Infiltration techniques for suppressing grain growth during the densification of submicron and nanophase alumina composites

Grain growth control during the final stages of densification is an issue with extreme technological significance. In the last decade, nanophase ceramic starting powders have become commercially available, and in order to take full advantage of the unique properties nanophase ceramics potentially offer, grain growth control during processing of these materials must be addressed. In this research study the author addresses the topic of suppression of grain growth in submicron and nanophase alumina using novel processing techniques. The approach used to decrease grain boundary motion during processing involves the addition of an intermediate step between initial powder pressing and sintering. During the intermediate step either solute amounts of an infiltrant are incorporated along grain boundary junctions through a vapor-phase technique or, second phase inclusions are formed within the pore structure of the ceramic porous compacts via a liquid-phase technique.; Using the vapor-phase technique, silicon nitride was introduced into porous submicron and nanophase alumina compacts. It was determined that silicon nitride in amounts below the solubility limit is an effective grain growth inhibitor. In the submicron pellets studies, three regions of grain growth were observed: abnormal, suppressed, and normal. Using a mathematical model developed for simulating the vapor infiltration process, a range of silicon concentrations (70 to 50 ppm) was determined to correspond to the suppressed grain growth region. In the nanophase silicon nitride infiltrated pellets, suppression of grain growth was observed throughout the infiltrated specimen, however the suppression effects were not great enough to retain the nanosized nature of the alumina grains.; Using the liquid-phase technique, zirconia particles were formed within the pore structure of submicron and nanophase alumina compacts. A comparison between suppression effects in compacts created by liquid-phase and by powder-mixing techniques illustrated the superiority of this processing method. It was determined that liquid-phase infiltration processing produced a greater number of evenly distributed inclusions than traditional powder-mixing. The distribution and greater number of particles at the same volume fraction of zirconia created a greater drag force on the alumina grain boundaries, which resulted in increased grain growth suppression.