CERAMICS AND CERAMIC COMPOSITES AS HIGH-TEMPERATURE STRUCTURAL-MATERIALS - CHALLENGES AND OPPORTUNITIES

Perspectives are presented on the development of ceramics and ceramic matrix composites (CMCS) for high-temperature structural components. The emphasis is on design requirements and their role in directing research toward actual products. An important theme concerns the relative roles of fracture toughness and inelastic strain (ductility) in the application of materials in primary structures. Ceramics with high toughness have been developed, but macroscopic inelasticity has not been achieved. Robust design procedures have yet to be developed for such materials. This deficiency, as well as relatively high manufacturing and qualification costs, has retarded their commercial exploitation. Strategies for addressing this problem are considered. CMCs are more 'design friendly' because they exhibit appreciable inelastic strain, in shear and/or in tension. Such strain capacity is an efficient means of redistributing stress and eliminating stress concentrations. The design process thus has commonality with that used for metallic components. Also, processing approaches that provide acceptable manufacturing costs have been devised. The sources of the inelastic strain are examined and models that lead to a constitutive law are described. Some examples are given of its FEM implementation for design calculations. A limitation on the extensive exploitation of CMCs in high temperature systems has been the existence of degradation mechanisms. These include a 'pest' phenomenon, manifest as oxidation embrittlement in non-oxide CMCs, as well as excessive creep in oxide-oxide CMCs. These degradation mechanisms are discussed, and pathways to affordable high-temperature CMCs are analysed.