Bonding mechanisms of carbon fiber-reinforced plastic/aluminum alloy interface during friction lap welding via silane coupling treatment

Silane coupling pretreatment could usually increase the strength of metal/plastic-based material joints, however, the detailed chemical bonding mechanism has not been clarified so far, which has become a key bottleneck for the further expanding application of this technology. Here, a specially designed silane coupling treatment was conducted on the aluminum alloy surface, and a strong friction lap welding (FLW) joint of carbon fiberreinforced thermoplastic (CFRTP) to aluminum alloy was obtained. The average tensile shear force of 6.83 kN (-30.36 MPa) was obtained, approximately 140% higher than the untreated joint, which was higher than the results obtained via FLW ever reported. The detailed bonding mechanism at the interface was studied by highresolution transmission electron microscopy, x-ray photoelectron spectroscopy, and Fourier transform infrared reflection. It was found that the coupling agent layer observed at the interface acted as a bridge to achieve a tight bond with aluminum alloy and CFRTP. The bonding between the coupling agent layer and the aluminum alloy was achieved via the Si-O-Al and Si-O-Mg bonds, and the covalent bonding of C(=O)-N was formed by the chemical reaction between the C--O bonds of CFRTP and the amino groups (-NH2) of the coupling agent layer, resulting in the tight joining at the atomic scale. These chemical bonds contributed to the joint strength, which provided a better understanding of the joining mechanisms of plastic-based materials to metals via silane coupling treatment.