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Silicon Carbide – SiC

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).

Silicon carbide sintered without pressure (SSiC) In case of SSiC, densification is achieved through pressureless solid phase sintering at high temperatures between 1900°C (3452°F) and 2300°C (4172°F) in a vacuum or in an argon, nitrogen or similar atmosphere. As the sintering activity of SiC is comparatively low, fine-grained powders having a grain size of normally less than one micrometer are used for densification. In addition, small amounts of free carbon and small amounts of a boron compound (mostly boron carbide) have to be used as additives. SSiC ceramic products have a maximum density of 97%. The high strength of SSiC remains almost unchanged up to about 1600°C (about 2900°F). Corrosion resistance is also very good; however, it varies depending on grain size.

Hot-pressed silicon carbide (HPSiC) / hot isostatically pressed SiC (HIPSiC) This method involves densification through axial (HPSiC) or isostatic (HIPSiC) pressing of SiC powder with simultaneous delivery of thermal energy. The densification rate is higher than with pressureless sintering at the same temperature, resulting in altogether higher densities. With optimised process control, complete densification can be achieved. This leads to even better mechanical properties than for SSiC; durability is also increased once again. The method is, however, limited to relatively simple shapes and also very costly so that HPSiC and HIPSiC are selected only for application under extreme conditions.

Recrystallized Silicon Carbide (RSiC) Unlike the types of SiC mentioned before, the aim here is not to achieve a high final density. The porosity of sintered RSiC is usually about 10 to 20%. For manufacture, a compact green having a bimodal grain size distribution is used in which SiC particles of about 100µm size with fine particles having a diameter of some micrometers are present in a homogeneous mixture. During sintering at a temperature of between 2200°C (3992°F) and 2500°C (4532°F), coarse grains grow at the expense of fine particles, resulting in material densification. Sintering contraction will not occur in this process so that big and complex geometries can be produced from RSiC. Strengths, however, are considerably lower than those of dense variants. However, its extraordinary thermal spalling resistance forms an advantage.

Liquid-Phase Sintered Silicon Carbide (LPSiC) LPSiC is made from a mixture of SiC and one or more oxidic powders. In pressure sintering, a liquid mixed phase is formed which wets particles and ensures densification. LPSiC is generally non-porous and shows considerable fracture toughness besides high strength. However, its production is costly due to the manufacturing process required.

Besides that, there are further variants where bonding in ceramics will not occur through SiC either, but through an additional phase, such as SiC bonded to nitride or silicate.

Typical applications of SiC ceramics comprise especially components subjected to tribological heavy loads, such as friction bearings, pumps, collars or gears, and components under high thermal loads, such as kiln furniture and burner components and those used in semiconductor technology, just to mention some of the most important fields of application.

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