EFFECT OF B2O3 AND HBN CRYSTALLINITY ON CBN SYNTHESIS

Four kinds of BN powders-amorphous BN with B2O3, partially crystallize d BN without B2O3, well-crystallized hBN with B2O3, and well-crystalli zed 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 tem perature (1350-1450-degrees-C). The amorphous BN with B2O3 easily crys tallized and transformed to cBN in the presence of AlN catalyst, while the partially crystallized BN without B2O3 did not. The well-crystall ized hBN transformed very slowly to cBN without B2O3, in contrast to f ast 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 c an be synthesized only from crystallized hBN under the experimental co nditions used. The formation of cBN from amorphous BN is possible thro ugh its prior crystallization, which can occur in the presence of B2O3 .