Papers

ICMCTF2008 Session A3-3: Thermal Barrier Coatings

Thursday, May 1, 2008 8:00 AM in Room Sunrise

Thursday Morning

Start	Invited?	Item
8:00 AM		A3-3-1 Influence of Reducing Atmosphere on the Luminescence of Eu-doped 7YSZ Sensor Layers in the Thermal Barrier Coatings Y. Shen, M.D. Chambers, D.R. Clarke (University of California, Santa Barbara) The influence of reducing atmospheres on the luminescence of Eu-doped 7YSZ sensor layer in a thermal barrier coating fabricated by electron-beam deposition has been investigated. The luminescence spectra and lifetimes were recorded from the coatings using both 248 nm and 532 nm excitation after annealing at 1100°C under different oxygen partial pressures. Complementary diffuse reflectance measurements were also made in order to assess the effect of the reducing treatments on the optical absorption. The measurements indicate that annealing even in a reducing atmosphere with oxygen partial pressures of less than 10-15 ppm has no adverse affect on the luminescence from the Eu-doped YSZ sensors measured at room temperature although it causes a slight darkening of the coating. These measurements indicate that Eu-doped 7YSZ sensors are insensitive to any reducing atmospheres likely to be encountered in gas turbines and can be used with confidence for temperature monitoring as well as erosion and wear sensing.
8:20 AM		A3-3-2 Luminescence Temperature Sensor Systems for Environmental Barrier Coatings M.D. Chambers (University of California, Santa Barbara, United States); P.A. Rousseve, D.R. Clarke (University of California, Santa Barbara) With the ongoing development of Environmental Barrier Coatings (EBC) technology for use at high temperatures in the presence of water vapor there is a growing need for accurate temperature measurements in EBC systems. Luminescence thermometry presents a non-contact, fast technique for measuring temperature precisely at the surface as well as at depth, and can often be applied simply by doping materials already present in EBC systems. This contribution presents a demonstration of the thermometric capabilities, based on luminescent lifetime, of three prospective EBC materials, Y₂SiO₅, Y₂Si₂O₇, and Ca₂Gd₈Si₆O₂₆, each doped with Eu, Dy, or Tb. Their respective advantages are assessed both as stand-alone sensor materials and as part of a multi-layer system for measuring thermal gradients across an EBC coating. The lifetime-temperature behavior of Ca₂Gd₈Si₆O₂₆ is further considered with respect to its at-temperature Raman spectrum, and a demonstration of measurement of a thermal gradient is made by the characterization of the thermal conductivity of these materials.
8:40 AM		A3-3-3 On the Determination of the Mechanical Properties of Aluminide Bondcoat Materials by High Temperature Instrumented Microindentation A. Villemiane, B. Passilly, P. Kanoute (Onera, France); R. Kouitat (ENSMN / INPL, France); R. Mevrel (ONERA, France) The knowledge of the mechanical behaviour of bondcoats is necessary to describe the stress field present in thermal barrier systems. Beta-(Ni,Pt)Al alloys, in bulk form and as bondcoats, have been tested with a high temperature instrumented microindentation technique which can operate up to 1000°C¹. The technique is original and enables to investigate a large number of compositions and conditions within a reasonable time. From the information obtained in these experiments, the mechanical behaviour of these materials is identified, as a function of the metallurgical evolution with temperature of bondcoats alloys, by an inverse method based on the finite element simulation of the test. The results are discussed in relation to the mechanical properties obtained by other techniques^2,3. ¹B. Passilly, P. Kanouté, F.-H. Leroy, R. Mévrel, Phil. Mag. 86 (33-35), 5739-5752 (2006). ² D. Pan, M.W. Chen, P.K. Wright, K.J. Hemker, Acta mater. 51, 2205-2217 (2003). ³M.P. Taylor, H.E. Evans, E.P. Busso, Z.Q. Qian, Acta Mater. 54, 3241-3252 (2006).
