Variability of Stress in Alumina Corrosion Layers Formed in Thermal-Barrier Coatings

Residual stress distributions have been measured and mapped usingphotoluminescence, piezo spectroscopy in thermally grown alumina oxides(TGO) formed on platinum-aluminide bond coats with thermal-barrier coatings(TBC) of thickness 0, 3, and 200 μm. When there is a 3-μm TBC or noTBC, the residual stress varies substantially with position and the meanstress is much lower than expected from simple thermoelastic, plane-stressanalysis. This is partly explained by the stress being relaxed by surfaceroughness, but stress mapping indicates that local fracture hasoccurred. The stress in the TGO formed under 200 μm TBCs appear muchmore uniform and the mean value is approximately equal to the estimatedthermoelastic plane stress. This could be due to the extra constraintimposed by the TBC. The luminescence line width is much greater forspecimens with 200-μm TBC and is too large to be explained by varyingmacroscopic stresses due to interface roughness. The effects of oxidationtime and reactive-element additions to the substrate at the level of30 ppm, on the residual stress, are relatively minor over the rangesexplored in these experiments.