Papers

ICMCTF1999 Session A3: Thermal Barrier Coatings

Tuesday, April 13, 1999 8:30 AM in Room Council/Chamber/Cabinet

Tuesday Morning

Start	Invited?	Item
8:30 AM		A3-1 Relationships Between Bond Strength, Bond Stress and Spallation Mechanisms of Thermal Barrier Coatings M. Gell, E. Jordan (University of Connecticut); D.C. Clarke (University of California) Bond strength and bond stress were determined for five production thermal barrier coatings as a function of furnace thermal cycling to 1121oC (2050oF). Of the five coatings, two were deposited by electron beam physical vapor deposition and three by air plasma spray. Bond coats include vacuum plasma sprayed MCrAlYs and a platinum aluminide. Bond strengths were measured as a function of thermal cycling using a modified ASTM direct pull test. The spallation failure mode in the direct pull test duplicated the failure mode in the thermal cycle test and in field service for each of the coatings. Bond stresses were determined in the thermally grown oxide as a function of thermal cycling using laser photostimulated luminescence. Changes in bond strength and stress will be related to localized compositional and microstructural changes, and to initiation and progression of interface debonding.
9:10 AM		A3-3 Upper Temperature Limitation for the Functionality of Yttria Partially Stabilized ZrO₂ for Thermal Barrier Coatings M. Juez-Lorenzo, V. Kolarik (Fraunhofer Institut für Chemische Technologie (ICT), Germany); M. Sommer, H.J. Schmutzler (ABB Corporate Research Heidelberg, Germany) In the further development of stationary gas turbines for electric power generation an increase of the gas inlet temperature and a reduction of the cooling air flow are envisaged in order to increase turbine efficiency. Thermal barrier coatings (TBC), which currently consist of yttria partially stabilised ZrO₂, are an essential tool for achieving these requirements. A detailed knowledge and understanding of the upper temperature limits for the application of such TBC´s is of great importance for the design of future stationary gas turbines. TBC samples exposed in air for 2000 h to 1200 and 1300 C showed considerable amounts of monoclinic phase increasing with temperature. Additionally, a cubic phase was observed, increasing in amount with the monoclinic phase. Samples exposed to 1100 C showed the t´-phase only. The TBC samples were studied in situ by high temperature X-ray diffraction on heating from room temperature to their exposure temperature and on subsequent cooling. The monoclinic phase in the 1200 and 1300 C samples disappeared on heating and was formed again on cooling in a reversible way. The 1100 C sample, however, showed no change in the phase composition on heating and cooling. The mechanism for the observed degradation phenomena and the upper temperature limitation for yttria partially stabilised ZrO₂ is discussed.
9:30 AM		A3-4 Developments in Materials and Manufacturing Technologies for Thermal Barrier Coatings Resulting From the Advanced Turbine Systems Program I.G. Wright, B.A. Pint, J.A. Haynes (Oak Ridge National Laboratory) During the course of the U.S. Department of Energy-sponsored Advanced Turbine Systems Program there have been significant developments in materials and manufacturing methods to provide thermal barrier coatings systems for use in land-based turbines. The exacting operating environments that will be experienced in the next generation of land-based gas turbines place demands on TBC systems that are different from those in aircraft engines. While the TBCs for the demonstration ATS machines are based on extensions of current materials and processes, as deemed appropriate by the engine manufacturers, improvements in processing and coating performance are needed to ensure that component longevity goals can be met. Data intended to provide improved understanding of the modes of degradation of TBC systems as an underpinning for approaches to increasing TBC operating lifetimes and to increasing the realism of TBC lifing models are being generated in programs conducted by national laboratories and universities. Significant results from these various parts of the ATS program will be highlighted, with particular emphasis on results that provide insight on factors that control the service lifetime of TBCs, and on potential routes for the translation of such improvements into practice.
10:30 AM		A3-7 Thermophysical Properties of Thermal Barrier Coatings R.E. Taylor (TPRL Inc.) Thin layers of thermal barrier coatings (TBCs) are applied to metallic components of heat engines to reduce metal temperatures and to provide environmental protection. This results in increased engine efficiency and prolonged operational life. Of special current interest is the use of TBCs in aircraft engines. The TBCs, often yttria-stabilized zirconia (YSZ), are deposited on nickel- or cobalt-based superalloy components used in high temperature environments. The thermophysical properties (especially thermal conductivity) of the coatings are extremely important since they and coating thickness control the temperature drop across the coating. Accurate determinations of the thermal conductivity of the coating are critical in designing the engines and in research aimed at decreasing the thermal conductivity of TBCs. Such research includes very thin multiple layers, compositional changes and deposition techniques. The number of potentially applicable techniques is limited because of the sample configurations. Consequently the reproducibility of results from a technique or agreement between the results from different techniques often are not satisfactory. Typical results are presented.
