High resolution erosion detection in thermal barrier coatings using photoluminescent layers

Accurate life prediction of thermal barrier coatings (TBCs) becomes increasingly important as they progress towards prime reliant status. Quantitative non-destructive evaluation techniques, which can enable remaining life assessments, are integral to achieving this goal. Although not the primary failure mechanism, in certain operating conditions, degradation to the TBC occurs by gradual erosion from the surface. The introduction of rare earth ions in discrete layers in the ceramic coating enables this form of degradation to be detected by analysis of the inherent phosphorescence. It has been shown that this technique can quantify thickness reductions with a resolution greater than the thickness of the doped layers and estimated to be ± 5 μm. A model has been developed to predict the relationship between coating thickness and emission intensity based on absorption and scattering coefficients derived from the literature. The model suggests that the relationship is linear and this has been validated using the experimental data. Sample TBCs, largely comprised of YSZ with europia and dysprosia doped layers, were eroded using a particle laden jet and the phosphorescent emission was imaged. Through bespoke image processing the data was reconstructed into a three-dimensional coating profile that correlates well with that expected for the applied erosion method. Further validation was achieved by comparing the surface profile with data taken using a confocal microscope. A protrusion approximately 0.3 mm wide from the erosion crater was identified by both techniques and is indicative of the fidelity of the technique. Secondly, the profile over the erosion crater derived from the phosphorescence image is within the error range, ± 5 μm, of the reconstructed confocal microscope data.