Features of a cBN-to-graphite-like BN phase transformation under pressure

Phase transformations of cubic boron nitride to graphite-like forms (cBN -- > gBN) have been studied within the 1670-2970 K temperature range and at hi gh pressures, which suppresses the dissociation decomposition of the compou nd, by using a recessed-type high pressure apparatus with a toroid-type loc k. To clarify the degree of structure imperfection effect on the type and m echanism of transformation, various types of bulk cBN initial materials wer e used. A partial or complete cBN --> gBN transformation has been observed in the studied temperature range. It has been found that in a pure recrysta llized material with a low level of the structural imperfection, a two-stag e transformation occurs to form an intermediate metastable rhombohedral pha se (rBN). Transformation into the hexagonal modification of graphite-like b oron nitride (hBN) is realized only in material with a highly imperfect str ucture. From the analysis of the crystallographic correspondence between cB N, rBN and hBN lattices, and the differences in structure between the initi al and forming materials, the possible structural mechanisms of the cBN --> gBN transformation were proposed. (C) 2000 Elsevier Science S.A. All right s reserved.