Formation of diamond-like BN phases under shock compression of graphite-like BN with different degree of structural ordering

The formation of diamond-like modifications of BN under high-temperature shock compression (P = 33 GPa and T up to 3500 K) has been studied. The powders of graphite-like BN with different degree of three-dimensional ordering of structure (D3) were used as starting materials. To increase shock temperature and preserve dense phase formed, boron nitride powder mixed with alkali halide salt was compressed in cylindrical ampoule. The recovered samples were studied by X-ray diffraction and electron microscopy. It was revealed that the phase composition of shock compression products depends on the degree of three-dimensional ordering of initial graphite-like structure. Turbostratic BN with disordered structure (D3 = 0) transformed mainly into cubic cBN by diffusion mechanism, whereas highly ordered graphite-like BN (D3 = 0.95) transformed only into wurtzite wBN by martensitic mechanism. Both cubic and wurtzite dense modifications are formed from partly ordered initial structures (for which D3 was 0.45 ÷ 0.7). The both diamond-like phases have nanocrystalline structure.