Effect of CMAS infiltration on the optical properties of thermal barrier coatings: Study of the mechanisms supported by FDTD simulations and ALD

Thermal barrier coatings (TBCs), which protect metallic components in aircraft engines thanks their low thermal conductivity, must also be effective blockers of radiative heat. While their porous microstructure makes them highly reflective to visible and infrared light through scattering, it also renders them susceptible to degradation, particularly due to calcium-magnesium-alumino-silicate (CMAS) infiltration. This study explores its effect on the optical scattering coefficient of TBC yttria-stabilized zirconia (YSZ) topcoats deposited by atmospheric plasma spray (APS) with two different microstructures. Different CMAS compositions are investigated by isothermal melting into the coatings, resulting in a significant decrease of their reduced scattering coefficients by around 50%. To further study the evolution of their performance as the pores are filled, atomic layer deposition (ALD) is used to mimic CMAS infiltration in a controllable fashion. The results show that most of the performance loss occurs with very little material inserted into the pores and that a saturation point is quickly reached. This is explained by two mechanisms: pores approximately 2 µm in diameter and less are responsible for most of the optical performance and are filled up rapidly, while the refractive index contrast at every pore's interface diminishes when material fills the voids. The obtained minimum scattering coefficient value is approximately half that of a pristine sample and matches with the values obtained by the CMAS melting approach. Finite-difference time-domain (FDTD) modeling is also shown to corroborate the observed saturation behavior and demonstrated to be a suitable tool for the design and optimization of future TBCs.