Microstructure, Texture, and Thermal Conductivity of Single‐Layer and Multilayer Thermal Barrier Coatings of Y2O3‐Stabilized ZrO2 and Al2O3 Made by Physical Vapor Deposition

Abstract

Thermal conductivity is one of the most important properties in the design of thermal barrier coatings (TBCs) for high‐temperature gas‐turbine blades. In the present study, the thermal conductivities of several TBCs were studied. Single‐layer and multilayer coatings consisting of Al2O3 and ZrO2 partially stabilized with 8 wt% Y2O3 (8YSZ) were deposited on nickel‐base superalloy substrates by electron‐beam physical vapor deposition (EB‐PVD). A columnar microstructure with (200) texture and [200] direction oriented nearly perpendicularly to the plane of the coating was observed in the single‐layer 8YSZ coating. In multilayer coatings with alternating layers of Al2O3 and 8YSZ, the tendency to develop columnar structure, as well as the intensity of (200) texture, decreased as the number of layers increased. In both the single‐layer and multilayer coatings, the 8YSZ layer consisted almost entirely of tetragonal (t) phase with a negligible amount of monoclinic (m) phase. This was in contrast to the microstructure of bulk 8YSZ (source material used for deposition), which contained a significant amount of m phase, in addition to t phase. Whereas bulk Al2O3 consisted of α‐phase, only γ‐phase was found in the Al2O3 layers of coatings. The thermal conductivities of these coatings were measured by the laser flash method. The thermal conductivity data of single‐layer 8YSZ coating compared well with the data on bulk ZrO2 stabilized by 5.3 wt% Y2O3 reported in the literature. This was due to the similarity of microstructures between the two. The thermal conductivity data of multilayer coatings were well described by a series heat‐transfer model calculation within the limits of experimental errors involved in the measurements.