Preparation of hexagonal BN

The nitridation or ammonolysis of boron trioxide produces hexagonal boron nitride. H-BN parts can be made by hot-pressing with subsequent machining; the machining cost is low due to the mechanical hardness similar to graphite. The parts are made from boron nitride powders, using boron oxide as a sintering agent. Thin boron nitride films can be obtained by chemical vapor deposition from boron trichloride and nitrogen precursors. Industrial production is based on two reactions: melted boric acid with ammonia and boric acid or alkaline borates with urea, guanidine, melamine, or other suitable organic nitrogen compounds in a nitrogen atmosphere—combustion of boron powder in nitrogen plasma at 5500 °C yields ultrafine boron nitride for lubricants and toners. Cubic boron nitride is extremely hard, although less so than diamond and some related materials. Also, like diamonds, cubic boron nitride is an electrical insulator but an excellent conductor of heat. This diamond-like polymorph, known as cubic boron nitride, c-BN, β-BN, or z-BN (after zinc blende crystalline structure), is widely used as an abrasive for industrial tools.[citation needed] Its usefulness arises from its insolubility in iron, nickel, and related alloys at high temperatures, whereas diamond is soluble in these metals to give carbides. Polycrystalline c-BN abrasives are used for machining steel, whereas diamond abrasives are preferred for aluminum alloys, ceramics, and stone. Like diamonds, cubic BN has good thermal conductivity caused by phonons.

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