Oxygen-induced contact evolution enables friction reduction in MoS₂/Ag composite films

Molybdenum disulfide (MoS₂) is widely utilized as a lubricant in aerospace, machinery, and electronics applications because of its unique layered structure and interlayer slip characteristics. Understanding the tribological behavior of MoS₂-based films under various atmospheric conditions, including oxidizing and specialized atmospheres, is crucial for developing environmentally adaptive lubricants. Here, we fabricated pure MoS₂ and Ag-doped MoS₂/Ag composite films via magnetron sputtering, focusing on their tribological performance in argon, CO₂, and O₂ atmospheres. Our results demonstrate that the friction coefficients of both films in argon and CO₂ are comparable to those in vacuum, with these environments promoting the formation of a continuous tribofilm on the counterpart ball surface, thereby reducing the wear rates. Remarkably, in an oxygen environment, the MoS₂/Ag composite film results in a ~50% reduction in the friction coefficient (0.027) and a threefold decrease in the wear rate compared with vacuum conditions. This exceptional performance is attributed to the friction-induced metal oxide nanoparticles coated with Ag, which form a “brick‒mud” structure that slides with MoS₂ (002) nanosheets to achieve low friction and wear. Furthermore, the addition of Ag enhances the ability of the film to repair sliding interfaces, mitigating abrasive wear. Our study elucidates the mechanisms driving the low-friction behavior of MoS₂-based films in atmospheric environments, offering valuable insights for the development of high-performance lubricants for extreme conditions.