XPS MAPPING TECHNIQUE FOR THE IDENTIFICATION OF ADHESION LOSS MECHANISMS

An x-ray photoelectron spectroscopy (XPS) mapping technique was developed to define spatially the interfacial chemistry of adhesive-adherend debonding. The technique was used to identify mechanisms of corrosion-induced bond failure for an epoxy adhesive applied to electrogalvanized steel. The specimens studied were lap-shear bonds exposed simultaneously to a static load and a corrosive environment. The bond failure surfaces were examined after testing. X-ray photoelectron spectroscopy elemental maps were generated over the entire area (approximately 12.5 mm x 25.0 mm) of the bond failure on both corresponding surfaces at a spatial resolution of 0.4 mm x 0.8 mm. The mapping was accomplished using a fixed small-spot-size source in conjunction with computer-controlled sample stage movement. The XPS elemental maps, and in one case a functional group map, were generated for the constituents of the adhesive (C, O, Si), the adherend (Zn, O) and the aggressive environment (Na, Cl, O, carbonate). The raw XPS intensity data were converted into units of at.% and displayed, using a bilinear interpolation scheme, as three-dimensional chemical images with a pseudo-color scale representing variations in elemental surface composition. The images provided detailed evidence for specific sites of anodic and cathodic corrosion processes, and facilitated identification of the locus of adhesive failure.