Aluminum oxide, also called corundum, is a naturally occurring stone. Aluminum is naturally found in emeralds and rubies, and industrial grades of these stones are used to make high-quality aluminum oxide sanding discs. Silicon carbide is a mixture of sand, rock salt and coke, a form of coal. This mixture is heated in a furnace until it melts and fuses together.
Friability refers to how easily the particles fracture when the disc is used. Aluminum oxide is more friable than silicon carbide. When the particles fracture, microscopic sharp edges are formed. Aluminum oxide therefore is highly self-sharpening when the disc being used. Because silicon carbide is not as friable, it is less self-sharpening.
Silicon carbide is harder and more brittle than aluminum oxide. Because of its friability and brittleness, silicon carbide wears out more quickly than aluminum oxide when sanding soft materials. When sanding harder materials, aluminum oxide wears out more quickly because it's softer than silicon carbide.
Each type of grit can work on different materials, but if you use the wrong type of grit for the material being sanded, the disc wears and goes dull more quickly. Use silicon carbide for sanding metals, plastics and other hard substances like fiberglass. Use aluminum oxide on softer substances like wood, soft plastics and soft metals like pewter.