Enhancing the shear strength of adhesive bonded compression-molded CFRP laminates using selective UV picosecond laser treatment

Carbon fiber reinforced polymers (CFRP) are widely recognized for their exceptional strength-to weight ratio, making them ideal for advanced applications requiring superior mechanical performance and low weight. However, their heterogeneous composition poses challenges in both processing and surface treatment, which are crucial to improve the strength of bonded joints involving CFRP adherends. Laser processing technology offers several advantages in the processing of CFRPs, such as the effective removal of contaminants (e.g. mold release compounds) which hamper adhesion to structural adhesives. In this work, a UV picosecond laser is used to perform surface preparation of CFRP compression molding laminates. The proposed treatment not only cleans the matrix of release agents but also enables selective matrix removal without damaging the carbon fibers. Pre- and post-treatment surface morphology and chemistry are analyzed using a stereomicroscope, a digital microscope, a scanning electron microscope, and X-ray spectroscopy. The results are an improvement in the ultimate tensile strength of CFRP/CFRP single lap joints by more than 300% compared to the baseline untreated material. Furthermore, the fracture mode for laser-treated samples changes from adhesive to cohesive/fiber tearing, since the adhesive penetrates between the bare fibers.