Functionalized plasma polymer coatings for improved durability of aluminium-epoxy adhesive joints: fractography

The effectiveness of carboxylic acid functionalized plasma polymers (PPs) as adhesion promotion pretreatments for aluminium is investigated. Plasma polymers of acrylic acid have been deposited onto AA6016 aluminium alloy and subsequently bonded with an amine-hardened epoxy-based adhesive formulation to make single lap shear (SLS) joints. Bond durability was investigated by testing joints after storage in water at 60 degrees C for 1 week. A combination of tensile testing of the SLS joints and locus of failure determination, by transmission electron microscopy and imaging XPS, was used to assess the PP pretreatment performance relative to the commercially applied chromic acid anodizing process. All non-aged joints failed in the adhesive, but after ageing of the joints in water a reduced failure stress was measured and areas of failure near to the aluminium oxide surface were visible. The locus of failure was determined through comparison of transmission electron micrographs, of ultramicrotomed sections transverse to the fracture surface, and XPS images of the surface. Failure was identified at both the PP/aluminium oxide and the PP/adhesive interfaces. It is anticipated that further application of this fractography approach will facilitate the optimization of PP adhesion promotion pretreatments to Improve on the promising results obtained in this initial study, Copyright (C) 2000 John Wiley & Sons, Ltd.