Nano-indentation, scratching and atomic force microscopy for evaluating the mar resistance of automotive clearcoats: study of the ductile scratches

Abstract

Automotive clearcoats are subjected to different kinds of environmental damage: UV radiation, acid rain, tree sap, birds droppings, stone chipping and marring. Marring, which is mainly due to car wash brushes, decreases the gloss of automotive paint and alters the colour. Optical and atomic force microscope imaging of clearcoats samples brushed by an automated car wash reveals both plastic scratches and fractures. The same type of scratches has been drawn in a controlled manner: the applied load is controlled, the deformation and the strain rate are known. First, the Young's modulus and the hardness of the clearcoats are calculated from indentation tests at different strain rate values. Next, scratching tests providing greater strain rates allowing us to extend the hardness results from indentation tests. These experiments carried out at room temperature have been supplemented by indentation tests performed at different temperatures (from −10°C to 45°C). Thus, the evolution of the indentation hardness vs. strain rate at different temperatures has been measured. A time–temperature superposition curve has been constructed. This hardness master curve as a function of reduced frequency (reduction temperature of 20°C) allows us to define, for each clearcoat, the activation energy of the viscoplastic process. The found values are close to those of the secondary β transition, usually attributed to local molecular motion, like rotations of side groups or limited motions within the chain backbone. It turns out that mar resistance would largely be dependent of this transition temperature position in comparison with the main mechanical relaxation (α transition) temperature.