NEW SCOPE OF HIGH-PRESSURE HIGH-TEMPERATURE SYNTHESIS OF CUBIC BORON-NITRIDE

The stability region of cubic boron nitride (c-BN) has been determined by high pressure-high temperature experiments. The P-T regions of c-BN formation by catalytic conversion and by direct transformation were reviewed and indicated that a temperature-independent threshold does exist between 4.6 and 5.0 GPa for most catalysts of alkali and alkaline earth boron nitrides. The corresponding solid state transformation from hexagonal BN (h-BN) to wurtzite BN showed a threshold at about 6.5 GPa. The threshold pressure of the formation region of c-BN is discussed based on the simple energy diagram for activation of sp(2) bonding of h-BN.