Tribological and chemical effects of long term humid air exposure on sputter-deposited nanocomposite MoS2 coatings

Sputter-deposited nanocomposite MoS2-based lubricant coatings have attractive properties for niche spacecraft applications, because they are thinner, exhibit lower friction, have excellent endurance, and can handle higher loads than bonded coatings. However, questions have been raised regarding the potential degradation of their tribological performance due to oxidation during prelaunch storage in air. We have studied the tribological and surface chemical changes occurring for coatings stored in air at 59% RH for extended periods. Pin-on-disk friction testing showed that the endurance of Ni-MoS2 coatings dropped significantly during storage of up to 4.2 years, but Sb2O3-containing coatings (Sb2O3-MoS2 and Au-Sb2O3-MoS2) exhibited much smaller drops in endurance over the same period. The differences in endurance correlate with X-ray Photoelectron Spectroscopy results that measured the conversion of MoS2 to nonlubricating MoO3 during storage.