Ilia Ponomarev

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.

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.

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.

Cyclopentasilane (CPS) has been studied as an liquid precursor for the deposition of thin silicon films for printed electronics and related applications. The processing involves a UV-induced prepolymerization of CPS followed by liquid deposition and low-temperature thermolysis. An insight into the oligomer and polymer formation including crosslinking in solution using Si-29 NMR spectroscopy and electron spin resonance spectroscopy is reported. Formation of SiH (T-units) and SiH3 (M-units) is observed as well as short-lived paramagnetic species. Additionally, the polymerization is followed by Raman spectroscopy. Reactive molecular dynamics simulations are applied to develop a theoretical model for the CPS-ring-opening and crosslinking steps. The experimental and computational data correspond well to each other and allow insight into the mechanism of polymer formation. The processing steps include spin-coating, thermal drying, and conversion to amorphous silicon, H-passivation, and fabrication of a CPS-derived thin-film transistor (TFT), without intermediate silicon crystallization. Further improvement is gained by using tetralene as a solvent, leading to a reduction of the time-consuming polymerization step by one order of magnitude compared to cyclooctane. The overall quality and characteristics of the CPS-derived spin-coated silicon thin films correspond to standard plasma enhanced chemical vapor deposition-derived devices with respect to performance levels.

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.