BORON NITRIDE

(chemical formula BN), synthetically produced crystalline compound of boron and nitrogen, an industrial ceramic material of limited but important application, principally in electrical insulators and cutting tools. It is made in two crystallographic forms, hexagonal boron nitride (H-BN) and cubic boron nitride (C-BN). Comparison of the hexagonal structures of graphite (left)and boron nitride (right). H-BN is prepared by several methods, including the heating of boric oxide (B2O3) with ammonia (NH3). It is a platy powder consisting, at the molecular level, of sheets of hexagonal rings that slide easily past one another. This structure, similar to that of the carbon mineral graphite (see the Figure), makes H-BN a soft, lubricious material; unlike graphite, though, H-BN is noted for its low electric conductivity and high thermal conductivity. H-BN is frequently molded and then hot-pressed into shapes such as electrical insulators and melting crucibles. It also can be applied with a liquid binder as a temperature-resistant coating for metallurgical, ceramic, or polymer processing machinery. C-BN is most often made in the form of small crystals by subjecting H-BN to extremely high pressure (six to nine gigapascals) and temperature (1,500 to 2,000 C, or 2,730 to 3,630 F). It is second only to diamond in hardness (approaching the maximum of 10 on the Mohs hardness scale) and, like synthetic diamond, is often bonded onto metallic or metallic-ceramic cutting tools for the machining of hard steels. Owing to its high oxidation temperature (above 1,900 C, or 3,450 F), it has a much higher working temperature than diamond (which oxidizes above 800 C, or 1,475 F).