Structural Transitions in Boron Carboxynitride (BNCO) during High-Pressure High-Temperature (HPHT) Synthesis and Sintering

The paper presented some results from the electron microscopic study of structure formation processes in boron carboxynitride (BNCO) during its synthesis on the basis of initial boron oxide and melamine in the form of a powder and a coating on single-crystal diamond particles. Structural studies were performed by transmission electron microscopy (TEM) including high-resolution TEM in combination with elemental microanalysis. Turbostratic BNCO with a different degree of ordering was established to be a major component during the synthesis of a single-phase powder in the temperature region T = 700-1200 degrees C. At a temperature of 1200 degrees C, the formation of an ordered phase occurred in the particles together with inclusions shaped as nanotubes, onions, and prisms with a size of up to 100 nm. BNCO precipitations on diamond particles were monolithic and have continuous interfaces with diamond. The sintering (p = 7.7 GPa, T = 1700 degrees C) of BNCO as a single phase and in composition with diamond (both particles with a coating and in the form of a powder) was accompanied by the formation of the sphalerite phase with a graphite-like residue. The sphalerite phase was a major component of sintered samples only when diamond powders with a grit size of 0.1/0 were used.