Molybdenum disulfide (MoS2) shows excellent lubricating properties in high vacuum condition due to its unique layered structure. But passivation to oxygen atmosphere like ambient air makes tribological performance lower. Although the a-C:H films exhibit super-low friction coefficient and wear rate in N2 condition, the long duration for super-low friction seems to be an important issue for its application in high vacuum. This paper aimed to combine MoS2 and a-C:H films to meet the requirements of high vacuum condition and super-low friction coefficient and long wear life under practical working conditions. Based on this, firstly, MoS2-C composite films were deposited by closed field unbalanced magnetron sputtering technology. The tribological properties of the films were tested by HVTRB in vacuum condition. The structure changes of the film before and after friction were analyzed by Raman, XRD, TEM and other analytical techniques. And the superlubricity mechanism was investigated in this study. It suggested that the composite films exhibit a dense "nanocrystalline/amorphous" microstructure, which maintain super-low friction coefficient (0.006~0.009) and wear rate 1.026×10-7 mm3/(N·m), reaching a superlubricity state. The wear tracks and wear scars were observed by a three-dimensional profiler. It revealed that the contact area was covered with thin transfer film, while a small amount of wear debris distributed around. Furthermore, different from the as-deposited films, it was observed by
Raman that MoS2 peak around 380 cm-1 and 410 cm-1 of the wear track was obviously enhanced. A broad peak at 1 200~1 600 cm-1, indentified as amorphous carbon, was observed on the wear scar. During the friction process, C was selectively transferred to the surface of the steel ball to form an amorphous carbon transfer film. Meanwhile, the relatively disordered structure became more ordered, resulting in the presence of well aligned MoS2 in the interface, which was prone to shear the basal plane oriented along the sliding direction. The friction occurred between the ordered MoS2 crystal and the amorphous carbon transfer film. The heterostructure leaded to incommensurate contact between the frictional interfaces, resulting in establishing superlow friction during the friction period. In addition, MoS2-Ti film and a-C:H film were deposited. The friction behaviors of steel /MoS2-Ti and a-C:H/MoS2-Ti were very different under the same condition. For steel/MoS2-Ti friction pair, it was found that the transformation of amorphous MoS2 into re-ordered MoS2 on the wear track after friction. The MoS2 crystal was transferred to the steel ball. Consequently, the friction occurred between MoS2 and MoS2, which cannot achieve superlubricity. For a-C:H/MoS2-Ti friction pair, the crystallinity of MoS2 on the wear track became better after friction. The ordered MoS2 formed incommensurate contact with the a-C:H film on the steel ball, which achieved superlubricity. The above-mentioned superlubricity mechanism was proved by the different experiments.