A comparison of low-pressure CVD synthesis of diamond and c-BN

The two superhard materials diamond and cubic boron nitride, are quite similar in many ways, such as for structure and other properties. Their industrial use is widespread today. A difference of major importance for industrial applications is that c-BN - in contrast to diamond - is not dissolved by iron and steel if used as a high duty tool at elevated temperatures. This difference could make c-BN a much more widely usable coating if appropriate low-pressure layer coating technologies could be developed. New findings give some hope in view of achieving this goal. Since the early successful high-pressure/high-temperature (HPHT) synthesis of diamond in 1955 and cubic boron nitride (c/BN) in 1957 the pressure/temperature phase diagrams were supposed to be quite similar. Recent results - based on calorimetric measurements of combustion enthalpy of c-BN with fluorine revealed however that in contrast to the diamond/graphite system c-BN - and not h-BN - is the stable phase under standard conditions. Considering these new facts a comparison between the possibilities of low pressure synthesis of diamond and boron nitride is given. So far the metastable diamond deposition from the gas phase seems however more easy than the formation of the stable c-BN. The main reasons seem to be: differences in the atomic attachment kinetics during crystal growth, caused by the anisotropy of the c-BN crystals, i.e. formation of completely N- or B- terminated facets; as well as differences of the bonding energies and of the bond lengths of B-N, B-B, and N-N. Thermodynamic equilibrium calculations for c-BN and h-BN in different gas atmospheres revealed that a selective reaction reducing the formation of the h-BN phase can be expected by addition of excess BCl3 during a CVD reaction. Preliminary experiments indeed indicated formation of some c-BN together with h-BN, as predicted by the thermodynamic data. However much further work needs to be done to reach for c-BN a state of the art similar to the today's low-pressure diamond coating technology.