PLASTIC RATCHETTING AS A MECHANISM OF METALLIC WEAR

Many researchers have observed metallic wear debris in the form of very thin platelets. In particular Akagaki & Kato (1987) revealed how such debris can be formed by progressive plastic extrusion from the edges of the irregularities on the softer of two sliding surfaces. This behaviour has been reproduced in experiments reported here, in which a single soft asperity is modelled by a blunt copper wedge in sliding contact with a flat hard steel surface under conditions of boundary lubrication. This progressive extrusion with continuous sliding is attributed to 'plastic ratchetting' and two ways in which the process can be driven have been identified: (i) pummelling of the soft surface by the asperities of the hard surface and (ii) cyclic stressing of the soft surface by the stress concentrations which occur at the edges of a hard slider. The kinematical shakedown theorem from the theory of plasticity is used to determine the asperity contact pressure necessary to drive these ratchetting processes. A significant feature of this mechanism is that it can occur under frictionless conditions.