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 ins ulation techniques. The sintering characteristics in terms of densification , alpha --> beta transformation, microstructural development and power requ irement were studied and compared to identical samples sintered conventiona lly. All of the microwave sintered materials could be sintered to near theo retical density and a full alpha --> beta transformation obtained at temper atures 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 f rom the very early stage of transformation. Samples sintered using a powder bed insulation technique, achieved full alpha --> beta transformation at t he lowest temperature, but a tendency to thermal runaway using this method of insulation meant the samples were susceptible to cracking. Samples sinte red using silicon carbide plates as a low temperature microwave absorber re ached full density and onset of transformation earlier than the other mater ials. In addition, the maximum power requirement for these samples was arou nd 1/3 of that required for other insulation techniques, and more uniform h eating meant that these samples were free from cracking.