Bioinspired Surface Engineering of Carbon Fibers for Enhanced Interfacial Adhesion in Epoxy Composites

Robust load transfer in carbon fiber-reinforced polymer (CFRP) laminates relies on a strong fiber–matrix interphase, yet the chemically inert carbon fiber (CF) surface often limits interfacial adhesion to thermoset resins. For this issue, a facile, eco-friendly, and bioinspired codeposition strategy was developed using soy protein isolate (SPI), gallic acid (GA), and ferric chloride to modify CF surfaces. The SPI–GA–Fe 3+ complex was codeposited onto the CF surface through covalent, hydrogen, and coordination bonding under mild alkaline conditions, forming a robust adhesive coating analogous to mussel adhesive protein. The introduced catechol–Fe 3+ coordination network provided abundant active sites and improved interfacial compatibility with the epoxy matrix. Characterization results confirmed that the coating was uniformly distributed and significantly enhanced the surface roughness and wettability of CF. Consequently, the flexural strength and interlaminar shear strength of CF–(SPI–GA)–Fe/EP are improved by 20.45% and 17.60%, respectively, compared with those of sized CF/EP. This work demonstrates a sustainable and efficient bioinspired modification approach that effectively strengthens interfacial adhesion and mechanical integrity in CF composites, offering new insight into the design of advanced composites with superior mechanical properties.