Advanced Spectroscopic Residual Stress Mapping in Thermal Barrier Coatings

Thermal Barrier Coatings (TBCs) are critical for gas turbine engine longevity and efficiency, protecting components from extreme temperatures. Monitoring their degradation, particularly residual stress evolution, is crucial for reliability. While Raman, and Photoluminescence (PL) spectroscopy are promising non-destructive techniques, their widespread industrial adoption is hindered by a lack of quantitative performance evaluation. This study addresses this gap, focusing on Raman spectroscopy for yttria-stabilized zirconia (YSZ) topcoats and Cr³⁺ PL spectroscopy for the thermally grown oxide (TGO) layer in air plasma sprayed (APS) YSZ TBCs. For YSZ, stress-free reference peak positions were established by correlating Raman peak shifts of powdered YSZ against thermal exposure, using the Hollomon-Jaffe parameter. The E_g Raman mode at 640 cm⁻¹ proved most sensitive to stress. Excellent agreement was found between Raman and XRD measurements incorporating thermal exposure-corrected parameters. Spatial and depth-resolved Raman mapping revealed stress variations and gradients. The technique demonstrated robustness, with an estimated stress uncertainty of ±74 MPa for rapid acquisition. For TGO, reliable Cr³⁺ PL stress data was obtained through YSZ topcoats exceeding 200 µm, requiring a confocal depth of at least 150 µm. TGO stress showed strong dependence on topcoat thickness, attributed to substrate-induced thermal force sharing. PL measurements demonstrated excellent repeatability with an uncertainty of ±48 MPa. Integrating both techniques enables simultaneous, co-located through-thickness stress mapping, offering valuable insights into residual stress distributions in real-world TBC systems, thereby advancing their reliability and applicability.