Multilayer, Multimaterial Thermal Barrier Coating Systems: Design, Synthesis, and Performance Assessment

Abstract

Thermal barrier coatings (TBCs) are increasingly playing a vital role in enhancing efficiency and performance of gas turbine engines. As engine operating temperatures rise, yttria-stabilized zirconia (YSZ), the currently principal TBC material, reaches its operational limits. Gadolinium Zirconate (GDZ)-based pyrochlore oxides are now emerging contenders, not only due to their lower thermal conductivity, but also their ability to resist attack by silicate deposits. However, GDZ cannot be directly substituted for YSZ due to its incompatibility with the thermally grown alumina layer, therefore requiring to be a component of multilayer system. Although industry has already adopted these materials in various applications, a number of fundamental issues arise with respect to layered-coating design, their properties and processing dependence. In this study several multilayer architectures, based on the YSZ-GDZ system, have been developed and tested for durability under furnace thermal cycling conditions. Coating designs considered optimization of microstructure and properties of individual layers based on their location within the top-coat thickness to address competing interests of thermal conductivity, compliance, and resistance to silicates. A large variance in durability was observed in coatings made with different multilayer designs. The results and the associated failure mechanisms are rationalized through preliminary evaluation of elastic energies at the failure locations and corresponding energy release rates. The results point to new strategies in the design and manufacturing of optimal multilayer coatings.

Mechanical Properties of Double-Layer and Graded Composite Coatings of YSZ Obtained by Atmospheric Plasma Spraying

“…the layer obtained from the nano-structured feedstock lies on the bond coat. This finding confirms the need of depositing the most dense and tough layer on the bond coat when a multilayer composite coating is designed since the bond coat-top coat interface is considered as a high stress zone [10,12]. On contrary, the impairing of mechanical properties associated with the layers obtained from the nanostructured feedstock is, as expected, more gradual in the composite coatings, in particular for the G1 coating in which 100% nano-structured feedstock was deposited on the top of the coating, i.e far away from the bond coat.…”

“…Thus new materials, stable at higher temperatures and with lower thermal conductivity than YSZ, multilayer systems with different functions or graded structures changing the composition from interface with bond coat to the surface are now under intense research activity [8][9][10][11][12]. The present research study is focused on controlling the coating architecture by tailoring its microstructure, keeping zirconia as coating material.…”

Orientation‐dependent mechanical and thermal properties of plasma‐sprayed ceramics

“…Common variants, including coatings with degrees of porosity and dense vertically cracked (DVC) coatings have allowed for an expansion of microstructurally linked, performance driven applications . Emerging of late is the application of multiple coating layers, tailored specifically to address disparate threats, including oxidation induced failure, erosion, and ash/sand particle damage, many of which occur simultaneously, engendering a need for multi‐functionality in performance …”

Atmospheric Plasma Spraying of Single Phase Lanthanum Zirconate Thermal Barrier Coatings with Optimized Porosity

“…However, the thermal expansion coefficient of 9.1 × 10 −6 K −1 (30-1000 • C) [5] is low in relation to bond coats and Ni-base alloy substrates (~15 × 10 −6 K −1 ), and the toughness is poor [7]. For this reason, pyrochlores are applied in combination with YSZ in double-layer TBC systems [8][9][10][11]. YSZ is applied as the first ceramic layer, since TBC failure is often initiated by cracks occurring close to the bond coat.…”