Boron nitride is a synthetic material, which although discovered in the early 19th century was not developed as a commercial material until the latter half of the 20th century. Boron and nitrogen are neighbours of carbon in the periodic table - in combination boron and nitrogen have the same number of outer shell electrons - the atomic radii of boron and nitrogen are similar to that of carbon. It is not surprising therefore that boron nitride and carbon exhibit similarity in their crystal structure.

In the same way that carbon exists as graphite and diamond, boron nitride can be synthesised in hexagonal and cubic forms.



The synthesis of hexagonal boron nitride powder is achieved by nitridation or ammonalysis of boric oxide at elevated temperature. Cubic boron nitride is formed by high pressure, high temperature treatment of hexagonal BN.



Hexagonal boron nitride (h-BN) is the equivalent in structure of graphite (see figure 1). Like graphite its plate like microstructure and layered lattice structure give it good lubricating properties. h-BN is resistant to sintering and is usually formed by hot pressing.



Figure 1. h-BN powder (photo courtesy of Ceram Research Ltd)



Cubic boron nitride (C-BN) has the same structure as diamond and its properties mirror those of diamond. Indeed C-BN is the second hardest material next to diamond. C-BN was first synthesised in 1957, but it is only in the last 15 years that commercial production of C-BN has developed.
Key Properties
Hexagonal Boron Nitride (h-BN)

· h-BN has excellent lubricating properties

· In the hot pressed state, h-BN is readily machinable using conventional metal cutting techniques, hence complex shaped components can be fashioned from hot pressed billet.

· Providing oxidation of the surface can be prevented, h-BN is not wetted by most molten metals, glasses and salts and hence has a high resistance to chemical attack.

· High dielectric breakdown strength

· High volume resistivity

· Good chemical inertness