A Detailed Molecular Dynamics Simulation and Experimental Investigation on the Interfacial Bonding Mechanism of an Epoxy Adhesive on Carbon Steel Sheets Decorated with a Novel Cerium–Lanthanum Nanofilm

The influences of steel surface treatment by a novel cerium–lanthanum (Ce-La) nanofilm on the adhesion mechanism of an epoxy adhesive were studied through experimental and modeling approaches. The surface morphology and microstructure of the film deposited were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The surface free energy and work of adhesion values were evaluated through contact angle analysis. Also, the interfacial adhesion strength between the epoxy adhesive and steel surface, together with failure forms, were examined through pull-off test, under dry and wet conditions, and Fourier transform infrared (FT-IR) spectroscopy. The results obtained from experiments revealed that depositing a Ce-La nanofilm on the steel surface increased its roughness and surface free energy, and strengthened the epoxy coating adhesion. It was also observed that the epoxy adhesion on the Ce-La treated steel was stronger, compared to the Ce-treated surface. Furthermore, the adhesion extent and surface bonding mechanism of aminoamide-cross-linked epoxy resin were computationally modeled by applying atomistic molecular dynamics (MD) and electronic density functional theory (DFT) methods. The modeling results evidenced that epoxy resin adhered more strongly to the conversion layer (represented by CeO₂ and La₂O₃), compared to an untreated steel surface (i.e., pure and oxidized iron). The epoxy binding onto CeO₂(111), La₂O₃(001), Fe₂O₃(110), and Fe₃O₄(100) almost occurred via electrostatic interactions, while its adhesion mechanism over FeO(100) and Fe(110) surfaces was based on van der Waals forces. The computations also demonstrated that the epoxy adsorption energy decreased in wet environments, because of solvent affinity toward the epoxy and the surface, but the rate of reduction was smaller over CeO₂ and La₂O₃, compared to that observed with iron oxides. These modeling outcomes were consistent with our experiments and proposed the superior epoxy adhesion on modified steel sheets.