Laser surface pretreatment for structural bonding and coating

Kurzfassung

Effective surface pretreatment is the key to long-term stable and durable bonds between different materials, e.g., used in structural bonding and coatings. Its purpose, besides a general cleaning of a joint surface, is to structure and functionalise the interface. Particularly attractive and fast, laser surface pretreatment enables simple, reproducible and waste-free processing of surfaces of different materials (e.g. metals, fibre-reinforced plastics). The laser pretreatment, the requirements for obtaining suitable surfaces as well as the optimisation for structural bonding and coating will be highlighted using different material combinations and application examples. For structural applications, metal-polymer and -FRP joining is very common. Metals are pretreated with a pulsed Nd:YAG laser and bonded with thermoset or thermoplastic matrices. For the purpose of forming protective metallic coatings on FRP components, the laser can be employed similarly. Various microanalytical, surface and mechanical characterisation methods are used to study the joint interfaces. Laser pretreatments of titanium, aluminium and steel resulted in nano- and microstructures that consist of solidified melt structures covered by oxidic films. Depending on the alloy, the morphology and chemistry of the surface structures varied. The optimal laser pretreatment leading to a stable interface depends strongly on the alloy itself, i.e., the reflectivity of the surface, the presence of contaminations, and the particular laser parameters. Generally, the formation of a stable, homogeneous nanostructure is a prerequisite for long-term stable adhesive bonding. Laser pretreatment of FRP, in contrast, leads to structuring of the surface by ablation of the matrix, thus exposing the underlying fibres. The metallic coating on the fibres is substantially more reliable than a coating deposited purely on the polymer matrix. Therefore, the laser pretreatment of metals and FRP for bonding and coating follow different concepts. The pretreated surfaces allow mechanical interlocking and chemical interactions leading to long-term stable bonds between different materials.