9:00 AM		A3-3-4 A Numerical Model of Time Dependent Processes Influence on Geometry and Microstructural Features in Thermal Barrier Coating Systems J. Frachon, J. Besson, V. Maurel, E. Busso (Ecole Nationale Supérieure des Mines de Paris - Paristech, France) A finite-element-based mechanistic study of stress development within a Thermal Barrier Coating system consisting of an Electron Beam Physical Vapour Deposition topcoat has been undertaken. The objective is to propose a global model incorporating the complex interaction between interfacial geometry and microstructural features and time-dependent processes. The role of material properties has been taken into account, especially the typical columnar structure of the ceramic topcoat. A new model is proposed for the anisotropic sintering of Yttrium stabilised Zirconia and the stiffening of the material due to sintering. Using homogenization approach, the ageing of the bond coat is investigated for the elastic-visco-plastic mechanical properties. The oxidation kinetic of the thermal grown oxide, given by experimental data, is implemented in the global model. A carefully look has been taken to determine the stress field closed to the moving TGO/YSZ and TGO/BC interfaces. Actually cracks nucleation appears on this critical zone of the TBC. Anisotropic volumetric strain associated with the growth of the TGO at the BC/YSZ interface has been incorporated in finite strain in the FE model and the ceramic-metal interface morphology has been modelled in 3D. A primary study has been undergone to show the influence of each mechanism (sintering of the ceramic, thermal gradient field in the TBC, ageing of BC, roughness of the ceramic-metal interface) on stress fields on the TBC. A secondary study deals with FE convergence closed to the ceramic-metal interface and the necessity of the 3D approach. A TBC lifetime prediction methodology will be proposed using a probabilistic approach.
9:20 AM		A3-3-5 A Mesh-Independed Finite Element Crack Analysis for TBC Systems T.S. Hille (Netherlands Institute for Metals Research, Netherlands); S.R. Turteltaub, A.S.J. Suiker (Delft University of Technology, Netherlands) Research on TBC systems has identified various mechanisms that lead to their failure¹. Common to all mechanisms is the spallation of parts of the zirconia top coating. This is the case when microcracks in the top coating and at the thermally grown oxide (TGO) layer have merged to form macrocracks. Therefore the present contribution focuses on fracture modeling of TBC systems. In finite element analyses cracks are described as discontinuities in the displacement field. At present, the partition of unity method is regarded as the most powerful approach to model the crack kinematics independendly from the underlying mesh. This concept is combined with a cohesive zone approach to describe the traction separation response at the crack faces. Conceptually, fracture processes are admitted anywhere in the modeled domain. The emerging crack patterns are therefore a pure result of the local stress fields in combination with the fracture strength and toughness properties of the coating components. A robust method to solve this problem² will be presented. The simulated TBC system is composed of three domains: The top coating, the TGO scale and the bond coating. The columnar structure of EB-PVD deposited top coatings is modeled by a transversely isotropic formulation. The growth of the TGO layer is simulated by a diffusion driven growth process. An elasto-plastic model with exponentially saturating hardening is ascribed to capture the ductile material response of the BC. The sensitivity of the crack patterns to the constitutive behavior of the TBC components is analyzed and a comparative study of typical crack patterns emerging at different stages during the growth of the TGO layer is presented. ¹ A.G. Evans, D.R. Mumm, J.W. Hutchison, G.W. Meier and F.S. Pettit, Mechanisms controlling the durability of thermal barrier coatings, Prog. Mater. Sci., 46, 505-553, 2001. ²T.S. Hille, A.S.J. Suiker and S.R. Turteltaub, Initially-rigid crack initiation in thermal barrier coating systems, Eng. Frac. Mech., submitted for publication.
10:00 AM		A3-3-7 Characterization of Thermal Barrier Coatings Using Diffuse Optical Reflectance A. Limarga, D.R. Clarke (University of California, Santa Barbara) Yttria-stabilized zirconia thermal barrier coatings, deposited by both EB and plasma-spraying, appear white to the eye because they scatter incident light. More precisely, porosity, cracks and gaps in the coatings scatter light at all the wavelengths that the eye is sensitive to. The intensity of scattered light can be quantified by measurements of the diffuse optical reflectance suggesting that diffuse reflectance may be a viable, non-destructive and non-contact method of characterizing thermal barrier coatings and their evolution under service conditions. In this contribution we describe the method, interpretation of the reflectance spectra and how it can be employed to characterize coatings as a function of thermal cycling and sintering.
