Asperity Pressures in Elastic–Plastic Rough Surface Contacts

Abstract Asperity pressures and deformations are key factors in determining the friction and wear behavior of tribological contacts. Although numerical contact models to predict these pressures have existed since 1980s, their accuracy is limited by our lack of understanding of asperity plastic behavior. This study uses a combination of indentation experiments and corresponding numerical rough contact simulations to ascertain the actual asperity pressures in elastic–plastic rough surface contacts covering a range of roughnesses, loads, and contacting material hardnesses. The indentation experiments were designed so that roughness profiles are measured before and after indentation in the same location, which enables the amount of asperity plastic deformation to be directly observed. The results show that asperities can carry much higher plastic pressures than the material bulk hardness, a behavior termed “asperity persistence,” with mean asperity pressures in heavily loaded plastic contacts established to be 1.5 times the bulk material hardness. For plasticity indices less than 1.5, the contacts operated in elastic–plastic regime where the mean asperity pressures were seen to increase linearly with the plasticity index; for plasticity indices greater than 1.5, the contacts operated in dominantly plastic regime and the mean asperity pressures leveled off at 1.5 times the bulk material hardness. It was shown that an elastic–perfectly plastic numerical contact model, which uses the herein established asperity plasticity limit of 1.5 times bulk hardness is able to provide relatively accurate predictions of real asperity pressures. The insights provided here can be used to improve the elastic–plastic contact models and to better understand the asperity pressures and deformations in rough surface contacts, which are crucial in controlling contact friction and wear.