Detection of Micro-Damage Evolution of Air Plasma-Sprayed Y₂O₃-ZrO₂ Thermal Barrier Coating through TGO Stress Measurement

Abstract

The micro damage evolution behavior in air plasma-sprayed 8 mass% Y₂O₃ stabilized ZrO₂ thermal barrier coating (APS-TBC) after isothermal heat exposure at 1150°C up to 200 h was observed. Residual stress of thermally-grown-oxide (TGO) layer was measured through the TBC layer using Cr³⁺ luminescence spectroscopy and the measured stress levels were compared with micro damage evolution behavior stored in the TBC system. With heat exposure time of longer than 10 h, microfracture behaviors were noted in the TBC layer. In-plane residual stress of the TGO layer increased for up to 50 h 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, \\barh_tgo, i.e., dσ_tgo⁄d\\barh_tgo is a useful indicator of the damage evolution in TGO: dσ_tgo⁄d\\barh_tgo>0: slight damage stage, dσ_tgo⁄d\\barh_tgo<0: micro fracture evolution stage. The result suggests that Cr³⁺ 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.