Silicon Carbide – SiC-1

From a technical point of view, silicon carbide (SiC) is certainly the most important non-oxidic ceramic material. It generally excels on account of its high hardness and good wear resistance, high strength even at high temperatures and good thermal spalling resistance combined with low thermal expansion, but high thermal conductivity. However, the high covalent bond, which is responsible for this, also impedes the production of SiC as its tendency towards sintering is distinctly lower in comparison with oxide ceramics. This means that a great number of different techniques is used for manufacturing SiC ceramic products, differing not only in method control but also with regard to the properties of the SiC produced. The forming operation may utilize all ceramic methods known; slip and die casting, axial and isostatic pressing, injection moulding, extrusion moulding, film casting and other methods are used. Differences arise in the final compaction, which may be achieved in greatly varying ways. The following types can be distinguished

Reaction-bonded silicon-infiltrated silicon carbide (SiSiC) In case of SiSiC, densification is not achieved through sintering a SiC green compact, but through impregnating a porous skeleton consisting of SiC and free carbon with molten silicon. This involves the conversion of carbon and part of the silicon into SiC, thus providing a bond matrix; the rest of the silicon will remain in unbonded form. SiSiC accordingly contains about 5 to 15% of free metallic silicon in its final state. Natural raw materials, such as wood or paper, may also be used as a matrix, which is initially pyrolized to form carbon and is then converted with the molten silicon. A great advantage of SiSiC is the lack of contraction during densification. On account of this dimensional stability can be ensured and large-scale components and complex geometries can be produced. Moreover, the material is completely sealed and, thus, also quite resistant to corrosive attacks. Its thermal shock resistance is excellent due to its very high thermal conductivity. Due to the free silicon, however, the operation temperature is limited to about 1380°C (2516°F).