Anomalous wear behavior of MoS2 films in moderate vacuum and dry nitrogen

440C steel thrust ball bearing races lubricated with 1 m thick sputtered films of MoS₂ were tested in the unidirectional and oscillatory modes against bare steel balls in moderate (10^–4–10^–5 Pa 10^–6–10^–7 Torr) vacuum and in 1 atmosphere of 99.999% pure ( 1 ppm water) N₂ in the same unbaked environmental chamber. Over 90% of the residual gases in the chamber vacuum consisted of H₂O vapor. The bearings operated in N₂ showed substantially longer lives compared to the specimens tested in vacuum. Scanning electron microscope (SEM) tribometry was also performed on an MoS₂ film powder-burnished onto a 440C flat. This flat was repeatedly oscillated against bare, hemispherical-tipped 440C pins on fresh wear tracks in the same type of N₂ and column vacuum of ~10^–3 Pa 10^–5 Torr itself containing over 90% residual H₂O. The SEM-generated results on the burnished film confirmed the same, atmosphere-dependent difference in wear life observed with the sputtered layers. Varying the moisture content of the burnished flat and its immediate environment by cryosorption predictably manipulated the coefficient of friction and wear life of MoS₂. The various possible causes of this perplexing phenomenon are reviewed, and a plausible hypothesis is offered attributing the unexpected wear life reduction to the physico-chemical consequences of residual H₂O hydrogen-bonding to the oxidized and/or hydrated edge and basal plane sites of MoS₂ in moderate vacuum. The site-specific sorption of water is severely hindered in 1 atm N₂ by the gas molecules disrupting the H-bonding mechanism.