The existence of a hexagonal (wurtzite) form of silicon, similar to that form of diamond (carbon) observed in meteorites and in the laboratory, has been identified by x-ray diffraction in reaction-bonded silicon nitride containing unreacted silicon. The presence of this phase is due to stresses created in the silicon by the nitridation reaction.

This paper reports on models which have been developed to explain the formation mechanism and kinetics of the major microconstituents of reaction-bonded silicon nitride. These models are based on information obtained from a detailed microstructural study during various stages of reaction and a knowledge of the reaction conditions which encourage formation of each particular microconstituent. It has become clear that there are at least two independent mechanisms and that they are governed by independent rate laws. The kinetics of nitridation of a silicon compact is, therefore, the superposition of at least two independent rate laws. Experimental evidence obtained thus far is in good agreement with this hypothesis.

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