High pressure low temperature sintering of nanocrystalline alumina

Bulk nanocrystalline alpha-Al2O3 samples with a relative density > 98% and a grain size < 50 nm have been produced by high pressure/low temperature sintering, using a toroidal-type high pressure apparatus. Nanocrystalline (n-) alumina powder with metastable gamma phase was used as the starting material. During sintering, the gamma phase transforms to alpha phase. The transformation temperature decreases from similar to 1075 degrees C at ambient pressure to about 460 degrees C: at 8 GPa. Grain growth is limited by the low sintering temperature, and a multiplicity of nucleation events in the parent gamma phase at very high pressure creates a nanoscale alpha grain size. The average grain size of the alpha-Al2O3 increases from 18 nm in the original powder to only about 49 nm in the sintered compact (98.2 % dense). In addition, we found that high pressure could increase the nucleation rate while reducing the growth rate of the transformed alpha phase so that its grain size decreased with sintering pressure under the same sintering temperature and time. Due to its high surface area, n-Al2O3 powder readily absorbs chemical species from the environment. Alumina hydrates,formed by the reaction of Al2O3 with chemisorbed OH- species during sintering, had a profound influence on sintering and phase transformation behaviors of n-Al2O3, To control grain size of the transformed alpha-phase, it is essential to eliminate the hydrates before sintering. (C) 1998 Acta Metallurgica Inc.