Current Research Issues in Silicon Nitride Structural Ceramics

Abstract

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.