Tribological Properties of MoS₂-Based Composite Films at Different Temperature in Vacuum and Its Mechanism

Molybdenum disulfide (MoS₂) is an excellent solid lubricating material, which has been used extensively in space applications, because of its special structure. However, the resistance of MoS₂ coatings against high temperatures is inadequate and the tribological properties of MoS₂ coatings are degraded in high temperatures conditions, resulting in an increase of the friction coefficient and a decrease in its working lifetime. This paper aimed to improve the high-temperature resistance performance of Molybdenum disulfide and enable it to be used in a wide temperature range, the Ti and TiB₂ were adoped in the MoS₂ films, and the morpholgy, structure and tribology performance were investigated in detailed. Fristly, three kinds of films were successfully prepared by closed-field non-equilibrium magnetron sputtering technology, namely pure MoS₂ films, MoS₂-Ti composite films, and MoS₂-Ti-TiB₂ composite films. XRD and Raman spectroscopy were used to analyze the physical structure and composition of thin films, and the results demonstrated that the texture of the films changed from random mixed orientation with (002) and (100) planes of the pure MoS₂ films to the preferred (002) orientation with the adoping of Ti, while the MoS₂-Ti-TiB₂ composite films were amorphous. The results of the Raman spectroscopy analysis indicated that the MoS₂-Ti-TiB₂ composite films had the the best ability of antioxidant than the other two kinds of films. The microstructure of the film was characterized by SEM and FIB-TEM, and the analysis showed that the surface of the pure MoS₂ films were loosed, and that of the composite films were smooth and dense, and the cross section morphology were consistent with it. Thus, the hardness of the MoS₂-Ti-TiB₂ composite films were the highest, and the pure films with loose structure were the lowest. The tribological properties from 25 to 300 ℃ in vacuum environment were investigated by using a vacuum high temperature friction tester. The pure MoS₂ film growed in (002) and (100) crystalline orientations had very short friction life at various temperatures (25~300℃) in vacuum. The MoS₂-Ti films had excellent tribological properties at low temperatures of 25 and 100 ℃, however, the friction efficient and the wear rate were decreased dramatically when the test temperature beyond 200 ℃. Though the wear rate of the MoS₂-Ti-TiB₂ composite films were higher than that of MoS₂-Ti films at room temperature, the wear rate and the friction coefficient of the MoS₂-Ti-TiB₂ composite films maintained low in the full temperature range of 25~300 ℃. That was mainly due to the dense amorphous structure of the composite film that prevented the diffusion of oxygen and the oxidation of the film. Besides, the morphology of the grinding crack showed that abrasive wear was the main mechanism for MoS₂-Ti-TiB₂ composite films at low temperature. At high temperature, a sliding interface with an ordered (002) crystal structure was formed during the friction process, and the friction mechanism was mainly scratch and interlayer transfer peeling.