Interaction mechanisms in UV-laser pre-treatment of CFRP parts for adhesive applications

Due to increasing demands for weight reduction, the use of carbon fiber-reinforced plastics (CFRPs) is steadily growing in the field of transportation applications like automotive and aircraft. An important method for joining these materials is adhesive bonding, which requires an appropriate pre-treatment of the surface to provide high bond strength. State of the art for CFRP pre-treatment is manual grinding of the surface in combination with wet-chemical cleaning. Depending on the size and geometry of the component this may result in a time and labor consuming process and require partially environmentally harmful solvents or chemicals. In addition, the quality and reproducibility of this mechanical treatment depends strongly on the respective user, which may also lead to an exposure and / or damage of the carbon fibers. The latter can negatively affect the strength of the adhesive bond or the mechanical stability of the CFRP component itself. An environmentally friendly and in-line compatible alternative is the surface pre-treatment with laser. Laser radiation in the UV range is well-suited because the penetration depth of the laser radiation in the epoxy based matrix material is small. For KrF excimer laser operating at UV wavelength of l = 248 nm the measured per shot ablation depth for epoxy based materials are a few 10 nm to 100 nm. This extremely shallow laser penetration into the material results in a very controllable layer-by-layer ablation with no extreme heating of the bulk material. By removing the matrix layer-by-layer the surface will be cleaned and structured. Similarly, the carbon fibers can be exposed for subsequent adhesive bonding. Due to the lower ablation threshold of the epoxy resin compared to the carbon fibers, damages to the fibers can be avoided. Results of UV laser processing carbon fiber-reinforced plastics at l = 248 nm with different laser parameters are presented. Aim of the study is to optimize the laser parameter for adhesive bonding. Ablation thresholds, ablation rate, reflectivity and absorption have been identified for the epoxy based material with and without fibers. The chemical and topographical changes of the laser treated surface were studied. The analytical findings are discussed in context with adhesive bonding tests of pre-treated CFRP samples with different adhesives. Also the results of the laser pre-treatment are compared to other methods for adhesive pre-treatment like CO2-snow + atmospheric pressure plasma and corundum blasting.