Tribological properties of a-C/MoS2:a-C multilayer composite films under vacuum and high-temperature environments

The a-C/MoS₂:a-C multilayer composite film was fabricated using the optimized Cr/Cr_xC_y gradient transition layer and MoS₂:a-C composite film. The a-C layer and MoS₂:a-C layer were prepared by sputtering graphite and MoS₂ targets, respectively, via the magnetron sputtering method. Multilayer films with varying modulation periods were fabricated by alternately depositing the layers for different durations. The microstructure, mechanical properties, and tribological performance of the films were evaluated. The tribological behavior of the multilayer composite films under varying environmental conditions was examined. The results indicated that the multilayer structure design of a-C/MoS₂:a-C enhanced the mechanical properties of the film, including nano-hardness, elastic modulus, and film-substrate adhesion. The modulation period had no substantial effect on the tribological properties of the film. The a-C/MoS₂:a-C multilayer composite film demonstrated favorable tribological performance with low friction coefficient and wear rate in ambient air at temperature ranging from room temperature to 200 °C, as well as in a vacuum at room temperature. However, in a 300 °C high-temperature environment, all films underwent lubrication failure, resulting in a marked decrease in durability and lubricity. The thermal stress induced by elevated temperatures caused instability in the film structure, with severe degradation occurring in the non-friction areas of certain film samples. In the 300 °C high-temperature environment, the films failed to form a stable and adequate transfer film to reduce friction and resist wear. In the film structure design, the thinness of the MoS₂:a-C composite layer hindered the generation of a stable and friction-reducing transfer film during the friction process, which was the primary factor of lubrication failure in the 300 °C high-temperature environment.