Tailored Aluminum surfaces for adhesive bonding - joint properties and moisture induced damage mechanisms

Abstract

For the energy efficiency of future vehicles and aircrafts, weight reductions through the use of alternative materials and joining techniques are essential. Structural adhesive bonding is one technique to join dissimilar materials, avoid the need for additional fasteners like bolts or rivets, and enable a uniform stress distribution in the bonded materials.

However, the fundamental understanding of the relationships between the physical and chemical surface properties of the interfaces and the resulting joint properties still requires furthering. Particularly damage occurring due to exposition to moisture and mechanical loads pose major challenges and limit the widespread use of adhesive bonding technologies in safety relevant applications. Since damage events are often very difficult to detect non-destructively, further research on the occurring damage mechanisms that can help to devise long-term durable adhesive joints may enable a wider spread use of bonding.

Pulsed laser surface pretreatments of metal surfaces prior to adhesive bonding significantly increase the mechanical strength and the long-term durability of metal-polymer joints. This phenomenon is often attributed to cleaning effects and generating structures with suitable roughness on the metal surface, allowing mechanical interlocking with the polymer matrix. However, the enlarged, roughened metal oxide surface also offers more potential surface sites for chemical bonding with the polymer adhesive. The enhanced aging resistance after laser surface pretreatment is still a matter of investigation. It is often attributed to increased mechanical or chemical bonding, but also diffusion can play a role. Since the joint interface is buried and the hydrogen and oxygen atoms from water molecules cannot be distinguished easily from those in the surface oxide film and polymer, it is extremely difficult to study the diffusion in the joints.

Our study focuses on a new approach to investigate the structure-property relations and the role of diffusion for differently laser-pretreated adhesive joints. In scanning electron microscopy (SEM) investigations, the different laser-generated surface structures are characterized and important surface features are quantified. The analysis of the diffusion profiles of water in the polymer-metal interface region and the bulk polymer was achieved with micro x-ray photoelectron spectroscopy (µ-XRF) and secondary ion mass spectrometry (SIMS). Additionally, single-lap shear tests of unaged and hydrothermally aged specimens with and without laser-pretreatment are performed in order to be able to link surface features and diffusion lengths with resulting mechanical strength of the specimens. This allowed gaining new insights in the occurring mechanisms responsible for the loss of mechanical joint performance due to moisture and the role of interface features that can increase long-term durability of the adhesive joints for future lightweight solutions.