Crystalline Silicon Monocarbide, SiC

The discovery of Crystalline Silicon Monocarbide (Carborundum), SiC, by Acheson in 1891 was an outcome of the experiments of Moissan on the electric furnace. It may be prepared by fusing silicon with carbon, or silica with calcium carbide, in an electric furnace, or by the combination of carbon and silicon vapours produced therein. It is manufactured, however, by heating together in an electric furnace, built of firebrick and provided with carbon terminals, a mixture of coke, silica, salt, and sawdust. The water-power of Niagara furnishes the energy. The reactions are:

SiO₂ + 2C = Si + 2CO; Si + C = SiC.

The salt acts as a flux till it is volatilised, and the sawdust yields gaseous decomposition products which keep the mass porous. When the furnace is opened the carborundum is found surrounded with a zone of the less valuable amorphous monocarbide, formed at a lower temperature.

Carborundum crystallises in hexagonal plates of density 3.12. The crystals are lustrous, iridescent, and often translucent, and of a colour varying from sapphire blue to greenish brown. The hardness of this substance approaches that of diamond, being 9.5 on Moh's scale; so that it will scratch ruby. The heats of formation and combustion are respectively

Si (cryst.) + C (amorph.) = SiC + 2000 calories
SiC + 2O₂ = SiO₂ + CO₂ + 238,800 calories.

Carborundum is practically unaltered by ignition at 1000° C. in air or oxygen. Neither hydrochloric nor sulphuric acid, nor even a mixture of hydrofluoric and nitric acids, attacks this substance; sulphur and fused nitre are without action upon it; chlorine decomposes it, slightly at 600° C., completely at 1200° C.; fused lead chromate oxidises it, and fused caustic alkali converts it into carbonate and silicate. Carborundum is used, on account of its excessive hardness, as an abrading, cutting, and polishing agent. It is employed in the iron and steel and other metal industries, and for smoothing pottery and polishing granite and glass. It is thus a substitute for emery.