The parameters of equilibrium between oxygen solid solutions in cubic and graphite-like hexagonal boron nitrides

A decrease in the onset temperature of the cBN-to-hBN solid-phase transform ation with increase in the oxygen content of cBN samples has been found exp erimentally. This dependence might be attributable to the existence of oxyg en solid solutions in cBN and hBN. From the experimental data and taking in to account the kinetics of solid-phase transformations, the equilibrium ons et temperature of the cBN-to-hBN transition has been derived for samples wi th different oxygen contents. In the framework of phenomenological thermody namics, the model parameters have been determined and the lines of equilibr ium between oxygen solid solutions in cubic and graphite-like hexagonal bor on nitride have been calculated. It has been shown that a decrease in the o xygen content of the boron nitride-solvent system usually used for cBN synt hesis should induce an increase in the thermodynamic stimulus to the hBN-to -cBN transformation at high pressures. (C) 1999 Elsevier Science S.A. All r ights reserved.