Insights on the High-Temperature Operational Limits of ZrO₂-Y₂O₃ TBCs Manufactured via Air Plasma Spray

The effective high-temperature operation limit of a ZrO₂-7-8 wt.%Y₂O₃ (YSZ) thermal barrier coating (TBC) manufactured via air plasma spray (APS) is considered to be ~1300 °C. This is related to the metastable tetragonal t′-phase formed during the rapid quenching of the YSZ particles during spraying. The t′-phase transforms into the equilibrium tetragonal and cubic phases at temperatures ≥~1300 °C, which can lead to the formation of the monoclinic phase of YSZ upon cooling to room temperature. This formation of the monoclinic phase is accompanied by a volume expansion that leads to TBC failure due to extensive micro-cracking. To further investigate this limitation, an APS YSZ TBC was sprayed on a CMSX-4 substrate. By using a thermal (laser) gradient cyclic testing, a temperature gradient was generated across the TBC/substrate system. The YSZ T-front and substrate backside T-back temperature levels were ~1500 and ~1000 °C, respectively. In cycle conditions (5-min or 1-h hot and 2-min cool), no TBC failure has been observed. This behavior was partially attributed to the unexpected absence of the monoclinic phase of the YSZ in the cycled coatings. Although preliminary, these results are promising regarding increasing the effective high-temperature operational limits of APS YSZ TBCs.