Non-metallic crystals undergoing cumulative work-hardening and wear due to softer lubricated metal sliding surfaces

Abstract

A purely mechanical mechanism of the room-temperature wear of hard non-metallic crystals, when traversed by considerably softer lubricated metal cones, has been observed. The contact pressure transmitted to the crystal is directly related to the flow stress of the slider and is therefore dependent on the metal used. It is shown that when this pressure is above a certain threshold level, i. e. just enough to exceed the critical resolved shear stress within the crystal, then dislocations move and multiply in the hard solid. The full dimensional extent of this movement, the dislocated volume, is essentially developed on the first load cycle and is determined in a given crystal primarily by the magnitude of the applied load. Repeated traversals increase the density of defects in this dislocated volume and produce significant work-hardening. The level of work-hardening saturates within comparatively few traversals (typically about twenty) and the degree of work-hardening is limited by the ultimate hardness of the slider ( H u ), or is curtailed by the onset of fracture. A linear relation between H u , varied by using sliders of different metals, and the subsequent hardness of the work-hardened surface ( H t ) is observed for the (001) surfaces of crystals of magnesium oxide, nickel oxide and titanium carbide. The flow stress of the softest metal to induce work-hardening may be used to estimate the critical resolved shear stress of the hard crystal, as shown here for titanium carbide. Further traversals, beyond those necessary to develop a saturation level of work-hardening, generally lead to the formation of surface cracks and catastrophic fragmentation. The number of traversals ( N c ) required to cause this surface fragmentation and accelerated wear is inversely related to the ultimate hardness of the slider, is independent of the normal load, but is dependent on the crystallographic plane and direction of sliding. A plot of H u : N c resembles a conventional fatigue curve and indicates that the surface will not fail when deformed by sliders below a certain limiting value of H u . The cracks that initiate final failure are shown to be generated at the surface, while subsurface cracks, lying on crystallographic planes parallel to the surface, may be formed during initial traversals well before N c . The wear resistance of nickel oxide is superior to that of MgO, although NiO is softer than MgO. Finally, the mechanism of work-hardening is discussed in terms of the interaction of dislocations on obliquely intersecting slip planes, as in conventional stage II work-hardening. The localized stress concentration around the dislocation debris produced by these interactions, in the near-surface layers, is considered to be responsible for the initiation of the cracks that precipitate fragmentation.