Variability of stress in alumina corrosion layers formed in thermal-barrier coatings

Residual str ess distributions have been measured and mapped using photolum inescence, piezo spectroscopy in thermally grown alumina oxides (TGO) forme d an platinum-aluminide bond coats with thermal-barrier coatings (TBC) of t hickness 0, 3, and 200 mum. When there is a 3-mum TBC or no TBC, the residu al stress varies substantially with position and the mean stress is much lo wer than expected from simple thermoelastic, plane-stress analysis. This is partly explained by the stress being relaxed by surface roughness, but str ess mapping indicates that local fracture has occurred The stress in the TG O formed under 200 mum TBCs appear much more uniform and the mean value is approximately equal to the estimated thermoelastic plane stress. This could be due to the extra constraint imposed by the TBC. The luminescence line w idth is much greater for specimens with 200-mum TBC and is too large to be explained by varying macroscopic stresses due to interface roughness. The e ffects of oxidation time and reactive-element additions to the substrate at the level of 30 ppm, on the residual str ess, are relatively minor over th e ranges explored in these experiments.