Design Of Multiphase Rare Earth Aluminate Zirconate Thermal Barrier Coating Materials For Enhanced Reactivity With Molten Silicates.

Abstract

The hot-section components in turbine engines rely on ceramic thermal and environmental barrier coatings (T/EBCs) for insulation and corrosion protection. The ability of these coatings to mitigate premature failure caused by calcium-magnesium-aluminosilicate (CMAS) based corrosive deposits is critical to ensure the desired component lifetimes. This work proposes advanced multiphase rare earth (RE) aluminate zirconates as candidate coating materials to promote a more predictable and consistent coating-CMAS reaction response against a range of deposit compositions. An integrated process using experiments and thermodynamic modelling tools was used to accelerate coating design. Understanding reactions between coating materials and CMAS deposits is important to design next generation coatings that can withstand CMAS attack to higher temperatures. This need was addressed through three experimental thrusts. The first focused on understanding the intrinsic stability of multi-cation, mixed oxide/sulfate deposits. The results showed that specific reactions between sulfates and CMAS oxides drive rapid decomposition of the sulfate, implying that the deposits inducing coating degradation would be primarily oxides. The second thrust improved the understanding of the temperature- and composition-dependent extent of the rare earth aluminosilicate garnet phase field, and the influence of RE ion identity on the equilibrium transitions between silicate apatite and garnet phases in Gd/Y/Yb+CMAS systems. Guided by computational design tools developed using results from the early experiments, the third thrust evaluated the performance of RE- rich multiphase aluminate zirconate novel candidate coating materials. The inclusion of alumina in traditional Y or Gd zirconate coating compositions was hypothesized to stabilize garnet as a CMAS reaction product, promoting more consistent reaction sequences across a variety of deposit compositions, and improving the reliability of coating materials in different service environments. The extent of reactive melt spreading and coating-CMAS reaction depths in conjunction with microchemical analyses of reaction products were used to evaluate the response of sintered pellets of candidate coating materials against model deposit compositions. As hypothesized, the addition of alumina to the coating material resulted in garnet formation along with a range of crystalline products which maximized the reactive consumption of molten deposits. This work has established an efficient protocol towards utilizing targeted experiments integrated with thermodynamic computations to accelerate materials discovery.