Effect of Al₂O₃ and SiC Content in Pack Cementation Powders on the Microstructure of SiC Coatings on HTR Graphite Spheres

To improve the anti-oxidation ability of graphite, Al₂O₃ and SiC as additives, a uniform SiC coating was prepared on the surface of HTR graphite spheres by the pack cementation and sintering process. The microstructure of SiC coating was characterized by XRD, Raman spectroscopy, scanning electron microscopy, and the effect of Si/C ratio, Al₂O₃ and SiC content in the starting powder, sintering process conditions on the microstructure of SiC coating was also analyzed. The results show that the SiC coating on the surface of spherical graphite sample can be obtained with Si/C ratio higher than 3: 1, and the SiC coatings with different raw materials possess different microstructure and phase constitutes. The SiC coating is a kind of clear porous microstructure, while the main composition of the coating is β-SiC with some α-SiC and unreacted graphite. It is found that the content of Al₂O₃ has a significant influence on the penetration into the SiC coating, while the SiC content has a great influence on the microstructure between SiC coating and the substrate. When Al₂O₃ content is 10%, the bonding between the SiC coating and the substrate is better. When SiC content is 20%, the SiC coatings obtained have good densification and less lamellar accumulation of on the surface of the coatings. The SiC coatings sintered in Ar gas at 1750 °C have larger thickness and porous porosity microstructure, and the maximum thickness, about 400 ∼ 600 μm, can be achieved with the 2h of sintering time. The obtained SiC coatings sintered in vacuum at 1750 °C have a plate-like dense microstructure, in which the main composition of the coating is free Si, α-SiC and β-SiC, and the transitional area between SiC coating and the substrate is not obvious.