Adhesive bond strength of PEO coated AA6060-T6

Adhesive bonding is a key joining technique for aluminium alloy components in the automotive industry. Plasma Electrolytic Oxidation (PEO) is an emerging surface treatment for the protection of metal alloys including those of aluminium. This work set out to assess the adhesive bond strength and behaviour of PEO coated aluminium alloy AA6060-T6. Thin (15 to 25 μm) oxide coatings were produced on AA6060-T6 by Plasma electrolytic Oxidation (PEO) in phosphate and silicate-phosphate electrolytes. The coatings were characterised extensively, then adhesively bonded in lap-shear joints and loaded to failure under tensile mode to evaluate the bond strength of adhesively bonded coated samples. Surface fracture analysis was carried out to determine the failure modes of the lap-shear joints. The failure mechanism and strength of the coated joints was compared to the characteristic features of the coatings, in an attempt to identify governing properties. It was found that PEO coatings could withstand high loading; however, the strength of the bonded joints was governed by the cohesive strength of the oxide layers. All coatings produced in the phosphate and thicker coatings produced in the silicate-phosphate electrolyte ultimately experienced cohesive failure within the oxide region. Thinner silicate-phosphate coatings electrolyte displayed a favourable failure mechanism with fracture occurring within the adhesive, which highlights the possibility of PEO technique to be used for adhesively bonded aluminium alloy parts.