Sintering Silicon Nitride Ceramics in Air

Abstract

Silicon nitride (Si3N4) ceramics, prepared with Y2O3 and Al2O3 sintering additives, have been densified in air at temperatures of up to 1750°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 α‐ to β‐Si3N4 transformation can be achieved at temperatures above 1650°C, although a low volume fraction of Si2N2O is also observed to form below 1750°C. Partial crystallization of the residual grain‐boundary glassy phase was also apparent, with β‐Y2Si2O7 being noted in the majority of samples. The microstructures of the sintered materials exhibited typical β‐Si3N4 elongated grain morphologies, indicating potential for low‐cost processing of in situ toughened Si3N4‐based ceramics.