Impurity Oxygen-Triggered α- → β-Si₃N₄ Phase Transformation at 1900 °C

Oxide additive-free α- → β-Si3N4 phase transformation of a high-purity commercial α-Si3N4 powder was investigated at 1600 to 1900 °C under a nitrogen pressure of 980 kPa. The XRD analysis revealed that the α- → β-Si3N4 phase transformation proceeded mainly at 1900 °C, and was completed by the extensive 1900 °C heat treatment for 20 h. This phase transformation temperature was 33 °C lower than the theoretical α-Si3N4 dissociation temperature and was confirmed as completely different from that often discussed for the liquid-phase sintering of α-Si3N4 powder by direct comparison with the phase transformation behavior of a reference powder, α-Si3N4 powder doped with 1 mol% Y2O3. The unique α- → β-Si3N4 phase transformation was further studied by a set of characterization techniques including elemental analysis, HAADF-STEM and STEM-EDS analyses. The results strongly suggested that the oxide additive-free α- → β-Si3N4 phase transformation was governed by the formation of a metastable solid solution between α-Si3N4 and impurity oxygen of approximately 0.6 wt%, which promoted the dissociation below the theoretical α-Si3N4 dissociation temperature to afford thermodynamically favorable β-Si3N4. Along with the β-Si3N4 formation, the impurity oxygen concentrated at the grain boundaries was released from the sample via the grain boundary diffusion to afford high-purity β-Si3N4.