Fundamental Research in Structural Ceramics for Service Near 2000°C

Abstract

Structural ceramics for high‐temperature applications should embody the following properties: oxidation resistance, chemical stability, low volatility, resistance to creep deformation, resistance to creep cavitation at interfaces, sufficient toughness at ambient temperature, and thermalshock resistance. These criteria lead to five themes for fundamental research and design of high‐temperature structural ceramics: chemical and environmental stability, grain‐boundary sliding and cavitation, single‐crystal microstructure design, room‐temperature mechanical properties, and thermal shock. It is recommended that research that is confined to any one of these five areas takes into consideration the broader implications of research results. For example, microstructure designs that require weak interfaces for obtaining toughness at room temperature directly or indirectly conflict with creep and cavitation resistance needed for long‐term service at high temperatures. New research should be directed at mechanisms that can simultaneously achieve good mechanical properties over a wide range of temperatures. This paper addresses the following recommendations: (i) although non‐oxide systems can be viable for structural applications below 1500°C, oxidebased ceramics are necessary for service above 1500°C; (ii) microstructure designs based on acicular grain morphologies and/or single‐crystal fiber reinforcements have the potential for meeting the mechanical property requirements from room temperature up to very high temperatures; (iii) for fundamental studies of mechanical properties at high temperatures, simple uniaxial tension experiments should be used in tandem with four‐point bending and uniaxial compression experiments; (iv) the study of the reinforcement phase should center on very pure, highly stoichiometric materials in the case of non‐oxides, and on mixed and alloyed single crystals of cubic symmetry, or crystals having isotropic properties, and large unit cells in the case of oxides; (v) the study of interfaces in non‐oxides should focus on the chemistry of the intergranular glass phase, particularly the control of the oxygen content and the crystallization of this phase for improvement of high‐temperature properties; (vi) the study of interfaces in oxides is best directed at the relationship between interface structure, defect chemistry, and interfacial mechanical properties over a wide range of temperature; (vii) the understanding of the micromechanisms of thermal‐shock failure and the application of this understanding for designing graded interfaces that may be able to cope with thermal‐expansion stresses without leading to microfracture and cavitation is important in all classes of ceramic materials, and is of critical importance in the development of oxides for very‐high‐temperature applications; and (viii) research in processing science should emphasize the study of basic mechanisms that lead to in‐situ growth of acicular and fibrous microstructures.