Danilenko A. Phase transformations of graphite-like structures of boron nitride under high temperature shock compression.

The phase transformations have been investigated within boron nitride under high temperature shock compression (HTSC) at pressure 33 GPa and temperatures in the range 1000 to 3500 K, which in accordance with the method of HTSC was regulated by the amount of the specially added alkaline-haloid salts. The optimum compression conditions which provided the maximal output of dense phases of boron nitride (BNd) with the structures of sphalerite (BNc) and wurtzite (BNw) were determined by systematic examination of the degree of phase transformations depending on different parameters (type and amount addition, density of mixture). Under these conditions of HTSC the dependence of mechanism and extent of transformations on the degree of three-dimensional ordering (P3) of initial graphite-like structures has been investigated. On the basis of the results obtained the diagram for phase transformations within BN under HTSC has been plotted. The especial attention was paid to study the real structure of dense phases BNc and BNw synthesized by HTSC. It was shown X-ray diffractometry and electron microscopy that both dense phases exhibit nanocrystalline structure and high level of microstrain of crystalline lattice, caused by the very non equilibrium conditions of shock compression. The wurtzite phase which is formed via the puckering of graphite-like planes by martensitic mechanism, exhibit the defects of packing in the plane of (001) and (100) type, which directly concerned with the mechanism of BNw formation. The powders of dense phases synthesized by the HTSC method were used for sintering them at high static pressures. The possibility for conservation of initial nanocrystalline structure of BNd having relatively high hardness of the obtained polycrystal has been demonstrated.