Micropitting in Hertzian Contacts

Abstract

This paper shows that both micropits and spalls occur near pure rolling conditions. The first are initiated at the surface and are related to asperity interaction. The second are well described in the literature and will not be discussed here. A model capable of predicting the mechanical conditions under which micropits are generated is presented. In this model the pressures on asperities are analyzed for given separations of the surfaces; elastic, elasto-plastic, or plastic deformations are deduced. Master curves can be obtained if a pressure index is used, the model requires a comprehensive description of the surface as it calls for the distributions of both profiles height and asperity tip radii of curvature. A new approach was proposed to define the peaks which carries the load in the contact. In this model a peak is defined with respect to the separation of the surfaces. Good correlation between theory and experiment was obtained for materials as different as mild and case hardened steels and with ground and “as received” treated specimen surfaces. The rate of micropit formation which depends necessarily on running-in cannot be predicted at this time. Results however clearly show that when running-in takes place, i.e. when the pressure on asperity tips decreases to a value compatible with elastic deformation, the rate of formation of micropits with time tends to zero and of course fatigue life is increased.