CMAS/Volcanic Ash Infiltration Performance of Yttria Rich-Zirconia Thermal Barrier Coatings Produced by Electron Beam Physical Vapor Deposition

For many years the use of thermal barrier coatings (TBC) in gas turbine engines has allowed a significant increase in engine operating temperatures which are above the Ni-based super alloys’ melting point. The base material for TBCs is the state of the art 7 wt % yttria stabilized zirconia (YSZ) which provides the thermal protection. Current industry demands for increased engine efficiency has pushed TBCs to the limits where significant technological constrains have been identified. One of the main ones represents the high temperature corrosion of the TBC material due to the ingestion of silicate based airborne particles commonly referred as CMAS into the engine. Their main constituents represent CaO-MgO-Al2O3 and SiO2 (or CMAS to simplify). The most common CMAS sources represent sand, dust, environmental pollution, runway debris, fly ash and volcanic ash. As they are ingested into the engine, they melt and infiltrate the porous TBC which produces significant corrosion damage to the TBC material and loss of the TBC strain tolerance. These combined CMAS attack mechanisms lead to reduced coating lifetime or even TBC premature failure. Several approaches have been proposed to counteract this corrosion damage. Among one of the most successful approach is based on the use of a sacrificial top coating based on a rare earth (RE) material oxide which can react with the CMAS melt inducing its crystallization into stable products. Therefore, the CMAS glass can be arrested into only a few microns of infiltration. This thesis explores the CMAS and volcanic ash (VA) infiltration performance of a RE based coating made of 65 wt % Y2O3-ZrO2 balanced (65YZ). Electron beam physical vapor deposition (EB-PVD) methods were used to produce the 65YZ coatings. The studies are based on the effects of chemical composition of the RE material and the microstructural influence of the coating for infiltration. The proposed coatings showed promising results as a CMAS/VA resistant material for high temperature regimes (1250 and 1300 °C) by forming stable apatite and garnet crystalline phases. A detailed study on yttria-zirconia ratios is presented for an optimal CMAS resistant coating. The influence of RE oxide and melt chemistry in the phase equilibra and kinetics of reaction was studied in detail for this proposed coating. A corrosion mechanism study is presented by applying basicity index estimations in the melt which proves to be a good indicator to predict reaction phases and corrosion damage. In addition, comparative experiments with Gd2Zr2O7 (GZO) coatings are performed were the 65YZ coating exhibited higher CMAS/VA resistance. The microstructural influence of the 65YZ coatings with infiltration were also assessed showing that microstructures can be tuned specifically to further improve the infiltration resistance of the coating. Additionally, real engine conditions e.g. thermal gradient infiltration testing were studied for a full proposed CMAS/VA resistant coating system which included the Ni-based super alloy, bond coat, thermally grown oxide (TGO), YSZ TBC and CMAS resistant TBC. Finally, an overview of the mechanical properties of the proposed 65YZ coatings (i.e. erosion, thermal conductivity and toughness) is presente