Papers

ICMCTF2008 Session A3-2: Thermal Barrier Coatings

Wednesday, April 30, 2008 1:30 PM in Room Sunrise

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		A3-2-1 Cyclic Behavior of EB-PVD Thermal Barrier Coating Systems With Modified Bond Coat U. Schulz (German Aerospace Center-DLR,Institute of Materials Research, Germany) Thermal barrier coatings (TBCs) offer the potential to significantly improve efficiency of aero engines as well as stationary gas turbines for power generation. The properties of their constituents that are bond coat, thermally grown oxide, ceramic top and even the substrate are key elements for lifetime of the coatings. Further efficiency improvements require TBCs being an integral part of the component which, in turn, requires reliable and predictable TBC performance. In the present paper NiCoCrAlY(X) deposited by LPPS and EB-PVD (electron-beam physical vapour deposition) as well as NiPtAl bond coats underneath conventional EB-PVD yttria stabilized zirconia top coats were investigated. Several bond coat treatments such as polishing, annealing in vacuum or in air, and grit blasting were employed in order to study TBC life, and particularly the underlying mechanisms of TGO (thermally grown oxide) formation. Phase composition, microstructure and roughness of the treated bond coats were analyzed by SEM, XRD, and roughness measurements. All cylindrical samples were thermally cycled at 1100°C and 1150°C. Spallation of the TBCs is mainly correlated with TGO formation that is induced by bond coat type and its pre-treatment. Special attention was paid to the influence of rare earth elements such as Y and Hf in the bond coats on cyclic TBC life. The longest lifetimes were achieved by an Hf-doped EB-PVD NiCoCrAlY system that showed a differing TGO formation and failure mechanism. A new approach to evaluate failure phenomena based on energetic criterions is introduced which allows to identify key failure mechanisms within these complex coating systems.
2:10 PM		A3-2-3 Damage Evolution and Spallation Mechanisms for APS-TBCs at Lower Temperatures S. Sanyal, K. Anand (GE Global Research, India); I. Giovannetti, F. Cappuccini, M. Giannozzi (GE Oil & Gas, Italy) Thermal barrier coating systems have been utilized in industrial gas turbines for over thirty years, primarily to protect the existing materials under the demands for increased levels of gas turbine firing temperature. Atmospheric Plasma Spray TBC commonly used for hot combustion chambers components of advanced gas turbines are exposed to thermo-mechanical loads, which may lead to failure in form of macroscopic spallations from the metallic component. The durability of TBC is limited by the interaction of different processes and parameters, such as bond coat oxidation, cyclic strains, visco-plastic and relaxation properties, interface roughness and others. The performance of TBCs at higher operating temperatures (1000 C and above) has been extensively investigated in earlier studies. This work focuses on air plasma sprayed TBC-BC systems that operate at temperatures less than 900°C. As expected, it was found that the kinetics of TGO growth were slower; but there was a significant contribution from internal oxidation and spinel formation in these systems over extended periods of exposure. This work explores the mechanisms of failure & damage evolution, with special relevance to APS-TBC/APS-MCrAlY coating systems.
2:30 PM		A3-2-5 Monitoring Delamination of Thermal Barrier Coatings by Near-Infrared and Upconversion Luminescence Imaging J.I. Eldridge (NASA Glenn Research Center); R.E. Martin (Cleveland State University); J. Singh, D.E. Wolfe (Penn State University) Previous work has demonstrated that TBC delamination can be monitored by incorporating a thin luminescent sublayer that produces greatly increased luminescence intensity from delaminated regions of the TBC. Initial efforts utilized visible-wavelength luminescence from either europium or erbium doped sublayers. This approach exhibited good sensitivity to delamination of electron-beam physical-vapor-deposited (EB-PVD) TBCs, but limited sensitivity to delamination of the more highly scattering plasma-sprayed TBCs due to stronger optical scattering and to interference by luminescence from rare-earth impurities. These difficulties have now been overcome by new strategies employing near-infrared (NIR) and upconversion luminescence imaging. NIR luminescence at 1550 nm was produced in an erbium plus ytterbium co-doped yttria-stabilized zirconia (YSZ) luminescent sublayer using 980-nm excitation. Compared to visible-wavelength luminescence, these NIR emission and excitation wavelengths are much more weakly scattered by the TBC and therefore show much improved depth-probing capabilities. In addition, two-photon upconversion luminescence excitation at 980 nm wavelength produces luminescence emission at 562 nm with near-zero fluorescence background and exceptional contrast for delamination indication. The ability to detect TBC delamination produced by Rockwell indentation and by furnace cycling is demonstrated for both EB-PVD and plasma-sprayed TBCs. The relative strengths of the NIR and upconversion luminescence methods for monitoring TBC delamination are discussed.
