Characterization of Surface Pretreatments on Al/Li Alloy and Related Mechanical Properties of Polysulfone Adhesive Bonds

Abstract An investigation of polysulfone-Al/Li alloy interaction involved single lap shear joints and wedge samples following an FPL etch, sulfuric acid anodization (SAA) and phosphoric acid anodization (PAA). The study of the Al/Li surfaces involved the determination of the elemental composition and morphological features of the pretreated adherend before bonding and following fracture. When polysulfone was either thermally pressed or primed onto the microporous surface, the polysulfone indeed penetrated into the porous oxide and thereby provided a mechanical means of adhesion. The wedge test results for the adherend pretreated by PAA and SAA were superior to those for the FPL etched adherend. The failure path for the FPL etched samples was at the adhesive/oxide interface whereas the failure path for the PAA samples was within the adhesive but with occasional divergence of the crack into the oxide. The porous oxides on Al/Li alloy formed after PAA and SAA treatment were shown to undergo dramatic changes in morphology on short term (< 95 hrs) exposure to 71°C and 100% R.H. environment. The mechanism of failure was due to moisture which caused hydration and subsequent weakening of the surface oxide layer and the bonded joint. Lithium was not surface concentrated in the PAA treated Al/Li alloy as shown by AES depth profiling and therefore the effect of Li on the durability of the bonded alloy is considered minimal.