50-150 and 3-10 nm. Compared to unreinforced Al, the addition of 3% SiNxOy increased hardness by 464%, tensile strength by 103% (25 degrees C), 84% (300 degrees C), and 86% (500 degrees C), compressive strength by 200% (25 degrees C), 164% (300 degrees C), and 192% (500 degrees C), and impact wear resistance by 33-46%. TEM microstructure analysis after deformation revealed the types of defects and helped to elucidate the deformation and strengthening mechanisms. The obtained results are important for the development of Al-based composites capable of operating in an extended temperature range.

Analysis of hypervelocity impacts: the tungsten case

  • DOI: 10.1088/1741-4326/ac42f6
  • Link: https://doi.org/10.1088/1741-4326/ac42f6
  • Department: Department of Control Engineering
  • Annotation:
    The atomistic mechanisms of damage initiation during high velocity (v up to 9 km s(-1), kinetic energies up to 200 keV) impacts of W projectiles on a W surface have been investigated using parallel molecular-dynamics simulations involving large samples (up to 40 million atoms). Various aspects of the high velocity impacts, where the projectile and part of the target material undergo massive plastic deformation, breakup, melting, and vaporization, are analyzed. Different stages of the penetration process have been identified through a detailed examination of implantation, crater size and volume, sputtered atoms, and dislocations created by the impacts. The crater volume increases linearly with the kinetic energy for a given impactor; and the total dislocation length (TDL) increases with the kinetic energy but depends on the size of the impactor. We found that the TDL does not depend on the used interatomic potential. The results are rationalized based on the physical properties of bcc W.

Characterizing heavy ions-irradiated Zr/Nb: Structure and mechanical properties

  • Authors: Dr. Nabil Daghbouj, Ph.D., Sen, H., Cizek, J., Lorincik, J., Karlík, M., Callisti, M., Čech, J., Havránek, V., LI, B., Krsjak, V., Liedke, M.O., Butterling, M., Wagner, A., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Materials & Design. 2022, 219 ISSN 0264-1275.
  • Year: 2022
  • DOI: 10.1016/j.matdes.2022.110732
  • Link: https://doi.org/10.1016/j.matdes.2022.110732
  • Department: Department of Control Engineering
  • Annotation:
    In this work, the radiation responses of Zr/Nb nanostructured metallic multilayers (NMMs) are studied. The nanostructures with different layer thicknesses were deposited on Si (111) substrate by using magnetron sputtering and were subjected to heavy-ion irradiation at room temperature with different fluences. Nanoindentation, XRD, DFT, SIMS, and Variable Energy Positron Annihilation Spectroscopy (VEPAS) techniques were used to study the type and distribution of defects, and strain within the material as well as the changes in the hardness of the structures as a function of damage. Our results suggest that the strain and the irradiation hardening are layer thickness- and damage-dependent while they are independent of the type of irradiated ions. The magnitude of hardening decreases with decreasing individual layer thickness indicating that the number of interfaces has a direct effect on the radiation tolerance enhancement. For thin layers with a periodicity of 27 nm (Zr/Nb27), a transition from hardening to softening occurs at high fluence, and a saturation point is reached in thick layers with a periodicity of 96 nm (Zr/Nb96). The as-deposited thin multilayers presented a significantly higher atomic-scale disorder which increases with ion irradiation compared to the thick multilayers. VEPAS reveals the vacancy defects before and after irradiation that contribute to the presented strain. Based on the findings, thin nanostructured Zr/Nb multilayered structures possess excellent radiation resistance due to the high density of interfaces that act as sinks for radiation-induced point defects.

Design Guidelines for Two-Dimensional Transition Metal Dichalcogenide Alloys

  • Authors: Silva, A., Cao, J., prof. Ing. Tomáš Polcar, Ph.D., Kramer, D.
  • Publication: Chemistry of Materials. 2022, 34(23), 10279-10290. ISSN 0897-4756.
  • Year: 2022
  • DOI: 10.1021/acs.chemmater.2c01390
  • Link: https://doi.org/10.1021/acs.chemmater.2c01390
  • Department: Department of Control Engineering
  • Annotation:
    Two-dimensional (2D) materials and transition metal dichalcogenides (TMD) in particular are at the forefront of nanotechnology. To tailor their properties for engineering applications, alloying strategies-used successfully for bulk metals in the last century-need to be extended to this novel class of materials. Here we present a systematic analysis of the phase behavior of substitutional 2D alloys in the TMD family on both the metal and the chalcogenide site. The phase behavior is quantified in terms of a metastability metric and benchmarked against systematic computational screening of configurational energy landscapes from First-Principles. The resulting Pettifor maps can be used to identify broad trends across chemical spaces and as starting point for setting up rational search strategies in phase space, thus allowing for targeted computational analysis of properties on likely thermodynamically stable compounds. The results presented here also constitute a useful guideline for synthesis of binary metal 2D TMDs alloys via a range of synthesis techniques.

Engineering width and directness of the band gap in diamond-based materials: An ab initio investigation towards electron-structure features control

  • DOI: 10.1016/j.diamond.2022.109237
  • Link: https://doi.org/10.1016/j.diamond.2022.109237
  • Department: Department of Control Engineering
  • Annotation:
    Diamond-based compounds are ideal materials to build nanoengineered devices with wide applicability in nanophotonics, optomechanics, photovoltaics and electronics, where tuning the width and the character of the electronic band gap is paramount. While the available information is focused on specific aspects of the material preparation and response, a general understanding of how the entangled geometric and electronic properties determine the characteristics of the band gap is missing. The present work aims to tackle this challenge by means of first principle simulations. We show that specific charge distributions in the ion environment determine the width of the band gap; in order to control it, we suggest how to select suitable dopant atomic types and how to impose specific structural deformations. We also propose different routes to switch the character of the band gap from indirect to direct. The results pave new avenues aimed to design diamond-based nanostructured materials with targeted optical and electronic properties. The outcomes of the present work are general and can therefore be promptly applied to the study of optical and electronic materials irrespective of their chemical composition and atomic topology.

Femtosecond laser texturing of DLC-based coatings by DLW method with sub-micrometer precision

  • Authors: Cermak, A., Simonovic, K., Bondarev, A., Kozmin, P., Syrovatka, S., prof. Ing. Tomáš Polcar, Ph.D., Syrovatka, J.
  • Publication: The International Journal of Advanced Manufacturing Technology. 2022, 121(11-12), 8479-8494. ISSN 0268-3768.
  • Year: 2022
  • DOI: 10.1007/s00170-022-09855-0
  • Link: https://doi.org/10.1007/s00170-022-09855-0
  • Department: Department of Control Engineering
  • Annotation:
    Laser texturing of surfaces may provide specific functionality, such as altering hydrophobicity, changing optical properties, or reducing friction and wear. For the latter, surfaces of parts are often coated with protective and/or solid lubricant coating, and texturing may further improve the tribological properties in both dry or lubricated sliding. New laser technologies, such as the direct laser writing (DLW) method using a femtosecond laser, allow the production of extremely precise textures directly into the coating. Here, we describe a method of preparing ultra-precise textures into diamond-like carbon (DLC) coatings on a large area. The textured topography was assessed by 3D laser scanning microscope, which confirmed the repeatability of fs laser processing. Raman spectroscopy mapping, SEM, and XPS were combined to investigate the effect of laser processing on DLC coating in terms of oxidation or structural changes. Traditional process (i.e., coating textured surface) often results in coating adhesion/cohesion failure due to deposition of sharp edges produced by texturing, whereas our approach eliminates this issue. Even complex textures inside the coating are fabricated with a fast speed of 10 s per mm(2) and a high precision in texture depth (tens of nanometers), unlocking many application fields in tribology or microfluidics.

Friend or Foe? Revising the Role of Oxygen in the Tribological Performance of Solid Lubricant MoS2

  • DOI: 10.1021/acsami.2c15706
  • Link: https://doi.org/10.1021/acsami.2c15706
  • Department: Department of Control Engineering
  • Annotation:
    Molybdenum disulfide (MoS2) is a solid lubricant used in various forms, such as a dry lubricant by itself or as a component of a more complex coating. In both these forms, the effect of oxygen contamination on the sliding properties of the MoS2 coatings is traditionally considered detrimental, resulting in expensive technological processes to produce pure MoS2. Here, it is shown that the high oxygen content does not necessarily hinder the solid lubricant properties and may even result in a lower friction and wear when compared to pure MoS2. Mo-S-O coatings were fabricated by unbalanced magnetron sputtering and tribologically tested under vacuum conditions. Oxygen caused amorphization of the as-deposited coatings but did not prevent the triboactivated formation of an ultra-thin crystalline MoS2 tribolayer with the incorporated oxygen. Such an imperfect tribolayer was found to reduce the coefficient of friction to 0.02, a value lower than that of pure MoS2. Moreover, owing to the higher density and hardness of oxygen-containing films, the wear rate was also found to be lower. Molecular dynamics simulations performed using a newly developed Mo-S-O force field confirmed that such an imperfect tribolayer can mitigate friction in a manner comparable to that of MoS2.

Interface-Driven Strain in Heavy Ion-Irradiated Zr/Nb Nanoscale Metallic Multilayers: Validation of Distortion Modeling via Local Strain Mapping

  • Authors: Sen, H., Dr. Nabil Daghbouj, Ph.D., Callisti, M., Vronka, M., Karlík, M., Duchoň, J., Čech, J., Lorinčík, J., Havránek, V., Bábor, P., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: ACS Applied Materials & Interfaces. 2022, 14(10), 12777-12796. ISSN 1944-8244.
  • Year: 2022
  • DOI: 10.1021/acsami.1c22995
  • Link: https://doi.org/10.1021/acsami.1c22995
  • Department: Department of Control Engineering
  • Annotation:
    Nanolayered metallic alloys are promising materials for nuclear applications thanks to their resistance to radiation damage. Here, we investigate the effect of ion (C, Si, and Cu) irradiation at room temperature with different tluences into sputtered Zr/Nb metallic multilayer films with periods 27 nm (thin) and 96 nm (thick). After irradiation, while a high strain in the entire thin nanoscale metallic multilayer (NMM) is observed, a quite small strain in the entire thick NMM is established. This difference is further analyzed by a semianalytical model, and the reasons behind it are revealed, which are also validated by local strain mapping. Both methods show that within a thick layer, two opposite distortions occur, making the overall strain small, whereas in a thin layer, all the atomic planes are affected by the interface and are subjected to only a single type of distortion (Nb-tension and Zr-compression). In both thin and thick NMMs, with increasing damage, the strain around the interface increases, resulting in a release of the elastic energy at the interface (decrease in the lattice mismatch), and the radiation-induced transition of the Zr/Nb interfaces from incoherent to partially coherent occurs. Density functional theory simulations decipher that the inequality of point defect diffusion flux from the inner to the interface-affected region is responsible for the presence of opposite distortions within a layer. Technologically, based on this work, we estimated that Zr/NbSS with thicknesses around Zr = 24 nm and Nb = 31 nm is the most promising multilayer system with the high radiation damage resistance and minimum swelling for nuclear applications.

New Reactive Force Field for Simulations of MoS2 Crystallization

  • DOI: 10.1021/acs.jpcc.2c01075
  • Link: https://doi.org/10.1021/acs.jpcc.2c01075
  • Department: Department of Control Engineering
  • Annotation:
    We present a new reactive force field (ReaxFF) parameter set for simulations of Mo−S structures. We compare our parameterization to the state-ofthe-art ones in their performance against density functional theory (DFT) benchmarks and MoS2 crystallization simulations. Our new force field matches DFT data significantly better than any previously published force fields and provides a realistic layered MoS2 structure in crystallization simulations. It significantly improves the state-of-the-art force fields, which tend to crystallize in the experimentally unknown rock-salt MoS structure. Therefore, our new force field is a good candidate for further development and inclusion of other practically relevant elements, such as O, C, N, and H, which can be used to study the formation and tribological or catalytical properties of molybdenum disulfide.

Pettifor maps of complex ternary two-dimensional transition metal sulfides

  • DOI: 10.1038/s41524-022-00868-7
  • Link: https://doi.org/10.1038/s41524-022-00868-7
  • Department: Department of Control Engineering
  • Annotation:
    Alloying is an established strategy to tune the properties of bulk compounds for desired applications. With the advent of nanotechnology, the same strategy can be applied to 2D materials for technological applications, like single-layer transistors and solid lubricants. Here we present a systematic analysis of the phase behaviour of substitutional 2D alloys in the Transition Metal Disulfides (TMD) family. The phase behaviour is quantified in terms of a metastability metric and benchmarked against many-body expansion of the energy landscape. We show how the metastability metric can be directly used as starting point for setting up rational search strategies in phase space, thus allowing for targeted further computational prediction and analysis of properties. The results presented here also constitute a useful guideline for synthesis of TMDs binary alloys via a range of synthesis techniques.

Revealing nanoscale strain mechanisms in ion-irradiated multilayers

  • DOI: 10.1016/j.actamat.2022.117807
  • Link: https://doi.org/10.1016/j.actamat.2022.117807
  • Department: Department of Control Engineering
  • Annotation:
    Tailoring interfaces is a powerful way of reducing the accumulation of radiation defects. Understanding strain evolution induced by ion bombardment in nuclear materials with high interface density is crucial for next-generation reactors since induced defects are responsible for volumetric swelling and catastrophic failure. X-ray and selected-area diffraction patterns (SADPs) measurements reveal, after Cu implantation, that a relatively high out-of-plane strain is created in thin Zr/Nb-6 multilayers, while thick Zr/Nb96 is barely distorted. The absence of layer deformation in Zr/Nb-96 is explained by local TEM strain mapping showing the presence of two oppositely distorted regions (inner and interface-affected regions) within one layer producing only a small overall strain, whereas the whole individual layers of Zr/Nb-6 are affected by the interface manifesting high strain. Using MD simulations, the types of defects responsible for layer distortion are identified. The opposite distortion within the layer is attributed to the inequality of the defect flux from the inner to interface-affected region due to the difference in migration energy barriers of the point defects. Moreover, the interface sink efficiency (defect annihilation) is determined for Zr/Nb as an illustration which provides a strategy for designing new derivate structures of multilayers with high radiation damage resistance.

Thermal behavior of iron in 6H-SiC: Influence of He-induced defects

  • DOI: 10.1016/j.scriptamat.2022.114805
  • Link: https://doi.org/10.1016/j.scriptamat.2022.114805
  • Department: Department of Control Engineering
  • Annotation:
    SiC is considered a perspective material in advanced nuclear systems as well as for electronic or spintronic applications, which require an ion implantation process. In this regard, two sets of 6H-SiC samples were implanted with i) 2.5 MeV Fe ions and ii) 2.5 MeV Fe ions and co-implanted 500 keV He ions at room temperature and then annealed at 1500 degrees C for 2 h. The microstructure evolution and Fe diffusion behavior before and after annealing were characterized and analyzed. After annealing, Fe concentration is enhanced close to the surface in the Fe-implanted sample, whereas in the co-implanted system, Fe atoms are redistributed into two distinct, spatially separated regions (close to the surface, and around the He-induced defects). The reason behind this finding is explained from an energetic point of view by using ab initio simulations. Technologically, the preexisting cavities can be used to control the Fe diffusion.

Tribological behaviour of W-S-C coated ceramics in a vacuum environment

  • DOI: 10.1016/j.triboint.2021.107375
  • Link: https://doi.org/10.1016/j.triboint.2021.107375
  • Department: Department of Control Engineering
  • Annotation:
    Sliding of the WSC coated ceramics (ball-on-disc configuration) was investigated under vacuum condition at multiple loads (2–18 N), and each experiment was followed by detailed Raman and SEM analysis of both the wear track and the wear scar (on the ball). Main finding is that under low loads (up to 8 N), wear is polishing, carbon structure becomes more ordered and the number of WS2 monolayer increases. Above 8 N, wear transitions into an abrasive regime interrupting the aforementioned processes increasing both friction and wear. Furthermore, roles of the coating components were differentiated: WS component is responsible for the low friction, whereas the carbon part is responsible for excellent wear properties.

Tribological properties of vanadium oxides investigated with reactive molecular dynamics

  • DOI: 10.1016/j.triboint.2022.107795
  • Link: https://doi.org/10.1016/j.triboint.2022.107795
  • Department: Department of Control Engineering
  • Annotation:
    We present a reactive molecular dynamics study on tribological properties of five vanadium oxides (𝑉2𝑂3, 𝑉3𝑂5, 𝑉8𝑂15, 𝑉9𝑂17, 𝑉 𝑂2) under elevated temperatures and pressures. All considered stoichiometries provide lubrication with a comparatively low coefficient of friction (𝐶𝑂𝐹 ∼ 0.2 at 600 K, 𝐶𝑂𝐹 < 0.2 at 800 and 1000 K) which is a valuable information relevant for the design of coatings containing vanadium as a lubricious agent. An overall tendency of the decrease of friction coefficient with the increase of temperature represents a tribological effect useful for self-adjusting lubrication. We observed the increasing trend of adhesion-related offset of the friction force with the decrease of oxygen content in vanadium oxides.

UItra-low friction and edge-pinning effect in large-lattice-mismatch van der Waals heterostructures

  • Authors: Liao, M., Nicolini, P., Du, L., Yuan, J., Wang, S.P., Yu, H., Tang, J., Cheng, P., Watanabe, K., Taniguchi, T., Gu, L., Claerbout, V., Silva, A., Kramer, D., prof. Ing. Tomáš Polcar, Ph.D., Yang, R., Shi, D.X., Zhang, G.Y.
  • Publication: NATURE MATERIALS. 2022, 21(1), 47-53. ISSN 1476-1122.
  • Year: 2022
  • DOI: 10.1038/s41563-021-01058-4
  • Link: https://doi.org/10.1038/s41563-021-01058-4
  • Department: Department of Control Engineering
  • Annotation:
    MoS2/graphite and MoS2/h-BN interfaces are shown to have ultra-low friction coefficients, whereas edges and interface steps mainly contribute to the friction force. Two-dimensional heterostructures are excellent platforms to realize twist-angle-independent ultra-low friction due to their weak interlayer van der Waals interactions and natural lattice mismatch. However, for finite-size interfaces, the effect of domain edges on the friction process remains unclear. Here we report the superlubricity phenomenon and the edge-pinning effect at MoS2/graphite and MoS2/hexagonal boron nitride van der Waals heterostructure interfaces. We found that the friction coefficients of these heterostructures are below 10(-6). Molecular dynamics simulations corroborate the experiments, which highlights the contribution of edges and interface steps to friction forces. Our experiments and simulations provide more information on the sliding mechanism of finite low-dimensional structures, which is vital to understand the friction process of laminar solid lubricants.

A new protocol for the identification of singlet fission sensitizers through computational screening

  • Authors: López-Carballeira, D., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Journal of Computational Chemistry. 2021, 42(31), 2241-2249. ISSN 0192-8651.
  • Year: 2021
  • DOI: 10.1002/jcc.26753
  • Link: https://doi.org/10.1002/jcc.26753
  • Department: Department of Control Engineering
  • Annotation:
    Although singlet fission presents deep advantages or the generation of solar energy, the list of efficient singlet fission sensitizers is still very short, encouraging the theoreticians to focus their efforts on selecting and designing new candidates. Here, it is presented a computational protocol for the efficient screening of databases to select those species matching the energy requirements for singlet fission. Hence, out of the initial 29,123 species, 254 molecules (0.87%) were found to match singlet fission energy conditions. The consideration of practical concerns such as availability or stability reduced the number to just 24 (0.08%), among which the aminoanthraquinone derivatives are found particularly promising. The proposed protocol correct the deficiencies of the preceding ones, reaching DFT accuracy and minimizing the risk of getting false negatives especially at the early stages of the screening. In addition, this protocol can be used in future high-throughput investigations using datasets composed of millions of species.

Blister formation in He-H co-implanted InP: A comprehensive atomistic study

  • DOI: 10.1016/j.apsusc.2021.149426
  • Link: https://doi.org/10.1016/j.apsusc.2021.149426
  • Department: Department of Control Engineering
  • Annotation:
    The blistering efficiency in He-H-ions co-implanted and annealed InP has been found to peak and vanish in a narrow range of ion fluence ratio (?H/?He = 1.5?3.5) with a fixed He fluence of 2 ? 1016 He+/cm2. The blisters are formed at low fluence (?H/?He = 1.5), peaked in the middle (?H/?He = 2.5), and disappeared at the high fluence ratio (?H/?He = 3.5). To get a fundamental understanding of blister formation in nanoscale, the defect profiles were studied by various experimental techniques combined with FEM and ab-initio simulations. Crosssection TEM images showed that at a low fluence ratio, He and H are stored in microcracks and bubbles whereas, at a high fluence ratio, the ions are trapped only inside bubbles. These atomic processes that occur during and after co-implantation and annealing are presented together with detailed scenarios in an attempt to explain our results. Based on DFT simulations, the de-trapping of He atoms from the small clusters is energetically cheaper compared to the migration of He from the large clusters formed at high fluence. Moreover, at a high fluence ratio, the presence of large clusters inhibits the He diffusion to the small clusters (precursor of blisters) by capturing migrating He atoms.

Effect of Noninteracting Intercalants on Layer Exfoliation in Transition-Metal Dichalcogenides

  • DOI: 10.1103/PhysRevApplied.15.064041
  • Link: https://doi.org/10.1103/PhysRevApplied.15.064041
  • Department: Department of Control Engineering
  • Annotation:
    The control of friction at the atomic scale is fundamental to optimize the exfoliation of layered materials. To this aim, we report a density-functional investigation of how intercalated molecules affect the nanoscale friction of van derWaals transition-metal dichalcogenides.We find that the molecule does not interact with the electronic density of the layers directly; nonetheless it determines the features of the valence band of the system. In particular, the valence-band width appears to be a promising parameter to correlate the electronic properties with the nanofrictional response; it then constitutes a guide for the automatic search of intercalation molecules suitable for layer exfoliation. The present outcomes also constitute a theoretical tool for future investigations of the effect that intercalated species have on the nanoscale friction in layered materials.

Effect of substrate bias voltage on the mechanical properties and deformation mechanisms in the nanostructured Ti-22Nb-10Zr coating

  • Authors: Frutos, E., Karlík, M., Jimenez, J. A., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Surface & Coatings Technology. 2021, 405 ISSN 0257-8972.
  • Year: 2021
  • DOI: 10.1016/j.surfcoat.2020.126674
  • Link: https://doi.org/10.1016/j.surfcoat.2020.126674
  • Department: Department of Control Engineering
  • Annotation:
    The manufacturing of future implants based on NiTi shape memory alloys, like stents, orthodontic archwires, intracranial aneurysms..., clips aims to eliminate the cytotoxicity and allergic problems associated with the release of nickel ions. In this context, the design of beta-rich Ti-22Nb-10Zr (mass.%) coating by magnetron sputtering, with a non-linear elastic behavior and tunable mechanical properties, offers a novel approach for the development of a new biocompatible implants with self-adjusting mechanical properties. The XRD and TEM analyses reveal that microstructure consisting of hexagonal close-packed (alpha-phase), prevalent body-centered cubic (beta-phase) and orthorhombic (alpha ''-phase) nanograins, which can be tuned during the deposition by a single parameter - applied bias voltage as a result of the activation of a stress-induced martensitic transformation (beta -> alpha ''). We found that the minimum stress value to trigger the SIM transformation must be higher than 712 MPa (bias -63 V). The application of higher bias voltage values alters the main deformation mechanism, from a combination of reversible SIM transformation and mechanical twinning to dislocation slip, causing the compressive residual stresses to decrease from 712 to 120 MPa, the hardness increases from 2,1 to 4,1 GPa, and the coating stops showing low Young's modulus (<50 GPa) and non-linear elastic behavior.

Exploring Nanoscale Lubrication Mechanisms of Multilayer MoS2 During Sliding: The Effect of Humidity

  • DOI: 10.3389/fchem.2021.684441
  • Link: https://doi.org/10.3389/fchem.2021.684441
  • Department: Department of Control Engineering
  • Annotation:
    Solid lubricants have received substantial attention due to their excellent frictional properties. Among others, molybdenum disulfide (MoS2) is one of the most studied lubricants. Humidity results in a deterioration of the frictional properties of MoS2. The actual mechanism at the nanoscale is still under debate, although there are indications that chemical reactions are not likely to occur in defect-free structures. In this study, we performed nonequilibrium molecular dynamics simulations to study the frictional properties of multilayer MoS2 during sliding in the presence of water. Moreover, we also investigated the effect of sliding speed and normal load. We confirmed earlier results that a thin layer of water organizes as a solidified, ice-like network of hydrogen bonds as a result of being confined in a two-dimensional fashion between MoS2. Moreover, we found that there exists an energy-driven, rotational dependence of the water network atop/beneath MoS2. This orientational anisotropy is directly related to the dissipative character of MoS2 during sliding. Finally, three distinct frictional regimes were identified, two for a thin layer of water and one for bulk water. In the case of a thin layer and low coverage, water represents a solid-like contaminant, causing high energy dissipation. For a thin layer and high coverage, water starts to act as a solid-like lubricant, reducing dissipation during sliding. Finally, a regime where water acts as a liquid lubricant, characterized by a clear velocity dependence was found.

Failure of Solid Lubricant W-S-C Coatings under Boundary Lubrication Conditions

  • Authors: Simonovic, K., Harvey, T. J., Vítek, J., Callisti, M., Cavailero, A., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Journal of Materials Engineering and Performance. 2021, 30(6), 3990-3999. ISSN 1059-9495.
  • Year: 2021
  • DOI: 10.1007/s11665-021-05740-1
  • Link: https://doi.org/10.1007/s11665-021-05740-1
  • Department: Department of Control Engineering
  • Annotation:
    A boundary lubrication regime imposes a significant challenge on mechanical systems in terms of both friction and wear. A standard approach to the boundary lubrication regime problems involves coating the contacting parts and lubrication with additivated lubricant. The presented paper explores the application of TMD-based coatings combined with a base oil and an organic-based friction modifier to provide a more environmentally friendly and cheaper solution. An investigation has been performed using a reciprocating tribometer across a range of applied loads, with post-test characterisation performed by a 3D optical profilometer, Raman microscopy, SEM, and TEM. Results show that the coating with higher hardness performs better and that the organic friction modifier increases coefficient of friction but decrease wear in the boundary lubrication regime. Moreover, it was established that without an external source of sulphur TMD-based coatings do not have a satisfactory performance in the boundary lubrication regime.

Fine control of lattice thermal conductivity in low-dimensional materials

  • DOI: 10.1103/PhysRevB.103.035406
  • Link: https://doi.org/10.1103/PhysRevB.103.035406
  • Department: Department of Control Engineering
  • Annotation:
    Optimal regulation of lattice thermal conductivity in low-dimensional materials is fundamental to obtain highly efficient miniaturized devices. To this aim, we use quantum-mechanical based analyses to understand how atomic type and structural geometry determine electron density and lattice dynamic features ruling the thermal conduction. As a case study, we consider layered van der Waals transition metal dichalcogenides with a finite number of layers. We find that a large thermal conductivity is realized when the atomic bonds display highly covalent character, promoting fast motions of the cations in correspondence of the low-frequency phonon band. Such an effect is the result of the entangled electronic and phonon features, which are captured by the covalency and cophonicity metric. The investigation protocol that we present has general applicability and can be used to design novel thermal low-dimensional materials irrespective of the kind of atomic topology and chemical composition.

