Corundum, Artificial

(also fused alumina or artificial aluminum oxide), a synthetic abrasive that consists mainly of crystallized alumina, or aluminum oxide, in the a form (corundum) and also contains oxides of silicon, titanium, calcium, and iron. Artificial corundum is obtained by melting an alumina-containing raw material in an arc furnace and then allowing the melt to crystallize.

The density of artificial corundum, except for the spheroidal variety, is 3.9–4.0 g/cm³. Its microhardness is 19–24 giganewtons per m². Depending on the alumina content and the characteristics of the melting process, the following varieties of artificial corundum are distinguished: regular alumina, white alumina, modified alumina, single-crystal alumina, and spheroidal alumina.

Regular alumina consists of up to 95 percent α-alumina and small amounts of slag and a ferroalloy as impurities. It is widely used to machine metals.

White alumina is obtained by remelting pure aluminum oxide in the γ form. It contains 98–99 percent α-alumina and a relatively small amount of impurities. In its properties and chemical composition, white alumina is more homogeneous than regular alumina; its microhardness is somewhat higher than that of regular alumina. White alumina is used to machine high-strength alloys and in high-speed and precision grinding.

Modified alumina may contain chromium oxide, titanium oxide, or zirconium oxide. Its properties depend on the composition and content of the impurities. Abrasive tools made from modified alumina are used to machine parts made of structural steel or certain tool steels.

Single-crystal alumina consists of isometric tabular single crystals of α-alumina and a small amount (2–3 percent) of impurities. It is obtained by fusing bauxite and iron sulfide. Abrasive tools made from single-crystal alumina are used to grind, for example, heat-resistant or structural alloy steels or other alloys that are difficult to machine.

Spheroidal alumina is obtained from alumina as hollow spherules of α-alumina. Its density is 2.2 g/cm³, and it contains a small amount (< 1 percent) of impurities. Abrasive tools made from spheroidal alumina are used to process soft or viscous materials, such as nonferrous metals, plastics, rubber, or leather.

Artificial corundum crystals and powders, including very fine powders, account for about 80 percent of the total production of abrasives. Owing to its high refractoriness, its resistance to acids and alkalis, its good thermal conductivity, and its low thermal expansion and electrical conductivity, artificial corundum is widely used to manufacture refractories, chemically inert products, and ceramic parts for insulators and electron-tube devices. It is also used as a filler in high-temperature concretes and in ramming mixes for the crucibles of induction furnaces. In ferrous metallurgy, a considerable amount of artificial corundum is used to produce synthetic slags for the refining of liquid steel.

The fields of application for artificial corundum continue to expand.