Mechanisms of blistering and chipping of a scratch-resistant coating

In most cases, scratching of the surface of a polymeric glass elicits brittle behavior. Industrial solutions have been successfully used to improve the scratch resistance of polymeric glasses and a common way is to coat the substrate with a thin film. However, one of the limitations of this method is the risk of cracking and chipping. The origin of the success of the coating technique is still of great research interest and further work will be required to explain the improvement in scratch resistance and predict the cracking in anti-scratch coatings. 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 (3.5 μm) nano-composite coating deposited on a viscoelastic–viscoplastic substrate (polycarbonate) was investigated under different conditions of temperature and scratching speed. Four types of fracture mechanisms were observed, depending on these two variables. The processes involved in deformation of the system were: (i) delamination (blister formation) and fracture (chipping) of the coating and (ii) viscoelastic–viscoplastic deformation of the substrate. Image analyses were performed on video sequences of the different processes leading to damage of the film. The quantitative results are discussed in terms of the damage mechanisms involved.