SiOC-based Ceramics Modified Carbon Fiber Framework Composites And Its Oxidation Resistant Coatings

Thermal insulation materials were usually composed of a framework with low thermal conductivity, reinforced phase and ceramic coating with high emittance to decrease the temperature of aircraft. The existing thermal protection system cannot meet the demands of aircraft, such as high temperature, oxidation-resistant, light-weight and long time. In this thesis, novel carbon bonded carbon fiber composites were prepared as the framework. Si OC polymer derived ceramics were synthesized as the reinforced phase of the framework. Ta Si₂ based coating was used for oxidation resistance. The structural design, fabrication process, properties and ablation behavior of the thermal insulation materials were carefully investigated. Firstly, CBCF materials were prepared through pressure filtration technique, which achieve a homogeneous microstructure without agglomeration and layering. The mechanical and thermal physical properties of CBCF were studied. Secondly, the synthetic process of Si OC ceramics was optimized. B or Zr element was introduced into the Si OC ceramics. The high temperature structure evolution and thermal stability was carefully investigated through various characterization techniques. Thirdly, the Si（M）OC/CBCF composites were prepared by PIP method. The mechanical and thermal physical properties as well as oxidation-resistant were studied in detail. Finally, the Ta Si₂ based coating was prepared and the ablation behavior of coating was examined through oxygen acetylene and high frequency plasma arc wind tunnel.Carbon-bonded carbon fiber composites（CBCF） were produced by a novel pressure filtration technique. This method consists of dispersing carbon fibers and phenolic resin to make slurry and then pressing the water out through a filter cloth, and finally carbonizing the composites at 1000°C in nitrogen. A h omogeneous microstructure without agglomeration and layering was achieved. A consequence of the pressure filtration process is that carbon fibers are oriented preferentially in xy direction and the microstructure and properties are anisotropic. The compressive strength for the CBCF composites with the density of 0.17⁰.3g/cm3 were 0.25⁰.75 MPa and 0.5¹MPa for z sample and xy sample, respectively. The compressive modulus for the CBCF composites with the density of 0.17⁰.3g/cm3 were 7³3MPa and 11³1MPa for z sample and xy sample, respectively. The compressive properties increases with density and the compressive properties for xy sample is larger than that for z sample, which is associated with the anisotropic microstructure. The thermal conductivity for the composites of 0.23g/cm3 was 0.2⁰.43W?m^-1?K^-1 and 0.11⁰.35W?m^-1?K^-1 for xy sample and z sample, respectively, at 25¹300°C. The thermal expansion coefficient of CBCF for both xy sample and z sample is 1.2×10-6/K^-1 at the temperature ranged from room temperature to 350°C.Si OC ceramics were prepared through two synthesis pathways, where the catalyst for crosslinking was acid and alkali, respectively. The crosslinking and pyrolysis mechanism were investigated in detail. The Si OC ceramics catalyzed by acid had a dense microstructure, but broke to pieces during drying and pyrolysis due to the large shrinkage. The Si OC ceramics catalyzed by alkali had a porous microstructure consisting of spherical ceramic particles, and it is feasible to prepare monolithic Si OC. At above 1400°C, the carbothermal reduction started up and led to the thermal decomposition of Si OC ceramics, especially in the porous Si OC owing to its high specific area. Therefore, the Si OC ceramics with acid catalyst exhibited higher thermal stability.In order to further improve the thermal stability of Si OC ceramics, B and Zr were introduced into Si OC ceramics. The prepared Si（M）OC gels were pyrolyzed in the temperature range 1000¹600°C to investigate the role of B and Zr on the structural evolution of Si OC ceramics at high temperature. It is found that the incorporation of B into the Si OC led to the increased sp3 carbon content and ceramic yield, and also promoted the precipitation of Si C nanocrystal as well as the graphitization of free carbon. While the incorporation of Zr led to the decreased sp3 carbon content and the crystallization of Zr O₂ nanoparticles. The simultaneous incorporation of B and Zr into Si OC led to the graphitization of free carbon and the crystallization of Zr O₂ and Zr Si O4 nanoparticles. Both the graphitization of free carbon and the crystallization of nanoparticles led to the improved thermal stability of Si OC ceramics.The Si OC-based ceramics reinforced CBCF composites were prepared through precursor infiltration ptrolysis. After two infiltration/pyrolysis cycles, the density of Si OC/CBCF, Si BOC/CBCF, Si Zr OC/CBCF and Si BZr OC/CBCF composites was 0.6, 0.88, 0.87 and 0.87g/cm3, respectively. The compressive strength of Si BOC/CBCF is the highest among these composites, which is 4.5 and 5MPa in the z and xy direction, respectively. The thermal expansion coefficients in both directions for all the composites were in the range of 1.2².9×10-6K^-1. The thermal conductivity of Si OC/CBCF and Si BOC/CBCF in the z direction was 0.25 and 0.3 3 W?m^-1?K^-1, respectively. The thermal stability of Si（M）OC/CBCF composites in inert and oxidizing atmosphere were studied. The results shows the B or Zr modified-Si OC/CBCF composites have better thermal stability than the Si OC/CBCF composites.In order to improve the oxidation resistance of ceramics carbon-bonded carbon fiber composites, a multi-composition coating consisting of multilayer Mo Si₂-Si O₂·B₂O₃-Si Cw/Ta Si₂-Mo Si₂-Si O₂·B₂O₃-Si Cw was prepared on the surface of Si OC/CBCF composites by slurry brushing. The emissivity of the coating was 0.8⁰.9 in the wave range of 8²5μm under 800°C. Ablation behavior of the oxidation protective coating was investigated through oxygen acetylene and high frequency plasma arc wind tunnel at the temperature of 1500¹700°C. After ablation, the coating consisted of dense layer and porous layer. The dense layer consisted of Si O₂ and Ta₂O₅. The porous layer consisted of Mo₅Si₃ and Mo_4.8Si₃C₆. At longer ablation time, the oxygen diffused through the glass phase and led to the oxidation of matrix. The ceramic carbon matrix was also oxidized and pores were formed. At higher temperature（1600°C, 600s）, the dense layer contained only Si O₂ glass phase, and the thickness of dense layer rapidly decreased. Pores and crack were observed in the interface of coating and matrix. At higher temperature（1700°C）, the macro defect was found in the surface of coating due to the volatilization of Si O₂ glass. The coating eventually departed from the matrix and was completely destroyed.