Papers

ICMCTF2009 Session A3-2: Thermal Barrier Coatings

Friday, May 1, 2009 8:00 AM in Room Royal Palm 1-3

Friday Morning

Start	Invited?	Item
8:00 AM		A3-2-3 Moisture-Induced Spallation of Thermal Barrier Coatings M. Rudolphi, D. Renusch, M. Schütze (DECHEMA e.V., Germany) Spallation of thermal barrier coatings (TBCs) usually occurs during cool down when thermal expansion mismatch stresses are maximal. However, sometimes a delayed failure of the coating is observed at the end of their life-time, which is the spallation of the coating after the sample is cold. This so called Desktop Effect is believed to be strongly influenced by the presence of water (i.e. water vapor) in the ambient environment. While the influence of water / water vapor on corrosion rates at high temperatures has been under investigation for several decades, the detrimental effect of water on oxide scale adhesion has received only little attention or even remained unnoticed. In order to develop a deeper understanding of the underlying mechanisms of moisture induced spallation a series of experiments was designed that incorporates elemental analysis as well as crack detection. Hydrogen concentration depth profiles derived from nuclear reaction analysis were measured on A PS TBCs after pre-oxidation in dry and humid environments to clarify the role of hydrogen and possible transport mechanisms. Acoustic emission measurements and metallographic investigations were employed to investigate the micro-crack evolution.
8:20 AM		A3-2-4 Thermal Barrier Coatings Adherence and Spallation : Interfacial Indentation Resistance and Cyclic Oxidation Behaviour Under Thermal Gradient J. Sniezewski, V. Vidal, Y. Le Maoult, P. Lours (Université de Toulouse, France) Thermal barrier coatings adherence and spallation : interfacial indentation resistance and cyclic oxidation behaviour under thermal gradient Thermal barrier coatings (TBC) are complex multi-materials systems used in hot parts of gas turbines. In service, one of the most critical damage results from the spallation of the yttria stabilized zirconia (YSZ) layer, leading to a detrimental temperature increase of the underlying superalloy. The toughness of the interface between the bond coat and the top coat is the main parameter that controls the adherence of the zirconia layer. This parameter can be straightforwardly approached by loading the interface using an appropriate indentation technique and determining the critical force required to cause the delamination of the TBC. The resulting interfacial toughness is calculated for as-deposited TBC and cyclically oxidized TBC. The cyclic oxidation of TBC is performed using a dedicated equipment, recently developed in order to r eproduce as close as possible the real in-service thermal conditions of gas turbines. Specifically, the cyclic oxidation test permits to generate and control a thermal gradient through the TBC system. In addition, the equipment is instrumented with a video camera to monitor in situ the interfacial crack propagation and the resulting spallation of the TBC. An analysis of the interfacial toughness evolution as the function of the type of cyclic oxidation is proposed. Namely, the number of cycles, the oxidation temperature and the holding time at high temperature as well as the magnitude of the thermal gradient are considered and thoroughly studied. Concomitantly, the mechanisms of crack initiation and growth and the modes of spallation are investigated and critically discussed.
8:40 AM		A3-2-5 Correlation of Mechanical Properties and Electrochemical Impedance Spectroscopy Analysis of Thermal Barrier Coatings J. Gómez-García, A. Rico, C.J. Múnez, P. Poza, V. Utrilla (Universidad Rey Juan Carlos, Spain) Thermal barrier coatings (TBC) are widely used in turbine engines for propulsion and power generation where materials to withstand increasing operating temperatures, mechanical loads and chemical degradation are required. The aim of these coatings is to insulate the metallic components from the aggressive environment at high temperature increasing turbine entry gas temperature, which promotes overall engine efficiency. TBCs comprise at least two layers: a ceramic top coating and a metallic bond coat. This metallic layer increases the adhesion of the ceramic coating and provides enhanced oxidation and corrosion resistance. To this end, these coatings should be capable to develop a surface oxide layer thermodynamically stable, slow growing and adherent. During service at high temperature a thermally grown oxide layer (TGO) is formed between the metallic overlay coating and the ceramic top coat. TGO growing is the most important phenomenon controlling TBC durability. Out of plane stresses are developed at the TGO - bond coat and top coat interfaces. These stresses increase as TGO thickens promoting fracture at the interfaces and cracking within the brittle ceramic coating, modifying the top coat apparent mechanical properties. Finally, the system fails by spalling. For these reasons, it should be interesting to analyse the degradation status during service by a non destructive method. Electrochemical impedance spectroscopy (EIS) analyse the system impedance which will be modified by TGO thickening and top coat cracking. The aim of this work is to analyse by EIS two TBCs after isothermal oxidation in air and correlate the electrical parameters, which characterize the system status, with the apparent mechanical properties. The TBCs analysed were a CaZrO₃ top coat, sprayed onto an AISI 304 substrate using NiAlMo as overlay coating, and a ZrO₂(Y₂O₃) ceramic coating, plasma sprayed onto an Inconel 600 substrate using NiCrAlY as bond coat.
