Enhancing bond strength and long-term stability of titanium-15-3-polymer-bonds by laser pretreatment of the metal

Abstract

Key to high performance bonded joining technology is the adhesive interface between dissimilar materials. With adhesive joints and hybrid materials such as fiber-metal-laminates, weight savings and superior mechanical properties (e.g. fatigue strength or stiffness) can be achieved. One major challenge for adhesive joining is the development of suitable adhesive interfaces between polymer matrix and (metal) surfaces that enable long-term stable, degradation resistant bonding. Ideally, these joints also exhibit high bond strengths capable of carrying high loads for structural applications. To a certain degree, the strength requirement of the joints can be tackled by specific design (e.g. dimensioning, tapering, load-adapted design). However, degradation resistance towards humidity or corrosive media such as aqueous salt solutions is much harder to address by design measures (e.g. non-permeable paints on bond edges). Both strength and long-term stability can very efficiently be realized by means of a laser surface pre-treatment. For Ti15-3/PEEK and Ti15-3/Epicote 600 joints, excellent shear strengths and long-term stabilities were found after pre-treating the metal with a pulsed Nd:YAG laser as evidenced in single-lap shear tests after accelerated aging. Investigation of the laser-treated surfaces revealed a micrometer-level structure covered by a nanoporous oxide. The laser pre-treatment leads to higher long-term stability than e.g. grit blasting, although initially the bond strengths are comparable. Our current research focuses on bonding mechanisms and their importance for degradation resistance. Through further experiments modifying the structures at both scales, we show that the aging resistance is due to the nanoporous oxide. Removal of the nanostructure also eliminates the degradation resistance. Conversely, enhancing the micron-level structuring by creating trenches and undercuts does not yield increased bond strengths or degradation resistance. Combining measured shear strengths with joint and fracture surface analysis allows for a deeper understanding of the role of nano-structuring for bond strength, long-term stability and failure mechanisms.