Study On ZrC-SiC Modified C/C Composites And Their Silicon-Based Ceramic Coatings

Carbon/carbon（C/C）composites have a series of good properties,such as low density,high specific strength,high specific modulus,excellent high temperature mechanical properties and so on,thus being regarded as one of the key materials in the field of aerospace.However,high oxidation sensitivity of carbon limits the applications of C/C composites in the aerospace field.To broaden the applications of composites in the high-temperature environments,C/C-ZrC-SiC composites were prepared by precursor infiltration and pyrolysis（PIP）method using 2D needle-carbon felts as preforms.The ablation properties of C/C-ZrC-SiC composites with a small angle were fully investigated in different heat fluxes.Results showed that the oxidation properties of C/C-ZrC-SiC composites were not good enough.To solve the above problems,coatings were prepared on C/C-ZrC-SiC by chemical vapor deposition（CVD）,pack cementation（PC）,and supersonic atmosphere plasma spray（SAPS）methods.For restraining the cracking of the coating caused by the mismatch of thermal expansion coefficient,the interlocking interfacial structure was constructed between the coating and substrate.In addition,Si C nanowire/pyrolytic carbon（SiCnw/PyC）core-shell structures were introduced to improve the interfacial adhesion of the coating and to alleviate the thermal mismatch between the coating and the substrate,by means of micron-scale mechanical interlocking and nano-scale bridging of SiCnw/PyC core-shell structures.The microstructure and phase composition of C/C-ZrC-SiC composites and coating were analyzed by XRD,SEM,and TEM.The ablation properties of C/C-ZrC-SiC and coated C/C-ZrC-SiC samples were investigated using an oxyacetylene ablation tester.Thermal shock,thermogravimetric analysis and isothermal oxidation tests were also carried out.The growth and strengthening mechanism of SiCnw/PyC core-shell structure was analyzed.The main results are as follows:The oxyacetylene ablation performance of C/C-ZrC-SiC with a small angle was studied in two heat fluxes（HF₁-2.38mw/m² and HF₂-4.18mw/m²）.During ablation a dense double-layer Zr O₂/Si O₂ structure could be formed on the surface of C/C-ZrC-SiC in HF₁ condition,resulting in excellent ablation resistance of the composites.The linear abltion rate of the composites was only 1.33±0.17μm/s.While in HF₂ condition a loose and porous Zr O₂ was generated on the surface of composites,which could not effectively resist the mechanical scouring and high-temperature corrosion caused by oxyacetylene flame.Therefore the linear ablation rate reached6.16±0.47μm/s in HF₂ condtion.Meanwhile in the isothermal static oxidation testing at 1500℃,Si O₂ could not be fully spread owing to the alternating carbon-rich/ceramic-rich laminated structures in the composites,reducing the oxidation resistance of the composites.The effect of pre-oxidation on the microstructure of C/C-ZrC-SiC was investigated.The results showed that pre-oxidation could open the channels between carbon fiber and PyC on C/C-ZrC-SiC so that carbon nanotubes（CNT）could grow inside the pores,which is helpful to the subsequent formation of coating with interlocking interface.The weight loss rate of the coating samples was reduced by64.46%after constructing the micron-nano multi-scale interface（weight loss of the samples after 25 time-thermal cycles between 1500℃and room temperature was7.58%）.On the one hand,the microstructure and composition transition were formed at the interface between the substrate and coating,alleviating the interfacial stress between them;on the other hand,the crack generation and propagation were suppressed by strengthening mechanism such as pulling out and bridging of CNT.This could improve the thermal shock properties of the coated samples.Si Cnw was grown in situ on C/C-ZrC-SiC by CVD method,followed by the deposition of PyC.The results showed that PyC deposited in situ on Si Cnw could grow uniformly around the Si Cnw by a screw dislocation growth model.By controlling the deposition process,SiCnw/PyC core-shell structures with uniform diameter and pore distribution could be obtained.Additionally,the deposition of PyC can bind the adjacent Si Cnw to form a complete skeleton with high toughness.Meanwhile at high temperatures,PyC shell could put thermal pressure on the Si Cnw core because of the difference of thermal expansion coefficient between PyC and Si C,hindering the phase transition and recrystallizing of Si Cnw,and avoiding the high temperature phase transition of Si Cnw.SiCnw/PyC reinforced CVD-Si C coating with composition,thermal expansion coefficient,elastic modulus and hardness gradients could be got by introducing SiCnw/PyC core-shell structures into the coating.The gradients obviously alleviated the interfacial stress caused by the mismatch of thermal expansion coefficient between the coating and substrate,and improved the thermal shock performance of the coating.After 8-time thermal cycles between 1500℃and room temperature,the weight loss of the samples was only 0.072%.In addition,the multi-interface pulling-out and crack deflection induced SiCnw/PyC core-shell structures made the coating tougher（increasing from 8.5±0.3N to 16.8±0.7N）,and thus improved the mechanical properties of the coating.The introduction of SiCnw/PyC core-shell structures into the SAPS-Zr B₂-Zr C coating could enhance the interfacial adhesion strength of the coating,Also,SiCnw/PyC structures could serve as channels for heat transference,which reduced the surface temperature of the coating,and improved the ablation performance of the coating.