| Abstract | The oxygen permeability of ytterbium disilicate (YbDS) topcoat (TC) and silicondioxide (SiO₂)thermally grown oxide (TGO) is evaluated. The primary goal is toelucidate the oxidation mechanisms in environmental barrier coatings (EBCs).For this purpose, oxidant diffusion is investigated using physics- based and numer-ical modeling. The oxygen permeability constants are systematically evaluated andquantified in terms of thermodynamics using defect reactions and the parabolicrate constant (kp), respectively. Dry oxygen and wet oxygen conditions as well asdifferent temperatures, partial pressures, and topcoat modifiers are investigated.The results offer evidence that the oxygen permeability constant for the YbDS top-coat is an order of magnitude higher than for the TGO. As such, the TGO hindersthe oxidant diffusion stronger, proving to be the diffusion rate- controlling layer.Moreover, water vapor strongly increases the oxidant permeation with defect reac-tions playing akey role. It is suggested that the mass transfer through the topcoatis primarily by outward ytterbium ion diffusion and inward oxygen ion movement,with the latter being dominant, particularly in wet environments. The effect oftopcoat modifiers on oxidant permeation is composition sensitive and seems to berelated to their interaction with oxygen ions andtheir mobility. |
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