The development of strength in reaction sintered silicon nitride

Abstract

The development of strength in reaction sintered silicon nitride has been investigated by determining the elastic moduli, fracture mechanics parameters, strengths and critical defect sizes of silicon compacts reacted to various degrees of conversion using “static” or “flowing” nitrogen. The relationship between each property and the nitrided density is shown to be independent of the green silicon compact density but is influenced by the nitriding conditions employed. Young's moduli, rigidity moduli and strengths vary linearly with the nitrided density. After an initial period when increases may occur, the critical defect sizes in both “static” and “flow” materials decrease continuously with increasing nitrided density, although at any particular density they are larger in material produced under “flow” conditions. A model is suggested for the development of the structure of reaction sintered silicon nitride involving the development of a continuous silicon nitride network within the pore space of the original silicon compact. The experimental data are discussed in terms of the proportion of silicon nitride which contributes effectively to the continuous network.