The disclosure refers to the art of remarkably increasing the hardness of alumina materials, such as alumina abrasives, alumina ceramics or aluminum oxide single crystal (sapphire), containing aluminum oxide (Al.sub.2 O.sub.3) as the main component, or the hardness of products using the same....http://www.google.com.au/patents/US5804522?utm_source=gb-gplus-sharePatent US5804522 - Hardened alumina material
