Residual Stress Evolution and Failure Modes in a Thermal Barrier Coating Deposited on a Curved Substrate

A set of thermal barrier coated (TBC) specimens was made from CMSX-4 superalloy containing curved surfaces that are representative of typical turbine blades with a bond coat (BC) of HVOF ‘Amdry 995’ and a top coat (TC) that was air plasma sprayed 7 wt% Y₂O₃ stabilised ZrO₂. The specimens were thermally oxidised at 925 °C for times between 100 and 10,000 h. The residual stresses, both in the TC and the thermally grown oxide (TGO) that formed between the TC and BC upon thermal exposure, were measured on the cross-sections using Raman spectroscopy and photo-stimulated luminescence spectroscopy, respectively. A constraint upon the residual stress in the TC in the vicinity of the interface was found to be due to the growth of TGO. The corresponding microstructural evolution of the TBC and TGO was investigated using scanning electron microscopy to assist understanding of the residual stress distribution within the TC thickness and the undulating TGO. The effect of substrate curvature on the strain levels in the TC was evaluated and considered with respect to the macro-scale failure modes of the coating system.