Mechanical analysis of the blistering of a thin film deposited on a glassy polymer

In most cases, scratching of the surface of a polymeric glass elicits brittle behavior. A common way to improve the scratch resistance of a sensitive surface is to coat it with a thin film. Further work is required to explain the improvement in scratch resistance due to coating technique and predict the cracking in anti-scratch coatings. Moreover, the substrate/thin film adhesion must be well controlled and measurable. The present study contributes to these aims. Using a single-asperity scratching device allowing in situ observation of the scratch, the fracturing of a thin nano-composite coating deposited on a polycarbonate substrate was investigated under different conditions of temperature and scratching speed. Four types of fracture mechanisms were observed, depending on these two variables. A global energy balance model of the blistering process which is obtained for some experimental conditions permits one to determine the adhesion of the system. The adhesion can be measured by following the delaminated area (quantified by image analysis) as a function of the scratching distance during blistering. The particular case of an experimental stable blistering process was studied and the corresponding substrate/thin film adhesion was derived using the global energy balance model.