Production of nano- and micropowders of cubic BN from mechanically activated graphite-like BN under high pressures and temperatures using aluminium as a phase convertion initiator

Abstract

An effect of mechanical activation of graphite-like (hexagonal) boron nitride (hBN) on catalytic synthesis of nano- and micropowders of cubic boron nitride (cBN) under conditions of high pressures and temperatures is shown in the article. It has been suggested that nuclei of dense phases of boron nitride (cubic and wurtzite) are formed in the hBN structure during mechanical activation they serve as crystallization centers stimulating the formation of cBN crystals under subsequent thermobaric treatment. Additional chemical-thermal modification of hBN with aluminum as a catalyst (initiator) for the phase transformation of hBN into cBN leads to a significant increase in the content of cBN along with an increase in the synthesis pressure. Thus, the introduction of Al additives in the amount of 10 wt.% leads to an increase in the content of the cBN phase from 10–15 % under synthesis pressure of 2.5 GPa up to 90 % under pressure of 5.5 GPa. In this case, the grain size of cBN, estimated by scanning and atomic force microscopy, is mainly 200–400 nm and 40–120 nm for the samples synthesized under pressure of 2.5 and 5.5 GPa, respectively. The increase in the content of the initiator from 10 to 40 wt.% with an increase in isothermal holding time from 15 to 60 s in the studied range of pressures and temperatures leads to the formation of intergrowths of cBN grains in nano- and submicron sizes and individual cBN single crystals of a cubic habit with a grain size of 1–4 µm, as well as polycrystalline cBN particles from 10 to 50 µm. In this case, the maximum size distribution of cBN micropowders in the range up to 50 µm falls on particles up to 5 µm (~ 70 %). In the submicron grain size range the maximum yield (~ 50 %) is marked for cBN particles 0.5–0.7 µm in size. The obtained powders can be used to make abrasive and cutting tools.