We studied the tribological properties of amorphous molybdenum sulfide (MoSx) thin-film coatings during sliding friction in an oxidizing environment at a low temperature (−100 °C). To obtain films with different sulfur contents (x ~ 2, 3, and 4), we used reactive pulsed laser deposition, where laser ablation of the Mo target was performed in H2S at various pressures. The lowest coefficient of friction (0.08) was observed during tribo-testing of the MoS3 coating. This coating had good ductility and low wear; the wear of a steel counterbody was minimal. The MoS2 coating had the best wear resistance, due to the tribo-film adhering well to the coating in the wear track. Tribo-modification of the MoS2 coating, however, caused a higher coefficient of friction (0.16) and the most intensive wear of the counterbody. The MoS4 coating had inferior tribological properties. This study explored the mechanisms of possible tribo-chemical changes and structural rearrangements in MoSx coatings upon contact with a counterbody when exposed to oxygen and water. The properties of the tribo-film and the efficiency of its transfer onto the coating and/or the counterbody largely depended on local atomic packing of the nanoclusters that formed the structure of the amorphous MoSx films.

The thickness profiles and compositional distributions of MoSx films deposited from a plume generated by pulsed laser irradiation of the MoS2 target were investigated at a varying fluence and constant laser pulse energy. It was shown that films with stoichiometric composition were formed at sufficiently low fluence (near the ionization threshold), and increasing fluence caused intricate nonmonotonic variations of the compositional distribution. A substantial deviation of the film composition from stoichiometric and a significant radial gradient of the sulfur concentration over the substrate surface (1<x<3, where x is the ratio of concentrations of S and Mo atoms, x=S/Mo) were found. These phenomena were caused by: (1) the incongruent target evaporation; (2) the mass dependence of the angular distribution of the ablated particles; and (3) the selective sulfur sputtering and desorption induced by energetic particles (ions, excited atoms) of the laser-generated plume. When the laser fluence was low, films of stoichiometric composition were bombarded by energetic particles with a low intensity, and the number of displacements was inadequate for formation of the basal-oriented structure (≪1 displacement per atom). Raising the laser fluence, it is possible to increase the bombardment dose; however, the effect of the factors (1)–(3) has turned the selection of optimal conditions for stoichiometric film deposition into a rather complicated problem. A new technique is proposed for pulsed laser deposition of high-purity MoSx films with varying sulfur concentration and the required ion-irradiation dose. The technique consists in accelerating the ion component of the plume by a pulsed electric field up to an energy of a few tens of kilo-electron-volts, thus making it possible to significantly increase the number of displaced atoms in the MoSx film without noticeable change in its composition and to induce ion mixing at the MoSx-substrate interface. The positive voltage pulse of up to 100 kV amplitude was applied to the target during the plume expansion from the target towards the substrate which was grounded. The modification of the MoSx films by accelerated ions resulted in noticeable improvement of their lubrication performance.

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Using in situ x-ray photoelectron spectroscopy we have investigated the possibility of photochemical organic contaminant removal from a silicon surface at room temperature in oxygen and fluorine containing atmospheres (O2, NF3/H<thin>2, O2/NF3/H2). In contrast to UV irradiation in O2 and NF3/H2 reagents, the possibility of complete organic contaminant removal has been observed in O2/NF3/H2 gas mixture.

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