Role of specimen insulation on densification and transformation during microwave sintering of silicon nitride

Silicon nitride ceramics, with Y2O3, Al2O3 and MgO as sintering aids, have been sintered in a 28 GHz microwave applicator using a number of sample insulation techniques. The sintering characteristics in terms of densification, alpha --> beta transformation, microstructural development and power requirement were studied and compared to identical samples sintered conventionally. All of the microwave sintered materials could be sintered to near theoretical density and a full alpha --> beta transformation obtained at temperatures around 200 degreesC lower than the materials sintered conventionally. In addition, the microstructural development showed important differences, with the selective development of elongated beta -grains being observed from the very early stage of transformation. Samples sintered using a powder bed insulation technique, achieved full alpha --> beta transformation at the lowest temperature, but a tendency to thermal runaway using this method of insulation meant the samples were susceptible to cracking. Samples sintered using silicon carbide plates as a low temperature microwave absorber reached full density and onset of transformation earlier than the other materials. In addition, the maximum power requirement for these samples was around 1/3 of that required for other insulation techniques, and more uniform heating meant that these samples were free from cracking.