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.