Scratching of polymeric coatings: experimental study and mechanical analysis

摘要

Coating with polymers is a common way of improving the scratch resistance of polymeric glasses. It is generally accepted that the critical load generating the first damage in a scratch test is representative of the behaviour of the coating. As the properties of polymers are time and temperature dependent, a single value of the critical load nevertheless cannot describe the overall mechanical response. A new scratch apparatus has been built that allows scratching velocities in the range 1 to 10~4 μm/s at temperatures varying from -70 to +120℃. The unique feature of this experimental set-up is a built-in imaging system to record realtime pictures of the in-situ contact during scratching. Damage to the coating can thus be observed during the process. In the case of optical coatings, if damage occurs then cracking appears before flaking, irrespective of the quality of the adhesion between substrate and coating. The flaking mechanism has been found to depend on the quality of the layer adhesion. However, the damage mechanism is not related to the adhesion in a simple way. It has also been observed that cracking of the coating appears within the contact area and King and Sullivan have predicted that this behaviour will be obtained when the ratio of the elastic modulus of the coating to that of the substrate is 2. Scratching of polymeric coatings was studied over wide ranges of tip velocity and temperature to find a set of critical parameters controlling the onset of damage. The ratio of the contact radius to the radius of the tip proved to be a pertinent parameter, which indicates that the mean strain in the contact area at the onset of damage does not depend on the scratch velocity or temperature.