Process gas-assisted surface modifications on Zn-Mg-Al alloy-coatings using atmospheric pressure plasma for improved adhesion to structural adhesives

Adhesion between Zn-Mg-Al (ZMA) alloy-coated steel sheets and epoxy is critical for automotive applications, yet surface oxides often hinder bonding efficacy. This study systematically investigates atmospheric pressure plasma treatment (APPT) using argon, oxygen, nitrogen, and compressed air as process gases to enhance the bonding between ZMA coatings and epoxy adhesives. APPT operates under ambient conditions, eliminating the need for vacuum chambers and enabling scalable integration into industrial workflows. The nozzle-based plasma system allows uniform treatment of large steel sheets, making it suitable for roll-to-roll processing in high-throughput production lines. Electron microscopic analysis revealed that the composition of process gases significantly influences surface modifications. Surface characterization techniques confirmed the ability of air plasma to reduce carbon contamination and enhance adhesion strength compared to untreated ZMA coatings. Air plasma uniquely generates hydroxyl-rich compounds, such as Mg(OH)₂, AlO(OH), and Zn₅(CO₃)₂(OH)₆, which increase surface polarity and wettability while creating nano-roughened spherical morphologies conducive to mechanical interlocking. Single-lap shear tests demonstrated that air-plasma-treated samples exhibited superior adhesion strength, achieving 701 MPa, which was more than twice that of untreated samples. Cohesive zone modeling simulations validated these findings by demonstrating uniform stress distribution at the coating-substrate interface, minimizing delamination risks under mechanical loading. By addressing critical challenges in adhesion reliability, scalability, and cost-effectiveness, this study establishes APPT as a transformative solution for advancing high-performance ZMA coatings in automotive applications while meeting the demands of modern manufacturing processes.