Synthesis of cubic boron nitride

Hexagonal boron nitride (hBN) is transformed to cubic boron nitride (cBN) on a tetrahedral anvil apparatus for high pressure and temperature using Mg and AlN as catalysts. The mixtures of hBN:Mg = 3:1 and hBN:AlN = 10:1 were employed as starting compositions to obtain cBN. The specimens thus obtained were examined by X-ray diffraction technique and scanning electron microscopy. The pressure-temperature regions for the formation of cBN were determined. In the presence of Mg catalyst, the minimum condition to form cBN was 60Kb and 1,3000C. In the presence of AlN catalyst, the minimum pressure and temperature to form cBN were 55Kb at 1,6000C and 1,1000C at 65Kb, respectively. The formation process of cBN in each case were assumed to be as follows; in the case of Mg catalyst, melted Mg was diffused into hBN to form Mg-BN compound which was oxidized and then decomposed to MgO and cBN. In the case of AlN catalyst, AlN-BN solid solution was formed and then decomposed to AlN and cBN. The grains of cBN having the crystal habit of diamond tended to develop in the experiments with Mg catalyst, while in the experiments with AlN catalyst, the typical grains of cBN having the crystal habit of its own were developed. (auth.)