Oxygen transport by gas permeation through the zirconia layer in plasma sprayed thermal barrier coatings

The specific permeability of oxygen in plasma sprayed zirconia has been measured to be approximately 10−16 m2, with some dependence on microstructure, and hence on spraying conditions. Permeability rises with increasing temperature, as expected, and differences of a factor of approximately 4 were observed between room temperature and 600 °C. Maximum oxygen fluxes through the top coat via gas permeation, estimated for typical operating conditions of a thermal barrier coating (TBC) in a gas turbine, have been estimated and compared with those expected via ionic diffusion, obtained using published data. It is shown that gas permeation is normally expected to dominate. However, the rate of oxide growth at the bond coat/top coat interface of a TBC is controlled by diffusion through the oxide layer and either mechanism of oxygen transport through the top coat could generate fluxes sufficient to maintain this condition. This explains why oxide growth rates are similar whether or not the top coat is 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.