  In this paper we have theoretically found the activation energy
($420.38kJ/mol$) for the transformation from hBN to cBN in the microscopic
viewpoint. We have introduced an analytical formula representing the dependence
of nucleus formation time on the activation energy, synthesis pressure and
temperature. We have theoretically determined the boundary line of cBN nucleus
formation region in the $P-T$ phase diagram on the basis of the diffusion
mechanism of cBN nucleus formation. We have found that the cBN crystal nucleus
formation time is less than $300ms$ by comparing of the theory and experiment.

Materials Science