Preparation and properties of MoS₂/a-C films for space tribology

Abstract

MoS2/a-C composite films with various (Mo+S)/C ratios were deposited by medium frequency unbalanced magnetron sputtering. The effects of MoS2 doping on the microstructure, mechanical and vacuum tribological properties of the films were investigated. Results show that the sp2 carbon content in the film increases with increasing (Mo+S)/C ratio from 0 to 0.19, and MoS2 nanocrystallines are formed in film as (Mo+S)/C ratio increased to 0.07. Consequently, the composite film exhibits decreasing hardness (from 5.2 to 2.5?GPa) and elastic modulus (from 116 to 24?GPa). As the (Mo+S)/C ratio increases from 0 to 0.19, the friction coefficient of the films decreases from 0.18 to 0.008 and sliding time increases from 3?s to more than 3600?s in vacuum. This is mainly attributed to that the films with higher (Mo+S)/C ratios (at least 0.12) are inclined to form the lamellar MoS2 with low shear strength on the counterface. Furthermore, the composite film with (Mo+S)/C ratio of 0.12 has been investigated in various test environments (air, N2, vacuum). The average friction coefficient is lower than 0.035 and it exhibits little sensitivity to the test environment. However, the highest and lowest wear rate (about 4???10?7?mm3?Nm?1 and 0.8???10?7?mm3?Nm?1) are obtained in vacuum and N2, respectively. The environment dependence of the tribological behaviours is related with the lattice orientation of MoS2 crystalline and the graphitization degree of the film.