Development of microstructure during the fabrication of Si3N4 by nitridation and pressureless sintering of Si:Si3N4 compacts

The technique for the fabrication of Si3N4 which was investigated involves the nitridation of Si:Si3N4 powder compacts containing additions of sintering aids (e.g. Y2O3 and Al2O3) followed by pressureless sintering. The development of microstructure during fabrication by this method has been followed by X-ray diffraction and analytical electron microscopy. As well as being important for the sintering process, it was found that the sintering aids promote nitridation through reaction with the surface silica on the powder particles. During nitridation extremely fine grained Si3N4 forms at silicon powder particle surfaces and at tunnel walls extending into the interior of these powder particles. Secondary crystalline phases which form during nitridation are eliminated from the microstructure during sintering. Theα- toΒ-Si3N4 phase transformation is completed early in the sintering process, but despite this the fully sintered product contains fineΒ-Si3N4 grains. TheΒ grains are surrounded by a thin intergranular amorphous film.