EFFECT OF SMALL AMOUNTS OF HYDROCHLORIC-ACID ON THE SYNTHESIS OF TRANSLUCENT SINTERED CUBIC BN

The transformation behavior from hexagonal boron nitride (h-BN) to cub ic boron nitride (c-BN) was investigated under high pressures and high temperatures of 5.5-6.5 GPa and 1600-1900-degrees-C for 20 min in the presence of hydrochloric acid. To decrease the oxygen impurity, the s tarting h-BN was treated in advance at 2100-degrees-C for 2 h in a nit rogen atmosphere. A translucent c-BN sintered body was synthesized by reaction sintering from the system h-BN and 0.3 wt.% 11.6 N HCI soluti on under 6.5 GPa and 1900-degrees-C for 20 min. It is clear from chara cterization of the sintered body that the translucent c-BN with a grai n size ranging from submicrometer to 5 mum is colorless, of high purit y, with good light transmissivity, and of high hardness above 40 GPa o n the Knoop hardness scale.