Transformation-toughened zirconia ceramics

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Abstract
An assessment has been made of the current interest and potential future of zirconia ceramics for structural applications. Interest in the mechanical properties of ceramics based on, or containing, ZrO2 centres on the manipulation of the tetragonal to monoclinic martensitic phase transformation. It will be demonstrated that the maintenance of a metastable tetragonal phase is one of the most important factors in the optimum design of a strong and tough ceramic. There are several microstructural approaches that can be taken to achieve toughening based on either precipitate-hardened or particulate-dispersed systems, and a number of different toughening mechanisms can operate, either singly or together, to produce different combinations of properties. The potential engineering future of three ZrO2 ceramics: partially stabilized zirconia, tetragonal zirconia polycrystals, and zirconia-toughened alumina, is examined in this context. The important relationship between microstructure and mechanical properties is explored for these materials and current applications are cited. The significance of grain boundaries and grain-boundary phases in affecting the way in which the microstructures evolve during sintering and/or aging, and hence in affecting the final properties of the ceramic, are highlighted. The properties of ZrO2 ceramics in current use are then compared with other common engineering materials, and the problems of property reliability are stated. Finally, an opinion is expressed regarding the deficiency of present scientific effort in ceramics, and some predictions are made concerning the future of ZrO2 ceramics. MST/201

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