The effect of anodic oxide chemistry and morphology on adhesion and durability in aerospace structural adhesive bonding

摘要

Long-term service of adhesively bonded aircraft structures relies upon durability of adhesion between the aluminium substrates and adhesives. The major challenge in the mechanistic understanding of the adhesion is to differentiate the adhesion due to mechanical and physical and/or chemical forces. Therefore two types of specimens have been investigated. Featureless barrier-type oxides were used to test the effect of chemical composition while excluding mechanical interlocking, whereas porous films were used to study morphological effects. A correlation between the type of Cr(VI)-free anodising pre-treatment and the nature of adhesion was investigated with PAA, SAA and PSA. To relate these fundamental properties to bond performance, adhesion strength and durability of bonded panels were measured by Bell peel and bondline corrosion tests. Results on barrier-type oxides show that the amount of surface hydroxyls, as calculated by XPS, plays an important role in the wet adhesion of epoxy-, but not of phenolic- based adhesive [1,2]. TEM and EDS investigation of porous oxides cross-sections indicate that anodizing conditions that lead to moderate dissolution of the top part of the oxide promote adhesion due to extended resin penetration. The porediameter treshhold for minimal dry peel strength is 10 15nm, while the level of wet adhesion is also a function of the anodizing temperature.