The effects of temperature and substrate curvature on TBC lifetime and residual stress in alumina scales beneath APS YSZ

In order to assess the role of temperature on the lifetime of thermal barrier coatings (TBCs) and the development of residual stresses in Al₂O₃ scales grown under yttria-stabilized zirconia (YSZ) top coatings, two vacuum plasma sprayed (VPS) NiCoCrAlYHf bond coating (BC) compositions, with and without Si, were deposited on Hf-rich directionally-solidified (DS) 247 substrates and then coated with air plasma sprayed (APS) YSZ. Samples were thermally-cycled at temperatures ranging from 1075 to 1150 °C with 1-h cycles in air with 10% H₂O. Photo-stimulated luminescence spectroscopy (PSLS) was used to map residual stresses in the Al₂O₃ scale at the YSZ/BC interface from the same region at regular cycling intervals. All samples exhibited similar stress distributions after 100 1-h cycles with the exception of the YHfSi BC cycled at 1150 °C which had a lower average compressive stress with more delaminations. This specimen also had the shortest TBC lifetime which shows that the PSLS measurements correlate well to interfacial damage accumulation in TBCs. The presence of Si had no effect on lifetime or residual stress but was found to reduce the amount of internal oxidation. The effect of sample curvature on the interfacial stress and TBC lifetime was also assessed by comparing TBC-coated rod specimens to flat buttons. Rod specimens failed much earlier than flat specimens and had lower Al₂O₃ residual stress.