Raw materials for aluminum oxide include bauxite (aluminum ore), alunite (alum stone), and nepheline (potassium and sodium aluminosilicate). For the production of high-strength corundum ceramics, aluminum oxide powder obtained by thermal decomposition of some aluminum salts of varying degrees of purity is used. Aluminum oxide obtained by decomposition of salts is a highly dispersed powder γ-Al2O3 (when calcined up to 1200°C) and has a high chemical activity.
Synthetic α-aluminum oxide (corundum) is used as: an intermediate in aluminum production, for refractory, chemical resistant and abrasive materials, in the production of components for lasers, for the manufacture of synthetic gemstones, etc. Electrocorundum is mainly used for sharpening, both on electric equipment and sharpeners, and for hand sharpening. Electrocorundum (alund, aloxite) is a crystalline aluminum oxide that is artificially produced by melting alumina. This is done in a continuous process in arc furnaces with subsequent crystallization of the substance. After baking, the synthesized corundum acquires a very high hardness, second only to diamond. The Mohs hardness index for electrocorundum is 9, which is practically marginal. The more aluminum oxide is contained in electrocorundum, the harder, stronger and lighter it becomes.
The most commonly used electrocorundum for sharpening is normal (alund). It is a type of electrocorundum containing 91% to 96% Al2O3 in its composition. It is smelted by reductive smelting from bauxite containing aluminum. This electrocorundum abrasive has high hardness and is suitable for grinding a wide variety of metals. The density of electrocorundum ranges from 3.8 g/cm³ to 3.9 g/cm³; the microhardness is approximately 18.6 GPa (Pascal) to 19.6 GPa (1900 kgf/mm² to 2000 kgf/mm²). The color of corundum depends on the impurity content. Unlike silicon carbide, aluminum oxide can have a minimum grain size of less than 1 µm, which allows for more efficient fine finishing of the cutting edge. Sharpening on aluminum oxide abrasives is well suited for most kitchen and hunting knives as well as carpentry tools.
Aluminum oxide is cleaner than silicon carbide works on steels below 58 HRC, leaving a less coarse risk on the approach. Due to the fact that the corundum grains do not split in the process of work, as in silicon carbide, and rolled, reducing in size and losing the sharpness of the edges, this abrasive works softer. At the same time, the difference in working speed between aluminum oxide and silicon carbide depends mainly on the hardness of the bond. Oxide stones are created on a vitreous ceramic bond while carbide stones are created on a porcelain bond, which is much softer. In addition, aluminum oxide stones work with oil, while silicon carbide stones work with an aqueous suspension, which has a greater abrasive effect. However, this does not apply to the stones of Naniwa Professional series, which due to the very high quality of abrasive powder and finely dispersed suspension, are able to work quickly and efficiently on any steel, including those above 58 HRC.
Examples of aluminum oxide sharpening stones include:
1. Boride T2 stones – The Boride T2 series of American Boride stones are made of ceramic vitreous bonded oxide. This results in high performance and a lower than average wear rate. Boride stone manufacturers recommend T2 as the best series for stainless steel. When sharpening with Boride T2 series stones, both oil-based and water-based coolants can be used. The stone is cleaned in water, with a stiff brush and a soap solution. Traces of oil-based coolant are effectively and quickly removed with cleaning oils such as TSPROF abrasive cleaning oil. Stones on thick glass or mirror are leveled with silicon carbide powder.
2. Boride PC (Polisher’s Choice) stones are a series of synthetic aluminum oxide stones of exceptionally high quality. The name of the stones literally translates to “Polisher’s Choice”. PC series stones are designed as finishing stones for the final finishing of metal to a mirror shine. Boride PC stones are used only with the application of coolant.
3. Naniwa Professional stones -an improved series of Japanese Naniwa stones. This series uses magnesia-bonded aluminum oxide. The stones do not require soaking, slow salting and high performance. The stones work gently, yet are fast enough due to their suspension. Naniwa Professional are suitable for virtually all steels.
