Atomic-scale structural investigations on the nucleation of cubic boron nitride from amorphous boron nitride under high pressures and temperatures

By controlling the microstructure of the starting materials, i.e., by ball-milling a commercial hexagonal boron nitride (h-BN) to an amorphous boron nitride (a-BN), the subsequent high-pressure and high-temperature (HP-HT) induced phase transformation has been significantly facilitated. Namely, cubic boron nitride (c-BN) forms at 900 degreesC and achieves accomplishment at 1350 degreesC under 7.7 GPa, which are significantly less-extreme conditions than that of crystalline h-BN under similar HP-HT treatments. High-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) clarified the nucleation mechanism at an atomic scale. It demonstrated that the c-BN phase nucleates directly from the sp(3)-hybridized amorphous matrix, which is originally induced by ball-milling and is therefore responsible for the reduced HP-HT conditions. This c-BN nucleation mechanism is completely different from the so-called diffusionless "puckering" mechanism involved in the HP-HT experiments starting from crystalline h-BN but very similar to one of the proposed mechanisms involved in the chemical vapor deposition (CVD) of diamond and c-BN. It turns out that the present experimental results provide not only a less-extreme way to synthesize nanocrystalline c-BN material but also key clues to the understanding of the nucleation mechanisms involved in the CVD diamond or c-BN, which are still under controversy.