NATURE OF SUPER-LUBRICATING MOS2 PHYSICAL VAPOR-DEPOSITION COATINGS

MoS2 is a well-known solid lubricant used for space applications. Radi o frequency (rf) magnetron sputtered MoS2 filMS are being used increas ingly, due to an ultra-low friction behavior in high vacuum (friction coefficient: approximately 0.01). MoS2 coatings with a super-low frict ion behavior (friction coefficient: approximately 0.001) have been rec ently synthesized in an ultra-high vacuum tribometer equipped with a r f magnetron sputtering device. Physicochemical and structural characte rizations of the thin films have been carried out using XPS (x-ray pho toelectron spectroscopy) and AES (Auger electron spectroscopy) to dete rmine the extreme surface composition, HRTEM (high-resolution transmis sion electron microscopy), GXRD (grazing-angle x-ray diffraction) for structural investigations, RBS (Rutherford backscattering spectrometry ) for thickness and stoichiometry determination, and NBS (nuclear back scattering spectrometry) for quantitative analysis of oxygen contamina tion. Indentation tests at nanometric scale have also been performed t o compare the mechanical constants (Young modulus, hardness) of the sp uttered MoS2 films with the characteristics of molybdenite and to conf irm the overall data. The coatings are made of highly pure, stoichiome tric MoS2, with a polycrystalline microstructure (grain size on the or der of 10 nm). The stoichiometry is MoS1.97+/-0.1000.10+/-0.01. Most o f the grains are ''edge-oriented'' on the substrate surface [relative to their (002) basal planes], with an azimuthal disorder of die other crystallographic directions [(100) and (110)]. Consequently, and by co mparison with already published data, the super-low friction regime is found to be associated with the purity and microstructure of the coat ing.