(Abstract) Adhesive bonding of metals with polymeric resin has attracted attention as an innovative bonding technology to realize high functionality and weight reduction of various mechanical components and structural materials, and the demand for this technology has been increasing due to the rapid expansion of multi-materials in recent years. However, it has been reported that there are several reasons of degradation of adhesive (i.e. adhesive strength becomes lower), such as the formation of natural and hydrated oxide films on the metallic surface, and thus the degradation is accelerated by inherent water molecules at the interface. However, there are still many unclear points regarding the mechanism of lower adhesive strength. This study elucidates the mechanism of adhesion fracture between a metal and a resin using Molecular Dynamics (MD), and investigated how the moisture on the metallic surface affects the adhesion.(Simulation model) LAMMPS software was employed for the MD simulation, and the force field was ReaxFF, which is a typical reactive force field. This study prepared an alumina model, since a surface of aluminum alloy is usually oxidized. The alumina model exposed a pure (0001) surface. First, an arbitrary number of water molecules were allowed to approach this surface, resulting in O-H termination on the surface due to the reaction between water molecule and alumina surface. This becomes a termination layer on the alumina surface. When the number of water molecules added to the surface increases, the O-H termination became saturated, and the water molecules remained on the surface. This is a water molecule layer due to un-reacting of termination. Here, we designated several models in terms of water molecule at the interface. The model without water molecules was designated as “Phase 1”, the alumina model with a termination layer was designated as “Phase 2”, and the alumina model with a termination layer and a water molecule layer was designated as “Phase 3”. For the model of the epoxy resin, the main agent molecule (DGEBA) and the curing agent molecule (DDM) were mixed and cross-linked reaction occurs. This epoxy resin model was placed on the alumina model to simulate the adhesion. By performing long-time relaxation, various chemical reactions occurred between the interfaces. For these interface models, tensile loading was applied the epoxy part to separate from the alumina surface, and then bonding strength of the interface was investigated.　　(Results and discussions) From tensile loading simulation, the peak loading stresses were used as the bond strengths. It is found that the bond strength decreases with the increase in the number of water molecules at the interface. To elucidate such this mechanism, interfacial energy contributing to the adhesion was calculated. The primary energy is non-boding potential one, but the other energy is also critical in adhesion. It is noted that the number of covalent bonds at the interfaces is similar with the covalent energy, and the number of hydrogen bonds by RDF is similar with the hydrogen bond energy. Among three models, the covalent bond energy in “Phase 1” was the highest, and the hydrogen bond energy in “Phase 2” was the highest. When the number of water molecules at the interface increases, it is found that the number of covalent bonds decreases, resulting in lower adhesion. By comparing the energies of covalent bonds and hydrogen bonds, the covalent bond energy is larger and then the covalent bond energy contributes much more to the adhesive strength. These results suggest that increasing the amount of water molecules decreases the adhesive strength.

Fracture Behavior of Alumina/Epoxy Resin Interface and Effect of Water Molecules by Using Molecular Dynamics Using Reaction Force Field (ReaxFF)

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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