Traditional steel surface treatment (e.g., sand blasting, or silane treatment) was regarded as an effective method to improve the bonding strength of steel-epoxy single-lap joints. In the present study, a new steel surface treatment method was developed. With this method, the steel surfaces were treated with suspensions of nano-sized and micro-sized Al2O3 particles in ethanol/water mixture using the dip-coating method. Both Al2O3 particle sizes were previously treated or not treated with silane. Single-lap shear tests of the steel-epoxy bonds were conducted to compare the effects of the treating methods. According to the testing results, the highest increase in the bonding strength (by 51.8%) was found for the steel coated with the suspension of silane treated nano-Al2O3 particles. Scanning electron microscopy (SEM) analysis and energy dispersive spectrometer (EDS) analysis indicates that the nano-Al2O3 particles were clearly attached to the treated steel surfaces. Moreover, the steel surface with the silane-treated nano-Al2O3 particles was found to clearly enhance the contact angle between the steel and epoxy resin. The fracture morphology analysis of the single-lap shear testing specimen shows that the bonding between the steel and adhesive changed from steel-epoxy interfacial failure to cohesive failure when the steel surfaces were treated with the nano-Al2O3 particles suspension. The developed steel surface treatment method with the suspension of nano-particles proves to be effective and reliable in enhancing the bonding strength of the steel-to-epoxy adhesives.
Keywords: adhesion; interface mechanical properties; nanostructures; shear strength.