Influence of Interface Roughness, Substrate and Oxide-Creep on Damage Evolution and Lifetime of Plasma Sprayed Zirconia-based Thermal Barrier Coatings

AbstractThe stress state in thermal barrier coating (TBC) systems for gas turbine blades during thermal cycling is mainly governed by (i) the thermal and mechanical mismatch between the ZrO2-base topcoat and the Ni-base substrate alloy, (ii) the roughness profile at the interface between MCrAlY-bondcoat (BC) and TBC, and (iii) plastic deformation and creep in the BC and in the oxide scale growing at the interface between TBC and BC. In the present study, the influence of (ii) and (iii) on the thermal cycling lifetime was studied using a simplified model system consisting of (a) FeCrAlY substrates with different creep strength, i.e. a conventional and an oxide dispersion strengthened alloy, (b) oxide layers with different grain size, i.e. a coarse grained version applied by oxidation and a nanocrystalline version applied by sputtering and (c) a plasma sprayed ZrO2-TBC stabilized by 8% Y2O3. A trend of decreasing lifetime with increasing creep strength of the FeCrAlY was observed. Infrared pulse thermography analysis of delamination crack growth during thermal cycling tests showed that in case of low creep strength substrates, small delaminations grow continuously, link to each other and finally lead to spallation of the TBC, whereas in the case of high creep strength substrates fast crack growth and spallation occurs as soon as one delamination exceeds a diameter of approx. 2mm. A fracture mechanics model for delamination crack growth is able to roughly describe this behavior. The influence of oxide creep seems to be less pronounced than that of substrate creep. However, the highest lifetimes were observed for the combination of Fecralloy substrate and nanocrystalline oxide layer. The influence of roughness on lifetime remains unclear due to a wide lifetime scatter