As-deposited mixed zone in thermally grown oxide beneath a thermal barrier coating

Gas turbine designers are increasingly using electron-beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBC) to meet the challenge of higher efficiency gas turbine engine requirements. A key feature for expanding the use of TBCs is increased spallation life and reduced spallation life variability. Such a coating system comprises a substrate (Ni-based single crystal alloy), a bond coat (diffusion aluminide or MCrAlY), a ceramic (7 wt.% yttria stabilized zirconia), and a thin thermally grown oxide (TGO) between the bond coat and the ceramic. The TGO is intended to be α-alumina, but evidence reported by other researchers suggests that in some cases the as-deposited TGO may not be entirely α-alumina. The thin nature of the TGO in as-deposited TBCs (<0.5 μm) makes analysis of the phases present and morphology difficult. Advancements in transmission electron microscopy (TEM) sample preparation and photo-stimulated luminescence spectroscopy (PSLS) have allowed higher quality and easier characterization of the TGO. In this study, EB-PVD TBCs were applied to platinum–aluminide bond coats on a Ni-based superalloy. Three types of coatings were produced by changing one PVD process variable. The as-processed TGO layer was characterized utilizing scanning transmission electron microscopy (STEM) and PSLS for each of the three coating process variables used. Results of this work found that the TGO comprised two sublayers; (1) a continuous layer of γ-Al2O3 between the mixed oxide zone and the bond coat; and (2) a mixed oxide zone between the continuous γ-Al2O3 and the TBC layer. An explanation for the creation of the mixed oxide zone found in these TGO morphologies is presented.