Failure Mechanism for Thermal Fatigueof Thermal Barrier Coating Systems

Thick thermal barrier coatings (TBCs), consisting of a CoNiCrAlY bond coat and yttria-partiallystabilized zirconia top coat with different porosity values, were produced by air plasma spray (APS). Thethermal fatigue resistance limit of the TBCs was tested by furnace cycling tests (FCT) according to thespecifications of an original equipment manufacturer (OEM). The morphology, residual stresses, andmicromechanical properties (microhardness, indentation fracture toughness) of the TBC systems beforeand after FCT were analyzed. The thermal fatigue resistance increases with the amount of porosity in thetop coat. The compressive in-plane stresses increase in the TBC systems after thermal cycling; neverthelessthe increasing rate has a trend contrary to the porosity level of top coat. The data suggest that thespallation happens at the TGO/top coat interface. The failure mechanism of thick TBCs was found to besimilar to that of conventional thin TBC systems made by APS.