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Ceramics have good chemical stability and high wear resistance, and are relatively inexpensive. Due to its sensitivity to shocks, vibrations, and sudden temperature changes, as well as its low thermal conductivity, it can only be used in corrosive media with low PV values. Commonly used ceramics include alumina (Al2O3), silicon nitride (Si3N4), boron carbide (B4C), titanium carbide (TiC), and silicon carbide (SiC), among others. Some are made as a single ceramic ring, while others have a layer of ceramic sprayed on the surface of the sealing ring. Alumina ceramics possess excellent corrosion resistance, capable of withstanding the corrosion caused by various acids, bases, salts, various organic solvents, and strong oxidizing agents. Its performance is related to the content of alumina; a higher alumina content results in better performance and greater wear resistance. The hardness of alumina is second only to that of diamond and boron carbide, reaching RC85~86; its mechanical strength is similar to that of ordinary cast iron, and it is available from a wide range of raw materials with a simple manufacturing process. Due to its brittleness and poor thermal conductivity, it is often used as a spraying material. It should be noted that both the stationary and rotating rings cannot be made of alumina, as there is a risk of generating static electricity. Silicon nitride is a newly developed material for sealing rings; in addition to its excellent corrosion resistance, it boasts much better thermal shock resistance than alumina due to its low linear expansion coefficient (2.5x10-6~2.8x10-6 per 1/°C). In air, within the range from room temperature to 1000°C, it can withstand dozens to hundreds of cycles of rapid cooling and heating without cracking. Silicon nitride has good wear resistance and a low coefficient of friction. Silicon nitride exists in two forms depending on its manufacturing method: reaction-sintered silicon nitride and hot-pressed silicon nitride. Hot-pressed silicon nitride has high strength and is used for parts that do not require machining; reaction-sintered silicon nitride is widely used due to the characteristics of its manufacturing process. The physical and mechanical properties of these ceramics are shown in Table 18.