Formation of Solid Lubricants during High Temperature Tribology of Silver-Doped Molybdenum Nitride Coatings Deposited by dcMS and HIPIMS

  • DOI: 10.3390/coatings11111415
  • Link: https://doi.org/10.3390/coatings11111415
  • Department: Department of Control Engineering
  • Annotation:
    The coating system MoN-Ag is an interesting candidate for industrial applications as a low friction coating at elevated temperatures, due to the formation of lubricous molybdenum oxides and silver molybdates. Film deposition was performed by high-power impulse magnetron sputtering and direct current magnetron sputtering. To facilitate a future transfer to industry Mo-Ag composite targets have been sputtered in Ar/N2 atmosphere. The chemical composition of the deposited MoN-Ag films has been investigated by wavelength dispersive X-ray spectroscopy. Morphology and crystallographic phases of the films were studied by scanning electron microscopy and X-ray diffraction. To obtain film hardness in relation to Ag content and bias voltage, the instrumented indentation test was applied. Pin-on-disc tribological tests have been performed at room temperature and at high temperature (HT, 450 °C). Samples from HT tests have been analyzed by Raman measurements to identify possible molybdenum oxide and/or silver molybdate phases. At low Ag contents (≤7 at.%), coatings with a hardness of 18–31 GPa could be deposited. Friction coefficients at HT decreased with increasing Ag content. After these tests, Raman measurements revealed the MoO3 phase on all samples and the Ag2Mo4O13 phase for the highest Ag contents (~23–26 at.%).

Friction-Induced Chemical and Structural Modifications of Molybdenum Disulphide Thin Films

  • DOI: 10.1007/s11665-021-05928-5
  • Link: https://doi.org/10.1007/s11665-021-05928-5
  • Department: Department of Control Engineering
  • Annotation:
    The present work investigates chemical and structural modifications of molybdenum disulphide thin coatings induced by macroscale friction in both reactive (air) and inert conditions. Chemical and micro-structural modifications were analyzed in detail by Raman spectroscopy. We found no traces of oxide formation even when sliding was performed in the air. However, the formation of a well-adhered tribofilm, related to superior lubricating properties, was detected only in inert conditions. We can conclude that, in ambient condition, it is water physisorption and not oxidation, which impairs good lubrication. At the same time, Raman spectra indicated a re-crystallization effect induced by sliding.

Helium migration in Zr-Nb multilayers under electric field

  • DOI: 10.1016/j.jnucmat.2021.153133
  • Link: https://doi.org/10.1016/j.jnucmat.2021.153133
  • Department: Department of Control Engineering
  • Annotation:
    The alpha particles, which appear during the nuclear reactions and are transformed into He atoms, are often implanted into (or produced in) the structural materials causing deterioration of the mechanical properties. Moreover, they precipitate into high-pressure He bubbles, further decreasing the strength. Metallic alloys with a high density of interfaces and/or grain boundaries can reduce or even heal the defects caused by irradiation; nevertheless, the detrimental effect of He atoms is still a concern and diffusion of He atoms in the vicinity of interfaces is the subject of active research. Here, we present an ab initio study to explore how the electric field influences the diffusion of He atoms in (HCP-BCC) Zr-Nb metallic multilayer composite before they start to agglomerate. Motivated by our previous work, where we showed that He atoms tend to agglomerate into the regions with low electron density, we utilized the shift of electrons inside Zr-Nb multilayer system due to the external field to manipulate the migration of He atoms in a preferred direction. The results indicate that He atoms can be guided along the direction of the field out of the system, especially along [00 02] direction in HCP-Zr. (C) 2021 Elsevier B.V. All rights reserved.

Insight into high temperature performance of magnetron sputtered Si-Ta-C-(N) coatings with an ion-implanted interlayer

  • Authors: Bondarev, A., Antonyuk, M. N., Kiryukhantsev-Korneev, Ph, prof. Ing. Tomáš Polcar, Ph.D., Shtansky, D.V.
  • Publication: Applied Surface Science. 2021, 541 ISSN 0169-4332.
  • Year: 2021
  • DOI: 10.1016/j.apsusc.2020.148526
  • Link: https://doi.org/10.1016/j.apsusc.2020.148526
  • Department: Department of Control Engineering
  • Annotation:
    Challenges related to the application of wear resistant coatings at high temperatures require the development of novel materials with an exceptional combination of mechanical, chemical and tribological properties. The present paper is focused on understanding of relationships between structure, composition and high-temperature performance of the Si-Ta-C-(N) coatings. The coatings were produced using combined magnetron sputtering (MS) and ion implantation (CMSII) technique. It was found that ion implanted coatings demonstrated better thermal shock resistance compared to MS Si Ta C (N) coatings. The Si-Ta-C-(N) coatings revealed a nanocomposite structure consisting of 2-3 nm fcc TaC(N) grains and amorphous a-Si and a-SiC(N) phases. The composition and structure of amorphous matrix and nanocrystallites strongly affected tribological performance of the Si-Ta-C-(N) coatings. The N-doped coatings exhibited exceptionally good tribological performance due to a higher ductility of N-rich amorphous a-SiCN and a-SiNx matrix, and fcc Ta(C,N)-based crystallites compared with the a-Si + a-SiC, and fcc TaC-based phases in N-free coating. The Si-Ta-C-(N) coatings easily withstood oxidation annealing at 800 degrees C due to the formation of a 200 nm protective TaSiOx amorphous layer. Oxidation annealings revealed that under thin protective TaSiOx layer crystalline components of coatings did not change when Si and C from the amorphous matrix started to diffuse towards the substrate at 800 degrees C but even after redistribution of elements and formation of oxide scale the coatings demonstrated reasonably high hardness - 13-16 GPa. Triboactivated formation of TaSiOx fibers which could slide/roll against the same TaSiOx tribolayer during high-temperature tribotests resulted in low coefficient of friction values (0.23 at 800 degrees C) and absence of wear.

Interphase boundary layer-dominated strain mechanisms in Cu+ implanted Zr-Nb nanoscale multilayers

  • DOI: 10.1016/j.actamat.2020.10.072
  • Link: https://doi.org/10.1016/j.actamat.2020.10.072
  • Department: Department of Control Engineering
  • Annotation:
    Sputter-deposited Zr/Nb nanoscale metallic multilayers with a periodicity of 27 (thin) and 96 nm (thick) were subjected to Cu + implantation with low and high fluences and then studied using various experimental techniques in combination with DFT calculations. After Cu + implantation, the thinner multilayer exhibited a tensile strain along c-axis in Nb layers and a compressive strain in Zr layers, while the thicker multilayer showed a compressive strain in both layers. The strain is higher in the thin multilayer and increases for higher fluences. We developed a mathematical method for the fundamental understanding of the deformation mechanisms in metallic multilayers subjected to radiation damage. In the model, the cumulative strain within a layer is described as the combination of two contributions coming from the interfacial region and the inner region of the layers. The semi-analytical model predicts that the interfacial strain is dominant and extends over a certain region around the interface. Predictions are well supported by ab-initio calculations which show that in the vicinity of the interface and in the Zr side, vacancies and interstitials (low energy barriers) exhibit high mobility compared to the Nb side, thus resulting in a high recombination rate. As a consequence, less strain occurs in the Zr side of the interface compared to the Nb side. The density and distribution of various types of defects along the ion profile (low and high damaged regions) are obtained by combining DFT results and the predictions of the model. (c) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Nanotribology of transition metal dichalcogenide flakes deposited by chemical vapour deposition: The influence of chemical composition and sliding speed on nanoscale friction of monolayers

  • DOI: 10.1016/j.apsusc.2021.149762
  • Link: https://doi.org/10.1016/j.apsusc.2021.149762
  • Department: Department of Control Engineering
  • Annotation:
    We present nanoscale frictional analysis of three commonly used transition metal dichalcogenide (TMD) monolayers, WS2, MoSe2 and WSe2, deposited by chemical vapour deposition (CVD). The monolayers were characterised by Raman spectroscopy, photoluminescence spectroscopy (PL), and X-ray spectroscopy (XPS), to determine the composition of the coating and confirm monolayer structure. Nanoscale frictional analysis was performed by atomic force microscopy (AFM). Load-dependent frictional behaviour was measured at different sliding speeds to quantitatively assess friction on each sample. All samples experienced low nanoscale friction, with the lowest friction observed on WSe2. The friction was independent of sliding speed within the analysed range. Furthermore, monolayer TMDs significantly increase the operational load range by at least one order of magnitude when compared to SiO2 substrate.

Phase behaviour of (Ti:Mo) S2 binary alloys arising from electron-lattice coupling

  • DOI: 10.1016/j.commatsci.2020.110044
  • Link: https://doi.org/10.1016/j.commatsci.2020.110044
  • Department: Department of Control Engineering
  • Annotation:
    While 2D materials attract considerable interests for their exotic electronic and mechanical properties, their phase behaviour is still largely not understood. This work focuses on (Mo:Ti) S2 binary alloys which have captured the interest of the tribology community for their good performance in solid lubrication applications and whose chemistry and crystallography is still debated. Using electronic structures calculations and statistical mechanics we predict a phase-separating behaviour for the system and trace its origin to the energetics of the d-band manifold due to crystal field splitting. Our predicted solubility limits as a function of temperature are in accordance with experimental data and demonstrate the utility of this protocol in understanding and designing TMD alloys.

Phototribology: Control of Friction by Light

  • Authors: Perotti, B.L., doc. Antonio Cammarata, Ph.D., Cemin, F., de Mello, S.R.S., Leidens, L.M., Echeverrigaray, F.G., Minea, T., Alvarez, F., Michels, A.F., prof. Ing. Tomáš Polcar, Ph.D., Figueroa, C.A.
  • Publication: ACS Applied Materials & Interfaces. 2021, 13(36), 43746-43754. ISSN 1944-8244.
  • Year: 2021
  • DOI: 10.1021/acsami.1c13054
  • Link: https://doi.org/10.1021/acsami.1c13054
  • Department: Department of Control Engineering
  • Annotation:
    In dry sliding, the coefficient of friction depends on the material pair and contact conditions. If the material and operating conditions remain unchanged, the coefficient of friction is constant. Obviously, we can tune friction by surface treatments, but it is a nonreversible process. Here, we report active control of friction forces on TiO2 thin films under UV light. It is reversible and stable and can be tuned/controlled with the light wavelength. The analysis of atomic force microscopy signals by wavelet spectrograms reveals different mechanisms acting in the darkness and under UV. Ab initio simulations on UV light-exposed TiO2 show a lower atomic orbital overlapping on the surface, which leads to a friction reduction of up to 60%. We suggest that photocontrol of friction is due to the modification of atomic orbital interactions from both surfaces at the sliding interface.

Revisiting the electronic nature of nanodiamonds

  • DOI: 10.1016/j.diamond.2021.108627
  • Link: https://doi.org/10.1016/j.diamond.2021.108627
  • Department: Department of Control Engineering
  • Annotation:
    Nanodiamonds, commonly described as fragments of diamond, have been theoretically found to have lower HOMO-LUMO energy splitting compared to the bandgap of bulk diamond. This apparent lack of correlation between theory and experiment is caused by the position of the LUMO, which is placed in the surface of the ND. An eventual enlargement of the ND towards a macroscopic size will turn the LUMO into the unoccupied surface states, which are not accounted if the bandgap of a bulk material is measured. Here, the electron structure of the nanodiamonds is evaluated, demonstrating that due their nature they should be described as discrete systems instead of bulk materials. Hence, the word bandgap should be avoided in the case of the nanodiamonds, using HOMO-LUMO gap instead. Additionally, our obtained ionization potentials show a satisfactory degree of correlation with the experiment, while the electron affinities are found to be positive. Although this feature fits the estimation performed from experimental data, it opposes the generally accepted idea of a negative electron affinity for hydrogenated nanodiamonds. The present article clarifies common misunderstandings regarding the electronic nature of the NDs, and provides some guidelines for the correct computation of this systems. Finally, as a helpful tool, an estimation of the content of carbon atoms and its surface to volume ratio is provided starting from the diamond unit cell.

Structure, mechanical and tribological properties of MoSe2 and Mo-Se-N solid lubricant coatings

  • Authors: Hudec, T., Izai, V., Satrapinskyy, L., Huminiuc, T., Roch, R., Gregor, M., Grančič, B., Mikula, M., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Surface & Coatings Technology. 2021, 405 ISSN 0257-8972.
  • Year: 2021
  • DOI: 10.1016/j.surfcoat.2020.126536
  • Link: https://doi.org/10.1016/j.surfcoat.2020.126536
  • Department: Department of Control Engineering
  • Annotation:
    Mo-Se-N films were prepared by pulsed direct current (DC) High Target Utilisation Sputtering (HiTUS) in reactive Ar + N-2 atmosphere. Here, the effect of nitrogen doping was studied. MoSex film with Se/Mo atomic ratio similar to 2 exhibited polycrystalline structure and the lowest coefficients of friction (COFs) in humid air from 0.025 to 0.1 for loads in the range 2-45 N. Mo-Se-N coatings were deposited with N concentrations ranging from 1 to 40 at.%, whereas Se/Mo ratio varied from 0.6 to 2. Mo-Se-N coatings formed amorphous structures for the N contents above 7 at.% and increased hardness proportional with the N content up to 9 GPa. The addition of nitrogen also resulted in a general decrease in wear rate of two orders of magnitude when compared to pristine films while retaining a reasonably low coefficient of friction. Mo-Se-N films showed notable COF values in humid environment ranging from 0.22 to 0.015 when tested using loads from 2 to 45 N. The excellent friction properties of Mo-Se-N films were associated to the crystallisation of a MoSe2 tribofilm in the wear scar. Moreover, we showed that modern pulsed DC HiTUS technology represents a suitable way of producing thin films with a variety of elemental compositions and desired mechanical and tribological properties, even from sensitive, semiconducting and extremely low thermally conductive MoSe2 targets.

Titanium doped MoSe2 coatings - Synthesis, structure, mechanical and tribological properties investigation

  • Authors: Hudec, T., Bondarev, A., Izai, V., Sroba, V., Satrapinskyy, L., Roch, T., Turiničová, V., Grančič, B., prof. Ing. Tomáš Polcar, Ph.D., Mikula, M.
  • Publication: Applied Surface Science. 2021, 568 ISSN 0169-4332.
  • Year: 2021
  • DOI: 10.1016/j.apsusc.2021.150990
  • Link: https://doi.org/10.1016/j.apsusc.2021.150990
  • Department: Department of Control Engineering
  • Annotation:
    In this work, we doped sputtered MoSe2 coatings with Ti to improve their mechanical and tribological properties. Mo-Se-Ti coatings were prepared onto steel discs by High Target Utilisation Sputtering (HiTUS). The Ti was in the range 7-31 at.%, which led to Se/Mo ratios of 1.7-2.7. The Mo-Se-Ti coatings possessed a polycrystalline structure for the lower Ti contents and became amorphous when Ti content was above approx. 18 at.%. The hardness of the initially soft (0.3 GPa) pure MoSe2 coatings was dramatically improved with Ti additions (up to 7.5 GPa). Sliding experiments confirmed excellent tribological properties: coefficient of friction values from 0.02 to 0.08 and very low wear rates (8 x 10(8) mm(3)N(-1)m(-1)) of the Ti-containing coatings tested in the humid air at the loads in the range of 2-45 N. The outstanding self-lubricant properties of Mo-Se-Ti coatings were attributed to the transformation of the topmost amorphous part of the coating into crystalline MoSe2 tribofilm while preserving inner parts of the coating in as-deposited state ensuring good load-bearing capacity. There is strong experimental evidence that Ti has a gettering effect in the tribological contact, which protects MoSe2 tribofilm against oxidation.

Toughening mechanisms in V-Si-N coatings

  • DOI: 10.1016/j.matdes.2021.109961
  • Link: https://doi.org/10.1016/j.matdes.2021.109961
  • Department: Department of Control Engineering
  • Annotation:
    Microstructural evolution and deformation mechanisms of magnetron sputtered V-Si-N coatings with various Si contents are investigated by transmission electron microscopy, X-ray absorption spectroscopy, and ab initio calculations. A small amount of Si atoms was dissolved into the cubic VN lattice, locally reducing the neighboring V-N p-d hybridization near the Si site. The Si content was found to impact the architecture of coating significantly. With increasing Si content, the microstructure evolved through three different architectures: (i) highly textured columnar grains, (ii) refined columnar grains, and (iii) nanocomposite structures where elongated grains were bounded by vein-like boundaries. Enhanced damage tolerance was observed in the nanocomposite structure, where multiple toughening mechanisms become active. Ab initio calculations revealed that the incorporation of Si monolayer in the (111) -oriented VN resulted in the formation of weaker Si-N bonds compared to V-N bonds, which allowed a selective response to strain and shear deformations by assisting the activation of the slip systems. (c) 2021 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Tribological behaviour of Mo-S-N solid lubricant coatings in vacuum, nitrogen gas and elevated temperatures

  • Authors: Hudec, T., Roch, T., Gregor, M., Orovcik, L., Mikula, M., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Surface & Coatings Technology. 2021, 405 ISSN 0257-8972.
  • Year: 2021
  • DOI: 10.1016/j.surfcoat.2020.126722
  • Link: https://doi.org/10.1016/j.surfcoat.2020.126722
  • Department: Department of Control Engineering
  • Annotation:
    Molybdenum disulphide (MoS2) is well known for the exceptional tribological properties in inert and high vacuum environments. On the other hand, these properties rapidly degrade in humid and higher temperature (>300 degrees C) environments which cause increase of the friction coefficient and substantial increase of the wear. Apart from this, MoS2 films also suffers of porous structure, very low hardness and low load-bearing capacity which limits its applications in terrestrial atmosphere and more demanding conditions. Mo-S-N coatings published in previous studies revealed greatly improved mechanical and tribological properties in humid environment - low coefficient of friction, very low wear rates and one order of magnitude higher hardness. However, to this date, knowledge about sliding response of these Mo-S-N coating is mainly limited to humid air. In this work, we tested Mo-S-N coatings with different nitrogen contents from 0 to 40 at.% of N in vacuum (10(-2) Pa), nitrogen atmosphere and elevated temperatures. The coatings were deposited by High Target Utilisation Sputtering method. All tribological tests were performed using ball-on-disc testing rig. A maximum hardness of 8 GPa was measured for the 19 and 31 atomic % N-doped coatings. In all testing conditions, the pure MoS2 films had COFs in the range of 0.02-0.15 and wear rates of 1.2-22x10(-6) mm(3) N-1 m(-1). The COF and specific wear rates decreased with N additions. Mo-S-N films had COFs between superlubric 0.007 and 0.13 and wear rates of 0.08-3x10(-6) mm(3)N(-1) m(-1). The best overall tribological performance was shown to be for the Mo-S-N films containing 31 at.% of N. This study concluded that Mo-S-N coatings performed better than MoS2 in all testing conditions.

Tribological properties of Mo-S-C coating deposited by pulsed d.c. magnetron sputtering

  • DOI: 10.1016/j.wear.2021.203939
  • Link: https://doi.org/10.1016/j.wear.2021.203939
  • Department: Department of Control Engineering
  • Annotation:
    Transition metal dichalcogenides such as MoS2 are widely used as solid lubricants for vacuum applications. On the other hand, diamond-like carbon coatings exhibit excellent sliding properties in the ambient environment. Our Mo-S-C coatings deposited by pulsed d.c. sputtering combine both structures to obtain stable properties regardless of the testing conditions. The coatings were studied using HR-TEM and Raman spectroscopy, revealing amorphous nature of the coating. The tribological properties were evaluated by pin-on-disc method. The results showed high lubrication ability in all the testing conditions. HR-TEM and Raman spectroscopy were employed to show the structural characteristics of the wear traces. Our results indicate that the low-friction effect should be attributed to carbon structure re-arrangement since expected wear-induced MoS2 formation was not observed.

Tribological Properties of V2O5 Studied via Reactive Molecular Dynamics Simulation

  • DOI: 10.1016/j.triboint.2020.106750
  • Link: https://doi.org/10.1016/j.triboint.2020.106750
  • Department: Department of Control Engineering
  • Annotation:
    Providing lubrication at high temperature in the presence of oxygen is a significant technological challenge. We apply reactive molecular dynamics to study tribological properties of vanadium pentoxide, an active lubricious component of oxidation-resistant hard coatings. We explore sliding on V2O5 in a wide range of conditions and note its tendency to melt in sliding conditions at elevated temperatures and/or pressures. We observe a stick-slip-like collective mechanism of sliding in crystalline V2O5 at room temperature, that requires further exploration from theoretical prospective. We find that even a single layer of V2O5 on the surface of the coating is an effective lubricant at high temperatures, which is a vital piece of information for coating design.

A 2D finite element approach for predicting the machining performance of nanolayered TiAlCrN coating on WC-Co cutting tool during dry turning of AISI 1045 steel

  • DOI: 10.1016/j.ceramint.2020.06.294
  • Link: https://doi.org/10.1016/j.ceramint.2020.06.294
  • Department: Department of Control Engineering
  • Annotation:
    The hard coatings have proved very effective in industrial machining applications, and the development of new coatings relies more on simulations of the cutting process. In this regard, the present work investigates the machining performance of newly developed TiAlCrN multi-layered coating on tungsten carbide cutting tool during dry turning of AISI 1045 steel. A finite element model with damage evolution based on fracture energy was developed with the help of Abaqus/Explicit software. Later, the numerical results were validated by experimental measurements. The application of coating not only helped in the reduction of friction and tool temperatures at different cutting speeds but also affected the chip formation mechanism significantly. The secondary shear plane (SSP) was generated while machining with both uncoated and coated cutting tools. However, the SSP appeared only at higher cutting speeds in the case of the coated cutting tool. The proposed finite element model is able to predict the instability and formation of secondary serrated teeth with good accuracy.

Ab initio description of nanodiamonds: A DFT and TDDFT benchmark

  • DOI: 10.1016/j.diamond.2020.107959
  • Link: https://doi.org/10.1016/j.diamond.2020.107959
  • Department: Department of Control Engineering
  • Annotation:
    In this article 15 DFT functionals were evaluated in order to find a good description of the electronic features and geometries of the nanodiamonds. With this in mind, two sets of molecules were designed, one of them composed by well-known organic molecules and other by diamondoids. The main parameters considered for comparison in this work are the excitation energies, ionization potentials, and the energies of the frontier orbitals. Moreover, absorption distances and binding energies were also considered to assess the quality of the description of the physisorption process. The best overall performance was found on hybrid functionals and more specifically on those with low HF percentage (TPSSH and O3LYP), despite reasonable results with lower computational cost can be obtained with GGA or LDA methods. The use of a correction for the dispersion interaction is mandatory except when a Truhlar functional, LDA or ωB97X functional is used. An augmented triple zeta basis set is recommended, especially for the description of the nanodiamond model, while an augmented double zeta is enough to yield a converged geometry.

Comparative Study of DC and RF Sputtered MoSe2 Coatings Containing Carbon-An Approach to Optimize Stoichiometry, Microstructure, Crystallinity and Hardness

  • DOI: 10.3390/coatings10020133
  • Link: https://doi.org/10.3390/coatings10020133
  • Department: Department of Control Engineering
  • Annotation:
    Low stoichiometry, low crystallinity, low hardness and incongruencies involving the reported microstructure have limited the applicability of TMD-C (Transition metal dichalcogenides with carbon) solid-lubricant coatings. In this work, optimized Mo-Se-C coatings were deposited using confocal plasma magnetron sputtering to overcome the above-mentioned issues. Two different approaches were used; MoSe2 target powered by DC (direct current) or RF (radio frequency) magnetron sputtering. Carbon was always added by DC magnetron sputtering. Wavelength dispersive spectroscopy displayed Se/Mo stoichiometry of similar to 2, values higher than the literature. The Se/Mo ratio for RF-deposited coatings was lower than for their DC counterparts. Scanning electron microscopy showed that irrespective of the low carbon additions, the Mo-Se-C coatings were highly compact with no vestiges of columnar growth due to optimal bombardment of sputtered species. Application of substrate bias further improved compactness at the expense of lower Se/Mo ratio. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy confirmed the presence of MoSe2 crystals, and (002) basal planes. Even very low carbon additions led to an improvement of the hardness of the coatings. The work reports a comparison between RF and DC sputtering of MoSe2 coatings with carbon and provides a guideline to optimize the composition, morphology, structure, and mechanical properties.

Control of energy dissipation in sliding low-dimensional materials

  • DOI: 10.1103/PhysRevB.102.085409
  • Link: https://doi.org/10.1103/PhysRevB.102.085409
  • Department: Department of Control Engineering
  • Annotation:
    Frictional forces acting during the relative motion of nanosurfaces are the cause of energy loss and wear which limit an efficient assembly and yield of atomic-scale devices. In this research, we investigate the microscopic origin of the dissipative processes as a result of the frictional response, with the aim to control them in a subtle way. We recast the study of friction in terms of phonon modes of the system at the equilibrium, with no need to resort to dynamics simulations. As a case study, we here consider layer sliding in transition metal dichalcogenides thin films. We find that the population of specific atomic orbitals and the relative contribution of the atomic type to selected system vibrations are the crucial quantities which determine the frictional response in tribological conditions. A reduced amount of energy dissipation is found when the bond character is more ionic and the layer sliding is realized by a faster motion of the chalcogen atoms. The individuated relevant parameters governing the energy dissipation can be used as descriptors in high-throughput calculations or machine learning engines to screen databases of frictional materials. The presented framework is general and can be promptly extended to the design of tribological materials with targeted frictional response, irrespective of the chemistry and atomic topology.

Effect of electric fields in low-dimensional materials: Nanofrictional response as a case study

  • DOI: 10.1103/PhysRevB.102.155433
  • Link: https://doi.org/10.1103/PhysRevB.102.155433
  • Department: Department of Control Engineering
  • Annotation:
    A proper control of nanoscale friction is mandatory for the fabrication and operation of optimal nanoengineered devices. In this respect, the use of electric fields looks to be promising, since they are able to alter the frictional response without imprinting permanent deformations into the structure. To this aim, we perform ab initio simulations to study the microscopic mechanisms governing friction in low-dimensional materials in the presence of electrostatic fields. We consider MX2 transition metal dichalcogenides as a case study. By applying an electric field along an axis orthogonal to the atom layers, we induce a transfer of charge along the same axis; this transfer modifies the interatomic forces, leading, in general, to easier relative layer motion. The reported outcomes constitute a starting point to study the effect of the field direction on the intrinsic friction in future investigations. Finally, the present results can be used to predict the preferential electronic redistribution in nanostructured devices where metal-to-insulator transitions may occur in working conditions.

Elucidating the role of TiCl(4)post-treatment on percolation of TiO(2)electron transport layer in perovskite solar cells

  • DOI: 10.1088/1361-6463/ab938c
  • Link: https://doi.org/10.1088/1361-6463/ab938c
  • Department: Department of Electrotechnology, Department of Control Engineering
  • Annotation:
    The ideal electron transport layer of a high performance perovskite solar cell should have good optical transparency, high electron mobility, and an energy level alignment well-matched with the perovskite material. In this work, we investigate the role of TiCl(4)post-treatment of the mesoporous TiO(2)electron transport layer by varying the concentration of TiCl(4)and characterizing optical and electrical properties, charge carrier dynamics, and photovoltaic performance of mesoscopic CH(3)NH(3)PbI(3)solar cells. It is found that the TiCl(4)treatment provides an additional interconnection between the TiO(2)particles, leading to better percolation as evident from high resolution cross-section images and chemical maps. This enhances effective electron mobility in the material as well as significantly reduces average sub-bandgap absorption due to defects and electronic disorder determined by photothermal deflection spectroscopy.

