F. Blanchard, B. Baloukas, M. Azzi, M. Kadi, J.E. Klemberg-Sapieha, L. Martinu, Polytechnique Montréal, Montreal, Canada
As aircraft engine operating temperatures increase, so must the thermal insulation capabilities of the thermal barrier coatings (TBCs) used to shield metallic components in the combustion chamber and high-temperature turbine areas. Heat transfer from the hot gases to the engine components occurs through two main mechanisms: conduction and radiation. Considerable efforts have been deployed over the years to ensure TBCs have low thermal conductivity, thanks to a porous microstructure generally achieved by thermal spray or EB-PVD techniques. The radiative component of heat transfer, however, has been comparatively largely ignored in TBC design. This aspect is however very important due to an exponential increase in radiative heat transfer for higher gas temperature. While TBCs are naturally reflective of radiative heat due to light scattering, this property is vulnerable to degradation.
Degradation of TBCs over their lifetime is related to microstructural change mainly due to high temperature exposure and CMAS (Calcium-Magnesium-Alumino-Silicate) attack. Basic understanding of the underlying mechanism is an important aspect of the design and development of high performance TBCs. In this work, the effects of high temperature cycling and CMAS infiltration on the optical performance of Yttria-stabilized zirconia (YSZ) coatings prepared by atmospheric plasma spray (APS) were systematically investigated. Absorption and scattering coefficients have been extracted from spectrophotometry integrating sphere measurements in a novel way via the inverse adding-doubling (IAD) method. The microstructure was analysed using scanning electron microscopy (SEM) and mercury infiltration porosimetry (MIP) in an attempt to establish a relationship between the evolving microstructure and the optical properties. Both were found to have a significant impact on performance, with CMAS infiltration having the biggest effect. 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. In addition, a finite-difference time-domain (FDTD) model was developed to predict the optical performance of TBCs before and after degradation with good agreement with experimental data. This model can now be applied to investigate ways to mitigate the degradation processes.