Adhesion improvement and strengthening mechanisms of ultrasonic adhesive-impact bonding process for CFRP/Ni bonded joints

The adhesively bonded structures of carbon fiber-reinforced polymers (CFRPs) and metals are extensively applied in various engineering fields. However, the inferior adhesion between metals (particularly such inert metals as nickel) and adhesives remains a significant challenge. In addition to surface pretreatment technology, ultrasonic vibration has been demonstrated to effectively enhance the flowability and permeability of liquid. In this paper, by implementing a novel ultrasonic adhesive-impact bonding process, the ultrasonic vibration was directly acted on adhesives to generate a violent high-frequency impact on the adherend surfaces of CFRP/Ni joints before the adhesive bonding process, resulting in an improvement of 17.2 % in the bonding strength. The optimal process parameters were determined through an orthogonal experiment, and the failure modes of joints were analyzed. The strengthening mechanisms were investigated by analyzing the microstructure and chemical components of both adherend surfaces and adhesive interfaces. The SEM image of the interface indicated that the highfrequency impact of adhesive caused by ultrasonic vibration facilitated its penetration into the microgrooves on the nickel surface, thereby increasing the effective bonding area and enhancing the mechanical anchoring. Furthermore, the FTIR spectra of the interface indicated the enlarged bonding area and the intense highfrequency collision between the adhesive molecules and the polar chemical groups on the nickel surface, which was introduced by atmospheric plasma treatment, enhanced the probability of the chemical reactions between them, thus strengthening the chemical bonding.