Ceramics are covalent compounds that generally contain metal and nonmetal atoms. Titanium carbide and aluminum oxide are examples of ceramics. Ceramics typically have high melting points, high hardness, excellent chemical corrosion resistance, excellent erosion resistance, and good mechanical properties at elevated temperatures. Some ceramics are good thermal or electrical conductors, whereas others are good insulators or dielectrics.
The brittleness of ceramics leads to their typical use as coatings or reinforcement with fibers. When used as a coating, ceramics provide wear resistance and/or oxidation/corrosion protection. For example, cutting tools can be coated with titanium carbide, which, because of its hardness, results in excellent wear resistance and extended tool life. Graphite components can be coated with silicon carbide to provide oxidation resistance. Other coatings, such as Ultramet’s hafnium carbide/silicon carbide, are customized to provide maximal protection against chemical and oxidative attack at elevated temperatures.
Because ceramics typically have lower densities than refractory metals, interest in replacing heavy, metallic components with ceramics is great. To address the brittleness, ceramics are reinforced with high-strength fibers to provide fracture toughness and minimize brittle behavior. Fiber-reinforced ceramics are most frequently made from silicon carbide because of its low density, high strength, and excellent oxidation/corrosion resistance. For more demanding applications, fiber-reinforced zirconium carbide is used. Ceramics make excellent high temperature filters and catalyst supports because they demonstrate impressive properties at high temperature.