Synthesis of Superhard Materials Based on Sphalerite Boron Nitride Using Carbon Nanoparticles as a Phase Conversion Catalyst

Using modern ideas about the form of the phase diagram of boron nitride, the paper considers thermodynamic parameters and mechanisms of the synthesis of dense phases of boron nitride under equilibrium and nonequilibrium conditions. It has been verified that nanodiamonds, like fullerenes and carbon nanotubes, have catalytic properties and contribute to the solid-state conversion of graphite-like boron nitride to sphalerite modification at high pressures and temperatures. We propose a mechanism for the interaction of nanodiamond under high pressures with the surface of graphite-like boron nitride, which leads to a change in the type of electronic bond in its lattice from sp to sp with the formation of boron nitride with a wurtzite structure and its subsequent transformation into sphalerite boron nitride by the shear mechanism. The use of carbon-coated nanodiamonds resulted in an increase in the catalytically active centers of phase transformation in boron nitride in comparison with unmodified nanodiamonds, which was manifested in an increase in the content of sphalerite boron nitride in materials obtained under comparable technological synthesis conditions. Modified nanodiamonds also contribute to the intensification of the synthesis of superhard polycrystals as compared to uncoated nanodiamond additives, both due to the diffusion of carbon atoms along the grain boundaries of sphalerite boron nitride and due to the rearrangement of graphite microgroups into a diamond structure and sintering of the obtained diamond blocks with grains of sphalerite boron nitride. The process parameters of obtaining superhard polycrystals based on sphalerite boron nitride with the addition of detonation nanodiamond powders after chemical cleaning and surface modification with non-diamond forms of carbon were found.