Abstract
Residual defects in alumina green compacts are observed by liquid-immersion technique. A high purity (of 99.99%) and fine (of 0.2μm) alumina powder is dispersed in the ion-exchanged water and compacted by High Speed Centrifugal Compaction Process (HCP) and pressure casting (PC). Green compacts are pre-sintered at 1073 K for 3.6ks and subsequently sliced into 500μm thick disks. The disk are immersed in diiodmethane and observed by optical microscopy with transmission light.
Although HCP and PC green compacts show almost the same green densities and sintered densities, only PC compacts contain many residual bubbles remaining as huge pores after pre-sintering. The residual bubbles in PC compacts become larger as the viscosity of slip increases. The de-bubbling effect of HCP is far greater than that of vacuum de-bubbling, whereas its efficiency is also affected by the condition of the compaction. In HCP, the bubbles can be removed more effectively under greater centrifugal force using a slip with low viscosity.
