Ceramic Materials for Gas Turbine Environment

WDER COMPACTS WERE FIRST EXPOSED TO NITROGEN, INHIBITING THE SILICON-ALUMINUM MELT REACTION. The high (negative) free energy of formation of AlN relative to Si3N4 appeared to be responsible for the early nitriding of aluminum. The reaction sintering kinetics were not materially affected by aluminum because of the initial formation of the stable aluminum nitride compound. The bend strength and the amount of alpha silicon nitride were, however, reduced by the aluminum additions. Conditions that favored melt formation before the aluminum was fixed as a nitride (high aluminum concentration and rapid heating) lead to segregation of the constituents and growth of porosity and silicon grains. The results confirm the importance of determining the effects of metal solutes in silicon on the properties of silicon nitride and on the technology of reaction sintering. (Author)