10:20 AM		A3-3-8 Luminescence Spectroscopy for Stress Measurement and Lifetime Prediction: a Computer Simulation Approach A. Luz, D. Balint, K. Nikbin (Imperial College London, United Kingdom) Thermal Barrier Coatings have been used for almost three decades for heat insulation in high-temperature components to increase efficiency. Reliable diagnostic techniques that are practical to implement are needed to identify the location and severity of degradation in TBCs to protect against premature TBC failure. Photo luminescence piezo spectroscopy has been utilized in detecting early damage, as it exhibits monotonic changes in the spectral characteristics with damage. But so far information based only on the mean equivalent residual stress (i.e. the luminescence peak shift) has been used for this purpose. However, this is not straightforward because the stress changes can have a variety of causes. Other spectral parameters should provide extra information. In this work a finite element model has been developed to analyze the influence of several non-linear mechanisms in the TBC lifetime, like beta to gamma prime transformation, martensitic transformation, oxide growth and sintering of the top coat. To simulate damage accumulation and delamination of the YSZ/TGO and/or TGO/BC interfaces the finite element model has been used to run 100 thermal cycles during which nucleation, propagation and coalescence of cracks is incorporated. A variety of cracking scenarios have been studied. The stress field of the oxide layer determined by the finite element model is subsequently converted into luminescence spectrum. Each stress level is represented by a Lorentzian shape function and by adding all the different stresses within the probed volume, the whole spectrum is obtained. The predicted luminescence spectrum for several cracks was used to identify characteristic spectral features that can be employed to predict TBC lifetime.
10:40 AM		A3-3-9 Influence of Bondcoat Surface Finish on the Residual Stresses Generated in he Thermally Grown Oxide for an EB-PVD TBC, Deposited on PtAl and Pt-Diffused Bondcoats L. Chiviri (Cranfield University, United Kingdom); X. Wang, A. Atkinson (Imperial College London, United Kingdom); J. Nicholls (Cranfield University, United Kingdom) Thermal barrier coatings (TBCs) are well established as protective systems for gas turbine hot path components, due to their ability, with substrate cooling, to reduce the maximum surface temperature experienced by the metal component. In this study, the influence of substrate surface roughness on the stress, and distribution of stress, generated in the TGO has been studied for two EB-PVD TBC systems; a zirconia 8wt% yttria topcoat on a platinum aluminide bondcoat and a zirconia 8wt% yttria topcoat on a platinum diffused, +’, bondcoat. The stress distributions were mapped using luminescence piezo-spectroscopy, for the as-manufactured coating and after 20h cyclic oxidation at 1150°C. Various surface finishes have been investigated, two uni-directional-one course and one fine - and two grit blasted - one low impact energy and the second high impact energy. The methodology, measurement of surface roughness and the evaluation and evolution of stress distributions with oxide time will be discussed in this paper. It is shown that the residual stresses in the TGO are greater for the platinum diffused, +’, bondcoat than for the platinum aluminide bondcoat. That stresses are greater for the fine finished surface than course surface. The course surface finish has a bimodal distribution of residual stress, while the fine surface finish appears mono-modal, with the stress mapping reflecting the interfacial morphology.
11:00 AM		A3-3-10 Revisiting the Structural Characterization of 8YSZ Under Isothermal Aging Y. Gao, D. Lipkin, C. Johnson (GE Global Research); W. Nelson (GE Energy); L.F. Bonetti (INPE/MCT-Instituto Nacional de Pesquisas Espaciais, Brazil); C.G. Levi, R. Leckie (University of California, Santa Barbara) Stabilized zirconia has found broad applications for its superb thermal barrier, structural, and electronic properties. Over the last four decades, numerous studies have sought to elucidate the detailed structure of zirconia's metastable and equilibrium allotropes. We will review key features of diffraction-based structural refinement of the yttria-stabilized zirconia (YSZ) system, focusing on the evolution occurring upon thermal aging of the metastable tetragonal phase, t'-YSZ. Recent data on the diffusion-driven decomposition of t'-YSZ reveals new insights as well as new challenges in the analysis and interpretation of the structural evolution in this system.