Discussion: the difference between silicon nitride ceramics and silicon carbide ceramics
Everyone knows that silicon nitride ceramics is an inorganic material ceramic that does not shrink during sintering, and has high strength, especially hot-pressed silicon nitride, which is one of the hardest substances in the world, with high strength and low density. High temperature resistance and other characteristics. Silicon carbide ceramics are hot-pressed sintered, pressureless sintered, hot isostatic pressed sintered materials, and their high-temperature strength can be maintained up to 1600 ° C, which is a material with better high-temperature strength among ceramic materials. Where is the difference between the two? Let's find out together!
Silicon nitride ceramics are high-temperature insoluble compounds with no melting point and strong high-temperature creep resistance. The softening point of reaction-sintered silicon nitride without binder is above 1800°C. The thermal conductivity of silicon nitride ceramics is 18.42 W/ m K , so it has excellent thermal shock resistance, second only to quartz and glass-ceramics. Experimental reports show that the reaction-sintered silicon nitride test with a density of 2500 kg/m3 The sample was cooled from 1200 ℃ to 20 ℃ for thousands of thermal cycles, and it still did not break. The thermal stability of silicon nitride ceramics is good.
Silicon nitride ceramics is a covalent bond compound, the basic structural unit is [SiN4] tetrahedron, the silicon atom is located in the center of the tetrahedron, and there are four nitrogen atoms around it, respectively located at the four vertices of the tetrahedron, and then Every three tetrahedrons share the form of an atom, forming a continuous and solid network structure in three-dimensional space. Many of the properties of silicon nitride are due to this structure. Pure Si3N4 is 3119, has two crystal structures of α and β, both of which are hexagonal crystals, and its decomposition temperature is 1800°C in air and 1850°C in 011MPa nitrogen. Si3N4 has a low thermal expansion coefficient and high thermal conductivity, so it has excellent thermal shock resistance. The hot-pressed sintered silicon nitride will not break after being heated to 1000°C and put into cold water. At not too high temperatures, Si3N4 has high strength and impact resistance, but above 1200°C, it will be damaged with the increase of service time, reducing its strength, and it is more prone to fatigue damage above 1450°C, so Si3N4 The operating temperature generally does not exceed 1300°C. Due to the low theoretical density of Si3N4, which is much lighter than steel and engineering super-heat-resistant alloy steel, it is very suitable to replace alloy steel with Si3N4 ceramics in places where materials are required to have high strength, low density, and high temperature resistance. .
Silicon carbide, commonly known as corundum, also known as moissanite, is a typical covalent bond compound, which hardly exists in nature, while silicon carbide is a compound with strong covalent bond, and the ionic type of its Si--C bond is only 12%. Therefore, it also has excellent mechanical properties, excellent oxidation resistance, high wear resistance and low friction coefficient. The biggest feature of silicon carbide is its high strength at high temperature. The strength of ordinary ceramic materials will be significantly reduced at 1200 ~ 1400 degrees Celsius, while the flexural strength of silicon carbide remains at a relatively high level of 500 ~ 600MPa at 1400 degrees Celsius, so its working temperature can be Up to 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 the fracture toughness is low, that is, the brittleness is relatively large. For this reason, multiphase ceramics based on SiC ceramics, such as fiber (or whisker) reinforcement, heterogeneous particle dispersion strengthening, and gradient functional materials have been successively developed. Appears, improving the toughness and strength of the monomer material.
Silicon carbide ceramics are widely used in aviation, machinery, automobile, metallurgy, chemical industry, electronics and other fields. Silicon carbide can be divided into recrystallized silicon carbide ceramics, reaction sintered silicon carbide ceramics, pressureless sintered silicon carbide ceramics, hot pressing sintered silicon carbide ceramics, high temperature hot isostatic pressing sintered silicon carbide ceramics and chemical vapor deposition silicon carbide. The properties of silicon carbide prepared by various processes are quite different, so various processes can be used to manufacture silicon carbide ceramics with different properties to meet different needs.