Some New Aspects of Microstructural Development During Sintering of Silicon Nitride

The mechanical properties of silicon nitride ceramics strongly depend on their microstructure. However, there is still a lively discussion about the parameters controlling the microstructural development. The research was stimulated by the observation that a bimodal grain-size distribution in dense silicon nitride has a very beneficial effect on the mechanical properties, especially on the fracture toughness. The work focused on the relationship between the alpha - beta -transformation and the densification of silicon nitride powders with different characteristics and sintering additives. Effects of beta -grains originally present in the silicon nitride powder, of added beta -silicon nitride seeds and of beta -crystals formed by the alpha / beta -transformation on the resulting microstructure and on the properties are discussed. The results are summarized in a model describing prerequisites and processing conditions, which are necessary to achieve a bimodal microstructure, i.e. a self-reinforced silicon nitride ceramic.