Investigation of TGO stress in thermally cycled plasma-spray physical vapor deposition and electron-beam physical vapor deposition thermal barrier coatings via photoluminescence spectroscopy

Abstract

Gas turbine engines for aircraft propulsion require thermal barrier coatings (TBCs) to protect metallic components in the hot section from the extreme temperatures of operation, which may exceed 1200 ∘C. One standard method of coating deposition is electron-beam physical vapor deposition (EB-PVD), which produces a high quality and strain tolerant coating with a smooth surface, which does not impair aerodynamical flow. Plasma-spray physical vapor deposition (PS-PVD) is a promising technique that offers several advantages over EB-PVD, including lower cost, shorter coating time, customization of microstructure by varying processing parameters, and the possibility of non-line-of-sight coating. However, the properties and microstructure of PS-PVD coatings and their effect on performance are not fully understood. This work investigates the residual stress in the thermally grown oxide (TGO) layer of samples made by EB-PVD and PS-PVD, thermally cycled at multiple times (uncycled, 300 cycles, and 600 cycles), to evaluate the effects of these two deposition methods on coating lifetime. Area stress maps of the samples were made with photoluminescence piezospectroscopy (PLPS). The PS-PVD samples exhibited stress relaxation with cycling time, suggesting stress relief from damage, and the PS-PVD coatings exhibited greater stress relaxation between 300 and 600 cycles; however, no samples exhibited spallation. SEM imaging revealed different damage modes for the two coating types. These results reveal PS-PVD's potential as a candidate deposition method for coatings in aero-engine applications.