Exploring the Stability of Twisted van der Waals Heterostructures

  • Authors: Silva, A., Claerbout, V., prof. Ing. Tomáš Polcar, Ph.D., Kramer, D., Nicolini, P.
  • Publication: ACS Applied Materials & Interfaces. 2020, 12(40), 45214-45221. ISSN 1944-8244.
  • Year: 2020
  • DOI: 10.1021/acsami.0c13971
  • Link: https://doi.org/10.1021/acsami.0c13971
  • Department: Department of Control Engineering
  • Annotation:
    Recent research showed that the rotational degree of freedom in stacking 2D materials yields great changes in the electronic properties. Here, we focus on an often overlooked question: are twisted geometries stable and what defines their rotational energy landscape? Our simulations show how epitaxy theory breaks down in these systems, and we explain the observed behavior in terms of an interplay between flexural phonons and the interlayer coupling, governed by the moiré superlattice. Our argument, applied to the well-studied MoS2/graphene system, rationalizes experimental results and could serve as guidance to design twistronic devices.

Mechanisms of friction and wear reduction by h-BN nanosheet and spherical W nanoparticle additives to base oil: Experimental study and molecular dynamics simulation

  • Authors: Bondarev, A.V., Fraile, A., prof. Ing. Tomáš Polcar, Ph.D., Shtansky, D.V.
  • Publication: Tribology International. 2020, 151 ISSN 0301-679X.
  • Year: 2020
  • DOI: 10.1016/j.triboint.2020.106493
  • Link: https://doi.org/10.1016/j.triboint.2020.106493
  • Department: Department of Control Engineering
  • Annotation:
    Hexagonal boron nitride (h-BN) nanosheets, spherical W nanoparticles, and their combinations were utilized as lubricant additives to synthetic PAO6 oil. The addition of W NPs led to a decrease in the coefficient of friction and wear rate. Molecular dynamics (MD) simulations and in situ TEM mechanical tests showed that the positive effect of adding spherical W NPs can be attributed to their rolling and sliding in the tribological contact zone. Adding BN nanosheets to PAO6 also improved the tribological performance of friction pairs: MD simulations suggest that the exfoliation and sliding of BN layers under tribological contact can contribute to the reduction of friction and wear. Moreover, a synergistic effect from the simultaneous addition of W and BN nanoparticles was observed: the CoF and wear reached minimum values among all tested suspensions. The formation of W/BN core/shell structures by wrapping of W nanoparticles by h-BN sheets provided superior macroscale lubricity.

Microstructure and mechanical properties of nanostructured ti-22nb-10zr coatings

  • Authors: Frutos, E., Karlík, M., Jiménez, J.A., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: METAL 2020 - Conference Proceedings. Ostrava: TANGER, 2020. p. 661-666. 1st. ISSN 2694-9296. ISBN 978-80-87294-97-0.
  • Year: 2020
  • DOI: 10.37904/metal.2020.3540
  • Link: https://doi.org/10.37904/metal.2020.3540
  • Department: Department of Control Engineering
  • Annotation:
    The design of implants and functional prostheses requires superficial modifications that promote fast and lasting osseointegration. Magnetron sputtering enables to design nanostructured and textured ß-Ti rich Ti-22Nb-10Zr (wt.%) coatings with variable mechanical properties (hardness and Young's modulus). Depending on the magnitude of the bias voltage used during deposition of the coating, martensitic transformation from the unstable ß (bcc) to a” (orthorhombic) phase is activated. This transformation induces compressive residual stresses modifying the tensile strength, hardness and Young's modulus. The residual stresses were measured by nanoindentation, the microstructure and phase evolution were characterized by X-ray diffraction. The spatial phase distribution was determined by transmission electron microscopy. The calculated real hardness increases from 2.1 to 4.1 GPa as the bias voltage is increased from 0 to -148 V. The calculus confirms that the coating has a non-linear elastic behavior.

Nanotribological Investigation of Sliding Properties of Transition Metal Dichalcogenide Thin Film Coatings

  • Authors: Rapuc, A., Simonovic, K., Huminiuc, T., Cavaleiro, A., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: ACS Applied Materials and Interfaces. 2020, 12(48), 54191-54202. ISSN 1944-8244.
  • Year: 2020
  • DOI: 10.1021/acsami.0c16789
  • Link: https://doi.org/10.1021/acsami.0c16789
  • Department: Department of Control Engineering
  • Annotation:
    Transition metal dichalcogenide (TMD)-based coatings are known for their low friction performance, which is attributed to the formation of a tribolayer consisting almost exclusively of pure well-ordered TMD. However, the formation of such a tribolayer and its wear track coverage is still unknown. In this study, we employed surface mapping and nanotribological techniques to study the properties of the wear tracks of composite W-S-C coatings. Our analysis revealed that the as-deposited coating consisted of two phases, with significantly different nanoscale frictional properties. We attributed the phases to nanocrystalline WS2 (low friction) and amorphous solution of carbon and WS2 (high friction). The two phases wear at different rates, especially at lower loads, where we observed faster depletion of nanocrystalline WS2. In the wear track, sparse flat WS2 flakes were identified, suggesting that the recrystallization of the WS2 phase occurs only at the spots where the contact pressure is the highest.

Precise control of the interlayer twist angle in large scale MoS2 homostructures

  • Authors: Liao, M., Wei, Z., Du, L., Wang, Q., Tang, J., Yu, H., Wu, F., Zhao, J., Xu, X., Han, B., Liu, K., Gao, P., prof. Ing. Tomáš Polcar, Ph.D., Sun, Z., Shi, D., Yang, R., Zhang, G.
  • Publication: NATURE COMMUNICATIONS. 2020, 11(1), ISSN 2041-1723.
  • Year: 2020
  • DOI: 10.1038/s41467-020-16056-4
  • Link: https://doi.org/10.1038/s41467-020-16056-4
  • Department: Department of Control Engineering
  • Annotation:
    Twist angle between adjacent layers of two-dimensional (2D) layered materials provides an exotic degree of freedom to enable various fascinating phenomena, which opens a research direction-twistronics. To realize the practical applications of twistronics, it is of the utmost importance to control the interlayer twist angle on large scales. In this work, we report the precise control of interlayer twist angle in centimeter-scale stacked multilayer MoS2 homostructures via the combination of wafer-scale highly-oriented monolayer MoS2 growth techniques and a water-assisted transfer method. We confirm that the twist angle can continuously change the indirect bandgap of centimeter-scale stacked multilayer MoS2 homostructures, which is indicated by the photoluminescence peak shift. Furthermore, we demonstrate that the stack structure can affect the electrical properties of MoS2 homostructures, where 30 degrees twist angle yields higher electron mobility. Our work provides a firm basis for the development of twistronics. Interlayer twist angle between vertically stacked 2D material layers can trigger exciting fundamental physics. Here, the authors report precise control of interlayer twist angle of stacked centimeter scale multilayer MoS2 homostructures that enables continuous change in their indirect bandgap, Moire phonons and electrical properties.

Room and High Temperature Tribological Performance of Multilayered TiSiN/TiN and TiSiN/TiN(Ag) Coatings Deposited by Sputtering

  • Authors: Fernandes, F., AL-Rjoub, A., Cavaleiro, D., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Coatings. 2020, 10(12), 1-13. ISSN 2079-6412.
  • Year: 2020
  • DOI: 10.3390/coatings10121191
  • Link: https://doi.org/10.3390/coatings10121191
  • Department: Department of Control Engineering
  • Annotation:
    In this study, we compare the tribological performance of a multilayer TiSiN/Ti(Ag)N coating with a TiSiN/TiN coating with a similar Si content in order to demonstrate the effect of the solid lubricant phase, silver. For Al2O3 balls, the hardness and reduced modulus determine the tribological performance of the coatings for tests conducted at room temperature (RT) against Al2O3 balls. At 550 degrees C, the TiSiN/TiN coating failed, whereas the Ag-containing coating performed better due to the presence of Ag in the contact, which decreased the shear stress and, consequently, the friction. For tests against TiAl6V4 balls, the Ag-containing coating was always better than the TiSiN/TiN one. At 550 degrees C, Ag in the wear track prevented the adhesion of the oxidized Ti-alloy wear debris in the contact, favoring the adhesion of wear debris from the coating to both the coating and counterpart surfaces. No wear could be measured for the 700 degrees C tests for both coatings due to different reasons: (i) the presence of oxidized adhered material from the ball to the reference TiSiN/TiN coating surface protected from wear and (ii) the presence of Ag-agglomerated particles decreased the friction and minimized the adhesion wear of the counterpart for the TiSiN/TiN(Ag) coating.

Synthesis, microstructure and mechanical properties of W-S-C self-lubricant thin films deposited by magnetron sputtering

  • Authors: Vuchkov, T., Evaristo, M., Bin Yaqub, T., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Tribology International. 2020, 150 ISSN 0301-679X.
  • Year: 2020
  • DOI: 10.1016/j.triboint.2020.106363
  • Link: https://doi.org/10.1016/j.triboint.2020.106363
  • Department: Department of Control Engineering
  • Annotation:
    W-S-C thin films were deposited by magnetron sputtering in a semi-industrial deposition unit. Various parameters like the composition, crystallinity, morphology and hardness were studied. A single coating was selected for tribological studies in different testing environments. The tribological response during ambient air testing was governed mainly by carbon-based phase at low normal loads and both carbon-based and WS2 rich tribofilms at higher loads. Very low coefficients of friction (similar to 0.01) were obtained during testing at elevated temperature due to accelerated formation of WS2 rich tribofilms. The tribological response in a vacuum against a steel-based counterbody was unsatisfactory due to the presence of a carbon-phase at the sliding interface. Discrepancies were observed between the tribological response in dry N-2 and vacuum environments.

The structural evolution of light-ion implanted 6H-SiC single crystal: Comparison of the effect of helium and hydrogen

  • DOI: 10.1016/j.actamat.2020.02.046
  • Link: https://doi.org/10.1016/j.actamat.2020.02.046
  • Department: Department of Control Engineering
  • Annotation:
    The microstructure evolution of hydrogen-implanted 6H-SiC at different temperatures and fluences is investigated by using various experimental techniques. In H-implanted samples with relatively low fluence at RT, dense blister cavities are observed after annealing at 1100 degrees C, while no visible blister cavities appear after annealing at 1100 degrees C in the sample implanted at RT with high fluence. The absence of blister cavities is due to the loss of elastic energy during the crystalline-to-amorphous transition. With a further increase of implantation temperature to 450 and 900 degrees C, amorphization did not occur and H-containing microcracks grew laterally below the surface. Thus, blisters appeared on the surface of the samples implanted at 900 degrees C even without annealing. The results are compared to the microstructural evolution of He-implanted 6H-SiC which was explored in our previous work. The behavior of hydrogen and helium ions in 6H-SiC lattice was rather different. For He implantation, regardless of the fluence and implantation temperature, blisters did not form. The mechanism of migration and coalescence of nanoscale bubbles that are responsible for blistering were studied via density functional theory calculations, which well-supported the presented results. We found that both mechanisms (migration and coalescence) are energetically cheaper in the case of H compared to He. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Volume and pressure of helium bubbles inside liquid Pb16Li. A molecular dynamics study

  • DOI: 10.1088/1741-4326/ab73c2
  • Link: https://doi.org/10.1088/1741-4326/ab73c2
  • Department: Department of Control Engineering
  • Annotation:
    The behaviour of helium impurities inside metals has been well studied in the last 30 years, however, little attention has been devoted to helium atoms inside liquid metals. Here we have investigated the nucleation and coalescence processes of helium atoms inside liquid eutectic lithium–lead alloys using atomistic simulations. Several key findings regarding He bubbles inside liquid PbLi eutectic are presented. The radius versus the number of atoms has been calculated in the temperature range 600–1000 K. The trend can be fitted and likely extrapolated to larger bubbles (micrometer size). The value of thermal expansion of He bubbles is given as well and compared to the thermal expansion of bulk He. The pressure inside He bubbles has been calculated as a function of bubble size. Finally, the importance of accurate interatomic potentials for the He–metal interaction is discussed.

Atomic-scale design of friction and energy dissipation

  • DOI: 10.1103/PhysRevB.99.094309
  • Link: https://doi.org/10.1103/PhysRevB.99.094309
  • Department: Department of Physics, Department of Control Engineering
  • Annotation:
    Study of friction and energy dissipation always relied on direct observations. Actual theories provide limited prediction on the frictional and dissipative properties if only the material chemistry and geometry are known. We here develop a framework to study intrinsic friction and energy dissipation based on the only knowledge of the normal modes of the system at equilibrium. We derive an approximated expression for the first anharmonic term in the potential energy expansion which does not require the computation of the third-order force constants. Moreover, we show how to characterize the frequency content of observed physical quantities and individuate the dissipative processes active during experimental measurements. As a case study, we consider the relative sliding motion of atomic layers in molybdenum disulfide dry lubricant, and we discuss how to extract information on the energetics of sliding from atomic force microscopy signals. The presented framework switches the investigation paradigm on friction and energy dissipation from dynamic to static studies, paving avenues to explore for the design of alternative anisotropic tribological and thermal materials.

Controllable Tunneling Triboelectrification of Two-Dimensional Chemical Vapor Deposited MoS2

  • DOI: 10.1038/s41598-018-36830-1
  • Link: https://doi.org/10.1038/s41598-018-36830-1
  • Department: Department of Control Engineering
  • Annotation:
    Tunneling triboelectrification of chemical vapor deposited monolayer MoS2 has been characterized at nanoscale with contact-mode atomic force microscopy (AFM) and Kelvin force microscopy (KFM). Although charges can be trapped on insulators like SiO2 by conventional triboelectrification, triboelectric charges tunneling through MoS2 and localized at the underlying substrate exhibit more than two orders of magnitude longer lifetime. Their polarity and density can be modified by triboelectric process with various bias voltages applied to Pt-coated AFM tips, and the saturated density is almost 30 times higher than the reported result of SiO2. Thus, the controllable tunneling triboelectric properties of MoS2 on insulating substrates can provide guidance to build a new class of two-dimensional (2D) MoS2- based nanoelectronic devices.

Deformation-Controlled Design of Metallic Nanocomposites

  • Authors: Yavas, H., Fraile, A., Huminiuc, T., Sen, H.S., Frutos, E., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: ACS Applied Materials & Interfaces. 2019, 11(49), 46296-46302. ISSN 1944-8244.
  • Year: 2019
  • DOI: 10.1021/acsami.9b12235
  • Link: https://doi.org/10.1021/acsami.9b12235
  • Department: Department of Control Engineering
  • Annotation:
    Achieving the theoretical strength of a metallic alloy material is a demanding task that usually requires utilizing one or more of the well-established routes: (1) Decreasing the grain size to stop or slow down the dislocation mobility, (2) adding external barriers to dislocation pathways, (3) altering the crystal structure, or (4) combining two of the previous discrete strategies, that is, implementing crystal seeds into an amorphous matrix. Each of the outlined methods has clear limitations; hence, further improvements are required. We present a unique approach that envelops all the different strength-building strategies together with a new phenomenon–phase transition. We simulated the plastic deformation of a Zr–Nb nanolayered alloy using molecular dynamics and ab initio methods and observed the transition of Zr from hexagonal close-packed to face-centered cubic and then to body-cenetered cubic during compression. The alloy, which was prepared by magnetron sputtering, exhibited near-theoretical hardness (10.8 GPa) and the predicted transition of the Zr structure was confirmed. Therefore, we have identified a new route for improving the hardness of metallic alloys.

Effect of implantation of C, Si and Cu into ZrNb nanometric multilayers

  • Department: Department of Control Engineering
  • Annotation:
    Sputter-deposited Zr/Nb nanometric multilayer films with a periodicity (L) in the range from 6 to 167 nm were subjected to carbon, silicon and copper ion irradiation with low and high fluences at room temperature. The ion profiles, mechanical proprieties, and disordering behavior have been investigated by using a variety of experimental techniques (Secondary Ion Mass Spectrometry - SIMS, nanoindentation, X-ray diffraction - XRD, and scanning transmission electron microscopy - STEM). On the STEM bright field micrographs there is damage clearly visible on the surface side of the multilayer; deeper, the most damaged and disordered zone, located close to the maximum ion concentration, was observed. The in-depth C and Si concentration profiles obtained from SIMS were not affected by the periodicity of the nanolayers. This is in accordance with SRIM simulations. XRD and electron diffraction analyses suggest a structural evolution in relation to L. After irradiation, Zr (0002) and Nb (110) reflexions overlap for L=6 nm. For the periodicity L> 6 nm the Zr (0002) peak is shifted to higher angles and Nb (110) peak is shifted to lower angles.

Microstructural evolution of helium-irradiated 6H–SiC subjected to different irradiation conditions and annealing temperatures: A multiple characterization study

  • DOI: 10.1016/j.actamat.2019.09.027
  • Link: https://doi.org/10.1016/j.actamat.2019.09.027
  • Department: Department of Control Engineering
  • Annotation:
    The microstructural phenomena occurring in 6H–SiC subjected to different irradiation conditions and annealing temperatures were investigated to assess the suitability of 6H–SiC as a structural material for nuclear applications. To this aim, a single crystal of 6H–SiC was subjected to He+ irradiation at 300 keV with different fluences and at temperatures ranging from 25 to 750 °C. Rutherford backscattering/channeling (RBS/C), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses were combined to shed light on the microstructural changes induced by irradiation and subsequent annealing (750 to 1500 °C). At room temperature, amorphization starts to occur at a fluence of 2.5 × 1016 cm−2 (0.66 dpa). On the contrary, amorphization was prevented at high irradiation temperatures and fluences. Furthermore, a thin and highly strained region located around the maximum He concentration (Rp) formed. This region results from the accumulation of interstitial atoms which are driven toward the highly damaged region under the actions of a strain gradient and high temperature. Regardless of the fluence and irradiation temperature, the material stores elastic energy, which leads to the trapping of He in dissimilar defect geometries. For irradiation temperatures below 750 °C, helium was accumulated in bubbles which coarsened after annealing. On the other hand, for an irradiation temperature of 750 °C, helium was trapped in platelets (even for medium fluence), which evolved into a homogeneous dense array of cavities during annealing. DFT calculations show that the bubbles are under high pressure and contribute to developing the overall tensile strain in the single crystal 6H–SiC.

Nanoscale frictional properties of ordered and disordered MoS2

  • DOI: 10.1016/j.triboint.2019.03.004
  • Link: https://doi.org/10.1016/j.triboint.2019.03.004
  • Department: Department of Physics, Department of Control Engineering
  • Annotation:
    The present work aims to understand the sliding of ordered/disordered molybdenum disulfide against itself by combination of nanoscale sliding experiments and atomistic simulations. Tribological experiments were performed using lateral force microscopy with tips covered by a thin sputtered MoS2film. Nanoscale contact area between the MoS2-coated tips and MoS2 samples opened up the possibility for close comparison with classical molecular dynamics simulations. Our simulations replicated well the coefficient of friction obtained by experiments for various contact conditions and shed light on nanoscale sliding of both crystalline and amorphous MoS2. Experimental sliding at humid environment demonstrated detrimental effect of water molecules on friction. However, such effect was much less pronounced when compared to that observed in macroscopic sliding experiments.

Structure, mechanical and tribological properties of Mo-S-N solid lubricant coatings

  • Authors: Hudec, T., Mikula, M., Satrapinskyy, L., Roch, T., Truchlý, M., Švec, P., Huminiuc, T., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Applied Surface Science. 2019, 486 1-14. ISSN 0169-4332.
  • Year: 2019
  • DOI: 10.1016/j.apsusc.2019.03.294
  • Link: https://doi.org/10.1016/j.apsusc.2019.03.294
  • Department: Department of Control Engineering
  • Annotation:
    Solid lubricant coatings deposited by plasma-assisted deposition techniques represent modern way to reduce the coefficient of friction in lubricant-free sliding contacts. Molybdenum disulphide (MoS2) is perhaps the most known and applied solid lubricant coating; however, its use is strictly limited by low hardness and environmental sensitivity. To improve mechanical and tribological properties, we doped MoS2 coating with nitrogen. Mo-S-N self-lubricant films were deposited by pulsed d. c. High Target Utilisation Sputtering (HiTUS) in reactive atmosphere. The effect of deposition conditions on chemical composition, structure and mechanical properties of MoSx and Mo-S-N coatings was studied; films with the most promising properties have been selected for tribological testing. MoSx film with the elemental composition sulphur to molybdenum (S/Mo ratio) 1.6 exhibited the coefficients of friction (COFs) in humid air 0.17 and 0.07 for loads 2 and 15 N, respectively. Mo-S-N films were prepared with nitrogen content in a range of 11 to 50 at., whereas S/Mo ratio varied from 1.35 to 0.4. Mo-S-N films were amorphous or nanostructured with nanograins of molybdenum disulphide. Hardness increased with N content up to 14 GPa for film with the highest content of nitrogen. Friction behaviour in humid air was evaluated using a ball-on-disk tribometer. Globally, the doping with N resulted in hardness in Mo–S–N films one order of magnitude higher and wear rate two orders of magnitude lower than in an undoped one, keeping the friction coefficient at the same level or even lower. These coatings showed remarkable friction coefficients in humid air from 0.28 to 0.05 with loads from 2 to 15 N, respectively. The excellent friction properties were attributed to the formation of a thin molybdenum disulphide tribofilm at the sliding interface. HiTUS represents a very promising way of producing thin films on the thermally sensitive substrates (e.g.bearing steel) with desired properties.

Superlubricity achieved for commensurate sliding: MoS2 frictional anisotropy in silico

  • Authors: Claerbout, V., prof. Ing. Tomáš Polcar, Ph.D., Nicolini, P.
  • Publication: Computational Materials Science. 2019, 163 17-23. ISSN 0927-0256.
  • Year: 2019
  • DOI: 10.1016/j.commatsci.2019.03.019
  • Link: https://doi.org/10.1016/j.commatsci.2019.03.019
  • Department: Department of Control Engineering
  • Annotation:
    In the past decades, MoS2 has received substantial attention from the tribology community due to its excellent frictional properties. However, a fundamental understanding of the mechanisms determining friction at the nanoscale is yet to be achieved. The general consensus is that one of the ingredients required for obtaining the superlubric regime when sliding layers of MoS2 atop each other is incommensurability. Herein, we report on a molecular dynamics study focused on studying the effects of the sliding direction on the frictional properties of commensurate MoS2. It is found that different types of dynamics are obtained with different sliding directions, with the stick-slip dynamics characterized by a highly dissipative behavior. Also, it is found that there is a relation linking the mismatch angle between the driving force and the actual trajectory and the load-dependence of the frictional force. We show, for the first time with a computational study, that incommensurability is not required for achieving the superlubric behavior of MoS2.

The role of α″ orthorhombic phase content on the tenacity and fracture toughness behavior of Ti-22Nb-10Zr coating used in the design of long-term medical implants

  • DOI: 10.1016/j.apsusc.2018.09.017
  • Link: https://doi.org/10.1016/j.apsusc.2018.09.017
  • Department: Department of Control Engineering
  • Annotation:
    Tenacity and fracture toughness of a novel β/α″ Ti-22Nb-10Zr (wt.%) coating processed by magnetron sputtering were modified as a result of the martensitic transformation (β → α″) activated by the presence of compressive residual stresses when the coating deposition is performed at high bias voltage values. Mechanical properties, such as hardness, H, and Young's modulus, E, values, and therefore elastoplastic response of the coating were characterized through H/E, and H3/E2 ratios as a function of the extent of the martensitic transformation. These ratios were correlated to the elastic response and to the resistance to plastic deformation of a surface subjected to sliding mechanical contact, respectively. The usefulness of both ratios to design “hard and tough” coatings, suitable for enhancing of its wear resistance, is compared with the tenacity, G, the semi-quantitative, FT, and the quantitative, KI, fracture toughness values obtained from nano-scratch characterization. Results show that Ti-22Nb-10Zr (wt.%) coating with the highest and lowest hardness and Young's modulus values, and therefore the highest H/E and H3/E2, has the highest cracking resistance and fracture toughness. Under linearly ramped loading from 0.1 to 5 and 100 mN it was impossible to produce fracture of the coating when it was deposited with a bias voltage of −63 V. In return, the coating deposited with a bias voltage of −148 V shows an almost complete elastic recovery until the moment of its fracture and delamination, which is an evidence of its high tenacity and superior fracture toughness. The KI value is ∼21 MPa∗m1/2, which is higher than typical values of bio-ceramics (Al2O3 and ZrO2) used in medical applications, demonstrating that this coating could be used in components subjected to high wear and cyclic impacts, e.g. on femoral heads in artificial hip joints.

The wettability and tribological behaviour of thin F-doped WS2 films deposited by magnetron sputtering

  • Authors: Rodrigues, S.P., prof. Ing. Tomáš Polcar, Ph.D., Carvalho, S., Cavaleiro, A.
  • Publication: Surface and Coatings Technology. 2019, 378 ISSN 0257-8972.
  • Year: 2019
  • DOI: 10.1016/j.surfcoat.2019.125033
  • Link: https://doi.org/10.1016/j.surfcoat.2019.125033
  • Department: Department of Control Engineering
  • Annotation:
    This work aims to study the influence of the thickness on the wettability behaviour and on the tribological performance of thin WS2–C coatings doped with fluorine (F) deposited by magnetron sputtering. The morphology of the coatings did not change with the thickness decrease, revealing an increase on both the compactness and on the smoothening of the coating surface with F increase. Such an evolution led to more hydrophilic and less oleophilic surface behaviour. XRD results showed that the coatings start growing with an alignment of the (002) planes parallel to the substrate, which changed for randomly oriented crystals with the thickness increase. F insertion induced a decrease in the crystallinity, more evident in the case of thinner coatings. This result was corroborated by Raman analysis which showed a broadening of the main peaks assigned to WS2. Tribological tests carried out at room temperature without lubrication showed lower friction coefficient for F-doped coatings, especially for the thinner one (in the overall test, average 0.03). Regarding the lubricated tests performed with pure PAO (polyalphaoleophin) oil, both thin coatings presented similar friction coefficients (∼0.07) in the steady-state regime. The tribolayer based on oriented W–S layers formed on the wear track for the F-doped coating can justify the very low friction achieved when testing in non-lubricated conditions.

