Residual Stresses in As-Sprayed and Heat Treated TBCs. Measurements and FEM Calculations

Abstract

The first part of this paper concerns measurement of through-thickness residual stresses in TBCs by the hole-drilling method. The influences of top coat thickness and different thermal histories (furnace and burner rig) were determined. Low tensile stresses prevailed in the as-sprayed state, and low compressive stresses after annealing in a furnace. In both cases the stresses tended to decrease with increasing coating thickness. The specimens tested in the burner rig showed a stress state between those of as-sprayed and annealed specimens. Moreover, the stress sign depended on top coat thickness. Finally, some experimental problems are discussed. The second part describes model prediction of residual stresses in a TBC after cooling from an initial stress-free state at high temperature. Compressive in-plane stresses were predicted, and these decreased with coating thickness. However, the calculated stresses were much higher than the measured stresses in an annealed specimen. This discrepancy between experiment and model is attributed to stress relaxation in the real TBC system. Also, calculated shear stresses along the top coat / bond coat interface at the free edge of the specimen explain the delamination often observed during furnace cycling.