Sintering of silicon carbide industrial ceramics - Reaction sintering

Reaction sintering is the last sintering method for silicon carbide industrial ceramics. Compared with the other three sintering methods, its characteristics are more significant. In this article, Kezhong Ceramic Factory will analyze the different properties of silicon carbide ceramics sintered by these four sintering methods.

First, let's understand what reactive sintering is.

Reaction sintering:

The reaction sintering method of SiC was first successfully studied in the United States. The process of reaction sintering is as follows: first α－ SiC powder and graphite powder are mixed evenly in proportion and made into porous bodies through dry pressing, extrusion, or grouting methods. At high temperatures, when in contact with liquid Si, the C in the green body reacts with the infiltrated Si to form β－ SiC, combined with α－ By combining SiC, excessive Si is filled into the pores, resulting in a dense reactive sintered body without pores. Reactive sintered SiC typically contains 8% free Si. Therefore, to ensure the complete infiltration of Si, the blank should have sufficient porosity. Generally, by adjusting the initial mixture α－ The content of SiC and C, α－ The particle size distribution of SiC, the shape and particle size of C, and the molding pressure are used to obtain appropriate green density.

Reaction sintered SiC, also known as self bonded SiC, is a process in which porous billets undergo chemical reactions with gas or liquid phases to increase the quality of the billets, reduce porosity, and sinter into finished products with certain strength and dimensional accuracy. It is caused by α— SiC powder and graphite are mixed in a certain proportion to form a green body, and heated to around 1650 ℃. At the same time, Si is melted or infiltrated into the green body through gas-phase Si, causing it to react with graphite to generate β— SiC, replace the existing α— SiC particles combine. If Si infiltration is complete, a completely dense and non dimensional shrinkage reactive sintered body can be obtained. Compared with other sintering processes, the size change of reaction sintering during the densification process is small, and precise products can be manufactured. However, the presence of a considerable amount of SiC in the sintered body results in poor high-temperature performance of reaction sintered SiC ceramics.

Reaction sintering or reaction forming is a process in which porous billets undergo chemical reactions with gas or liquid phases, resulting in an increase in the quality of the billets, a decrease in porosity, and sintering into finished products with certain strength and dimensional accuracy. Compared with other sintering processes, reaction sintering has the following characteristics:

① During reactive sintering, the mass increases, and chemical reactions may also occur during ordinary sintering processes, but the mass does not increase.

② Sintered blanks do not shrink and their dimensions remain the same, thus enabling the production of products with precise dimensions. Ordinary sintered blanks generally experience volume shrinkage.

③ The migration of ordinary sintered materials occurs between particles within the particle scale range, while the migration process of reactive sintered materials occurs over long distances, and the reaction rate depends on the mass and heat transfer processes.

④ The liquid phase reaction sintering process is similar in form to the melt leaching method in powder metallurgy. However, the liquid phase and solid phase in the melt leaching method do not undergo chemical reactions, nor do they undergo mutual dissolution, or only slight solubility is allowed.

Reaction sintered silicon nitride industrial ceramics are formed by the reaction of porous silicon powder billets with nitrogen gas at around 1400 ℃. During the reaction process, as the number of connected pores decreases, nitrogen diffusion becomes difficult and the reaction is difficult to complete. Therefore, the thickness of reaction sintered silicon nitride billets is limited, and the relative density is also difficult to reach 90%. The factors that affect the reaction process include the original density of the billet, the particle size of silicon powder, and the thickness of the billet. For coarse-grained silicon powder, the diffusion channels of nitrogen are few, and it takes a long time to diffuse to the center of the silicon particle. Therefore, the reaction weight increases less, the thickness of the reaction is thin, and the original density of the billet is high, which is not conducive to the reaction.

Reaction sintered silicon oxynitride blanks are composed of Si, SiO2 and CaF2 (or CaO, MgO, etc.), which react with nitrogen to generate Si2ON2. During reaction sintering, CaO and MgO form glass phases equivalent to SiO2. Nitrogen is dissolved in molten glass, and Si2ON2 crystal precipitates from nitrogen saturated glass phase. The density of reaction sintered silicon oxynitride can be more than 90%. Silicon oxynitride has good corrosion resistance to chlorides and oxygen, and has been used as the lining of electrolytic cell for aluminum production by AICI3 electrolysis and zinc production by ZnCl2 electrolysis. Reaction sintered silicon carbide is a SiC-C porous billet made by impregnating it with liquid phase silicon.

SiC ceramics produced by pressureless sintering, hot pressing sintering, hot isostatic pressing sintering, and reaction sintering have various performance characteristics. In terms of sintering density and bending strength, hot press sintering and hot isostatic pressing sintering have relatively more SiC ceramics, while reaction sintering has relatively lower SiC.

On the other hand, the mechanical properties of SiC ceramics also vary with different sintering additives. Pressure free sintering, hot pressing sintering, and reaction sintered SiC ceramics have the performance of corrosion resistant ceramics and have good resistance to strong acids and alkalis, but reaction sintered SiC ceramics have poor corrosion resistance to super strong acids such as HF. In terms of the performance comparison of high-temperature resistant ceramics, almost all SiC ceramics have improved strength when the temperature is below 900 ℃; When the temperature exceeds 1400 ℃, the bending strength of reaction sintered SiC ceramics sharply decreases. (This is due to the presence of a certain amount of free Si in the sintered body, which causes a sharp decrease in bending strength when the temperature exceeds a certain level.) For SiC ceramics sintered without pressure and hot isostatic pressing, their high-temperature resistance is mainly affected by the type of additives.

Website: https://www.zhengtaifeng.com/