Laser-induced chemical modification and nanostructuring of metal surfaces for optimal adhesive bonding to thermoplastic polymers: the interface between TWIP steel and PEEK

Kurzfassung

Surface preparation methods play a major role in many technical applications such as paint and anti-corrosion coating or adhesive joining. They are crucial for producing reliable surfaces that allow the formation of adhesive bonds with high initial strengths and high long-term stability in degrading environments. The surface preparation methods depend strongly on the material combination and the operating conditions that the joint will be exposed to. Bonding metals to thermoplastic polymers (e.g. PEEK) is a particular challenge that poses high requirements towards the chemical and structural surface properties. Contrary to chemically consolidating adhesives, thermoplastic polymers are commonly assumed to bond only via mechanical interlocking and weaker physical interactions. Previous studies showed that Nd:YAG-laser-treatments allow the formation of strong Titanium-PEEK-interfaces with high degradation resistance against humidity. The goal of our present research aims at transferring the methodology to steel-PEEK interfaces and developing a fundamental understanding of the underlying processes and problems. The knowledge of the interface’s and the laser-generated oxide film’s properties, the nature of mechanical, physical and chemical bonds formed, and the degradation processes is a prerequisite for systematic furthering of adhesive technologies for new materials and applications. In this study we report the first results obtained on laser-treated surfaces of highly ductile high-strength TWIP-steel and the adhesion properties to PEEK. Fracture-mechanical studies by means of single-lap-shear (SLS) and degradation experiments were performed with regard to the laser process parameters. Electron-microscopy showed the formation of oxide films upon laser irradiation in ambient atmosphere. Further investigation of the SLS fracture surfaces implied the occurrence of different failure mechanisms. Chemically-sensitive scanning electron microscopy allowed the differentiation between “apparent” adhesive failure as detected with a light-microscope and pseudo-adhesive failure in the PEEK-interface region by means of residual PEEK. These, besides large areas of cohesive failure, indicate a strong bonding to the functionalized metal.