Facilitated synthesis of cubic boron nitride by a mechanochemical effect

Abstract

A superhard cubic boron nitride (c-BN), which is now an industrially important material used in preference to diamond especially for cutting and lapping steel products, has so far been synthesized by heating a starting powder of hexagonal boron nitride (h-BN) under a high pressure at a high temperature. In the present study we have mechanically milled h-BN starting material to introduce lattice defects prior to its conversion to c-BN. The synthesis of c-BN has then been facilitated prominently owing to a mechanochemical effect; under 7.7 GPa the formation of c-BN starts at 1250°C with the pre-milled h-BN powders, and at 1450°C with non-milled powders. High resolution transmission electron microscopy has clarified that a variety of defects form in h-BN during milling. The h-BN changes to w-BN by compression in the c direction, and a slight tilt at the interface between (0002)_h and (0002)_w partially compensates for the lattice misfit between the basal planes of both phases. Characteristic stacking faults appear in h-BN on heating under a high pressure. They are succeeded by the formation of wurtzite-type boron nitride during the compression and form nucleation sites for c-BN.