Investigation of the synergistic effect of surface treatment methods on the bond strength of aluminum alloys

With the advancement of material technologies, the importance of various surface preparation techniques has been steadily increasing, particularly to enhance the quality and durability of adhesive bonding methods used in structural components. In the bonding of metal surfaces with epoxy-based adhesives, surface preparation processes, both chemically and mechanically, are decisive factors in determining the final adhesion performance. In this study, the effects of different combinations of surface modification processes applied to aluminum plates on the adhesion strength of joints formed with a Bisphenol A–based epoxy adhesive were comprehensively investigated. For surface preparation, mechanical (sanding), chemical (etching with sodium hydroxide and nitric acid), electrochemical (phosphoric acid anodization), and physical (radiofrequency vacuum plasma) treatments were applied in a planned sequence. In addition, silanization (e.g., using TEOS) was performed to enhance chemical bonding on the surface. By applying surface preparation methods individually and in combination, different surface morphologies and chemistries were obtained. In the experimental process, tensile tests on control specimens without any surface modification showed a low strength value of 3.4 MPa. In contrast, the combination of sanding and plasma treatment increased this value by nearly threefold to 10.4 MPa. When silanization was also included, functional groups on the surface were enriched, and surface energy was increased, resulting in adhesion strengths exceeding 15.8 MPa. The highest value was obtained by sequentially applying chemical etching, phosphoric acid anodization, silanization, and vacuum plasma treatment, reaching 32.1 MPa. These results reveal the synergistic effect of multiple surface preparation methods and highlight that both surface topography and chemical properties are critical in adhesion performance. After aging tests (thermal cycling and humidity exposure), surfaces treated with single-step processes exhibited significant reductions in strength, whereas multi-process combinations—especially those including silanization—largely retained high adhesion values. This finding demonstrates the crucial role of chemical bonds in long-term stability. To characterize the effectiveness of the surface treatments in detail, Scanning Electron Microscopy (SEM) analyses were performed on aluminum plates obtained after each process. SEM images showed that while mechanical treatments significantly roughened the surface, chemical etching and anodization produced nano-porous structures.