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The Influence of Carbon Content on the Microstructure of Reaction-Sintered Silicon Carbide
Release time:
2022-01-09
The carbon content in the fracture surfaces of each sintered specimen varies. Within the range of a-2.5 wt.%, a dense material with virtually no porosity is formed, consisting of uniformly distributed silicon carbide particles and free silicon. As the carbon addition increases, Reaction-sintered silicon carbide The content of carbon gradually increases, and the particle size of silicon carbide grows larger, with silicon carbide particles interconnected in a skeletal structure. However, excessive carbon content tends to result in residual carbon within the sintered body. When the amount of carbon black is further increased to 3a, the sample exhibits incomplete sintering, and black “layers” appear internally.
When carbon reacts with molten silicon, its volume expands by 234%, which means that the microstructure of reaction-sintered silicon carbide is closely related to the carbon content in the green body. When the carbon content in the green body is relatively low, the silicon carbide formed through the carbon-silicon reaction is insufficient to fill the pores surrounding the carbon powder, resulting in a large amount of free silicon remaining in the sample. As the carbon content in the green body increases, Reaction-sintered silicon carbide The carbon powder can fully fill the pores surrounding it, thereby bonding the original silicon carbide particles together. At this stage, the content of free silicon in the sample decreases, and the density of the sintered body increases. However, when the green body contains more carbon, the secondary silicon carbide formed by the reaction between carbon and silicon rapidly surrounds the carbon powder, making it difficult for the molten silicon to come into contact with the carbon powder, thus leaving residual carbon in the sintered body.
According to the XRD test results of the samples from each group, the phase composition of reaction-sintered silicon carbide consists of α-SiC, β-SiC, and free silicon.
During high-temperature reaction sintering, carbon atoms migrate from the β-SiC phase on the SiC surface to the initial state via the melting of silicon α-phase. Since the silicon-carbon reaction is a typical exothermic reaction that releases substantial heat, rapid cooling following a brief period of spontaneous high-temperature reaction increases the supersaturation of carbon dissolved in the liquid silicon. As a result, β-SiC particles precipitate in the form of carbon, thereby enhancing the material’s mechanical properties. Consequently, the refinement of secondary β-SiC grains contributes to an improvement in flexural strength. In the Si-SiC composite system, the content of free silicon in the material decreases as the carbon content in the raw materials increases.
Conclusion:
⑴ The viscosity of the reaction-sintered slurry prepared increases with the increase in carbon black content; its pH value is alkaline and gradually rises.
⑵ As the carbon content of the green body increases, the density and flexural strength of reaction-sintered ceramics prepared by the slip-casting method first increase and then decrease. When the amount of carbon black is 2.5 times the initial dosage, the three-point flexural strength and bulk density of the green body after reaction sintering are remarkably high, reaching 227.5 MPa and 3.093 g/cm³, respectively.
(3) When sintering green bodies with excessive carbon content, cracks and dark “sandwich”-like regions will appear inside the green body. The cause of cracking is that the silicon oxide gas generated during reactive sintering is difficult to escape, gradually accumulating and increasing in pressure. This rising pressure then causes the green body to crack. In the dark “sandwich”-like regions within the sintered ore, there is a large amount of carbon that has not participated in the reaction.
(4) Reaction-sintered silicon carbide The phase composition of α-SiC, β-SiC, and free silicon. As the carbon content increases, the SiC content rises after reaction sintering, the SiC particle size grows larger, and the free silicon content decreases, all of which contribute to improving the mechanical properties of the reaction-sintered ceramics.
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