SEGREGATIVE CRYSTALLIZATION OF SEVERAL DIAMOND-LIKE PHASES FROM THE GRAPHITIC BC2N WITHOUT AN ADDITIVE AT 7.7 GPA

A graphite relative of composition BC2N has been subjected to high-pressure (7.7 GPa) and high-temperature (2000-2400-degrees-C) conditions to explore the possibility for the formation of a cubic phase via direct transformation. Several cubic phases with a diamond-like structure were confirmed in the products above 2150-degrees-C by the powder X-ray diffraction patterns. The deconvolution of the broad and partly split diffraction lines suggested that the products obtained at 2150-2300-degrees-C consisted of cBN, ''diamond'' (containing minor amount of B and N), and a cubic B-C-N substance. At 2400-degrees-C, however, the cubic products tended to segregate into two major phases assigned as cBN and ''diamond''. This tendency was strongly supported by the microstructural and elemental observation of the products using a high-resolution scanning electron microscope and Auger electron spectroscopy. The present study concludes that not a cubic B-C-N compound but a mixture of cBN and diamond exists as the thermodynamically stable phases in the ternary system under the conditions employed.