Modeling oxygen permeability through topcoat and thermally grown oxide in dense Yb2Si2O7 environmental barrier coatings

The oxygen permeability of ytterbium disilicate (YbDS) topcoat (TC) and silicon dioxide (SiO2) thermally grown oxide (TGO) is evaluated. The primary goal is to elucidate the oxidation mechanisms in environmental barrier coatings (EBCs). For this purpose, oxidant diffusion is investigated using physics-based and numerical modeling. The oxygen permeability constants are systematically evaluated and quantified in terms of thermodynamics using defect reactions and the parabolic rate constant (kp), respectively. Dry oxygen and wet oxygen conditions as well as different temperatures, partial pressures, and topcoat modifiers are investigated. The results offer evidence that the oxygen permeability constant for the YbDS topcoat is an order of magnitude higher than for the TGO. As such, the TGO hinders the oxidant diffusion stronger, proving to be the diffusion rate-controlling layer. Moreover, water vapor strongly increases the oxidant permeation with defect reactions playing a key role. It is suggested that the mass transfer through the topcoat is primarily by outward ytterbium ion diffusion and inward oxygen ion movement, with the latter being dominant, particularly in wet environments. The effect of topcoat modifiers on oxidant permeation is composition sensitive and seems to be related to their interaction with oxygen ions and their mobility.