Study On Preparation And Properties Of C/SiC-ZrB₂（ZrC） Composites Via Slurry Infiltration Method

The needs of some thermal structure parts in hypersonic aircrafts, such as nose cone and leading edge of the wings, put forward higher requirements in ultra-high-temperature materials with excellent mechanical properties, oxidationresistance and anti-ablation behaviors. In this paper, continuous fiber reinforced ultra high temperature ceramics matrix composites, with obvious gradient structure, was prepared via slurry infiltration method.Gradient structure in these composites makes it possible to make full use of excellent performance at high temperatures. The stability and fluidity of the slurry,gradient composition,infiltration process, structure and performance of the composites, such as mechanical properties, high-temperature oxidation resistance and anti-ablation behavior were investigated.First, the relationshipsbetweencomposition and stability or fluidity of the composites were investigated. Compositions of slurry systems with good stability and fluidity were dertermined for the following preparation of composites via slurry infiltration mehod. These compositions arelisted as follows: mPCS: mXylene:mSiC=1:1:0.3(SC-15), 1:1:0.6(SC-30), 1:1:0.9(SC-45);mPCS:mXylene:mZrB2=0.8:1.2:0.1(ZB-5), 0.8:1.2:0.2(ZB-10), 0.8:1.2:0.4(ZB-20);mPCS:mXylene:mZrC=0.8:1.2:0.1(ZC-5), 0.8:1.2:0.2(ZC-10), 0.8:1.2:0.4(ZC-20).Effects of infiltration method on preparation process, composition and performance of C/SiC were evaluated. SC-15, SC-30 and SC-45 samples were prepared via vacuum bag-pressureinfiltration and vacuum-pressure infiltration, respectively. The former method shows a higher infiltration efficience in the first three rounds, the density of C/SiC compositescan reach 1.58g/cm3(85% ofthe final composites’ density). There is no difference in performance and content of SiC powder of C/SiC between these two methods. For these samples prepared by vacuumbag-pressure impregnation, bending strengths were 389.8MPa, 368.4MPa and 351.9MPa, respectively; densities of composites were 1.82g/cm3, 1.85g/cm3, 1.84g/cm3, respectively; volume fractionsof carbon fiber were about 40%, volume fractions of SiCp were 10.27%, 13.88%, 18.74%, respectively; SiC matrix volume fractions were 18.63%, 16.75%, 12.86%, respectively; porosity rates were about 29%; while for those samples prepared by vacuum-pressure impregnation, bending strengths were 410.8MPa, 397.0MPa and 344.5MPa, respectively; densities of compositeswere 1.84g/cm3, 1.88g/cm3, 1.85g/cm3, respectively; volume fractions of carbon fiber of about 40%, volume fractions of Si Cpwere 9.84%, 14.35%, 18.87%, respectively; volume fractions ofSiC matrix were 19.69%, 15.75%, 13.02%, respectively; porosity rates were about 29%. There is almost no difference in structure and performance compositesprepared by vacumm bag-pressure infiltration and vacumm-pressure infiltration. Vacuum bag-pressure process is complex, and there is a serious waste in slurry, vacuum-pressure infiltration process was used in subsequent experiments. Meanwhile, density gradient and porosity distribution of C/SiC were investigated. The results showed that obvious density gradient and porosity gradient structure exist in surface area of about 1mm thickness, so that the effective powder impregnation depth is 1mm.Effects of the Zr B2 p content in slurry on fabrication process, structure and properties of C/SiC-ZrB2 composites were studied. With the increase in powder content, density and bending strength decreased, and while content of ultra-high temperature ceramic powders increased,high-temperature oxidation resistance and ablation resistance improved. The flexural strength with three contents(ZB5, ZB10, ZB15) were 438.6MPa, 417.9MPa, 386.7MPa, respectively; densities were 1.89g/cm3, 1.97g/cm3, 1.94g/cm3, respectively; volume fractions of carbon fiber were about 42%, volume fractions of ZrB2 p were 4.05%, 6.57%, 9.36%, respectively; volume fractions of SiC matrix were 36.07%, 32.26%, 26.95%, respectively; the porosity rates of the material were 17.07%, 19.56% and 22.59%, respectively. After oxidation in 1600℃ for 20 min, mass retention rate and strength retention rate were 96.99% and 90.4%, respectively. During oxyacetylene flame ablation process, the mass ablation rate and linear ablation rate were 0.0030g/s and 0.0047mm/s, respectively. With SEM and EDS analysis of the cut surface of C/Si C-ZrB2, gradient distribution of ultra-high temperature ceramic powder was investigated.Results indicate that almostall ultra-high temperature ceramic powder enriched in surface area of the composites, relatively dense shell was formed, content of ultra-high temperature ceramic inside compositeswasvery low. Comparative analysis of the samples cut and fracture surface morphology before and after oxidation, ZrO2-B2O3-Si O2 glass withgood mobility was formed during oxidation, and spread on the surfaceof composites.Effects of ZrCp content in slurry on 3D C/Si C-ZrC composites” structure and properties were investigated. With the powder content increased, the density of the sample, the content of SiC matrix and bending strength decreases composites ultra high temperature ceramic powders increased oxidation resistance and improved ablation resistance, bending strength were 431.1MPa, 416.2MPa, 378.4MPa, densitieswere 1.96g/cm3, 2.01g/cm3, 2.12g/cm3, respectively; volume fractions of carbon fiber were about 43%, volume fractions of ZrCpwere4.31%, 6.34%, 8.96%, respectively; volume fraction of SiC matrix were 31.83%, 29.73%, 27.88%, respectively; the porosity of the material was 19.44%, 21.02% and 20.96%. After oxidation in 1600℃ for 20 min, mass retention rate and strength retention rate were 96.08% and 78.6%. After oxyacetylene flame ablation, mass ablation rate and linear ablation rate were 0.0025g/s and 0.0017mm/s. With SEM and EDS analysis of the cut surface of C/SiC-ZrC, gradient distribution of ultra-high temperature ceramic powder was investigated.Results indicate that almostall ultra-high temperature ceramic powder enriched in surface area of the composites, relatively dense shell was formed, content of ultra-high temperature ceramic inside compositeswasvery low. Comparative analysis of the samples cut and fracture surface morphology before and after oxidation,fluidity of ZrO2 is not so good as Zr O2-B2O3 galss, can not spread well on the surfaceof composites.