MICROSTRUCTURAL CHARACTERIZATION OF A MICROWAVE-SINTERED SILICON-NITRIDE BASED CERAMIC

The microstructure of a microwave-densified silicon nitride based cera mic has been assessed in the as-sintered, post-sinter hot-isostaticall y pressed (HIPed) and annealed conditions. The grain size of the as-si ntered material, which is a low substitution beta'-Sialon, was signifi cantly finer than observed in conventionally processed materials of si milar composition. The as-sintered ceramic exhibits a reverse porosity gradient (with the highest porosity level at the surface) due to heat dissipation to the cooler surroundings during microwave processing. T his also results in a higher beta' aspect ratio close to the surface a rising from an increased glass viscosity (due to heat loss) and compos itional change in this region during sintering. HIPing results in remo val of all porosity from the sample core; however, a reduced porosity surface layer is retained. Significant beta'-Sialon grain growth is al so apparent after HIPing. A fine beta' grain structure was retained af ter annealing, with partial devitrification of the glassy grain bounda ry phase to beta-Y2Si2O7.