Work of fracture and fracture surface energy of magnesia-spinel composites

This study used a model system of fully dense high purity magnesia, incorporating fine grain spinel prepared by hot-pressing. Work of fracture ((gamma)WOF) and fracture surface energy ((gamma)i) were determined to be a function of spinel particle size and volume fraction. Predictions made from the thermal shock parameter, Rʹʹʹʹ, were tested by measurements made on (gamma) WOF/(gamma)i ratios of magnesia and magnesia-spinel composites. The results obtained from (gamma)WOF/(gamma)i ratios matched the predictions. On the basis of (gamma)WOF/(gamma)i ratios, the optimum spinel content and particle size were determined. Thermal shock data confirmed that higher values of the (gamma)WOF/(gamma)i ratios were considered to be a good indicator for thermal shock resistance. Higher values of (gamma)WOF were associated with the occurrence of more intergranular fracture with increasing spinel additions. Crack propagation was found to be much more difficult in magnesia-spinel composites, than the initiation of cracks.