Microwave sintering of alumina made by high-speed centrifugal compaction process

High purity (99.99%) and fine (0.2μm) alumina powder was prepared as aqueous slip and compacted by High-Speed Centrifugal Compaction Process (HCP) with a centrifugal acceleration of above 10,000g. Green compacts were then sintered by high frequency (28 GHz) microwave furnace with a holding temperature of 1423-1523 K. No sintering aid was added into the compacts in any processing steps. Microwave sintering in vacuum condition improved densification of HCP alumina and it reached almost full density at 1443 K, which was about 70 K lower than conventional sintering. On the other hand, the concurrent grain growth during microwave sintering was suppressed, thus a finer microstructure was achieved in comparison with conventionally sintered ones. The hardness and four-point strength of microwave sintered alumina, Hv 2160 and 1130 MPa respectively, surpassed those of conventionally sintered HCP alumina as well as many of other alumina ever reported.