Improving interface bonding strength of laser-welded steel/CFRTP hybrid joint via bilayer silane film

The gamma-aminopropyltrimethoxysilane (gamma-APS) and gamma-(2,3-epoxypropoxy) propyltrimethoxysilane (gamma-GPS) single or bilayer silane film was produced to improve the tensile property of laser-welded 304 stainless steel/CFRTP joint in this work. The bonding mechanism at joint interface was investigated through experiments as well as density functional theory (DFT) calculation. Si-O-Fe covalent bonds were formed between steel and different silane films. Hydrogen bonds were induced at the interfaces of silane coating/polymer sides. The binding energy calculation results suggested that the binding between gamma-APS silane coupling agent and steel was more stable than that of gamma-GPS and steel. The binding energy of -3.37 eV between amino group in gamma-APS and carbonyl group in CFRTP was greater than the binding energy of -4.84 eV between epoxy group in gamma-GPS and amide group in CFRTP, which indicated stronger interaction was obtained between gamma-GPS and polymer compared to the interaction between gamma-APS and polymer. The bilayer silane coating could combine the dominant functional groups of silane monolayer film to obtain stable bonding on both metal side and CFRTP side. Moreover, compared with silane single film, longer silane oligomer molecular chain length was provided by bilayer silane film, resulting in more entanglement sites between silane film and CFRTP and the increased joint mechanical property. The steel/CFRTP hybrid joint reached maximum tensile shear strength of 17.2 MPa, which was further increased by 115.0% and 79.2% than that obtained by gamma-APS and gamma-GPS silane single coating, respectively.