Understanding of automotive clearcoats scratch resistance

Abstract Micrometric scratches, mainly due to car wash brushes, decrease the gloss of automotive clearcoats, alter their appearance and prevent them from protecting the car in the best way. Scratch resistance is estimated from the visible damage caused on the clearcoats. As the eye is more sensitive to brittle than to ductile scratches (Microstructures and Microtribology of Polymer surfaces (2000) 428), one way to improve scratch resistance it to decrease the proportion of brittle scratches and to favor healing of ductile scratches. An analysis of the contact during a scratch test shows that the ductile/brittle transition is controlled by both an energetic and a size criterion. The energetic criterion is related to the tensile stress located at the rear of the contact between the indenter and the clearcoat. This stress can enlarge existing cracks, leading to fracture. It depends on the clearcoats viscoelastoplastic properties, the mean deformation imposed by the indenter and the environment (dry tests or tests in water or soapy water). The effect of these parameters on scratch resistance has been studied: indentation and scratch tests with a Nano-indenter XP have been carried out. The healing of ductile scratches has been estimated from scratched samples of clearcoats. The samples have been heated at different temperatures and the remaining scratch depth measured with an atomic force microscope.