PLASMA ACTIVATED SINTERING OF NANOCRYSTALLINE GAMMA-AL2O3

Plasma activated sintering of nanocrystalline gamma-Al2O3 has been stu died 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 w ith both the applied stress and temperature. Depending on the temperat ure and applied stress, the density varied from 57 to 96% of theoretic al. The grain growth was excessive in specimens with 87-96% density. C onsidering that the aim was to obtain nanocrystalline alumina, the bes t possible combination of density (76% theoretical) and grain size (80 nm) was obtained at 1833 K and 66 MPa of applied stress. This is comp arable with the recently reported results on microwave sintering of ga mma-Al2O3. It is shown that the sintering of gamma-Al2O3 is more diffi cult than other nanocrystalline oxides (ZrO2, TiO2 and Y2O3), perhaps because of the gamma to alpha phase transformation and associated text ure. 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 d uring 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 nanocryst alline gamma-alumina.