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

440C steel thrust ball bearing races lubricated with 1 mu 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 approximate to 10(-6)-10(-7) Torr) vacuum and in 1 atmosphere of 99.999% pure (<= 1 ppm water) N-2 in the same unbaked environmental chamber. Over 90% of the residual gases in the chamber vacuum consisted of H2O vapor. The bearings operated in N-2 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 N-2 and column vacuum of similar to 10(-3) Pa approximate to 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 fiat 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 N-2 by the gas molecules disrupting the H-bonding mechanism.