Microstructural evolution during sintering of aluminum nitride ceramics doped with alumina and yttria

The microstructural evolution of AIN sintered at > 1950 degreesC was studie d in a specimen doped with 10 wt% Al2O3 and 5 wt% Y2O3. The constituent pha ses of the specimen were AW, YAG, gamma -AlON, and AlON polytypoids (compos itional polytypes). Transmission and scanning electron microscopy revealed the microstructural characters: platelike 7AlN.Al2O3 first crystallized wit h concurrent formation of a residual Liquid, then spherical ALN crystals fo rmed. The liquid itself changed composition with the progress of the crysta llization and reached the eutectic composition in the pseudobinary system A lN-YAG, and crystallized to an aggregate of AlN and YAG during cooling. As a product of the reaction of 7AlN.Al2O3, gamma -AlON was formed.