Thermal Barrier Coatings
Wednesday, May 2, 2001 8:30 AM in Room Royal Palm Salon 1-3
A3-3-1 Microstructural Evaluation of Thermally Cycled TBCs
K.L. More, J.A. Haynes, K.S. Trent, D.W. Coffey, M.J. Lance (Oak Ridge National Laboratory); R. Darolia (General Electric Aircraft Engines); B.A. Nagaraj (General Electric Aircraft Engine Business Group)
Thermal barrier coatings (TBCs) have been evaluated using several electron microscopy techniques after cyclic oxidation at two different temperatures, 1135@super o@C and 1163@super o@C. The TBCs used for this work were deposited by EBPVD onto CVD aluminide bond coats on Rene' N5 single crystal button substrates (~1" diameter). In order to understand the progressive microstructural changes that contributed to TBC failure during thermal cycling, it was necessary to evaluate similarly processed TBC buttons following different numbers of cycles at each temperature. For instance, at 1135@super o@C, samples were prepared for microstructural analysis after 10, 20, 40, 100, and 200 cycles and failure occurred after 380 cycles. At 1163@super o@C, TBC failure was observed after 240 cycles. In order to prepare useful and fully intact samples for bulk microstructural characterization from the cycled TBC button specimens (using scanning electron microscopy and electron probe microanalysis) and specimens for high-resolution TEM analysis, several innovative preparation techniques have been pioneered and reproducibly employed at Oak Ridge National Laboratory. An Hitachi HB-2000 focused ion beam (FIB) instrument was used to prepare the cross-section TEM specimens. The FIB was used to produce TEM samples having large, intact thin areas through the different layers of the TBC button samples (including the TBC, the alumina scale, and bond coat). These carefully prepared TEM specimens allowed for viewing of all the critical interfaces where failure typically occurs in TBCs (metal/oxide and oxide/ceramic interfaces). This work will focus on the microstructural degradation of the TBC specimens as a function of thermal cycling conditions and correlate these observations with possible failure mechanisms of the TBCs.
A3-3-3 Influence of Substrate Material on Oxidation Behavior and Cyclic Lifetime of EB-PVD TBC Systems
U. Schulz, M. Menzebach, Ch. Leyens (DLR, Germany); Y.Q. Yang (Northwestern Polytechnical University, PR China)
Current thermal barrier coatings (TBCs) used for protection of turbine blades in aero engines and stationary gas turbines are composed of a ceramic top coating and an MCrAlY-type or aluminized bond coat, deposited onto a superalloy substrate. Although proper processing of the bond coat-TBC system is a key point for lifetime of the coating, also the substrate material can play a dominant role for TBC spallation life. Since it is widely known that failure of EB-PVD TBCs predominantly occurs at or near the thermally grown oxide (TGO), a basic understanding of the formation and evolution of this "bonding zone" is necessary. In the present study a NiCoCrAlY EB-PVD bondcoat was deposited on various substrates, namely on IN 100, Rene 142, MAR M002, PWA 1483, and CMSX-4. All samples got a EB-PVD P-YSZ TBC on top. Cyclic tests were performed on cylindrical specimens under thermal load at 1100°C using 1 hour cycles. Coatings were investigated after fixed intervals and after failure of the TBC by metallography and SEM. Thickness and composition of TGO, depleted layer, and interdiffusion zone were compared for all different substrate materials. The growth kinetics of the TGO was correlated with the time to failure. The longest TBC lifetime was observed for MAR M002 and Rene 142 substrates while TBCs on single crystalline substrate materials suffered from early failure. In order to better understand TGO formation and the reason for failure a detailed TEM study was performed on one TBC system. This NiCoCrAlY-PYSZ system was characterized after isothermal exposure to air at 1050°C for up to 1013h. With increasing exposure time, a pure alumina layer forms beneath a mixed layer of alumina and zirconia that was present already after coating deposition. After the longest times, mixed oxides containing Cr and Ni have been identified. The growth kinetics of the TGO was correlated with the data from the 1100Â°C cyclic tests.
