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Silicon nitride ceramic performance process

Release time:2020-07-14
    Silicon nitride ceramics are inorganic materials which do not shrink when sintered. Silicon nitride is very strong, especially hot pressed silicon nitride, and is one of the hardest substances in the world. High strength, low density, high temperature resistance and so on.
    Si3N4 ceramics is a covalent bond compound, the basic structure unit for [SiN4] tetrahedron, silicon atom is located in the center of the tetrahedron, there are four nitrogen atoms around it, which located in tetrahedral four vertices, then to every three  tetrahedron Shared in the form of an atom, the three dimensional space form a continuous and strong network structure.
    Many of the properties of silicon nitride are due to this structure. Pure Si3N4 is 3119 and has and crystal structures, both of which are hexagonal in shape. Its decomposition temperature is 1800℃ in air and 1850℃ in 011MPa nitrogen. Si3N4 has low thermal expansion coefficient and high thermal conductivity, so it has excellent heat shock resistance. The hot pressed sintered silicon nitride will not break when heated to l000℃ and put into cold water. At a moderate temperature, Si3N4 has a high strength and impact resistance. However, when the temperature is above 1200 ° C, damage will occur with the increase of service time, which will reduce its strength. Fatigue damage is more likely to occur above 1450 ° C, so the service temperature of Si3N4 generally does not exceed 1300 ° C. Because Si3N4 has a low theoretical density and is much lighter than steel and engineering super-heat-resistant alloy steels, Si3N4 ceramics are ideal to replace alloy steels where high strength, low density, and high temperature resistance are required.
    Si3N4 ceramic material is an excellent high temperature engineering material, and its application in the high temperature field can give full play to its advantage. The development direction of Si3N4 in the future is: (1) give full play to and use the excellent characteristics of Si3N4 itself; (2) To develop new fluxes for sintering of Si3N4 powder and to study and control the optimal composition of existing fluxes; (3) Improve the powder making, molding and sintering process; (4) Research and Si3N4 and SiC and other materials compound, in order to make more high-performance composite materials. The application of Si3N4 ceramics in automobile engine has created a new situation for the development of new high-temperature structural materials. Automobile industry itself is a set of multidisciplinary nature of industry, science and technology of China is a civilization with a history of, once in the history of ceramic made a brilliant performance, as the process of reform and opening up, one day, China must also car among big industrial nations in the world and create new splendor for the development of ceramics.
    It is extremely resistant to high temperature, its strength can be maintained to the high temperature of 1200℃ without decreasing, will not melt into a body after being heated, will decompose until 1900℃, and has amazing chemical corrosion resistance, can withstand almost all inorganic acids and less than 30% of caustic soda solution, but also can withstand the corrosion of many organic acids; It is also a kind of high performance electrical insulating material.
    It is made of silicon powder as raw material, and the required shape is first made by the usual forming method. Then, it is preliminarily nitrified in nitrogen and at 1200℃, and part of the silicon powder reacts with nitrogen to form silicon nitride. At this time, the whole billet body has a certain strength. Then in 1350℃~1450℃ high temperature furnace for a second nitriding reaction to silicon nitride. The theoretical density of silicon nitride was 99% by hot pressing sintering.
    Of Si3N4 ceramics preparation technology developing rapidly in the last few years, the preparation process are mainly concentrated in the reaction sintering process, air pressure and normal pressure sintering, hot-pressing sintering, the sintering process and so on. Because of the different preparation technology, various types of silicon nitride ceramics with different microstructure (such as porosity and the pore morphology, grain morphology, intergranular morphology and the content of intergranular phase 2, etc.). Thus the various properties vary greatly. To get a good performance of Si3N4 ceramics materials, preparation of high quality should first Si3N4 powder. Use different methods for preparation of Si3N4 powder quality is not exactly the same, which leads to the differences on purpose, many ceramic material application failure, often attributed to developers do not understand the difference between various kinds of ceramics powder, insufficient understanding of its nature. Generally speaking, high quality Si3N4 powder should have high content of phase, uniform composition, less impurities and uniform distribution in ceramics, small particle size, narrow particle size distribution and good dispersion. Good Si3N4 powder should contain at least 90% phase. This is because part of the phase is converted into phase during the sintering process of Si3N4. Without sufficient content, the strength of ceramic materials will be reduced.
