Cumulative deformation of magnesium oxide crystals by softer sliders

Abstract

MECHANISMS postulated for the wear of hard crystalline surfaces by softer solids include thermal degradation¹, diffusion and solution²; and self abrasion of the harder solid when its wear debris is picked up by the softer surface³. A mechanism based purely on mechanical aspects has not yet been established but there has been the suggestion of an ‘incubation period’ during which the structure of the hard solid has been modified before the onset of visible wear⁴. In earlier publications^5,6, we have established that a dislocated zone was formed in some hard crystals by a softer slider, whenever the pressure between the contacting surfaces was greater than a certain threshold value which was thought to be related to the critical resolved shear stress. For a given load, and in these conditions, the depth of this zone was independent of the slider hardness—regardless of whether the point itself was blunted or visible macroscopic deformation and penetration of the harder crystal was obtained. Then, the depth of the dislocated zone was determined only by the magnitude of the applied load. There were two principal objectives to this particular study. First, to measure the effect of slider hardness on the number of traversals necessary to initiate visible fragmentation and wear in the harder crystal. Second, to use dislocation etching and microhardness techniques to follow the deformation of the harder crystal during the microdeformation preceding fracture—the incubation period.