Ceramic materials in general offer inany unique properties to the materials engineer, including low density, extreme hardness and wear resistance, and excellent resistance to corrosion and chemical attack. Often these properties are retained to much higher temperatures than in other materials. However, traditional engineering ceramics such as AI2O3MgO and mullite, are more brittle than other engineering materials. This is of particular concern in applications in which they are subject to shock loading of either mechanical or thermal origin. The fracture behaviour of materials is commonly discussed in terms of the fracture toughness (I), and several mechanisms for improving it have been discovered. Pioneering work by Garvie, Hannink and Pascoe (2) identified ‘transformation toughening’ as a means to improve fracture toughness of ceramics, and their work has led to an entirely new class of engineering ceramic, based on zirconia, and termed ‘ceramic steel’.