Aluminum Oxide Al2O3
Alumina – aluminum oxide (Al2O3, often abbreviated as simply AO) – comprises over 15% of the earth’s crust (compared to about 6% iron oxides). Sixty-five million tons of it are mined each year as the aluminum-bearing ore bauxite with 90% of that being used to make aluminum metal. The pure, natural form of aluminum oxide is a white compound called corundum, which, when contaminated with about 2.5% chromic oxide becomes a ruby. If contaminated with other elements such as iron, titanium and chromium, corundum is called sapphire which, while most commonly blue, can be almost any color but red (which is exclusive to rubies).
Aluminum Oxide, Al2O3. Image courtesy Ben Mills
Alumina is also the major component of emery, a natural compound and one of the most commonly available abrasives until man-made aluminum oxide and silicon carbide abrasives of greater purity and uniformity were developed. There are eight or so different crystalline forms of AO manufactured today, each with different degrees of friability. AO easily passes the primary test of an abrasive: it is hard and sharp. While not as hard as silicon carbide, with a Mohs hardness of 9 (Knoop 2100), AO is one of the hardest substances around. However, not all of the crystal forms of aluminum oxide have silicon carbide’s friability to refresh its sharp points. Aluminum oxide is tougher – harder to crush – than SiC, but it will eventually dull. The tendency of AO to dull is usually addressed by formulating grinding wheels with the AO grains in a friable bond. As a grain dulls, pressure against it increases until the grain breaks free of the bond, which exposes a new, sharp grain that continues cutting. Friable grinding media are truly self-sharpening.
15 micron Aluminum Oxide grains at 1000x magnification.
AO crystals with a greater friability, which allows them to crush to new sharpness, are also employed and available in the marketplace. Norton makes a seeded gel grinding wheel with friable AO grains and 3M offers their sol-gel to the same end. Specific grain sizes of AO are usually obtained by crushing larger chunks into smaller ones, while the seeded-gel and sol-gel grains are literally grown from seed crystals until they become the desired size.
Aluminum oxide is the work horse of the tool-sharpening industry. The large majority of abrasive media in use daily throughout the world is made from one form or another of aluminum oxide.