Block Cracking in Surface Coatings of Polymeric Substrates

Block cracking is commonly observed in form of a series of interconnected cracks that divide the surface of multilayered materials into approximately rectangular or square pieces. A three-dimensional (3D) elastic fracture model is developed to study the block cracking in thin film/substrate structures which consist of a thin protective metal oxide coating fully bonded to a polymer substrate. Under a temperature change, fracture occurs in the coating due to the thermal stress caused by the material mismatch. Using the plane assumption and non-shearing assumptions, the displacement and stress fields in this substrate-coating system under thermal loading are explicitly solved and verified with the finite element (FE) simulation results. Therefore, the energy release rate (ERR) can be calculated from the work done by the stress on the crack opening before crack and has been used to study the fracture initiation, infilling and saturation. Additionally, the theoretical fracture analysis results are verified by FE simulation using the cohesive zone model (CZM) and experimental data from literature. The results show that the fracture model presented in this study is able to capture the displacement and stress distributions in the thin hard oxide film fully bonded to a polymer substrate accurately and predicts the fracture initiation, infilling and saturation successfully.