Transparent nanocrystalline bulk alumina obtained at 7.7 GPa and 800 °C

Transparent nanocrystalline bulk alumina was obtained at 7.7 GPa and 800 °C. The alumina has an average grain size of ∼150 nm and an in-line transmission of 71% for a wavelength of 640 nm and a thickness of 0.8 mm. The values of microhardness and fracture toughness were found to be 25.5 ± 0.3 GPa and 2.9 ± 0.3 MPa m1/2, respectively. The hardness value is consistent with the previously determined Hall–Petch relation and fracture toughness is independent of grain size.