Lifetime evaluation of various new EB-PVD and APS TBCs in thermal in thermal gradient and FCT
June 24-29, 2018
Thermal barrier coatings (TBCs) are applied to increase lifetime and efficiency of highly loaded turbine components. Degradation of those coatings by volcanic ash (VA) and calcium-magnesium-alumino-silicate (CMAS) deposits is now recognized as an increasingly important fundamental degradation mechanism. The presentation attempts to quantify the damage and reduction in lifetime resulting from infiltration of TBCs by deposits. Comparing TBCs deposited by either electron-beam physical vapor deposition (EB-PVD) or air plasma spraying, the 7YSZ gets infiltrated in both versions at a comparable rate. Depending on the applied experimental condition such as CMAS composition, temperature, and time, the infiltration depth varies in different TBC morphologies.
Upon thermal cycling the infiltration causes severe mechanical stresses within the TBC, subsequently leading to crack formation and TBC spallation. Most of the life time assessment tests so far were conducted under isothermal conditions which generally does not represent real engine atmospheres. In this presentation a combined approach utilizing thermal gradient test rig and furnace cyclic testing has been used to study the TBC life time behavior under the influence of CMAS and volcanic ash. EB-PVD 7YSZ coatings on Ni-based superalloys protected by a NiCoCrAlY bond coat were infiltrated with CMAS and subsequently tested under isothermal and thermal gradient conditions. The CMAS infiltration depth has a direct impact on the life time of the coatings. Full infiltration leads to a life time reduction up to 90% compared to a non-infiltrated coating while the life time of a half infiltrated coating is reduced by approximately one third. Spallation occurs at the interface between infiltrated and un-infiltrated regions. This failure location is associated with local stress peaks arising from the infiltration.
New topcoat compositions such as Gadolinium zirconate (GZO) that mitigate damage by deposits were investigated as well. The presentation provides results on several new TBCs, especially on their behavior under the influence of deposits and under thermo-cyclic loading. The interaction of the coatings with volcanic ash and with CMAS was investigated at temperatures between 1200 ºC and 1250 ºC using several compositions of the deposits. The pyrochlore TBCs rapidly form crystalline phases which provides a potential for damage mitigation. The new topcoats were investigated as single and double layers in comparison to standard 7YSZ coatings. In the double-layered TBC systems, a thin layer (~25 to 30µm) of 7YSZ was used between the new topcoat material and the bond coat. All new coatings were deposited by EB-PVD. Special emphasis was put on the engineering of the interface between both layers that was systematically varied. Selected TBCs were tested in FCT at 1100°C. Both single and double-layered GZO have shown longer lifetimes than the standard YSZ samples. Changes in microstructure, growth of the TGO layer, and diffusion of elements are discussed.
Finally, a more realistic test scenario is introduced by using a small-scale micro gas turbine that burns kerosene. A volcanic ash test stand has been built with a precise particle feeder, a concentration measuring unit and a full TBC system on the blisk of the turbine. This setup allows testing of melting, sticking, corrosion, and erosion behavior of volcanic ash in-situ at high temperature in a realistic manner in a gas turbine. Both 7YSZ and GZO TBC have been investigated showing distinct differences in erosion resistance.
Uwe Schulz, Ravisankar Naraparaju, Lukas Hochstein, and Berndhard Kanka, "Lifetime evaluation of various new EB-PVD and APS TBCs in thermal in thermal gradient and FCT" in "Thermal Barrier Coatings V", Prof. Dr. Robert Vaßen, Forschungszentrum Jülich GmbH, Germany Brian Hazel, Pratt & Whitney, USA Prof. Dr. Uwe Schulz, German Aerospace Center, Germany Dr. Michael J. Maloney, Pratt & Whitney, USA Dr. Ram Darolia, GE Aviation (Retired), USA Eds, ECI Symposium Series, (2018). https://dc.engconfintl.org/tbcv/52