Silicon carbide is a colorless crystal, commonly known as carborundum. It is industrially produced by reacting quartz sand with excess coke at a high temperature of 2000-2500°C. It has a diamond-like structure and a hardness close to diamond. It is a widely used industrial abrasive. If it is doped with some impurity elements, it will also become a semiconductor, which can be used as a high-temperature semiconductor for electric heating elements. It has good thermal and chemical stability, high mechanical strength and low thermal expansion coefficient, and is also widely used in high-temperature structural ceramic materials. High temperature pyrolysis of silane containing organic groups can produce silicon carbide ceramic fibers with good mechanical properties.
Silicon nitride is a new type of ceramic material. It is an off-white solid. Like silicon carbide, it has high thermal and chemical stability. The friction coefficient is only 0.1 to 0.2, which is equivalent to a lubricated metal surface. The hardness is second only to For diamond and cubic boron nitride. Its biggest application field is also as a high-temperature structural ceramic material. A small amount of aluminum oxide is added to silicon nitride to make a new type of ceramic, its trade name is sialon. Sailong ceramics have been widely used in gas turbine blades, machine casings, engine parts, rocket tail nozzles, bearings and cutting tools.
1. Silicon carbide
Silicon Carbide Silicon Carbide is green, black and yellow due to impurities. Industrially, it is divided into two types: green silicon carbide and black silicon carbide according to their color. The SiC content is ≥99% and 98.5% respectively. Silicon carbide is a kind of covalent bond compound, with strong bonding force between atoms, high melting point, high hardness, high strength and low expansion, high thermal conductivity, high conductivity, strong chemical stability, so it is a good raw material for refractory materials . However, in an oxidizing atmosphere, silicon carbide is easily oxidized, and only when the SiO2 protective film is formed, can the oxidation be slowed down.
2. Silicon nitride
Silicon nitride is gray, the content of Si3N4 is 98.5%, and the free silicon is 1.5%. Its melting point is 1900°C (the molecular formula of silicon nitride is Si3N4, and the crystal types are α-type and β-type, replacing the hexagonal crystal system. Sublimation decomposition), high strength, low thermal expansion, degradation, and strong corrosion resistance .
