Dependence of mechanical properties on sintered microstructure of high purity alumina made by high-speed centrifugal compaction process

Influence of microstructures on mechanical properties of high performance alumina is investigated focusing on density and mean grain size as microstructure characteristics. An alumina powder with a purity of 99.99% and mean grain size of 0.22μm is dispersed in 25 mass % of ion-exchanged water, compacted applying a centrifugal force of 10,000-20,000 g for 3 ks, and sintered at 1423-1773 K for 0-172.8 ks in air. It is shown that strength and hardness have one peak value respectively as microstructure develops during sintering, and those peaks appear on the same specimen with a relative density of about 99% and grain size of about 1μm. For the specimens with density lower than 99%, the strength and hardness decrease as density decrease, whereas for the ones with density higher than 99%, those values decrease as grain size increase. Furthermore, two empirical equations, which show the dependence of bending strength and hardness on both porosity and grain size, are derived by multivariate analysis. These equations provide contour lines which represent microstructures having the same strength or hardness on the grain-size/relative-density diagram. Superposing a trajectory line derived from grain size and density of various specimens on the diagram, the point where this trajectory and one of contour lines contact represents the best couple of grain size and relative density for the mechanical properties.