The Measurement of Residual Stresses in Thermal Barrier Coated Model Aerofoils

In this paper, we explored the relationship between thermal aging and the residual stresses developed within a thermal barrier coating (TBC) and the underlying thermally grown oxide (TGO). Superalloy model specimens (CMSX4) with a curved geometry designed to simulate key features of turbine blades coated with air plasma sprayed (APS) 7 wt.% Y₂O₃-stabilised ZrO₂ applied to an Amdry 995 bond coat were oxidised in the temperature range 900°C to 975°C for various lengths of time. Stress measurements on both the TBC and the underlying TGO were made at positions of known curvature using Raman and photo-stimulated piezo-luminescence spectroscopy (PLPS). The residual stress values reflect the interaction between TGO and TBC, which has the potential to be used as an indicator of service lifetime. The results are discussed with respect to the origin of the stress changes and the long term integrity of the coating.