Enhancing the mechanical performance of CF/epoxy composites via a sizing method based on multiscale characterization

Carbon fiber (CF)/epoxy composites are widely used in aerospace and automotive applications but still suffer from insufficient interfacial adhesion due to the chemically inert and smooth CF surface. To address this limitation, this work proposes a matrix-compatible sizing strategy based on bisphenol-A epoxy resin (E-51), with optional incorporation of multiwalled carbon nanotubes (CNTs). The novelty of this study lies in linking the interfacial enhancement mechanism across multiple scales, from surface chemistry and interphase structure to mechanical performance, thus establishing design guidance for next-generation sizing systems. Two E-51-based sizing agents were prepared and applied to CFs. Their effects were evaluated using a comprehensive suite of characterization techniques, including Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), atomic force microscope (AFM) quantitative mapping, monofilament debonding, single-fiber fragmentation tests, and laminate-level mechanical testing. The optimized sizing formulation containing 0.3 wt% E-51 and 1 wt% CNTs introduced reactive functional groups (e.g., epoxy, C–O, O–C = O, –CH 2 , C = O, C = C, and benzene rings) and increased nanoscale roughness, leading to significantly improved interfacial adhesion. Consequently, the flexural strength and interlaminar shear strength (ILSS) of the composites increased by 48.4% and 23.2%, respectively, compared with the unsized laminates. This study demonstrates an effective and industrially relevant route for enhancing CF/epoxy interfaces, and provides new insights into the multi-scale reinforcement mechanism of CNT-modified sizing layers for high-performance composite applications.