Residual [alpha]-Si[sub 3]N[sub 4] in O[prime] crystals in CeO[sub 2]-doped O[prime] + [beta][prime] SiAlON ceramics

The microstructure of a pressureless sintered (1,605 C, 90 min) O[prime] + [beta][prime] SiAlON ceramic with CeO[sub 2] doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O[prime] grains (20--30 [mu]m long), and a continuous matrix of small rodlike [beta][prime] grains (< 1.0 [mu]m in length). Many [alpha]-Si[sub 3]N[sub 4] inclusions (0.1--0.5 [mu]m in size) were found in the large O[prime] grains. CeO[sub 2]-doping and its high doping level as well as the high Al[sub 2]O[sub 3] concentration were thought to be the main reasons for accelerating the reaction between the [alpha]-Si[sub 3]N[sub 4] and the Si-Al-O-N liquid to precipitate O[prime]-SiAlON. This caused the supergrowth of O[prime] grains. The rapid growth of O[prime] crystals isolated the remnant [alpha]-Si[sub 3]N[sub 4] from the reacting liquid, resulting in a delay in the [alpha] [yields] [beta]-Si[sub 3]N[sub 4] transformation. The large O[prime] grains and the [alpha]-Si[sub 3]N[sub 4] inclusions have a pronounced effect on the strength degradation of O[prime] + [beta][prime] ceramics.