Sintering of Si3N4 with Al2O3 and Y2O3.

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.