Effect of femtosecond laser surface treatment on the interfacial adhesion between carbon fiber and epoxy resin

This study systematically investigated the effectiveness of femtosecond laser modification in enhancing the interfacial adhesion between carbon fibers and an epoxy matrix. Both sized and desized T700 carbon fibers were treated using a femtosecond laser at fluences of 0.34 J/cm² and 0.85 J/cm², enabling a clear assessment of the individual and combined effects of surface sizing and laser processing on fiber surface morphology, tensile properties, and interfacial shear strength. Surface morphology of the treated fibers was characterized using field-emission scanning electron microscopy (FESEM), and interfacial adhesion was quantitatively evaluated through microdroplet tests. FESEM observations revealed that femtosecond laser processing generated ablation zones on the carbon fiber surfaces compared to the smooth surface of untreated fibers. The ablation length increases as the intensity of fluence increases. The microdroplet test results indicated that conventional sizing treatment alone increased the interfacial shear strength by approximately 10%. In contrast, femtosecond laser surface treatment resulted in a significant enhancement in interfacial shear strength of desized fiber up to 35.8%. Furthermore, when laser treatment was combined with fiber sizing, the interfacial shear strength improvement reached approximately 50%. These results demonstrate that femtosecond laser surface modification is an effective approach for enhancing interfacial adhesion between the carbon fiber and epoxy matrix.