One compound, two careers
Silicon carbide, formula SiC, is a compound of silicon and carbon. Few materials have such separate working lives. In one, it is a loose or bonded grit that grinds and cuts other materials. In the other, it is a dense, solid engineering ceramic used for parts that must stay stiff and stable at high temperature. Both roles come from the same underlying features: strong covalent bonding, high hardness and good thermal stability.
Structure and properties
In silicon carbide, each silicon atom bonds to four carbon atoms, and each carbon atom bonds to four silicon atoms, in an arrangement resembling diamond’s. This gives it a high hardness, though well below diamond’s, along with good stiffness, strong thermal conductivity for a ceramic, a low thermal expansion compared with many metals and strong resistance to chemical attack.
The compound exists in many crystal forms called polytypes, which differ in how the atomic layers are stacked. The two most often mentioned are the cubic form and a set of hexagonal forms. For most industrial uses the polytype matters less than the grain size, purity and density of the finished material.
Silicon carbide as an abrasive
Silicon carbide was among the first manufactured abrasives, developed in the late nineteenth century. It is typically made in large electric resistance furnaces from silica sand and a carbon source such as petroleum coke, which react at very high temperature. The resulting crystalline mass is crushed and sorted. Industry commonly talks about black and green silicon carbide, with green being the purer and somewhat harder, more brittle form.
Its abrasive uses include:
- Coated abrasives such as sandpaper and abrasive belts.
- Bonded grinding wheels for non-ferrous metals, cast iron, stone, glass and ceramics.
- Loose grit for lapping and for wire sawing of materials such as silicon.
- Blasting media and polishing compounds.
Silicon carbide is a good fit for materials that are hard and brittle or low in tensile strength. It is not usually chosen for steel, because at grinding temperatures it tends to react with iron and wear rapidly. That limitation is one reason cubic boron nitride exists for steel grinding. Compared with the superabrasives, it is much cheaper, and it fills the middle ground between conventional alumina and diamond. The grit is sorted by size through grit grading, and held by bond types that suit the work.
Silicon carbide as a structural ceramic
When silicon carbide powder is formed and densified rather than crushed as grit, it becomes a structural ceramic. Dense parts can be made through several routes:
- Sintered silicon carbide. Fine powder with small additions is pressed and sintered at very high temperature. The route gives dense, pure material with good corrosion resistance.
- Reaction-bonded silicon carbide. A porous carbon and silicon carbide shape is infiltrated with molten silicon, which reacts to form more silicon carbide. It leaves some free silicon in the structure and allows large and complex shapes with limited shrinkage.
- Hot-pressed and other pressure-assisted types. These can reach high density but tend to be limited to simpler forms.
Because the compound resists diffusion, densification is difficult, and understanding the sintering process is central to making good parts. Strong covalent bonding is the reason.
Where structural silicon carbide is used
- Mechanical seals and bearings. Its hardness and chemical resistance make it a common face material in pump seals that handle abrasive or corrosive fluids.
- Kiln furniture and furnace parts. Setters, beams, rollers and tubes keep their shape at high temperatures, a good match for refractory materials and other high-temperature materials.
- Heat exchangers. Corrosion resistance and good thermal conductivity make it useful for tubes handling aggressive fluids.
- Semiconductor process equipment. High-purity or coated silicon carbide appears in fixtures that hold wafers during hot processing.
- Armor and wear liners. It is also used in ceramic armor and in lining equipment that handles abrasive slurries.
- Brake discs and mirrors. Specialty versions are used in certain high-performance braking systems and in lightweight, stiff optical structures.
Silicon carbide is also a semiconductor, and single-crystal wafers are used in power electronics. That is a separate subject from the ceramic and abrasive uses described here.
Strengths and limits
Silicon carbide handles heat well and tolerates thermal shock better than many ceramics, because of its high thermal conductivity and low expansion. It resists most acids and many molten metals. It is brittle, so its fracture toughness is modest, and the usual care applies to design: avoid sharp corners and impact loads and keep the part in compression where possible. In very hot, oxidizing conditions it forms a thin protective layer of silica, which protects it up to a point, but extended exposure to steam or certain molten salts can damage it.
Compared with neighbors
Silicon carbide is often compared with silicon nitride, which is tougher and favored for rolling bearings, and with alumina ceramics, which are cheaper but less thermally conductive. Boron carbide is harder and lighter but more expensive. Selection among these technical ceramics comes down to temperature, chemistry, loading and cost.