Materials based on silicon carbide (SiC, also known as silicon carbide) have only recently been discovered as construction materials for mechanical engineering and energy technology. Silicon carbide is the lightest (its density is between 2.6 and 3.2 g/cm³ depending on the porosity), but also the hardest ceramic material. It is often and rightly called "almost like a diamond". Its hardness can reach 9.6 according to the Mohs scale (approx. 2600 HV). Silicon carbide-based ceramics have outstanding properties. These include high strength and a high modulus of elasticity of 380 to 430 GPa. They are characterized by a high thermal conductivity of 120 to 200 W/m K (twice as high as steel at room temperature) and a low thermal expansion of 3.6 to 4.1x10-6/K (at 20 to 400°C) and thus good thermal shock resistance. Silicon carbide ceramic can be used up to 1400°C. In addition, this ceramic has very good corrosion resistance and semiconductor properties. High-purity silicon carbide is colorless, while technical grade silicon carbide is often black, due to the use of alumina-containing excipients in its manufacture. Silicon carbide is similar to diamond in structure and properties because silicon and carbon are in the same main group and adjacent periods of the periodic table, and the atomic diameter of silicon is only slightly larger.
Image: Silicon carbide bushes for sheet metal forming
A typical area of application for silicon carbide components is dynamic sealing technology with plain bearings and mechanical seals, for example in pumps and drive systems. Compared to metals, silicon carbide enables very economical solutions with longer service lives, even when used in aggressive and high-temperature media. Silicon carbide ceramics are also ideally suited for demanding operating conditions in ballistics, chemical production, energy technology, paper production and as components for piping systems. Because of its hardness and high melting point, silicon carbide is used as an abrasive for optical mirrors and lenses, and as a component for refractory materials. Less pure silicon carbide is called metallurgical SiC and is used in large quantities to alloy cast iron with silicon and carbon. Mixed with other materials, it is used as a hard concrete aggregate to make industrial floors abrasion-resistant or to make vaults resilient. The rings of high-quality fishing rods are also usually made of silicon carbide. Here, too, the advantage over other materials lies in the hardness, which prevents the fishing line from cutting a notch in the ring under high loads and ultimately breaking due to abrasion. Because of its low thermal expansion, silicon carbide is used in space telescope mirrors. Carbon fiber reinforced SiC ceramic is used to manufacture high performance brake discs. Silicon carbide is toxicologically harmless, which allows it to be used in food applications. SiC ceramics are usually classified according to the microstructure (open-pore or dense) and the type of bonding between the SiC grains (type-specific or non-type-specific bonding). Different manufacturing processes lead to further variants of the material, each with different properties due to different SiC content and porosity. The technically most important types of silicon carbide are:
Silicate-bonded SiC, which is mainly used in refractory technology.
Recrystallized silicon carbide (RSiC), which has very low shrinkage during sintering of components due to its high open porosity.
Nitride-bonded silicon carbide (NSiC) with a binder matrix of silicon nitride around the SiC grains and a fine-pored microstructure.
Pressureless sintered silicon carbide (SSiC), which is sintered from fine SiC powders at high temperatures in protective gas and has a very uniform structure with diamond-like hardness and wear resistance.
Hot-pressed silicon carbide (HPSiC) and hot isostatically pressed silicon carbide (HIPSiC) almost non-porous with the best properties but high shrinkage.
Reaction-bonded, silicon-infiltrated silicon carbide (SiSiC) with virtually no shrinkage but with a lower melting temperature.
Liquid Phase Cemented Silicon Carbide (LPSiC), a relatively new grade with improved fracture toughness (SiC ceramics are typically very brittle).
Silicon carbide is a widely used technical ceramic today due to its hardness and low weight as well as its high availability and large variety of types.
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