Effects of surface modification on the adhesive and structural strength of CFRP-to-metal laminates using A5052 and SUS304

This study systematically evaluates the effects of surface treatment methods and adhesive types on the bonding strength between carbon fiber reinforced polymer (CFRP) and two commonly used metals: stainless steel (SUS304) and aluminum alloy (A5052). Three surface conditions—untreated, sanded, and atmospheric pressure plasma treated (APPJ)—were applied, and five adhesives (four epoxy-based and one acrylic-based) were assessed using lap shear tests in accordance with JIS K6850. To elucidate the mechanisms underlying strength improvement, the chemical states of the bonding interfaces were analyzed using X-ray photoelectron spectroscopy (XPS). The results showed that SUS304 achieved high bonding strength even without surface treatment, whereas A5052 required surface modification to ensure adequate adhesion. Sanding enhanced bonding via physical roughening and contaminant removal, while APPJ treatment introduced oxygen-containing functional groups (e.g., C=O, O=C–O) and eliminated surface-adsorbed water, thereby chemically activating the surface. Notably, the combination of sanding and APPJ treatment exhibited a synergistic effect, resulting in further strength improvement. Furthermore, three-point bending tests on CFRP-metal sandwich structures revealed that appropriate surface treatment of the metal layer suppressed interfacial delamination and provided bending strength and stiffness comparable to or exceeding those of monolithic CFRP. In particular, the A5052–CFRP combination demonstrated a favorable balance between weight reduction and mechanical performance. Overall, this work presents a comprehensive evaluation of dissimilar material bonding strategies, offering practical insights for lightweight structural design in automotive, aerospace, and renewable energy applications.