Microstructure and sintering of ceramic green compacts described in a constitutive model

The effect of forming pressure on the internal structure and the sintering behavior of green compacts made from ceramic granules was investigated with the aim of describing those by a constitutive model. Alumina granules were compacted by cold isostatic pressing(CIP) at three different pressures of 98, 196 and 294 MPa. Examined were the initial pore structure, shrinkage and densification of the granular compacts during sintering at four temperatures in the range between 1100 and 1400°C. Three distinct regions in the microstructure with fractured, unfractured granules and crack-like cavities were identified as the three peaks observed in the pore size distribution. The ratio of the respective pore volumes in these three regions was independent of the forming pressure within the range under examination, while the pore size decreased as the pressure increased. The sintering rate of the compacts formed at 98 MPa was similar to that at higher pressures, because the dominant process responsible for the densification during sintering is the shrinkage of small pores in a dense region. However, the final density after sintering of the compacts formed at 98 MPa was somewhat lower than that of the compacts formed at higher pressures. This stems from large crack-like pores around the unfractured granules remained even at 1400°C. The shrinkage behavior was modelled with a set of constitutive equations by taking account of the pore size distribution of granular compacts. The sintering behavior related to the forming pressure could be well described by the present model.