The Effect of Bond Coat Creep on Residual Stresses and Debonding in Plasma Sprayed Thermal Barrier Systems

Abstract

Abstract Test pieces were machined from vacuum plasma sprayed (VPS) deposits of the bond coat materials Ni-22Cr-10Al-l Y and Co-32Ni-21Cr-8Al-0.5Y. Changes in gauge length at high temperatures under various applied stresses were measured using a scanning laser extensometer. In this way, isothermal creep data and, using no applied load, expansivity data were obtained. The CoNiCrAlY creeps faster than the NiCrAlY at low stresses, but the reverse is true at high stresses. The CoNiCrAlY has an appreciably higher expansivity than the NiCrAlY. These data were used in a numerical process model to evaluate the effect of bond coat creep on the stress state of a TBC system. Comparisons between measured and predicted curvature histories during deposition were used to evaluate the quenching stress for the two materials. This is considerably higher for the CoNiCrAlY. Although creep generally results in reduced stress levels at service temperatures, it can generate residual stresses which are raised after cooling down to ambient temperature, particularly for the CoNiCrAlY. Evaluation of the strain energy release rates associated with various stress distributions, and comparison with measured interfacial fracture energy values, confirmed that debonding will tend to occur at the top coat / bond coat interface, rather than between the bond coat and the substrate. However, bond coat spallation is more likely with CoNiCrAlY than with NiCrAlY.