Vacuum tribological properties of a composite Mo–MoS 2 film after atomic oxygen exposure irradiation

Abstract

A composite Mo–MoS₂solid-lubrication film measuring approximately 2-µm thick was prepared using a two-step composite process involving magnetron sputtering of Mo film followed by low-temperature ion sulfurization. Microstructure of the said film was characterized using scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, Auger electron spectroscopy, and X-ray photoelectron spectroscopy, and nanomechanical properties of the Mo and Mo–MoS₂film were tested using a nanoindenter. The Mo–MoS₂film and GCr15 substrate were irradiated by atomic oxygen with the self-developed MSTS-1 space tribometer system equipped with an atomic oxygen beam at a flux of 9 × 10¹⁹atoms/cm². Subsequently, vacuum tribological properties of the said substrate and film were examined and compared. Results demonstrated that the Mo film possessed a smooth and dense structure with stable nanomechanical properties. Part of the elemental Mo translated into MoS₂post sulfurizing, thereby forming the Mo–MoS₂composite film with an alternating soft and hard structure. The Mo–MoS₂film demonstrated a low and stable friction coefficient of approximately 0.15 along with only a slight wear in vacuum. The film surface could be oxidized and eroded when exposed to the highly active and energetic atomic oxygen. Post atomic oxygen erosion, the film thickness demonstrated a decrease, and small amounts of MoO₃were observed on the surface. However, all structural and property changes were limited to the superficial layer. The excellent tribological performance of the film could be restored when the surface layer was removed after a certain period of sliding friction.