C/SiC复合材料具有低密度、耐高温、高强度、抗氧化等优异性能，在高超声速飞行器的热防护系统中具有广阔的应用前景。随着高能激光技术的发展，高功率密度的激光能否对高超声速飞行器的热防护系统构成有效破坏，成为人们关注的一个热点问题。本文对C/SiC复合材料在不同环境下的激光烧蚀行为进行研究，旨在阐明高超声速来流条件下激光对C/SiC复合材料的热力烧蚀机理与多场耦合行为，为高超声速飞行器热防护系统的激光攻防策略提供基础性支撑。

当C/SiC复合材料受激光辐照时，高温下各组分会发生一系列的物理化学反应，如碳纤维和SiC基体的升华、分解、氧化等。在高速来流下，气动力诱导出的机械剥蚀效应显著，会改变烧蚀形貌，加快激光烧蚀过程。此外，高速气流条件下激光对C/SiC复合材料的热力烧蚀涉及了流场、传热、结构、烧蚀之间的相互作用，是一个复杂的多物理场耦合过程。本文主要从烧蚀试验、机理模型和多场耦合数值的角度，对C/SiC复合材料的激光烧蚀机理与多场耦合效应进行研究工作，并揭示了高速来流条件对激光烧蚀行为的影响规律，以及高超声速来流条件下“雪崩”破坏发生的机理与条件。

在烧蚀试验的研究层面，本文进行了C/SiC复合材料在静态空气、超声速风洞和高超声速风洞环境下的激光烧蚀试验。通过C/SiC复合材料烧蚀形貌演化、烧蚀深度和烧蚀质量损失等关键烧蚀数据，研究了在不同环境下材料的激光烧蚀行为。高速来流在烧蚀表面诱导出机械剥蚀效应，从而改变了烧蚀形貌，并加速了C/SiC复合材料的激光烧蚀。随着来流马赫数的增大，激光对C/SiC复合材料的线烧蚀速率增大，质量烧蚀速率的显著增加。在高超声速气流环境下，激光诱导的质量烧蚀速率是静态环境下的5~9倍，我们称之为高超声速气流作用下激光烧蚀的“雪崩”加速现象。特别地，在长时间高超声速气流作用下，C/SiC复合材料的质量烧蚀速率比短时间高超声速气流条件下提升了约70 %。上述实验结果表明，气动剥蚀效应和气动加热效应都会对“雪崩”加速破坏构成显著贡献。

在机理模型的研究层面，对C/SiC复合材料的激光烧蚀机理进行分析，并在理论模型上表征了C/SiC复合材料的烧蚀行为。研究发现，高超声速气流导致烧蚀区域的烧蚀机制发生改变，烧蚀坑内机械剥蚀效应比较严重。在高功率激光作用下，C/SiC复合材料展现出独特的微观烧蚀形貌：在静态空气环境下，碳纤维呈现出细长的“针笋状”烧蚀形貌；而在高速气流的作用下，碳纤维由“针笋状”呈现为“破损”状态。这种微观结构的差异反映出高速气流给烧蚀过程带来了新的机制。根据C/SiC高温时的各相物理化学反应，给出了描述各个反应机制的表征模型。给出了“针笋状”碳纤维烧蚀形貌的理论表征，并分析了几类无量纲参数对碳纤维烧蚀高度的影响以及原因。提出了“烧蚀-剥蚀-退化”循环烧蚀状态假设，从而建立起表述纤维/基体剥蚀速率的理论模型。烧蚀导致的退化速率差异与其在壁面温度环境下的烧蚀状态有关，随着温度升高，剥蚀速率由基体剥蚀逐渐转化为纤维剥蚀，并随之增大。

在多场耦合数值计算层面，针对激光烧蚀过程中烧蚀表面的界面退化以及多场耦合效应，进行了模型表征、耦合算法以及数值仿真工作。考虑了来流特性对升华、氧化和机械剥蚀等各种机制的影响，建立对应的烧蚀模型来表征对应的界面退化。提出了改进的ALE网格变形处理方法，有效实现局部大变形条件下的界面捕捉与载荷加载，建立了适用于C/SiC复合材料激光烧蚀的多物理场耦合数值仿真模型。基于该数值模型，完成了不同环境下C/SiC复合材料的激光烧蚀行为数值仿真模拟，定量化地给出了高超声速气流对激光烧蚀行为的影响规律。数值计算结果表明，升华增强和剥蚀加速的综合效应是C/SiC复合材料在高超声速气流下激光烧蚀“雪崩”加速现象的主要原因，而激光辐照前的长时间高超声速气动加热带来的高结构初始温度会进一步加剧这一现象。

总地来说，C/SiC复合材料在高速气流环境下的激光烧蚀行为，是耦合了升华反应、氧化反应和机械剥蚀效应等多种烧蚀机制综合作用的结果，而升华增强和剥蚀加速的综合效应是C/SiC复合材料在高超声速气流条件下激光烧蚀“雪崩”加速现象的主要原因。

Due to the excellent material properties, such as low density, high-temperature resistance, high strength and oxidation resistance, C/SiC composites have broad application prospects in the field of thermal protection systems for hypersonic vehicles. With the development of high-energy laser technology, whether high-power density laser can effectively damage the thermal protection system of hypersonic vehicle has become a hot issue. In this paper, the laser ablation behaviors of C/SiC composites in different environments are studied, to clarify the thermal ablation mechanism and multi-field coupling behavior of laser on C/SiC composites under hypersonic airflow, so as to provide the basic support for the laser attack and defense strategy of hypersonic vehicles thermal protection systems.

