Capturing the Competing Influence of Thermal and Mechanical Loads on the Strain of Turbine Blade Coatings via High Energy X-rays

This paper presents findings of synchrotron diffraction measurements on tubular specimens with a thermal barrier coating (TBC) system applied by electron beam physical vapor deposition (EB-PVD), having a thermally grown oxide (TGO) layer due to aging in hot air. The diffraction measurements were in situ while applying a thermal cycle with high temperature holds at 1000 degrees C and varying internal air cooling mass flow and mechanical load. It was observed that, during high temperature holds at 1000 degrees C, the TGO strain approached zero if no mechanical load or internal cooling was applied. When applying a mechanical load, the TGO in-plane strain (e22) changed to tensile and the out of plane TGO strain (e11) became compressive. The addition of internal cooling induced a thermal gradient, yielding a competing effect, driving the e22 strain to compressive and e11 strain to tensile. Quantifying TGO strain variations in response to competing factors will provide a path to controlling the TGO strain, and further improving the lifetime assessment and durability design strategies for TBC systems.