Phase equilibria pertinent to the growth of cubic boron nitride

Abstract

The practical synthesis of cubic boron nitride (cubic BN) requires the simultaneous application of hig temperatures and pressures. Since the discovery of cubic BN more detailed understanding of the chemistry of the process has been acquired from equilibria or “approach-to-equilibria” studies in the Li-B-N system. This paper describes some of these results and considers their application to the growth of borazon.The principal compound that coexists with cubic BN at high pressures and temperatures is a high pressure modification of Li3BN2 (called Li3BN2(W)). It has been shown that cubic BN does not form when this compound melts or decomposes. When BN is added to Li3BN2 in excess of the composition 53 mole% BN: 47 mole% Li3N, then cubic BN precipitates from solution. At 55 kb, the system Li3BN2-BN has a eutectie at 6% BN and 1610°c and the upper limit of the cubic BN plus liquid region is at 1750°C. The solubility curve is a very steep function of temperatures and consequently the system is not too favourable for crystal growth by slow cooling.These new results can be compared with previous results by plotting a series of P-T curves defining the stability region for cubic BN under different conditions. Good agreement is found between the solidus determined in the Li3BN2-BN study and the lower limit of the catalyst growth area. In effect, this lower limit is directly related to the initial formation of liquid in the Li3BN2-BN portion of the system. The lower temperature limit of growth of cubic BN reported from Li-BN compositions is below that of the present study and the catalyst growth area previously defined. This result would have some fundamental basis in fact if a lower temperature reaction involving cubic BN occurred in the ternary system Li-B-N.