Influence of thermal cycle on surface evolution and oxide formation in a superalloy system with a NiCoCrAlY bond coat

A material system comprising a NiCoCrAlY bond coat deposited on a superalloy substrate has been subjected to thermal cycling. The assessment contrasts the influence of simple and stepwise (intermediate temperature hold) thermal cycles on the undulation of the surface and on the evolution of residual compressive stress in the thermally-grown oxide (TGO) layer. Stress-mapping of the TGO was performed using luminescence spectroscopy. Regions of interest were cross-sectioned using focused ion beam techniques to enable sub-surface examination by scanning electron microscopy. The investigation revealed that the surface develops undulations upon stepwise cycling, but not for either simple cycling or isothermal exposure (at comparable TGO thickness). This behavior has been related to the rapid creep displacements occurring in the bond coat during the intermediate temperature hold, because it is subject to large stress at this temperature. When the undulations attain sufficient amplitude, creep cracks form along the ridges, causing the stress to locally relax. For situations that do not cause undulations, areas of reduced residual compression appear in the TGO. Yttria-rich particles were invariably present in these regions.