Nanocrystalline alumina by high pressure sintering

Nanocrystalline alumina has been sintered using the Boyd-England piston-in-cylinder apparatus in the temperature range of 923-1323 K and at a nominal pressure of 1 GPa. Gas condensation synthesized gamma-Al2O3 with an average particle size of 15 nm was used for this study. The gamma-Al2O3 to alpha-Al2O3 transformation temperature changed from 1473 K without pressure to similar to 1023 K at 1 GPa confining pressure. This has allowed us to obtain high density nanocrystalline alumina for the first time (99.2% of theoretical density and 93 +/- 34 nm grain size). Full density was achieved in 10 minutes above 1223 K. The experimental results agree well with the theoretical predictions of densification by the dislocation creep mechanism. Nanocrystalline alumina shows significantly higher hardness (25.3 +/- 0.8 GPa) than the hardness values reported previously for fine grained alumina (18-20 GPa).