Reaction sintering method (RS)
    Is to use the general forming method, the silicon powder is pressed into the desired shape of the green, put into the nitriding furnace after pre-nitriding (partial nitriding) sintering treatment, pre-nitriding has a certain strength, can be a variety of mechanical processing (such as car, planer, milling, drilling). Finally, in the melting point of silicon temperature above; The raw material is completely nitrided and sintered again, and the product with small size change is obtained (that is, after the raw material is sintered, the shrinkage rate is very small, the linear shrinkage rate is < 011%). The product generally can be used without grinding processing. The reaction sintering method is suitable for the manufacture of complex shape and precise size parts.
Hot pressing sintering method (HPS)
    Si3N4 powder and a small amount of additives (such as MgO, Al2O3, MgF2, Fe2O3, etc.) were pressed and sintered at pressure above 1916 MPa and temperature above 1600 ℃. Some companies in the UK and the US use hot-pressed sintered Si3N4 ceramics with strengths of up to 981MPa and above. During sintering, additives and phase composition have great influence on product performance. Due to the strict control of the composition of grain boundary phase and proper heat treatment after sintering of Si3N4 ceramics, Si3N4 ceramic materials whose strength (up to 490MPa and above) will not significantly decrease even when the temperature is up to 1300 ℃ can be obtained, and their creep resistance can be increased by three orders of magnitude. If the Si3N4 ceramic material is pre-oxidized at 1400-- 1500 ℃, the Si2N2O phase is formed on the surface of the ceramic material, which can significantly improve the oxidation resistance and high temperature strength of Si3N4 ceramics. The mechanical properties of Si3N4 ceramics produced by hot pressing sintering method are superior to those of reactive sintering, with high strength and density. However, due to the high manufacturing cost and complex sintering equipment, the size accuracy of the products is limited due to the large shrinkage of the sintered body, which makes it difficult to manufacture complex parts, only parts with simple shapes can be manufactured, and the machining of the workpiece is also difficult.
Atmospheric sintering method (PLS)
    In terms of increasing the pressure of sintering nitrogen atmosphere, the properties of Si3N4 decomposition temperature rising (usually under N2 = 1atm pressure, starting from 1800℃) were used to conduct atmospheric sintering in the temperature range of 1700-- 1800℃, and then atmospheric sintering in the temperature range of 1800-- 2000℃. The purpose of this method is to use air pressure to promote the densification of Si3N4 ceramics, thus improving the strength of ceramics. The disadvantage of this method is similar to hot pressing sintering.
    Pneumatic sintering (GPS)
    In recent years, a great deal of research has been done on pneumatic sintering, and great progress has been made. Air pressure sintering of si3N4 is carried out at 1 ~10MPa and around 2000℃. High nitrogen pressure inhibited the high temperature decomposition of silicon nitride. Add less due to using high temperature sintering, the sintering AIDS cases, also to promote Si3N4 grain growth, and obtain the density > containing 99% of the in-situ growth of columnar grain and high toughness ceramic. So air pressure sintering both in laboratory and get more and more attention in the production. Air pressure sintered silicon nitride ceramics with high toughness, high strength and good wear resistance, can be directly from close to the final shape of all kinds of complicated shape products, thus can greatly reduce the production cost and processing cost. And its production process is close to cemented carbide production technology, suitable for mass production.
    Research status of folding edit this section
    For Si3N4 and Sialon ceramic sintered bodies, a superplastic forming process that does not form composites but remains in a single state has been provided, and a sintered body formed according to this process has been provided. Si3n4 and Sialon sintered bodies with relative density above 95% and linear density within 120~250 of the length of 50 m on the two-dimensional cross section of the sintered body; It is molded by plastic deformation at a strain rate less than 10-1/ s by stretching or compression at 1300~1700℃. The sintered body has excellent mechanical properties at room temperature.
    Si3N4 ceramics is an important structural material. It is a super hard material with lubricity and abrasion resistance. Except hydrofluoric acid, it does not react with other inorganic acid, strong corrosion resistance, oxidation resistance at high temperature. And it also can resist cold and hot shock, in air heated to above 1000 ℃, a sharp cooling heating sharply again, also won't fracture. It is because of Si3N4 ceramics has such excellent properties, people often use it to manufacture bearings, steam turbine blades, mechanical seal, mechanical components, such as permanent mould. If use high temperature resistant and not easily heat transfer of silicon nitride ceramic to make engine parts of heating surface, can not only improve the quality of diesel engine, save fuel, China, the United States, Japan and other countries have developed such diesel engines.