C/C复合材料预制体中碳纳米管可控生长与强韧机制

The low strength and great brittleness of carbon matrix result in the weak interlaminar shearing resistance and unsatisfied mechanical properties of carbon/carbon composites (C/Cs), especially for the aculeated or thin C/C parts. In order to resolve these problems, this project proposes the idea about applying carbon nanotubes (CNTs) with controllable morphology and microstructure as the secondary reinforcements to stiffen and toughen carbon matrix, which are in-situ grown in the performs of C/C composites by catalytic chemical vapor deposition (CCVD). The final aim is to improve and control the mechanical property of carbon fiber-based multiscale hybrid C/Cs, i.e. CNT-C/Cs. This project will be focused on the process mechanism of CNT nucleation and growth in the complicated carbon fiber preform based on the CCVD monitoring technology and the interaction among CNT growth and the structure of carbon fiber perform and CCVD parameters. Based on this research, the theoretical and technological base for the active control of CNT morphology and microstructure, and the uniform CNT growth in large-sized carbon fiber preform with complicated structures will be founded. Furthermore, the general law of action between CNTs and mechanical properties of C/Cs will be explored through investigating the effects CNTs with different morphologies and microstructures on the densificaton process, the microstructures, the micro-mechanical and macro-mechanical properties of C/Cs and the related mechanism will be established, which will be used as the guidance for the optimization of the morphology and microstructure parameters of CNTs based on the design and regulation of the service behavior of CNT-C/Cs. The present work will provide a meaningful way for the development of C/Cs with high strength and excellent toughness, and also enrich the design and reinforcement theory of carbon fiber-based multi-scale composites.

针对碳/碳(C/C)复合材料基体碳强度低、脆性大及其导致的材料抗层间剪切强度差，尖角、锐形、薄壁区域力学性能薄弱等问题，本项目提出一种多尺度增强方法，即在碳纤维预制体中原位生长“形性”可控的碳纳米管(CNT)，围绕CNT生长机理、均匀性分布控制及强韧化机制开展深入研究工作，旨在借助“形性”可控CNT与碳纤维的协同增强增韧，实现C/C复合材料的性能提升与调控。拟系统研究CNT在C/C复合材料预制体中的形核机制和生长过程，揭示不同 “形性”CNT的生长机理，实现其可控生长，阐明CNT对C/C复合材料致密化过程和碳基体微观结构的影响规律，揭示C/C复合材料中CNT与碳纤维的协同强韧化机制，为航空航天用高强韧C/C复合材料的制备奠定理论基础，同时发展复合材料的设计和强韧化理论。

我国航空航天及国防科技的快速发展，对高强韧薄壁、锐形C/C复合材料提出了迫切需求。针对传统C/C强度低、脆性大，薄壁、锐形区域力学性能薄弱等问题，本项目旨在在碳纤维预制体中原位生长“形性”可控的CNT，在微纳尺度上对C/C补强增韧、进而大幅提高其力学性能。主要开展CNT形貌与结构优化设计、CNT“形性”可控、均匀生长机理、CNT对C/C致密化与石墨化过程影响、CNT-碳纤维的协同强韧机制、CNT-C/C服役环境结构演变及模拟构件考核等研究。研究主要结果如下：优化获得了C/C强韧用CNT应具有的最佳结构与形貌特征，即平直管身和高结晶度管壁结构；提出了“液相催化生长”模型，在碳纤维表面实现了径向直立CNT和定向CNT阵列的原位生长，解决了传统研究中因CNT形貌卷曲、团聚、自身强度低等导致的强韧效果不足问题，使得C/C的基体主导力学性能提高了100-200%以上，该成果使得CNT的强韧效果相比传统水平提高了3-5倍；通过CVD温度场和流场优化，突破了传统CVD只能在预制体表面或近表面生长CNT的瓶颈，使得复杂结构碳纤维预制体内CNT的最大生长深度由传统研究的0.5mm提高到40mm以上；相比卷曲CNT，直立CNT可提高C/C复合材料全程致密化速率、可更好地抵御基体石墨化裂纹出现；直立CNT可通过“径向缝合热解碳”、“径向支撑碳纤维”、“细化热解碳”等多途径对复合材料实施强韧。发现、论证了CNT界面压应力诱导裂纹转向的强韧新机制，较准确揭示了CNT的间接强韧途径；发展了以Cu为催化剂生长直立CNT的方法，解决了传统CNT催化剂加速CNT-C/C烧蚀的问题，改善了CNT-C/C的抗烧蚀性能。制备了CNT-C/C尖劈构件，进行了燃气风洞考核。在来流总焓为2650 KJ/kg、气流速度为2040 m/s条件下，经3*700 s考核后，线烧蚀率仅为8.1×10-5 mm/s，性能显著优于其他碳陶和纯陶瓷材料，满足可重复使用M5-M6飞行器前缘构件的性能要求。项目研究成果可指导高强韧大推重比航空发动机叶片、高超声速空天飞行器热端前缘、固体火箭发动机大尺寸薄壁喷管等国防急需装备构件的研制。