3:10 PM		A3-2-7 Recent Advances in Plasma Spray Process Control Towards Optimizing Compliance and Thermal Conductivity of Yttria Stabilized Zirconia Coatings S. Sampath (State University of New York (SUNY)) Plasma sprayed TBCs continue to play an important role in both aerospace and power generation industry and plasma spraying has become the process of choice particularly for very large power gas turbine components. The cost effective nature of the process, flexibility with respect to materials, and ease of applicability are all attributes that has enabled this growth. In the past, plasma sprayed TBCs have been known to be less reliable and more prone to process induced variability. This is related to very large variations in feed stock materials, process parameters and location to location variances. However, much has changed in recent years. A number of robust process sensors have become available to characterize the plasma spray process and understand the process variability. In addition, significant progress has been achieved in scientific understanding of the complex process- coating - performance relationships and allowing perhaps for the first time the ability to meet stringent microstructure design criteria. This paper will discuss these advancements through an integrated investigation of the critical processing parameters on the thermal and mechanical properties of the YSZ coatings. Recent observations have indicated that the thermal conductivity of the coatings is strongly influenced by the nature of the splat-splat interfaces and can be manipulated through feedstock characteristics and processing parameters. Furthermore, it is now possible to quantitatively describe the coating compliance through characterization of the non-linear stress-strain response of plasma sprayed YSZ coatings. Finally, these new approaches also allow critical assessment of coating reliability. This presentation will highlight salient results of this multi-year investigation which not only enhances the current YSZ based application but also provides a framework for processing strategies for potential new compositions.
3:50 PM		A3-2-9 Residual Stress in TGO on Different Bond Coats in TBC Systems X. Wang, A. Atkinson (Imperial College London, United Kingdom) Photoluminescence piezospectroscopy (PLPS) has long been used to determine the residual stresses in thermally grown oxide (TGO) for thermal barrier coatings (TBCs). However, PLPS cannot be as powerful as is expected in studying TBCs degradation mechanism and in predicting the TBCs lifetime if a better understanding is not achieved regarding how the stresses are originated and in what ways the stresses are relaxed. In this contribution, a variety of thermal barrier coating systems with different types of bond coat were investigated. The residual stress in the TGO was found to be dependent not only on heat treatment, but also on the types of the bond coat, surface roughness and morphology. The stress relaxation mechanisms under different circumstances were investigated in detail. This research sheds new light on TBCs degradation mechanism.
4:10 PM		A3-2-10 Optical Measurement of the Thermal Diffusivity of Intact Thermal Barrier Coatings B. Heeg (Metrolaser, Inc.); D.R. Clarke (University of California, Santa Barbara) An all-optical approach is described for determining the thermal diffusivity of thermal barrier coating (TBC) attached to an underlying alloy or turbine component. The method does not require knowledge of either the thermal properties of the alloy or its temperature. The method combines heating of the coating surface with detection of the temperature variation in the thermally grown oxide, formed during service, from laser induced Cr³⁺ luminescence signal generated in the thermally grown oxide (TGO) layer at the bondcoat - TBC interface. The thermal diffusivity is measured by means of the temporal phase lag between a surface heating laser pulse and the resulting traveling heat pulse arriving at the interface. The interface temperature variations are measured by means of a laser induced Cr³⁺ luminescence signal generated in the thermally grown oxide (TGO) layer at the bondcoat - TBC interface. The surface heating is induced by a modulated CO₂ laser. A comparison between experimental data and a heat equation model show a reasonably good correlation. The technique may be extended to measure thermal conductivity, which requires the top surface temperature to be measured as well. The method is well suited for measurement on coated parts of complex shape, such as a turbine blade.