THE ROLE OF ELECTRON-MICROSCOPY IN THE STUDY OF ADHESION TO ALUMINUM SUBSTRATES

The analysis of the bondline in structural aluminium joints is taken as an example to illustrate how electron microscopy can be used to pin a new insight into the phenomenon of adhesion. Scanning electron microscopy and transmission electron microscopy of ultramicrotomed sections has been used to characterize the adhered surfaces generated by the pretreatment processes prior to bonding. It is shown how a simple extension of these techniques can be used to analyse and characterize the interfacial and interphasial morphologies of the bonded joint both before and after exposure to 'hot/wet' environments. Results have shown that the importance of the so-called mechanical interlocking phenomenon is related to the durability resistance of the joint rather than enhancement of the 'dry-state' mechanical properties. It is also concluded that the formation of the voluminous hydrated alumina at the interface occurs post-rupture and is not a prima facie cause. From the observed interphasial morphologies a hypothesis is advanced to explain the effect of various pretreatments on joint durability. Other analytical techniques, such as 'parallel' sections, AES, XPS and electron energy-loss spectroscopy (EELS), are discussed and it is shown how these methods can be used, not only to confirm the interphasial structures but also to generate new data in their own right. A greater understanding, therefore, of the effect of adhesive theology, environmental exposure, adherend composition and pretreatment on the interface between adhesive and adherend, can be obtained using these techniques.