Coating is a common way of improving the scratch behaviour of polymeric glasses. A thin coating will prevent the micro-scratches created by the roughness of a diamond tip at the surface of the macro-groove left on the substrate and allow recovery of the groove. The ratio of the thickness of the coating to the roughness of the tip is a critical parameter, which enables one to increase the scratch resistance in the case of a thin coating. A single value of the critical load cannot describe the damage behaviour of a coating on a polymeric material. It is generally assumed that cracking appears at the rear edge of the contact area and is related to the tensile stress, whereas we observed that cracking of the coating appears within the contact area. The mechanical behaviour of a coating on a viscoelastic material should be analysed in terms of the shape of the stress field, modified by the effect of the local friction between a scratching tip and the coat, where this local friction will depend on the roughness of the tip. Some numerical simulations were performed to attempt to elucidate the cracking mechanism.

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