Preparation And Tribological Properties Of MoS₂-C Lubricant Films

As a candidate of excellent solid lubricants, MoS2 film has been widely used in the space field due to its excellent lubricant property in vacuum environment. In spite of its successful application in aerospace industry, the pure MoS2 film generally suffers from the humid air environment, which is attributed to its loose structure, low hardness, low adhesion and high chemical activity to oxygen. To improve tribological performance in different environment, doping a third element into MoS2 film has been considered as an effective strategy. Among these doped elements, MoS2 doped with C acquires increasing attention because of its homogeneous and dense structure as well as low friction coefficient in ambient air environment. In this study, MoS2-C film is prepared by direct current magnetron sputtering using MoS2 and graphite targets, while Ti interlayer is fabricated by high power impulse magnetron sputtering (HIPIMS) to improve the adhesion. The influence of carbon on microstructure and properties of the MoS2-C film have been investigated. Specifically, we studied the tribological performance in different environments and test parameters, the wear mechanism are discussed in terms of the microstructure of the deposited films.By varying the number of MoS2 plates bonded to the carbon targets, the carbon content of the composite film changes from 40.9 at.% to 73.1 at.%. The results show that the structure of MoS2-C film exhibit amorphous, homogeneous and dense. The amorphous structure could prevent the intergranular slip under contact loads and thus increase the hardness of film. The amorphous carbon matrix not only improves the mechanical properties, but also protects MoS2 from oxidation effectively and decreases the friction coefficient and wear rate under humid air. Furthermore, compared to other technique, the Ti bufferlayer deposited by HIPIMS technique could significantly improve the interface adhesion between substrate and film. The critical load of the composite film increases from 25N to 56N, compared to direct current magnetron sputtering.Friction test environment and parameters have a great influence on the tribological performance of MoS2-C film. The composite film displays low friction coefficient (below 0.1) and wear rate (10-7 mm3/Nm) under different relative humidity, since the dense carbon matrix protecting MoS2 from exposing directly to the ambient air. N2 could inhibit the chemical reaction during sliding in N2 atmosphere and the steady continual transfer film formed which is propitious to low friction coefficient (0.02) and wear rate (10-8 mm3/Nm). The superiror tribological behavior of the film in oil environment is resulted from the solid film lubricant and oil lubricant. High temperature could make MoS2 crystallize to improve lubrication and increase lifetime, but also result in the pulverization of film surface. With increasing friction load, the friction coefficient and wear rate of the film decreased, implying the good lubricant performance for the application with high load. Besides, MoS2-C film also has a steady tribological property and a long lifetime.