What Silicon Carbide Is
Silicon carbide is a compound of silicon and carbon with the formula SiC. It crystallises in a very hard, covalently bonded structure, so it resists abrasion and keeps its strength at high temperature. The material is brittle rather than ductile, it conducts heat well and it has a low coefficient of thermal expansion, which together give it good resistance to thermal shock in furnace service.
Manufacture of Silicon Carbide
Industrial silicon carbide is made in an electric resistance furnace. A mixture of high purity quartz sand and petroleum coke, with a small amount of salt or sawdust to control the reaction, is heated to above 2200 degrees Celsius. The charge reacts to form a cylinder of crystalline silicon carbide surrounded by partly reacted material. The ingot is then crushed, milled and graded, and the purity of each fraction depends on how far it came from the core of the ingot.
Grades and Purity
Silicon carbide is graded by silicon carbide content, crystal type and grain size. Black silicon carbide is the general purpose abrasive and refractory grade, while green silicon carbide is produced from a purer charge and gives a sharper, harder grain for precision finishing. Metallurgical grades are supplied at defined levels of silicon carbide content, and refractory producers buy graded powders and fines for castables, bricks and monolithic linings. GB/T 2480 covers silicon carbide for abrasive use, and grit sizes are commonly quoted to FEPA or to similar national sieve series.
Refractory Applications
Silicon carbide is a key raw material for high duty refractories because it combines refractoriness with abrasion resistance and thermal shock resistance. It is used in blast furnace linings and hearths, in iron and steel ladles, in ceramic kiln furniture and saggars, in incinerator linings and in parts exposed to rapid temperature cycling. It is applied as a bonded brick, as an aggregate in castables and as a coating or ramming mix, and it is often combined with oxide or nitride bonds to control oxidation at high temperature.
Abrasive and Metallurgical Uses
As an abrasive, silicon carbide is formed into grinding wheels, segments, coated papers and blasting media for grinding, cutting and finishing cast iron, cemented carbide, stone, glass and ceramics. In metallurgy it serves as a deoxidiser and as a carrier of silicon and carbon in foundry and steelmaking practice, and because it supplies both elements it can replace part of a separate ferrosilicon and carbon addition. It is also used in wear resistant ceramics, in heating elements and, at high purity levels, in electrical and semiconductor applications.
Handling and Selection
Selection should begin with the duty of the part, the operating temperature and the furnace atmosphere. Oxidation resistance is better in oxidising conditions than in strongly reducing ones at very high temperature, so the bond system and the silicon carbide content must be matched to the environment. Finer grades give higher strength in the installed lining but more shrinkage, while coarser grades improve thermal shock resistance. Moisture content and dust should be controlled during storage and mixing.
Frequently Asked Questions
Q: What is silicon carbide used for?
It is used as a refractory raw material, as an abrasive and as a metallurgical additive that supplies silicon and carbon to steel and foundry melts.
Q: What is the difference between black and green silicon carbide?
Black silicon carbide is the general purpose grade, while green silicon carbide is made from a purer charge and gives a sharper, harder grain for precision work.
Q: Is silicon carbide a refractory material?
Yes, it is used in bricks, castables and monolithic linings where abrasion, thermal shock and high temperature are combined.
Q: How is silicon carbide graded?
It is graded by silicon carbide content, by crystal type and by grain size, and the grade is chosen from the duty of the application.
Q: What should be checked before purchase?
Confirm the silicon carbide content, the grain size, the moisture level and the certificate of analysis for the lot.
