The effects of hardness, toughness, microstructure and thermomechanical heating on the erosion of ceramic and ultrahard materials

Master Thesis


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University of Cape Town

Nine different ceramic and ultrahard materials have been eroded by four different erodents under standardised experimental conditions. The target materials range from the soft stabilised zirconias to the very hard polycrystalline diamond composites. The four erodent particles used were: soft, friable silica, sharp alumina, tough silicon carbide and very hard, synthetic industrial diamonds. The steady state erosion rates of the different target/erodent combinations were measured. The erosion for each combination was studied by examining the target surfaces at progressive stages of erosion and the erodent particles after impact. Special attention has been paid to the morphology of impact sites, the amount of material lost and the mechanisms of erosion. The ceramic materials eroded by an elastic/plastic process: irreversible deformation is followed by lateral fracture. The ultrahard materials erode by a complex process involving deformation and extrusion of the softer phases and fracture and loss of the harder crystallites. The relative hardness of the target and erodent is a determining factor in erosion. When the hardnesses are similar, the ease of initiation and propagation of lateral fracture determines the rate of material removal. When the erodent particles are much harder than the target material, initiation of fracture is inevitable and the ease of crack propagation determines the rate of material removal. This is controlled by the sharpness of the particles and the microtoughness of the target material. The microtoughness is a function of grain size, porosity and defect density. Melting features, seen in many of the eroded surfaces, are thought to arise from a combination of plastic deformation and frictional heating under high contact stresses. The amount of heat dissipated is determined by the erodent and target hardness and friction coefficients. The rate at which the heat diffuses away from the impact zone is determined by the thermal diffusivities, heat capacities and densities of the target and the erodent.