Sintering of Si₃N₄ with Al₂O₃ and Y₂O₃

Abstract

The sintering of Si₃N₄ with 10-50% (in weight) of equi-molar mixture of Al₂O₃ and Y₂O₃ was investigated at 1700°C and 1800°C. During heating, the dissolution of Si₃N₄ into Al₂O₃-Y₂O₃ reduces the melting point of the phase. The glass phase liquidized at sintering temperature and enhanced the liquid phase sintering. During sintering, a part of the liquid crystallized to α′-Si₃N₄ (solid solution of Si₃N₄ and Al₂O₃ with α-Si₃N₄ structure) and/or Si₃N₄⋅Y₂O₃.
The minimum porosity performed was 13.3% and 3.8% in pressureless sintering at 1700°C and gas pressure sintering under 10 atm N₂ at 1800°C, respectively.
The glass phase crystallized to α′-Si₃N₄ and Si₃N₄⋅Y₂O₃ by annealing at 1600°C. The crystallization of β′-Si₃N₄ and Si₃N₄⋅Y₂O₃ from glass phase was found in Al₂O₃ rich and Y₂O₃ rich compositions, respectively. The occurrence of α′-Si₃N₄ during sintering was attributed to non-equilibrium conditions.