Macroscopic superlubricity enabled by the tribopair of nc-Ag/MoS2 and hydrogenated graphitic-like carbon films under high contact stress

Abstract

Solid lubricants have superior environmental adaptability and can be widely used in space machinery under extreme harsh working conditions, including high contact pressure, high or cryogenic temperature, long working duration, etc. Loose-structured molybdenum disulfide (MoS2) film usually has poor load-bearing capacity, while low-hydrogenated graphitic-like carbon (GLCH) film is difficult to construct low-sheared frictional interfaces under dry inert atmosphere. In this paper, the heterogeneous interface was built up by combining nano-crystallized silver-doped MoS2 (nc-Ag/MoS2) and GLCH films to achieve superlubricity (coefficient of friction, COF = 0.006) under the ultra-high contact stress of 2.33 GPa. The results emphasize that the GLCH film with a low hydrogen content among the hydrogenated amorphous carbon (a-C:H) films is more likely to be low-friction and anti-wear when paired with nc-Ag/MoS2 film. Moreover, the nc-Ag/MoS2 film has potential to obtain the high load-bearing capacities and induce an appropriate graphitization of GLCH film in the heterogeneous interface system, obtaining superlow friction in contrast to the homogeneous self-mated GLCH tribopair. Additionally, this heterogeneous interface can achieve superlubricity (COF = 0.007) for more than 50,000 sliding cycles without failure. This study has important application significance in space machinery and can provide theoretical guidance and new paths for future solid lubrication technology.