Investigation of CMAS infiltration into EB-PVD Thermal Barrier Coatings

Abstract

Molten CMAS infiltration into thermal barrier coatings (TBCs) of gas turbines causes loss of strain tolerance and delamination of the ceramic topcoat. To develop efficient mitigation strategies, it is crucial to understand CMAS infiltration dynamics into the porous topcoat. An integrated model is introduced incorporating simultaneous droplet spreading, liquid flow in unsaturated porous structures, heat transfer, and temperature-dependent viscosities, to study CMAS infiltration through TBCs grown by the electron beam physical vapor deposition (EB-PVD) method. Additionally, a micro-CT imaging methodology is presented that can visualize the final infiltration profile of a droplet in a porous medium non-destructively. EB-PVD TBC topcoats are characterized by highly anisotropic columnar structures. The effects of different CMAS compositions, temperature gradients across the topcoat, coating microstructures are investigated. Simulation shows that CMAS infiltration exhibits significantly nonlinear dynamics with a fast infiltration rate at the early stage due to high temperature, high pressure gradients, and low viscosity. Neglecting heat transfer enhancement from CMAS by approximating the temperature distribution as linear underestimates the infiltration rate. The anisotropy determines the final infiltration profile. Fine porous microstructures slow infiltration. Bilayer or multilayer structures, consisting of variable column and pore sizes, combine the advantages of an increased hydraulic resistance to infiltration and lower capillary pressures. Such heterogeneous structures can delay early-stage infiltration by manipulating the layer thickness and arrangement. The wetting interactions between the CMAS and topcoat material have a weak effect on infiltration dynamics but are important to the droplet spreading behavior and, as a result, the infiltrated region. A CMAS droplet infiltrating a partially wetted TBC topcoats experiences slower infiltration and greater spreading. We anticipate that the quantitative information and advanced understanding obtained would benefit the development of CMAS-resistant EB-PVD TBC topcoats.