Growth of elongated grains in alpha-SiAlON ceramics

The microstructural evolution of pressureless sintered alpha -SiAlON, which was made from different particle sizes of alpha -Si3N4 as starting powders have been investigated. The microstructure of the specimens prepared from as-received powder of 0.55 mum in average size, which was sintered at 1850 degreesC, consisted of equiaxed alpha -SiAlON grains. However, by decreasin g the initial alpha -powder size to 0.27 mum, the growth of elongated grain s was enhanced, resulting in a microstructure with elongated grains of a hi gh aspect ratio. In addition, when the specimen prepared from as-received a lpha -Si3N4 powder of 0.55 mum in average size was heat treated at 1600 deg reesC for 10 h, and then sintered at 1950 degreesC for 1 h, elongated grain s were also formed, probably due to the nuclei size reduction by the dissol ution of remaining alpha -Si3N4 during intermediate heat treatment. The spe cimen prepared from finer powders showed increased values of hardness and f racture toughness as a result of the microstructure containing elongated gr ains. (C) 2001 Elsevier Science B.V. All rights reserved.