The Name and the Chemical Identity
The letters SiC are the chemical symbols for silicon and carbon, so SiC is silicon carbide, a covalent compound of the two elements in a one to one ratio and with a molar mass of about 40 grams per mole. It occurs naturally only rarely, as the mineral moissanite, and essentially all commercial material is synthetic. Silicon carbide is extremely hard, with a Mohs hardness of about 9 to 9.5 and a Knoop hardness near 2,500, and it retains strength at high temperature and resists oxidation because a protective silica layer forms on its surface. Its theoretical density is about 3.21 grams per cubic centimetre, and it is electrically semiconducting in the pure form, which is the basis of its use in power electronics.
How Silicon Carbide Is Produced
Bulk silicon carbide is made in an Acheson furnace, where a mixture of high purity quartz sand and carbon, usually petroleum coke, is heated by passing current through a graphite core until the charge reaches the reaction temperature above 2,000 degrees Celsius. The product forms as a large ingot that is crushed, milled, screened and graded, and the process is energy intensive, which is why silicon carbide production is concentrated where power is cheap. Fine powders for ceramics and for semiconductor work are made by other routes, including the reaction of silicon with carbon and vapour phase processes that give very high purity material with controlled particle size.
Black and Green Abrasive Grades
The two familiar abrasive grades are black silicon carbide, made from quartz sand and petroleum coke and often containing a lower silicon carbide content that suits general purpose work, and green silicon carbide, made from a purer charge that produces a higher silicon carbide content and a sharper, more friable grain. Black grain is used for cast iron, stone, glass and non ferrous metals in blasting, grinding wheels and coated abrasives, while green grain is preferred for cemented carbide and other hard, brittle materials and for work where heat generation must be limited. Both are graded in mesh sizes, with each mesh number defining a particle size band, and the silicon carbide content of the grade determines how much non abrasive mineral is present.
Refractory and Ceramic Forms
Silicon carbide is used in refractories as a raw material for bricks, castables and kiln furniture because of its high thermal conductivity, resistance to thermal shock and chemical stability. Its high thermal conductivity and high temperature strength make it valuable in heat exchangers and in burner components. Bonded and sintered silicon carbide ceramics serve as mechanical seals, pump components, wear plates, nozzles and armour, where hardness and wear resistance are the deciding properties. Reaction bonded material is made by infiltrating a silicon carbide and carbon preform with molten silicon, while sintered material is densified with additives, and the two processes give different porosity, strength and cost profiles.
Electrical and Semiconductor Uses
Silicon carbide is a wide bandgap semiconductor, and its ability to operate at high voltage and high temperature with low switching loss has made it important in power electronics. Devices are built on monocrystalline wafers grown from high purity powder by physical vapour transport, and the wafers must meet very tight limits on micropipe defects, dislocations and dopant concentration. The same high purity powder is used for semiconductor grade ceramics and for the heating elements found in high temperature furnaces. In all of these uses the purity requirement is far beyond that of abrasive grade material, which is why the price per kilogram differs by orders of magnitude between blasting grit and electronic grade wafers.
Frequently Asked Questions
Q: What does SiC stand for?
SiC stands for silicon carbide, the compound of silicon and carbon, and the letters are simply the chemical symbols of the two elements in a one to one ratio.
Q: What is silicon carbide used for?
It is used as an abrasive for grinding and blasting, as a refractory raw material, as a structural ceramic for seals and wear parts, and as a wide bandgap semiconductor in power electronics.
Q: What is the difference between black and green silicon carbide?
Black grain is made from a less pure charge and suits general abrasive work on cast iron, stone and non ferrous metals, while green grain is purer, sharper and preferred for hard materials such as cemented carbide.
Q: How hard is silicon carbide?
It has a Mohs hardness of about 9 to 9.5 and a Knoop hardness near 2,500, which places it among the hardest bulk industrial materials after diamond and cubic boron nitride.
Q: Is silicon carbide natural or synthetic?
Nearly all commercial silicon carbide is synthetic, produced in an Acheson furnace from quartz sand and carbon, although a rare natural form exists as the mineral moissanite.
Q: Why is electronic grade silicon carbide so expensive?
Because semiconductor wafers must be grown from very high purity powder and meet extremely tight limits on crystal defects and dopants, far beyond the requirements of abrasive or refractory grade material.