Influence of atmosphere and sintering additives on clay-silicon nitride densification

摘要

The reaction of silicon nitride with clays and refractory cements is an economic route to produce low grade sialon phases suitable for bonding purposes in refractory mixes. By such fabrication routes it is possible to produce a range of nitride bonded materials which are now finding increased use in high temperature gas and liquid metal handling applications. The reactions which occur, however, in these rather complex oxide-nitride systems are not well characterised and, in particular, the shrinkage and sintering phenomena are not well known. The present work gives the results of thermo-mechanical analysis studies of reactions in mixtures of silicon nitride with lime, kaolinitic china clay, and calcium aluminate refractory cement. The results are interpreted in terms of the effect of the various additives to silicon nitride and the role of nitrogen gas in the sintering atmosphere. Nitrogen dissolved in the liquid phases which form is considered for its effect on softening points and rate of shrinkage. The α-β silicon nitride transformation is also considered. Precalcination of the clay is shown to be an important factor. The results show that at the relatively low sintering temperature studied here there is only limited transformation of silicon nitride but that the shrinkage and rate of densification can be controlled by variation of the starting composition to give materials suitable for a range of nitride bonded refractory applications.