Detection of micro-damage evolution of air plasma-sprayed Y_2O_3-Zro_2 thermal barrier coating through TGO stress measurement

The micro damage evolution behavior in air plasma-sprayed 8mass% $Y_{2}0_{3}$ stabilized Zr$O_{2}$ thermal barrier coating (APS-TBC) after isothermal heat exposure at 1150℃ up to 200h was observed. Residual stress of thermally-grown-oxide (TGO) layer was measured though the TBC layer using Cr^3+ luminescence spectroscopy and the measured stress levels were compared with microdamage evolution behavior stored in the TBC system. With heat exposure time of longer than 1Oh, microfracture behaviors were noted in the TBC layer. In-plane residual stress of the TGO layer increased for up to 50h of heat exposure time, and thereafter, diminished with further increase. These behaviors are strongly correlated with the thickness and the residual stress of the TGO layer. This decrease in the TGO stress well contrasts with the evolution of the micro fracture behavior, which is also related to the thickness and undulation of the formed TGO layer. The change of TGO stress $σ_{tgo}$, depending on the average thickness of the_TGO layer, $overline{h}_{tgo}$, {\iti.e.}, d$σ_{tgo}/d$overline{h}_{tgo}$ is a useful indicator of the damage evolution in TGO： d$σ_{tgo}/d$overline{h}_{tgo}$ > 0: slight damage stage, d$σ_{tgo}$/d$overline{h}_{tgo}$ < 0: micro fracture evolution stage. The result suggests that Cr^3+ luminescence spectroscopy is a good indicator for micro damage evolution in the TBC layer and is a useful tool for nondestructive evaluation (NDE) of APS-TBC systems