Effect of SiC-ZrC Coating Prepared by SiZr Liquid Phase Sintering on the Oxidation Resistance of C/SiC Composites
PI Huilong1, SHI Xiaolei2(), XU Xingxiang2
1.State Key Laboratory of Thermostructural Composite Materials, Northwestern Polytechnical University, Xi'an 710072, China 2.Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
PI Huilong, SHI Xiaolei, XU Xingxiang. Effect of SiC-ZrC Coating Prepared by SiZr Liquid Phase Sintering on the Oxidation Resistance of C/SiC Composites. Acta Metall Sin, 2021, 57(6): 791-796.
Owing to their low density and good mechanical properties at high temperatures, C/SiC composites are increasingly used in the aerospace industry. They are also being proposed as thermal-structural materials in the hypersonic field; however, C/SiC composites are easily oxidized in high-temperature air environments. In this study, a C/SiC composite was coated with a SiC-ZrC oxidation-resistant layer by a two-step sintering method using Si-25%Zr (mass fraction) alloy, and the phase evolution of the coating was studied during the sintering. The oxidation resistance of the material was then tested at 1400oC in an air environment. The microstructural changes of the coating before and after oxidation and the effect of oxidation on the bending properties of C/SiC were analyzed. After the reaction with carbon, Si and ZrSi2 disappear in the coating, leaving only pure ZrC and SiC. The ZrC phase refined the structure of the reactive SiC layer. The grain size of the sintered SiC was 2 μm, versus 5-20 μm for SiC sintered from pure Si. The refined grains created a dense and continuous SiO2 film during the oxidation process. As the oxidation time was increased at 1400oC, the C/SiC composite with the SiC-ZrC coating began losing weight at 200 s, but began gaining weight at 500 s as a dense SiO2 film was formed. After 1000 s of oxidation, the flexural strength of the C/SiC composites was 335 MPa, only 5% lower than that of the initial C/SiC composite. According to this result, the sintered SiC-ZrC oxidation-resistant film effectively protected the mechanical properties of the C/SiC composite during the oxidation process.
Fig.1 SEM image of cross-section morphology of C/SiC composite by reactive infiltration process
Fig.2 Cross-section (a) and surface (b) SEM images of SiC coating formed on the surface of C/SiC-Si sample
Fig.3 Cross-section (a) and surface (b, c) SEM images of SiC-ZrC coating formed on the surface of C/SiC-SiZr sample (Fig.3c shows the BSE image)
Fig.4 XRD spectra of coatings formed on the surface of C/SiC-Si (a) and C/SiC-SiZr (b) samples
Fig.5 Mass ratios of C/SiC composite samples with different coatings before and after oxidation at 1400oC for different time (m1 showed the mass after oxidation and m2 showed the initial mass)
Fig.6 Surface SEM images of C/SiC-Si (a) and C/SiC-SiZr (b) samples after oxidation at 1400oC for 1000 s
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