Ceramic–Polymer Continuous Gradient Thermal Protection Structure And High Temperature Performance

With the rapid development of hypersonic vehicles,thermal protection system of aircrafts put forward higher requirements for thermal insulation and carrying capacity.The traditional combined thermal protection system has the problems of performance mismatch between different materials and low structural efficiency,which seriously affects the reliability of the thermal protection system.Therefore,the integrated structure of heat insulation and bearing has become a hot research topic of the thermal protection system.Antioxidant ceramic materials have good high temperature oxidation resistance and mechanical properties,but poor thermal insulation properties.Resin matrix composites have good thermal insulation properties,but there is a problem of carbonization and ablation under high temperature conditions.Therefore,giving full play to the advantages of anti-oxidation ceramic materials and resin matrix composites,anti-oxidation ceramic materials are used at the thermal protection end,and resin matrix composites are used at the thermal insulation end.A continuous gradient structure is formed between the two to achieve a smooth transition from anti-oxidation ceramic materials to resin matrix composites,so as to alleviate the thermal short circuit and thermal mismatch between different anti-insulation materials,and achieve the purpose of integrated heat insulation / bearing structure,which is the main direction of the current development of thermal protection materials.However,there is a lack of integrated molding research on this continuous gradient structure of ceramic-to-resin matrix composites,and a lack of systematic understanding of its high-temperature performance.In this paper,the ceramic precursor was ceramicized in different degrees by controlling the temperature gradient distribution to solve the problem of ceramic-polymer molding process under the condition of large temperature gradient.The continuous gradient transition structure between ceramic and polymer meeting certain size requirements was prepared by optimizing the molding process and fiber texture form,and its heat insulation and high temperature mechanical properties were characterized,analyzed and evaluated.It can promote the development and engineering application of integrated thermal protection structure with heat insulation and bearing.(1)According to the continuous variation characteristics of gradient structure components,the basic properties of polysiloxane and its composites at different positions of gradient structure were systematically studied.Firstly,the basic properties of polysilazane precursor at different temperatures were studied to determine the performance variation of matrix components in the gradient structure.Secondly,three-dimensional five-directional carbon fiber braided preform is introduced into the matrix.The thermophysical properties and mechanical properties of the composites under different transformation degrees were characterized and modeled to obtain the performance variation of the composites under different transformation degrees.This provides data support for optimal design of continuous gradient structure.(2)In view of the complex modeling of continuous gradient structure and the difficulty of integrated molding of ceramic polymer,the hierarchical gradient model is used to optimize the distribution of gradient structure.The forming method of continuous gradient structure is designed and realized.Firstly,the layered modeling method is used to establish the gradient layer distribution model,and the effects of heating rate and surface temperature on the gradient distribution form are studied.The optimal scheme of gradient transition distribution form is determined by heat transfer and stress response analysis.Secondly,the heating device and forming process of continuous gradient structure are proposed and designed.The polymer-derived ceramics method(PDCs)was combined with the impregnation-pyrolysis method(PIP),and the continuous gradient structure was effectively prepared when the temperature difference between ceramic and polymer molding was greater than 600℃,which verified the feasibility of ceramic-polymer integrated molding.(3)In view of the thickness size limitation of thermal protection structure,under the condition of satisfying the optimized gradient structure transition form,the molding process was improved,and the five-dimensional gradient thermal protection structure with a thickness of 40 mm was obtained by integrated preparation.Aiming at the problem of insufficient mechanical properties of five-direction gradient structure along gradient direction,three-dimensional seven-direction carbon fiber braided preform and temperature gradient control device are improved.The seven-direction gradient thermal protection structure of20 mm × 20 mm × 40 mm sample and 100 mm × 100 mm × 40 mm plate was fabricated.Aiming at the difficulty of high temperature mechanical properties test of continuous gradient structure,the high temperature compression properties of composites with different conversion degrees were tested by layered test method.The strength properties of composites were obtained,and the high temperature compression properties of continuous gradient structure were analyzed.(4)In order to verify the ablation and thermal insulation performance of continuous gradient structure,the thermal insulation performance tests of five-directional gradient structure and seven-directional gradient structure were carried out under different ablation environments using oxyacetylene ablation and arc wind tunnel ablation.A variety of analysis and test methods are used to discuss the ablation results and obtain the ablation and thermal insulation performance of continuous gradient structure in different environments.It is verified that the seven-directional gradient structure has good ablation and thermal insulation properties at 1750 ℃,and the ablation and thermal insulation mechanism of the continuous gradient structure under the synergistic effect of amorphous SiCN ceramics and carbon fiber braid is clarified.This provides basic data for the response analysis of ceramic-polymer continuous gradient structure under high temperature load environment.