Influence of aging on structure and thermal conductivity of Y-PSZ and Y-FSZ EB-PVD coatings

The surface temperature of the ceramic layer of thermal barrier coatings is likely to increase in future, due to the introduction of systems with lower thermal conductivity. This study aims at describing and understanding how the crystallographic structure and the microstructure of EB-PVD coatings can change after aging at high temperature, and how the thermal conductivity of the ceramic layer is affected. The structures of two EB-PVD yttria-zirconia coatings have been studied (one partially stabilized, the other fully stabilized) by XRD, SEM and TEM after heat treatments at temperatures ranging between 1100 and 1500 °C up to 100 h. Their thermal diffusivity has been determined up to 1000 °C by a laser flash method. The results show that: (1) no or little monoclinic phase is detected in the Y-PSZ coatings after aging; (2) phase transformations in Y-PSZ coatings lead to the formation of a band structure; and (3) shrinkage due to the constrained sintering of the coating results in transverse cracks. The thermal conductivity results are discussed in relation with these structural evolutions.