Triboelectrification of Two-Dimensional Chemical Vapor Deposited WS2 at Nanoscale

  • DOI: 10.1038/s41598-019-49107-y
  • Link: https://doi.org/10.1038/s41598-019-49107-y
  • Department: Department of Control Engineering
  • Annotation:
    Triboelectric properties of chemical vapor deposited WS2 nanoflakes have been characterized in nano-range by atomic force microscopy (AFM) and Kelvin force microscopy (KFM). The triboelectric process is dependent on the thickness of WS2 nanoflakes, and it is sensitive to the adsorbates like water molecules, as well as transferred Pt from the tip on the sample. The density of tribo-charge can be modified by applying various biases to the conductive Pt-coated tip during the frictional process. Tunneling of the tribo-charge into the gap between WS2 and the underlying substrate results in a long lifetime, which is about 100 times longer than conventional triboelectric charges. Moreover, we observe a positive correlation between the layer number and resistance to charge dissipation. Our finding can become the driving force for a new category of two-dimensional (2D) WS2 triboelectrically controllable nanodevices.

Vacancy-interface-helium interaction in Zr-Nb multi-layer system: A first-principles study

  • DOI: 10.1016/j.jnucmat.2019.02.030
  • Link: https://doi.org/10.1016/j.jnucmat.2019.02.030
  • Department: Department of Control Engineering
  • Annotation:
    Driven by our previous experimental findings, we have performed ab-initio simulations to investigate the mechanical properties of and vacancy-interface-helium interaction at Zr-Nb (HCP-BCC) multi-layer composite at Burgers orientation ((0 0 0 2)//(1 1 0) - < 2 -1 -1 0 > // < 1 -1 1 >). HCP-BCC interfaces have not been previously modelled using density functional theory. We present many aspects of this interfacial system such as elastic constants, charge densities and densities of states which, although being crucial in understanding the properties of the materials with interfaces at the atomistic level, are usually ignored by similar works. A thorough analysis of the mechanical properties of Zr-Nb multi-layers has been performed and compared with the behaviour of their constituents in bulk form. The results are promising for applicational purposes as the multi-layers retain most of the mechanical properties of the bulk forms except for the anisotropy index. Furthermore, we present electronic structure analysis which reveals formation of bonding between opposing Zr and Nb atoms and transfer of some net charge at the interface. These results, together with the energetics of the systems, are used to understand the interaction among vacancy, He and the interface which gives insight for foreseeing the behaviour of the system in the presence of He atoms. (C) 2019 Elsevier B.V. All rights reserved.

Viewpoint: Atomic-Scale Design Protocols toward Energy, Electronic, Catalysis, and Sensing Applications

  • Authors: Belviso, F., Claerbout, V., Comas-Vives, A., Dalal, N.S., Fan, F., Filippetti, A., Fiorentini, V., Foppa, L., Franchini, C., Geisler, B., Ghiringhelli, L.M., Groß, A., Hu, S., Íñiguez, J., Kauwe, S.K., Musfeldt, J.L., Nicolini, P., Pentcheva, R., prof. Ing. Tomáš Polcar, Ph.D., Ren, W., Ricci, F., Ricci, F., Sen, H.S., Skelton, J.M., Sparks, T.D., Stroppa, A., Urru, A., Vandichel, M., Vavassori, P., Wu, H., Yang, K., Zhao, H.J., Puggioni, D., Cortese, R., doc. Antonio Cammarata, Ph.D.,
  • Publication: Inorganic Chemistry. 2019, 58(22), 14939-14980. ISSN 0020-1669.
  • Year: 2019
  • DOI: 10.1021/acs.inorgchem.9b01785
  • Link: https://doi.org/10.1021/acs.inorgchem.9b01785
  • Department: Department of Control Engineering
  • Annotation:
    Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic, and optoelectronic applications. However, because of the increase of technological needs, it is essential to identify new functional materials and improve the properties of existing ones. The objective of this Viewpoint is to examine the state of the art of atomic-scale simulative and experimental protocols aimed to the design of novel functional nanostructured materials, and to present new perspectives in the relative fields. This is the result of the debates of Symposium I “Atomic-scale design protocols towards energy, electronic, catalysis, and sensing applications”, which took place within the 2018 European Materials Research Society fall meeting.

Competing mechanisms on the strength of ion-irradiated Zr/Nb nanoscale multilayers: Interface strength versus radiation hardening

  • DOI: 10.1016/j.scriptamat.2018.03.039
  • Link: https://doi.org/10.1016/j.scriptamat.2018.03.039
  • Department: Department of Control Engineering
  • Annotation:
    The structural stability and mechanical properties of sputter-deposited Zr/Nb nanoscale multilayers subjected to Si-ion irradiation were investigated in relation to the individual layer thickness. The interface density distribution played a major role on the nature and amount of accumulated radiation damage. The multilayer with a smaller periodicity experienced a significantly higher atomic-scale disorder and radiation hardening compared to the multilayer with thicker individual layers. In the latter case, an enhanced radiation damage tolerance was achieved due to the balance between competing deformation mechanisms.

Development of new beta/alpha ''-Ti-Nb-Zr biocompatible coating with low Young's modulus and high toughness for medical applications

  • Authors: Frutos, E., Karlík, M., Jimenez, A.J., Langhansova, H., Lieskovska, J., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Materials & Design. 2018, 142 44-55. ISSN 0264-1275.
  • Year: 2018
  • DOI: 10.1016/j.matdes.2018.01.014
  • Link: https://doi.org/10.1016/j.matdes.2018.01.014
  • Department: Department of Control Engineering
  • Annotation:
    Ideal biomaterials to fabricate orthopedic implants, especially for load-bearing joint replacements, should include only non-toxic elements with good biocompatibility, high corrosion resistance and surface bioactivity, together with a good combination of mechanical properties. Based on these criteria, a manufacturing approach based on sputtering techniques can be ideal to develop coatings free of toxic elements tailored for advanced applications on pure titanium or titanium alloys used in biomedical applications. In this work, the ternary Ti-Nb-Zr system was used to develop non-toxic beta-rich Ti coatings with several complex microstructures by careful control of Nb and Zr concentration and deposition parameters, such as bias voltage. Depending on the alloy chemistry and processing, the coating included variable amounts of alpha-, alpha '' and beta-phases of Ti with different morphologies and crystallographic texture. Mechanical properties of every coating is largely determined by the micro-structure present, which is directly related to bias voltage used during sputtering process. Thus, hardness values change as a function of the compressive residual stresses magnitude and Young's modulus decreased from 63 GPa, at 0 V, to 47 GPa, at -63 V, being this value close to human bone (similar to 30 GPa). After that, Young's modulus progressively increases to 89 GPa, at -148 V. On the other hand, bioactivity of the coating is practically doubled when compared to Ti6AL4V alloy.

Effect of layer thickness on the mechanical behaviour of oxidation-strengthened Zr/Nb nanoscale multilayers

  • Authors: Monclús, M.A., Calisti, M., prof. Ing. Tomáš Polcar, Ph.D., Yang, L.W., Molina-Aldareguía, J.M., Llorca, J.
  • Publication: Journal of Materials Science. 2018, 53(8), 5860-5878. ISSN 0022-2461.
  • Year: 2018
  • DOI: 10.1007/s10853-017-1665-6
  • Link: https://doi.org/10.1007/s10853-017-1665-6
  • Department: Department of Control Engineering
  • Annotation:
    The effect of bilayer thickness (L) reduction on the oxidation-induced strengthening of Zr/Nb nanoscale metallic multilayers (NMM) is investigated. Zr/Nb NMMs with L = 10 and 75 nm were annealed at 350 °C for a time ranging between 2 and 336 h, and the changes in structure and deformation behaviour were studied by nanoscale mechanical testing and analytical electron microscopy. Annealing led to the transformation of the Zr layers into ZrO2 after a few hours, while the Nb layers oxidised progressively at a much slower rate. The sequential oxidation of Zr and Nb layers was found to be key for the oxidation to take place without rupture of the multilayered structure and without coating spallation in all cases. However, the multilayers with the smallest bilayer thickness (L = 10 nm) presented superior damage tolerance and therefore structural integrity during the oxidation process, while for L = 75 nm the volumetric expansion associated with oxidation led to the formation of cracks at the interfaces and within the ZrO2 layers. As a result, the nanoindentation hardness increase after annealing was significantly higher for the nanolaminate with L = 10 nm. Comparison between nanoindentation and micropillar compression behaviour of the oxidised NMMs demonstrates that the hardness increase upon oxidation arises from the contribution of the residual stresses associated with the volume increase due to oxidation and to the higher strength of the oxides.

Structural Ordering of Molybdenum Disulfide Studied via Reactive Molecular Dynamics Simulations

  • Authors: Nicolini, P., Capozza, R., Restuccia, P., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: ACS Applied Materials & Interfaces. 2018, 10(10), 8937-8946. ISSN 1944-8244.
  • Year: 2018
  • DOI: 10.1021/acsami.7b17960
  • Link: https://doi.org/10.1021/acsami.7b17960
  • Department: Department of Control Engineering
  • Annotation:
    Molybdenum disulfide (MoS2) is a well-known and effective lubricant that provides extremely low values of coefficient of friction. It is known that the sliding process may induce structural transformations of amorphous or disordered MoS2 to the crystalline phase with basal planes oriented parallel to the sliding direction, which is optimal for reducing friction. However, the key reaction parameters and conditions promoting this structural transformation are still largely unknown. We investigate, by employing reactive molecular dynamics simulations, the formation of MoS2 layers from an amorphous phase as a function of temperature, initial sample density, and sliding velocity. We show that the formation of ordered crystalline structures can be explained in the framework of classical nucleation theory as it predicts the conditions for their nucleation and growth. These results may have important implications in the fields of coating and thin-film deposition, tribology, and in all technological applications where a fast and effective structural transition to an ordered phase is needed.

Tribological and cutting performance of TiAlCrN films with different Cr contents deposited with multilayered structure

  • DOI: 10.1016/j.triboint.2017.11.008
  • Link: https://doi.org/10.1016/j.triboint.2017.11.008
  • Department: Department of Control Engineering
  • Annotation:
    Increasing requirements on high speed and dry cutting applications open up new demands on the quality of cutting tool materials. In this study, we evaluated the effect of Cr additions on the properties of Ti(Al)N/Cr(Al)N multilayered films deposited be magnetron sputtering. In particular, we studied the high temperature tribological properties and the cutting performance in real service drilling tests and compared the results with a monolayer Ti0.47Al0.46N reference film. Dry sliding experiments were performed using a pin-on-disc tribometer at room temperature and at 650 °C with Al2O3 balls as counterparts. The drilling performance of the coatings was evaluated at cutting speeds in the range 50–200 m/min. Wear of the drills in different zones (chisel edge and flank edge) was mapped after each 15 holes.

2H -> 1T Phase Engineering of Layered Tantalum Disulfides in Electrocatalysis: Oxygen Reduction Reaction

  • Authors: Luxa, J., Mazánek, V., Pumera, M., Lazar, P., Sedmidubský, D., Callisti, M., prof. Ing. Tomáš Polcar, Ph.D., Sofer, Z.
  • Publication: Chemistry - A European Journal. 2017, 23(33), 8082-8091. ISSN 0947-6539.
  • Year: 2017
  • DOI: 10.1002/chem.201701494
  • Link: https://doi.org/10.1002/chem.201701494
  • Department: Department of Control Engineering
  • Annotation:
    Tremendous attention is currently being paid to renewable sources of energy. Transition-metal dichalcogenides (TMDs) have been intensively studied for their promising catalytic activities in the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR). In this fundamental work, we explored the catalytic properties of TMD family members 2H TaS2 and 1T TaS2. Our findings reveal that both polytypes exhibit poor HER performance, which is even more pronounced after electrochemical reduction/oxidation. Our experimental data show that 1T TaS2 has a lower overpotential at a current density of −10 mA cm−1, despite theoretical DFT calculations that indicated that the more favorable free energy of hydrogen adsorption should make “perfect” 2H TaS2 a better HER catalyst. Thorough characterization showed that the higher conductivity of 1T TaS2 and a slightly higher surface oxidation of 2H TaS2 explains this discrepancy. Moreover, changes in the catalytic activity after electrochemical treatment are addressed here. For the ORR in an alkaline environment, the electrochemical treatment led to an improvement in catalytic properties. With onset potentials similar to that of Pt/C catalysts, TaS2 was found to be an efficient catalyst for the ORR, rather than for proton reduction, in contrast to the behavior of Group 6 layered TMDs.

Combined size and texture-dependent deformation and strengthening mechanisms in Zr/Nb nano-multilayers

  • DOI: 10.1016/j.actamat.2016.11.007
  • Link: https://doi.org/10.1016/j.actamat.2016.11.007
  • Department: Department of Control Engineering
  • Annotation:
    A combination of transmission electron microscopy analyses and nanomechanical measurements was performed in this study to reveal deformation and strengthening mechanisms occurring in sputtered Zr/Nb nanoscale metallic multilayers (NMMs) with a periodicity (L) in the range 6–167 nm. Electron diffraction analyses revealed a change in the crystallographic orientation of α-Zr when L ≤ 27 nm, while Nb structure retained the same orientations regardless of L. For L > 60 nm, the strengthening mechanism is well described by the Hall-Petch model, while for 27 < L < 60 nm the refined CLS model comes into picture. A decrease in strength is found for L < 27 nm, which could not be simply explained by considering only misfit and Koehler stresses. For L ≤ 27 nm, plastic strain measured across compressed NMMs revealed a change in the plastic behaviour of α-Zr, which experienced a hard-to-soft transition. At these length scales, the combination of two structural factors was found to affect the strength. These relate to the formation of weaker interfaces which extend the effective distance between strong barriers against dislocation transmission, thus producing a softening effect. The second effect relates to the crystallographic orientation change exhibited by α-Zr for L < 27 nm with a consequent change of the dominant slip system. The actual strength at these smaller length scales was effectively quantified by taking these structural aspects into account in the interface barrier strength model.

Influence of Cr additions on the structure and oxidation resistance of multilayered TiAlCrN films

  • DOI: 10.1016/j.surfcoat.2017.01.053
  • Link: https://doi.org/10.1016/j.surfcoat.2017.01.053
  • Department: Department of Control Engineering
  • Annotation:
    Ti1 - xAlxN coatings with NaCl-type structure have been widely used in advanced machining and other high temperature applications due to their excellent mechanical, thermal and tribological properties. Here, we investigated the influence of Cr additions on the structure, mechanical properties and oxidation resistance, including the oxide scale characterization, of multilayered TiAlN/CrAlN coatings deposited by magnetron sputtering. The properties of Cr rich coatings were compared to a TiAlN film deposited as reference. XRD diffraction analysis revealed that all coatings showed an fcc NaCl-type structure. However, for the TiAlN monolayer Ti and Al are forming a solid solution whilst, for Cr rich coatings, a multilayer structure alternating TiAlN and CrAlN layers, also forming solid solutions, was grown as a result of the geometry of the targets distribution inside the deposition chamber in combination with a slow rotation of the sample holder and a sufficiently high deposition rate. TGA measurements showed that Cr additions increased the oxidation performance of the coatings. For Ti0.47Al0.46N, dual oxide layers occur when tested at 800 degrees C, being the porous inner one of TiO2 and the outer a compact and continuous layer of Al-oxide which protects the coating from the oxidation. At 900 degrees C the oxidation resistance of this film degraded due to the fast Ti ions diffusion to the surface which impedes the formation of the continuous and protective Al oxide layer.

Overcoming nanoscale friction barriers in transition metal dichalcogenides

  • DOI: 10.1103/PhysRevB.96.085406
  • Link: https://doi.org/10.1103/PhysRevB.96.085406
  • Department: Department of Control Engineering
  • Annotation:
    We study the atomic contributions to the nanoscale friction in layered MX2 (M = Mo, W; X = S, Se, Te) transitionmetal dichalcogenides by combining ab initio techniques with group-theoretical analysis. Starting from stable atomic configurations, we propose a computational method, named normal-modes transition approximation (NMTA), to individuate possible sliding paths from only the analysis of the phonon modes of the stable geometry. The method provides a way to decompose the atomic displacements realizing the layer sliding in terms of phonon modes of the stable structure, so as to guide the selection and tuning of specific atomic motions promoting MX2 sheets gliding, and to adjust the corresponding energy barrier. The present results show that main contributions to the nanoscale friction are due to few low frequency phonon modes, corresponding to rigid shifts of MX2 layers. We also provide further evidences that a previously reported Ti-doped MoS2 phase is a promising candidate as new material with enhanced tribologic properties. The NMTA approach can be exploited to tune the energetic and the structural features of specific phonon modes, and, thanks to its general formulation, can also be applied to any solid state system, irrespective of the chemical composition and structural topology.

Selective oxidation-induced strengthening of Zr/Nb nanoscale multilayers

  • Authors: Monclús, M.A., Callisti, M., prof. Ing. Tomáš Polcar, Ph.D., Yang, L.W., Llorca, J., Molina-Aldareguía, J.M.
  • Publication: Acta materialia. 2017, 122 1-10. ISSN 1359-6454.
  • Year: 2017
  • DOI: 10.1016/j.actamat.2016.09.021
  • Link: https://doi.org/10.1016/j.actamat.2016.09.021
  • Department: Department of Control Engineering
  • Annotation:
    The paper presents a new approach, based on controlled oxidation of nanoscale metallic multilayers, to produce strong and hard oxide/metal nanocomposite coatings with high strength and good thermal stability. The approach is demonstrated by performing long term annealing on sputtered Zr/Nb nanoscale metallic multilayers and investigating the evolution of their microstructure and mechanical properties by combining analytical transmission electron microscopy, nano-mechanical tests and finite element models. As-deposited multilayers were annealed at 350 °C in air for times ranging between 1 and 336 h. The elastic modulus increased by ∼20% and the hardness by ∼42% after 15 h of annealing. Longer annealing times did not lead to changes in hardness, although the elastic modulus increased up to 35% after 336 h. The hcp Zr layers were rapidly transformed into monoclinic ZrO2 (in the first 15 h), while the Nb layers were progressively oxidised, from top surface down towards the substrate, to form an amorphous oxide phase at a much lower rate. The sequential oxidation of Zr and Nb layers was key for the oxidation to take place without rupture of the multi-layered structure and without coating spallation, as the plastic deformation of the metallic Nb layers allowed for the partial relieve of the residual stresses developed as a result of the volumetric expansion of the Zr layers upon oxidation. Moreover, the development of residual stresses induced further changes in mechanical properties in relation to the annealing time, as revealed by finite element simulations.

Vibrational Contributions to Intrinsic Friction in Charged Transition Metal Dichalcogenides

  • DOI: 10.1039/c7nr04034b
  • Link: https://doi.org/10.1039/c7nr04034b
  • Department: Department of Control Engineering
  • Annotation:
    Vibrational contributions to intrinsic friction in layered transition metal dichalcogenides (TMD) have been studied at different charge content. We find that any deviation from charge neutrality produces complex rearrangements of atomic positions and electronic distribution, and consequent phase transitions. Upon charge injection, cell volume expansion is observed, due to charge accumulation along an axis orthogonal to the layer planes. Such accumulation is accounted by the d3z2-r2 orbital of the transition metal and it is regulated by the Pt2g,eg orbital polarization. The latter, in turn, determines the frequency of the phonon modes related to the intrisic friction through non-trivial electro-vibrational coupling. The bond covalency and atom pair cophonicity can be exploited as a knob to control such coupling, ruling subtle charge flows through atomic orbitals hence determining vibrational frequencies at specific charge content. The results can be exploited to finely tune vibrational contributions to intrinsic friction in TMD structures, in order to facilitate assembly and operation of nanoelectromechanical systems and, ultimately, to govern electronic charge distribution in TMD-based devices for applications beyond nanoscale tribology.

A comparison of empirical potentials for sliding simulations of MoS2

  • DOI: 10.1016/j.commatsci.2016.01.013
  • Link: https://doi.org/10.1016/j.commatsci.2016.01.013
  • Department: Department of Control Engineering
  • Annotation:
    In recent years, a lot of effort has been devoted to the investigation of the properties of molybdenum disulfide by using both experimental and theoretical/computational techniques. In fact MoS2 applications include catalysis, nanoelectronics and lubrication. Among other things, this is also reflected in the number of empirical force fields for MoS2 that have been developed and proposed in literature. However, as in any classical molecular dynamics simulations, the accuracy of the calculated properties strongly depends on the reliability of the empirical potential employed. In this article we review critically all the available force fields for MoS2, focusing on the calculation of the properties that are more involved in determining the tribological behavior. On the basis of the presented results, we provide recommendations for a proper choice of the empirical potentials to use in a computational study, as well as indications for future refinements of the existing force fields.

Broadband Optical Absorption Caused by the Plasmonic Response of Coalesced Au Nanoparticles Embedded in a TiO2 Matrix

  • Authors: Borges, J., Pereira, R.M.S., Rodrigues, M.S., Kubart, T., Kumar, S., Leifer, K., Cavaleiro, A., prof. Ing. Tomáš Polcar, Ph.D., Vasilevskiy, M.I., Vaz, F.
  • Publication: Journal of Physical Chemistry C. 2016, 120(30), 16931-16945. ISSN 1932-7447.
  • Year: 2016
  • DOI: 10.1021/acs.jpcc.6b03684
  • Link: https://doi.org/10.1021/acs.jpcc.6b03684
  • Department: Department of Control Engineering
  • Annotation:
    The effect of Au nanoparticles' (NPs) concentration, size, and spatial distribution within a TiO2 dielectric matrix on the localized surface plasmon resonance (LSPR) band characteristics was experimentally and theoretically studied. The results of the analysis of the Au NPs' size distributions allowed us to conclude that isolated NPs grow only up to 5 to 6 nm in size, even for the highest annealing temperature used. However, for higher volume fractions of Au, the coalescence of closely located NPs yields elongated clusters that are much larger in size and cause a considerable broadening of the LSPR band. This effect was confirmed by Monte Carlo modeling results. Coupled dipole equations were solved to find the electromagnetic modes of a supercell, where isolated and coalesced NPs were distributed, from which an effective dielectric function of the nanocomposite material was calculated and used to evaluate the optical transmittance and reflectance spectra. The modeling results suggested that the observed LSPR band broadening is due to a wider spectral distribution of plasmonic modes, caused by the presence of coalesced NPs (in addition to the usual damping effect). This is particularly important for detection applications via surface-enhanced Raman spectroscopy (SERS), where it is desirable to have a spectrally broad LSPR band in order to favor the fulfillment of the conditions of resonance matching, to electronic transitions in detected species.

Bubbles formation in helium ion irradiated Cu/W multilayer nanocomposites: Effects on structure and mechanical properties

  • DOI: 10.1016/j.jnucmat.2016.02.006
  • Link: https://doi.org/10.1016/j.jnucmat.2016.02.006
  • Department: Department of Control Engineering
  • Annotation:
    This study investigates the effects of He bubbles on structural and mechanical properties of sputter-deposited Cu/W multilayers. A multilayer with a periodicity of 10 nm was deposited and subjected to helium ion irradiation with two different fluences. He bubbles formed mostly in Cu layers and their distribution was affected by He concentration and radiation damage. According to SRIM calculations, in low He concentration regions bubbles formed mostly along interfaces, while more homogeneously distributed bubbles were found in Cu layers and along columnar grain boundaries in higher He concentration regions. We suggest that the capability of interfaces to annihilate point defects is weakened by the He bubbles shielding effect. Nanoindentation tests revealed a hardness decrease amounting to similar to 0.5 and similar to 1 GPa for low and high fluences, respectively. The observed softening effect is attributed to He storage-induced changes in residual stresses and columnar grain boundary/interfacial sliding facilitated by He bubbles. (C) 2016 Elsevier B.V. All rights reserved.

Fracture toughness determination by repetitive nano-impact testing in Cu/W nanomultilayers with length-scale-dependent films properties

  • Authors: Frutos Torres, E., Karlík, M., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Procedia Structural Integrity 2. Kidlington Oxford OX GB: Elsevier, 2016. p. 1391-1404. ISSN 2452-3216.
  • Year: 2016
  • DOI: 10.1016/j.prostr.2016.06.177
  • Link: https://doi.org/10.1016/j.prostr.2016.06.177
  • Department: Department of Control Engineering
  • Annotation:
    Nanoscale metallic multilayers based on Cu/W have been considered as a potential material for structural applications in nuclear reactors and for the cladding of storage tanks for advanced fuels kept at high temperatures. The understanding of how mechanical properties change in relation to periodicity, is required in order to use CU/W nano-multilayers as a protective coating against radiation damage.

Layering effects on low frequency modes in n-layered MX2 transition metal dichalcogenides

  • DOI: 10.1039/c5cp06788j
  • Link: https://doi.org/10.1039/c5cp06788j
  • Department: Department of Control Engineering
  • Annotation:
    n-Layered (n = 2, 3, 4) MX2 transition metal dichalcogenides (M = Mo, W; X = S, Se, Te) have been studied using DFT techniques. Long-range van der Waals forces have been modeled using the Grimme correction to capture interlayer interactions. We study the dynamic and electronic dependence of atomic displacement on the number of layers. We find that the displacement patterns mainly affected by a change in the layer number are low-frequency modes at Γ and A k-points; such modes are connected with the intrinsic tribological response. We disentangle electro–phonon coupling by combining orbital polarization, covalency and cophonicity analysis with phonon band calculations. We find that the frequency dependence on the number of layers and the atomic type has a non-trivial relation with the electronic charge distribution in the interlayer region. We show that the interlayer electronic density can be adjusted by appropriately tuning M–X cophonicity, acting as a knob to control vibrational frequencies, hence the intrinsic frictional response. The present results can be exploited to study the electro–phonon coupling effects in TMD-based materials beyond tribological applications.

Nanomechanical characterization of alumina coatings grown on FeCrAl alloy by thermal oxidation

  • Authors: Frutos, E., González-Carrasco, J.L., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Journal of the Mechanical Behavior of Biomedical Materials. 2016, 57 310-320. ISSN 1751-6161.
  • Year: 2016
  • DOI: 10.1016/j.jmbbm.2016.01.027
  • Link: https://doi.org/10.1016/j.jmbbm.2016.01.027
  • Department: Department of Control Engineering
  • Annotation:
    This work studies the feasibility of using repetitive-nano-impact tests with a cube-corner tip and low loads for obtaining quantitative fracture toughness values in thin and brittle coatings. For this purpose, it will be assumed that the impacts are able to produce a cracking, similar to the pattern developed for the classical fracture toughness tests in bulk materials, and therefore, from the crack developed in the repetitive impacts it will be possible to evaluate the suitability of the classical indentation models (Anstins and Laugier) for measuring fracture toughness. However, the length of this crack has to be lower than 10% of the total coating thickness to avoid substrate contributions. For this reason, and in order to ensure a small plastic region localized at the origin of the crack tip, low load values (or small distance between the indenter tip and the surface) have to be used. In order to demonstrate the validity of this technique, repetitive-nano-impact will be done in a fine and dense oxide layer (α-Al2O3), which has been developed on the top of oxide dispersion strengthened (ODS) FeCrAl alloys (PM 2000) by thermal oxidation at elevated temperatures. Moreover, it will be shown how it is possible to know with each new impact the crack geometry evolution from Palmqvist crack to half-penny crack, being able to study the proper evolution of the different values of fracture toughness in terms of both indentation models and as a function of the strain rate, ε, decreasing. Thereby, fracture toughness values for α-Al2O3 layer decrease from ~4.40 √m , for high ε value (103 s-1), to ~3.21MPam, for quasi-static ε value (10-3 s-1). On the other hand, ε a new process to obtain fracture toughness values will be analysed, when the classical indentation models are not met.

