Plasma electrolytic oxidation coating for aluminium and glass-fibre polyamide 6 bonding

Achieving strong and reliable adhesion between metals and thermoplastic composites (TPCs) is essential for lightweight structural applications. This study explores the role of plasma electrolytic oxidation (PEO) coatings in enhancing the bonding between aluminium and glass-fibre polyamide 6 unidirectional tape (GFPA6 UD tape). Specifically, we examined the effect of morphology and chemistry of PEO coatings, influenced by sodium metasilicate concentration in the electrolyte, on bonding performance. The PEO coatings were characterised in terms of surface roughness and elemental composition, followed by an evaluation of their physical interactions with polyamide 6 (PA6). After direct bonding, i.e., co-consolidation of GFPA6 UD tape with aluminium, fracture toughness was measured using peel test, with subsequent fractography analysis. Results show that PEO coatings promote bonding strength through undercut surface features that enhance mechanical interlocking and an aluminium oxide layer that improves wettability with PA6. Adding sodium metasilicate in the electrolyte increases surface roughness of PEO coatings, strengthening their mechanical interlocking with PA6. Moderate silicate concentrations (1–2.5 g/L) yield joint fracture toughness values comparable to the interlaminar toughness of the TPC, with an intraply failure mode. However, a porous silicon-rich outer layer developed at higher silicate contents create stress concentration points, leading to coating failure. These findings allow tailoring PEO coatings for improved metal-polymer bonding in lightweight hybrid structures.