Reaction sintered silicon nitride

The strength, composition and structure of reaction sintered silicon nitride formed in the presence of oxygen or water vapour have been investigated. It is shown that chemical contamination of the nitriding gas by oxygen and water vapour to relatively high concentrations is not the cause of abnormally low strength silicon nitride. The coefficient of variation in strength and apparent crystallite size for a particular batch of samples are related to the water vapour concentration in the nitriding gas. The formation of - and -silicon nitride is discussed and it is suggested that compositional variations in -silicon nitride must be small.