Mixed lubrication model for microbearings containing microasperities of defined geometry

Abstract

For applications involving small-size bearings, the hydrodynamic lubrication conditions are not fulfilled and a mixed lubrication model, involving geometrical characteristics of the microasperities which are responsible for abrasion, is presented. In micromechanical systems, the mixed lubrication condition is assumed to be a steady-state regime. Abrasion is considered as the dominant phenomenon and its dependence on the geometry of microasperities is studied. It is assumed that the lubricant properties do not significantly influence the lubrication conditions in this model. The degree of lubrication is averaged by a modifier. Conical, pyramidal and spherical shapes of microasperities are assumed. It is found that for conical and pyramidal microasperity shapes, the friction coefficient depends only on the tip, angle of the protrusions. For a spherical shape, the friction coefficient depends also on the depth of the wear plough. Experiments done with macroscopic samples for dry and mixed lubrication show no detectable correlation between the macroshape of the test sample and the friction coefficient. Assuming in turn, for the same samples, uniform and regular shapes of the microasperities, geometric characteristics of these regular shapes can be determined. Reciprocal, for a certain configuration of the microasperities, the friction coefficient can be anticipated. This model provides a quantitative guide to the proper design of surface topography in order to get a minimal friction coefficient during the mixed lubrication regime.