Nature of super-lubricating MoS2 physical vapor deposition coatings

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MoS2 is a well-known solid lubricant used for space applications. Radio frequency (rf) magnetron sputtered MoS2 films are being used increasingly, due to an ultra-low friction behavior in high vacuum (friction coefficient: ∼0.01). MoS2 coatings with a super-low friction behavior (friction coefficient: ∼0.001) have been recently synthesized in an ultra-high vacuum tribometer equipped with a rf magnetron sputtering device. Physicochemical and structural characterizations of the thin films have been carried out using XPS (x-ray photoelectron spectroscopy) and AES (Auger electron spectroscopy) to determine the extreme surface composition, HRTEM (high-resolution transmission electron microscopy), GXRD (grazing-angle x-ray diffraction) for structural investigations, RBS (Rutherford backscattering spectrometry) for thickness and stoichiometry determination, and NBS (nuclear backscattering spectrometry) for quantitative analysis of oxygen contamination. Indentation tests at nanometric scale have also been performed to compare the mechanical constants (Young modulus, hardness) of the sputtered MoS2 films with the characteristics of molybdenite and to confirm the overall data. The coatings are made of highly pure, stoichiometric MoS2, with a polycrystalline microstructure (grain size on the order of 10 nm). The stoichiometry is MoS1.97±0.10O0.10±0.01. Most of the grains are ‘‘edge-oriented’’ on the substrate surface [relative to their (002) basal planes], with an azimuthal disorder of the other crystallographic directions [(100) and (110)]. Consequently, and by comparison with already published data, the super-low friction regime is found to be associated with the purity and microstructure of the coating.