Construction of nanocomposite interphase with controllable thickness to relieve stress concentration and boost stress transfer from carbon fiber/epoxy resin interface

Carbon fiber/epoxy resin composites are susceptible to stress concentration at the interface due to significant differences in the physical and chemical properties of the fibers and resin, resulting in severe degradation of the composite properties. In this paper, the construction of an interfacial transition layer with controllable thickness was achieved for the first time by in situ growth of amino-functionalized metal–organic framework materials with different particle sizes on the surface of carbon fibers and the stress concentration mitigation effect of the modified composites was investigated. Nanoindentation tests showed that interfacial transition layers with thicknesses of 500, 800, 1100 and 1600 nm were successfully constructed. The maximum improvement in interfacial shear, interlaminar shear and flexural properties of the modified composites were 115.0 %, 18.7 % and 26.7 % respectively, compared to the pure carbon fiber composites. Finite element simulations indicated that the stress concentration of the composites decreased by 16.7 %, 33.3 %, 58.3 % and 66.7 % with increasing the thickness of the interfacial transition layer respectively. The electron micrographs after failure showed that the failure mode of the composites changed from interfacial debonding to cohesive failure (fiber and matrix fracture), and the failure plane moved slightly from the interface to the inside of the epoxy resin, which indicated that the introduction of the interfacial transition layer improved the stress transfer between the fibers and the matrix, thus improving the mechanical properties of the composites. This finding enriched the theory of interfacial design for composites and contributed to the breakthrough improvement of their mechanical properties.