WEAR THROUGH THE PLASTIC INTERACTION OF CYLINDRICAL ASPERITIES IN SLIDING

Wear under mild conditions is known to occur by the progressive extrusion of thin silvers and their subsequent break-off. In an experiment on a large scale model asperity, Kapoor and Johnson [Proc. R. Soc. Lond. 445, 367 (1994)] have found that the process of progressive extrusion is driven by pummelling of the soft surface by the roughness of the harder surface. The case of pummelling by a hard surface with isotropic roughness, i.e. in which the asperity crests may be considered hemispherical, was analysed using the classical kinematical shakedown theorem and the ratchetting thresholds, i.e. the asperity contact pressure above which progressive extrusion of silvers takes place, were obtained. However, most bearing surfaces are machined by using turning and grinding, which produces asperities which are elongated in the direction of machining rather than hemispherical as assumed by Kapoor and Johnson. In the present paper, we assume the hard surface to have such long asperities and analyse the process of pummelling. A striking feature of the results is that the shakedown limit and its dependence on the coefficient of friction is not very sensitive to the detailed morphology of the contacting surfaces.