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

Abstract

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.