Sintering silicon nitride ceramics in air

Silicon nitride (Si3N4) ceramics, prepared with Y2O3 and Al2O3 sintering additives, have been densified in air at temperatures of up to 1750 degrees C using a conventional MoSi2 element furnace. At the highest sintering temperatures, densities in excess of 98% of theoretical have been achieved for materials prepared with a combined sintering addition of 12 wt% Y2O3 and 3 wt% Al2O3. Densification is accompanied by a small weight gain (typically < 1-2 wt%), because of limited passive oxidation of the sample. Complete alpha- to beta-Si3N4 transformation can be achieved at temperatures above 1650 degrees C, although a low volume fraction of Si2N2O is also observed to form below 1750 degrees C. Partial crystallization of the residual grain-boundary glassy phase was also apparent, with beta-Y2Si2O7 being noted in the majority of samples. The microstructures of the sintered materials exhibited typical beta-Si3N4 elongated grain morphologies, indicating potential for low-cost processing of in situ toughened Si3N4-based ceramics.