The role of plastic deformation in compaction and decompaction processes in sintering materials based on wurtzitic boron nitride

Abstract

The evolution of the microstructure of wurtzitic boron nitride-based polycrystals with sintering time (τ) increasing from 15 to 240 s at p = 7.7 GPa and T = 1800°C has been TEM examined. It has been concluded that the decisive role in compaction below 90 s has been played by the plastic deformation of initial particles caused by the basal slip and irregular, i.e. non-crystallographic, rotation. A dispersion of particles into disoriented fragments without distortion of their continuity, which defines their further deformation due to slipping along the fragment boundaries, is promoted by the deformation via the irregular rotation. The strengthening of the orientation factor for the development of basal slip and, hence, the activation of the wBN transition into the cubic phase is facilitated by the fragment disorientation. A decompaction of polycrystals in sintering for τ > 90 s is caused by the evolution of plastic deformation via the creep initiated by the formation of areas of the microstructure of a material based on the perfect singlephased (cBN) grains of sizes below 1 μm with well-developed (with no relief) boundaries between them. The creep process is assumed to proceed owing to intergranular slipping combined with diffusion atom-by-atom mass transfer in grain boundaries.