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-degrees-C). The amorphous BN with B2O3 easily crystallized and transformed to cBN in the presence of AlN 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 AlN 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.