What is SiC(Silicon Carbide)?

It is a heat-resistant and high-strength material with various features including hardness,
good heat resistance, and good heat conductance.

Silicon carbide (SiC) is a covalent compound rarely found in nature, wherein carbon (C) and silicon (Si) are bound to each other on a one-to-one basis.
SiC has high hardness, good heat resistance, and good heat conductance and is therefore used as an abrasive and a refractory. Furthermore, it has received a lot of attention recently as a material for semiconductors, etc. because of its exceptional heat resistance at high temperatures.

Characteristics of SiC

High hardness

New Mohs Hardness 13 The third hardest on earth
SiC is the third hardest compound on earth, making it less susceptible to deformation caused by compression. Its new Mohs hardness is 13, with only diamond (new Mohs hardness: 15) and boron carbide (new Mohs hardness: 14) having higher hardness. By comparison, alumina, which is also used for abrasives and fire-resistant materials, is 12.
New Mohs scale of mineral hardness
[New Mohs scale of mineral hardness]
New Mohs hardness Substance
1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase
7 Fused quartz
8 Quartz
9 Topaz
10 Garnet
11 Fused zircon
12 Fused alumina
13 Silicon carbide
14 Boron carbide
15 Diamond
*New Mohs hardness: a number of minerals of known hardness are selected and used to see if they can scratch another mineral of unknown hardness.

Low thermal expansion

thermal expansion of 4.5 x 10-6/°C
SiC is highly resistant to changes in length and volume caused by increases in temperature. Its resistance against thermal shock is extremely high with a lower rate of thermal expansion of 4.5 x 10-6/ °C, compared with metals, coupled with heat conductance. (*2)
Coefficient of thermal expansion
[Coefficient of thermal expansion]
Substance Composition Coefficient of thermal expansion [10-6/°C]
293K(20°C)
Fused silica SiO2 0.5 to 1.4
Diamond C 1.0
Hexagonal boron nitride h-BN 1.5 to 3
Silicon nitride SiN 2 to 3
Zinc oxide ZnO 2.9
Silicon carbide SiC 4.5
Aluminum nitride AlN 5
Crystalline silica SiO2 5
Alumina Al2O3 6 to 9
Titanium oxide TiO2 9
Iron Fe 11.8
Magnesium oxide MgO 11 to 15
Copper Cu 16.5
Silver Ag 18.9
Aluminum Al 23.1
Magnesium Mg 24.8
Zinc Zn 30.2

Good oxidation resistance

Oxidization start temperature at 700 degrees or more. *In the case of granular silicon carbide
Granulated SiC starts to oxidize at 700 °C or more, a temperature which is dependent upon whether it is a sintered object or powder, and if in powder form, its size.
The oxidative product generated by the reaction between SiC and oxygen is SiO2 (silicon dioxide), which is dense enough to protect SiC from oxidation by covering the entire surface of SiC such that oxidation is restricted (protective oxidation in high partial pressure of oxygen).
At a high temperature and low partial pressure of oxygen, SiC changes into SiO and evaporates, decreasing in amount but not generating SiO2. (*1)

Outstanding heat resistance

Decomposition temperature: 2,545°C
Silicon carbide (SiC) is highly resistant to heat. It is stable up to temperatures near 1,600 °C in air, and its decomposition temperature is 2,545 °C. Particularly notable is its exceptional resistance to high temperatures. (*1)
Refractoriness of SiC and other ceramics
[Refractoriness of SiC and other ceramics]
Name Refractoriness
Crystalline alumina › SK40 mp. 2,050 °C
Fused alumina › SK40 mp. 2,050 °C
Fused silica › SK32 mp. 1,720 °C
Silicon carbide › SK40 Decomposition temperature 2,545 °C
Silicon nitride SK38 Decomposition temperature 1,850 °C
*SK number: a number assigned to indicate the temperature at which pyrometric cones soften and bend. SK32 is 1,710 °C; SK38 is 1,850 °C; and SK40 is 1,920 °C.

Good heat conductance

thermal conductivity of 270 W/m·K *Sintered bodies
SiC is a good conductor of heat. Sintered bodies have a thermal conductivity of 270 W/m·K, which is comparable to the metalloids. (*1)
Thermal conductivity of silicon carbide and other ceramics & metals
[Thermal conductivity of silicon carbide and other ceramics & metals (W/(m·k))]
Thermal conductivity of silicon carbide and other ceramics & metals

Note: numbers in parentheses are thermal conductivity values.
See thermal conductivity values, document values.

Semiconductor properties

Electric resistance: Heating element ⇔ Insulation
SiC is a semiconductor with an electric resistance that undergoes changes greater than 10 figures, ranging from a zone of resistance at which it can be used as a heating element to a zone of near insulation. Further, research and development is actively proceeding on a power device such as a high-temperature semiconductor which can be used up to approximately 500 °C due to its high electric field for insulation breakdown and high heat conductance with a wide bandgap (two or three times wider than Si). (*1)

Good chemical resistance

Chemically stable
Silicon carbide (SiC) is chemically stable, and resists corrosion by acids and bases. It is unaffected by acid mixtures of heated hydrofluoric acid plus nitric acid, nor by concentrated sodium hydroxide. It can, however, be broken down by a melting method using sodium carbonate, or by a high-pressure acid decomposition method using a mixture of hydrofluoric acid, nitric acid and sulfuric acid.
Bibliography:
*1 "New ceramic material of SiC series" (Editor: Japan Society for the Promotion of Science) issued in 2001
*2 "Practical Refractory Course", the 47th issue of Refractory in 1995 (p40 to 53)
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