The Lubricating Properties Of MoS₂-Based Composite Films Under High/Low Humidity Environments

Over the past years, there was increasing demand for solid lubrication materials in space area alongside the development of science and technology, especially the rapid development of technology and the needs of aerospace technology. As a solid lubricant, MoS₂ film plays a significant role in target application of friction mitigation in space, especially in strict requirements of special conditions such as high speed, high load, high/low temperature, high vacuum and space radiation and many other conditions. MoS₂ has good tribological performance in vacuum and inert environment owing to its layered structure. But it was easily oxidized and affected the tribological performance when exposed to the air. For instance, the satellites and satellite launch vehicles wait for extended periods of time in humid coastal environment prior to launch, potentially exposing the moving mechanical assemblies to humidity, such as Hainan Wenchang Satellite Lunch Center. Therefore, it is of great significance in improving the tribological performance of MoS₂ film test in different relative humidity environment, especially in high humidity environment.In this thesis, MoS₂ film and MoS₂/a-C, MoS₂/Si, MoS₂/Pb, MoS₂/Ti composite films in different contents were grown on either 304 stainless steel or?100? silicon wafer by unbalanced magnetron sputtering, respectively. Films with various doping elements category and chemistries are discussed along with their characterization by various investigated such as chemical composition, structure and morphology of surface and cross section, hardness and elastic modulus and other mechanical performance, as well as the tribological performance under different high and low humidity environment. Finally, the effects of doping element category and chemical components on the film antioxidant activity provided a reference for MoS₂-based composite film in a relative field application and promotion. The major objective of this thesis indicated that:?1? The mechanical properties of the MoS₂/a-C film doped with non-metallic carbon element were significantly increased compared with MoS₂ film. The surface roughness of the composite film decreased, the density of the cross section structure increased significantly, and the crystal structure of the thin film varied from orientation of?002? basal plane parallel to the sliding direction to amorphous structure. Always, the friction coefficient and wear rate of the composite films were significantly lower in the high/low humidity environment. In the environment of the humidity changes between high and low value, the composite film provided a corresponding switch in friction response.?2? The structure of the MoS₂/Si composite film doped with different content of non-metallic silicon element was improved in some degree, such as the refinement of grain size, the reduction of surface roughness and densification of loose column structure. But the columnar structure of the MoS₂/Si composite film was still quite obvious. In the high/low humidity environment, the friction coefficient curve was smooth. Conversely, the wear rate was increase. The composite film has a poor adaptability to the environment of humidity changes. This may be related to the forming of silicon or silicon oxide and other substances in the course of tribotest.?3? Although the grain size of Pb doped MoS₂/Pb composite films was decreases, the surface roughness and the structure induced density of the films were also greatly improved, but its mechanical properties were not so outstanding as other element dopant composite films. The respective of tribology was greatly improved. Pb can not only reduce the friction coefficient in high/low humid environment, but also stabilize the tribological process, and reduce the wear rate. But Pb was harmful to health, people had to take the factor of health into consideration when prepared and mad use of MoS₂/Pb composite film.?4? It was not hard to find that metallic element Ti had obviously effect on mechanical properties of MoS₂/Ti composite film. The surface roughness of the composite film was decreased, as well as grain size was refined. The films possess columnar structure, but the density of the structure was increased. The phenomenon of element Ti oxidized preferential in the process of friction had played a key role in the oxidation resistance of thin films. But due to the generation of TiO₂,MoS₂/Ti composite film only effective in a certain range to reduce the wear rate.By general comparison, MoS₂/Pb composite film had the best tribological performance in high / low humidity environment, but the performance of MoS₂/a-C composite film was also performed well, so MoS₂/a-C composite film seemed to be a good choice.