Microwave sintering of silicon nitride ceramics was examined to identify aspects that would advantageous over conventional sintering. Alumina and yttria dopped silicon nitride was sintered in a multimode microwave applicator with 2.45 GHz power generator. The sintering additive content was varied 0 to 12 wt%. Temperature was controlled by a molybdenum sheathed C-type thermocouple surrounded by the specimens. Conventional sintering was done in a graphite element furnace. Heating rates were 25℃ /min to the sintering temperature.
Higher densities and β-phase fractions were achieved by microwave sintering of silicon nitride powder compacts as compared to those fired in a conventional furnace. The heating rate of specimens in the constant microwave power increased with the content of sintering additives. The heating rate maximized at temperature of formton of liquid phase.
The microwave enhancement of sintering of silicon nitride ceramics may be explained by high dielectric loss in intergranular liquid phase. Silicon nitride is densified by liquid phase sintering and this is also true for the microwave sintered materials. But silicon nitride based materials in the microwave field heat by coupling of the microwave energy to the sintering additives. The coupling of microwave with formed liquid will increase the ion mobility in the intergranular phases. Thus microwave sintering can greatly decrease the sintering temperature and increase the rate of α to β phase transformation.
