SLIDING FRICTION MECHANISMS OF BOUNDARY-LUBRICATED LAYERED SURFACES .2. THEORETICAL-ANALYSIS

A new friction theory of boundary-lubricated layered surfaces based on the qualitative aspects of the prevailing friction mechanisms and the phenomenological observations is presented. The cases of elastic and plastic contacts are investigated analytically, and mathematical friction models which yield relationships between the coefficient of friction and critical parameters are developed. For the case in which elastic deformation prevails at the asperity junctions and when shearing of the lubricant film is the predominant friction mechanism, a friction model which accounts for the shear properties of the lubricant, the ratio of the layer thickness to the size of the asperity contacts, and the elastic properties of the layer and substrate media is presented, and the concept of boundary lubrication "friction space" is introduced. Futhermore, a friction model for plastically deformed contacts which accounts for interfacial adhesion and plowing effects is derived. It is shown that the friction coefficient is a function of the relative size and shape of the plowing hard wear particles (and/or work hardened asperities), the interfacial shear stress, the shear strength and strain hardening properties of the substrate, and the steady-state surface topography statistics. Predictions of the analytical friction models are shown to be in fair agreement with the friction coefficients obtained from sliding experiments with boundary-lubricated layered surfaces.