Kinetic Modeling of Grain Boundary Diffusion: Typical, Bi-Modal, and Semi-Lamellar Polycrystalline Coating Morphologies

Abstract

Polycrystalline coatings and materials are widely used in engineering applications. Therefore, it is important to know their kinetics and mass transport mechanisms. The effect of grain boundaries (GBs) on diffusion in thin films with different morphologies lacks understanding. Numerical studies are necessary to study GB kinetics but are limited to simplified cases. The present work addresses the lack of diffusion studies in more complex morphologies. Diffusion in two-dimensional polycrystalline coatings of typical, bi-modal, and semi-lamellar morphologies was modeled and the influence of the microstructure on the diffusion regimes and the overall rate was identified. Different morphologies with similar diffusion coefficients provided different regimes. The regime depends not only on the total diffusivity and grain/GB diffusivities, but also on the morphological features of the surface. While the fast diffusion pathways of GBs accelerated diffusion, the level of acceleration depends on the morphology since fast pathways and flux areas are limited to GBs. GB distribution is important to the mass transfer process, as GBs accelerate diffusion locally. The overall diffusion rate is generally dependent on the diffusion coefficients ratio. Nevertheless, the level of this dependence relies on the morphology.