Abstract: In order to alleviate the brittleness of C/C composites, the unidirectional carbon fiber preform was treated with Boehm sol, and the Al₂O₃ coating was prepared on the fiber surface. The preform was densified by self-made thermal gradient chemical vapor infiltration (TG-CVI) deposition equipment to obtain a dense C/C composite. The bonding strength of the interface and the graphitization degree of the matrix carbon were further adjusted by high-temperature heat treatment.The density of the composite material was tested by drainage method, while tensile properties were assessed with the universal material testing machine. The sample was treated with the visualization of graphene texture (VGT). The microstructure, interfacial, and cross-sectional morphologies of the composite were studied using polarizing microscope (PLM), scanning electron microscope (SEM), and X-ray diffraction (XRD) to analyze phase composition. The results showed that the C/C composites coated with Al₂O₃ coating transform into rough layer (RL) texture in the late deposition stage. After high-temperature heat treatment, the graphitization degree of the carbon matrixis increased. As a result of this, the fracture mechanism of the C/C compositeis significantly changed. The elongation is improved from the initial brittle fracture of the composite to the quasi-ductility. The peak stress of the C_f(Al2O3)/C-3 sample reached 77.3 MPa, and the elongation reached 15%.

Key words: C/C composites, Al₂O₃, interfacial modification, pyrolytic carbon matrix, microstructure