Analysis Of Temperature,Stress Field And Ablation Process Of Ultra High Temperature Anti-oxidation Coating Systems

Carbon fiber toughened silicon carbide?C/SiC?composite is regarded as one of the important candidate materials for aerospace thermal components due to their low density,high strength,excellent corrosion resistance and better fracture toughness.However,the rapid development of solid rocket engines demands high-performance parameters and brought about important problems such as ablation in an oxygen-rich environment.As one of the effective surface modification methods,Ultra-high temperature ceramics?UHTC?coatings are ideal materials to further improve the ablation resistance of C/SiC composites.However,due to the numerous factors affecting the ablation process,the ultra-high temperature oxidation coating system still lacks a perfect ablation mathematical model and theoretical calculation method.Therefore,this thesis uses finite element simulation sof tware COMSOL Multiphysics to simulate the physical field distribution and the ablation process of ultra-high temperature oxidation resistant coating system,and these results provide certain reference value for coating selection and system design optimizat ion.Based on the control equations,two models established by the software physics module were discussed.It is found that the HfO₂-ZrC-SiC-C/SiC model system has better thermal insulation ability and smaller stress value than the HfC-SiC-C/SiC model system.Based on the XY shear stress cloud map,we found that the coating interface in the former model has a less tendency to break.Then we performed an in-depth discussion to explore the internal oxidation component diffusion and to establish a crack model for calculating the physical fields.We found that the crack slightly accelerates the temperature conduction,and it can cause a large stress concentration which exceeds the maximum stress value of the non-crack model 200MPa.The crack model will be the oxygen diffusion channel leading the oxygen concentration at the bottom of the model to six times in the non-crack model.The mechanism of the ablation process was further analyzed.The J-integral method was used to establish the corresponding model to calculate the stress intensity factor of the crack propagation criterion.Based on the mass,energy conservation and physical property equations,the gas turbulence models in the engine nozzle were established.The line ablation rate of the nozzle base and that in each coating was calculated by numerical simulation method and the accuracy of the model was verified.By comparing the ablation resistance of different kinds of coatings and the matching between different coatings,a multi-component composite coating system was established,and the ablation behavior and ablation mechanism of the system were analyzed.The HfO ₂-ZrC-SiC-C/SiC quaternary system's line ablation rate at different temperature was calculated.The main results and conclusions are as follows.The maximum line ablation rate is between 0.3 and 1.2?m*s^-1,which reflects the excellent oxidation and ablation performance of Hf-and Zr-containing coatings.HfO₂ has good ablation resistance and self-stability.Compared with other systems,the ablation rates of the throat and diffuser part of HfO₂-ZrC-SiC-C/SiC system nozzle are lower.At 7 MPa,the line ablation rate was calculated at 1700K,2100K,2500K and 2900K,and the maximum line ablation rate region move along gas direction.The ablation rate of each temperature gradient line was increased by 174%,20.22%and18.04%,respectively.It is indicated that HfO₂ can effectively reduce the ablation rate of the nozzle converging section and is suitable as the outermost layer of the composite coating system.The increase in temperature significantly aggravates chemical ablation and mechanical ablation,and mechanical ablation at high temperatures is a major factor in UHTC ablation.