CARBIDE

any of a class of chemical compounds in which carbon is combined with a metallic or semimetallic element. Calcium carbide is important chiefly as a source of acetylene and other chemicals, whereas the carbides of silicon, tungsten, and several other elements are valued for their physical hardness, strength, and resistance to chemical attack even at very high temperatures. Iron carbide (cementite) is an important constituent of steel and cast iron. On the basis of their atomic structures, the carbides fall into three groups: ionic (saltlike), interstitial (metallic), and covalent (diamond-like). Metals of groups I, IIa, and IIIb of the periodic table commonly form ionic carbides, pure samples of which are transparent solids that are poor conductors of electricity; treatment with acidsor, in certain cases, even waterdecomposes them into hydrocarbons and metal hydroxides. Metallic carbides most often are formed from metals of groups IVb, Vb, VIb, VIIb, and VIII; these compounds are very hard and electrically conductive; several of them, especially tungsten carbide and the carbides of titanium, tantalum, and niobium, are important components of the composite materials called cermets. Boron carbide and silicon carbide (Carborundum) resemble diamond in structure and approach it in hardness; they are highly resistant to chemical attack and are used as abrasives. Carbides usually are prepared from carbon and either a metal or a metal oxide at temperatures of 1,0002,800 C (1,8005,100 F); in certain cases, a hydrocarbon or hydrogen is used to reduce a metal oxide or chloride.