Ordered coalescence of nanocrystallites contributing to the rapid anisotropic grain growth in silicon nitride ceramics

Microstructural characterization is performed on two dense Si3N4 ceramic samples consolidated by spark plasma sintering (SPS): one fabricated using α-Si3N4 and the other using β-Si3N4 as the starting powder. A novel mechanism is revealed where ordered coalescence of nanoβ-crystallites accelerate the rapid β-Si3N4 anisotropic grain growth. The rapid α- to β-Si3N4 phase transformation via a high supersaturation of dissolved Si3N4 in the melt favors this mechanism. The high heating rate by SPS is essential for achieving such supersaturation.