Biological behaviour of thin films consisting of Au nanoparticles dispersed in a TiO2 dielectric matrix

  • Authors: Borges, J., Costa, D., Antunes, E., Lopes, C., Rodrigues, M.S., Apreutesei, M., Alves, E., Barradas, N.P., Pedrosa, P., Moura, C., Cunha, L., prof. Ing. Tomáš Polcar, Ph.D., Vaz, F., Sampaio, P.
  • Publication: VACUUM. 2015, 122(1), 360-368. ISSN 0042-207X.
  • Year: 2015
  • DOI: 10.1016/j.vacuum.2015.03.036
  • Link: https://doi.org/10.1016/j.vacuum.2015.03.036
  • Department: Department of Control Engineering
  • Annotation:
    In this work it was studied the possible use of thin films, composed of Au nanoparticles (NPs) embedded in a TiO2 matrix, in biological applications, by evaluating their interaction with a well-known protein, Bovine Serum Albumin (BSA), as well as with microbial cells (Candida albicans). The films were produced by one-step reactive DC magnetron sputtering followed by heat-treatment. The samples revealed a composition of 8.3 at.% of Au and a stoichiometric TiO2 matrix. The annealing promoted grain size increase of the Au NPs from 3 nm (at 300 °C) to 7 nm (at 500 °C) and a progressive crystallization of the TiO2 matrix to anatase. A broad localized surface plasmon resonance (LSPR) absorption band (λ = 580-720 nm) was clearly observed in the sample annealed at 500 °C, being less intense at 300 °C. The biological tests indicated that the BSA adhesion is dependent on surface nanostructure morphology, which in turn depends on the annealing temperature that changed the roughness and wettability of the films. The Au:TiO2 thin films also induced a significant change of the microbial cell membrane integrity, and ultimately the cell viability, which in turn affected the adhesion on its surface. The microstructural changes (structure, grain size and surface morphology) of the Au:TiO2 films promoted by heat-treatment shaped the amount of BSA adhered and affected cell viability.

Effect of the substrate dilution on the room and high temperature tribological behaviour of Ni-based coatings deposited by PTA on grey cast iron

  • Authors: Fernandes, F., prof. Ing. Tomáš Polcar, Ph.D., Loureiro, A., Cavaleiro, A.
  • Publication: Surface and Coatings Technology. 2015, 281 11-19. ISSN 0257-8972.
  • Year: 2015
  • DOI: 10.1016/j.surfcoat.2015.09.034
  • Link: https://doi.org/10.1016/j.surfcoat.2015.09.034
  • Department: Department of Control Engineering
  • Annotation:
    In the present investigation the effect of the substrate dilution on room and high temperature (550 and 700 °C) tribological behaviour of nickel based hardfaced coating deposited by plasma transferred arc onto a grey cast iron was investigated and compared to the uncoated grey cast iron. At room temperature, the wear loss of coatings was independent of the substrate dilution and similar to the grey cast iron. At high temperatures, coating pro- duced with high dilution displayed the highest wear resistance between all the samples. This is attributed to the formation of a protective tribo-layer resulting from the agglomeration of a high amount of oxide debris due to its lower oxidation resistance when compared to the sample produced with low dilution.

Electro-vibrational coupling effects on "intrinsic friction" in transition metal dichalcogenides

  • DOI: 10.1039/c5ra24837j
  • Link: https://doi.org/10.1039/c5ra24837j
  • Department: Department of Control Engineering
  • Annotation:
    We propose a protocol to disentangle the electro-vibrational structural coupling contributing to the intrinsic tribologic properties of layered MX2 transition metal dichalcogenides (M = Mo, W; X = S, Se, Te) under load. We employ ab initio techniques to model how changing the interlayer distance affects the electronic distribution and the vibrational properties of the system. We analyze the electro-vibrational coupling features by combining orbital polarization and mode Gruneisen parameters analyses with the recently developed bond covalency descriptor and the lattice dynamic metric named cophonicity. We find that intralayer charge distribution depends on the interlayer distance, determining, in turn, a shift of specific vibrational frequencies. We finally suggest a route to control the frequency shift, thus the bulk response to the load, in transition metal dichalcogenides through a proper selection of the atomic type.

First-principles comparative study on the interlayer adhesion and shear strength of transition-metal dichalcogenides and graphene

  • DOI: 10.1103/PhysRevB.92.085434
  • Link: https://doi.org/10.1103/PhysRevB.92.085434
  • Department: Department of Control Engineering
  • Annotation:
    Due to their layered structure, graphene and transition-metal dichalcogenides (TMDs) are easily sheared along the basal planes. Despite a growing attention towards their use as solid lubricants, so far no head-to-head comparison has been carried out. By means of ab initio modeling of a bilayer sliding motion, we show that graphene is characterized by a shallower potential energy landscape while more similarities are attained when considering the sliding forces; we propose that the calculated interfacial ideal shear strengths afford the most accurate information on the intrinsic sliding capability of layered materials. We also investigate the effect of an applied uniaxial load: in graphene, this introduces a limited increase in the sliding barrier while in TMDs it has a substantially different impact on the possible polytypes. The polytype presenting a parallel orientation of the layers (R0) bears more similarities to graphene while that with antiparallel orientation (R180) shows deep changes in the potential energy landscape and consequently a sharper increase of its sliding barrier.

Friction Force Microscopy Analysis of Self-Adaptive W-S-C Coatings: Nanoscale Friction and Wear

  • Authors: Zekonyte, J., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: ACS Applied Materials and Interfaces. 2015, 7(38), 21056-21064. ISSN 1944-8244.
  • Year: 2015
  • DOI: 10.1021/acsami.5b05546
  • Link: https://doi.org/10.1021/acsami.5b05546
  • Department: Department of Control Engineering
  • Annotation:
    Transition metal dichalcogenides (TMD) are increasingly popular due to unique structural and mechanical properties. They belong, together with graphene and similar 2D materials, to a small family of solid lubricants with potential to produce ultralow friction state. At the macroscale, low friction stems from the ability to form well-oriented films on the sliding surface (typically up to 10 nm thick), with the TMD basal planes aligned parallel to the surface. In this study, we quantitatively evaluate tribological properties of three sputtered tungsten-sulfur-carbon (W-S-C) coatings at a nanoscale using friction force microscopy. In particular, we investigate possible formation of well-ordered tungsten disulfide (WS2) layers on the coating surface. The coefficient of friction decreased with increasing load independently of coating composition or mechanical properties. In contrast, hard coatings with high tungsten carbide content were more resistant to wear. We successfully identified a WS2 tribolayer at the sliding interface, which peeled off as ultrathin flakes and attached to AFM tip. Nanoscale tribological behavior of WSC coatings replicates deviation of Amonton's law observed in macroscale testing and strongly suggests that the tribolayer is formed almost immediately after the start of sliding.

Length-scale-dependent mechanical behaviour of Zr/Nb multilayers as a function of individual layer thickness

  • Authors: Frutos, E., Callisti, M., Karlík, M., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Materials Science and Engineering A - Structural Materials: Properties, Microstructure and Processing. 2015, 632 137-146. ISSN 0921-5093.
  • Year: 2015
  • DOI: 10.1016/j.msea.2015.02.071
  • Link: https://doi.org/10.1016/j.msea.2015.02.071
  • Department: Department of Control Engineering
  • Annotation:
    Nanostructured metallic multilayers systems continue to garner interest because of their promising mechanical properties, exploitable in the field of materials engineering. These novel materials show high yield strengths, anomalous Young's modulus values and even superior radiation tolerance for layer thicknesses up to a few tens of nanometers. However, there are still many unknowns related to the deformation mechanisms operating at the nanoscale because of deformation mechanisms, in this nanoscale, depend directly on the layer thickness and the combination of different or similar crystal structures in the interface. The objective of this work is to produce Zr/Nb multilayers and investigate the dependence of deformation mechanisms when the layer thickness is reduced. Nanoindentation hardness as a function of periodicity, lambda, has been measured for Zr/Nb multilayers. It has been found that for decreasing h the yield strength values, sigma(2.7), do not increase. For lambda=60 nm and lambda=30 nm, sigma(2.7) values are almost constant: 1.97 and 1.93 GPa, respectively, whereas for lambda=10 nm, the yield strength shows a decrease to 1.79 GPa. The mismatch between sigma(2.7) and sigma(CLS) values for any core cut-off, alpha, condition (0.2 and 1) and for any eta ratio (eta= h(Zr)/h(Nb)), indicates that the strain mechanism based on CLS did not occur for any period studied; therefore, the strain mechanism based on IBS is suggested, in accord with the activation of a pyramidal slip system {11 (2) over bar2}((11) over bar 23), along Zr layer, even for thickness up to 30 nm. Thereby, dislocation loop glide is not confined to an isolated layer, changing the plastic behaviour of the nano-multilayer.

LUBRICIOUS TiSi(V)N FILMS FOR HIGH TEMPERATURE APPLICATIONS WITH CONTROLLED RELEASE OF THE LUBRICIOUS AGENT DEPOSITED BY DEEP OSCILLATION MAGNETRON SPUTTERING (DOMS) MODE

  • Authors: Fernandes, F., Oliveira, J.C., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Proceedings of the 6th International Conference on Mechanics and Materials in Design. Porto: Universidade de Porto, 2015. pp. 2271-2272. ISBN 978-989-98832-3-9.
  • Year: 2015
  • Department: Department of Control Engineering
  • Annotation:
    This work is focused on the study of the influence of V additions on the structure, mechanical properties, oxidation resistance and tribologial behaviour of TiSiN films deposited by deep oscillation magnetron sputtering (DOMS) mode. Through the change of the peak power on the DOMS power supply, different structures could be reached to control the V ions diffusion. V additions significantly improve the mechanical and tribological properties of TiSiN films; however, it has a detrimental effect on their oxidation resistance. The improvement of the tribological properties with V incorporations was related to V2O5 formation on the sliding contact that acted as a lubricious tribo-film, decreasing the friction and protecting the coating from wear.

MATERIALS INCOMPATIBILITY AS A MAJOR CAUSE OF HIP PROSTHESES REJECTION

  • Authors: Carvalho, I., prof. Ing. Tomáš Polcar, Ph.D., Henriques, M., Carvalho, S.
  • Publication: Reviews on Advanced Materials Science. 2015, 42(1), 36-49. ISSN 1606-5131.
  • Year: 2015
  • Department: Department of Control Engineering
  • Annotation:
    The development of new multifunctional coatings to apply on medical biomaterials continues to be required, since materials commonly used in hip prostheses still presenting failures. Multifunctionality is the result of a synergy, on the nanoscale level, of good corrosion, mechanical and tribological properties. Additionally, a biomaterial must always be biocompatible. Besides these properties, the major challenge would be to get a material that also has antimicrobial activity. In this context, the development of advanced materials with the ability to present these properties is being regarded as a strategy to prevent the colonization of implant and biofilm formation by bacteria. So, in this review, the attention is focused on the description of the fundamental points of the natural synovial joint, since, its mechanical and tribological characteristics are the main causes that lead to the necessity of its replacement by an implant. Moreover, a contextualization was also performed on the hip replacement surgery and the biomaterials used, with a focus on their mechanical and tribological properties. Finally, it is explained the need of surface modification and the potential of TiCN coatings doped with silver.

Ni-Ti(-Cu) shape memory alloy interlayers supporting low friction functional coatings

  • DOI: 10.1016/j.triboint.2015.03.018
  • Link: https://doi.org/10.1016/j.triboint.2015.03.018
  • Department: Department of Control Engineering
  • Annotation:
    The effects of Ni-Ti(-Cu) interlayers on the mechanical and tribological behaviour of W-S-C/Ni-Ti(-Cu) and W:DLC/Ni-Ti(-Cu) bilayer coatings have been investigated. The mechanical properties of the bilayers, measured at different depths by nanoindentation, exhibited a clear dependence on the combination between different Ni-Ti(-Cu) interlayers and top layers. The tribological performance of the bilayers was evaluated by pin-on-disk under different load conditions. The use of Ni-Ti(-Cu) interlayers improved the adhesion of the top layers and the resistance against surface damage such as delamination and cracking. Transmission electron microscopy analyses performed in the wear tracks of W-S-C/Ni-Ti and W:DLC/Ni-Ti bilayers tested with high load revealed that the formation of stabilised martensite was limited when a much harder top functional layer was employed.

Oxidation and diffusion processes during annealing of TiSi(V)N films

  • Authors: Fernandes, F., Morgiel, J., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Surface and Coatings Technology. 2015, 275(15), 120-126. ISSN 0257-8972.
  • Year: 2015
  • DOI: 10.1016/j.surfcoat.2015.05.031
  • Link: https://doi.org/10.1016/j.surfcoat.2015.05.031
  • Department: Department of Control Engineering
  • Annotation:
    The degradation of self-lubricant hard coatings applied in tools for high-speed cutting or dry drilling operations occurs by a combination of wear, oxidation and diffusion. The aim of this investigation was to study the effect of V additions on the diffusion processes and on the oxide scale formation during annealing of TiSiVN coatings. Relation of these results with those achieved for a reference Ti0.80Si0.15N coating with similar Si content is also presented. The structure evolution of the Ti0.65Si0.11V0.15N film was assessed by an in-situ hot-XRD device. A dual layer oxide was formed in the case of Ti0.80Si0.15N coating with a protective Si-O layer at an oxide/coating interface; however, in zones of film defects a complex oxide structure was developed. V additions increased the oxidation rate of the coatings as a result of the V ions diffusion throughout the oxide scale, which inhibited the formation of a continuous protective silicon oxide layer.

PVD-grown antibacterial Ag-TiN fi lms on piezoelectric PVDF substrates for sensor applications

  • Authors: Marques, S.M., Carvalho, I., Henriques, M., prof. Ing. Tomáš Polcar, Ph.D., Carvalho, S.
  • Publication: Surface and Coatings Technology. 2015, 281 117-124. ISSN 0257-8972.
  • Year: 2015
  • DOI: 10.1016/j.surfcoat.2015.09.057
  • Link: https://doi.org/10.1016/j.surfcoat.2015.09.057
  • Department: Department of Control Engineering
  • Annotation:
    This work reports on Ti-1 _ Ag-x(x) and Ag-TiNx electrodes deposited by dc/pulsed dc magnetron sputtering at room temperature on poly (vinylidene fluoride) (PVDF) with the purpose of developing sensors for prosthesis. In Ti-1 _ Ag-x(x) electrodes, silver content was varied from 0-100 at.%; and for Ag-TiNx electrodes, the nitrogen content was varied between 34 and 43 at.% and the Ti/Ag ratio changed from 12.2 to 5.2. The antibacterial activity of the samples was assessed by the agar diffusion method. Interestingly, samples from the Ag-TiNx series presented antibacterial activity, in contrast to the samples from Ti-1 _ Ag-x(x) series. XPS results showed that the as-deposited samples from the Ag-TiNx series exhibited silver clusters smaller than 4 nm. In order to deepen the results obtained, samples were analysed after the Halo test (antibacterial test), and XPS analyses showed the disappearance of these Ag clusters in agreement with the SEM results, which displayed a visible aggregation and a significant decrease in the number of Ag clusters, suggesting that silver nanoparticles aggregated and diffused through the agar killing bacteria or inhibiting their growth in the vicinity. All results show that Ag-TiNx electrodes are more promising candidates to be used in PVDF sensor protection than Ti-1 _ Ag-x(x) electrodes.

Smart Surfaces for Lubrication: Solid Lubricants and Adaptive Texture

  • Authors: prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Coating Technology for Vehicle Applications. Basel: Springer, 2015. p. 203-214. ISBN 978-3-319-14770-3.
  • Year: 2015
  • DOI: 10.1007/978-3-319-14771-0_12
  • Link: https://doi.org/10.1007/978-3-319-14771-0_12
  • Department: Department of Control Engineering
  • Annotation:
    Modern thin films, although still being developer at laboratory scale, promise revolutionary changes in surface engineering. For automotive industry, we will focus on two classes of thin films with the potential to decrease or even eliminate oil additives, reduce friction, and improve control of tribological proces.

Structural and mechanical properties of nanocrystalline Zr co-sputtered a-C(:H) amorphous films

  • Authors: Escudeiro, A., Figueiredo, N.M., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Applied Surface Science. 2015, 325(C), 64-72. ISSN 0169-4332.
  • Year: 2015
  • DOI: 10.1016/j.apsusc.2014.11.015
  • Link: https://doi.org/10.1016/j.apsusc.2014.11.015
  • Department: Department of Control Engineering
  • Annotation:
    The aim of this study was to investigate the effect of Zr as alloying element to carbon films, particularly in respect to film structure and mechanical properties. The films were deposited by magnetron sputtering in reactive (Ar + CH4) and non-reactive (Ar) atmosphere with different Zr contents (from 0 to 14 at.%) in order to achieve a nanocomposite based films. With an increase of Zr content a broad peak was observed in X-ray diffraction spectra suggesting the presence of nanocrystalline (nc) ZrC phase for the coatings with Zr content higher than 4 at.%. The application of Scherrer formula yielded a grain sizes with a dimension of 1.0-2.2 nm. These results were supported by X-ray photoelectron spectroscopy showing typical charge transfer at Zr C nanograins and carbon matrix interface. The nc-ZrC phase was also observed by transmission electron microscopy. The hardness of the coatings was approximately independent of Zr content. However, the Young modulus increased linearly. The residual stress of the coatings was strongly improved by the presence of nc-ZrC phase embedded in the a-C matrix. Finally, the incorporation of H into the matrix led to denser and harder films.

Structure, mechanical and tribological properties of self-lubricant W-S-N coatings

  • Authors: Mutafov, P., Evaristo, M., Cavaleiro, A., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Surface and Coatings Technology. 2015, 261 7-14. ISSN 0257-8972.
  • Year: 2015
  • DOI: 10.1016/j.surfcoat.2014.11.074
  • Link: https://doi.org/10.1016/j.surfcoat.2014.11.074
  • Department: Department of Control Engineering
  • Annotation:
    W-S-N self-lubricant coatings were deposited by d.c. reactive magnetron sputtering at two total discharge pressures of 0.6 and 1.2 Pa. The nitrogen content and S/W ratio varied from 0 to 30 at.%, and 0.6 to 1.6, respectively. The effect of deposition conditions on chemical composition, structure and mechanical properties was studied; coatings with the most promising properties have been selected for tribological testing. Coatings were nanostructured with nanograins of tungsten disulfide; the size of which decreased with increasing nitrogen content. Hardness increased with N content up to 7 GPa for coating deposited with a discharge pressure of 1.2 Pa; coatings deposited at lower pressure exhibited hardness around 9 GPa. Friction and wear behavior in humid air and dry nitrogen was evaluated using a pin-on-disk tribometer. The wear tracks and the ball wear scars were analyzed by Raman spectroscopy and scanning electron microscopy. Although the friction coefficient in dry nitrogen was significantly lower than in humid air, the wear rate was similar for both environments. Detailed wear track surface analysis showed that the tribolayer formed on the surface contained mostly tungsten disulfide and tungsten oxide.

Tailoring Nanoscale Friction in MX2 Transition Metal Dichalcogenides

  • DOI: 10.1021/acs.inorgchem.5b00431
  • Link: https://doi.org/10.1021/acs.inorgchem.5b00431
  • Department: Department of Control Engineering
  • Annotation:
    Lattice dynamics of MX2 transition metal dichalcogenides (M = Mo, W; X = S, Se, Te) have been studied with density functional theory techniques to control the macroscopic tribological behavior. Long-range van der Waals forces have been modeled with Grimme correction to capture the interlayer interactions. A new lattice dynamic metric, named cophonicity, is proposed and used in combination with electronic and geometric descriptors to relate the stability of the lattice distortions with the electro-structural features of the system. The cophonicity analysis shows that the distortion modes relevant to the microscopic friction can be controlled by tuning the relative MIX atomic contributions to the phonon density of states. Guidelines on how to engineer macroscopic friction at nanoscale are formulated, and finally applied to design a new Ti-doped MoS2 phase with enhanced tribologic properties.

THE EFFECT OF CHROMIUM CONTENT ON CUTTING PERFORMANCE AND OXIDATION RESISTANCE OF TiAlCrN COATINGS

  • Department: Department of Control Engineering
  • Annotation:
    Improvement of the durability and cutting performance of coatings designed for tools is an ongoing engineering challenge. There are two main paths to achieve such ambitious goal: to enhance oxidation resistance and/or to decrease friction. In this investigation we is focused on laboratory and industrial performance of TiAlN coating with higher oxidation resistance due to various content of chromium. The objective of the work is to increase the cutting speed of drillers. TiAlCrN coatings with different content of chromium were deposited by unbalanced pulsed magnetron sputtering CemeCon 880 MLT industrial apparatus. TiAlN coatings were deposited as reference. The coatings were deposited on WCCo standard drills and cutting inserts to test their performance either in laboratory by drilling high-speed steel and Inconel as in real production by industrial partner. To measure oxidation resistance, oxidation speed was measured by thermogravimetric analysis (TGA); in this case fecralloy substrates were used. Tribological measurements were performed on CSM tribometer at temperatures up to 800 degrees C. The worn surfaces, both from tribometer and real tools, were investigated by scanning electron microscopy equipped with Energy-dispersive X-ray spectroscopy (EDX) and by Raman spectroscopy. The wear was measured by 3D white light optical profilometry. Oxidation tests and tribological properties obtained in laboratory were compared with the behavior of the coatings deposited on tools.

The influence of nitrogen and oxygen additions on the thermal characteristics of aluminium-based thin filmsOriginal

  • Authors: Borges, J., Macedo, F., Couto, F.M., Rodrigues, M.S., Lopes, C., Pedrosa, P., prof. Ing. Tomáš Polcar, Ph.D., Marques, L., Vaz, F.
  • Publication: Materials Chemistry and Physics. 2015, 163 569-580. ISSN 0254-0584.
  • Year: 2015
  • DOI: 10.1016/j.matchemphys.2015.08.015
  • Link: https://doi.org/10.1016/j.matchemphys.2015.08.015
  • Department: Department of Control Engineering
  • Annotation:
    The ternary aluminium oxynitride (AlNxOy) system offers the possibility to obtain a wide range of properties by tailoring the ratio between pure Al, AlNx and AlOy and therefore opening a significant number of possible applications. In this work the thermal behaviour of AlNxOy thin films was analysed by modulated infrared radiometry (MIRR), taking as reference the binary AlOy and AlNx systems. MIRR is a non-contact and non-destructive thermal wave measurement technique based on the excitation, propagation and detection of temperature oscillations of very small amplitudes. The intended change of the partial pressure of the reactive gas (N2 and/or O2) influenced the target condition and hence the deposition characteristics which, altogether, affected the composition and microstructure of the films. Based on the MIRR measurements and their qualitative and quantitative interpretation, some correlations between the thermal transport properties of the films and their chemical/physical properties have been found. Furthermore, the potential of such technique applied in this oxynitride system, which present a wide range of different physical responses, is also discussed. The experimental results obtained are consistent with those reported in previous works and show a high potential to fulfil the demands needed for the possible applications of the systems studied. They are clearly indicative of an adequate thermal response if this particular thin film system is aimed to be applied in small sensor devices or in electrodes for biosignal acquisition, such as those for electroencephalography or electromyography as it is the case of the main research area that is being developed in the group.

Thin films composed of Ag nanoclusters dispersed in TiO 2 : Influence of composition and thermal annealing on the microstructure and physical responses

  • Authors: Borges, J., Rodrigues, M.S., Lopes, C., Costa, D., Couto, F.M., Kubart, T., Martins, B., Duarte, N., Dias, J.P., Cavaleiro, A., prof. Ing. Tomáš Polcar, Ph.D., Macedo, F., Vaz, F.
  • Publication: Applied Surface Science. 2015, 358(Part B), 595-604. ISSN 0169-4332.
  • Year: 2015
  • DOI: 10.1016/j.apsusc.2015.08.148
  • Link: https://doi.org/10.1016/j.apsusc.2015.08.148
  • Department: Department of Control Engineering
  • Annotation:
    Noble metal powders containing gold and silver have been used for many centuries, providing different colours in the windows of the medieval cathedrals and in ancient Roman glasses. Nowadays, the interest in nanocomposite materials containing noble nanoparticles embedded in dielectric matrices is related with their potential use for a wide range of advanced technological applications. They have been proposed for environmental and biological sensing, tailoring colour of functional coatings, or for surface enhanced Raman spectroscopy.

Thin films composed of gold nanoparticles dispersed in a dielectric matrix: The influence of the host matrix on the optical and mechanical responses

  • Authors: Borges, J., Rodrigues, M.S., Kubart, T., Kumar, S., Leifer, K., Evaristo, M., Cavaleiro, A., Apreutesei, M., Pereira, R.M.S., Vasilevskiy, M.I., prof. Ing. Tomáš Polcar, Ph.D., Vaz, F.
  • Publication: Thin Solid Films. 2015, 596 8-17. ISSN 0040-6090.
  • Year: 2015
  • DOI: 10.1016/j.tsf.2015.08.058
  • Link: https://doi.org/10.1016/j.tsf.2015.08.058
  • Department: Department of Control Engineering
  • Annotation:
    Gold nanoparticles were dispersed in two different dielectric matrices, TiO 2 and Al 2O3, using magnetron sputtering and a post-deposition annealing treatment. The main goal of the present work was to study how the two different host dielectric matrices, and the resulting microstructure evolution (including both the nano-particles and the host matrix itself) promoted by thermal annealing, influenced the physical properties of the films. In particular, the structure and morphology of the nanocomposites were correlated with the optical response of the thin films, namely their localized surface plasmon resonance (LSPR) characteristics.

Tribological properties of self-lubricating TiSiVN coatings at room temperature

  • Authors: Fernandes, F., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Surface & Coatings Technology. 2015, 267 8-14. ISSN 0257-8972.
  • Year: 2015
  • DOI: 10.1016/j.surfcoat.2014.10.016
  • Link: https://doi.org/10.1016/j.surfcoat.2014.10.016
  • Department: Department of Control Engineering
  • Annotation:
    In the last years, vanadium rich coatings have been introduced as possible candidates for self-lubrication due to their optimum tribological properties. In the present investigation, the influence of V incorporation on the wear performance of TiSiN films deposited onto WC substrates by d.c reactive magnetron sputtering is reported. The results achieved for TiSiVN films were compared and discussed in relation to Ti0.80Si0.15N, TiN and Ti0.82V0.15N coatings prepared as references. The tribological properties of the coatings were evaluated at room temperature on a pin on disk tribometer equipment using two different counterparts: Al2O3 and HSS balls. The wear tracks, ball-wear scars and wear debris were characterized by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). Tribological tests indicated that the wear rate and the friction coefficient of Ti0.30Si0.15N coating decreased with continuous increase of V content being the overall behavior strongly dependent on the counterpart ball material. For Al2O3 balls the wear rate and friction coefficient of coatings were much lower compared to sliding against HSS steel balls. Ti0.80Si0.15N showed the lowest wear resistance among all tested coatings, independently of the counter-body. For V rich coatings tested with Al2O3 balls the polishing wear mechanism was observed, whereas adhesion wear took place when tested against HSS balls.

