Residual stresses and elastic modulus of thermal barrier coatings graded in porosity

Zirconia (ZrO2) stabilized with 8 wt.% Y2O3 is the most common material to be applied in thermal barrier coatings (TBC) owing to its excellent properties: low thermal conductivity; high toughness and thermal expansion coefficient similar to iron. Nevertheless, in order to increase the coatings lifetime, improvements in their thermomechanical behavior are still needed. With that purpose, we propose in this paper a graded ceramic coating. These TBC have been produced by depositing a conventional NiCoCrAlY bond coat on a Inconel 738 LC substrate followed by an atmospheric plasma sprayed top coat of ZrO2–8wt.%Y2O3 with a porosity gradient along the cross section. The aim of the present contribution is to study residual stresses and elastic properties of the coating as a function of the porosity gradient. For the characterization of the TBCs residual stresses, we have used Raman and X-ray diffraction (XRD) in different thermal conditions: as sprayed, after thermal shock at 1000 8C, and annealing at 1100 8C in air during 100 h. The top coatings show compressive stresses near the interface with the bond coat. A decrease of the stress level is observed along the cross section towards the surface. The residual stresses increase after annealing, however, have smaller variations after thermal shock. The elastic properties were evaluated by Brillouin scattering: the scattering of laser light by acoustic waves in the GHz frequency range. The spectra at different depths indicate that in the annealed condition the acoustic velocity increases when approaching the external surface.