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Ceramics have long been known for their refractoriness, or ability to bear loads at elevated temperatures. However, until the 1960s, the predominant refractory ceramics were oxide-based materials such as silica, zirconia, alumina, mullite, magnesia, and their combinations, including silicates. These ceramics were, and still are, used for firebrick furnace linings, crucibles and liquid metal carrier liners, regenerators, and recuperators. However, these materials all possess some characteristic which precludes their use for very high stress, high temperature applications. Typically, the silicates form viscous liquids which allow creep, while zirconia and alumina suffer from poor thermal shock resistance, and magnesia possesses a large thermal expansion coefficient. Consequently, for heat engine applications which involve high temperatures, high stresses, sudden temperature changes (e.g., startup), and may. involve the maintenance of tight operating tolerances, a new family of materials is required. The requisite properties for heat engine applications may be found in certain non-oxide materials, namely silicon nitride and silicon carbide. They possess high strength even at high temperatures, low thermal expansion coefficient, and excellent thermal shock resistance. These materials are not thermodynamically stable in air at elevated temperatures and will eventually react to form oxides. Nonetheless, they possess excellent oxidation resistance by virtue of protective silica-based glass oxidation layers. Many non-oxide ceramics exhibit further characteristics which make them useful to industry in other applications. For instance, silicon nitride, silicon carbide, boron carbide, titanium diboride, etc. have very high hardness values, making these materials useful as abrasives and cutting and milling tools. Some also exhibit low coefficients of friction, which, in concert with their hardness, make them suitable for wear-resistant applications such as bearings and seals. Some properties of these materials are listed in Table I. For several of the properties in Table I, a range of values is shown because often several types of each material exist. They may differ either in composition or consolidation technique, or both (e.g., reaction-bonded, sintered, hotpressed, or HIPed varieties), and they will

exhibit different properties. Indeed, the ability to fabricate ceramics with properties tailored to suit the applications is one of the advantages of these materials. Sintered silicon nitride, for example, has been prepared at GTE Laboratories with thermal conductivity values ranging from about 5 W/m-K1 to about 70 W/m-K2 simply by varying the amount and composition of the grain boundary phase. The low conductivity type is useful in low-heat-rejection diesel engine applications in which minimum heat loss through the cylinder liner is desired. The high conductivity type is applicable to electronic substrate manufacture in which heat is to be conducted away from the chip. This article will concentrate, however, on the silicon ni