Adsorption of bovine serum albumin on Zr co-sputtered a-C(:H) films: Implication on wear behaviour

  • Authors: Escudeiro, A., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Journal of the Mechanical Behavior of Biomedical Materials. 2014, 39 316-327. ISSN 1751-6161.
  • Year: 2014
  • DOI: 10.1016/j.jmbbm.2014.08.001
  • Link: https://doi.org/10.1016/j.jmbbm.2014.08.001
  • Department: Department of Control Engineering
  • Annotation:
    The use of protective coatings in biomedical field is an ongoing scientific challenge. Among different materials, carbon-based coatings are considered a potential surface treatment for orthopaedic implants. In this study, the effect of Zr incorporation in amorphous carbon coatings on the wear behaviour under protein containing lubrication was investigated. The coatings were deposited by dc unbalanced magnetron sputtering in Ar (non-hydrogenated) and Ar+CH4 (hydrogenated) discharges onto Ti based biomedical substrate. To improve the adhesion between the film and substrate a functional gradient Ti based layer was deposited (~550nm). The surface wettability was evaluated to assess the effect of the Zr and hydrogen content. The films with Zr were found to be hydrophobic enhancing the protein adsorption onto the surface; no significant differences were found when H was incorporated in the films. The adsorption layer characterized by X-ray photoelectron spectroscopy showed a well defined nitrogen peak originating from the organic layer. The tribological properties of the film were evaluated by unidirectional pin-on-disc testing with diluted bovine serum lubrication and physiological solution at 37±3C°. The friction and the wear of the coatings were very low compared to uncoated substrates in both lubrication conditions. The ability of the surfaces to adsorb proteins was considered as the driving force for wear resistance acting as a protecting layer. In addition, the incorporation of Zr decreased the wear of the counterbody (Ti alloy) due to higher albumin adsorption.

Effect of rough surface patterning on the tribology of W-S-C-Cr self-lubricant coatings

  • DOI: 10.1016/j.triboint.2013.09.004
  • Link: https://doi.org/10.1016/j.triboint.2013.09.004
  • Department: Department of Control Engineering
  • Annotation:
    In this work, self-lubricant W-S-C films were alloyed with Cr by co-sputtering chromium and composite WS2-C targets. Besides the usual physical, chemical and mechanical characterization, including the evaluation of the chemical composition, the structure, the morphology, the hardness and the cohesion/adhesion, special attention was paid to the friction and wear analyses of the film deposited on intentionally roughened surfaces. The substrates were steel polished disks with different types of patterns produced by micromachining. To analyze the sliding process, in-situ techniques were applied, such as optical microscopy and Raman spectroscopy monitoring of the wear track. The surfaces in the contact were then analyzed by scanning electron microscopy (SEM) and 3D profilometry. The results showed that W-S-C-Cr coating exhibited remarkable ability to reduce the detrimental effect of deep grooves with irregular shape. For specific patterns on the substrate the friction and wear was even slightly lower compared to a polished sample. The grooves were filled with self-lubricant material and acted as reservoirs supporting formation of a low-friction tribolayer.

Frictional properties of self-adaptive chromium doped tungsten-sulfur-carbon coatings at nanoscale

  • DOI: 10.1016/j.apsusc.2014.03.010
  • Link: https://doi.org/10.1016/j.apsusc.2014.03.010
  • Department: Department of Control Engineering
  • Annotation:
    Transition metal dichalcogenides (TMD) are excellent dry lubricants forming thin (∼10 nm) tribolayer that simultaneously protects the coating from environmental attack and provides low friction. In this paper, we focus on nanoscale frictional properties of chromium doped tungsten–sulfur–carbon (WSC–Cr) coatings with various Cr content. Friction force microscopy was used to investigate friction force as a function of load. A non-linear contact area dependence on the normal force was observed. The calculated interfacial shear strength was relatively low in the region of 70–99 MPa. Friction coefficient decreased with increased applied load independently of chromium content in the coatings.

High temperature behavior of nanolayered CrAlTiN coating: Thermal stability, oxidation, and tribological properties

  • DOI: 10.1016/j.surfcoat.2014.07.053
  • Link: https://doi.org/10.1016/j.surfcoat.2014.07.053
  • Department: Department of Control Engineering
  • Annotation:
    Hard protective nitride coatings are often applied to cutting tools operating at high temperature. To further develop and optimize their performance, in-situ investigation of structure, oxidation, mechanical and tribological properties at elevated temperature is required. In this study we focus on the high temperature behavior of a nanolayered CrAlTiN coating deposited on WC substrates by cathodic arc evaporation. The coating's chemical composition and the bonding state were evaluated by electron probe microanalysis and by X-ray photoelectron spectroscopy (XPS). The structure of as-deposited and annealed samples was analyzed using X-ray diffraction. The adhesion was investigated by scratch test and the mechanical properties were studied by depth sensing nanoindentation. The main objective of this work was to have a detailed analysis of friction and wear properties tested by high temperature tribometer (pin-on-disc) with alumina balls as counterparts in the temperature range of 20–800 °C. Selected wear track cross-sections were prepared by focused ion beam and analyzed by transmission electron microscopy; the wear track was investigated as well by XPS (chemical depth profile) and by Raman spectroscopy. The coating showed an excellent thermal stability and wear resistance. The friction reached a maximum at 500 °C and then decreased, whereas the wear rate was negligible up to 600 °C and then increased significantly for higher temperatures. Oxidation of the worn surfaces was surprisingly low even at the highest temperature corroborating results of oxidation tests. The main identified wear mechanism was polishing combined with a nanoscale delamination of thin coating layers; nanoscale multilayer proved to be a vital factor blocking vertical crack propagation.

Microstructural investigation on the grain refinement occurring in Cu-doped Ni-Ti thin films

  • DOI: 10.1016/j.scriptamat.2014.01.021
  • Link: https://doi.org/10.1016/j.scriptamat.2014.01.021
  • Department: Department of Control Engineering
  • Annotation:
    The mechanism of grain refinement in Cu-doped Ni–Ti thin films has been investigated by transmission electron microscopy. Sputter-deposited (Ni,Cu)-rich Ni–Ti–Cu thin films exhibited a columnar structure consisting of grains whose lateral size decreased with increasing Cu content. Cu-rich grain boundary segregation was found to become prominent in films containing higher Cu contents. This segregation was attributed to a non-polymorphic crystallization process which lowered the grain growth rate as the Cu content in the films increased.

Microstructure and mechanical properties of physical vapor deposited Cu/W nanoscale multilayers: Influence of layer thickness and temperature

  • Authors: Monclús, M.A., Karlík, M., Callisti, M., Frutos Torres, E., Llorca, J., prof. Ing. Tomáš Polcar, Ph.D., Molina-Aldareguía, J.K.
  • Publication: Thin Solid Films. 2014, 571 275-282. ISSN 0040-6090.
  • Year: 2014
  • DOI: 10.1016/j.tsf.2014.05.044
  • Link: https://doi.org/10.1016/j.tsf.2014.05.044
  • Department: Department of Control Engineering
  • Annotation:
    Based on our previous knowledge on Cu/Nb nanoscale metallic multilayers (NMMs), Cu/W NMMs show a good potential for applications as heat skins in plasma experiments and armors, and it could be expected that the substitution of Nb by W would increase the strength, particularly at high temperatures. To check this hypothesis, Cu/W NMMs with individual layer thicknesses ranging between 5 and 30 nm were deposited by physical vapor deposition, and their mechanical properties were measured by nanoindentation. The results showed that, contrary to Cu/Nb NMMs, the hardness was independent of the layer thickness and decreased rapidly with temperature, especially above 200 °C. This behavior was attributed to the growth morphology of the W layers as well as the jagged Cu/W interface, both a consequence of the low W adatom mobility during deposition. Therefore, future efforts on the development of Cu/W multilayers should concentrate on optimization of the W deposition parameters via substrate heating and/or ion assisted deposition to increase the W adatom mobility during deposition.

Sliding Properties of MoS2 Layers: Load and Interlayer Orientation Effects

  • Authors: Levita, G., Cavaleiro, A., Molinari, E., prof. Ing. Tomáš Polcar, Ph.D., Righi, M.C.
  • Publication: The Journal of Physical Chemistry C. 2014, 118(25), 13809-13816. ISSN 1932-7447.
  • Year: 2014
  • DOI: 10.1021/jp4098099
  • Link: https://doi.org/10.1021/jp4098099
  • Department: Department of Control Engineering
  • Annotation:
    Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disulfide has the most consolidated application outcomes in tribological fields. However, despite the growing usage as nanostructured solid lubricant due to its lamellar structure, little is known about the atomistic interactions taking place at the interface between two MoS2 sliding layers, especially at high loads. By means of ab initio modeling of the static potential energy surface and charge distribution analysis, we demonstrate how electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads. As such, they discriminate the relative stability and the frictional behavior of bilayers where the two monolayers defining the interface have a different relative orientation. In particular, for antiparallel sliding layers we observed a load-induced increase of both the depth of the minima and the height of the energy barriers compared to parallel ones, which may have important consequences for the fabrication of more efficient ultralow friction devices at the nanoscale.

Sliding properties of Zr-DLC coatings: The effect of tribolayer formation

  • DOI: 10.1016/j.surfcoat.2014.08.003
  • Link: https://doi.org/10.1016/j.surfcoat.2014.08.003
  • Department: Department of Control Engineering
  • Annotation:
    Amorphous carbon-based coatings (DLC) exhibit excellent mechanical and tribological properties such as high hardness, high elastic modulus, low friction and low wear. Reduced friction is often related to the formation of a low-friction tribolayer, which is formed during sliding and transferred to the counterpart. Here, we investigate the sliding of hydrogenated and non-hydrogenated DLC coatings alloyed with zirconium; pure DLC films are used as reference. The coatings were deposited by magnetron sputtering in Ar (non-hydrogenated) and Ar/methane atmosphere (hydrogenated) onto steel substrates and silicon wafers. The total thickness of the coatings was around 1.5 μm including a complex Ti/TiN/TiCN adhesion-improving interlayer with a thickness of 450 nm. All deposited coatings were amorphous, Zr/C ratio was approx. 0.05. The hardness was in the range of 9-13 GPa. Tribological tests were carried out in humid air at room temperature, at 100 °C and in nitrogen environment using pin-on-disk. Intermittent tribological test analysis has been performed to understand running-in behaviour. The worn surfaces and wear debris were analysed by Raman spectroscopy. Coatings alloyed with Zr showed lower friction and wear at room temperature compared to pure DLC. In general, Zr-doped hydrogenated coating outperformed the non-hydrogenated one when tested in an inert nitrogen atmosphere or at elevated temperature (100 °C), exhibiting almost super-low friction (μ = 0.03 in the steady-state regime) due to the formation of a homogenous, thick and stable tribolayer.

The effect of increasing V content on the structure, mechanical properties and oxidation resistance of Ti-Si-V-N films deposited by DC reactive magnetron sputtering

  • Authors: Fernandes, F., Loureiro, A., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Applied Surface Science. 2014, 289 114-123. ISSN 0169-4332.
  • Year: 2014
  • DOI: 10.1016/j.apsusc.2013.10.117
  • Link: https://doi.org/10.1016/j.apsusc.2013.10.117
  • Department: Department of Control Engineering
  • Annotation:
    In the last years, vanadium rich films have been introduced as possible candidates for self-lubrication at high temperatures, based on the formation of V2O5 oxide. The aim of this investigation was to study the effect of V additions on the structure, mechanical properties and oxidation resistance of Ti–Si–V–N coatings deposited by DC reactive magnetron sputtering. The results achieved for TiSiVN films were compared and discussed in relation to TiN and TiSiN films prepared as reference. All coatings presented a fcc NaCl-type structure. A shift of the diffraction peaks to higher angles with increasing Si and V contents suggested the formation of a substitutional solid solution in TiN phase. Hardness and Young's modulus of the coatings were similar regardless on V content. The onset of oxidation of the films decreased significantly to 500 °C when V was added into the films; this behaviour was independent of the Si and V contents. The thermogravimetric isothermal curves of TiSiVN coatings oxidized at temperatures below the melting point of α-V2O5 (∼685 °C) showed two stages: at an early stage, the weight increase over time is linear, whilst, in the second stage, a parabolic evolution can be fitted to the experimental data. At higher temperatures only a parabolic evolution was fitted. α-V2O5 was the main phase detected at the oxidized surface of the coatings. Reduction of α-V2O5 to β-V2O5 phase occurred for temperatures above its melting point.

The role of Ni-Ti-(Cu) interlayers on the mechanical properties and nano-scratch behaviour of solid lubricant W-S-C coatings

  • DOI: 10.1016/j.surfcoat.2014.06.028
  • Link: https://doi.org/10.1016/j.surfcoat.2014.06.028
  • Department: Department of Control Engineering
  • Annotation:
    A self-lubricant W-S-C coating with different Ni-Ti-(Cu) interlayers was fabricated by magnetron sputtering following a three-step process. It consisted of deposition and annealing of Ni-Ti-(Cu) layers with different Cu contents and subsequent deposition of the transition metal dichalcogenide W-S-C coating bonded to the Ni-Ti-(Cu) layer through a gradient Cr-based thin layer. Doping the Ni-Ti interlayer with Cu led to significant microstructural changes which influenced mechanical properties, such as the H/E-r ratio and the resistance against plastic deformation. The response of the bilayers, i.e. W-S-C/Ni-Ti-(Cu) coatings, to indentation was affected by the interlayer material. The resistance to adhesion damage of the W-S-C coating was improved by using Ni-Ti-(Cu) interlayers. It was found that interlayers with lower H/E-r ratio showed an improved capability to increase the adhesion of the functional top layer.

Tribological analysis of thin films by pin-on-disc: evaluation of friction and wear measurement uncertainty

  • DOI: 10.1016/j.triboint.2014.02.011
  • Link: https://doi.org/10.1016/j.triboint.2014.02.011
  • Department: Department of Control Engineering
  • Annotation:
    Pin-on-disc is widely used to evaluate tribological properties of thin films. However, the results are often present without standard uncertainties; moreover, in many cases the standard uncertainty is replaced by standard deviation, which is strong underestimation of real uncertainty. In thus study we have followed ISO and NIST guidelines to investigate possible sources of uncertainties related to friction and wear rate measurement and to apply them on two selected coating systems – TiN and DLC. We show that influence of operator is a significant contribution to the uncertainty of the wear rate, particularly in the case of very low wear of DLC coatings. We discuss why variance should be used instead statistic deviation and suggest method to calculate uncertainties in case of small number of measurements. The paper could be used as a guide to evaluate friction and wear data of thin films and coatings using pin-on-disc technique.

Tribological behaviour of W-alloyed carbon-based coatings in dry and lubricated sliding contact

  • DOI: 10.1002/ls.1259
  • Link: https://doi.org/10.1002/ls.1259
  • Department: Department of Control Engineering
  • Annotation:
    Carbon-based coatings with different W contents were deposited by direct current magnetron sputtering in reactive and non-reactive atmospheres. All deposited coatings have compact morphologies with amorphous (tungsten-free) or nanocrystalline structures (tungsten-doped). The latter one was indicated by very broad peaks in X-ray Diffraction spectra in the position of tungsten carbide suggesting W-carbide nanoparticles embedded in an amorphous carbon matrix. The hardness increased from 10 to 15 GPa with increasing W content. The coatings were tribological tested at dry and lubricated conditions with increasing temperature in a coating/steel configuration. In dry sliding, the friction coefficient increases with the increase of the temperature reaching values higher than 1.0. The friction is significantly lower in lubricated contact using three different oils: poly-alpha-olefin, paraffin and olive oil. The olive oil shows promising lubricating properties at the temperature lower than 70 degrees C; however, at higher temperature, the coatings were quickly worn through.

a-C(:H) and a-C(:H)_Zr coatings deposited on biomedical Ti-based substrates: Tribological properties

  • DOI: 10.1016/j.tsf.2012.12.086
  • Link: https://doi.org/10.1016/j.tsf.2012.12.086
  • Department: Department of Control Engineering
  • Annotation:
    Amorphous carbon (a-C) based coatings are possible candidates as a surface treatment for various biocompatible materials used in medicine. In this study, the carbon coatings co-sputtered with Zr, deposited by dc unbalanced magnetron sputtering in Ar (non-hydrogenated, i.e. a-C/Zr) and Ar + CH4 (hydrogenated, i.e. a-C:H/Zr) discharges, were investigated and compared with pure carbon films. Polished pure commercial Ti grade 2 and Ti grade 5 ELI (Ti6Al4V) discs were used as substrates. To improve the coating/substrate adhesion, a gradient Ti-based interlayer was deposited (~ 450 nm). The coating structure was characterized by X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy; the chemical composition was measured by electron probe microanalysis. The adhesion was evaluated by scratch-test and the hardness was measured by nanoindentation. Tribological testing of the coatings was carried out using unidirectional pin-on-disc experiments; pure titanium balls were used as counterparts. The wear rate of the coatings was negligible except for the films with the highest Zr content deposited on Ti grade 2 substrates.

Effects of Cu on the microstructural and mechanical properties of sputter deposited Ni-Ti thin films

  • Authors: Callisti, M., Tichelaar, F.D., Mellor, B.M., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Surface and Coatings Technology. 2013, 237 261-268. ISSN 0257-8972.
  • Year: 2013
  • DOI: 10.1016/j.surfcoat.2013.06.040
  • Link: https://doi.org/10.1016/j.surfcoat.2013.06.040
  • Department: Department of Control Engineering
  • Annotation:
    The microstructure of sputter deposited Ti-rich Ni-Ti thin films doped with Cu in the range 0–20.4 at.% and annealed for 1 h at 500 and 600 °C has been investigated and correlated with the mechanical properties of the films measured by depth-sensing nanoindentation. X-ray diffraction analysis showed the microstructural evolution of Ni-Ti thin films when doped with Cu and annealed at different temperatures. Heat treatments promoted the nucleation and growth of Ti2Ni precipitates in Ti-rich Ni-Ti thin films, which affected the stability of austenitic and martensitic phases at ambient temperature. Doping with Cu caused the formation of Ti(Ni, Cu)2 plate precipitates, which became more finely and densely dispersed in the grains with increasing Cu content. TEM analysis showed a columnar grain morphology extended through the whole films thickness, while with increasing Cu content a noticeable lateral grain refinement was induced by segregation of a (Ni, Cu)-rich phase to grain boundaries. The nano-hardness increased almost linearly with increasing Cu content owing to this grain refinement, though differences between samples annealed at different temperatures were found which could be related to the evolution of Ti(Ni, Cu)2 plate precipitates with annealing temperature and Cu content. The Young's modulus exhibited a similar dependence on Cu content as nano-hardness, though no significant differences were observed with increasing annealing temperatures.

Fluence scan: an unexplored property of a laser beam

  • Authors: Chalupský, J., Burian, T., Hájková, V., Juha, L., prof. Ing. Tomáš Polcar, Ph.D., Gaudin, J., Nagasono, M., Sobierajski, R., Yabashi, M., Krzywinski, J.
  • Publication: Optics Express. 2013, 21(22), 26363-26375. ISSN 1094-4087.
  • Year: 2013
  • DOI: 10.1364/OE.21.026363
  • Link: https://doi.org/10.1364/OE.21.026363
  • Department: Department of Control Engineering
  • Annotation:
    We present an extended theoretical background of so-called fluence scan (f-scan or F-scan) method which is frequently being used for offline focused short-wavelength (XUV, soft X-ray, and hard X-ray) laser beam characterization (Chalupský et al. 2010 Opt. Express 18 27836). The method exploits ablative imprints in various solids to visualize iso-fluence beam contours at different fluence and/or clip levels. By varying the pulse energy, an f-scan curve (clip level as a function of the contour area) can be generated for a general non-Gaussian beam. The fluence scan method greatly facilitates transverse characterization of focused non-Gaussian beams and provides important information about energy distribution within the beam profile. Here we for the first time discuss fundamental properties of the f-scan function and its inverse counterpart (if-scan or iF-scan). Furthermore, we extensively elucidate how it is related to the effective beam area, energy distribution, and to the so called Liu’s plot (Liu 1982 Opt. Lett. 7 196). A new method of effective area evaluation based on weighted inverse f-scan fit is introduced and applied to real data obtained at the SCSS (Spring-8 Compact SASE Source) facility.

Frictional behavior of self-adaptive nanostructural Mo-Se-C coatings in different sliding conditions

  • Authors: Gustavsson, F., Jacobsson, S., Cavaleiro, A., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Wear. 2013, 303(1-2), 286-296. ISSN 0043-1648.
  • Year: 2013
  • DOI: 10.1016/j.wear.2013.03.032
  • Link: https://doi.org/10.1016/j.wear.2013.03.032
  • Department: Department of Control Engineering
  • Annotation:
    Sliding properties of Mo–Se–C coatings with two different carbon content deposited by magnetron sputtering were investigated in different sliding environments (argon, nitrogen, dry and humid air). Both coatings had a structure that was identified as randomly oriented structures of MoSe2 embedded into amorphous carbon matrix. The worn surfaces, i.e. the wear tracks and the wear scars of the balls, were analyzed by optical microscopy, Raman spectroscopy and scanning electron microscopy. The material transferred to the ball steel surfaces was almost exclusively MoSe2, whereas the wear tracks on the coatings were more complex, with areas rich in MoSe2 and areas similar to that of as-deposited coatings.

High temperature properties of the Cr-Nb-Al-N coatings with increasing Al contents

  • Authors: Li, W.Z., prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M., Cavaleiro, A.
  • Publication: Surface and Coatings Technology. 2013, 228 187-194. ISSN 0257-8972.
  • Year: 2013
  • DOI: 10.1016/j.surfcoat.2013.04.027
  • Link: https://doi.org/10.1016/j.surfcoat.2013.04.027
  • Department: Department of Control Engineering
  • Annotation:
    Cr-Nb-Al-N coatings with Al content from 0 to 12 at.% were deposited by d.c. reactive magnetron sputtering. The coatings were annealed in protective atmosphere at 800 and 900 degrees C for 1 h and exposed to air at 800,900 and 1200 degrees C for different times. The chemical composition, structure, microstructure, hardness and adhesive/cohesive strength of the coatings, in as-deposited and annealed conditions, were investigated and the oxidation resistance was evaluated. As expected, the Al content increased gradually in the coating with increasing power density in the Al target; nitrogen was kept approximately constant. All coatings exhibited columnar cross section morphology and fcc NaCl-type B1 phase structure. After thermal annealing, an increase in the grain size and a decrease in the lattice parameter were observed, which led to either a decrease in the hardness or lower adhesion/cohesion strength than the as-deposited coatings. The onset oxidation temperature was approx. 900 degrees C for all coatings. High Al content coatings showed minimum oxidation when exposed to 800 degrees C for 2 h and to 900 degrees C for 0.5 h in air. The Al-free and low Al coatings exhibited lower oxidation resistance; Cr2O3 and CrNbO4 coexisting together although signs of the cubic nitride phase were still detected. The increase of Al was beneficial to the improvement of the thermal stability and oxidation resistance of the Cr-Nb-Al-N coatings.

Influence of Ag content on mechanical and tribological behavior of DLC coatings

  • Authors: Manninen, N.K., Ribeiro, F., Escudeiro, A., prof. Ing. Tomáš Polcar, Ph.D., Carvalho, S., Cavaleiro, A.
  • Publication: Surface and Coatings Technology. 2013, 232 440-446. ISSN 0257-8972.
  • Year: 2013
  • DOI: 10.1016/j.surfcoat.2013.05.048
  • Link: https://doi.org/10.1016/j.surfcoat.2013.05.048
  • Department: Department of Control Engineering
  • Annotation:
    Ag-DLC coatings with Ag contents ranging from 1.3. at.% to 13.1. at.% were deposited by DC magnetron sputtering. The coatings were characterized with respect to their structure (by means of XRD and Raman spectroscopy), mechanical and tribological properties (by scratch test, nanoindentation, residual stress measurements and pin-on-disk test). The incorporation of 13.1. at.% Ag resulted in the formation of Ag grains of 2-3. nm which promoted the increase of graphite like bonds organized in rings. Regarding the mechanical properties, no variations were found for films with Ag contents lower than 13. at.%; a reduction of both hardness and compressive residual stress was then observed for higher values. Pin-on-disk tests were performed at two different contact pressures (690. MPa and 1180. MPa) in dry sliding conditions against a zirconia counterpart. For the lower contact pressure the variations in the wear rate are well correlated with the coating structure and mechanical properties, while for the higher contact pressure the presence of Ag is relevant and Ag-DLC coatings are showing higher wear rate than DLC one. SEM analysis revealed the formation of Ag aggregates on the wear track and adhesion of silver to the counterpart.

Optimum high temperature strength of two-dimensional nanocomposites

  • Authors: Monclús, M.A., Zheng, S.J., Mayeur, J.R., Beyerlein, I.J., Mara, N.A., prof. Ing. Tomáš Polcar, Ph.D., Llorca, J., Molina-Aldareguía, J.M.
  • Publication: APL Materials. 2013, 1(1), ISSN 2166-532X.
  • Year: 2013
  • DOI: 10.1063/1.4828757
  • Link: https://doi.org/10.1063/1.4828757
  • Department: Department of Control Engineering
  • Annotation:
    High-temperature nanoindentation was used to reveal nano-layer size effects on the hardness of two-dimensional metallic nanocomposites. We report the existence of a critical layer thickness at which strength achieves optimal thermal stability. Transmission electron microscopy and theoretical bicrystal calculations show that this optimum arises due to a transition from thermally activated glidewithin the layers to dislocation transmission across the layers.We demonstrate experimentally that the atomic-scale properties of the interfaces profoundly affect this critical transition. The strong implications are that interfaces can be tuned to achieve an optimum in high temperature strength in layered nanocomposite structures.

Solid Lubricants, Layered-Hexagonal Transition Metal Dichalcogenides

  • DOI: 10.1007/978-0-387-92897-5_1260
  • Link: https://doi.org/10.1007/978-0-387-92897-5_1260
  • Department: Department of Control Engineering
  • Annotation:
    Transition metal dichalcogenides (TMD) are among the best alternatives as solid lubricants for tribological applications, particularly in dry and vacuum environments. Their excellent lubricating properties are based on the extreme crystal anisotropy; under ideal conditions (very dense material deposited and tested in ultra-high vacuum) they can be considered as "frictionless," i.e., with friction coefficient as low as 0.001. Pure TMD are sensitive to the presence of water vapor, exhibiting high friction coefficient. For tribological purposes, TMD are prepared in the form of powder, burnished films, thin films, inorganic fullerene-like particles, or nanotubes.

Tribological properties of the two-step thermally deposited chromium films

  • Authors: Lazauskas, A., Baltrusaitis, J., Grigaliūnas, V., Baltušnikas, A., Abakevičienė, B., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Applied Surface Science. 2013, 283 1089-1095. ISSN 0169-4332.
  • Year: 2013
  • DOI: 10.1016/j.apsusc.2013.07.083
  • Link: https://doi.org/10.1016/j.apsusc.2013.07.083
  • Department: Department of Control Engineering
  • Annotation:
    Chromium thin films were prepared on glass substrate via a two-step thermal deposition and their structural, chemical and tribological properties were determined. The X-ray diffraction pattern of the two-step thermally deposited chromium film showed the presence of well-defined body-centered cubic Cr metal structure with a prominent (1 1 0) plane. X-ray photoelectron spectroscopy depth profile showed a presence of ∼2% of oxygen in the film within the first five etching cycles showing oxygen incorporation. Dry sliding wear experiments at temperatures ranging from 20 to 200 °C were conducted. The thin films were worn with either 100Cr6 or 440C bearing balls using a ball-on-disk sliding configuration at a contact load of 1 N. Friction coefficient and coating wear rates were measured and wear tracks were analyzed using optical microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Thin films exhibited low wear rates at room temperature and an unexpected wear behavior at the elevated temperatures. Only the first half of the metallic chromium film was worn out after ∼18 m of dry sliding at room temperature, whereas increase in temperature resulted in a decrease in wear resistance of the film.

