Research On ZrB₂ Based Ceramic Modified Carbon-bonded Carbon Fiber Composites And Its Oxidation Resistant Coatings

With the rapid development of the aero and space technology, the performance of the thermal protection material for aircraft needs to be gradually improved. Ceramic insulating tile with the shortcomings of brittleness and high density is difficult to meet the requirements for future development of aircraft. Si-O-C ceramic modified carbonbonded carbon fiber（CBCF） composites and anti-oxidation coating have been caused the attention of more and more researchers recently. It is a new generation of thermal protection and heat insulated integration material with super lightweight, high stability, non-ablative, toughening and resistant to high temperature. However, the carbon thermal reduction reaction between Si-O-C ceramic and carbon fibers will decress the stability of the substrate above 1400 oC. In this study, to improve the stability of the substrate, the CBCF was modified by ZrB₂ ceramic via a precursor infiltration and pyrolysis method. Firstly, in order to improve the adsorption ability to the liquid precursor for carbon fiber, the Si C particles modified carbon fibers（CBCFs） were prepared to increase the surface roughness of carbon fibers. Secondly, the CBCFs composites modified by ZrB₂（CBCFs/ZrB₂） were prepared by precursor infiltration and pyrolysis method. The microstructure, mechanical, and thermal properties of CBCFs/ZrB₂ composites were investigated in detail. In addition, the mechanical enhancement of lightweight ZrB₂-modified CBCFs composites by in-situ grown carbon nanotubes（CBCFs-CNT） was studied. Finally, the the multi-composition coating for CBCFs/ZrB₂ and CBCFs-CNT was prepared and oxidation behavior of the as-prepared coating were also studied.To improve the surface roughness of carbon fibers, the Si C particles modified carbon fibers were prepared by effective dispersion and flocculent approach for the first time. The adsorption of the carbon fiber for ZrB₂ polymeric precursor could be promoted by modified with Si C particles. The effects of PEI and PAM on slurry dispersibility and stability and the dispersion behavior of slurry were studied. The optimum amounts to achieve a stable and homogeneous Si C dispersion for preparing CBCFs are 0.6wt.% PEI and 0.8wt.%PAM.The CBCFs composites modified by ZrB₂ were prepared by precursor infiltration and pyrolysis method. The pyrolysis behavior of the Zr and B-containing hybrid polymeric precursor was investigated in detail at different temperature. The effects of ZrB₂ content on the density, microstructure, mechanically and thermal properties of CBCFs/ZrB₂ composites were further studied. The results demonstrate that the properties of lightweight CBCFs/ZrB₂ composites show anisotropic between x/y direction and z direction due to the anisotropic structures. The compressive strength ranges from 1.36 MPa to 2.83 MPa and from 0.61 MPa to 1.63 MPa for x/y direction and z direction, respectively, with different densities. The thermal conductivity for the composites of CBCFs/ZrB₂ composites with density of 0.77g/cm^-3 is 0.524⁰.883 and 0.237⁰.473 Wm^-1K^-1 for the directions perpendicular and parallel to the pressure, respectively, from 25.5°C to 1001.7°C. The thermal expansion coefficients at z direction and x/y direction is from 0.14×10-6/K to 1.19×10-6/K and from 0.17×10-6/K to 0.69×10-6/K, respectively, from 67 oC to 400 oC.The oxidation behavior of CBCFs/ZrB₂ composites was studied. The TGA-DSC results show that the beginning oxidation temperature of ZrB₂ coated is raised from 517 oC to 742 oC and the largest weight loss of composites without ZrB₂ coating is near 736 oC, whilst the ZrB₂ coated ones are near 1025 oC. The isothermal oxidation tests show that the complete oxidation of CBCFs composites is about at 30 min, while the CBCFs/ZrB₂ composites is not complete oxidation even at 60 min at 800 oC. The coating of infusible compound ZrB₂ on the CBCFs composites can improve the oxidation resistance.The in-situ self-growing CNTs were prepared by an original catalytic method based on ZrB₂ contained precursors and mixed structured CNTs-ZrB₂-Zr O2 was synthesized. The microstructure evolution and formation mechanism of in-situ self-growing CNTsZrB₂-Zr O2 mixed structures were discussed. Moreover, in-situ strategy to prepare carbon nanotubes（CNTs） and ZrB₂-modified CBCFs composites were studied. Results show that the in-situ self-growing CNTs play a significant role in improving the mechanical properties of CBCFs composites by pulling-out and bridging effect compared with CBCFs composites without CNTs. This leads to pseudo-plastic behavior of CBCFsCNTs. The CNTs with pullout behavior and tensile fracture behavior which change and enlarge the crack propagation path can absorb more energy under bending tests. The compress strength increase by 11.03% at x/y direction and 18.12% at z direction than CBCFs/ZrB₂ without CNTs. Moreover, the flexural strength increase by 13.30% at x/y direction and 60.01% at z direction than CBCFs/ZrB₂ without CNTs. A larger increase in thermal conductivity and thermal expansion coefficients is observed for CNTs modified CBCF/ZrB₂ composites. The results show that the thermal conductivity is much larger in z direction than in x/y direction due to the growth direction of CNTs. Moreover, the thermal expansion coefficient of composites in x/y direction is close to the z direction by in-situ growth CNTs.Finally, the ZrB₂-based anti-oxidation coating was successfully fabricated on the surface of CBCFs/ZrB₂ and CNTs modified CBCFs/ZrB₂-Zr O2 composites, respectively. The influence of temperature on the emissivity of coating was studied. And the ablation behavior of ZrB₂-based coating coated composites was studied by isothermal oxidation tests, oxyacetylene torch tests and high frequency plasma wind tunnel tests. The results showed that the emissivity of coatings increases with increasing of Mo Si2 content. Compare to single coatings, the double layer coatings have excellent antioxidants. In addition, the in situ growth CNTs in the matrix does not impact the ablation behavior of ZrB₂-based coating. With the increasing ablation temperature, the grey Si- rich containing glass phase on the coating surface reduced and disappeared after ablation at 1700 oC under an oxyacetylene flame. However, the carbon fibers in the interface between the coating and CBCFs composites were coated by Si O2, which protect carbon fibers from oxidation again. The ablation of coatings becomes serious with the increasing test time at 1600 oC and the carbon fiber was oxidized for 800 s. The high frequency plasma wind tunnel tests demonstrated that the ZrB₂-based ceramic coating can provide erosion protection for CBCF composites in combustion gas under wind tunnel tests.