Bonding and aging mechanisms of polymers on titanium alloys

Abstract

Adhesive bonding is a key technology for structural joining as well as for modern lightweight multimaterials (e.g. fiber-metal laminates) in aerospace or automobile applications. It allows joining different materials with more homogenous load pathways compared to mechanical joining. However, it is quite demanding since service conditions often include both mechanical and environmental/chemical loads, for example humidity, salt-water or deicing-agents. Achieving high-performance joints with sufficient degradation resistance depends strongly on the surface pretreatment, especially for thermoplastics like PEEK (poly-ether ether ketone). The goals of the current investigations are two-fold: (i) To study the bonding of selected thermoplastic and thermoset polymers to titanium alloys and the aging mechanisms, and (ii) to develop surface pretreatments suitable for specific aerospace material combinations. Specimens have been investigated with various microanalytical, surface and mechanical characterization methods. Nd:YAG laser treatments as well as anodization allow for long-term stable joints between PEEK or epoxies like RTM6 and CP-Ti, different Ti-Al-V alloys or Ti-15V-3Al-3Sn-3Cr. Results show that the presence of a nanostructure with suitable morphology and chemistry on the metal surfaces is crucial for slowing degradation. Special experiments have been designed to separately modify surface structures at different length scales or the surface chemistry in order to elucidate the dominating bonding mechanisms. Chemical interactions are most important in the initial stage of the bonding for wetting and infiltration of the surface structures. For polymers largely "inert" against water, they account for the observed aging effects until only (nano-mechanical interlocking remains. Suitable processing windows for laser pretreatment of different titanium alloys have been identified and the understanding of prevailing bonding and aging mechanisms been advanced.