A3-3-4 The Effect of Bond Coat Surface Treatment on the Oxidation Induced Failure of EB-PVD Thermal Barrier Coatings
V.K. Tolpygo, D.R. Clarke (University of California at Santa Barbara); K.S. Murphy (Howmet Castings)
Oxidation induced failure of EB-PVD thermal barrier coatings (TBC) deposited on a single-crystal superalloy with a platinum aluminide bond coat has been studied in order to determine the specific mechanisms leading to TBC spallation. Since the oxidation behavior of the bond coat exerts primary control over TBC life, such characteristics as microstructure and thickness of the thermally grown alumina scale, as well as stresses in the scale are expected to have significant effects. It is demonstrated that the method of the bond coat surface treatment prior to TBC deposition determines these characteristics and, in turn, affects the time to failure. In particular, for a given morphology of the oxide-metal interface, there exists a critical thickness of the scale at which TBC spallation is inevitable. Therefore, the methods which lead to a slowly growing alumina scale are expected to be beneficial, whereas if the oxide growth is accelerated, the TBC will fail earlier. An additional contributing factor is the morphology of the oxide-metal interface. Cyclic testing of the TBC-coated samples with different roughness of the bond coat surface shows that there may be either favorable or unfavorable interface morphology, depending on the bond coat treatment prior to TBC deposition. The development of the alumina scale under the TBC during cyclic oxidation can be non-destructively monitored using the luminescence spectroscopy and the spectral data can be standardized for a given TBC system and pre-treatment method. It is proposed that luminescence spectroscopy, performed directly after TBC deposition or after a short-term high temperature exposure, is a reliable indicator of life expectancy.
A3-3-5 Microstructural Characterization of Thermal Barrier Coatings on High Pressure Turbine Blades
Y.H. Sohn (University of Connecticut "now with" University of Central Florida); R.R. Biederman, R.D. Sisson, Jr. (Worcester Polytechnique Institute); E.Y. Lee (Andong National University); B.A. Nagaraj (General Electric Aircraft Engine Business Group)
Thermal barrier coated high pressure turbine blades before and after the test were characterized by microstructural analysis, and the compressive residual stress within the thermally grown oxide (TGO) was measured by the photostimulated luminescence piezo-spectroscopy (PLPS). Thermal barrier coatings (TBCs), in this study, consisted of electron beam physical vapor deposited (EB-PVD) yttria partially stabilized zirconia (YSZ; ZrO2-8wt.%Y2O3), chemical vapor deposited aluminide bond coat and Ni-base superalloy high pressure turbine (HPT) blades. Compressive residual stress in TGO, measured by photo-stimulated luminescence piezo-spectroscopy, was observed to be in the order of 2.5 GPa and varied slightly as a function of substrate geometry. X-ray diffraction and scanning electron microscopy equipped with energy dispersive x-ray spectroscopy were utilized to characterize the TBC coated HPT blades. The as-deposited non-equilibrium tetragonal (t') phase in the YSZ coatings was observed to decompose after the test, and the monoclinic (m) phase was found in the YSZ coatings with concave substrate curvature. Also, sintering of YSZ coating after the test was evident in the microstructure. Fracture within the TGO and within the YSZ ceramic was observed for pressure and suction side of tested HPT blade near the top, respectively.
