Thermal Barrier Coatings
Tuesday, May 1, 2001 1:30 PM in Room Royal Palm Salon 1-3
A3-2-1 Vapor Deposition of Thermal Barrier Coating Systems
H. Wadley, J. Groves, D. Hass (University of Virginia)
The preparation of thermal barrier coating systems is a multistep process with many attendant limitations upon the achievable composition, microstructure and morphology of coating constituents. We are exploring the development of a potentially low cost approach for the vapor deposition of complete coating systems in a single chamber reactor. The approach is based upon the use of electron evaporation in combination with rarefied helium gas jets for controlling transport of the vapor to a substrate. Using high scan speed electron beams and closely spaced evaporation sources, we are able to evaporate up to four materials simultaneously, fully mix the four vapor streams and deposit either alloy bond coats or the thermal insulating layer. The microstructure of both is controllable by the use of gas phase scattering to introduce high volume fractions of porosity (and attendant low thermal conductivity top coats) or plasma enhanced deposition (of bond coats) to form fully dense coatings without resorting to unacceptably high deposition temperatures.
A3-2-3 Zirconia-Based Thermal Barrier Coatings Deposited by PECVD
B. Preauchat, S. Landaisl, S. Drawin (ONERA, France)
Thermal barrier coatings (TBC) are used to protect turboengine metallic parts (blades and vanes) from the hot combustion gas flow. @paragraph@ A microwave PECVD apparatus, as an alternative to the industrially used EBPVD technique, is used to deposit 100 to 200 µm-thick zirconia-based coatings on planar or non-planar, ceramic or metallic specimens at temperatures between 300°C and 900°C with a good thickness uniformity. This process has been developed with the goal of a better surface coverage of complex-shaped parts by the possible control on the gas flows and prospective lower investment and running costs. @paragraph@ The extremely high deposition rates, more than 150 µm/h, and the coating morphology have been optimised at a deposition temperature of 900°C for suitable high temperature stability. @paragraph@ The performances of the PECVD TBCs are investigated in terms of thermal conductivity and resistance to thermal cycling tests at 1100°C. This study presents the results obtained for 6 - 10 mol% YO@sub 1.5@ partially stabilised zirconia coatings. The crystallographic properties have been analysed to explain the behaviour of our coatings at high temperatures through a comparison between the thermal cycling tests and the destabilisation observed during isothermal ageing. Moreover, to validate the process as a real alternative to EBPVD, we have compared these results with those obtained recently on EBPVD coatings, in terms of coating morphology, phase composition, phase stability and live time under thermal cycling conditions. @paragraph@ The versatility of the PECVD process allows also the deposition of newer concepts of TBCs: some properties of multilayer coatings and coatings with non-standard compositions will be presented.
A3-2-4 Microstructural Characterization and Deposition Rates for Electron Beam - Physical Vapor Deposition of Thermal Barrier Coatings (EB-PVD TBCs) on Curved Surfaces as a Function of Position in the Coating Chamber
R.D. Sisson, W.C.S. Weir (Worcester Polytechnic Institute); S. Bose (Pratt and Whitney)
The microstructure and deposition rates of EB-PVD partially stabilized zirconia have been experimentally determined as a function of position and curvature in multiple ingot coating chamber. The microstructures are presented in terms of apparent coating density, phase distribution, crystallographic texture and column dimensions. The coating thicknesses and deposition rates are compared with theoretical model prediction based on a modified Knudsen's cosine law. These results are compared to previous work on PSZ and other materials systems. Applications of these results to a rules-based software system to determine coating processing parameters will also be discussed. .
A3-2-5 Processing/Structure/Thermoluminescene Property of Eu Doped Y@sub 2@O@sub 3@-ZrO@sub 2@ Smart Coatings Deposited using ESAVD
K.L. Choy (Imperial College of Science, Technology and Medicine, United Kingdom); M. Wei, J.P. Feist, A.L. Heyes (Imperial College, United Kingdom)
Thermal barrier coatings such as Yttria Partially Stabilised Zirconia(YPSZ) have been doped with a rare earth element such as Europium to obtain a phosphor material capable of luminescene following excitation with UV light. Such Eu-doped YSZ thick films have been deposited using a novel and cost-effective Electrostatic Spray Assisted Vapor Deposition method (ESAVD) onto Ni-based superalloy substrates. The ESAVD process involves spraying atomised precursor droplets across an electric field where the precursor droplets undergo decomposition and chemical reaction near the vicinity of the heated substrate and deposit a stable solid film. The correlation of the lifetime decay and temperature change of the Eu-doped YSZ films as a function of film thickness was investigated. The suitability of Eu-YSZ coating as a thermal barrier coating and temperature sensor will be reported. .
A3-2-7 Architecture of Thermal Barrier Coatings Produced by Electron Beam-Physical Vapor Deposition (EB-PVD).
J. Singh (The Pennsylvania State University); G. Simpson (Navy Aviation Depot)
Extremely high temperatures and severe atmospheric conditions in the hot section of aircraft engine during operation result in degradation and structural failures of turbine components. Replacing these components is very expensive. Thermal barrier coatings (TBC) composed of ZrO2-8wt. % Y2O3 (8YSZ) applied by Electron Beam-Physical Vapor Deposition (EB-PVD) to turbine components offers excellent properties for thermal protection and resistant against oxidation, erosion, and corrosion. However, the life of turbine components is still limited due to premature failure of the TBC. It is hypothesized that the life of the coated components can be extended by lowering thermal conductivity of the TBC and reducing the oxygen transport through the coating by creating mulitple interfaces. TBC was applied by EB-PVD on various bond coated samples. Four sets of experiments were conducted. The first set had the standard 8YSZ coating, the second had standard 8YSZ coating with several interfaces, the third and fourth sets were composed of the standard 8YSZ and Niobium Oxide, but in different combinations. TBC samples were examined by various methods including, scanning electron microscopy (SEM), high resolution optical microscopy, X-ray diffraction (XRD), and thermal cycling tests. Multiple interfaced TBC exhibited better oxidation resistant properties as compared to standard and alloyed TBC.
