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 fiber-reinforced 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 high-resolution 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 CO 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.