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This report consists of a review and evaluation of the work presented in the literature over the past 40 years on the behavior of MO!% as a lubricant. The discussion is concerned with the behavior of MO& as a lubricant, its frictional characteristics, the apparent reasons for these characteristics, and the physical and chemical properties which are important to its use as a lubricant The low friction and easy cleavage of molybdenum disulfide is intrinsic to the material and a result of its crystal structure. Although it is clear that the presence of condensable vapors is not required for molybdenum disulfide to exhibit low friction, as it is in the case of graphite, it is true that condensable vapors play an important role in determining the friction and wear characteristics of a lubricant film of MoS2. The presence of oxidizing agents, particularly water vapor and oxygen, tends to reduce the wear life of the MO& film. This is particularly true in the case of burnished films. Although a great deal of knowledge exists concerning the behavior of molybdenum disulfide as a lubricant, there are still voids in our understanding of this subject. CONTENTS * This review is based on an earlier review (October, 1965) by the author which was given limited circulation by the Climax Molybdenum Company of Michigan. Wear, 10 (1967) 422-452 MECHANISM OF FRICTION IN MOLYBDENUM DISULFIDE Although molybdenum disulfide has long been known to exhibit a low coefficient of friction, the reason for this behavior has been the subject of much research and speculation. Because of the similarities between MO& and graphite the early scientists mistook one for the other, and, in recent years, some scientists have at

A shear rheological model based on primary laboratory data is proposed for concentrated contact lubrication. The model is a Maxwell model modified with a limiting shear stress. Three material properties are required: Low shear stress viscosity, limiting elastic shear modulus, and the limiting shear stress the material can withstand. All three are functions of temperature and pressure. In applying the model to EHD contacts the predicted response possesses the characteristics expected from several experiments reported in the literature and, in one specific case where direct comparison could be made, good numerical agreement is shown.

Measurements of lubricant shear rheological behavior in the amorphous solid region and near the liquid-solid transition are reported on three lubricants under pressure. Elastic, plastic and viscous behavior was observed. The maximum yield shear stress (limiting shear stress) is a function of temperature and pressure and is believed to be the property which determines the maximum traction in elastohydrodynamic contacts such as traction drives.

A limiting shear stress model of liquid lubricant shear rheology is offered which accurately represents all available primary data. The model is of the nonlinear Maxwell type with shear modulus taken into the time derivative and broadening of the viscous-plastic transition with pressure. Property relations for viscosity, limiting stress and shear modulus are refined for a polyphenyl ether in particular. The model, with simplifying assumptions, is compared with disk machine results. This model, with change of yield criterion, may be applicable to some shear thinning liquids at low pressure. Limiting shear stress varies with pressure in the same manner as the ultimate shear strength of solid polymers.