Apart from its main uses in metallurgy, abrasives and refractories, silicon carbide is also used in structural applications where high temperature strength or high stiffness to weight ratios are required.
The demand for more energy efficient engines and the need to replace critical metals drove the development of a number of ceramic materials and processes, particularly for high temperature gas turbine engines. Reaction bonded silicon carbide resulted from these investigations.
Processing
Although silicon carbide can be densified with high temperature and pressure, the process is not a viable commercial process.
Reaction bonded silicon carbide is made by infiltrating compacts made of mixtures of SiC and carbon with liquid silicon. The silicon reacts with the carbon forming silicon carbide. The reaction product bonds the silicon carbide particles. Any excess silicon fills the remaining pores in the body and produces a dense SiC-Si composite.
The ratio of SiC to carbon and particle size distribution varies widely in practice. Articles are produced with a wide range of compositions and properties. At one extreme, carbon fibre felt or cloth can be infiltrated with liquid silicon, whilst at the other extreme, an impervious silicon carbide body can be made with a small amount of carbon.
Most reaction bonded silicon carbide is made with formulations that contain an organic plasticiser, carbon and silicon carbide particles. This mixture is ideally suited to near net formation by pressing, injection moulding or extruding. Further, since the reaction process typically gives a dimensional change of <1%, manufacturers have excellent control of component tolerances.
Key Properties
The properties of silicon carbide components depend on the material grade. In the case of a fully dense SiC-Si composite, the material demonstrates good bend strength at room temperature (typically 400 MPa), which is maintained to the melting point of silicon (1410 ºC) where it decreases to around 250 MPa. Young’s modulus is typically in the range 350 - 400 GPa.
The properties that lead to selection of the material are:
- Resistance to wear
- Resistance to corrosion; the material tolerates a wide range of acids and alkalis
- Resistance to oxidation
- Abrasion resistance
- Good thermal shock resistance due to low thermal expansion coefficient and high thermal conductivity
- Strength at high temperature
- Good dimensional control of complex shapes
Table 1. Typical properties of a two phase reaction bonded silicon carbide (90%SiC, 10% Si)
| Property |
Value |
| Density (kg.m-3) |
3040 |
| Apparent Porosity (%) |
0 |
| Young’s Modulus (GPa) |
400 |
| Bend Strength (MPa) |
390 |
| Hardness (VHN) |
2500 |
| Thermal Expansion Coefficient (x 10-6/ºC) |
4.3 |
| Thermal Conductivity (W/m.K) |
145 |
| Maximum Use Temperature (ºC) |
1375 |
Applications
Kiln Furniture And Support Components
The high temperature strength, oxidation resistance and thermal shock resistance of RBSC enable manufacturers to produce low mass kiln supports compared to conventional kiln furniture materials such as cordierite. Kiln products include thin walled beams, posts, setters, burner nozzles and rolls. The components lower the thermal mass of kiln cars, result in energy savings and provide the possibility for faster product throughput. However, there a furnace support made from RBSC would be typically about four times more expensive than cordierite.
Wear Parts And Thrust Bearings
Good wear resistance, high temperature strength and corrosion resistance make RBSC an ideal material for wear components, such as screws, plates and impellers. It can also be used in thrust bearings that can carry extremely high loads in heavily contaminated liquids. The properties enable the production of compact bearings capable of operating in the temperature range -200°C and 400°C.
Mechanical Seals And Vanes
RBSC has been successfully used in mechanical seals and pump vanes with high abrasion resistance. Careful control of the free graphite level within the final body determines the material properties. RBSC with free graphite is used as a seal nose material contacting hardfaces in hostile operating environments.
Precision Components
The negligible volume change after reacting with liquid silicon means that components can be formed with complex shapes and to exacting tolerances. Examples of components made by this route include laser mirror blanks, wafer handling devices and optical benches. The components are lightweight and stiff with excellent thermal stability.