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Silicon carbide

The all-rounder of technical ceramics

Silicon carbide, a real all-rounder in engineering ceramics

Silicon carbide (SiC) is a high-performance ceramic material made of high-quality, non-oxide powders and can be manufactured to the specific requirements of a wide range of applications. Due to its excellent tribological properties in combination with low weight, universal chemical resistance and ready availability, SiC is a frequently used engineering ceramic with a highly diverse range of applications.

In mechanical engineering, as well as in the chemical industry and in plant and process engineering, SiC has gained great prominence in particular as a corrosion- and wear-resistant material in pumps, mechanical seals and bearings. The material is also used as an abrasive (carborundum) for blasting nozzles, pipes and guide and deflecting elements.

The capabilities of silicon carbide

The most noteworthy properties of silicon carbide are its good resistance to corrosion and excellent resistance to high temperatures and thermal shock. A high modulus of elasticity furthermore results in excellent dimensional stability. Additionally, there is the zero porosity and low pore density of the ceramic material. Due to its resistance to almost all chemicals as well as water/steam at several hundred degrees Celsius, SiC is superior to other ceramic and metallic materials. These properties make SiC ideal for use as a structural material in gas sealing rings, mechanical seals and bearings in aggressive and high-temperature media, even during temporary dry-running conditions or in environments where the media is the only lubrication.

Components of this kind that are made of silicon carbide (SSiC/SiSiC) can be found in, for example:

  • Hermetically sealed pumps (e.g. centrifugal and gear pumps)
  • Sealing and regulating disks
  • Valves
  • Sealing technology in automotive engineering
  • Switching valves in air conditioning and refrigeration systems (leak-free ceramic regulating disks and valves made of SiC show practically no wear and have good sliding behaviour)
  • Agitators and grinders
  • Expanders and extruders

Even canned motor pumps and magnetically coupled pumps benefit from using ceramic components made from optimised SiC bearing materials. These have proven to be effective in hot water circuits, support bearings for stirrer tanks and submersible pumps, as well as magnetic drives for agitators.

All of this is possible thanks to a special tribological optimisation of the sliding surface in SiC mechanical seals: microstructures introduced into the running surface create a thin “separation pad” that allows contact-free, smooth rotation. The microscopic surface structure is akin to a shark skin, which results in significantly reduced flow resistance. The sliding behaviour is comparable to a water ski, where good sliding is guaranteed even at low speeds. This requires the cavities to be of a micro-wedge shape of the necessary accuracy (e.g. flatness 0.6 μm, surface quality Ra 0.2 μm), which is achieved by special laser-structured processing techniques – a neodymium laser with a wavelength of 1064 nm and targeted laser pulses in the nanosecond range are used.

Due to their extremely low-wear property profile, mechanical seals and bearings made of silicon carbide are particularly suitable for media that are subjected to high levels of contamination, abrasion and/or corrosion – such as in the petroleum and gas processing industry for pumping out highly abrasive and corrosive oil sludge mixtures from the drill hole, for tank draining pumps or high-speed chemical pumps. Here, SiC mechanical bearings and seals are known for their impressively long service life.

