Localized Mechanical Response in a Thermal Barrier Coating System Determined Using High-Energy X-Rays

Thermal barrier coating (TBC) systems used in gas turbine applications are subjected to thermo-mechanical stresses that can ultimately lead to topcoat spallation following the development of a thermally grow oxide (TGO) at the bond coat/topcoat interface.  In order to better understand the mechanical response of a plasma-sprayed TBC, high-energy X-rays were used to probe the strain response of a YSZ/NiCoCrAlY TBC on Inconel 939 during mechanical testing.  As-sprayed and heat-treated samples were loaded at room temperature in four-point bending to produce tension and compression while X-ray strain measurements were made in situ.  This transmission technique allows for the determination of local strains through the thickness of the YSZ topcoat and NiCoCrAlY bond coat as well as strain in the TGO of heat-treated samples. 

Results for as-sprayed coatings illustrate the limited tensile load tolerance for YSZ compared with its compressive load tolerance.  As expected, the metallic bond coat is able to support a higher tensile load than YSZ.  Residual strains were found in heat-treated coatings resulting in a strain gradient in the YSZ with compressive strains at the YSZ/NiCoCrAlY interface and near-zero strain at the coating surface.  Subsequent loading of these samples demonstrated that higher applied tensile loads could be tolerated at the YSZ/NiCoCrAlY interface due to the in-plane residual compressive stresses.  Although extensive vertical cracking was observed in the mechanically-loaded samples, coatings heat treated at 1000°C and 1100°C remained well-adhered.  However, the thicker TGO of the coatings heat treated at 1200°C led to horizontal crack formation at the YSZ/NiCoCrAlY interface and ultimately, spallation of the coating at high tensile loads.  This set of experiments lays the groundwork for future exploration where mechanical testing will be conducted at elevated temperatures.