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

Abstract

Abstract Phase transformations of cubic boron nitride to graphite-like forms (cBN→gBN) have been studied within the 1670–2970 K temperature range and at high pressures, which suppresses the dissociation decomposition of the compound, by using a recessed-type high pressure apparatus with a toroid-type lock. To clarify the degree of structure imperfection effect on the type and mechanism of transformation, various types of bulk cBN initial materials were used. A partial or complete cBN→gBN transformation has been observed in the studied temperature range. It has been found that in a pure recrystallized material with a low level of the structural imperfection, a two-stage transformation occurs to form an intermediate metastable rhombohedral phase (rBN). Transformation into the hexagonal modification of graphite-like boron nitride (hBN) is realized only in material with a highly imperfect structure. From the analysis of the crystallographic correspondence between cBN, rBN and hBN lattices, and the differences in structure between the initial and forming materials, the possible structural mechanisms of the cBN→gBN transformation were proposed.