A3-2-8 Evolution of Photo-Stimulated Luminescence from Thermally Grown Oxide in EBPVD/MCrAlY/IN738 Thermal Barrier Coatings During Thermal Cyclic Oxidation at 1121@super o@C
Y.H. Sohn (University of Connecticut "now with" University of Central Florida); K. Vaidyanathan, M. Ronski, E. E. Jordan, M. Gell (University of Connecticut)
Photostimulated luminescence piezo-spectroscopy (PLPS) is being developed as a non-destructive inspection technique for thermal barrier coatings (TBCs). In this study, structural integrity of thermally grown oxide (TGO) and TBC/TGO/bond coat interfaces were examined by PLPS as a function of thermal cycles for a production TBC specimens that consists of electron beam physical vapor deposited (EB-PVD) ZrO2-7wt.%Y2O3 (7YSZ), MCrAlY bond coat and IN-738 superalloy substrate. Thermal cycling for TBC consisted of 10-minute heat-up, 40-minute-hold at 1121@super o@C and 10-minute quench to ambient temperature. Various characteristics of photo-stimulated luminescence spectra, including residual stress calculated from the peak-shifts and relative intensities of the photo-stimulated luminescence were examined as a function of thermal cycles and related to the spallation of TBCs which occurred at around 400 cycles. A gradual increase in the magnitude of compressive residual stress in the TGO with thermal cycles was observed up to 300 cycles, and then a sharp decrease in the magnitude of compressive residual stress was then observed along with the spallation of TBC near 400 cycles. Photo-stimulated luminescence spectra consisting of stressed and stress-free luminescence peaks (i.e., bimodal) were also frequently observed for spalled or nearly-failed TBCs. Physical basis for the evolution of photo-stimulated luminescence spectra is discussed and related to the damage and the final spallation of TBCs.
A3-2-9 Measuring Fatigue in Ceramic Coatings
J. Lira-Olivares (Centro de Ingenieria de Superficies, Venezuela); M. Brito (Universidad Simon Bolivar, Venezuela); Y. Mutoh (Nagaoka University of Technology, Japan)
A series of Mode I and Mode II fatigue tests were designed for Thermal Barrier Coatings (TBC) of PSZ applied by plasma spray. The tests were carried out at room temperature and 900°C, using original and thermally treated specimens. The results showed that the Mode I test specimen presented a crack propagation in the external TBC following a function that depended on the stress intensity factor. The crack propagation was influenced by previous deffects localized near the interface coating-substrate. In contrast, the Mode II tests results showed an extensive microcrack phenomena instead of the single large cracks observed in Mode I. In both cases, high temperature accented the crack propagation mode. Some fatigue crack mecanisms are advanced in this paper.
A3-2-10 Quantification of Damage Evolution in TBCs by Photo-Stimulated Luminescence Spectroscopy
J.A. Nychka, D.R. Clarke (University of California at Santa Barbara)
There is need for a non-destructive tool for evaluating the damage evolution and life expectancy of TBCs, especially if they are to be considered prime reliant. One particularly promising method is that of photo-stimulated luminescence spectroscopy (PSLS, since it is both non-destructive and produces a direct measure of local elastic strain energy in the thermally grown oxide (TGO). Another benefit of the PSLS technique is the variety of quantitative information that can be deduced from the spectra, including residual stresses, different stress states, and stress gradients. The desired quantitative information is obtained using curve-fitting techniques, and the existing literature treats curve fitting of PSL spectra as mainly a statistical task. In contrast, our work explains quantitative fitting based on known physical characteristics, and we describe the quantitative analysis of PSL spectra with the view of developing a physical mechanisms based approach.
A3-2-11 Sub-Micron Structured MCrAlY by Pulsed Electron Beams and its Influence on Oxide Scale Formation at Different Temperatures
W. Stamm (Siemens AG, Germany); V. Engelko (Efremov Institute St. Petersburg, Russia); G. Müller, G. Schumacher, D. Strauss (Forschungszentrum Karlsruhe, Germany)
The pulsed electron beam treatment of Low Pressure Plasma Sprayed MCrAlY surfaces leads to a significant structure refinement in the sub micron range. The obtained phase distribution approaches the ideal of a homogeneous alloy considering especially Aluminium and Yttrium, the most reactive elements typically found in MCrAlY coatings. During exposure to high temperatures a homogeneous oxide scale grows and recrystallisation of the underlying metal occurs. Oxide scale growth and morphology on MCrAlY at different temperatures were studied to qualify the effect and development of recrystallisation and oxide scale formation. Furthermore the Yttrium distribution in the electron beam treated MCrAlY was measured and compared to its concentration in the original LPPS-coating. The results are discussed emphasizing the potential of the treated MCrAlY as a bond coating for EBPVD-Thermal Barrier Coatings compared to untreated LPPS-MCrAlY. @paragraph@ Keywords: MCrAlY, bond coat, TBC, surface treatment