Comparative study on taper preparation techniques for direct internal structure analysis of organic coatings applied on galvanized steel

Depth-resolved analysis of organic coatings delivers crucial information on the interior material structure, e.g., on the formation of chemical gradients or on the impact of degradation processes. However, assessing the complex internal composition of modern heterogeneous paint systems requires a careful selection of suitable depth profiling methods. In this work, a comparative examination of four state-of-the-art preparation techniques for depth-resolved analysis, exemplified on a high-build polyester-polyurethane coating system applied on galvanized steel, is reported. The fabrication of wedge-shaped tapers by applying mechanical polishing, cryo-ultra-low-angle microtomy (cryo-ULAM), a standardised paint borer, and Ar+-ion flat milling was comprehensively evaluated with respect to determining preparation artefacts including smearing, plastic deformation or alterations of the exposed internal coating chemistry induced by the different techniques. To this purpose, 3D topographical imaging, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were employed to examine the resulting surface morphology of extended sections down to the steel substrate, which showed the displacement of material for the paint borer sections, while the polished samples contained grooves and smearing artefacts. Furthermore, correlative elemental and chemical analysis by Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) allowed to determine the impact of the respective preparation process on the bulk and surface chemistry of the exposed areas, e.g., the loss of chemical information after Ar+-ion flat milling. Overall, the fabrication of extended tapers for depth profiling by cryo-ULAM is favourably recommended by the comprehensive in-depth investigation performed in this study, which serves as a valuable basis for the future internal structure analysis of organic coatings.