Molecular Understanding of the Interfacial Interaction and Corrosion Resistance between Epoxy Adhesive and Metallic Oxides on Galvanized Steel.

The epoxy adhesive-galvanized steel adhesive structure has been widely used in various industrial fields, but achieving high bonding strength and corrosion resistance is a challenge. This study examined the impact of surface oxides on the interfacial bonding performance of two types of galvanized steel with Zn-Al or Zn-Al-Mg coatings. Scanning electron microscopy and X-ray photoelectron spectroscopy analysis showed that the Zn-Al coating was covered by ZnO and Al2O3, while MgO was additionally found on the Zn-Al-Mg coating. Both coatings exhibited excellent adhesion in dry environments, but after 21 days of water soaking, the Zn-Al-Mg joint demonstrated better corrosion resistance than the Zn-Al joint. Numerical simulations revealed that metallic oxides of ZnO, Al2O3, and MgO had different adsorption preferences for the main components of the adhesive. The adhesion stress at the coating-adhesive interface was mainly due to hydrogen bonds and ionic interactions, and the theoretical adhesion stress of MgO adhesive system was higher than that of ZnO and Al2O3. The corrosion resistance of the Zn-Al-Mg adhesive interface was mainly due to the stronger corrosion resistance of the coating itself, and the lower water-related hydrogen bond content at the MgO adhesive interface. Understanding these bonding mechanisms can lead to the development of improved adhesive-galvanized steel structures with enhanced corrosion resistance.

Interface Wetting Driven by Laplace Pressure on Multiscale Topographies and Its Application to Performance Enhancement of Metal-Composite Hybrid Structure.

Surface topography reconstruction is extensively used to address the issue of weak bonding at the polymer-metal interface of metal-composite hybrid structure, while enhancement from this approach is seriously impaired by insufficient interface wetting. In this study, the wetting behavior of polymer on aluminum surfaces with multiscale topographies was theoretically and experimentally investigated to realize stable and complete wetting. Geometric dimensions of multiscale surface topographies have a notable impact on interfacial forces at the three-phase contact line of polymer/air/aluminum, and a competition exists between Laplace pressure and bubble pressure in dominating the wetting behavior. Laplace pressure facilitates the degassing of trapped air bubbles in grooves, bringing more robust interfacial wettability to grooves than dimples and grids. Conversely, dimples with excessive dimensions generate interfacial pores, and this intrinsic mechanism is theoretically unraveled. Moreover, different degrees of interface wetting cause variations in bonding strength of polymer-aluminum interface, which changes from ∼18% improvement to ∼17% reduction compared to original strength. Finally, groove topography perfectly achieved complete wetting between polymer and aluminum and consequently improved flexure performance by over 11% for the aluminum-carbon fiber hybrid side impact bar, which verifies the importance of complete wetting at a part scale. This study deepens the understanding of wetting behavior and clarifies the intrinsic correlation between interfacial bonding performance and surface topography.

Spindle-Shaped Surface Microstructure Inspired by Directional Water Collection Biosystems to Enhance Interfacial Wetting and Bonding Strength.

Unique spindle microstructures with an apex angle of ∼20° bring the ability of directional water collection to various biosystems (i.e., spider silk and cactus stem). This has great potential to solve the insufficient interfacial wetting for mechanical interlocking formation between polymers and substrates. In this study, the bioinspired spindle microstructures were easily fabricated through the deposition of molten materials by a nanosecond laser with an overlap ratio of 21% between laser spots and achieved superior interfacial wetting for commercial epoxy adhesive on aluminum substrates. Detailed analyses show that there are four mechanisms responsible for the superior interfacial wettability of bioinspired spindle microstructures: the Laplace pressure difference, newly formed aluminum oxide, the capillary effect, and no extra pressure from a trapped atmosphere. Consequently, the bioinspired spindle surface microstructures achieve a maximum improvement of ∼16 and ∼39% in interfacial bonding strength before and after water soak exposure compared to the as-received condition. Moreover, the stable interfacial wettability of bioinspired spindle microstructures ensures that the improved joint strength varied little with an increase in surface roughness from ∼1.7 to ∼12.8 µm. However, the interfacial wettability of common dimple microstructures deteriorated with an increase in surface roughness, which is indicated by the decreasing rule in the quadratic polynomial function of the interfacial bonding strength as the surface roughness increases from ∼2.1 to ∼18.2 µm.

Effect of Atmospheric Pressure Plasma Treatment on Adhesive Bonding of Carbon Fiber Reinforced Polymer.

To improve the strength of the adhesive-bonded carbon fiber reinforced polymer (CFRP) joints, atmospheric pressure plasma treatment (APPT) was used to treat a CFRP substrate surface. This study investigated the effects of nozzle distance (i.e., the distance between plasma nozzle and CFRP substrate) and nozzle speed (i.e., the moving speed of plasma nozzle relative to CFRP substrate) of APPT on the lap-shear strength of adhesive-bonded CFRP joints. Results show that the lap-shear strength of plasma-treated CFRP joints increased to a peak value and then decreased as the nozzle distance increased, and the nozzle distance associated with the peaked joint strength depends on the applied nozzle speed. The lap-shear strength of plasma-treated adhesive-bonded CFRP joints reaches up to 31.6 MPa, compared to 8.6 MPa of the as-received adhesive-bonded CFRP joints. The surface morphology of plasma-treated CFRP substrates was investigated by scanning electron microscope observation, and the mechanism associated with the improved joint strength after applying APPT was revealed through surface chemistry analysis. It is found that APPT not only effectively removed the content of Si element and ⁻CH3 (i.e., the main compositions of release agent) from the as-received CFRP substrate surface, but also generated many polar groups (i.e., ⁻NH2, ⁻OH, ⁻COOH, etc.), which has a positive effect on increasing the wettability and interfacial bonding strength of CFRP substrates and consequently results in a significant improvement of lap-shear strength of plasma-treated CFRP joints. In addition, the result of differential scanning calorimetry (DSC) test shows that the surface temperature of CFRP substrate should not exceed 175.3 °C during APPT. In this study, an empirical model governing temperature, nozzle distance and nozzle speed was established to guide the selection of atmospheric pressure plasma treatment process parameters in industrial manufacture.