Research on Sintering Technology of Silicon Carbide Ceramic Material

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With the development of science and technology, especially the rapid development of modern national defense, aerospace, marine engineering and automobile industry, the market has higher and higher requirements for various properties of materials. Silicon carbide ceramics are widely used in precision bearings, seals, gas Turbine rotors, optical components, high temperature nozzles, heat exchanger components and atomic thermal reactor materials.

SiC is a compound with strong covalent bonds, and its covalent bond composition is as high as 88%. The high-temperature diffusion coefficient of SiC is very low. Even at a high temperature of 2100 C, the self-diffusion coefficient of Si and C is only 2.5×10 13 cm2/ s and 1.5×1010 cm2/s, so the densification must be achieved by means of sintering additives, external pressure and other methods. For this reason, sintering technologies such as reaction sintering, pressureless sintering, hot pressing sintering, hot isostatic pressing sintering, spark plasma sintering and oscillating pressure sintering of dense SiC ceramics have been developed, making it possible to prepare high-performance SiC ceramics. At present, the research and industrial application of silicon carbide reaction sintering and atmospheric pressure sintering have achieved great development; many achievements have also been made in the research fields of hot pressing sintering, hot isostatic pressing sintering, spark plasma sintering and oscillating pressure sintering.

Silicon Carbide Reaction Sintering 

The reaction sintering silicon carbide process is a near-net-size sintering process. There is almost no shrinkage and dimensional change during the sintering process. It has the advantages of low sintering temperature, compact product structure, and low production cost. It is suitable for the preparation of large-scale and complex-shaped silicon carbide ceramic products. . In the reaction sintering process, in order to ensure the complete siliconization, the green body (α-SiC+C) should have sufficient porosity, otherwise only the C in the surface area will be silicided, and there will still be unreacted carbon in the center. Therefore, the density of the green body must be strictly controlled. By adjusting the content of α-SiC and C in the initial mixture, the particle size distribution of α-SiC, the shape and particle size of C, and the molding pressure, an appropriate Green density. After the reaction is completed, the free Si between the SiC particles fills the residual pores, so that a non-porous and compact reaction sintered body is obtained. The flexural strength of reaction-bonded SiC is generally 350-500 MPa, and the fracture toughness is 4.0-5.0 MPa m1/2, but with the increase of Si content, the strength and fracture toughness decrease linearly; when the Si content is constant, with the SiC grain Reduced in size, increased in strength. Fracture toughness varies with temperature, from 4 MPa m1/2 at room temperature to 12 MPa m1/2 at 1200 C, due to the increased plasticity of free silicon at high temperatures.

Silicon Carbide Pressureless Sintering

The pressureless sintering technology of SiC is very mature. Its advantage is that it can adopt a variety of molding processes, break through the limitations of product shape and size, and obtain higher strength and toughness under the action of appropriate additives. In addition, the pressureless sintering of SiC is simple and cost-effective, and is suitable for mass production of ceramic parts of different shapes.

Silicon carbide hot pressing sintering

Hot press sintering is to fill dry silicon carbide powder into a high-strength graphite mold, apply an axial pressure while raising the temperature, and realize the sintering of silicon carbide under the control of appropriate pressure-temperature-time process conditions. Due to the simultaneous heating and pressing, the powder is in a thermoplastic state, which is conducive to the contact and diffusion of particles and the flow and mass transfer process. It can obtain fine grains and fine grains at a lower sintering temperature and a shorter sintering time. Silicon carbide ceramic products with high relative density and good mechanical properties. The disadvantage of this process is that the equipment and process are complicated, the mold material requirements are high, and only parts with simple shapes can be prepared, the production efficiency is low, and the production cost is high. However, in the field of semiconductor manufacturing, the performance requirements of ceramic materials required for the manufacture of precision instruments and components are very high. The importance of composition control, purity and densification of such ceramic materials is far higher than the consideration of economic costs. In addition, the added value of the product is high, which also makes hot pressing sintering particularly important.

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