A Probabilistic Life Model for EB-PVD Thermal Barrier Coatings on Diffusion Bondcoats

Published lifetime data for the cyclic oxidation life of an EB-PVD zirconia-8wt% yttria thermal barrier coating on a platinum aluminide bondcoat over the temperature range 1000-1200°C has been reviewed and compared with experimental data determined in this study at 1150°C.    It is shown that the cyclic lifetime of an EB-PVD TBC may be modelled in terms of the accumulated time at temperature; although at any given test temperature measured lifetimes may vary by x5 – x20 depending on the system studied and the substrate alloy composition.

In developing the life model, it is recognised that thermal cycles are necessary to provide sufficient strain energy to initiate failure, but that measured lifetimes (>20% ceramic loss) are stochastic, dependent on a distribution of initial defects introduced at manufacture and their likelihood of growth.  This uncertainty accounts for the observed range in measured lifetimes.

Such variability in lifetime has been modelled statistically using Weibull statistics, with the Weibull based life model sensitive to the substrate composition, bondcoat manufacturing route, bondcoat surface finish and ceramic deposition conditions.  The effect of each of these process variables on TBC performance will be examined.