A delamination mechanism for thermal barrier coatings subject to calcium–magnesium–alumino-silicate (CMAS) infiltration

When a turbine airfoil attains temperatures that allow calcium–magnesium–alumino-silicate (CMAS) infiltration into the thermal barrier coating (TBC), a new mechanism of in-service spalling may be activated. The mechanism is associated with cold shock of the infiltrated layer during shut down. It has been identified by inspecting an airfoil removed from service. The identification has been based on observations of sub-surface delaminations within infiltrated regions of the TBC. Three important aspects of the mechanism are as follows. (a) The sub-surface delaminations always initiate at surface-connected vertical separations. (b) They are fully-infiltrated with CMAS. (c) They are strictly mode I. A thermal shock analysis has been invoked to identify a critical infiltration thickness, above which delaminations are possible. The analysis also defines a characteristic depth beneath the surface at which the delaminations are most likely. The observations made on the airfoils are consistent with these two dimensions. A second mechanism has been explored as the potential cause of large spalled regions also observed on the airfoils. But it has not been possible to verify the mechanism using the current observations.