One basic strategy for the latter viewpoint is to perform high-temperature water vapor corrosion tests for single-phase polycrystals, as reported for R 2 Si 2 O 7 (R = Sc, Nd, Er, Yb, or Lu) ; this strategy aims to investigate structural changes, more-intrinsic recession rates, and relations to thermochemistry of volatilization . The results provide insights into the reaction with H 2 O on the surfaces and useful data to estimate recession rates. Meanwhile, oxygen permeability measurement of a single-phase polycrystalline wafer, which is a model of each layer that constitutes multilayered EBCs, is quite useful to better understand mass transfer along grain boundaries (GBs) in the layer under an oxygen potential gradient . The GB density, diffusion length, and oxygen potential gradient remain constant during the experiment. Hence, evaluating the interdiffusion of cations and anions along GBs in a wafer allows us to accurately characterize the diffusion properties. This measurement has been extended to an environment containing water vapor . Atomic diffusivity from oxygen permeation and tracer diffusion experiments could be used for the estimation of oxidation rates of an underlying bonding layer . For the latter purpose, an alternative approach based on ionic conductivity measurements has also been reported .