Effect of B2O3 and hBN Crystallinity on cBN Synthesis

Four kinds of BN powders—amorphous BN with B₂O₃, partially crystallized BN without B₂O₃, well-crystallized hBN with B₂O₃, and well-crystallized hBN without B₂O₃—were prepared to determine the effect of B₂O₃ on the crystallization of amorphous BN and the effect of BN crystallinity on the formation of cBN under high pressure (4–5 GPa) and at high temperature (1350–1450°C). The amorphous BN with B₂O₃ easily crystallized and transformed to cBN in the presence of A1N catalyst, while the partially crystallized BN without B₂O₃ did not. The well-crystallized hBN transformed very slowly to cBN without B₂O₃, in contrast to fast transformation with B₂O₃. It is thus found that the transformation from hBN to cBN in the presence of AIN catalyst is determined by the degree of BN crystallinity as well as the presence of B₂O₃. Cubic BN can be synthesized only from crystallized hBN under the experimental conditions used. The formation of cBN from amorphous BN is possible through its prior crystallization, which can occur in the presence of B₂O₃.