Features of formation of nanodisperse granular structure during РТ-sintering of hexagonal diamond (lonsdaleite) powder

Abstract

Results of an electron microscopic investigation of particles of lonsdaleite powder with additives of cubic diamond and polycrystalline specimens based on it at Р = 7,7 GPa in the temperature range 1700―1900 °С are presented. Lonsdaleite particles are characterized by a predominantly ternary texture [1120]l of different degree of perfection. Structural mechanisms of transformations in lonsdaleite particles, which cause the formation on nanograined structure in sintering, have been established. The initial stage is the mechanical dispersion of particles and dispersion as a result of plastic faulting deformation, which promotes their fragmentation without breakdown of continuity. The indicated processes lead to the destruction of texture in particles. Beginning from 1700 °С, the lonsdaleitecubic diamond phase transformation with the orientation ratio (111)dII (001)l occurs. It is realized within rods as elements of the substructure of lonsdaleite. At 1900 °С, the size of detected elements of the structure (grains) is 5―15 nm. Beginning from 2000 °С, the self-association of such grains into aggregates with sizes up to 70 nm and the subsequent process of coalescence of grains in aggregates with the formation of the monocrystalline state occur. The next stage of formation of the granular structure is caused by the formation of grain boundaries and development of collective recrystallization. After sintering at Т = 2100 °С, the grain size in specimens does not exceed 100 nm. It has been established that the transformation in lonsdaleite proceeds by structural mechanisms that are characteristic of wurtzite modifications of boron nitride and silicon carbide (strong disordering in the direction of the basal axis, plastic faulting deformation, and formation of multilayer polytypes during the hexagonal-to-cubic phase transformation). Keywords: lonsdaleite, particles, sintering, fragmentation, grains, coalescence.