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Reaction sintering of silicon carbide ceramics

Silicon carbide ceramics have excellent properties such as high mechanical strength, high temperature resistance, strong oxidation resistance, good thermal stability, high thermal conductivity, good wear resistance, good chemical corrosion resistance, high hardness, and good thermal shock resistance. SiC is one of the best oxidation resistance of all non oxide ceramics.

SiC ceramics not only have excellent mechanical properties at room temperature, but also have the best mechanical properties at high temperature (strength, creep resistance, etc.) among known ceramic materials. SiC is a material of hot pressing sintering, pressureless sintering, reactive sintering and hot isostatic pressing sintering. Its high temperature strength can be maintained up to 1600 ℃. It is the best material of high temperature strength among ceramic materials.

Silicon carbide ceramics not only play an important role in high-tech fields, but also have broad markets in such hot fields as metallurgy, machinery, energy, building materials and chemicals. With the continuous development of high and new technology, the requirements for silicon carbide ceramics are becoming higher and higher, requiring various products with different levels and properties.

Reaction sintering of silicon carbide:

The basic principle of reactive sintering is: the reactive liquid silicon or silicon alloy infiltrates into the carbon containing porous ceramic blank under the action of capillary force, reacts with the carbon in the blank to generate silicon carbide, the newly generated silicon carbide is in situ combined with the original silicon carbide particles in the blank, and the infiltrating agent is used to fill the remaining pores in the blank to complete the densification process. The disadvantage of reaction sintering is that the structure of green body and additives will affect the residual free silicon and voids in reaction sintered silicon carbide. Therefore, it is necessary to master the mechanism of reaction sintering to improve the production quality of reaction sintering.

Traditional reaction sintering

The preparation process of traditional reaction sintering is as follows：

SiC powder and C powder mixing → forming → drying → coke removal under atmosphere protection → high temperature silicon infiltration → subsequent processing.

SiC powder and C powder are mixed with organic binder to form, in which the mass fraction of SiC powder is more than 60%. SiC powder is prepared after forming, drying, coke removal and finally siliconizing. In general, the traditional reaction sintering method is the most widely used method to prepare reaction sintered SiC at home and abroad, but there are still several problems:

(1) Cost and energy consumption. The traditional sintering process has large energy consumption and high cost in the preparation process. The main reasons are that the preparation of raw material SiC powder has large energy consumption and high cost, the sintering cycle is very long, and the sintering temperature is high.

(2) The existence of free silicon in the material greatly limits the use temperature of the material, which is generally not lower than 1380 ℃.

(3) The reliability of materials is not high enough. The material is of poor homogeneity and brittle fracture, so it belongs to general brittle material.

Reaction sintering using vacuum sintering technology

Vacuum reaction sintered silicon carbide is a kind of engineering ceramics which is almost completely dense. The preparation process of reaction sintering is as follows: firstly, silicon carbide and adhesives (such as resin) are made into composite blanks by traditional ceramic forming methods (pressing, grouting and tape casting); Then the blank is heated in an inert atmosphere, and the binder is pyrolyzed into glassy carbon. At the same time, the blank becomes a composite of porous carbon and silicon carbide; The green body after pyrolysis is the green body. In order to make the shape closer to the final product, the blank is machined to reduce the workload of subsequent processing. Finally, the machined blank is heated in molten silicon under vacuum or inert atmosphere to complete the reaction sintering process. The process flow for preparing silicon carbide ceramics by reaction sintering is as follows:

SiC powder and binder → forming → blank → pre sintering → plain blank → machining → melting in silicon solution → reaction sintering → products.

Compared with the traditional sintering and pressured sintering preparation methods, the reactive sintering process has the following advantages:

1) The expensive pressure sintering equipment is not needed, and the preparation cost is greatly reduced.

2) The processing time of products is short.

3) The processing temperature is low. Silicon carbide is a covalent bond structure, and its sintering usually requires a preparation temperature higher than 1900 ℃, while reactive sintering can be carried out only at tens of degrees above the liquidus of the impregnating agent, hundreds of degrees lower than other sintering processes.

4) During sample processing, the size change is small (50.1%), and the product is completely dense.

5) It can prepare products with large size and complex shape. Theoretically, the sintering process does not require pressurization, so the size of the product is only limited by the size of the sintering furnace, and the complexity of its shape is determined by the forming process.

6) There are many forming methods. Such as extrusion, injection, pressing and pouring.

The traditional sintering method requires high quality of raw materials and large process shrinkage of silicon carbide, so it is costly and difficult to prepare products with complex shapes. It is difficult to prepare large and complex products by pressure sintering because of its high process requirements. Compared with the above two sintering methods, reactive sintering has the advantages of low cost, simple process and large production capacity.

Selection of sintering equipment: The RVS-S vacuum sintering furnace provided by SIMUWU is an excellent product for processing silicon carbide reaction sintering process. The product has the characteristics of good temperature uniformity, high temperature control accuracy, stable sintering, full automatic control, etc. SIMUWU provides a professional team of engineers who can solve various problems encountered in the production process, reduce the cost of trial and error in production, and devote themselves to giving customers the most convenient and fast experience.