The Role of Oxide and Sulfide Additions in Solid Lubricant Compacts

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Abstract
A study of antimony trioxide-molybdenum disulfide solid lubricating compacts found no evidence to support hypotheses that the tribologically beneficial role of antimony trioxide in those compacts results from sacrificial oxidation of antimony trioxide, direct retardation of molybdenum disulfide oxidation, or formation of eutectic of antimony and molybdenum oxides. Data do support a hypothesis that the role of antimony trioxide is to plastically deform at asperity flash temperatures permitting molybdenum disulfide crystallites attain and retain a preferred orientation to improve overall tribological condition. Studies of other and sulfides as bulk additions in lubricating compacts support the concept. Additional confirmation is provided by the finding that lower test temperatures are required to obtain the beneficial effect of antimony oxide additions to graphite as compared with additions to MoS2; evidently, higher flash temperatures are experienced by the graphite compact as shown by calculations. Excellent correlation of wear volume and poor correlation of friction coefficient with calculated additive shear strength indicates that the beneficial effect of additions is a bulk rather than interfacial phenomenon, which supports the concept of additive deformation permitting the solid lubricant to achieve preferred tribological position. Thus, lubricant compact properties and selection of beneficial bulk additions which deform at asperity contacts for improved tribological performance are determined by test or application conditions, shear strength of the additive and properties of the solid lubricant.