Multilayer, multimaterial thermal barrier coating systems: Design, synthesis, and performance assessment

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.