Low-pressure plasma treatment of CFRP substrates for epoxy-adhesive bonding: an investigation of the effect of various process gases

This work reports a systematic and quantitative evaluation of the effects induced on the adhesive properties of carbon fiber reinforced polymer (CFRP) substrates by various vacuum cold-plasma treatments. In particular, surface activation of the CFRP substrates was performed using several combinations of exposure time, plasma power, and processing gas (air, O-2, Ar and N-2). By comparing these plasma treatments with conventional techniques of abrasion and peel ply, it was possible to substantially increase the performance of the adhesively bonded joints made by overlapping the CFRP substrates with a structural epoxy resin. On each differently treated surface, measurements of roughness and of wettability were performed, allowing the evaluation of the increase in surface energy after the plasma treatment. XPS analyses allowed the identification of the chemical state of the substrates and showed an in-depth functionalization of the outer layer of the CFRP material. The experimental results show that an engineered plasma treatment of the CFRP substrates allows one to modify the surface morphology and both wetting and chemical activation properties of the treated surfaces, resulting in an increased mechanical shear strength of the joints.