Thermal Analysis of Calcium–Magnesium–Alumino–Silicate Infiltration Dynamics in Thermal Barrier Coatings

Abstract

Molten calcium–magnesium–alumino–silicate (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. This study introduces an integrated model, incorporating 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. The effects of different CMAS compositions, temperature gradients across the topcoat, and coating microstructures are investigated. Our 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. Fine porous microstructures slow infiltration, and 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. It is anticipated that the quantitative information and advanced understanding obtained would benefit the development of CMAS-resistant EB-PVD TBC topcoats.