Papers

ICMCTF2006 Session A3-3: Thermal Barrier Coatings

Thursday, May 4, 2006 8:30 AM in Room Sunrise

Thursday Morning

Start	Invited?	Item
8:30 AM		A3-3-1 Microstructural Observations of As-Prepared and Thermal Cycled NiCoCrAl Y Bond Coats K.J. Hemker (Johns Hopkins University) Technological benefits of thermal barrier coatings are derived from their ability to sustain high thermal gradients while maintaining structural integrity in extremely hostile environments. Final failure in many systems is associated with spallation of the top coat, but the overall performance and lifetime of a TBC is closely related to the behavior of its bond coat. This paper addresses the microstructural stability and mechanical properties of a commercial low pressure plasma sprayed (LPPS) NiCoCrAlY bond coat. The as-deposited microstructures of two-phase MCrAlY bond coats are generally described as containing two phases, β-NiAl and γ-solid solution Ni, but TEM micro-diffraction and energy-filtered imaging conducted in this study have uncovered the presence of very fine γ'-Ni₃Al precipitates and the formation of additional phases with thermal cycling. Microsample tensile testing is also employed to measure the elevated temperature mechanical behavior of this bond coat. Salient observations regarding the link between microstructure and mechanical behavior and the evolution of both, as a result of thermal cycling, will be outlined and discussed. The support of The Air Force Office of Scientific Research under the MEANS-2 Program (Grant No. FA9550-05-1-0173) is gratefully acknowledged.
9:10 AM		A3-3-3 USAXS-Analysis of Electron-Physical Vapour Deposited Thermal Barrier Coatings and Application of Void Structure Modeling to Determine the Influence of Process Parameters on the Thermal Conductivity A. Flores Renteria, B. Saruhan (DLR-German Aerospace Center, Germany); J. Ilavsky (Argonne National Laboratory); A.J. Allen (National Institute of Standards and Technology) Thermal barrier coatings (TBCs) deposited via Electron-Beam Physical Vapour Deposition (EB-PVD) are used to reduce the temperature on the metal blades situated at the high pressure sector of aircraft and stationary turbines. One of the most promising methods to improve the efficiency of these turbines is optimizing the TBCs properties through alteration of their microstructure. The EB-PVD TBCs contain open (inter-columnar gaps and voids between feather arms) and closed pores (intra-columnar pores) whose spatial and geometrical distribution can be altered by changing the process parameters, influencing additionally their thermal conductivity. Since such pores display large differences between their sizes, difficulties in accessibility and high shape and distribution anisotropy, their characterization requires complex measuring methods. In this work, three different microstructures were manufactured by varying the process parameters. Their corresponding thermal conductivities were measured in as-coated and after annealing conditions via Laser Flash Analysis Method (LFA). Analysis of the pore formation during processing was made by employing sophisticated USAXS-method supported with a modelling programme developed by researchers at APS/NIST, which enables the characterization of the size, shape, volume and orientation of all pores at EB-PVD TBCs. Measurements and modelling results indicate that microstructures containing intra-columnar pores and larger voids between feather-arms with oblate shapes oriented closer to the substrate plane enclose lower thermal conductivity values. Inter-columnar gaps exhibit no significant contribution in lowering the thermal conductivity. On heat-treatment, larger voids between feather-arms break up into arrays of nano-sized spheres and the fine, elongated and anisotropic shape of intra-columnar pores turns into quasi-spherical, becoming clearly less effective in reducing the thermal conductivity.
