Influence of internal stress in optical thin films on their failure modes assessed by in situ real-time scratch analysis

- A methodology allowing for an in-situ study of the scratch mechanisms of optical coatings is proposed.
- As an example, scratch behavior of TiO2 optical films presenting different levels of internal stress is investigated.
- The scratch sequence and the evolution of the stress field surrounding the contact region are observed in situ in real time
- Failure mode and internal stress of the thin films are linked together to extract yield strength of the coating.
- Scratch test operation in the decreasing load mode provides a significantly improved measurement reproducibility.

In this study we present a new in situ real-time approach to perform and analyze scratch tests of transparent coating/substrates systems. This method allows for the observation of the contact region during the scratching process. As an example, thin TiO2 layers exhibiting stress levels ranging from tensile to compressive were deposited by ion beam assisted evaporation onto plastic substrates. Failure processes obtained using an increasing and a novel decreasing load scratch sequences were then linked to the internal stress in the coatings allowing one to draw a stress management diagram and to evaluate the yield stress of the TiO2 layers. This work enhances the understanding of the optical films' failure mechanisms, and outlines a new pathway to increase measurement reproducibility.