Ultra-low friction W-S-N solid lubricant coating

  • Authors: Gustavsson, F., Jacobson, S., Cavaleiro, A., prof. Ing. Tomáš Polcar, Ph.D.,
  • Publication: Surface and Coatings Technology. 2013, 232 541-548. ISSN 0257-8972.
  • Year: 2013
  • DOI: 10.1016/j.surfcoat.2013.06.026
  • Link: https://doi.org/10.1016/j.surfcoat.2013.06.026
  • Department: Department of Control Engineering
  • Annotation:
    W-S-N films were deposited by reactive magnetron sputtering from WS2 target in Ar/N2 atmosphere. Besides the standard evaluation of composition, structure, morphology, hardness and cohesion/adhesion, the core objective of this paper was to analyze coating tribological behavior. The chemical composition was 34 at.% N, 12 at.% O, 29 at.% W and 25 at.% S, and the as-deposited films were completely amorphous. The film thickness was 2.3 μm, including the approximately 300-nm thick adhesion improving titanium interlayer. The friction coefficient was lower than 0.003 when sliding in dry nitrogen. The coating showed remarkable wear resistance surviving more than 2 million laps on pin-on-dics. The excellent friction properties were attributed to the formation of a thin tungsten disulfide tribofilm on the top of the wear track of the coating and on the counterpart surface. Moreover, the coating showed ability to replenish damaged areas with solid lubricant. We demonstrated that a structural transformation of the coating from an amorphous-like to a gradient quasi-ordered structure and an ordered transfer layer formation improved mechanical properties and radically decreased friction and wear.

Complex frictional analysis of self-lubricant W-S-C/Cr coating

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Gustavsson, F., Thersleff, T., Jacobson, S., Cavaleiro, A.
  • Publication: Faraday discussions. 2012, 156(1-2), 383-401. ISSN 1359-6640.
  • Year: 2012
  • DOI: 10.1039/c2fd00003b
  • Link: https://doi.org/10.1039/c2fd00003b
  • Department: Department of Control Engineering
  • Annotation:
    Transition metal dichalcogenides belong to one of the most developed classes of materials for solid lubrication. However, one of the main drawbacks of most of the self-lubricating coatings is their low load-bearing capacity, particularly in terrestrial atmospheres. In our previous work, alloying thin films based on tungsten disulfide with non-metallic interstitial elements, such as carbon or nitrogen, has been studied in order to improve tribological performance in different environments. Excellent results were reached with the deposited coatings hardness, in some cases, more than one order of magnitude higher than single W-S films. In this work, W-S-C films were deposited with increasing Cr contents by co-sputtering chromium and composite WS2-C and targets. Two films were prepared with approx. 7 and 13 at.% of Cr. Alloying with chromium led to dense films with amorphous microstructure; the hardness and adhesion was improved. Sliding tests were carried out in dry and humid air using a pin-on-disc tribometer with 100Cr6 steel balls as a counterpart. To analyse the sliding process, the surfaces in the contact were investigated by X-ray photoelectron spectroscopy (bonding), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. Surface and sub-surface structural modification of the coating and composition of the transferred tribolayer are discussed in detail. High friction in humid air was attributed to the absence of a well-ordered WS2 sliding interface. On the other hand, the existence of such an interface explained the very low friction observed in dry air.

Examination of the tribolayer formation of a self-lubricant W-S-C sputtered coating

  • Authors: Pimentel, J.V., prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M., Cavaleiro, A.
  • Publication: Tribology International. 2012, 47 188-193. ISSN 0301-679X.
  • Year: 2012
  • DOI: 10.1016/j.triboint.2011.10.021
  • Link: https://doi.org/10.1016/j.triboint.2011.10.021
  • Department: Department of Control Engineering
  • Annotation:
    A W-S-C self-lubricating coating was deposited by d.c. magnetron sputtering from carbon and tungsten disulfide targets on steel substrates. The coating showed a carbon content of 42 at.% and a S/W ratio of 1.26. The film was characterized (structure, hardness, adhesion) and tribologicaly tested using a pin-on-disc device. The load and number of cycles were varied and the corresponding wear tracks were thoroughly investigated by several techniques. Moreover, the wear tracks were monitored with Raman spectroscopy including in-situ measurements. The W-S-C coating exhibited friction and wear decreasing with the applied load. It has been demonstrated that the running-in process is related with changes in the Raman spectra on the wear tracks.

Potential Application of a Ti-C:H Coating in Implants

  • Authors: Feng, F., Zhou, Y., Yun, H., Rocha, A., prof. Ing. Tomáš Polcar, Ph.D., Cvrček, L., Liang, H.
  • Publication: Journal of the American Ceramic Society. 2012, 95(9), 2741-2745. ISSN 0002-7820.
  • Year: 2012
  • DOI: 10.1111/j.1551-2916.2011.05000.x
  • Link: https://doi.org/10.1111/j.1551-2916.2011.05000.x
  • Department: Department of Control Engineering
  • Annotation:
    A coating of Ti-C:H was investigated for potential applications as artificial joint materials. Herein, we conducted experimental study using fluid shear method and tribolgoical study to evaluate and analyze the adhesive strength of proteins on Ti- C:H coatings and Ti6Al4V. Sample surfaces were worn against a steel bearing (E52100) under the lubrication of egg white protein solution. The following wear track and debris analysis helped further understand their wear mechanism. Research results showed that Ti-C:H coatings on silicon substrate had a porous topography that enhanced its adhesion with the protein. The friction coefficient and wear rate for Ti-C:H was much lower than Ti6Al4V. This can be attributed to the chemical inertness and functional diamond-like features of Ti-C:H coatings enhancement in wear resistance, making it superior to Ti6Al4V.

Carbon-based coatings doped by copper: Tribological and mechanical behavior in olive oil lubrication

  • Authors: Balestra, R.M., Castro, A.M.G., Evaristo, M., Escudeiro, A., Mutafov, P., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Surface and Coatings Technology. 2011, 205(1), S79-S83. ISSN 0257-8972.
  • Year: 2011
  • DOI: 10.1016/j.surfcoat.2011.01.053
  • Link: https://doi.org/10.1016/j.surfcoat.2011.01.053
  • Department: Department of Control Engineering
  • Annotation:
    We deposited C-based films doped with Cu and tested their sliding properties in olive oil as environment-friendly lubricant, which can be used in many mechanical systems, particularly in agriculture engineering. The coatings were deposited in a four unbalanced magnetron sputtering device combining C and C/Cu targets; argon (hydrogen-free films) and Ar/CH4 (hydrogenated films) atmospheres were used. Cu content of the films was in the range 5-14 at.%. The hardness of the films was almost constant whatever the Cu content was. On the other hand, hydrogen-free coatings were much harder (about 15 GPa) than hydrogenated ones (about 4 GPa).

High temperature properties of CrAlN, CrAlSiN and AlCrSiN coatings - Structure and oxidation

  • DOI: 10.1016/j.matchemphys.2011.03.078
  • Link: https://doi.org/10.1016/j.matchemphys.2011.03.078
  • Department: Department of Control Engineering
  • Annotation:
    CrAlN, CrAlSiN and AlCrSiN coatings were deposited by cathodic arc deposition technique from composite targets. Three targets were used: i) Cr/Al ratio close to 1, ii) Cr/Al ratio close to 1 with Si addition, and iii) Cr/Al ratio close to 1/2 and Si addition. Nitrogen flow was kept constant during the depositions. The Cr/Al ratio of the films, measured by electron probe microanalysis (EPMA), was similar to that of the target and the silicon content was in the range 3-4 at.%.

High-temperature tribological properties of CrAlN, CrAlSiN and AlCrSiN coatings

  • DOI: 10.1016/j.surfcoat.2011.08.037
  • Link: https://doi.org/10.1016/j.surfcoat.2011.08.037
  • Department: Department of Control Engineering
  • Annotation:
    Cr-Al-Si-N coatings with high and low Cr/Al ratio (CrAlSiN and AlCrSiN, respectively) were deposited on WC substrates by cathodic arc and compared with a reference Cr-Al-N coating. The silicon content was close to 3 at.%. X-ray diffraction analysis showed that CrAlN and CrAlSiN coatings exhibited the cubic Cr(Al)N structure, whereas in AlCrSiN a mixture of cubic Cr(Al)N and wurzite-type AlN was identified. All three coatings showed excellent thermal stability and oxidation resistance up to 800 °C.

Properties of nanocomposite film combining hard TiN matrix with embedded fullerene-like WS2 nanoclusters

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Mohan, D.B., Sandu, C.S, Radnoczi, G., Cavaleiro, A.
  • Publication: Thin Solid Films. 2011, 519(10), 3191-3195. ISSN 0040-6090.
  • Year: 2011
  • DOI: 10.1016/j.tsf.2010.12.180
  • Link: https://doi.org/10.1016/j.tsf.2010.12.180
  • Department: Department of Control Engineering
  • Annotation:
    We have developed hard self-lubricant coatings combining a hard matrix (TiN) and a self-lubricant phase in the form of inorganic-like WS2 fullerene. The nanoparticles were injected from the preparation chamber directly to the sample surface during reactive sputtering from a Ti target in Ar/N2 atmosphere. The injection of the particles led to the local oxidation of the matrix due to the flow of residual oxygen from the preparation chamber; therefore, the final composite was TiN/Ti-O-WS2. The observation of the composite film by scanning and transmission electron microscopies showed the incorporation of the WS2 nanoparticles; however, their bonding with the matrix was weak. The analysis of the wear tracks did not show any presence of WS2 in the contact.

Review on self-lubricant transition metal dichalcogenide nanocomposite coatings alloyed with carbon

  • DOI: 10.1016/j.surfcoat.2011.03.004
  • Link: https://doi.org/10.1016/j.surfcoat.2011.03.004
  • Department: Department of Control Engineering
  • Annotation:
    In this paper, we review the results on the tribological behavior of nanocomposite coatings composed of nanoplatelets of transition metal dichalcogenides (TMD) immersed in a C-rich amorphous matrix. It is shown that such a microstructure produces low friction coefficients under different operating conditions such as air humidity, contact pressure or temperature. Special attention is paid to the analysis of the worn surfaces after the tests by Raman spectroscopy, Auger electron spectroscopy and transmission electron microscopy.

Self-adaptive low friction coatings based on transition metal dichalcogenides

  • DOI: 10.1016/j.tsf.2011.01.180
  • Link: https://doi.org/10.1016/j.tsf.2011.01.180
  • Department: Department of Control Engineering
  • Annotation:
    This paper deals with three fundamentally different concepts of self-lubricant coatings based on the transition metal dichalcogenides (TMD) and deposited by magnetron sputtering. The first two designs could be considered as traditional: the TMD films doped by carbon or nitrogen. The main attention is aimed at qualitative description of surface and sub-surface modification of the films as the result of the sliding process. Based on a thorough analysis of the worn surfaces, two features emerge: self-adaptability, where originally randomly oriented TMD phase becomes well oriented (i.e. with basal planes parallel to the surface), and limited role of carbon or nitrogen in the contact. The films doped with carbon are the best solution for humid environment, while those doped with nitrogen are ideal for use in dry environment or vacuum. The last concept deals with the combination of a hard TiN matrix deposited by reactive magnetron sputtering with fullerene-like TMD nanoclusters.

Structural, mechanical and tribological properties of Mo-S-C solid lubricant coating

  • Authors: Pimentel, J., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Surface and Coatings Technology. 2011, 205(10), 3274-3279. ISSN 0257-8972.
  • Year: 2011
  • DOI: 10.1016/j.surfcoat.2010.11.043
  • Link: https://doi.org/10.1016/j.surfcoat.2010.11.043
  • Department: Department of Control Engineering
  • Annotation:
    Mo-S-C self-lubricating coatings were deposited by d.c. magnetron sputtering from carbon and molybdenum disulphide targets. The power ratio of the targets was varied in order to prepare films with carbon content in the range 0-55 at.%. Whatever the carbon content, the S/Mo ratio was higher than 1.25. The hardness of the films increased almost linearly with the carbon content. X-ray photoelectron spectroscopy showed evidence of Mo-C bonds; nevertheless, the size of molybdenum carbide grains was expected to be very small, since X-ray diffraction did not reveal any peaks related to any Mo-C phase. The coatings tested by pin-on-disc exhibited low friction, decreasing with increasing carbon content, when humid air was present. In nitrogen, the friction of all films was lower than 0.02 except for the reference MoS2 (0.04). Mo-S-C outperformed the wear resistance of MoS2; on the other hand, the results were in some cases hindered by the low adhesion of the coatings.

Structure and tribological properties of AlCrTiN coatings at elevated temperature

  • DOI: 10.1016/j.surfcoat.2011.03.015
  • Link: https://doi.org/10.1016/j.surfcoat.2011.03.015
  • Department: Department of Control Engineering
  • Annotation:
    In this study, we analyzed the high temperature tribological behavior of AlCrTiN coatings deposited on WC substrates by low cathodic arc technique. The coatings chemical composition, Al 31 at.%, Cr 16 at.%, Ti 7 at.% and N 46 at.%, and the bonding state were evaluated by X-ray photoelectron spectroscopy. The mechanical properties of the coatings were studied by scratch-test and nanohardness depth sensing indentation. The morphology of the coatings surface, ball scars, wear tracks and wear debris as well as the oxidized samples was examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The structure was analyzed using X-ray diffraction (XRD). Wear testing was carried out using a high temperature tribometer (pin-on-disc) with alumina balls as counterparts.

Tribological behaviour a-C and a-C:H films doped with Ti in biological solutions

  • DOI: 10.1016/j.vacuum.2011.03.011
  • Link: https://doi.org/10.1016/j.vacuum.2011.03.011
  • Department: Department of Control Engineering
  • Annotation:
    The coatings were deposited in a multi magnetron Teer sputtering device with two C targets and a Ti target in Ar atmosphere, on steel and Si substrates. To deposit hydrogenated coatings methane was introduced in the discharge atmosphere. The films were deposited in a range of 7e14 at.% Ti content. X-ray diffraction patterns revealed the presence of a nanocrystalline TiC phase. The hardness varied from 8 to 9 GPa. The tribological tests were performed under dry sliding and lubricated (0.9% NaCl water solution, physiological solution, PS, and 10% fetal bovine serum dissolved in Ringer's saline solution, FBS) conditions, using a 100Cr6 steel balls with a diameter of 6 mm, in a pin-on-disc wear test apparatus.

Effects of carbon content on the high temperature friction and wear of chromium carbonitride coatings

  • DOI: 10.1016/j.triboint.2009.12.010
  • Link: https://doi.org/10.1016/j.triboint.2009.12.010
  • Department: Department of Control Engineering
  • Annotation:
    CrCN coatings were prepared by cathode arc evaporation technology using constant N2 flow and variable C2H2 flow. The coatings with a thickness of 3-4 μm were deposited on hardened steel substrates and high-temperature resistant alloy. The carbon content varied from 0 at.% (i.e. CrN) up to 31 at.%. The standard coating characterization included the nano-hardness, adhesion, chemical composition and structure (including hot X-ray diffraction). Wear testing was done using a high temperature tribometer (pin-on-disc); the maximum testing temperature was 700 oC. The coatings with carbon content 12-31 at.% showed almost identical tribological behaviour up to 700 oC.

Sliding mechanisms in tribological contact of TMD-C sputtered coatings

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Tribology and Design. Wessex: WIT PRESS, 2010. pp. 195-208. WIT Transactions. ISSN 1746-4471. ISBN 978-1-84564-440-6.
  • Year: 2010
  • Department: Department of Control Engineering
  • Annotation:
    In this paper, results on the tribological behavior of nanocomposite coatings composed by nanoplatelets of transition metal dichalcogenides (TMD) immersed in a C-rich amorphous matrix will be presented. It will be shown that this microstructure will allow low friction coefficients in wide range of surrounding environments, since TMD platelets are re-oriented in the C-matrix as a consequence of the mechanical energy liberated in the sliding contact. In this case, the oxidation of the TMD material is avoided due to the protection achieved by the C matrix. Examples of TMD-C coatings of the W-S-C and Mo-Se-C systems will be presented and discussed. After tribological characterization of the coatings in different environments under different loading conditions experimental evidence of the wear mechanisms will be given by cross section transmission electron microscopy, Raman spectroscopy and Auger electron spectroscopy.

Structure, mechanical properties and tribology of W-N and W-O coatings

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: International Journal of Refractory Metals and Hard Materials. 2010, 28(1), 15-22. ISSN 0263-4368.
  • Year: 2010
  • DOI: 10.1016/j.ijrmhm.2009.07.013
  • Link: https://doi.org/10.1016/j.ijrmhm.2009.07.013
  • Department: Department of Control Engineering
  • Annotation:
    We are presenting here a summary of our recent work on tungsten nitride (nitrogen content 0-58 at.%) and tungsten oxide (oxygen content 0-75 at.%) coatings deposited by reactive magnetron sputtering. Our aim has been the analysis of the connection of fundamental properties of these films, such as chemical composition, structure, hardness, Young's modulus and residual stress, with their tribological properties - friction coefficient and wear rate. We have been focussed mainly on the description of the dominant wear mechanisms influencing the tribological properties. The tribological tests have been carried out both at room and elevated temperature; the temperature was increased in steps until immediate coatings failure. The tungsten nitride coating with the "worst" parameters generally considered as vital for high wear resistance, such as hardness, were considered to have the best tribological performance.

Subject "Robots" at the CTU FEE in Prague - using LEGO robots to teach the fundamentals of feedback control

  • Department: Department of Control Engineering
  • Annotation:
    Study of technical subjects could be very difficult for new students entering the faculty with different background. Traditional study of theoretical disciplines without any clear relevance to "real" problems decreases motivation of many students, particularly those having difficulties with advanced mathematics. The core objective of our new subject Robots is to explore, in friendly way, students' independent thinking, and creativity and work in team. Although the theory is limited we believe that our students will eventually improve their theoretical knowledge as well. They start with something like child game trying to move robot from one place to another. Then, to succeed in more complicated tasks, they soon recognize that there is something behind the curtain and they start to ask how to solve the problems looking for a solution actively. If they reach this stage they are "trapped" - and become excellent and highly motivated students of control engineering.

Subject "Robots" at the CTU FEE in Prague - using LEGO robots to teach the fundamentals of feedback control

  • Department: Department of Control Engineering
  • Annotation:
    Study of technical subjects could be very difficult for new students entering the faculty with different background. Traditional study of theoretical disciplines without any clear relevance to "real" problems decreases motivation of many students, particularly those having difficulties with advanced mathematics. The core objective of our new subject Robots is to explore, in friendly way, students' independent thinking, and creativity and work in team. Although the theory is limited we believe that our students will eventually improve their theoretical knowledge as well. They start with something like child game trying to move robot from one place to another. Then, to succeed in more complicated tasks, they soon recognize that there is something behind the curtain and they start to ask how to solve the problems looking for a solution actively. If they reach this stage they are "trapped" - and become excellent and highly motivated students of control engineering

Can W-Se-C coatings be competitive to W-S-C ones?

  • Authors: Evaristo, A., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Plasma Processes and Polymers. 2009, 6(S), 92-95. ISSN 1612-8850.
  • Year: 2009
  • DOI: 10.1002/ppap.200930414
  • Link: https://doi.org/10.1002/ppap.200930414
  • Department: Department of Control Engineering
  • Annotation:
    W-Se-C coatings were deposited by co-sputtering from WSe2 and C targets. Carbon content varied from 25 at. % up to 70 at. %, Se/W ratio was between 0.9-1.0, with no significant influence of the carbon content on their variations. The hardness of the coatings was evaluated by nanoindentation with values between 3.8 - 5.6 GPa. The structure of the coatings analysed by X-ray diffraction in glancing mode could be defined as quasi-amorphous. The tribological properties were evaluated in a SRV machine using reciprocating ball on disk tests under increasing loads. In general, the friction decreased with increasing loads. The comparison with other transition metal dichalcogenides alloyed with carbon, namely Mo-Se-C and W-S-C systems, showed that W-Se-C coatings could be a good alternative with almost identical friction properties.

Comparative study of the tribological behavior of self-lubricating W-S-C and Mo-Se-C sputtered coatings

  • DOI: 10.1016/j.wear.2008.04.011
  • Link: https://doi.org/10.1016/j.wear.2008.04.011
  • Department: Department of Control Engineering
  • Annotation:
    In this paper, the general comparison between W-S-C and Mo-Se-C coatings is presented. The main effort is pointed on the tribological behavior of both systems when tested by pin-on-disk against steel counterpart balls under different testing conditions: applied normal loads, temperatures and relative humidity of the atmospheres. Both coatings were deposited by co-sputtering from a C target with a varying number of TMD pellets which could lead to C contents in the films in the range from 30 up to 70 at.%. A Ti interlayer was interposed between the films and the substrates for improving the adhesion. Typically, W-S-C films are harder than Mo-Se-C films. From the tribological point of view, W-S-C films are more thermally stable than Mo-Se-C films although the friction coefficients of these last ones are lower when tested in humid containing atmospheres.

High temperature tribology of CrN and multilayered Cr/CrN coatings

  • DOI: 10.1016/j.surfcoat.2009.04.005
  • Link: https://doi.org/10.1016/j.surfcoat.2009.04.005
  • Department: Department of Control Engineering
  • Annotation:
    Chromium nitride (CrN) and multilayers Cr/CrN coatings were deposited by low cathodic arc technology. The thickness of Cr and CrN layers was identical; two different multilayer coatings were deposited with layer thicknesses of 85 and 160 nm. The structural analysis showed that CrN coatings exhibited the cubic CrN phase, while a mixture of CrN, Cr2N and Cr phases was observed in the case of multilayers. The annealing of the samples at 800 and 900 °C in air led to: (1) the decomposition of the chromium nitride phases into chromium; (2) a moderate oxidation by forming a thin Cr2O3 layer on the coating surface and, (3) carbon diffusion from the steel substrate.

Mechanical and tribological properties of sputtered Mo-Se-C coatings

  • DOI: 10.1016/j.wear.2008.04.010
  • Link: https://doi.org/10.1016/j.wear.2008.04.010
  • Department: Department of Control Engineering
  • Annotation:
    In this study, Mo-Se-C films were deposited by sputtering from a C target with pellets of MoSe2. Besides the evaluation of the chemical composition, the structure, the morphology, the hardness and the cohesion/adhesion, special attention was paid to the tribological characterization. The friction coefficient of Mo-Se-C coatings increased with air humidity from 0.04 to 0.12, while it was as low as 0.02 at temperature range 100-250°C. The coatings were very sensitive to the elevated temperature being worn out at 300°C due to adhesion problems at coating-titanium interface.

Self-lubricating W-S-C nanocomposite coatings

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M., Cavaleiro, A.
  • Publication: Plasma Processes and Polymers. 2009, 6(6-7), 417-424. ISSN 1612-8850.
  • Year: 2009
  • DOI: 10.1002/ppap.200930005
  • Link: https://doi.org/10.1002/ppap.200930005
  • Department: Department of Control Engineering
  • Annotation:
    This paper is aimed on a perspective low friction coatings, the W-S-C system deposited by magnetron sputtering, which exhibits extremely low friction coefficient together with high load-bearing capacity. Special attention has been paid to the analysis of the frictional and wear mechanisms under different operating conditions, such as the contact pressure, the air humidity and the temperature. The formation of a thin self-lubricating WS2 tribolayer, which was observed regardless on the sliding conditions, is the driving force for the promising frictional properties of the coatings.

Synthesis and properties of W-Se-C coatings deposited by PVD in reactive and non-reactive processes

  • DOI: 10.1016/j.vacuum.2009.03.030
  • Link: https://doi.org/10.1016/j.vacuum.2009.03.030
  • Department: Department of Control Engineering
  • Annotation:
    The WSeC coatings were deposited by either co-sputtering from WSe2 and C targets or by working in reactive mode in an Ar + CH4 atmosphere. Carbon content varied from 25 at.% up to 70 at.%, Se/W ratio of co-sputtered coatings was between 0.9 and 1.0, with no significant influence of the carbon content on their variations, while in the reactive process it reached values higher than 1.7. The hardness of the coatings was evaluated by nano-indentation, with values between 1.6 and 5.6 GPa. The structure of the coatings was analysed by X-ray diffraction in glancing mode, showing that the coatings have a quasi-amorphous structure. Finally, the selected samples were tested on pin-on-disc showing low friction properties of WSeC co-sputtered coatings.

Tribological behaviour of nanostructured Ti-C:H coatings for biomedical applications

  • DOI: 10.1016/j.solidstatesciences.2008.10.006
  • Link: https://doi.org/10.1016/j.solidstatesciences.2008.10.006
  • Department: Department of Control Engineering
  • Annotation:
    The development of a mechanically stable, functionally graded Ti-doped a-C:H interface layer in combination with a functional a-C:H coating requires a reduction of the brittle phases which induce generally problems in the transitions from Ti to TiC/a-C:H. The core objective of this study was to develop an optimum interlayer between the substrate and the functional top layer for biomedical applications, namely for tooth implants. Since the interlayer may be exposed to the sliding process, in the case of local failure of the top layer it has to fulfil the same criteria: biocompatibility, high wear resistance and low friction.

Tribological Performance of CrAlSiN Coatings at High Temperatures

  • DOI: 10.1002/ppap.200932307
  • Link: https://doi.org/10.1002/ppap.200932307
  • Department: Department of Control Engineering
  • Annotation:
    Chromium aluminium nitride (Cr-Al-N) coatings produced by PVD are routinely deposited on tools and tested as components for machining and forming applications. However, the addition of a doping element, such as Si, can further improve the properties of such coatings. In this study, Cr-Al-N and Cr-Al-Si-N coatings with different Cr/Al ratio were prepared on WC substrates by low cathodic arc deposition and their structure and tribological properties analyzed in situ at high temperatures. In spite of excellent thermal stability and oxidation resistance, the hot-sliding tests showed unsatisfactory coating behavior.

A simple model for the deposition of W-O coatings by reactive gas pulsing process

  • Authors: Parreira, N.M.G., prof. Ing. Tomáš Polcar, Ph.D., Carvalho, N.J.M., Cavaleiro, A.
  • Publication: The European Physical Journal Applied Physics. 2008, 43(3), 321-325. ISSN 1286-0042.
  • Year: 2008
  • DOI: 10.1051/epjap:2008068
  • Link: https://doi.org/10.1051/epjap:2008068
  • Department: Department of Control Engineering
  • Annotation:
    The conventional and RGPP process parameters (target potential, total pressure and deposition rate) and the corresponding chemical composition were used to build a simple model predicting the deposition rate and the average chemical composition of the coatings deposited by RGPP as a function of the pulsing parameters. It was shown that the measured total pressure could be used to calculate the deposition rate and the chemical composition of the RGPP coatings with reasonable precision.

