Investigation of TGO stress in thermally cycled PS-PVD and EB-PVD 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. These coatings are typically 6-8% yttria-stabilized zirconia (YSZ), and the standard methods of coating deposition are air plasma spray (APS) and electron-beam physical vapor deposition (EB-PVD). EB-PVD produces a higher-quality coating than APS due to its resulting columnar microstructure that increases the strain tolerance of the coating. Plasma-spray physical vapor deposition (PS-PVD) is a promising technique that offers several advantages over EB-PVD, including lower cost, shorter coating time, customizability of microstructure by varying processing parameters, and possibility of non-line-of-sight coating. However, the effect of this process on the resulting microstructure, properties, and performance is not fully understood. In this work, TBCs manufactured by both EB-PVD and PS-PVD were investigated via photoluminescence spectroscopy after varying amounts of thermal cycling, to investigate the effect of deposition process on the stress state of the thermally grown oxide (TGO) layer. This work investigates the residual stress in the TGO of samples made by EBPVD and PS-PVD, thermally cycled in the same way at multiple lifetimes (asdeposited, 300 cycles, and 600 cycles), to evaluate the effects of Deposition method. As-deposited PS-PVD samples were given a 1-hour heat treatment, during which they developed a TGO. This was found to be in compression on the order of 2-3 GPa, consistent with published data on EB-PVD TBCs. Other PS-PVD samples were cycled for 300 and 600 hours, and these exhibited stress relaxation compared to the 1-h heat treated, suggesting some stress relief from damage. The findings from these investigations will be presented in the paper. In this study, we compare the TGO residual stress evolution in PS-PVD and EB-PVD TBCs to shed light on their respective responses to thermal cycling. These results contribute to understanding how the PS-PVD process compares to the EB-PVD process with respect to coating life and durability.