Investigation on the effect of low-pressure plasma treatment on the adhesion properties of different carbon-fiber-reinforced-polymer materials for structural adhesive bonding

This three-part study, developed with the collaboration of the Istituto Italiano di Tecnologia (IIT), reports a systematic and quantitative evaluation of the effects induced by various low-pressure plasma (LPP) treatments on the adhesive properties of Carbon Fiber Reinforced Polymer (CFRP) substrates. In particular, Part A of this work was focused on the surface activation of CFRP substrates, made via traditional vacuum-bag technique, which was performed using several combinations of LPP parameters. From the comparison with conventional pre-bonding preparations, it was possible to quantify the effectiveness of LPP in increasing the performance of adhesively-bonded CFRP joints. Further measurements of roughness and wettability were performed, and analyses via x-ray photoelectron spectroscopy (XPS) were also carried out, allowing identification of the morphological, physical and chemical phenomena involved in the treatments. Then, a quantitative evaluation of the aging behavior of the adhesively-bonded joints was the topic of the subsequent Part B. Four significant sets of LPP-treatment conditions were selected, and then subjected to accelerated temperature-humidity aging. To assess the durability of the CFRP-adhesive system under severe aging conditions, tensile shear strength (TSS) testing and wedge cleavage test (WT) were performed in parallel. The experimental findings showed that LPP treatment of the CFRP substrates results in increased short-term quality of the adhesive joint as well as in enhancement of its durability even under severe aging conditions. The last part of the work (Part C) was inspired by the recent developments in additive technologies for the manufacturing of structural thermoplastic-composite parts. In this context, the mechanical and failure behavior were investigated of continuous carbon-fiber (CCF) composite materials built via Fused Filament Fabrication (FFF) technology when used as substrates for bonded joints. Notably, the experiments were focused on verifying how the additively-manufactured substrates respond to adhesive bonding when the interface interactions are increased by preparing the surface with LPP treatment. This approach allowed detection of those criticalities that might limit the application of adhesive bonding to 3D-printed composite parts, with respect to that observed using traditional CFRP materials.