Modification Multi-scale Structure And Properties Of C/C-SiC-ZrC Composites

With the rapid development of aerospace industry,the performance of thermal protection materials applied in new high-speed vehicles and their engines have become increasingly demanding.Thermal protection materials are required to withstand ultra-high temperature,long-term oxidation and ablation,and have excellent mechanical properties.C/C-SiC-ZrC composites have become one of the most promising materials of ultra-high temperature thermal protection system because of their low density,high specific strength and modulus,good oxidation and ablation resistance,and thermal shock resistance.However,some key problems are still existed in the preparation and application of the C/C-SiC-ZrC composites,which limit their long-term application in extreme environments.For example,when ZrC is introduced by the precursor infiltration and pyrolysis(PIP),the ZrC precursor will react with pyrolytic carbon(PyC)interface/matrix and carbon fiber(Cf)during the high-temperature pyrolysis due to the carbon deficiency,thus resulting in the reduction of mechanical property.The introduction of SiC matrix by chemical vapor infiltration(CVI)is beneficial to improve the mechanical,anti-oxidation and anti-ablation properties of the C/C-SiC-ZrC composites,but the influence of their composition and content on these properties is not clear.Under the ultra-high temperature cyclic ablation environment,the cracking of the oxide film caused by ZrO2 phase transition and the formation of porous ZrO2 caused by the excessive volatilization of SiO2 limit their long-term reusability,and multi-component design and optimization are needed to improve the high temperature stability of the oxide product.In this paper,the idea of multi-scale structural modification was proposed,namely,nanoscale carbon was introduced during the PIP process to modify the interface by wrapping ZrO2 and restricting its reaction region,micrometer-scale SiC was introduced during the CVI process to modify the ratio of PyC and SiC layer on carbon fiber surface,and nanoscale rare earth oxides was introduced during the PIP process to stabilize the crystal structure of ZrO2 and reduce the violatilization of SiO2 in the ablation process.The main results are as follows:Carbon deficiency during the introduction of ZrC matrix through PIP process leads to the interface damage,for this problem,it was proposed to introduce a soluble and thermosetting pitch resin into the xylene solution of organic polymeric precursors to compensate carbon source,thus slowing down the etching of the PyC interface/matrix and Cf by carbothermic reduction reaction of ZrO2 during the precursor cleavage process and realizing the fabrication of the C/C-SiC-ZrC composites with low-damage interface.The results showed that when the mass ratio of PZC/PCS/resin was 20:1:5,highly pure ZrC-SiC powders with higher ZrC content could be obtained by the cleavage of precursor mixture at 1500℃.The flexural strength of the C/C-SiC-ZrC composites obtained by using the above-menttioned precursor solution was up to 247.4 MPa,which was 52.2%higher than that of the composites without carbon source compensation.This was mainly because that the added resin-derived carbon could evenly encapsulate the ZrO2 intermediate at a molecular level and react with it in-situ to form the ZrC matrix,thereby effectively preventing the damage of PyC and Cf.How to further improve the mechanical and oxidative ablation properties of the C/C-SiC-ZrC composites is also a key issue.Introduction of CVI-SiC matrix with high crystallinity,excellent mechanical and anti-oxidation properties into the composites was proposed to improve the performance of the composites and the effect of C-SiC volume ratio on the properties was investigated.The results showed that when the volume ratio of C-SiC was 0.91,the composite had the higher flexural strength of 355.4 MPa.This was mainly attributed to its relatively high content of CVI-SiC,low porosity,optimum interfacial bonding and reduced matrix micro-cracks.After oxyacetylene ablation at 2100℃ for 600 s,the composite with a C-SiC volume ratio of 0.12 possessed a better ablation resistance with linear and mass ablation rates of 0.94 μm/s and 0.13 mg·cm-2·s-1,respectively.This could be explained by the formation of the continuous oxide scale with the low oxygen diffusion rate of the formed glass and the supporting role of the solid ZrO2 skeleton for the glass retained.In view of the problem of the oxide film cracking caused by ZrO2 phase transition and the formation of porous ZrO2 caused by the excessive volatilization of SiO2 at ultra-high temperatures during the cyclic ablation,the rare earth oxides were introduced into the matrix of the C/C-SiC-ZrC composites using rare earth nitrate solution as precursor.The effect of rare earth oxides on the stabilization of oxidation products was studied.The results showed that the linear ablation rate of the composites containing 14 mol%Y2O3 after ablation at 1700℃ for 1000 s was 0.23μm/s,which was 28%lower than that of the composites without Y2O3 addition.With the increase of ablation cycles,the content of m-ZrO2 and the number of micro-cracks decreased because of the increasing solid solution of Y2O3 in ZrO2,thereby obtaining a lower linear ablation rate of 0.47 μm/s after ablation at 1900℃ for 1000 s.With the Y2O3 content increased from 14 mol%to 22 mol%and 26 mol%,the linear ablation rate first decreased and then increased,and the composite containing 22 mol%Y2O3 showed the lowest linear ablation rate of 0.43 μm/s.Under ablation at 1900℃ for 600 s,Yb2O3 showed a better modification effect than Y2O3,and the linear ablation rate of the composite decreased by 43.1%.The formation of the Zr-RE-O phase during the ablation process that avoided the generation of m-ZrO2 and inhibited the volume expansion of the oxide film during the cooling process,the formation of RE2Si2O7 phase with high stability and low oxygen diffusion rate and the stabilization of SiO2 by the dissolution of RE3+ in the Si-O bond network structure were the main reasons for the improvement of the cyclic ablation resistance.The idea and method proposed in this paper that using PZC/PCS/resin precursor solutions to prevent the carbon interface from damage was not only suitable for the preparation of carbon fiber reinforced ZrC matrix composites with high strength prepared by PIP process,but also applicable to the preparation of other carbide ceramic matrix composites.Besides,the effect of CVI-SiC matrix content,and RE2O3 type and content on the properties of the composites was systematically studied.The mechanical and ablation properties of the C/C-SiC-ZrC composites prepared by CVI-PIP process were optimized,the stabilization of RE2O3 on the oxide films under the ultra-high temperature cyclic ablation conditions was verified.The above work provided a new idea and method for improving the performance of carbon fiber reinforced ultra-high temperature ceramic matrix composites,and the support of key materials for the development of thermal protection systems for advanced vehicles.