Introduction

The boron nitride (BN) and carbon structural systems are similar in many aspects and form analogue allotropes. Boron nitride exists mainly in two crystalline structures hexagonal BN (hBN) and cubic BN (cBN). Hexagonal BN is trigonally sp² bonded structures which corresponds to graphite. Cubic BN is tetrahedrally sp³ bonded structures which is relevant to cubic diamond. Like carbon, BN also forms fullerenes and nanotubes. Such analogy in carbon and BN systems may also invoke the expectation of similar properties. The strong covalent bonds of cubic boron nitride with high ionicity results in structure with very close atomic packing; which gives rise to outstanding mechanical and thermal properties.

Properties of cBN

Cubic BN has the second highest atomic density (1.68 × 10²³ cm^-3) and the second highest hardness (70 GPa) and the second highest thermal conductivity (13 W/cmK) just next to diamond. Because of the strong covalent bonds and induced ionicity, cBN even surpasses the most extreme material, diamond, in chemical and thermal stability. Cubic BN resists oxidation at ambient environment up to 1200 ^oC which is the value as twice higher as the oxidation temperature of diamond. The transformation temperature of sp³ to sp² BN phase is 1550 ^oC whereas diamond transform to graphite at 1400 ^oC.

The hardness and thermal conductivity of different materials

Unlike diamond, cBN is chemically inert when it contacts with molten ferrous materials. These extreme properties make cBN the best materials for machining ferrous materials such as tool steels. In addition, cBN has the widest band gap (6.2 ± 0.2 eV) of all III-V compound materials and can be doped for both p- and n-type conductivity. The electronic properties and the thermal stability hint the possibility of utilizing cBN in high speed, high power and high frequency electronic devices.

Physical properties of diamond and cBN