Density gradients and the expansion–shrinkage transition during sintering

Links between density gradients, internal microstructure and in situ sintering shrinkage in compacts formed from spray-dried alumina powder are established using a laser dilatometer and X-ray computed tomography (CT). All samples initially have the same overall density but different internal structures. An expansion–shrinkage transition occurs between 1000 and 1100 °C. Forming conditions appear to play a role: the samples compacted at 25% RH (Relative Humidity) shrank more rapidly than those compacted at 98% RH below 1300 °C; above 1300 °C, however, the specimen formed at 98% RH shrank more rapidly. CT examination following sintering showed both preservation and exaggeration of the original density gradients. Microstructural connectivity apparently contributes to both the observed macroscopic expansion and the onset of shrinkage. Discrete element modeling clearly suggests that the effective `transmission' of particle-level behavior to the macroscopic level is controlled both by internal agglomerate density and initial agglomerate bonding.