Modeling Microstructural Evolution in Thin Films: Phase Transformations and Grain Growth

<p dir="ltr">The reliability of materials in critical applications, from microelectronic interconnects to energetic materials, depends fundamentally on microstructural evolution driven by coupled physical processes. This dissertation investigates how grain boundaries, phase transformations, and void dynamics control reliability in thin-film systems using phase-field methods as the modeling framework. In Cu-Sn solder joints, the formation of brittle intermetallic compounds (Cu₆Sn₅ and Cu₃Sn) during thermal aging leads to mechanical degradation. Using a phase-field fracture model validated against ball pull test experiments, we demonstrate that the transition from single-stack columnar Cu₃Sn grains to multi-stack grain structures, rather than simply IMC thickness, governs the reduction in joint strength. Under electromigration conditions, the anisotropic diffusion of Cu through tetragonal Sn creates orientation-dependent failure, with Cu depletion rates varying by factors of two to three depending on Sn grain alignment relative to current flow. For pentaerythritol tetranitrate (PETN) energetic thin films, we develop an integrated model coupling physical vapor deposition with thermomechanical grain coarsening. Thermal stresses induce void elongation along grain boundaries, accelerate coarsening kinetics, and promote growth of compliant (110)-oriented grains, while porosity acts as a drag on grain boundary migration. Across all systems studied, interfaces (grain boundaries, phase boundaries, and voids) emerge as the critical microstructural features controlling reliability. This work establishes phase-field modeling as a predictive framework for understanding coupled multiphysics degradation mechanisms in materials where anisotropy, interfaces, and complex loading conditions govern performance.</p>