This work studies the interfacial behavior of functionally graded coatings (FGCs) on homogeneous isotropic substrates under a pure mode-I loading condition, as useful aspect of resilient structural applications. The scheme of unsymmetric double cantilever beam (DCB) based on an enhanced beam theory (EBT) is here applied, accounting for the nonlinear elastic detachment of both sublaminates at their interface both in the normal and tangential directions. A systematic investigation has focused on the sensitivity of the fracturing response of the coating-to-substrate system. A closed form solution stemming from a Matlab subroutine is provided for the energy release rate and mode-mixity angle of the specimen, as monitoring parameters of the overall fracturing process. The functionally graded (FG) materials considered in this study account for a power law FG distribution law, based on 5 different key parameters. The numerical solutions from the EBT have been systematically compared to predictions from a simple beam theory (SBT), confirming the accuracy of the proposed approach, and the sensitivity of the interfacial behavior, the adhesive resistance and fracture toughness to different functional graduations of the material, with useful insights for design purposes of such customizable novel materials.