Surface treatment of stainless steel (SUS) to improve interfacial adhesion of SUS/polymer hybrid bilayer composites

The use of hybrid metal/polymer structure is an efficient strategy for weight reduction in automotive applications. The poor interfacial adhesion between metals and polymers can be solved by using coupling agent (compatibilizer) and through surface treatments, adding new functionalities and a rough surface. In this study, the stainless steel (SUS) surface treatments were systematically examined using various etching and functionalizing solutions, such as hydrochloric acid (HCl), nitric acid (HNO ₃), acetic acid (CH ₃COOH), phosphoric acid (H ₃PO ₄), copper sulfate (CuSO ₄), ethanol (C ₂H ₅OH), fluorozirconic acid (H ₂ZrF ₆), sodium hydroxide (NaOH), acetone (C ₃H ₆O), and their mixtures. An HCl/CuSO ₄ aqueous solution was the most efficient surface treatment for SUS, resulting in the highest interfacial adhesion with polyamide 6,6 (lab shear strength: from 1.7 to 12.7 MPa, and toughness: from 4.9 to 35.4 N/m ²). The etching process was further investigated by optimizing the etching time and temperature. The treated surfaces were characterized by surface roughness, morphology, contact angle, elemental analysis, and single–lap shear tests.