When C/SiC composites are irradiated by laser, a series of physical and chemical reactions will occur at high temperatures, such as sublimation, decomposition and oxidation of carbon fiber and SiC matrix. At the same time, under high-speed airflow, the mechanical erosion effect induced by aerodynamic force is significant, which will change the ablation morphology and accelerate the laser ablation process. In addition, the thermal ablation of C/SiC composites by laser under high-speed airflow involves the interaction between flow, heat transfer, structure and ablation, which is a complex multi-physics field coupling process. This paper mainly studies the laser ablation mechanism and multi-field coupling effect of C/SiC composites from the ablation test, mechanism model and multi-field coupling numerical solution, and reveals the influence law of high-speed airflow on the laser ablation behavior of materials, as well as the mechanism and conditions of 'avalanche' failure under hypersonic inflow.

The laser ablation tests of C/SiC composites in the static air, supersonic wind tunnel and hypersonic wind tunnel are carried out in this paper. Through the key ablation data such as ablation morphology evolution, ablation depth and ablation quality loss, the laser ablation behavior of materials in different airflow environments are studied. High-speed airflow induces mechanical erosion effect on the ablated surface, which changes the ablation morphology and accelerates the laser ablation of C/SiC composites. With the increase of Mach number of airflow, the linear ablation rate of C/SiC composites increases, especially the mass ablation rate. In the hypersonic airflow environment, the mass ablation rate induced by laser is 5~9 times higher than that in the static environment, which is called the 'avalanche' acceleration phenomenon of laser ablation under hypersonic airflow. Especially, the mass ablation rate of C/SiC composites is about 70% higher under the action of hypersonic airflow for a long time. The above experimental results show that both aerodynamic erosion effect and aerodynamic heating effect will make a significant contribution to the 'avalanche' acceleration phenomenon.

The laser ablation mechanism of C/SiC composites is analyzed, and the ablation behavior of C/SiC composites is characterized by the theoretical model. It is found that the ablation mechanism in the ablation area is changed due to the hypersonic airflow, and the mechanical erosion effect in the ablation pit is more serious. C/SiC composites exhibit unique micro ablation morphology under high-power laser ablation. On the micro-scale, the fiber presents a 'needle' ablation morphology, while under the action of high-speed airflow, the carbon fiber presents a 'tattered' state instead of 'needle' state. This difference in microstructure reflects that the high-speed airflow brings a new mechanism to the ablation process. According to the physicochemical reactions of C/SiC at high temperatures, a characterization model describing each reaction mechanism is given. The theoretical characterization of the ablation morphology of 'needle' carbon fiber is given, and the effects of several dimensionless parameters on the ablation height of carbon fiber and the reasons are analyzed. The hypothesis of the 'ablation-erosion-recession' cycle ablation state is proposed, and a theoretical model describing the erosion rate of fiber/matrix is established. The difference of the recession rate caused by ablation is related to the ablation state under the wall temperature environment. With the increase of temperature, the erosion rate gradually changes from matrix erosion to fiber erosion and increases.

The model characterization, coupling algorithm and numerical simulation are carried out, to study the interface recession and multi-field coupling effect of ablated surface in the process of laser ablation. Considering the influence of airflow characteristics on various mechanisms such as sublimation, oxidation and mechanical erosion, a corresponding ablation model is established to characterize the corresponding interface recession. An improved ALE mesh deformation processing method is proposed to effectively realize the interface capture and load loading under the condition of local large deformation, and a multi-physical field coupling numerical simulation model suitable for laser ablation of C/SiC composites is established. Based on the numerical model, the numerical simulation of laser ablation behavior of C/SiC composites in different environments is completed, and the influence law of hypersonic airflow on laser ablation behavior is given quantitatively. The numerical results show that The combined effect of accelerated sublimation and enhanced erosion is the main reason for the 'avalanche' acceleration of laser behavior of C/SiC composites under hypersonic airflow, which will be further exacerbated by the high initial temperature of the structure from the hypersonic aerodynamic for a long action time before laser irradiation.

In general, the laser ablation behavior of C/SiC composites in high-speed airflow environment is the result of the combined action of various ablation mechanisms such as sublimation reaction, oxidation reaction and mechanical erosion effect. The combined effect of accelerated sublimation and enhanced erosion is the main reason for the 'avalanche' acceleration phenomenon of laser ablation behavior of C/SiC composites in hypersonic airflow.