MoN-MoSx Coating Achieving Excellent Tribological Properties in Solid–Liquid Composite Lubrication through a Time-Dependent Tribofilm

In the present work, the MoN-MoSx composite coatings were prepared by reactive sputtering of Mo and MoS2 targets. The influence of MoS2 target power on the structure, morphology and mechanical properties of composite coatings was investigated, and the tribological properties of coatings were evaluated under PAO6 base oil lubrication. As the increase of MoS2 target power was from 0 to 1000 W, the highest coating hardness and the optimized tribological performances were achieved at 400 W. Different from pure MoN coating, it was observed that MoN-MoSx composite coatings exhibited a time-dependent friction behavior, the friction coefficients and wear rates slowly decreased with the extension of sliding time. As the sliding test lasted for 4 h, the MoN-MoSx coating with the highest hardness gave a lower friction coefficient (~ 0.06) and significantly higher wear resistance (1.1 × 10−9mm3/N·m). Worn surface analysis indicated that a tribofilm containing MoS2, Mo oxides, and sulfates on steel ball could be associated with the excellent friction and wear properties of MoN-MoSx coating solid–liquid composite lubrication. As a result, it can be concluded that such MoN-MoSx coating offers a new candidate for tribological engineering toward future green and high-performance applications.