A3-3-7 Influence of Substrate Composition on the High-Temperature Stability of EB-PVD TBCs
U. Kaden, Ch. Leyens (German Aerospace Center, Cologne, Germany)
Advanced turbine performance in jet engines and in land-based systems requires higher operating temperatures and a decreased flow of cooling air. For the most demanding applications, vanes and blades of the first turbine stages are made of SX alloys overlaid with an electron beam physical vapour deposition (EB-PVD) thermal barrier coating (TBC) system. Among others, the cyclic oxidation performance of coated airfoil materials can strongly depend on the substrate composition. For example, the number of cycles to failure of a system CMSX-4 / NiCoCrAlY / PYSZ is only half of that of an identical TBC system based on the substrate material IN100. Diffusion of substrate elements into the bond coat appears to affect the adhesion of the oxide scale, thus reducing the useful lifetime of the TBC system on the single crystal compared with the polycrystalline material. To investigate this effect more in detail, research was focused on model substrate materials consisting of vacuum-cast test pins (dia. 6mm x 75mm). The composition of the test pins was based on Ni-22Co-17Cr-13.5Al-0.18Y doped with potentially deleterious elements, such as Ta, W, Re and combinations thereof. These modified NiCoCrAlYs were coated with undoped EB-PVD Ni-22Co-17Cr-13.5Al-0.18Y bond coats (to allow only the element of particular interest to diffuse) and some of them were overlaid with 7YPSZ TBCs. Diffusion of these elements of interest through the bond coat to the TGO and their effects on oxide scale formation after isothermal oxidation tests at 1100°C in air for up to 625hr were investigated by XRD and SEM/EDS. Depending on the dopant, different oxide modifications were detected. Furthermore, doped model substrates coated with NiCoCrAlY bond coat and TBC were tested in a thermal cyclic rig (50 min at 1100°C/10 min forced air cooling) to investigate the influence of the doping elements on the stability of the TBC system.
A3-3-8 Growth and Adherence of Alumina Scales Within TBC Systems wWth Aluminide Bond Coatings
J.A. Haynes, M.J. Lance, K.L. More, B.A. Pint, I.G. Wright (Oak Ridge National Laboratory)
Failure of thermal barrier coatings (TBCs) deposited by electron beam-physical vapor deposition (EB-PVD) is closely associated with oxidation and deformation of the metallic bond coat. The specific degradation mechanisms of the thermally grown alumina scales that form along the bond coat-top coat interface continue to be the subject of intense research and debate. It is well known that oxide scale adherence is very sensitive to the presence of trace impurities, reactive elements and precious metal additions, but the physical mechanisms of these various effects are not well defined. The present study investigated alumina scale growth and degradation within commercial EB-PVD TBC systems with chemical vapor deposition (CVD) platinum aluminide bond coatings and single-crystal superalloy substrates. The oxidation behavior of these commercial coatings was compared with ultra-high purity, laboratory-processed aluminide and platinum aluminide coatings fabricated on superalloys and model NiCrAl alloys. The effects of substrate and bond coat impurities on the preliminary stages of oxide scale growth will be described. Characterization of oxide scale adherence on the various coating/substrate systems has provided substantial insight into the influences of Pt, S and Hf on aluminide bond coat scale growth and adherence. @paragraph@ Research sponsored by the U.S. Department of Energy, Assistant Secretary of Energy Efficiency and Renewable Energy, Advanced Turbine Systems Program under contract DE-AC05-00OR22725 with UT-Battelle, LLC.
A3-3-9 Thermal Conductivity of Zirconia- and Hafnia-Based Thermal Barrier Coatings
D. Zhu, J.I. Eldridge, R.A. Miller (NASA Glenn Research Center); J. Singh (The Pennsylvania State University)
Ceramic thermal barrier coatings have received increasing attention for advanced gas turbine engine applications because of their ability to effectively protect engine hot section components. Achieving low thermal conductivity and maintaining long-term high temperature stability are among the most critical requirements for developing advanced thermal barrier coatings. In this paper, a steady-state laser heat flux technique has been used to evaluate thermal conductivity of several plasma-sprayed and electron beam-physical vapor deposited ZrO2- and Hafnia-based thermal barrier coating systems under high temperature thermal gradient conditions. In both systems, the changes in thermal conductivity during high heat flux exposure are examined. The effects of coating composition, processing and structures on coating performance and stability are also discussed.