Residual stress in air-plasma-sprayed thermal barrier coatings under long-term high-temperature oxidation

The accumulated residual stress in yttria-stabilized zirconia top coat (YSZ TC) and thermally grown oxide (TGO) is regarded as the key driving force for the delamination of air-plasma-sprayed thermal barrier coating system (APS-TBCs). In this study, the synchronous evolution of residual stress of both YSZ TC and TGO, as well as the TGO thickness, are characterized via the Raman spectroscopy (RS) and photoluminescence piezo-spectroscopy (PLPS) methods. For the APS-TBCs where one surface is coated with YSZ TC, while the other surface remains directly exposed to high temperature, the service life can be categorized into four distinct stages: the early stage with tensile stress state (0 - 40 h), the medium stage with reversal compressive stress state (40 - 100 h), the late stage with re-inverted tensile stress (100 - 140 h), and the final stage with compressive stress state (140 - 300 h). Such stress reversal phenomenon may be attributed to the competition between the growth of TGO and substrate oxide (SO). It is advisable to apply an oxidation-resistant coating on the inner wall of high-temperature blade cooling passages, to prevent the formation of SO and consequently avoid complex stress transitions during hightemperature service of the APS-TBC, ultimately preventing local stress concentration-induced cracking and delamination.