9:30 AM		A3-3-4 Delamination-Indicating Thermal Barrier Coatings Using Rare-Earth-Doped Luminescent Sublayers J.I. Eldridge, T.J. Bencic, C.M. Spuckler (NASA Glenn Research Center); J. Singh, D.E. Wolfe (Penn State University) Nondestructive diagnostic tools that can reliably assess the location, extent, and severity of delamination of thermal barrier coatings (TBCs) are needed to protect against premature TBC failure by prompting TBC replacement when the progress of TBC delamination approaches unacceptable thresholds. A coating design for a TBC that is self-indicating for delamination has been developed by incorporating a rare-earth-doped yttria-stabilized zirconia (YSZ) luminescent sublayer beneath the overlying undoped YSZ TBC. The strategy for producing delamination-indicating TBCs relies on the contrast-producing mechanism of total internal reflection of both the excitation and emission wavelength light at the TBC/crack interface, resulting in substantial enhancement of the detected luminescence from delaminated regions. Dopants were selected that exhibit strong excitation and emission peaks that can be transmitted through the TBC. Both europium and erbium were found to meet this criterion and were incorporated into both plasma-sprayed and electron beam physical vapor deposited (EB-PVD) TBCs. Both scanning luminescence mapping as well as luminescence imaging revealed enhanced detected luminescence from delaminated regions. Imaging of the enhanced luminescence associated with TBC delamination was fast and simple to implement, therefore showing great promise as a practical tool for inspecting for TBC delamination. Results for the europium-doped and erbium-doped sublayers are compared, and issues associated with the much more highly optically attenuating plasma-sprayed TBCs are discussed.
9:50 AM		A3-3-5 Development of Residual Stress and Damage in Thermal Barrier Coatings G.Y.H. Lee (Imperial College, United Kingdom) Photo- luminescence piezo-spectroscopy through the electron beam deposited (EBPVD) yttria-stabilised zirconia (YSZ) thermal barrier coating has been used to study the evolution of residual stress due to the alumina thermally grown oxide (TGO) formed on Pt-Al bond coats on a CMSX-4 substrate. The coated specimens were thermally cycled between room temperature and 1150°C, holding for one hour at the top temperature. The experiments explored the effects of two different bond coat surface finishes and three different thicknesses of YSZ. Damage within the composite coating was observed by the luminescence peaks broadening and splitting due to stress relaxation caused by damage in or near the TGO. These local damage regions, identified by deconvolution of the luminescence spectra, are observed to grow as thermal cycling proceeds. In this paper, we show how YSZ thickness and surface roughness influences the evolution of residual stress and damage in the TGO and eventual failure of the coating. The behaviour of these β structure bond coats is compared and contrasted with that of similar bond coats having the γγ^' structure.
10:30 AM		A3-3-7 Evolution in Photoluminescence and Electrochemical Impedance with Microstructural Changes in Thermal Barrier Coatings Y.H. Sohn (University of Central Florida); B. Jayaraj (Mitsubish Power Systems); S. Laxman (University of Central Florida (now with Johnson Controls Incorporated, Mississaga, Ontario)); B. Franke (University of Central Florida (now with Solar Turbines Incorporated, San Diego, CA)); J.W. Byeon (University of Central Florida) Photostimulated luminescence (PL) and electrochemical impedance (EI) have been employed as complimentary non-destructive evaluation (NDE) techniques for life-remain assessment of thermal barrier coatings (TBCs) as a function of furnace thermal cycling test. The evolution in the residual stress of the thermally grown oxide (TGO) scale, the TGO polymorphic constituents, and the electrochemical impedance of the Yttria-Stabized-Zirconia (YSZ) topcoat and the TGO scale, examined by these NDE techniques, are related to the microstructural development and damage characteristics observed by using a variety of techniques including transmission and scanning transmission electron microscopy (TEM and STEM). Microstructural changes such as YSZ sintering and TGO growth are related to the changes in EI (i.e., resistance and/or capacitance), while damage characteristics such as racheting and cracking at bond coat ridges are related to the stress relief and/or relaxation of the TGO scale determined from PL.
