Sintering process of Y₂O₃ and Al₂O₃-doped Si₃N₄

Abstract

The sintering process of Y₂O₃- and Al₂O₃-doped Si₃N₄ has been investigated by dilatometry and microstructural observations. The densification progressed through three processes. The bulk density increased to 85% theoretical without the formation of β-Si₃N₄ in the initial process. The densification once terminated after the second process. The α/β transformation of Si₃N₄ and the related formation of prismatic grains reduced the densification rate in the second process, although the grain size and the aspect ratio were very small. The final process was the densification of β-Si₃N₄, where the fibrous grains grew remarkably. The kinetic order for the densification of α-Si₃N₄ indicated a diffusion-rate controlling mechanism with the activation energy of 244 kJ mol⁻¹ (<1450 ° C) and 193 kJ mol⁻¹ (>1450 ° C). The influence of heating rate on the grain growth was characterized by a parameter derived from kinetic parameters. The relationships between grain growth and densification behaviour have also been discussed.