Tailored Aluminum Surfaces for Adhesive Bonding – Joint Properties and Moisture induced Damage Mechanisms

For the energy efficiency of future vehicles and aircraft, weight reductions through the use of alternative materials and joining techniques will be essential. Structural adhesive bonding is one technique to join dissimilar materials, eliminating the need for additional fasteners like bolts or rivets while enabling a uniform stress distribution across the bonded materials.<br/>However, the fundamental understanding of the relationships between the physical and chemical surface properties of the interfaces and the resulting joint properties is needed. One major concern is joint weakening due to exposure to moisture, which has blocked the widespread uptake of adhesive bonding technologies in safety relevant applications [1]. Despite such aging process being difficult to detect non-destructively, further research into the mechanisms that cause such weakening is necessary if long-term durable adhesive joints are to be achieved.<br/>Pulsed laser surface pretreatment of metal surfaces prior to adhesive bonding can significantly increases the mechanical strength and the long-term durability of metal-polymer joints. This benefit is often attributed to cleaning effects while also generating structures with suitable roughness on the metal surface that allow a better mechanical interlocking with the polymer matrix [2]. However, an enlarged, roughened metal oxide surface will also offer more surface area for chemical bonding with the polymer adhesive. The reasons for the improved aging properties provided by such laser surface pretreatment remains a matter of investigation, and here atomic-scale diffusion can play a role. Since the joint interface is buried and since the hydrogen and the oxygen atoms from water molecules cannot be distinguished easily from those native to the surface oxide film and polymer, it is difficult to study the diffusion in the joints.<br/>Our study uses on a new approach for investigating the role of diffusion in modifying laser-pretreated adhesive joints. Scanning electron microscopy (SEM) can characterize the laser-modification of such surfaces down to the nanometer scale. We combined this information with observed diffusion rates determined by micro-x-ray fluorescence spectroscopy (µ-XRF) and secondary ion mass spectrometry (SIMS), which used elemental and ¹⁸O tracers, respectively. The preliminary results from our tracer studies indicate that significant diffusion in the polymer-metal interface and within the bulk polymer occurs. We also used single-lap shear tests of both unaged and hydrothermally aged specimens both with and without laser-pretreatment to link surface features and diffusion lengths with the mechanical strength of the specimens. This brought new insights concerning the mechanisms responsible for the loss of mechanical joint performance due to moisture and the role of interface features in suppressing this problem.<br/><br/><b>References</b><br/>[1] R. A., Pethrick; Design and ageing of adhesives for structural adhesive bonding – A review <i>Journal of Materials, Design and Applications</i>, <b>2015</b>, <i>229 (5)</i>, 349-379.<br/>[2] J. Min, H. Wang; Application of laser ablating in adhesive bonding of metallic materials: A review <i>Optics and Laser Technology</i>, <b>2020</b>, <i>128</i>, 1-23.