A brief introduction to the Properties and applications of Silicon Nitride Materials
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Update time : 2023-12-29 09:51:52
What is Silicon nitride?
Silicon nitride (Si3N4) ceramic is an inorganic, non-metallic, strong covalent compound; its basic structural unit is tetrahedron; the silicon atom is located in the center of tetrahedron, four nitrogen atoms are located at four vertices of tetrahedron, and then form a continuous and solid network structure in three-dimensional space in the form of one silicon atom shared by every three tetrahedron. Many properties of silicon nitride are due to this special structure. Silicon nitride (Si3N4) contains three crystal forms and two common crystal forms. One is α-Si3N4, an acicular crystal, white or grayish-white, and the other is β-Si3N4, which is a dark and dense granular polyhedron or short prism. Due to the longer stacking sequence, the hardness of the α phase is higher than that of the β phase. However, compared with the β phase, the α phase is chemically unstable. Therefore, at the high temperature of the liquid phase, the α phase always changes to the β phase. Therefore, β-Si3N4 is the main form used in silicon nitride ceramics.
Performance characteristics of Silicon Nitride Materials
Physical properties of silicon nitride:
1)Thermal properties: it is a high-temperature refractory material, has no melting point, decomposes at about 1900 ℃ under normal pressure, and has strong resistance to high-pressure creep. The load softening point of reaction sintered silicon nitride without binder can be as high as 1800 ℃. 2)Good thermal conductivity. 3)The coefficient of thermal expansion is small. 4) good electrical insulation, low dielectric coefficient and high breakdown voltage.
Chemical properties of silicon nitride
1) Oxidation resistance: no reaction with oxygen in a dry atmosphere below 800 ℃. 2) Corrosion resistance of molten metal: Silicon nitride does not infiltrate and corrode the melt of elemental metal (except copper). 3) corrosion resistance of acid, alkali and salt: it is soluble in hydrofluoric acid and does not work with dilute acid.
Mechanical properties of silicon nitride
The main results are as follows: 1) The high-temperature strength is good, and the high-temperature strength at 1200 ℃ has little attenuation compared with that at room temperature. In addition, its high-temperature creep rate is very low. The nature of strong covalent bonds determines all these.
Si3N4 has high hardness, second only to a few super hard materials such as diamond, cubic BN, B4C, and so on.
4) The friction coefficient is small and self-lubricating, similar to the metal surface of oiling. Brief introduction of Market Application of Silicon Nitride Ceramics. Automotive industry: sintered silicon nitride is mainly used in the automotive industry as an engine parts material. In spark ignition engines, silicon nitride is used for lower-wear rocker pads, turbochargers with lower inertia and less engine lag, and exhaust gas control valves to increase acceleration. 5)Bearing: compared with other ceramics, silicon nitride ceramic has good impact resistance. Therefore, ball bearings made of silicon nitride ceramics are used in performance bearings. A typical example is silicon nitride bearings in the main engine of NASA's space shuttle. Since silicon nitride ball bearings are harder than metal, this reduces contact with bearing tracks. Silicon nitride ball bearings can be found in high-end automotive, industrial bearings, wind turbines, racing sports, bicycles, skates and skateboards. High-temperature materials: Silicon nitride has been used in high-temperature applications for a long time. In particular, it has been identified as one of the few monolithic ceramic materials that can survive the severe thermal shock and thermal gradient produced by hydrogen/oxygen rocket engines. Medical: Silicon nitride has many orthopedic applications. The material is also a substitute for PEEK (polyether ether ketone) and titanium for spinal fusion devices. Compared with PEEK and titanium, the hydrophilic and microstructure surface of silicon nitride contributes to the material's strength, durability and reliability.
Metal cutting tools: bulk monolithic silicon nitride is used as a material for cutting tools because of its hardness, thermal stability and wear resistance. Especially recommended for high-speed machining of cast iron. Thermal hardness, fracture toughness, and thermal shock resistance mean that sintered silicon nitride can cut cast iron, hard steel, and nickel-based alloys. Electronic products: These are commonly used as insulators and chemical barriers to manufacture integrated circuits to isolate different structures electrically or as etching masks in bulk micromachining. As the passivation layer of microchips, it is better than silicon dioxide because it is a significantly better diffusion barrier to water molecules and sodium ions, the two main corrosion sources and instability of microelectronics. Core copper wire power cable, each core has an independent color insulation sheath, is included in the external protection sleeve.
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