Monitoring thermal barrier coating delamination progression by upconversion luminescence imaging

Upconversion luminescence imaging provided contrast for monitoring thermal barrier coating (TBC) delamination progression during interrupted furnace cycling to 1163 °C, with sensitivity even for early stages of subcritical delamination crack propagation. Image contrast for delamination monitoring is obtained because delamination cracks introduce an interface between a higher (TBC) and lower (air) index of refraction medium so that total internal reflection beyond a critical incident angle occurs for both the excitation and emission wavelengths. Therefore, considerably greater luminescence emission is observed from regions containing delamination cracks. Upconversion luminescence produces exceptional image contrast because background fluorescence is very low when the detected emission is at a substantially shorter wavelength than the excitation. The upconversion luminescence emission was excited within a 6 μm thick Er³⁺/Yb³⁺ co-doped yttria-stabilized zirconia YSZ (YSZ:Er,Yb) layer beneath a 130 μm thick undoped YSZ layer, sequentially deposited with no interruption by electron-beam physical vapor deposition (EB-PVD) onto a NiPtAl bond-coated superalloy substrate. Excitation at 980 nm produced upconversion luminescence emission from Er³⁺ at 562 nm after two sequential energy transfers from Yb³⁺ co-dopant ions. Upconversion luminescence imaging, supported by scanning electron microscope inspection of interfacial damage, revealed that delamination progression proceeded by the formation of isolated microdelaminations with large separations between the TBC and thermally grown oxide (TGO) along with small-separation microcracks that propagated in the regions between the microdelaminations. In addition, upconversion luminescence imaging of the extent of delamination produced by Rockwell indentation clearly showed that the TBC becomes increasingly susceptible to mechanically induced damage throughout its cyclic life.