Tribological Properties of MoS₂/a-C Composite Films under High/Low Humidity Environments

In this study, the MoS₂ film and MoS₂/a-C films with two C contents were deposited by unbalanced magnetron sputtering. The structure, mechanical and tribological properties were investigated by X-ray diffraction, scanning electron microscopy, nanoindentor and CSM tribometer. The results show that MoS₂ film presented loose columnar structure with well crystalline state, while the MoS₂/a-C composite films appeared amorphous state with much dense microstructure. Moreover, the MoS₂/a-C composite films possessed relatively higher hardness compared to the MoS₂ film. The friction coefficient and wear rate of the films were all found to be very low, and there were no obviously variations with the change of C content under low humidity (RH 15%). However, under high humidity (RH 75%), the friction coefficient of the MoS₂ film and the MoS₂/a-C composite film with low C content was relatively high and unstable with rather high wear rate as the serious oxidization. The MoS₂/a-C composite film with high C content exhibited relatively lower and stable friction coefficient, as well as relatively lower wear rate compared to the MoS₂ film and the MoS₂/a-C composite film with low C content. The film also provided stable performance and recovery of low friction in multiple humidity cycles. The excellent properties of the MoS₂/a-C composite film were attributed to the relatively denser microstructure, high hardness and excellent oxidation resistance properties.