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Everyone knows that silicon nitride ceramic is an inorganic material ceramic that does not shrink during sintering and has high strength, especially hot pressed silicon nitride. It is one of the harder substances in the world, with high strength, low density, and high temperature resistance. Silicon carbide ceramics, on the other hand, are materials that undergo hot press sintering, pressureless sintering, and hot isostatic pressing sintering. Their high-temperature strength can be maintained up to 1600 ℃, making them the material with the best high-temperature strength among ceramic materials. What is the difference between the two? Let’s take a look together!

Silicon nitride ceramics are high-temperature insoluble compounds with no melting point and strong resistance to high-temperature creep. The reaction sintered silicon nitride without binder has a load softening point above 1800 ℃. The thermal conductivity of silicon nitride ceramic is 18.42 W/m · K, so it has excellent thermal shock resistance, second only to quartz and microcrystalline glass. Experimental reports have shown that the reaction sintered silicon nitride sample with a density of 2500 kg/m3 has been cooled from 1200 ℃ to 20 ℃ for thousands of thermal cycles, but still does not break. The thermal stability of silicon nitride ceramic is good.

Silicon nitride ceramics are a covalent bonding compound with a basic structural unit of [SiN4] tetrahedron. Silicon atoms are located at the center of the tetrahedron, surrounded by four nitrogen atoms located at the four vertices of the tetrahedron. They then form a continuous and sturdy network structure in three-dimensional space in the form of one atom shared by every three tetrahedrons. Many of the properties of silicon nitride are attributed to this structure. Pure Si3N4 is 3119, with α and β Both crystal structures are hexagonal, with a decomposition temperature of 1800 ℃ in air and 1850 ℃ in 011MPa nitrogen. Si3N4 has a low coefficient of thermal expansion and high thermal conductivity, making it highly resistant to thermal shock. The silicon nitride sintered by hot pressing will not crack when heated to 1000 ℃ and put into cold water. At not too high temperatures, Si3N4 has high strength and impact resistance, but it will damage with the increase of usage time above 1200 ℃, causing its strength to decrease. Fatigue damage is more likely to occur above 1450 ℃, so the usage temperature of Si3N4 generally does not exceed 1300 ℃. Due to the low theoretical density of Si3N4, which is much lighter than steel and engineering superheat resistant alloy steel, it is most suitable to use Si3N4 ceramics instead of alloy steel in areas where materials are required to have high strength, low density, high temperature resistance, and other properties.
Silicon carbide, commonly known as carborundum or carborundum, is a typical compound with covalent bonds that are almost non-existent in nature. Silicon carbide is a compound with strong covalent bonds, and its Si-C bond ion type is only about 12%. Therefore, it also has excellent mechanical properties, excellent oxidation resistance, high wear resistance, and low friction coefficient. The biggest characteristic of silicon carbide is its high high-temperature strength. Ordinary ceramic materials will significantly reduce their strength at 1200~1400 degrees Celsius, while silicon carbide’s bending strength remains at a high level of 500~600MPa at 1400 degrees Celsius. Therefore, its working temperature can reach 1600~1700 degrees Celsius. Silicon carbide has excellent mechanical properties, excellent oxidation resistance, high wear resistance, and low friction coefficient. The disadvantage of silicon carbide ceramics is that their fracture toughness is low, which means they are relatively brittle. Therefore, multiphase ceramics based on SiC ceramics, such as fiber (or whisker) reinforcement, heterogeneous particle dispersion strengthening, and gradient functional materials, have emerged one after another, improving the toughness and strength of individual materials.

Silicon carbide ceramics are widely used in fields such as aviation, machinery, automotive, metallurgy, chemical engineering, electronics, etc. Silicon carbide can be divided into recrystallized silicon carbide ceramics, reaction sintered silicon carbide ceramics, pressureless sintered silicon carbide ceramics, hot pressed sintered silicon carbide ceramics, high-temperature hot isostatic pressing sintered silicon carbide ceramics, and chemical vapor deposition of silicon carbide. The properties of silicon carbide prepared by various processes vary greatly, so different processes can be used to manufacture silicon carbide ceramics with different properties to meet different needs.