Bi-layer YSZ TBCs: the influence of microstructure and powder chemistry on the performances of a gas turbine coating.

The adoption of Thermal Barrier Coating (TBC) systems with high-yttrium zirconia is constantly increasing in the Oil&Gas and aviation fields due to demanding turbine temperatures. High-yttrium compositions represent an economical and easy choice to obtain TBCs with better phase stability than “standard” 7-8YSZ at high temperatures and chemical resistance to molten silicate deposits (“CMAS”).

The aim of this work was to study the performances of ceramic bi-layer coating systems deposited by Atmospheric Plasma Spraying (APS) using an innovative cascade plasma torch. The bilayer systems comprised an inner layer of standard 7-8YSZ and a top layer of ZrO₂-20wt.% Y₂O_3.

The bottom and top layers were obtained using agglomerated powders of the respective compositions, producing systems with porous and Dense-Vertically Cracked (DVC) microstructures for both layers. Hastelloy-X substrates with a HVOF-APS “Flash” NiCoCrAlY bond coat were used for all the coating tested.

The thermomechanical behavior and bond strength of the samples were analyzed through Thermal Cycling Fatigue (TCF) and tensile adhesion (ASTM C633) tests.

Chemical attack by molten CMAS (CaO-MgO-Al₂O₃-SiO₂) deposit at high temperature was also performed. FEG-SEM, XRD and Raman Spectroscopy techniques were used to characterize the coating systems before and after TCF and CMAS-corrosion tests, elucidating their degradation mechanisms.