Oxygen Permeability and Its Role in Governing Life of YbDS Environmental Barrier Coatings

The growth kinetics of thermally grown oxide (TGO) silica in Yb-disilicate (YbDS) environmental barrier coatings (EBCs) significantly affects the durability of EBCs. The oxygen permeability can control the TGO growth kinetics and thus could play an essential role in determining EBCs life. Therefore, the oxygen permeability constant of YbDS and TGO is systematically evaluated in terms of thermodynamics using the parabolic rate constant (kp). Dry and wet oxygen conditions as well as different temperatures and partial pressures are investigated. The results offer evidence that the oxygen permeability constant for YbDS is an order of magnitude higher than for TGO. As such, the TGO hinders the oxidant diffusion stronger, proving to be the rate controlling layer. Moreover, water vapor strongly increases oxygen permeability with defect reactions playing a key role. It is suggested that oxygen primarily diffuses in ionic form, particularly in wet environments. Consequently, the stronger impact of the TGO in reducing the oxidant diffusion rate can be rationalized by its higher Si-O bond population as compared to the YbDS topcoat. Moreover, the effect of topcoat oxide modifiers on oxygen permeation is composition sensitive and seems to be related to their interaction with oxide ions and their mobility.