9:00 AM		A3-2-6 The Response of Zirconia Layers under External Thermal and Mechanical Solicitations: Microstructural Evolution and Mechanical Stability B. Benali, A.M. Huntz, M. Andrieux (University Paris Sud 11, LEMHE-ICMMO, France); F. Jomard (Groupe d’Etude de la Matière Condensée (GEMaC), France); M. Ignat (SIMAP Grenoble INP, France) The microstructural evolution and the mechanical response of zirconia coatings deposited by OMCVD on steel substrates, were analysed before and after being submitted to thermal and/or mechanical solicitations. The coatings were deposited at two different temperatures and with different partial pressures of oxygen. The as deposited zirconia films presented two phases: the monoclinic one, as the metastable tetragonal phase. With these obtained films on substrate systems, two kinds of in-situ experiments were performed: deflection under controlled atmosphere and temperature gradients and tension with and without heating. The experimental results showed a strong dependence of the mechanical stability of the films to their deposition conditions. Their microstructural evolution in terms of phase transformation, grain sizes, internal stresses was then analysed from XRD experiments. These analyses allowed defining the limits of the domains of the phases, and their amount of transformation.
9:20 AM		A3-2-7 Understanding and Modeling of Multicomponent Interdiffusion for Life Prediction and Life Extension of Thermal Barrier Coatings Y. Sohn (University of Central Florida) Solid-state multicomponent interdiffusion is a subject of great interest for its intellectual merit and practical applications in materials and coatings for high temperature applications. Along with a brief review of framework for phenomenological descriptions, this talk will survey the importance of multicomponent-multiphase interdiffusion with specific examples from superalloys and thermal barrier coatings (TBCs) used in gas turbine engines. Results and analysis from laboratory experiments, field applications and microstructural modeling are presented to highlight the cross-fertilization of science and applications. Experimental concentration profiles of individual components in experimental diffusion couples using NiAl, Ni3Al and various Fe- and Ni-base austenitic alloys and superalloys are examined to determine interdiffusion fluxes, which are then directly integrated to determine multicomonent interidiffusion coefficient. Understanding of diffusional interactions v ia quantified interdiffusion coefficients then can be employed to design and select alloys and coatings that will provide microstructural stability for durable applications at high temperature. Phase field modeling can also provide mechanistic understanding of multicomponent interdiffusion and microstructural development that can lead to life prediction models and life extension methodologies. Using available thermodynamics and mobility data, a phase field model has been utilized to simulate and predict interdiffusion behavior and microstructural evolution in multi-phase diffusion couples of Ni-Al and Ni-Cr-Al systems. Based on existing theories and observations of failure mechanisms, approaches to model critical phenomena associated with thermal barrier coating (TBC) failure, namely sintering of ytrria-stabilized zirconia (YSZ) topcoat, (t’→f+m) phase transformations in YSZ topcoat, high temperature oxidation (i.e., growth of thermally grown oxide, TGO) of bond coats, mu lticomponent-multiphase interdiffusion between bond coat and superalloy substrate, and mode-III fracture at the YSZ/TGO and TGO/bond coat interfaces are presented.
10:00 AM		A3-2-9 Comparison of the Oxidation Behavior of Beta and Gamma - Gamma Prime NiPtAl Coatings J.A. Haynes, B.A. Pint (Oak Ridge National Laboratory); Y. Zhang (Tennessee Technological University); I.G. Wright (Oak Ridge National Laboratory) A new class of gamma-gamma prime platinum diffusion coatings are receiving increasing attention for use as a bond coat in thermal barrier coating applications. This study investigated and compared the oxidation behavior of gamma-gamma prime NiPtAl coatings to the more traditional beta-phase NiPtAl bond coatings formed by chemical vapor deposition. Both types of NiPtAl coatings were fabricated on the same Ni-base superalloys (single crystal Rene N5 and CMSX-4) using the same electroplated Pt source. Coatings were exposed to identical cyclic oxidation tests at 1100 and 1150C. Compositions of selected coatings were characterized by electron microprobe analysis to evaluate distribution of Pt and Al after 1,000 oxidation cycles. Coatings and oxidation products were characterized by scanning electron microscopy. The thickness, integrity and microstructure of the alumina scales were compared as a function of temperature, coating type and superalloy composition.
10:20 AM		A3-2-10 Effect of Exposure Conditions on the Oxidation of MCrAlY-Bondcoats and Lifetime of Thermal Barrier Coatings J. Toscano, M. Subanovic, E. Wessel, D. Naumenko, L. Singheiser, J. Quadakkers (Research Center Juelich, Germany) The high temperature oxidation and corrosion resistance of TBC-coated blades in gas turbines must be guaranteed under varying conditions in power generation. The introduction of new CO₂-capture technologies implies the operation of these materials in combustion gases with higher CO₂and water vapor concentrations and possibly lower oxygen levels than in the conventional power generation technologies. Aiming to assess the oxidation behavior of the typically employed MCrAlYs under such varying conditions, different experiments were carried out in air and H₂O-containing gases at temperatures between 1000-1100°C. The long term behavior of overlay coatings and free-standing MCrAlYs with chemical compositions similar to those commonly used in gas turbine blades was investigated. The oxidation kinetics was evaluated through mass gain measurements on the latter after predetermined intervals. Additionally, the effect of MCrAlY oxidation behavior on th e lifetime of ceramic TBC coatings in the mentioned atmospheres was estimated. Subsequently, extensive characterization of the oxide scales formed and the internal oxidation encountered was performed using light microscopy, Laser Raman Spectroscopy and SEM/EDX. The results of the microstructural studies were correlated with the scale growth kinetics and the long-term TBC performance.