| With a series of excellent properties, such as low density, high modulus, good toughness, wear and corrosion resistant, carbon fibers reinforced silicon carbide composites(Cf/SiC) have found their wide application in Aeronautics and Astronautics areas. However, carbon fiber will be oxidized above 400?, which cause performance reduction of Cf/SiC, so the application of composites will be limited under high-temperature and oxidizing condition. One can conclude that the investigation of anti-oxidization coating to impel the application of Cf/SiC in aerospace field is of great importance.The slurry was prepared by mixing phenolic resin with different powders (B4C, B4C-50wt%SiC and graphite) homogeneously, and deposited on Cf/SiC substrate by rotating brushing. After curing and carbonization, we obtained porous precoating. Then silicon and zirconium powder mixtures with different ratio were used to process anti-oxidation coating by reactive melt infiltration. The ablation capability of the coated composites was tested under oxyacetylene torch flame. The mass ablation rates as well as strength retention were evaluated. The phase composition and micros true ture of the coating before and after ablation were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with attached Energy Spectrum Analysis (EDX). The microstructure and ablation mechanism of coating were explored. The main results are as follows:(1) The as-prepared precoating possesses smooth surface and dense structure while adding pure B4C powder, the thickness is 33-52?m after reactive melt infiltration and the composition of coating is uniform. However, there are microcracks on the precoating surface while adding powders of R4C-50wt%SiC, and the coating after infiltration has low intensity. Both of the two series of coating mentioned above forms SiC-ZrC-ZrB2 system. In the continuous area of SiC, the mixtures of ZrB2 and ZrC phase were found, similarly, in the mixed areas of ZrB2 and ZrC phase also exist SiC particles. Moreover, there are cracks in the coating while adding graphite powder, and the thickness is less than 20?m, the coating composition is SiC-ZrC, in which SiC and ZrC were distributed uniformly.(2) The ablation resistence of the coated composites was tested under oxyacetylene torch flame, the center temperature reached about 2300? and the duration time was 60s. The mass ablation rates as well as strength retention were evaluated. The results showed that adding pure B4C powder presented good ablation resistance. With the content of B4C varying from 30% to 50%, the mass ablation rates became smaller while strength retention increased. Furthermore, the ablation resistence is better when the content of Zr is 26.5wt% than Zr is 10wt%, namely, the coating has the best anti-ablation property while adding 50wt%RtC and conduct reactive melt infiltration with Si-26.5wt%Zr mixture (B50Z26 for short), the rates of mass ablation and strength retention is 0.97×10-4g/s and 67.98%.(3) The composition and microstructure of B50Z26 after ablation test was investigated, the results are as follows:two-layer structure which contained oxidation layer and slight-oxidation layer was observed. In the ablation center, oxide B2O3 volatilized dramatically due to the high temperature, which resulted in porous morphology, partial coating was peeled off and the exposed fibers became needle-shaped; In the transitional zone, mechanical washing became weak and the coating was covered by SiC-SiO2 mixed phase to protect Cf/SiC substrate from oxidization. On the ablation edge, the coating was oxidized slightly and the content of ZrO2 decreased while ZrB2 rised compared with the ablation center. |
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