PLASTIC RATCHETTING AS A MECHANISM OF METALLIC WEAR

Many researchers have observed metallic wear debris in the form of ver y thin platelets. In particular Akagaki & Kato (1987) revealed how suc h debris can be formed by progressive plastic extrusion from the edges of the irregularities on the softer of two sliding surfaces. This beh aviour has been reproduced in experiments reported here, in which a si ngle soft asperity is modelled by a blunt copper wedge in sliding cont act with a flat hard steel surface under conditions of boundary lubric ation. This progressive extrusion with continuous sliding is attribute d to 'plastic ratchetting' and two ways in which the process can be dr iven have been identified: (i) pummelling of the soft surface by the a sperities of the hard surface and (ii) cyclic stressing of the soft su rface by the stress concentrations which occur at the edges of a hard slider. The kinematical shakedown theorem from the theory of plasticit y is used to determine the asperity contact pressure necessary to driv e these ratchetting processes. A significant feature of this mechanism is that it can occur under frictionless conditions.