Multiscale Analysis and Modeling of Nano-coating Fracture Induced by Inhomogeneous Plastic Deformation of Polycrystalline Metallic Substrate

Nanocrystalline coatings are critical for extensive applications, yet their fracture on polycrystalline metallic substrates severely deteriorates the performance. Nevertheless, the underlying coating fracture mechanism correlated with inhomogeneous substrate plasticity remains ambiguous, and accurately predicting the crack formation is challenging. To address these issues, this study comprehensively characterized 100-nm niobium coating cracks on stainless-steel sheets and developed a multiscale model to predict coating fracture dominated by substrate plasticity. In particular, different coating cracks were identified and classified into three patterns based on their locations: on intragranular slip bands, grain boundaries, and twin boundaries of the substrate. Crystallographic calculations and statistical analyses demonstrated that the coating fractures were induced by grain and sub-grain scale strain localization of the substrate, which was incorporated within a multiscale modeling framework. For nanocrystalline coatings, molecular dynamics simulations were employed to derive the cohesive zone model in the extended finite element method. The coating fracture was subsequently simulated on a representative volume element of the substrate containing discrete slip bands, which was developed based on crystal plasticity and calibrated using slip steps. Microscopic substrate slips with Burgers vectors oriented at 30° to 50° relative to the surface were revealed to trigger coating cracks, which were generalized with a fracture parameter to be efficiently implemented in macroscopic simulations. Compared to traditional homogeneous models, the developed model enabled precise identification of all coating crack patterns in practical samples. This multiscale modeling procedure and these in-depth insights facilitate the prevention of failure in engineered components with nano-coatings.