Development of bondcoats for high lifetime suspension plasma sprayed thermal barrier coatings
June 24-29, 2018
A Thermal Barrier Coating (TBC) system is designed to protect gas turbines from high temperatures and harsh environments. Development of TBCs allowing higher combustion temperatures is of high interest since it results in higher fuel efficiency and lower emissions. It is well known that nano-structured TBCs produced by Suspension Plasma Spraying (SPS) have significantly lower thermal conductivity as compared to conventional systems due to their very fine porous microstructure. Improvement of TBCs manufactured by SPS is of high commercial interest as SPS has been shown capable to produce columnar microstructures similar to the conventionally used Electron Beam – Physical Vapor Deposition (EB-PVD) process. Moreover, SPS is a significantly cheaper process than EB-PVD and can produce coatings with much higher deposition rates than EB-PVD. However, lifetime of SPS coatings needs to be improved further for them to be applicable in commercial applications.
Lifetime of a TBC system is significantly dependent on topcoat-bondcoat interface and bondcoat treatments before spraying topcoat as they influence the growth rate of Thermally Grown Oxide (TGO) layer as well as the mismatch stresses generated in the topcoat during thermal cyclic testing. Grit blasting, shot peening, and controlled atmosphere heat treatment are some of the common methods used to modify the bondcoat surface topography and oxide growth characteristics. The bondcoat microstructure also affects the TBC lifetime significantly. The bondcoat layer in EB-PVD TBCs is typically deposited by Vacuum Plasma Spraying (VPS) in case of NiCoCrAlY bondcoats and Chemical Vapor Deposition (CVD) in case of PtAl bondcoats, while in case of Atmospheric Plasma Sprayed (APS) TBCs, the NiCoCrAlY bondcoats are typically deposited by High Velocity Oxy-Fuel (HVOF) spraying. It is not fully understood yet which bondcoat deposition process and interface topography would be most suitable for SPS TBCs in order to achieve high lifetime.
The objective of this work was to study the effect of different bondcoat treatments and spray processes on bondcoat microstructure, TGO growth rate and lifetime in SPS TBC systems. Commercial NiCoCrAlY bondcoat powder was deposited by high velocity air fuel spraying while axial-SPS was used for yttria stabilized zirconia topcoat deposition using the same spray parameters for all samples. Before spraying the topcoat, the bondcoats were subjected to either grit blasting, shot peening, vacuum heat treatment, or a combination of these three treatments. The lifetime of resultant samples was examined by thermal cyclic fatigue and thermal shock testing. The failure mechanism in each case was investigated. The TGO growth rate was studied by taking out the samples at regular intervals during thermal cyclic fatigue testing before failure. The effect of bondcoat deposition process and interface topography on TGO growth and failure mechanisms in each case will be discussed.
Mohit Gupta, Nicolaie Markocsan, Xin-Hai Li, and Björn Kjellman, "Development of bondcoats for high lifetime suspension plasma sprayed thermal barrier coatings" 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). http://dc.engconfintl.org/tbcv/86