NaOH etching and resin pre-coating treatments for stronger adhesive bonding between CFRP and aluminium alloy

Adhesive bonding between carbon fibre reinforced plastics (CFRPs) and aluminium alloys is extensively practised to achieve optimum lightweight and reliable structures in the aerospace and automobile industries. In this research, we study pre-treatments of aluminium substrates for stronger adhesion with CFRP. An ultrasonic etching process was carried out in alkaline solutions to investigate the influence of NaOH concentration on adhesive bonding characteristics. An ultra-thin layer of acetone-diluted resin pre-coating (RPC) without hardener was then applied to the etched substrates to seal micro-cavities before adhesive bonding. The single lap shear test was used to evaluate the adhesive bond strength under different surface conditions. The topography and chemistry of treated surfaces were characterised using various surface analytical tools including optical profilometry, scanning electron microscopy and X-ray microanalysis (SEM/EDS), contact angle goniometry and X-ray photoelectron spectroscopy (XPS). Experimental results showed a maximum 91% improvement in bond strength after alkaline etching treatments, which removed the weak passive oxide layers and allowed the formation of thin hydroxide layers through aluminium-water reactions. The wettability of etched specimens was also improved, indicating by their higher surface energy values. Variations of roughness parameters and tomography under different NaOH concentrations also affected the bond strength. The RPC treatments further increased the bond strength of NaOH etched specimens by 8.4-11.6%. The surface treatments reported in this work are very simple and cost-effective for producing durable adhesive joints in industrial applications.