Anisotropic shrinkage during sintering of particle-oriented systems - numerical simulation and experimental studies

High shear rates involved in processes such as tape casting and injection molding result in anisotropic shrinkage during subsequent densification of the green bodies. Anisotropic shrinkage with more than 14% difference in the shrinkages between the casting and transverse directions was observed in tape cast alumina. To explain the measured shrinkage anisotropy in tape cast alumina and its variation as densification progresses, sintering of oriented ellipses is simulated. The simulated shrinkage anisotropy decreases as sintering progresses because of the increasing neck length in the particle-oriented direction. Similar trends were seen for experimental results on tape cast alumina. The decrease is more significant when surface diffusion dominates over grain boundary diffusion. Similar atomistic models were also used to simulate the sintering shrinkages in liquid-phase sintered alumina. Again, anisotropy decreases as sintering progresses. The predictions from solid state sintering models were then validated by sintering alumina with 500-1000 ppm of yttria. It is proposed that the addition of yttria lowers the grain boundary diffusion rate with respect to the surface diffusion and hence lowers the shrinkage anisotropy in accordance with the simulation model. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.