The roles of brittle microfracture and plastic flow in the wear of MgO single crystals

Abstract

The roles of brittle microfracture and plastic flow in the wear of MgO single crystals were investigated using a repeated rubbing apparatus. Wear after a small number of rubbing cycles occurs by brittle microfracture under the severe contact stress associated with the small contact area of the slider, although a plastically flowed layer is formed locally in a small region of the wear surface. Plastic flow occupies a larger region of the wear surface as wear increases, owing to the decrease in contact pressure caused by an increase in the contact area of the slider after many repeated slidings. Wear during this period, in which there is a very low wear rate, is caused by removal of the plastically flowed layer from the surface and corresponds to wear under a steady state process. When the tip of the slider approaches a subsurface parallel crack, a rectangular particle is formed; this particle is surrounded by the subsurface parallel crack, by two normal cracks K 1, which coincide with chevron cracking, and by two normal cracks K 2, which reach the subsurface parallel crack by a fatigue process after many repeated rubbing cycles. The rectangular particle is removed from the wear track and the subsurface parallel crack plane is locally exposed. The plastically flowed layer is readily produced after the material freed from the matrix is removed by frictional action although a new subsurface parallel crack below the previous subsurface parallel crack can be formed by further rubbing.