PLASMA ACTIVATED SINTERING OF NANOCRYSTALLINE GAMMA-AL2O3

Plasma activated sintering of nanocrystalline gamma-Al2O3 has been studied over a temperature range of 1573-1943 K and a uniaxial stress of 29-66 MPa. A transformation of gamma-Al2O3 to alpha-Al2O3 occurred at all the test temperatures. The density of the sintered pellet varied with both the applied stress and temperature. Depending on the temperature and applied stress, the density varied from 57 to 96% of theoretical. The grain growth was excessive in specimens with 87-96% density. Considering that the aim was to obtain nanocrystalline alumina, the best possible combination of density (76% theoretical) and grain size (80 nm) was obtained at 1833 K and 66 MPa of applied stress. This is comparable with the recently reported results on microwave sintering of gamma-Al2O3. It is shown that the sintering of gamma-Al2O3 is more difficult than other nanocrystalline oxides (ZrO2, TiO2 and Y2O3), perhaps because of the gamma to alpha phase transformation and associated texture. The experimental densification rate is lower than the theoretical one by several order of magnitude. As-sintered pellet shows texture, which perhaps gives rise to abnormal stress-strain behavior observed during the compression testing of as-sintered specimen with 59% initial density. Therefore, significantly higher pressure is required during sintering to achieve the desirable level of consolidation in nanocrystalline gamma-alumina.