Ing. Tomáš Vítů, Ph.D.

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.

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.

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.%).

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.

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.

Salt degradation is a well-known but still poorly understood problem. Determination of crystallization pressure that growing crystals exert on the pore walls represents a challenge solved by authors from different points of view. Nevertheless, few papers are aimed at the experimental measurement of the crystallization pressure magnitude. A novel high precision device able to detect repulsive forces generated by a crystal at the crystal/glass interface has been designed. Although some problems with determining the correct contact area of the confined crystal surface, which is most probably not atomically smooth, still exist, the results are comparable to data from other experimental studies. Crystallization experiments were performed with sodium chloride under 30 + 2% and 60 + 2% relative humidity (RH) conditions and with sodium sulfate in 30 + 2% RH. The disjoining pressure values were variable but did not exceed 1 MPa. Special interest was aimed at determination of disjoining pressure of sodium sulfate during phase transition after wetting, since this phenomenon creates most damage during standard crystallization tests. The disjoining pressure values were between 0.957 and 3.159 MPa - sufficiently high to overcome the tensile strength of most of the porous building materials.

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.

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.

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.

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.

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 new tribological systems based on functionally graded metal doped hydrocarbon nanocomposite coatings (Me-C:H) have been developed. Tribological properties of Me-C:H coatings were investigated using pin-on-disc tribometer both in dry environment and in NaCl solutions.

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.

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.

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.