Scale influence in surface design regarding laser structures for adhesion enhancement in polymer-metal hybrids: A fractal dimension approach

Surface structuring enhances adhesion, particularly at the interface of polymer-metal hybrids. Therefore, laser beam machining is well-established. However, the influence of scaling of such structures was not sufficiently determined yet. Correlating surface structure to functional properties, especially the adhesion strength, offers opportunities for further optimization in surface design and in processing. Therefore, the fractal dimension was employed to establish a quantitative correlation and to investigate the influence of structure scaling on groove structures in aluminum 6082 specimens for subsequent joining to a polymer counterpart as well as on modeled groove structures. Regardless of the actual groove widths, similar values of the fractal dimension for similar aspect-ratios were calculated – approximately 1.09, 1.13, and 1.16 for the structure densities 10 %, 20 %, and 30 %, respectively. For both cases (virtual, experimental) the fractal dimension acts as a scale-independent measure. Finally, the adhesion strength from 3.5 MPa to 18.5 MPa in lap shear tests occurred – despite partially within the standard deviation – independently of the scaling as well. Furthermore, a direct correlation between fractal dimension and adhesion strength is indicated (R² > 0.94). In conclusion, the suitability of the fractal dimension as quantitative design criterion and the feasibility of scaling for structuring were substantiated.