Optimizing matrix and fiber/matrix interface to achieve combination of strength, ductility and toughness in carbon nanotube-reinforced carbon/carbon composites

The direct attachment of carbon nanotubes (CNTs) on carbon fibers (CFs) always leads to a decrease of fiber-dominated properties (e.g., flexural strength) and a brittle fracture behavior of C/Cs, although the matrix-dominated properties (e.g., compressive strength and interlaminar shear strength (ILSS)) exhibit an obvious enhancement. To achieve the combination of mechanical strength, ductility and toughness in C/Cs, in this work, efforts were spent on simultaneously optimizing the matrix and fiber/matrix (F/M) interface. CNTs with radial orientation were grown onto the CFs by double-injection chemical vapor deposition to modify the microstructure of matrix. Pyrocarbon was deposited on the surface of CFs before CNT growth to protect CFs and to weaken interfacial strength between CFs and CNT/matrix. These optimal designs create strengthening and toughness mechanisms such as crack deflection and long pullout of CFs in the failure process of composites, which endow C/Cs with improved flexural strength of 31.5%, flexural ductility of 118%, compressive strength of 81.5% and ILSS of 82%, accompanied by a clear change from brittle fracture to pseudo-plastic fracture during flexural test. This work may provide a meaningful way to not only enhance both the fiber- and matrix-dominated strength but to substantially improve the ductility and toughness of C/Cs.