Evaluation of Scratch and Mar Resistance in Automotive Coatings: Nanoscratching by Atomic Force Microscope

Abstract

Automotive coatings are designed to both protect the substrate from damage and corrosion and to provide color, gloss, and distinctness to the coated object. Mar resistance is a measure of the resistance of a surface to permanent deformation resulting from the application of a dynamic mechanical force. This chapter is devoted to the measurement of the marring by nano-indentation, optical imaging, and gloss measurements. It focuses on the measurement of the resistance of different coating systems to controlled scratching. It is demonstrated that scanning probe microscopes can be used to perform nanoscratching experiments to help quantify the scratch resistance of automotive clearcoats. The use of Scanning Probe Microscopy (SPM) is not widespread and requires extensive training and capital investment. The results presented here should not be used to promote one coating system over another. Coating performance is a complex phenomenon that involves resistance to many different types of environmental factors. For example, improved scratch, and mar resistance may lead to lower performance in chip resistance as well as acid etch resistance. Furthermore, the properties of these coating systems change with time and exposure. The polyurethane system used in this study shows poor performance compared to the others used. With exposure, the polyurethane systems generally improve in their resistance to scratching. Further, some clearcoats have the potential to reflow and some scratches may be fully or partially healed. This typically occurs slowly and by further polishing of the surface.