EXPERIMENTAL STUDY OF MOS2 COATED GEARS TESTED IN VACUUM AFTER EXPOSURE TO HUMIDITY

Abstract

Space Industry commonly uses molybdenum disulphide (MoS2) as a solid lubricant film on gears in a variety of mechanisms. These mechanisms are often exposed to humid air during fabrication, integration into higher-level assemblies, and storage. It is generally accepted that operating MoS2 films in humid environments has a detrimental effect on the film's lifetime compared to operation in vacuum. However, the effect of exposure of such films to humid air with subsequent operation in vacuum is not well understood. In this study, a sputtered MoS2 dry film lubricant coating was applied to steel gears, which were then tested in a vacuum environment. Half of the spur gears coated with MoS2 were exposed to approximately 58 percent relative humidity for up to 77 days before testing. The other half set of gears were not exposed to humid air before testing. All gears were tested in a vacuum gear test rig. Gear tooth forces and the radial displacement of the gears were recorded during testing. These data together with photos, videos, Scanning Electron Microscope (SEM) inspections of worn gears, and Energy-Dispersive X-ray (EDX) spectra were analysed. For purposes of this study, the coating durability was defined as the time until the start of the coating degradation as was evident from increase of friction. Assessment of the results obtained from all experiments indicate that gears exposed to humid air prior to operation in vacuum showed nearly 36 percent reduction in coating durability, on average. SEM and EDX analyses of wear debris indicate mainly MoS2 was present with minor traces of other elements. The worn surfaces of gears show significant content of MoS2 still remaining in the gear teeth's contact area.

Keywords

• molybdenum disulfide
• solid lubricant
• thin film
• surface coating
• spur gears
• rolling contact
• mild wear
• debris formation