Hot-Pressing of Nano-Size Alumina Powder and the Resulting Mechanical Properties

The optimum conditions for sintering nano‐phase alumina powder to theoretical density with high hardness and strength were determined. As‐received gamma alumina powder was transformed to alpha alumina by heating for 8 min at 1300°C, and 0.1 wt% MgO was added to the gamma powder. These conditions resulted in 100% alpha powder with minimum growth in particle size. The alpha powder was ball‐milled for 24 hr to further reduce its particle size. The resulting powder was hot‐pressed in vacuum at different temperatures from 1250°C to 1450°C. To achieve high hardness and strength, the minimum time to give theoretical density at each temperature was used. Under these conditions, a Vickers hardness of greater than 22 GPa and diametral strength of above 1 GPa were achieved, which are much greater than those properties of polycrystalline alumina with larger grain size. The activation energy of alpha alumina with a mean diameter of 46 nm to reach theoretical density was 366 ± 23 kJ/molK. From analysis of these results and those of other workers, we conclude that low porosity is a very important factor for increasing strength and hardness of alumina with small grains.