Different contents of carbon nanotubes (CNTs) were in situ grown on the surface of carbon fiber bundles by injection chemical vapor deposition. The fibers were then stacked into unidirectional preforms and densified by pyrocarbon (PyC) via chemical vapor infiltration. The effects of CNT on the strength and toughness of carbon/carbon composites (C/Cs) before and after heat treatment at 2100℃ were investigated. Results show that both the tensile strength and work of fracture achieves the optimum performance when the CNT content is 1.5 wt%. After heat treatment, the tensile strength increases by 25.68% for CNT reinforced C/Cs (CNTs-C/Cs), while only 4.36% for pure C/Cs. The refinement effect of CNT promotes the resistance of PyC matrix against destruction and makes it maintain the structural integrity and continuity even after heat treatment. Before heat treatment, the presence of CNT results in a decreased in-plane lattice size (La) compared with pure C/Cs. while interestingly, after heat treatment, La becomes larger due to the stress graphitization of CNT. The stress graphitization induced by CNT gives the carbon matrix a stronger ability to resist crack propagation, thereby enhancing the strength of the C/Cs. In addition, the existence of CNT changes the fracture mode of the C/Cs and increases the way of energy consumption during the tensile test. Thus, both fibers and the interface of the composites are fully utilized.

Introduction

Carbon/carbon composites (C/Cs) are widely used as ultrahigh temperature structural materials in aeronautics and astronautics[1, 2]. Due to the inherent features of carbon matrix including large brittleness and low strength, the mechanical properties of C/Cs are far from satisfactory for the application of advanced aculeated or thin components in the next generation of aerospace systems[3, 4]. Carbon nanotubes (CNTs) possess exceptionally high stiffness and strength[5-8], which are regarded as the efficient nanoscale reinforcements to strengthen and toughen C/Cs beyond microscale carbon fibers. At present, the most frequently used method for introducing CNT into C/Cs is in-situ growth method using catalysis chemical vapor deposition (CCVD)[9], which can improve uniformity of CNT on carbon fibers and control CNT yielding, length and orientation. But carbon fibers are damaged during CNT growth due to acid treatment and high-temperature etches of metal particles, which leads to the decrease of fiber dominated mechanical properties[10]. Therefore, most researches have been only focused on the effects of CNTs on matrix dominated mechanical properties of C/Cs[11, 12], and almost no work is available to fiber dominated mechanical properties, especially for the tensile performance which is really important to the employ of C/Cs. Therefore, it is urgent to study the effect of CNT on the tensile property of C/Cs and the influencing mechanism. Injection chemical vapor deposition (ICVD) has been proved to be an efficient process for synthesizing CNT with controllable length and orientation[13]. Compared with CCVD process, which needs catalyst to be carried on the fiber surface firstly, ICVD process involves the simultaneous pyrolysis of the catalyst precursor and the carbon source. Thus, avoiding acid erosion and high-temperature treatment, ICVD process can reduce mechanical degradation of carbon fibers as far as possible and improve the fiber dominated mechanical properties of composites. In principle, the great designability of CNT reinforced C/Cs would provide a large potential to achieve a simultaneous reinforcement to the whole matrix and the fiber and matrix (F/M) interface[14-16], and this can optimize the global performance of materials and promote their application in enlarged fields. To scientifically investigate the tensile property of CNT reinforced C/Cs (CNTs-C/Cs), a model of unidirectional C/Cs has been selected. Besides, graphitization as a post-treatment after the infiltration process has great influence on the structure change of pyrocarbon (PyC) and F/M interface binding force[17, 18]. A comparison of structure and properties of composites before and after heat treatment is beneficial to explain the strengthening and toughening mechanisms of CNT during the tensile tests. In this work, different contents of CNT are doped in composites to find the optimal containing of CNT reinforcement and maximize the tensile strength of C/Cs, and then detailed fracture mechanisms are illuminated.