What Is Cubic Boron Nitride?

Cubic boron nitride (CBN) is a synthetic chemical compound made up of boron and nitrogen atoms in a one-to-one ratio. Unlike other types of boron nitride, it exists as a cubic crystal lattice, with its atoms arranged in a symmetrical, cube-shaped structure like the crystalline structure of diamond. Its material properties include extreme hardness, stability under heat, and superior chemical resistance. Cubic boron nitride is the second-hardest known material after diamond, making it useful in a wide variety of industrial applications, where it is often used as an abrasive or cutting tool.

All boron nitride compounds are produced synthetically, since they are not found in nature. The process starts with boric acid, a compound of boron, oxygen, and hydrogen, or with boron trioxide, which is made up of boron and oxygen. Either of these compounds can be reacted with urea or ammonia, which contain nitrogen, to produce a powder of boron nitride. The powder is crystallized through intense heat to form hexagonal boron nitride, which has a hexagonal crystal structure analogous to that of graphite, a precursor of synthetic diamond. Hexagonal boron nitride is then heat-treated under high pressure to form cubic boron nitride.

CBN has several advantages as an industrial material. It is almost as hard as diamond, making it useful for cutting and abrasion of metals and other substances. The heat resistance of CBN is also quite high. Diamond begins to decompose to carbon dioxide in air at around 1,472°F (about 800°C), whereas cubic boron nitride is stable in air up to about 2,552°F (about 1,400°C). In industrial grinding applications and other processes where intense heat is generated, CBN is preferred over diamond.

Another advantage of CBN as an alternative to diamond is its chemical stability in the presence of nickel, iron, and related substances. Diamond is soluble in these metals, meaning that it reacts with them chemically and dissolves. This makes it useless as a tool for cutting iron or steel. CBN, on the other hand, does not react and can therefore be used to cut these materials.

The variety of CBN used in industrial cutting tools is known as polycrystalline cubic boron nitride (PCBN). PCBN is manufactured by sintering cubic boron nitride particles with ceramics. In sintering, the boron nitride is placed in a furnace under high heat and pressure that binds it with the ceramic, fusing into the extremely hard substance used for grinding and cutting.