Bonding mechanisms in directly bonded aluminium and glass-fibre polyamide 6 hybrids

• Various treatments were applied to modify aluminium surface morphology and chemistry. • Annealing yields a 5-fold rise in fracture toughness via interfacial interactions. • Grit blasting yields a 7-fold rise in fracture toughness via mechanical interlocking. • PEO yields a 9-fold toughness gain by increasing surface area and interlocking. • Fracture toughness linearly correlates with surface roughness and energy parameters. This study investigates the contribution of different bonding mechanisms to the fracture toughness of aluminium alloy and glass-fibre polyamide 6 (GFPA6) joints as influenced by surface treatments. Aluminium substrates were treated by two grit blasting conditions, two annealing durations, and a plasma electrolytic oxidation (PEO) coating process, yielding surfaces with distinct morphologies and chemistries. Surface morphology, chemistry, and wettability of the treated aluminium surfaces were characterised. Afterwards, the aluminium-GFPA6 joints were fabricated via hot pressing and evaluated by mandrel peel testing to determine fracture toughness, complemented by interfacial and fractographic analyses. All aluminium surfaces showed wettability by PA6 with contact angles <90°. Annealing increased surface free energy and improved interfacial interactions, while grit blasting and PEO increased surface area and enabled mechanical interlocking. Compared to the as-received surface, fracture toughness increased up to ∼5-fold by annealing, ∼7-fold by grit blasting, and ∼9-fold by PEO. The superior performance of PEO-treated joints is attributed to the highly porous and irregular coating morphology, which maximises interfacial area, and promotes both mechanical interlocking and interfacial interactions.