The Synergistic Effects of Graphite on the Friction and Wear of MoS2Films in Air

Abstract

Literature data are interpreted to formulate an original mechanism of MoS₂ film degradation by tribooxidation and elucidate the synergistic effects of graphite in extending the wear life of burnished or bonded MoS₂ layers. It is shown that the known but previously unexplained reduction in hexagonal platelet displacement in the direction of tangential shear and blister formation within the films are probably caused by oxidation. The molar volume reduction during the conversion of MoS₂ into its stable oxide results in roughening of the interlaminar surfaces by oxidative etch-pitting at the slip steps and at the basal plane defect sites of MoS₂ crystallites. It is proposed that interlocking of the pitted crystallites, combined with the formation of SO₂ trapped and frictionally heated along with other gases in the coalesced free volume regions, is responsible for reduced shear, the formation of blisters and catastrophic delamination of the films. It is further suggested that the much slower oxidizing graphite crystallites contain shallower etch pits (i.e., their basal planers remain smoother) and, when mixed with MoS₂, the smoother planes prolong interlaminar shear and postpone film interlocking (sintering). To a certain degree, graphite also acts as an antioxidant and moisture scavenger. It contributes to reduced MoS₂ oxidation in well-run-in films compacted into oxygen diffusion barrier layers, promoting lower friction and hindering blister formation. Although higher in friction than MoS₂, the stable oxide generated from MoS₂ is the soft, non-abrasive MoO₃ with a low shear strength (010) crystal plane. The relatively lubricious oxide film disturbed by tribological action is, at best, marginally protective against further oxidation of the MoS₂ substrate. It is hoped that the concepts presented in this paper may help promote the discovery of new antioxidant-synergistic agents for MoS₂ or other layered transition metal dichalcogenides.