A novel superlubricity DLC/MoS₂-Ag coating and its low friction mechanism in wide temperature range

To achieve reliable self-lubrication of machine components working under conditions of extreme low temperature, a carbon-based nanocomposite coating co-doped with molybdenum disulfide (MoS₂) and silver (Ag) was fabricated by high-power pulsed magnetron sputtering (HiPIMS). The microstructure, mechanical properties and tribological behaviors over wide temperature range were investigated. The results show the hardness and elastic modulus of DLC/MoS₂-Ag coating increase as the temperature decreases. At room temperature (RT) to -40°C, the coating demonstrates stable superlubricity in N₂ atmosphere with a low the coefficient of friction (COF) of 0.0075. This superlubricity is primarily attributed to the graphitization induced by friction and the “ordered Ag nanoparticle array” formed within the transfer film. As the temperature decreases from -40°C to -120°C, COF gradually increases to 0.0865, and the wear gradually increases. This degradation is closely related to the suppressed graphitization of the DLC coating and the deterioration of the low-friction “ordered Ag nanoparticle array” structure at interface. When the temperature is further reduced to -160°C, the graphitization of the coating is significantly inhibited, and the cryogenic mechanical performance is deteriorated, resulting in excessive consumption of Ag on the surface of the coating, which makes it difficult to maintain the special structure of the ordered Ag nanoparticle array at the friction interface. Consequently, the self-lubrication capability of the interface is further reduced, however the coating still presents COF below 0.1. These test results show that the DLC/MoS₂-Ag composite coating designed in the present study can give ultra-low COF in a wide temperature range, which provides valuable insights into the design of carbon-based coatings in extreme low temperature conditions.