What are the characteristics of silicon carbide and what should be noted during its use?

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Silicon carbide is a new type of ceramic and, to date, the best material available for use as a friction pair, possessing various excellent properties. For example, physical properties, thermal properties, and chemical properties – even radiation resistance is no exception. In terms of adhesion properties, silicon carbide forms a protective oxide film that can be wetted by most liquids. The thermal conductivity of silicon carbide is 10 times that of alumina. These properties, combined with its super hardness, give it and the grinding material a very long service life. It has excellent corrosion resistance; in strongly oxidizing media, it begins to react only at high temperatures of 500–600°C, and it is resistant to almost all acids and bases. Manufacturing methods can be divided into hot-press sintered SiC, atmospheric-pressure sintered SiC, reaction-sintered SiC, chemical vapor reaction SiC (also known as graphitized silicon), and chemical vapor deposition SiC. The first three are bulk silicon carbide, while the latter two are SiC coatings formed on a graphite surface. The properties vary depending on the manufacturing method: SiC synthesized by normal-pressure sintering and hot-press sintering achieves a density of over 97%; SiC produced by reaction sintering contains a certain amount of free silicon and pores; whereas SiC obtained through chemical vapor deposition and chemical vapor reaction has a SiC surface layer with a graphite matrix. Their common feature is that the end faces of the friction pairs are all made of dense and extremely hard SiC. Silicon carbide sintered by hot pressing has the best comprehensive performance indicators, but its price is also high. Although pressure-free sintered silicon carbide has slightly inferior performance compared to heat-pressed silicon carbide, its free silicon content is extremely low, it resists corrosion by acids and alkalis, and its price is much lower than that of heat-pressed silicon carbide; it is therefore widely used abroad. In China, reaction-sintered silicon carbide is mostly used; it is cheaper than pressure-sintered silicon carbide, and it can also be mechanically processed, allowing for more complex product shapes. But its corrosion resistance is slightly lower. Their performance is shown in Tables 19 and 20. Points to note when using SiC: Select different types of SiC depending on the application conditions. For large-volume products, normal-pressure sintered or reaction-sintered SiC is suitable; for products with complex shapes, normal-pressure sintered SiC, which is easy to shape and relatively inexpensive, should be used. If the medium is a strong oxidizing agent, either normal-pressure sintered or hot-press sintered SiC is appropriate; hot-press sintered SiC is used at high pv values. If the medium contains solid particles, SiC paired with itself or SiC paired with cemented carbide should be chosen. SiC is a brittle material with poor strength and impact toughness. When designing sealing rings, it is necessary to consider that the SiC ring should have a simple shape, with no grooves or holes made in it in order to avoid and reduce stress concentration. During grinding, a high rotational speed should be used, and the feed rate should not be too high to prevent chipping or corner damage. Hitting the ring is strictly prohibited during assembly, and shock-absorbing measures should be employed for packaging. In addition, careful installation and use are also required.