Influence of electron beam physical vapor deposited thermal barrier coating microstructure on thermal barrier coating system performance under cyclic oxidation conditions

The lifetimes of electron beam physical vapor deposited (EB-PVD) thermal barrier coating (TBC) systems with three different microstructures of the Y2O3-stabilized ZrO2 (YSZ) ceramic top layer were investigated in 1 h thermal cycles at 1100 and 1150°C in flowing oxygen. Single crystal alloys CMSX-4 and René N5 that had been coated with an EB-PVD NiCoCrAlY bond coat were chosen as substrate materials. At 1150°C all samples failed after 80–100 1 h cycles, predominantly at the bond coat–alumina interface after cooling down from test temperature. The alumina scale remained adherent to the YSZ after spallation. Despite the different YSZ microstructures, no clear tendency regarding differences in spallation behavior were observed at 1150°C. At 1100°C the minimum lifetime was 750 1 h cycles for CMSX-4, whereas the first René N5 specimen failed after 1750 1 h cycles. The longest TBC lifetime on CMSX-4 substrates was 1250 1 h cycles, whereas the respective René N5 specimens have not yet failed after 2300 1 h cycles. The failure mode at 1100°C was identical to that at 1150°C, i.e. the TBC spalled off the surface exposing bare metal after cooling. Even though not all specimens have failed to date, the available results at 1100°C suggested that both the substrate alloy chemistry and the YSZ microstructure significantly affect the spallation resistance of the TBC.