Antioxidant Coatings Design And Structure Optimization Of C/SiC Composites

Carbon fiber reinforced silicon carbide ceramic matrix composite?C/SiC?has become a strategic material in the aerospace field due to its outstanding characteristics such as high specific strength,high specific modulus,high temperature resistance,ablation resistance and excellent thermodynamic properties.However,with the harsher application environment of rocket engines in the aerospace field,the demands for their performance indicators are increasing.As one of the most effective surface modification methods,the preparation of anti-oxidation coating can further improve the oxidation and ablation resistance of C/SiC composites.However,due to the many types of ultra-high temperature ceramic coatings,the matching methods of composite coatings used in its framework are also diverse,and the existing anti-oxidation coating systems still lack a consummate theoretical calculation method and ablation model.Therefore,an analytical model of thermal shock is established in this paper,and the coating materials with the lowest thermal stress when matched with C/SiC composites are screened.The COMSOL Multiphysics software was used to calculate the physical field distribution and ablation process of the rocket nozzle and its anti-oxidation coating system.Finally,the composite coating system of C/SiC nozzle was optimized.Based on the thermal shock model of the single-layer coating,an analytical model of thermal shock stress was established,and the finite element model was used to verify it.Compared with the finite element model,this analytical model can more easily calculate the thermal stress and critical temperature difference of fracture of the coating material.Micro-cracks models with a length greater than 30?m or an appropriate density in the anti-oxidation coating are calculated,which helps reduce the thermal stress at the interface between the anti-oxidation coating and the substrate and improve the thermal shock resistance of the anti-oxidation coating.It is calculated that the ceramic coating material with the lowest thermal stress when matched with the C/SiC matrix is ZrO₂,and the second lowest material is HfO₂.Two structural models were established using finite element software.The results show that the film system?1??HfO₂-ZrC-SiC-C/SiC?has better heat insulation performance than the film system?2??TaC-ZrC-SiC-C/SiC?and?3??HfO₂-SiC-C/SiC?.Excellent.The stress development of different structural models was analyzed by the model stress field cloud diagram.The maximum shear stress inside Model 2 is1.2 times that of Model 1,and the maximum tensile stress is 1.3 times that of Model 1.In comparison,Model 1 has better thermal shock resistance.Based on the energy,mass conservation and physical property equations,a turbulent gas flow model in the engine nozzle was established.The basic data of the physical field environment in the nozzle pipe was calculated and verified.The flow field environment in the tube was introduced into the nozzle model,and the linear ablation rates of the substrate and each coating material were calculated.The ablation rates of the coating materials were calculated and compared,and the coating system was optimized for different areas of the nozzle.HfO₂-ZrC-SiC-C/SiC film system is used in the nozzle convergence area,and TaC-ZrC-SiC-C/SiC film system is used in the throat and diffusion area.The optimized line ablation rate of the nozzle composite coating at the throat position most prone to failure is approximately 0.18 mm/s.