In Situ Structural Evolution of Arc-Deposited Cr-Based Coatings

  • Authors: Neves, A.M., Severo, V., Cvrček, L., prof. Ing. Tomáš Polcar, Ph.D., Louro, C., Cavaleiro, A.
  • Publication: Surface and Coatings Technology. 2008, 202(22-23), 5550-5555. ISSN 0257-8972.
  • Year: 2008
  • DOI: 10.1016/j.surfcoat.2008.06.102
  • Link: https://doi.org/10.1016/j.surfcoat.2008.06.102
  • Department: Department of Control Engineering
  • Annotation:
    Cr-based coatings were prepared by cathode arc evaporation technology using N2 and C2H2 as reactive gases. Three compositions were investigated, Cr60N40, Cr41N27C32 and Cr67C33. The present investigation is centred on the structural stability via coatings tempering up to 1000°C, by in situ X-ray diffraction in inert atmosphere.

Nanoscale colour control: W-O graded coatings deposited by magnetron sputtering

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Kubart, T., Malainho, E., Vasilevskiy, M., Parreira, N.M.G., Cavaleiro, A.
  • Publication: Nanotechnology. 2008, 19(39), 395202-395206. ISSN 0957-4484.
  • Year: 2008
  • DOI: 10.1088/0957-4484/19/39/395202
  • Link: https://doi.org/10.1088/0957-4484/19/39/395202
  • Department: Department of Control Engineering
  • Annotation:
    A new design of decorative tungsten oxide coatings is presented. The coatings were deposited with a graded refractive index by magnetron sputtering from a tungsten target and pulsing the reactive gas. The controlled injection of the reactive gas can produce a concentration profile gradient from pure tungsten to tungsten trioxide, determining the final apparent colour of the coating. A dynamic sputtering model was built to simulate the growth of the coating during the reactive gas pulsing which was validated by direct measurement of the gradient of the oxygen content in the deposited coatings. Finally, these results were used for an optical model allowing the optical properties of the deposited tungsten oxide layer to be described, again validated by experimental analysis.

Nanoscale triboactivity: The response of Mo-Se-C coatings to sliding

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M., Colaco, R., Silviu Sandu, C., Cavaleiro, A.
  • Publication: Acta materialia. 2008, 56(18), 5101-5111. ISSN 1359-6454.
  • Year: 2008
  • DOI: 10.1016/j.actamat.2008.06.029
  • Link: https://doi.org/10.1016/j.actamat.2008.06.029
  • Department: Department of Control Engineering
  • Annotation:
    Mo-Se-C films were deposited by sputtering from a carbon target with pellets of MoSe2. In addition to the standard evaluation of their chemical composition, structure, morphology, hardness and cohesion/adhesion, the core objective of this paper was to analyze the tribological behavior of these films, particularly in the high-load regime. The carbon content varied from 29 to 68 at.% which led to a progressive increase of the Se/Mo ratio and the hardness. The friction coefficient of Mo-Se-C coatings clearly decreased with load from 0.15 to 0.05. The excellent friction properties were attributed to the formation of a thin molybdenum diselenide film on the top of the wear track of the coating and on the counterpart surface, while the role of the carbon in the sliding process is only secondary by increasing the coating hardness and thus its wear resistance.

Structural and tribological characterization of tungsten nitride coatings at elevated temperature

  • DOI: 10.1016/j.wear.2007.10.011
  • Link: https://doi.org/10.1016/j.wear.2007.10.011
  • Department: Department of Control Engineering
  • Annotation:
    The structure, hardness, friction and wear of tungsten nitride coatings with nitrogen content in the range 30-58 at.% prepared by dc reactive magnetron sputtering were investigated. The tribological tests were performed on a pin-on-disc tribometer in terrestrial atmosphere with Al2O3 balls as sliding partner. The coating wear rate was negligible up to 200 °C exhibiting a decreasing tendency; however, the wear dramatically increased at higher temperatures. The coating peeled off after the test at 600 °C, which is connected with the oxidation of the coating.

Structure and tribology of biocompatible Ti-C:H coatings

  • DOI: 10.1016/j.surfcoat.2008.06.040
  • Link: https://doi.org/10.1016/j.surfcoat.2008.06.040
  • Department: Department of Control Engineering
  • Annotation:
    Ti-C:H coatings with different carbon content for biomedical applications were deposited by PECVD. Ti was varied by magnetron sputtering a Ti-target with different power in a dc discharge regime having Ar in the atmosphere. Ti-C:H coating was tribologically tested reflecting its expected use as an interlayer for improving the adhesion of functional a-C:H coatings. The tribological properties were studied using a pin-on-disc CSM Tribometer in order to ensure stable tribological properties of the whole Ti-C:H/DLC system for any case of top layer failure.

Synthesis and Structural Properties of Mo-Se-C Sputtered Coatings

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M., Stueber, M., Cavaleiro, A.
  • Publication: Surface and Coatings Technology. 2008, 202(11), 2418-2422. ISSN 0257-8972.
  • Year: 2008
  • DOI: 10.1016/j.surfcoat.2007.08.019
  • Link: https://doi.org/10.1016/j.surfcoat.2007.08.019
  • Department: Department of Control Engineering
  • Annotation:
    Mo-Se-C coatings were prepared by non-reactive r.f. magnetron sputtering from carbon target with embedded MoSe2 pellets. The carbon content and Se/Mo ratio determined by electron probe microanalysis increased from 29 to 68 at.% and from 1.7 to 2.0, respectively, as a function of the decreasing number of pellets. The coating structure analyzed by X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy showed that Mo-Se-C was a mixture of amorphous carbon and Mo-Se phases, since no traces of molybdenum carbides were observed. Linear increase of the hardness from 0.7 (29 at.% C) to 4.1 GPa (68 at.% C) showed a significant improvement compared to values typical for pure MoSe2 coating.

TEM investigation of MoSeC films

  • Authors: Sandu, C.S., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Microscopy & Microanalysis. 2008, 14(supp 3), 7-10. ISSN 1431-9276.
  • Year: 2008
  • DOI: 10.1017/S1431927608089228
  • Link: https://doi.org/10.1017/S1431927608089228
  • Department: Department of Control Engineering
  • Annotation:
    Transition metal dichalcogenides (TMD) are widely used as self-lubricating material either as oil additive or directly as thin films. Magnetron sputtering is a deposition method allowing depositing such films with high density and adhesion. However, their spread use in practical applications is still hindered since their excellent sliding properties are deteriorated in the presence of humidity and under high contact pressures. MoSe2, one of the members of TMD family recently studied, has been co-sputtered with carbon in order to improve the mechanical and tribological properties when compared to pure MoSe2 films.

Tribological analysis of MoSeC coatings: from nano to macroscale

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M., Colaco, R., Cavaleiro, A.
  • Publication: Proceedings of the SAIT International Tribology Conference Tribology 2008. Pretoria: SAIT, 2008. pp. 20-29. ISBN 978-0-620-38082-9.
  • Year: 2008
  • Department: Department of Control Engineering
  • Annotation:
    The Mo-Se-C coatings were prepared by r.f. magnetron sputtering from pure carbon target with MoSe2 pellets placed in the erosion zone, while the second target, titanium, was used to deposit a thin interlayer improving coating-substrate adhesion. The chemical composition measured by electron probe microanalysis varied from 29 to 70 at.% of carbon as a function of number of pellets; the Se/Mo ratio was close to 2 and oxygen content decreased with carbon from 5 to 2 at.%. The coatings exhibited hardness increasing with carbon content reaching a maximum value of 5 GPa.

Tribological behaviour of C-alloyed transition metal dichalcogenides (TMD) coatings in different environments

  • Authors: Evaristo, M., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: International Journal of Mechanics and Materials in Design. 2008, 4(2), 137-143. ISSN 1569-1713.
  • Year: 2008
  • DOI: 10.1007/s10999-007-9034-2
  • Link: https://doi.org/10.1007/s10999-007-9034-2
  • Department: Department of Control Engineering
  • Annotation:
    W-S-C films were deposited by non-reactive sputtering from a carbon target incrusted with WS2 pellets in the eroded zone. This process allows depositing coatings with a wide range of compositions, with a precise control of their carbon content. Before the deposition, a Ti interlayer was interposed between the film and the substrate to improve the final adhesion. The carbon content in W-S-C system was varied from 29 at.% to 70 at.%, which led to an hardness enhancement from 4 GPa up to 10 GPa where the maximum hardness was reached in films with a carbon content between 40 at.% and 50 at.%. The tribological behaviour of the coatings was evaluated by pin-on-disk testing, in environments with different humidity levels.

W based PVD coatings as a solution for the problems of friction and wear in engineering

  • Authors: Cavaleiro, A., Louro, C., prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M., Moura, C.W.
  • Publication: A Enghenharia no combate a pobreza, pelo desenvolvimento e competitividade. Porto: Edicoes INEGI, 2008. pp. 695-696. ISBN 978-972-8826-19-2.
  • Year: 2008
  • Department: Department of Control Engineering
  • Annotation:
    Tungsten based PVD coatings as a solution for the problems of friction and wear in engineering. We showed different W-based coatings, mainly hard tungsten nitrides doped with additional element such Ti or Si, with improved tribological properties.

Characterization of W-O Coatings Deposited by Magnetron Sputtering with Reactive Gas Pulsing

  • Authors: Parreira, N.M.G., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Surface and Coatings Technology. 2007, 207(9-11), 5481-5486. ISSN 0257-8972.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    In this study, tungsten oxide coatings were deposited by dc magnetron sputtering onto steel, glass and silica substrates using either a conventional process (constant flow of oxygen) or pulsing of reactive gas (RGP). A square wave regulation signal with different pulsing period (T) and oxygen injection time (t(on)) was used in RGP. The partial pressure of argon was kept constant for all depositions. Three series of coatings were prepared: two with constant T and increasing t(on) and one with different T and constant t(on)/T ratio. The chemical composition, morphology and structure of these coatings were analyzed by electron probe microanalysis, scanning electron microscope (SEM) and X-ray diffraction, respectively. Preliminary studies have shown a linear relationship between t(on)/T and the oxygen content ratio (O/W) in the coatings.

Comparative Study of the Tribological Behaviour of Self-Lubricating W-S-C

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M., Cavaleiro, A.
  • Publication: ECOTRIB 2007. Ljubljana: Slovenian Society for Tribology, 2007. pp. 505-515. ISBN 978-961-90254-8-2.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    In this paper, the general comparison between W-S-C and Mo-Se-C coatings is presented. The main effort is pointed on the tribological behaviour of both systems when tested by pin-on-disk against steel counterpart balls under different testing conditions: applied normal loads, temperatures and relative humidity of the atmospheres. Both coatings were deposited by co-sputtering from a C target with a varying number of TMD pellets which could lead to C contents in the films in the range from 30 at.% up to 70 at.%. A Ti interlayer was interposed between the films and the substrates for improving the adhesion. Typically, W-S-C films are harder than Mo-Se-C films. From the tribological point of view, W-S-C films are more thermal stable than Mo-Se-C films although the friction coefficients of these last ones are lower when tested in humid containing atmospheres.

Comparison of W-O Coatings Deposited by DC Reactive Magnetron Sputtering in Continuous and Pulsing Mode

  • Authors: Parreira, N.M.G., prof. Ing. Tomáš Polcar, Ph.D., Vaz, F., Cavaleiro, A.
  • Publication: E-MRS Spring Meeting 2007 - Book of Abstracts. Strasbourg: E-MRS, 2007. pp. 93.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    The deposition of the coating where done keeping constant the argon pressure at 0.2 Pa and the target current density (JW = 6 mA.cm-2). To cover all composition range for tungsten oxide prepared by CP (i.e. from W100 to W25O75), the oxygen flow 0 - 20 sccm was used. During the RGP process, the oxygen flow was pulsed using a square regulation signal defined by a pulsing period (T) and an oxygen injection time (tON). During tON the oxygen flow was 20 sccm and during tOFF (= T - ton), the oxygen flow was 0 sccm. The chemical composition were analysed by electron probe microanalysis (EPMA), the chemical profile by glow discharge optical emission spectroscopy (GDOES), the morphology were analysed by scanning electron microscope (SEM) and the colour represented by the CieL*a*b* system was measured by colorimetry.

Friction and Wear Behaviour of CrN Coating at Temperatures up to 500 °C

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Parreira, N.M.G., Novák, R.
  • Publication: Surface and Coatings Technology. 2007, 201(9-11), 5228-5235. ISSN 0257-8972.
  • Year: 2007

Friction of Self-Lubricating W-S-C Sputtered Coatings Sliding Under Increasing Load

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M., Cavaleiro, A.
  • Publication: Plasma Processes and Polymers. 2007, 4(S1), 541-546. ISSN 1612-8850.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    W-S-C films were deposited by non-reactive magnetron sputtering from a carbon target with several pellets of WS2 incrusted in the zone of the preferential erosion. Their structural, mechanical and tribological properties were analyzed in detail.

Mechanical and Tribological Characterization of CNx Films Deposited by D.C.Magnetron Sputtering

  • Authors: Ipaz, L., Yate, L., prof. Ing. Tomáš Polcar, Ph.D., Camps, E., Escobar-Alarcon, L., Zambrano, G., Prieto, P.
  • Publication: Physica Status Solidi (c). 2007, 4(11), 4267-4274. ISSN 1610-1634.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    Carbon nitride (CNx) thin films were deposited onto silicon and steel substrates at 400 °C from a carbon target by d.c. magnetron sputtering system. The composition, structural, and mechanical properties of deposited films were investigated as a function of argon/nitrogen concentration and sputtering power

Mechanical and Tribological Properties of Sputtered Mo-Se-C Coatings

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M., Stueber, M., Cavaleiro, A.
  • Publication: ECOTRIB 2007. Ljubljana: Slovenian Society for Tribology, 2007. pp. 613-623. ISBN 978-961-90254-8-2.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    Mo Se C films were deposited by sputtering from a C target with pellets of MoSe2. Besides the evaluation of the chemical composition, the structure, the morphology, the hardness and the cohesion/adhesion, special attention was paid to the tribological characterization. The C content varied from 29 to 68 at.% which led to a progressive increase of the Se/Mo ratio. As a typical trend, the hardness increases with increasing C content. The coatings were tested at room temperature with different air humidity levels and at temperatures up to 500 oC on a pin-on-disc tribometer. The friction coefficient of Mo-Se-C coatings increased with air humidity from ~0.04 to ~0.12, while it was as low as 0.02 at temperature range 100-250 oC. The coatings were very sensitive to the elevated temperature being worn out at 300 oC due to adhesion problems at coating - titanium interface.

Modelling of Magnetron Sputtering of Tungsten Oxide with Reactive Gas Pulsing

  • Authors: Kubart, T., prof. Ing. Tomáš Polcar, Ph.D., Kappertz, O., Parreira, N.M.G., Nyberg, T., Berg, S., Cavaleiro, A.
  • Publication: Plasma Processes and Polymers. 2007, 4(S1), 522-526. ISSN 1612-8850.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    In this report, the concept of periodically switching the RG flow between two different values is applied to the deposition of tungsten oxide. The trends in the measured time dependent RG pressure and discharge voltage are reproduced by a dynamical model developed for this process.

Nanocomposite Coatings of Carbon-Based and Transition Metal Dichalcogenides Phases: A Review

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Nossa, A., Evaristo, M., Cavaleiro, A.
  • Publication: Reviews on Advanced Materials Science. 2007, 15(2), 118-126. ISSN 1606-5131.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    The results obtained at Instituto de Ciencias e Engenharia de Materiais e Superficies-ICEMS on the development of W-S-C coatings are reviewed and compared with literature data on the same system. The coatings have been deposited by r.f. magnetron sputtering using several approaches including reactive and co-sputtering processes. The chemical composition, structure and morphology of the coatings were analyzed and correlated with the mechanical properties. A complete detailed tribological characterization was performed by pin-on-disk tests varying the applied load, the humidity ratio and the temperature.

Optical and Electrical Properties of W-O-N Coatings Deposited by DC Reactive Sputtering

  • Authors: Parreira, N.M.G., prof. Ing. Tomáš Polcar, Ph.D., Martin, N., Banakh, O., Cavaleiro, A.
  • Publication: Plasma Processes and Polymers. 2007, 4(S1), 69-75. ISSN 1612-8850.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    Tungsten oxynitride films (W-O-N) were prepared by DC reactive magnetron sputtering in a wide range of compositions from tungsten nitride (WN) to tungsten oxide (WO3), by changing the oxygen and nitrogen flows, fO2 and fN2, respectively, and their electrical and optical properties were evaluated with respect to the structure.

Structure and Tribology of Biocompatible Ti-C:H Coatings

  • Authors: Ing. Tomáš Vítů, Ph.D., prof. Ing. Tomáš Polcar, Ph.D., Cvrček, L., Novák, R., Macák, J., Vyskočil, J., Cavaleiro, A.
  • Publication: Proceedings of Sixth Asian-European International Conference on Plasma Surface Engineering (AEPSE 2007). Nagasaki: Asian Joint Committee on Plasma Surface Engineering (AJC/PSE), 2007. pp. 361.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    Ti-C:H coatings with different carbon content for biomedical applications were deposited by PECVD. Ti was varied by magnetron sputtering a Ti-target with different power in a dc discharge regime having Ar in the atmosphere. The tribological properties were studied using a pin-on-disc CSM Tribometer in order to ensure stable tribological properties of the whole Ti-C:H/DLC system for any case of top layer failure. The sliding tests were carried out at room temperature in room environment with relative air humidity 40+-5%, in 0.9% NaCl water solution (physiological solution, PS) and in 10% fetal bovine serum (FBS) dissolved in Ringer's saline solution using 440C steel balls with a diameter of 8 mm. The Ti-rich coatings exhibited poor wear resistance, while the best tribological properties were achieved for TiC/a-C:H coatings deposited with the highest C2H2 flows.

Study of the Cathode Potential in a Sputtering Discharge by Pulsing the Reactive Gas: Case of a W Target in an Ar-O2 Atmosphere

  • Authors: Parreira, N.M.G., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Plasma Processes and Polymers. 2007, 4(1), 62-68. ISSN 1612-8850.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    The process we used was d.c. magnetron sputtering, and we studied both the conventional process, using a constant flow of oxygen, and the process in which we pulsed the reactive gas. Square regulation signal with different pulsing periods (T) and oxygen injection time (ton) was used in the reactive gas pulsing (RGP), while the partial argon pressure was kept constant for all depositions.

Synthesis and Structural Properties of Mo-Se-C Sputtered Coatings

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M., Stueber, M., Cavaleiro, A.
  • Publication: E-MRS Spring Meeting 2007 - Book of Abstracts. Strasbourg: E-MRS, 2007. pp. 150.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    Mo-Se-C coatings were prepared by non-reactive r.f. magnetron sputtering from carbon target with embedded MoSe2 pellets. The carbon content and Se/Mo ratio determined by electron probe microanalysis increased from 29 to 68 at.% and from 1.7 to 2.0, respectively, as a function of the decreasing number of pellets. The coating structure analyzed by X-Ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy showed that Mo-Se-C was a mixture of amorphous carbon and Mo-Se phases, since no traces of molybdenum carbides were observed. Linear increase of the hardness from 0.7 (29 at.% C) to 4.1 GPa (68 at.% C) showed a significant improvement compared to values typical for pure MoSe2 coating.

The Tribological Behavior of W-S-C Films in Pin-on-Disk Testing at Elevated Temperature

  • Department: Department of Control Engineering
  • Annotation:
    The coatings were tested by pin-on-disk from room temperature (RT) up to 400 °C. At RT, the friction coefficient was in the range 0.2-0.30. At temperatures higher than 100 °C, the friction is below 0.05 for all compositions. The tribological behavior of the coatings with increasing temperatures depended on the films carbon content. For low-carbon content up to 40%, the wear rate was almost independent of the temperature up to 300 °C, while it increased dramatically in the case of the coatings with high-carbon content. In general, the limiting temperature for W-S-C coatings is 400 °C.

Thermal Stability of Reactive Sputtered Tungsten Oxide Coatings

  • Authors: Parreira, N., prof. Ing. Tomáš Polcar, Ph.D., Cavaleiro, A.
  • Publication: Surface and Coatings Technology. 2007, 201(16-17), 7076-7082. ISSN 0257-8972.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    The thermal stability of different W-O coatings, W100, W90O10, W54O46, W30O70 and W25O75, were studied by in-situ X-ray diffraction at elevated temperatures up to 900 °C. The coatings were deposited by DC reactive magnetron sputtering from a pure tungsten target in an Ar + O2 atmosphere onto Fecralloy alloy. The evolution of the structure of the coatings was studied in both protective and oxidant atmospheres.

Tribological Behaviour of Nanostructured Ti-C:H Coatings Used for Biomedical Applications

  • Authors: Ing. Tomáš Vítů, Ph.D., prof. Ing. Tomáš Polcar, Ph.D., Cvrček, L., Novák, R., Vyskočil, J.
  • Publication: ABSTRACTS BOOK of 2nd International Conference on Surfaces, Coatings and Nanostructured Materials. Algarve: Society of Nanoscience and Nanotechnology, 2007. pp. 159.
  • Year: 2007
  • Department: Department of Control Engineering
  • Annotation:
    The tribological properties of Ti-C:H coatings were studied using pin-on-disc CSM Tribometer. The functional Ti-C:H layers were deposited by PECVD using magnetron sputtering of Ti-target in C2H2 + Ar atmosphere. The sets of coatings samples were prepared with varying content of C:H components controlled by C2H2 flow. The typical thickness of Ti-C:H coatings varied from 2.5 to 3.5 um. The sliding tests were carried out at room temperature (RT) and at 100 °C using HS steel and ceramic Al2O3 balls with a diameter of 6 mm. The results showed relative clear correlation between tribological properties and coatings composition. There is assumed the commercial production for biomedical applications using Ti-C:H coatings deposited on Ti-alloy. The results acquired in this study will support the research and development of deposited surgical tools and prostheses with improved sliding properties.

Tribological Characterization of Tungsten Nitride Coatings Deposited by Reactive Magnetron Sputtering

  • Department: Department of Control Engineering
  • Annotation:
    The structure, hardness, friction and wear of tungsten nitrides prepared by d.c. reactive magnetron sputtering were investigated. The coatings were deposited with different nitrogen to argon ratios; the total pressure was kept constant. The tribological tests were performed on a pin-on-disc tribometer in terrestrial atmosphere with 100Cr6 steel, Al2O3 and Si3N4 balls as sliding counter-bodies. The wear tracks, the ball-wear scars and the wear debris were analysed by scanning electron microscopy in order to characterize the dominant wear mechanisms.

Tungsten Oxide with Different Oxygen Contents: Sliding Properties

  • Department: Department of Control Engineering
  • Annotation:
    In this study, tungsten oxide coatings with 13 and 75 at% of oxygen were prepared by DC reactive magnetron sputtering from a pure W target in an Ar+O2 atmosphere. The coating hardness (H) decreased with increasing oxygen content from 25 to 7.7 GPa. The friction coefficient was rather high in case of the W87O13 coating reaching values in the range from 0.7 to 0.75 for both counterpart materials, and slightly lower for W25O75 (0.50).

Nanocomposite TM-dichalcogenides sputtered self-lubricating coatings alloyed with C

  • Authors: Cavaleiro, A., prof. Ing. Tomáš Polcar, Ph.D., Evaristo, M.
  • Publication: E-MRS 2006 Fall Meeting. Strasbourg: European Materials Research Society, 2006. p. 12.
  • Year: 2006

The Tribological Characteristics of TiCN Coating at Elevated Temperatures

  • DOI: 10.1016/j.wear.2004.12.031
  • Link: https://doi.org/10.1016/j.wear.2004.12.031
  • Department: Department of Control Engineering
  • Annotation:
    The tribological behaviour of TiCN coating prepared by unbalanced magnetron sputtering is studied in this work. The substrates made from austenitic steel were coated by TiCN coatings during one deposition. The measurements were provided by high temperature tribometer. The steel balls and the Si3N4 ceramic balls were used as counter-parts. The wear tracks were examined by optical methods and SEM. The surface oxidation at elevated temperatures and profile elements composition of the wear track were also measured. The experiments have shown considerable dependency of TiCN tribological parameters on temperature. Rise in temperature increased both friction coefficient and the wear rate of the coating in case of 100Cr6 balls. The main wear mechanism was a mild wear at temperatures up to 200 oC; fracture and delamination were dominating wear mechanisms at temperatures from 300 to 500 oC.

Comparison of Tribological Behaviour of TiN, TiCN and CrN at Elevated Temperature

  • Authors: prof. Ing. Tomáš Polcar, Ph.D., Kubart, T., Novák, R., Kopecký, L., Široký, P.
  • Publication: Surface and Coatings Technology. 2005, 193(1-3), 192-199. ISSN 0257-8972.
  • Year: 2005
  • Department: Department of Control Engineering
  • Annotation:
    PVD-coated instruments are often used at elevated temperatures. However, the friction and wear data of these coatings at temperatures exceeding room temperature are still lacking. In order to help remedy this lack, three coatings widely spread in industrial applications, TiN, TiCN and CrN, taken from standard production of customer's coating centre were comparatively measured up to 500 oC. The coatings were deposited on hardened steel substrates either by unbalanced magnetron sputtering or arc evaporation deposition. The measurements were done using a high temperature tribometer (pin-on-disc, CSM Instruments).

Temperature dependence of tribological properties of MoS2 and MoSe2 coatings

  • Authors: Kubart, T., prof. Ing. Tomáš Polcar, Ph.D., Kopecký, L., Novák, R., Nováková, D.
  • Publication: Surface and Coatings Technology. 2005, 193(1-3), 230-233. ISSN 0257-8972.
  • Year: 2005
  • Department: Department of Control Engineering
  • Annotation:
    This paper is aimed to the comparison of tribological properties of MoS2 and MoSe2 coatings measured in air of different humidity and at elevated temperatures. Both coatings were prepared by non-reactive DC magnetron sputtering and tested with ball-on-disc high temperature tribometer. The results of measurements of friction coefficient and wear rate vs. tribometer revolutions and the resulting dependencies of friction coefficient and wear rate on ambient air relative humidity are presented. These results show that the fiction coefficient of MoSe2 was not influenced by air humidity. Wear rate of MoSe2 in dry air was substantially higher than that of MoS2; in humid air, the MoS2 wear rate increased rapidly while wear rate of MoSe2 remained unchanged. The operating temperature of both coatings was limited to 350 degrees C.

Tribological Characteristics of CrCN Coatings at Elevated Temperature

  • DOI: 10.1016/j.vacuum.2005.07.033
  • Link: https://doi.org/10.1016/j.vacuum.2005.07.033
  • Department: Department of Control Engineering
  • Annotation:
    Dry sliding wear experiments at specific temperatures ranging from 20 to 500C were conducted on CrCN coatings deposited onto steel substrates. The coatings were worn against 100Cr6 bearing steel balls, Si3N4 and Al2O3 balls, using a pin-on-disc sliding configuration at a contact load of 5 N. Friction coefficient and coating wear rates were measured, wear tracks were analyzed using SEM equipped with EDX. Coatings exhibited good wear performance and unexpected behaviour at elevated temperatures.

Responsible person Ing. Mgr. Radovan Suk