Study On Ablation And High Temperature Compression Behavior Of Hybrid Continuous Fiber Reinforced Composites With Different Density

Ablative thermal protection system has advantages such as high heat-proof efficiency,high safety factor,high reliability and the adjustable density,which is one of thermal protection system with long history.The gradient design for Hybrid Continuous Fiber Reinforced?HCFR?composite is an effective method for the aircraft to cope with aerodynamic heating environment?high heat flux density peak and long heating time?,when reentry in the semi-ballistic-skip re-entry return orbit for aircraft.At present,the high-temperature evolution of HCFR composites is complex and lacking uniform testing methods of ablation resistance and mechanical properties.Besides,the basic data such as high-temperature mechanical properties are seriously insufficient.Therefore,gradient design of ablation materials faces great challenges.In this paper,the high temperature evolution,ablation resistance and mechanical properties of the carbon layer of HCFR composites with different density are analyzed.And the relationship of composition,structure and performance of the materials at different temperature is explored.The anti-ablation mechanism,the mechanical failure and enhancement mechanism for HCFR composites carbon layer are proposed,which lay the foundation for the subsequent gradient design for ablation material.Firstly,the pyrolysis process of the material and the structure and composition evolution of the carbon layer are analyzed.TGA-MS was used to analyze the dynamic pyrolysis process of materials in different oxygen ratios,the components and the temperature range of gas release during pyrolysis.It is proved that the high-density material has a higher mass residual rate at 1000°C,and oxygen plays a active role in the pyrolysis process of the phenolic resin of the material.The mass residual rate of the high-density material is higher due to the oxidation process of the inorganic filler.The porosity and opening diameter of the static pyrolytic carbon layer in N₂ atmosphere were analyzed by mercury intrusion method.The porosity and average pore diameter of low-density materials increased with temperature increase,and the average opening diameter of high-density materials increased with temperature decrease.The microstructure and composition of the carbon layer show that the carbon layer of the high-density material is more complete and the SiC ratio is higher.Secondly,the oxygen-propane ablation test was carried out to analyze the ablation resistance and heat insulation of the materials,the failure behavior of the surface layer.The anti-oxidation mechanism for high-density material was established.It was found that the surface of the low-density material after oxygen-propane ablation is rough,and the peeling phenomenon of carbon layer is serious.The main cause of surface ablation failure are particle ablation and subsurface relaxation.The ablation surface of the high-density material is flat and a molten protective layer is formed in the surface.the ablation line rate is significantly reduced,and the temperature difference between the ablated surface and the back surface is larger,which demonstrates that the heat insulation effect is better.Finally,the compression experiment of the carbon layer in vacuum environment was carried out at different temperature.The high temperature compression behavior of the carbon layers was analyzed.The high temperature compressive strength of the material increased firstly and then decreased with increasing temperature.The material has the lowest compressive strength at 900°C.In general,the high-density material has higher compressive strength than the low-density material.At lower temperatures,the inorganic filler acts as a framework.At high temperatures,the carbon layer inorganic filler reacts with the carbon layer by Si-C reaction,which increases the stiffness and strength of the carbon layer.