Crystallization behavior of cubic boron nitride from an amorphous BN precursor via high-pressure, high-temperature treatment with controlled water addition

Crystallization behavior of cubic boron nitride (c-BN) from amorphous boron nitride (a-BN) by high-pressure, high-temperature (HP-HT) treatment with the intentional addition of a controlled amount of water was studied. The a-BN precursor was prepared by pyrolysis of a boric acid-urea complex with urea/boric acid = 2 at 1000 °C for 3 h under an ammonia atmosphere. Hexagonal BN (h-BN) were initially crystallized from a-BN containing 2 mass% of water after HP-HT treatment at 7.7 GPa and 1200 °C, and the formation of c-BN was observed after 5 min. The crystallization of c-BN from a-BN containing 8 mass% of water after HP-HT treatment at 7.7 GPa and 1200 °C was observed even after 1 min, indicating that the crystallization of c-BN was promoted by increasing the amount of water added. In addition, since neither h-BN nor c-BN was crystallized from a-BN without intentional water addition after the same HP-HT treatment, the addition of water promoted the crystallization of both h-BN and c-BN from a-BN. Since no transformation from h-BN to c-BN was observed during HP-HT treatment at 7.7 GPa and 1200 °C, it was concluded that c-BN was directly crystallized from a-BN. After HP-HT treatment at 7.7 GPa and 1650 °C for 1 min, both h-BN and c-BN were crystallized from a-BN containing 8 mass% of water. These results clearly indicate that the addition of water has a positive effect on crystallization of c-BN from a-BN.