Transparent sub-micrometre alumina from lanthanum oxide doped common grade alumina powder

Common grade high purity alumina (99.99%) powder doped with lanthanum oxide was slip cast, dried and pressureless sintered to sub-micrometre grain sized alumina at 1380–1410 °C. Samples that attained >96% theoretical density (TD) were subsequently hot isostatically pressed to fully dense alumina having ≥99.95% TD. Sintered sub-micrometre alumina samples, ground to 2 mm thickness and polished to 0.5 μm surface finish, displayed >82% transmission in the 3–5 μm wavelength range. Samples of 0.5 mm thickness also exhibited 40–60% transmission in the visible wavelength region of 400–900 nm. Lanthanum oxide addition within 0.10 wt% was shown to be favourable for improved transparency in the visible region. The alumina samples also exhibited high Vickers hardness values of 2144+45 kg/mm2 at 5 kg indentation load.