11:10 AM		A3-9 The Measurement of Strains Within the Bulk of Aged and As-Sprayed Thermal Barrier Coatings Using Synchrotron Radiation J.T. Thornton (DSTO, Australia); D. Cookson (Australian Synchrotron Research Project, Australia); E. Pescott (DSTO, Australia) Typical Thermal Barrier Coatings (TBCs) consist of a 0.3mm layer of zirconia over a 0.2mm layer of NiCoCrAlY. They are used in gas turbine engines to reduce the temperature of components exposed to the hot combustion gases that power the engines. Reducing component temperatures lowers maintenance costs or enables engines to save fuel or produce greater specific thrust. However, TBCs are not used to their full potential because their lifetime cannot be predicted reliably. This is partly due to an incomplete understanding of why failure (the delamination of the zirconia layer) occurs. One important factor is the build-up of compressive stresses in the zirconia layer on cooling (engine shutdown) after a long period at high temperatures (engine operation). The two aims of this work were to predict the stresses produced and to measure them. To make the predictions it was assumed that at temperature creep relaxes all residual stresses, but during rapid cooling creep cannot prevent the contraction of the superalloy substrate and the NiCoCrAlY layer from placing the zirconia layer in compression. To measure the strains accurately within the layer using diffraction required a high flux of penetrating (λ= 0.0729nm) and parallel radiation, thus the measurements were performed at a synchrotron facility - the "Big Diff" at the Photon Factory in Japan. Information about the strains in the top 20 µm can be obtained with conventional laboratory sources; however, the compressive stresses will be diluted in the rough surface and consequently any measurements will be misleading. Average Elastic Moduli from four-point bend tests and the literature were used in the calculations. The compressive stresses in the zirconia layer on cooling from 1150°C were found to reach 320MPa. The predicted stress was 350MPa. In comparison, the stresses in the as-sprayed samples were found to be negligible. Also cross-sections of the measured samples were examined by SEM to establish if any sintering or cracking had occurred.
11:30 AM		A3-10 Physical Properties of Thermal Barrier Coatings Studied in Situ by High Temperature X-ray Diffraction N. Czech (Siemens AG, Power Generation Group (KWU), Germany); H. Fietzek, M. Juez-Lorenzo, V. Kolarik (Fraunhofer Institut für Chemische Technologie (ICT), Germany); W. Stamm (Siemens AG, Power Generation Group (KWU), Germany) To increase the power efficiency of stationary gas turbines the turbine inlet temperature has reached more than 1400 C recently. At the same time a reduction of the cooling air is envisaged for avoiding an increase of the NO_x level.Both higher inlet temperatures and reduction of cooling air is achieved by the use of thermal barrier coatings (TBC). Induced stresses in the TBC´s have an essential influence on their durability, in the case of Electron Beam Physical Vapour Deposited (EB-PVD) thermal barrier coatings also the growth of the TGO (Thermally Grown Oxide) plays an essential role. EB-PVD and APS TBC´s were studied under thermal cycling conditions using high temperature X-ray diffraction as an in situ method. Anisotropies in the thermal expansion of the tetragonal ZrO₂ unit cell were observed and studied as function of the number of thermal cycles.
11:50 AM		A3-11 Effects of Deposition Temperature and Thermal Cycling on Residual Stress State in Zirconia Based Thermal Barrier Coatings V. Teixeira (University of Minho, Portugal); M. Andritschky (Universidade do Minho, Portugal); W. Fischer, H.P. Buchkremer, D. Stoever (Forschungszentrum Juelich, Germany) Advanced ceramic multilayered coatigs are commonly used as protective coatings for engine metal components to inprove performance, eg thermal barrier coatings,TBCs. Zirconia based TBCs were produced by plasma spraying process and characteriyed in terms of microstruture, porosity, elastic modulus, adherence and residual stresses. In this contribution the residual stresses in multilayered coatings applied on Ni based superalloys for use as thermal barrier coatings were studied both by numerical modelling and experiemental stress measurement. The thermal residual stress generated during spraying of duplex thermal barrier coatings were simulated by using an heat transfer finite element program and an elasto-plastic biaxial stress model. The TBC system was subjected to different thermal cycling conditions (maximum temperature, heating up and cooling dowm time, dwell time at maximum temperature,etc.). The stress state after thermal cycling were also modelled..The stress state in the as deposited and in thermal cycled coatings was verified by x ray diffraction technique. The measurements were in good agreement with residual stress modelled results. It was observed that the residual stresses were dependent on the thermal history of the thermal barrier coating (as deposited and thermal cycled). Thermal cycling allowed the stresses to relax by microcracking and creep mechanisms at high temperature such that on cooling down to room temperature an in-plane biaxial compressive stress will arise on the zirconia top coating due to the difference on the coefficients of thermal expansion between substrate and coating.