High temperature oxidation of EB-PVD TBCs on Pt-diffused single crystal Ni superalloy
The paper presents the results of high temperature oxidation of EB-PVD (Electron Beam Physical Vapor Deposition) TBCs (Thermal Barrier Coatings) deposited on a Pt-diffused second generation single crystal Ni-based superalloy. The Pt-diffused bond coating was obtained using a PVD (Physical Vapor Deposition) method applying a Closed Hollow Cathode (CHC) for the deposition of 5 µm of Pt layer and subsequent annealing at 1140 °C for 2 h in vacuum. The TBC system was thermally cycled in 1.25 h cycles at 1100 °C for 730 h (584 cycles) until delamination of 10% of Yttria Stabilized Zirconia (YSZ) top coating. A non-destructive 3D optical scanning method was applied for a macroscopic examination of the YSZ delamination which allowed predicting and revealing of the regions of YSZ buckling prior to its spallation. The microstructure of the TBC system in the as-deposited and thermally cycled conditions was studied using FEG-SEM. The growth and evolution of the Thermally Grown Oxide (TGO) scale was performed using high resolution FEG-S/TEM and Focused Ion Beam (FIB) methods. The TGO in the as-deposited condition was found to consist of porous a-Al2O3 which grew to form distinctive dense and columnar grains with ionic diffusion of reactive elements (Hf and Y) through grain boundaries during high temperature exposure. Additionally the formation of nanometric NiAl2O4 islands as well as high volume oxides containing Ni was found to accelerate the spallation of the YSZ top coat.