Microstructure and mechanical properties of C/C composites with a mesophase pitch transition layer

Carbon/carbon (C/C) composites, with a mesophase pitch transition layer between carbon fiber and pyrocarbon, were prepared by two step technique of liquid impregnation-carbonization and chemical vapor infiltration (CVI). The microstructure and mechanical properties of the as-obtained composites were studied by polarized light microscope (PLM), scanning electron microscope (SEM), conventional and high resolution transmission electron microscope (TEM, HRTEM) and mechanical properties tests, respectively. Results show that the mesophase pitch carbon exhibits lamellar structure, while the pyrocarbon exhibits like-crinkle lamellar structure under SEM and the granular structure under TEM. Under HRTEM, the degree of alignment of lattice fringes decreases gradually for mesophase pitch carbon, pyrocarbon and carbon fiber. The non-brittle fracture behavior of the composites is related to multiple crack deflections caused by the multi-interface and micro-cracks within the matrix carbon layer. The flexural strength and the fracture toughness (KIC) of C/C composites are 244 MPa and 9.7 MPa·m^1/2, respectively.