Evaluation of surface treatment methods and their influence on the mechanical performance of hybrid metal-composite laminates

Fiber metal laminates (FMLs), which fuse the strengths of alloys and fiber-reinforced composites, find application in the aerospace and automobile sectors. Despite the benefits, functional performance depends on the quality of the bonding strength at the metal-composite interface. Therefore, in the present study, effect of mechanical abrasion (MA), nitric acid etching (NI), P2 etching (P2), sulfuric acid anodizing (SA), and electric discharge texturing (ED) surface treatments methods was evaluated by considering the surface morphology, roughness, wettability, and surface energy. Furthermore, the impact of surface treatment methods on the tensile and flexural behavior of Carbon Reinforced Aluminum Laminate (CARALL) fabricated using the treated aluminum substrate was studied and compared with untreated laminates. The results showed that all treatment methods increased the surface roughness of the aluminum substrate. MA and ED treatments imparted higher surface roughness (0.95 and 1.85 μm), with the surface displaying hydrophobic behavior, while chemical and SA treatments increased the surface energy (39.1 MJ m⁻², 45.2 MJ m⁻², and 60.1 MJ m⁻²) and produced surfaces exhibiting hydrophilic behavior. Surface treatments helped improve the ultimate tensile strength of CARALL laminates. The highest improvement, 61%, was achieved using SA treatment (921.43 MPa), while MA resulted in a modest 38% improvement (789.82 MPa). SA treatment increased the elastic modulus and toughness by 188% and 82% in comparison to the untreated specimens. A similar increase in flexural strength was noted, with a 35% increase in the case of MA and a maximum 104% increment from SA, as against untreated ones. Flexural modulus and toughness of 88.05 GPa and 32.39 MJ m⁻³ was attained with the help of SA treatment. Delamination was observed during the plastic deformation stage under the tensile loading conditions. However, delamination occurred during the end stages of failure under flexural loading, indicating a higher delamination resistance offering by CARALL with flexural loading conditions.