The specific permeability of O in plasma sprayed zirconia was measured to be approximately 10−16 m2, with some dependence on microstructure, and hence on spraying conditions. Permeability rose with increasing temperature, as expected, and differences of a factor of approximately 4 were observed between room temperature and 600C. Maximum O fluxes through the top coat via gas permeation, estimated for typical operating conditions of a thermal barrier coating in a gas turbine, were estimated and compared with those expected via ionic diffusion, obtained by using published data. It was shown that gas permeation was normally expected to dominate. However, the rate of oxide growth at the bond coat/top coat interface of a thermal barrier coating was controlled by diffusion through the oxide layer and either mechanism of O transport through the top coat could generate fluxes sufficient to maintain this condition. This explained why oxide growth rates were similar whether or not the top coat was present. The permeability data, and the dependence of permeability on spraying conditions, may be of interest for other applications, such as permeable fuel cell membranes.

Oxygen Transport by Gas Permeation through the Zirconia Layer in Plasma Sprayed Thermal Barrier Coatings. A.C.Fox, T.W.Clyne: Surface and Coatings Technology, 2004, 184[2-3], 311-21