To clarify the effect of surface characteristics on the interfacial properties of T1100-grade carbon fiber (CF)/bismaleimide (BMI) composites, three CFs (F1, F2, and F3) with different surface treatments and sizing agents were studied. Surface physicochemical properties and sizing–resin reaction behavior were characterized; nano-infrared spectroscopy was innovatively used to quantify interfacial structure. The correlation among surface features, interfacial structure, and mechanical properties was established. All dry-jet wet-spun T1100 CFs show smooth surfaces with similar roughness, and mechanical interlocking contributes little to interfacial adhesion. F3 possesses the highest active carbon, oxygen content, and epoxy value. Its sizing agent exhibits strong reactivity with BMI, forming a ~200 nm thick interface and the highest interfacial shear strength (IFSS) of 95.9 MPa. Constructing a “thick and strong” interface promotes shear failure from brittle to tough, significantly enhancing 90° tensile and interlaminar shear strength (ILSS). This work provides guidance for interface design and engineering applications of T1100/BMI composites in aerospace primary load-bearing structures.

It has long been desired to achieve mechanical enhancement and structural health monitoring by introducing carbon nanotubes (CNTs) into traditional carbon fiber (CF) composites. Herein, the initiation of micro-damage and crack propagation has been investigated by utilizing in situ electrical resistance changes in interlaminar hybrid CNT network/CF composites during the shear loading process. The results show a clear relationship between the crack propagation and the electrical resistance response particularly when approaching the failure of the single-layer CNT network hybrid composites. Furthermore, the chemically modified CNT network exhibits evident enhancement on main mechanical properties of the CF composites, superior to the thermoplastic toughening method. The characterizations manifest that the multiscale interlayered CNT/CF structure can simultaneously resist the crack propagation along both the in-plane direction and the cross-plane direction, which consequently enhances the flexural and compressive strengths of the composite material. This discovery provides a novel idea for the potential application of CNT network/CF hybrid composites in the integration of mechanical reinforcement and structural health monitoring, namely, that the CNT network acts not only as a reinforcing phase but also as a sensor for the structural health monitoring of the composites.