The diverse applications of silicon carbide

  • Blasting nozzles made of silicon carbide (SiC) are characterised by constant operating conditions and uniform blasting power as well as low maintenance intervals.
  • With its good resistance to oxidation and corrosion as well as resistance to temperature changes, this ceramic is also used as a component for refractory applications: burner nozzles, jet and flame tubes (e.g. silicon carbide thermocouple protection tubes made of SSiC) are used in corrosive and abrasive conditions under very high temperatures and at high flow rates. They’re therefore used in combustion rooms under extreme conditions or in flue gas desulphurisation plants.
  • High-performance SiC ceramic is also used in the manufacture of pipes.
  • In the chemical industry and process engineering, its mostly only welded heat exchangers made of high-alloy nickel or tantalum that are used for heat recovery from, for example, highly concentrated sulphuric acid or hot sodium hydroxide solutions. At the same time, heat exchanger plates made of SiC ceramic ensure a longer service life. In this scenario, the advantage is that silicon carbide is resistant to almost all acid and alkali mixtures – even at high process temperatures. That’s why SiC components are also suitable for separating corrosive vapours from carrier gases or for condensing corrosive vapours – in general it can be used with all media that’s sensitive to contact with metallic surfaces.
  • Their long filter life makes ceramic sand filter systems based on SiC ideal for use in oil and gas production. They’re extremely durable and highly insensitive to erosion, corrosion, acids, high temperatures and various borehole fluids.
  • Due to its high melting point and high degree of hardness, silicon carbide is also used as an abrasive (carborundum or carborundum). Its exceptional hardness also ensures that SiC plays an important role as an abrasive for lenses and mirrors in the optics sector.
  • High hardness, high modulus of elasticity and high compressive strength are required for hard armour ballistic protection. In this area of use, black-grey SiC ceramic manages to reliably stop a projectile during the penetration process. The remaining energy is then absorbed by a soft elastic polymer matrix. Thanks to silicon carbide, ballistic protection is achieved at a significantly lower product weight than armoured steel or aluminium oxide. This is particularly important for vehicle protection, where lightweight silicon carbide components have a positive effect on fuel consumption, range and operating costs.
  • SiC is used together with other materials as a hard concrete aggregate, for example to increase the abrasion resistance of industrial floors.
  • SiC is used in metallurgy to alloy cast iron with carbon and silicon.
  • SiC also acts as a fuel element insulation material in high-temperature reactors.
  • Ceramic guiding and deflecting elements are used in textile machine construction. In this application, ceramic yarn guides made of SiC can easily increase processing speeds to 8000 m/s. This ensures high wear resistance and a long service life. On fishing rods, rings made of SiC avoid the ring being cut when subjected to heavy loads.
  • Diesel particle filters filter outgoing soot particles through a porous ceramic filter wall made of SiC. Yet it works equally well in hot, dust-laden exhaust gases from metallurgy.
  • Due to its high rigidity, low weight and low thermal expansion, SiC is also used as the basis for space telescope mirrors. The 3.5 m-diameter mirror of the Herschel space telescope weighed only 350 kg, compared to a weight of 1.5 t when manufactured using standard technologies.
  • Semiconductor material: Compared to conventional Si power semiconductors, SiC power modules of the same size offer drastically higher switching capacities with lower switching losses. This results in power amplifiers with fewer components and smaller heat sinks. That’s why SiC is used not only as a semiconductor material for varistors, but also for very fast Schottky diodes, blue-light-emitting diodes and junction FETs. Due to the excellent thermal conductivity of SiC as a substrate, semiconductor circuits made of SiC allow temperatures of up to 600 °C / 1112 °F.

Production and forms of silicon carbide

Silicon carbide can be produced using chemical vapour deposition (CVD), among other things. The starting material consists mostly of carbosilanes.

SiC is a silicon-carbon compound from the carbide family. In its purest state, it consists of hexagonal/rhombohedral, mostly flaky crystal platelets (technical SiC in a grey to black colour). Silicon carbide comes in different forms:

  • Sintered silicon carbide (SSiC)
  • Silicon infiltrated silicon carbide (SiSiC)
  • Liquid-phase sintered silicon carbide (LPSSiC)
  • Hot pressed silicon carbide (HPSiC)

The variants SSiC (sintered silicon carbide) and SiSiC (silicon infiltrated silicon carbide) have become established in industrial industries. The latter is particularly suitable for the production of complex, large-volume components.

Properties

Properties of silicon carbide

 SymbolType 
Unit
LPSSiC
Liquid phase sintered silicon carbide
SSiC
Pressureless sintered silicon carbide
SiSiC
Silicon infiltrated silicon carbide
Mechanical:     

SiC content

 [%]>94>98,5>80

Open porosity

 [Vol.-%]<100,0

Density, min.

ρ

[g/cm³]3,2-3,243,08-3,153,05-3,12

Flexural strength (4-point)

σ[MPa]600260-500180-450

Young´s Modulus

E[GPa]420350-450270-400

Hardness

HV10[GPa]2223-2614-25

Fracture toughness

KIC[MPa√m]6,63,0-4,83-5
Electrical:     

Specific resistance at 20 °C

ρv>20[Ωm]103-104103-104101-103

Specific resistance at 600 °C

ρv>600[Ωm]102102103
Thermal:     

Average coefficient of linear expansion at 30-1,000 °C

α30-1.000[10-6K-1]4,14,0-4,84,0-4,8

Spec. heat capacity at 30-1,000 °C

Cp.30-1.000[Jkg-1K-1]600600-1.00650-1.300
Thermal conductivityλ30-100[Wm1K1]10040-120100-160
Resistance to temperature changes ratedgoodgoodtvery good
Max. operating temperatureT[°C]1.200-1.4001.400-1.7501.380

The maximum operating temperature of sintered silicon carbide is 1,800 °C / 3,272 °F (under protective gas). It has very good resistance to temperature changes (SiSiC), good sliding properties, low thermal expansion and is corrosion- and wear-resistant, even at high temperatures. SiC is resistant to almost all organic and inorganic chemicals such as phosphoric, sulphuric, nitric and hydrochloric acid.

From 3M™ Ceramics Experts: Non-binding advice for your company

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