Solid lubrication of ball bearings for spacecraft mechanisms

Abstract

Solid lubricant coatings are commonly employed in mechanical systems operating in environments where oil or grease lubrication is not feasible. In particular, solid lubricant coatings are currently being used in rolling element bearings operating in vacuum environments and/or at high temperatures. Although numerous studies have reported the tribological performance of these materials in sliding contact, few if any studies have reported their performance in rolling contact.Since the functional lifetimes of solid lubricant coatings in rolling element bearings are derived from sliding contact experiments, it is beneficial and important to understand if the actual performance of the coatings in rolling contact bears a relationship to their sliding contact performance. In this work, Sb2O3/Au-doped MoS2 coatings are deposited onto AISI 52100 and M50 specimens by magnetron sputtering. Mechanical properties are studied by scratch tests and nano-indentation techniques; microstructural analysis is undertaken with Raman spectroscopy and transmission electron microscopy. Friction and wear measurements are performed on specimens in both unidirectional and reciprocating sliding, using ball-on-flat tribometers in both vacuum and humid environments. Friction coefficient values as low as ~ 0.04 and as high as ~ 0.14 are measured at different contact stresses, and depend greatly upon the test environment. Functional lifetimes of the coatings are obtained from rolling contact tests performed in a thrust ball bearing tribometer in standard laboratory conditions.