11:10 AM		A3-3-9 Determination on Non-Contact Sensing of T BC/BC Interface Temperature in a Thermal Gradient M.M. Gentleman, D.R. Clarke (University of California, Santa Barbara); J.I. Eldridge, D. Zhu (NASA Glenn Research Center) Under steady state conditions, the heat flux through a coating subject to a temperature gradient can be ascertained from knowledge of the temperature gradient and the thermal conductivity of the coating. In recent years a high-power CO₂ laser-based method has been developed for testing thermal barrier coated alloys and evaluating their thermal conductivity. In this system, the temperature at the heated side of the coating is usually measured by infra-red pyrometry and the back-side of the metal alloy measured by either pyrometry or a thermocouple welded to the alloy. In this contribution, we describe preliminary results of the heat flux obtained using in-situ measurements of the temperature from thin-film luminescence sensors deposited on either side of a yttria-stabilized zirconia coating. The temperature determinations were made from non-contact photo-excited luminescence decay measurements in which a pulsed laser was directed at the sensors and the luminescence collected with a sapphire fiber-optic system connected to a photomultiplier and associated electronics. The thermal conductivities of the coatings studied were determined by separate measurements. Details of the measurements, the fabrication of the thin-film luminescence sensors as well as comparison with simultaneous optical pyrometry measurements of temperature will be described.
11:30 AM		A3-3-10 Effects of Long Term, High Temperature Aging on Luminescence from Doped YSZ Thermal Barrer Coatings M.D. Chambers, D.R. Clarke (University of California, Santa Barbara) In previously published work, we have described thin film temperature sensors fabricated from Eu-doped yttria-stabilized zirconia (YSZ) and Eu-doped zirconate pyrochlore materials. These sensors are based on the measurement of the temperature dependent luminescence lifetime of the photoluminescence from the Eu³⁺ ions and excited by a laser pulse. At high temperatures, the luminescence lifetime from these materials decreases exponentially with increasing temperature, providing a rapid and reliable method for non-contact temperature measurement. Currently, the maximum measurement temperature is limited by the electronics used to detect the photoluminescence, being about 1050°C for the Eu-doped YSZ and about 1200°C for the Eu₂Zr₂O₇ pyrochlore sensors. In this contribution we will describe on-going research exploring alternative luminescence sensor materials compatible with thermal barrier coatings as well as concentration effects on the temperature dependence of the luminescence lifetime.
11:50 AM		A3-3-11 Infiltration-Inhibiting Reaction of Gadolinium Zirconate TBCs with Molten CMAS S. Krämer, R.M. Leckie, J. Yang, C. Levi (University of California, Santa Barbara) Enhancing efficiency by increasing the operating temperature of gas turbine engines carries the unintended penalty of promoting the deposition and melting of siliceous debris ingested with the intake air. The resulting melts, generically known as CMAS (calcium-magnesium alumino-silicate) penetrate the open spaces in the TBC degrading its strain tolerance and chemically attacking the thermal barrier oxide. Gadolinium zirconate (GZO) is an attractive alternate material for thermal barrier coatings (TBCs) owing to its lower thermal conductivity, phase stability and higher resistance to morphological evolution than 7YSZ. Of particular interest to this work is the interaction of GZO with CMAS in the context of current experiences with 7YSZ. A model CMAS with the composition 35CaO-10MgO-7Al₂O₃-48SiO₂ (Tm~1240°C) was applied to the TBC surface in palletized form. After isothermal heat treatments at 1300°C changes in morphology and chemistry were characterized by SEM, FIB and TEM. After 4h of exposure, CMAS and GZO formed a fine-grained, 5µm thick reaction layer with facetted needles protruding into the CMAS which were identified as CaGd4Si₃O₁₃, an apatite-type phase with some Zr in solid solution. The reaction zone consists of a conglomerate of smaller apatite-like grains, cubic ZrO₂ with Gd and Ca in solid solution, and spinel as well as some residual CMAS. Most remarkably the majority of the columnar gaps underneath the reaction zone were not infiltrated by the CMAS and in no case the penetration continued further than ~15 µmm below the remaining column tips. The underlying mechanism and kinetics of the reaction will be discussed.