Fabrication And Property Of The Ablation-Resistant ZrC-SiC Multi-Phase Coatings For C/C Composites

ZrC-based coatings modify the C/C composites,which is one of the ideal materials for the components of hypersonic vehicle.However,the thermal expansion difference between the coating and the substrate is prone to form the cracks of the components at high-Mach speeds,which restricts the application of the material.The gradient coatings were designed by using the difference of thermal expansion coefficient of materials,which can improve the binding strength and thermal shock resistance of materials,thereby ameliorating the ablative resistance of materials.In this paper,the reinforced ZrC-SiC and HfC-SiC coatings with ceramic solid solutions were prepared by in-situ solution reaction process.Meanwhile,the reinforced ZrC-SiC coatings with carbon fiber were used to improve the thermal shock resistance of the coating material.Last,the multicomponent ceramic phases（ZrC,HfC,TiC,SiC）were introduced into the C/C composites modified by carbon fiber,and the solid solution gradient coatings with carbon fiber were prepared,aiming to improve the ablative resistance of the coating material.The main research conclusions are listed below:（1）ZrSi₂ and TiC（0-60 wt.%）ceramic powders were uniformly mixed,and the solid solution coatings with Zr_xTi_1-xC,(Ti_xZr_1-x)₃SiC₂ and SiC phases were obtained by in-situ solution reaction at high temperature,which have a dense structure and uniform phase distribution.When the TiC content is 20 wt.%,the ablative performance of the prepared solid solution coating is the best.After 90 s plasma ablation,the mass and linear ablation rates are 0.025±0.003 mg/s and 1.022±0.006μm/s,respectively.During ablation,Zr_xTi_1-xC phase was oxidized to form a continuous Zr O₂ framework,which can pin the Zr Ti O₄-Ti O₂-Si O₂ oxide film from the oxidation of the(Ti_xZr_1-x)₃SiC₂ and SiC,thereby improving the stability of the oxide film;Moreover,the Hf-Ti-Si-C solid solution coating was prepared by the same process.In-situ solution reaction reduced the pores content of the coating,which improved the density of the coating.After oxyacetylene ablation for 60 s,the mass and linear ablation rates of the coating were-0.78±0.24 mg/s and-3.83±0.12μm/s,respectively.During the ablation process,Hf_xTi_1-xC,(Ti_1-xHf_x)₃SiC₂,(Ti_xHf_1-x)Si₂ and SiC were oxidized to form dense Hf O₂-Hf Ti O₂-Ti O₂-Si O₂ oxide film,which hindered the diffusion of oxygen into the coating.（2）The reinforced ZrC-SiC ultra-thick coatings by carbon fibers with different thickness were prepared by PIP method,and their thermal conductivity and ablative properties were investigated.When the thickness of C_f-ZrC-SiC coating is 2 mm,the density and the porosity of the coating material were increased by 13.02%and decreased by 64%,respectively.And its maximum thermal conductivity was increased to21.34 W/（m·K）.The dispersed carbon fibers in the C_f-ZrC-SiC coating improved the toughness of the coating,and the contents of ceramic phase between the coating and the inner substrate presented a gradient distribution,which improved the thermal shock resistance of the materials.After 60 s oxyacetylene ablation,the mass and linear ablation rate of the materials were 0.87±0.06 mg/s和4.00±0.21μm/s,respectively,and the ablation resistance is good.During the ablation process,ZrC and SiC with the high content in the coating were oxidized to form a dense Zr O₂ outer layer and Zr O₂-Si O₂ intermediate layer,which effectively hindered the diffusion of oxygen into the matrix.（3）The thermal expansion coefficient and ablative resistance of C_f-ZrC-SiC composites can be improved effectively by designing SiC ceramic gradient coating structure on C_f-ZrC-SiC coatings.When the thickness of C_f-ZrC-SiC layer is 1.2 mm,the maximum thermal expansion coefficient of the coating material was increased to 2.32×10^-6K^-1,which alleviated the thermal expansion difference between the C_f-ZrC-SiC coating and the CVD-SiC coating.Meanwhile,the formation of SiC ceramic gradient structure between C_f-ZrC-SiC layer and CVD-SiC layer,which improved the thermal shock resistance of the coating materials.After 120 s ablation,the mass and linear ablation rate were3.46±0.14 mg/s和16.67±0.51μm/s,respectively.（4）The reinforced（Zr,Ti）C-SiC and（Zr,Hf,Ti）C-SiC gradient coatings with carbon fibers were prepared by in-situ solution reaction on gradient C/C-ZrC-TiC composites.Among them,the reinforced（Zr,Ti）C-SiC gradient coating prepared at 2400℃had better ablative performance.And its mass and linear ablative rate were 2.31±0.16 mg/s and 19.00±0.63μm/s respectively.Modified（Zr,Ti）C-SiC coating by using HfC,the prepared（Zr,Hf,Ti）C-SiC gradient layer had best ablative resistance.During the ablation,the Zr_xTi_1-xC and Zr_xHf_1-xC multiphase ceramics with high content and SiC were oxidized to form a dense（Zr,Ti）O₂ oxide film with a small amount of Hf O₂ and Si O₂ oxides,which reduced the oxygen diffusion channels.After 120 s ablation,its mass and linear ablation rate were 1.74±0.06 mg/s and 14.67±0.35μm/s,respectively.The ablative resistance of C/C composites can be improved effectively by constructing multi-coating structure with ceramic gradient.