Friction of lubricated layered surfaces

Mixed lubrication is the dominant mode in most machine elements, such as gears, rolling bearings, cams and followers, The Stribeck curve has been generally used to represent the lubrication performance of these tribological components. To evaluate the friction behavior, a deterministic, mixed model is presented, by which Stribeck type curves can be formulated. In the mixed model, the applied load is shared by lubricant film and asperity contact. Solutions for the lubricated areas and the asperity contact pressure are determined by using Reynolds equation and the Greenwood and Williamson theory. Based on Rabinowicz's findings of the relationship between the shear strength of liquid boundary lubricants and pressure and on the elastic analysis of stresses and displacements in a layered medium proposed by Gupta and Walowit, the coefficient of friction of layered elastic solids in boundary lubrication is calculated. The effects of the rheological behavior of the lubricant, the Young's modulus ratio of the layer to substrate and the ratio of the layer thickness to the size of the asperity contacts are considered. Modified Stribeck curves are obtained.