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By Trunnano | 27 September 2022 | 0 Comments

Si3N4/SiC composite ceramic material

Silicon nitride/silicon carbide composite ceramic material is a special silicon carbide product, which was widely used in abrasive and electric ceramic industry in 1970s.
 
Silicon nitride/silicon carbide composite ceramic materials have the characteristics of low expansion, high thermal conductivity, insulation and long service life. At the same time, this material has strong corrosion resistance, oxidation resistance, corrosion resistance and good insulation, and has a certain bearing capacity for general inorganic acid and alkali solutions and molten non-ferrous metals.
 
The density of silicon nitride and silicon carbide is similar, when the columnar silicon nitride is interspersed between silicon carbide particles and sintered, the toughening and strengthening effect is much better than that of a single material. Silicon nitride ceramics have high brittleness, which can be compounded with silicon carbide materials to improve brittleness and fracture toughness, while the insufficient properties of silicon carbide materials, such as mechanical properties, oxidation resistance, wear resistance and so on, can also be improved after compounding with silicon nitride.
 
The Mohs hardness of silicon nitride/silicon carbide composite ceramic material is about 9, second only to diamond, and the strength and hardness of the material are high at room temperature and can be maintained at 1200-1400 ℃. Due to the lack of glass phase in the sintering process, excessive high temperature deformation will not occur in practical use. As a result, silicon nitride/silicon carbide composite ceramic materials are widely used in high temperature fields such as large ironmaking furnace, aluminum reduction cell, ceramic kiln furniture, waste incinerator and Lurgi liquid slag discharge furnace.
 
In industry, silicon nitride / silicon carbide composite ceramics are formed with SiC and Si as the main components, added additives, and then placed in a nitriding furnace with high purity nitrogen, then nitrided and sintered at a certain temperature. This method enables Si powder to be nitrided directly to form silicon nitride in situ. During the growth process, Si3N4 crystals form a strong mechanical bond with SiC, resulting in the formation of large SiC particles surrounded by rod-like Si3N4 and some granular SiC matrix. Because the direct nitridation method cannot accurately control the crystal growth, the product is a two-phase mixture of α-Si3N4 and β-Si3N4. The content of α-Si3N4 crystal phase directly determines the quality of the composites.
 
Raw material.
The main raw materials involved in silicon nitride / silicon carbide composite ceramic materials are silicon carbide, silicon powder, nitrogen, additives and so on.
 
Silicon carbide, silicon powder, nitrogen.
The purity of silicon carbide should be more than 98.5%, the purity of silicon powder should be more than 99%, and the purity of nitrogen should be more than 99.9%.
In addition to the need for strict control of the purity of raw materials, because the green body does not shrink in the subsequent nitriding process, the density of nitrided products depends on the density of the green body, and the particle composition of the billet has a great influence on the density of the green body. In the production and processing process, the particle size and particle gradation of raw materials need to be strictly controlled, and the coarse, medium and fine SiC sand and Si powder should be weighed and mixed according to the formula.
 
Additives
Catalysts such as Fe, Co, Ni or Cr can promote the nitriding of silicon powder and the growth of silicon nitride whiskers, and improve the mechanical properties or corrosion resistance of the materials.
Sintering aids can promote the sintering densification of silicon nitride bonded silicon carbide materials and improve their mechanical properties. Oxide sintering aids are the most widely studied sintering aids in ceramic sintering, such as Al2O3, MgO, ZrO2, SiO2, RE2O3 (RE=La, Nd, Gd, Y, Yb, Sc), and non-oxide sintering aids such as YF3, YbF3, MgSiN2 and so on.
 
Proper amount of reinforcements can improve the mechanical properties and oxidation resistance of materials. The common reinforcements are Al2O3, ZrSiO4, B4C, nano-Si3N4, Ti-Si-Fe alloy powder, graphene or carbon nanotubes.
 
Binding agent.
The addition of temporary binder has two main effects. one is that it can help the fusion of raw materials to achieve homogenization, improve the dispersibility of raw material particles and create good conditions for embryo body forming. Second, silicon nitride bonded silicon carbide products will face the process of heating up in the process of drying and firing, while under the condition of high temperature, the temporary binder in silicon nitride bonded silicon carbide products will decompose, leaving a large number of network pore channels for silicon nitride bonded silicon carbide products in the process of gaseous volatilization, which is not only more beneficial to the filling of nitrogen, but also improves the reaction efficiency between silicon powder and nitrogen. And it can also be more conducive to the stability of the final product.
 
The main temporary binders are: organic dextrin, calcium lignosulfonate and German Sima chemical dispersant, etc. at present, the mass percentage of temporary binder in the industry is usually less than 5%.
 
Forming process.
At present, the forming processes of silicon nitride / silicon carbide composite ceramics can be divided into two types: semi-dry molding and grouting molding.
The application of semi-dry forming is more common, the advantage of this forming method is that it is efficient and suitable for large-scale production, but the disadvantage is that it can only produce products with simple and thick shape, but can not produce thin plates and complex products.
The grouting molding process is simple and the cost is low. However, the performance of the slurry must be good, and there are many factors that determine the quality of the slurry, among which the surface treatment of silicon carbide powder occupies a very important position; in addition, the green body strength is low and the water absorption process of the gypsum mold is easy to cause the density gradient distribution of the green body.
 
Gel casting is a new ceramic forming technology developed in recent years, which can form green bodies with complex shape and good uniformity, and has become an ideal forming technology for ceramic materials. The forming method is as follows: firstly, the ceramic powder is dispersed in the solution containing organic monomer and cross-linking agent to prepare a suspension slurry with high solid volume fraction, and then injected into a mold of a certain shape. under certain conditions, the organic monomer reacts with the cross-linking agent, and then the slurry is solidified in situ.
 
Sintering.
The sintering methods of silicon nitride / silicon carbide composite ceramics include reaction sintering, hot pressing sintering, air pressure sintering, hot isostatic pressing sintering, oscillating pressure sintering and so on. No matter what kind of sintering method, the sintering system of the material affects the nitridation rate of silicon powder. Choosing appropriate heating rate, sintering temperature and holding time will promote the nitriding of silicon powder and improve the density and strength of the material. When the sintering temperature is low, the nitriding of silicon powder is not complete, while when the sintering temperature is too high, silicon nitride will decompose, which will decrease the density of the material and damage its mechanical properties.
 
In the process of the reaction between silicon powder and nitrogen, it roughly goes through two temperature stages: the first is the heating stage, and then the nitriding stage of the raw material. In the heating stage, the temperature in the device increases from the initial temperature to about 1100 ℃, while the temperature in the nitriding stage of the raw material is 1100-1350 ℃.
 
Silicon nitride Price
The price is influenced by many factors including the supply and demand in the market, industry trends, economic activity, market sentiment, and unexpected events.
If you are looking for the latest Silicon nitride powder price, you can send us your inquiry for a quote. ([email protected])
 
Silicon nitride Supplier
Luoyang Tongrun Nano Technology Co. Ltd. (TRUNNANO) is a trusted Si3N4 manufacturer and Si3N4 supplier with over 12-year-experience. We ship our goods all over the world.
 
If you are looking for high-quality Si3N4 powder, please feel free to contact us and send an inquiry. ([email protected])

 

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