Surface and subsurface contributions of oxidation and moisture to room temperature friction of molybdenum disulfide

Molybdenum disulfide (MoS2), a lamellar solid lubricant, is used extensively in space applications due to its exceptional performance in vacuum and inert environments. The friction and wear of MoS2, however, increase in the presence of atmospheric contaminants, such as water. Despite numerous studies of the moisture-sensitive friction response of MoS2 over the decades, important fundamental questions remain unanswered. Two leading hypotheses suggest that water affects friction by causing the MoS2 to oxidize or by physically bonding to edge sites, and thereby disrupting easy lamellar shear. This paper presents a parametric study to (1) isolate the effects of water and oxygen on ambient MoS2 friction, (2) identify the effect of water and oxygen on MoS2 oxidation, and (3) distinguish between the effects of water diffusion and surface oxidation on the frictional response of MoS2 coatings. The experimental findings were used to develop a qualitative model for the effects of environment on MoS2 friction; the model is used to explain transients, hysteretic effects, oxidation effects, and effects of physically bound water.