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 si ntering, using a toroidal-type high pressure apparatus. Nanocrystalline (n- ) alumina powder with metastable gamma phase was used as the starting mater ial. During sintering, the gamma phase transforms to alpha phase. The trans formation temperature decreases from similar to 1075 degrees C at ambient p ressure to about 460 degrees C: at 8 GPa. Grain growth is limited by the lo w sintering temperature, and a multiplicity of nucleation events in the par ent 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 orig inal 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 wh ile reducing the growth rate of the transformed alpha phase so that its gra in size decreased with sintering pressure under the same sintering temperat ure and time. Due to its high surface area, n-Al2O3 powder readily absorbs chemical species from the environment. Alumina hydrates,formed by the react ion of Al2O3 with chemisorbed OH- species during sintering, had a profound influence on sintering and phase transformation behaviors of n-Al2O3, To co ntrol grain size of the transformed alpha-phase, it is essential to elimina te the hydrates before sintering. (C) 1998 Acta Metallurgica Inc.