Scratch damage resistance of silica-based sol–gel coatings on polymeric substrates

TLDR: In this article, a review of the mechanics of sliding contact, scratch failure modes and related mechanisms, and the parametric models related to these failure modes are discussed and the importance of characterizing the coating fracture toughness and interfacial fracture toughness between the coating and the substrate is highlighted.

Abstract: Sol–gel coatings on polymeric substrates have been widely used in optical lenses, automobiles, safety windows, and flexible display panels. The resistance to scratch damage is an important reliability consideration. This article begins with a review of the mechanics of sliding contact, scratch failure modes and related mechanisms, and the parametric models related to these failure modes. Generally failure modes can be categorized into coating cracking driven, and delamination driven. By analyzing the potential failure modes, the importance of characterizing the coating fracture toughness and interfacial fracture toughness between the coating and the substrate is highlighted. Controlled buckling test is introduced as an easy and appropriate test method that can be used for the measurement of both properties. Silica-based sol-gel coatings with different amounts of colloidal silica were prepared on polymeric substrates. Pencil scratch test was carried out following the ISO 15184 standard. The indentation hardness, elastic modulus and fracture toughness of the coatings were also determined and correlated with the observed pencil scratch hardness. The scratch failure was found to be tensile trailing coating cracking. Analysis shows that the main factors affecting scratch resistance are elasticity modulus, thickness and fracture toughness of the coating. Based on the current and other reported results, ways to improve scratch resistance for brittle coatings on compliant substrates are discussed.