Effect of structural transformations on compaction and decompaction processes under sintering of materials based on wurtzite boron nitride

Abstract

The results of TEM examination of the microstructure evolution in polycrystals based on wurtzite boron nitride with elongation of sintering time(τ) from 15 to 240 s at Р=7.7 GPа and Т=1800 ⁰ С are presented. It has been established that the determining role in compaction to 90 s is played by the plastic deformation of initial particles involved by the basal slip and law-free, i.e. non-crystal-graphic, rotation. Deformation via the law-free rotation promotes disintegration of particles into disoriented fragments without distortion of their continuity, which determines their further deformation owing to slipping along the fragment boundaries. The disorientation of the fragments involves strengthening of the orientation factor for evolution of basal slip and, as a result, activation of the BN_wur transition into a sphalerite phase. Decompaction of polycrystals under sintering at (τ)>90 s is related to the evolution of plastic deformation via the creep initiated by the formation of areas in the material microstructure on the basis of perfect single-phased (BN_sph) grains sizing under 1 μm with well-developed (with no relief) boundaries. The creep process is assumed to proceed owing to grain-on-grain slipping combined with diffusion atom-by-atom mass transfer on grain boundaries.