UV-laser cleaning and surface characterization of an aerospace carbon fibre reinforced polymer

In the recent aerospace generations, composite materials have shown that they can fulfil the increasing demand for structural efficiency. In this context adhesive bonding could be used to max out the material properties of carbon fibre reinforced polymers (CFRP). However, investigations have shown that residues of release agents on the CFRP surface can cause adhesion problems. Thus, a reliable surface cleaning procedure for removing residues prior to bonding is necessary. Analytical surface investigations of an Aerospace CFRP material revealed the thickness and morphology of the predominant adhesion-weakening contamination. The optical absorptions of the epoxy material and the release agent characterised by UV–vis measurements were considered to develop a cleaning model based on a ns-pulsed ultra-violet laser with 266 nm wavelength. This removal mechanism leads to chipping and peeling of the release agent layer and could be supported with light microscopy, scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS) investigations. Single lap shear tests of the CFRP samples in the pristine and contaminated state, pre-treated with UV-laser as well as with the reference process (grinding) show that the laser pre-treated samples lead to the highest joint strength in the study. The failure mode changed from adhesion failure in the pristine state to a failure in the fibre-resin interface after laser treatment. Comparing a laser parameter with the focus on a smooth cleaning process and a laser parameter that lead to a surface with exposed carbon fibres, the surfaces treated with the smooth laser parameter showed the higher joint strength. The measured joint strength was 100% higher than with grinding, which underlines the high potential of laser technology for CFRP surface pre-treatment.