Mechanical analysis of the damage of a thin polymeric coating during scratching: role of the ratio of the coating thickness to the roughness of a scratching tip

It is generally accepted that the critical load generating the first damage in a scratch test is representative of the behaviour of a coating. As the properties of polymers are time and temperature dependent, a single value of the critical load cannot describe the overall mechanical behaviour. A new scratch apparatus has been designed which allows scratching over a wide range of velocities and temperatures and record real time photographs of the in situ contact area. It was observed that cracking appears within the contact area. In the case of thin solid films, the ratio of the contact radius to the radius of the grooving tip proved to be a pertinent parameter to predict the damage and did not depend on the scratching velocity or temperature. The ratio of the thickness of the coating to the roughness of the tip is another critical parameter: the coating prevents the roughness of the diamond tip from creating micro-scratches at the surface of the macro-groove. Therefore, since the absence of micro-scratches is a condition for relaxation of the macro-groove, the thickness of the coat must be greater than the roughness of the tip.