Improved thermo-oxidative stability of three-dimensional and four-directional braided carbon fiber/epoxy hierarchical composites using graphene-reinforced gradient interface layer

In order to improve the thermo-oxidative stability of three-dimensional and four-directional braided carbon fiber/epoxy composites, we introduced a gradient interphase reinforced by graphene nanoplatelets (GN) between the carbon fiber and the matrix, with a liquid phase deposition strategy. Both the interlaminar shear strength and the flexural strength of the composites were improved after thermo-oxidative aging at 140 °C for various durations (up to 1200 h). The interfacial reinforcing mechanisms are explored by analyzing the structure of the interfacial phase, thermal conductivity, weight loss, surface topography, fiber/matrix interfacial morphology and thermomechanical properties of the composites. Results indicate that the GN-reinforced gradient interphase provides an effective shield against interface oxidation, assists in thermal stress transfer, and restricts the movement of the different phases of materials at the composite interface.