Effect of sintering atmosphere and HIP treatment on translucency and mechanical properties of alumina made by high-speed centrifugal compaction process

Complete pore elimination from alumina compacts is attempted under various sintering conditions. An alumina powder of purity of 99.99% and grain size of 0.2 μm is used as a raw material, and it is mixed with ion-exchanged water of 25 mass%, dispersing agent of 0.6 mass% and binder of 0.1 mass% to prepare slip. Compacting process is executed under high centrifugal force of 10,00-20,000 g. Dense and homogeneous green compacts are obtained and they are subsequently sintered in dry hydrogen, vacuum or HIPed. Although vacuum sintering allows the specimen attains slight higher density than air-sintered specimen, some pores remain in the structure and the specimen doesn't show translucency. HIP treatment of air-sintered specimen, on the other hand, realizes almost pore free microstructure and the specimen shows translucency. HIPed specimen shows higher strength (four-point bending strength of 750 MPa) in comparison with commercial translucent alumina, but no remarkable improvement of strength is marked in comparison with air sintered alumina. HIPed specimen loses translucency after annealing in air because of the restore of transgranular pores. In-line transmittance of the HIPed specimen decreases as the wave length decreases.