ICMCTF2001 Session A3-1: Thermal Barrier Coatings

Tuesday, May 1, 2001 8:30 AM in Room Royal Palm Salon 1-3

Tuesday Morning

Start	Invited?	Item
8:30 AM	Invited	A3-1-1 Failure Mechanisms and Models for TBCs A. Evans (Princeton University); J. Hutchinson (Harvard University) Thermal barrier materials are used in gas turbines to provide both thermal and oxidation protection. These materials consist of three basic layers. (i) An inner layer comprising a NiAl-based alloy is designed to resist visco-plastic flow while forming (ii) a thin thermally grown oxide (TGO, typically ?-alumina), that restricts oxygen ingress and protects the substrate from oxidation. (iii) An outer layer comprises a low thermal conductivity oxide, generally stabilized zirconia. This layer is referred to as a thermal barrier coating (TBC). It is designed to provide a temperature drop of about 100C on components subject to back-side air cooling. It must also have the strain tolerance to withstand the thermal expansion misfit with the substrate. The thermal barrier system is dynamic. The TGO thickens, the microchemistry and microstructure of the alloy layer changes as the Al is depleted, and, in some scenarios, the TBC microstructure changes. The durability of the system is linked to these changes. The major constituent governing performance is the TGO layer, which is highly stressed and which, while thin (a few microns), still represent a high energy density domain. The means whereby this energy density couples into the TBC to cause failure are described. Based on this understanding, approaches for addressing durability are discussed.
9:10 AM		A3-1-3 The Method of Nanoindentation for the Evaluation of Mechanical Properties of Electron Beam-Physical Vapor Deposition-Thermal Barrier Coatings (EB-PVD-TBCs) A. Etzkorn, E. Lugscheider, K. Bobzin (Materials Science Institute (WW), Germany) EB-PVD-zirconia coatings are well known as thermal barrier material for gas turbine applications. By using this material the gas turbine can work at higher temperatures and with it the turbine efficiency increases. Not only turbine efficiency but also reliability is a very important subject. Due to this, simulation and modeling becomes very important for life assessment and life prediction of thermal barrier coatings. Developing a model of an anisotropic EB-PVD-coating requires the knowledge of the mechanical values like Young’s modulus of the coating material. In the present paper the Young’s modulus was determined by nanoindentation on cross sections of mounted samples. The main subject of the work was to examine the influence of side issues. The main point of interest was the supporting behavior of mounting- and substrate material. Also the mounting technique was analyzed. As mounting material cold and hot hardening materials were used as well as UV hardening ones. To estimate the influence of the mounting material also a non-mounted sample was examined. The examined coatings were deposited with the EB-PVD technique on four different substrates: high-alloyed steel, aluminum, CMSX-4+MCrAlY and copper. For the evaluation of the substrate influence on the measurement results, the coatings were also detached from the substrate and examined.
9:30 AM		A3-1-4 Measurement and Prediction of Residual Stresses in NiCoCrAlY Bond Coatings on First Heating After Deposition R.O. Howells, R.I. Todd (University of Oxford, United Kingdom); J. Wigren, P. Bengtsson (Volvo Aero Corporation, Sweden) This study aims to characterise the microstructure and properties of 150 µm thick NiCoCrAlY bond coatings on Hastelloy X substrates, in order to fully understand how residual stresses arise in the coating during thermal cycling. The thermomechanical properties of NiCoCrAlY were measured and used to predict the stresses in the coating during the 1st thermal cycle to 1000°C after deposition. The predictions were then compared to the stresses measured using Stoney's method during the same thermal cycle. Microstructural characterisation by scanning electron microscope and microprobe showed that a metastable fine grained structure exists in the as-sprayed coating that develops during the 1st heating cycle to a microduplex structure based on γ-Ni₃Al and β-NiAl. Four point bending experiments showed that the as-sprayed coating and freestanding bond coat material had a Young's modulus of ~40GPa which increased significantly to ~120GPa after the 1st heating cycle. Isothermal curvature measurements and tensile testing showed that significant stress relaxation by creep was possible in NiCoCrAlY above 400°C during the heating phase due to the fine grained microstructure. Quenching stresses in the as-sprayed coatings were measured by preferentially dissolving the bond coat and the average result was found to be 94MPa ± 37 (1 s.d.). Peak, tensile, in-plane stresses of the order of +125MPa were recorded at around 200°C in the heating phase, which arise from purely elastic accommodation of the CTE mismatch between the coating and the substrate. From 400°C during heating creep reduced the stress in the coating. The stress then remained small up to 1000°C. From 500°C during cooling compressive stresses accrued in the coating resulting in a compressive stress of ~ -150MPa at the end of the 1st thermal cycle.
9:50 AM		A3-1-5 Influence of Thermal Cycling and Oxidation on the Microstructure, Properties and Residual Stresses in NiCoCrAlY Bond Coats H. Thompson, R.I. Todd (University of Oxford, United Kingdom); J. Wigren (Volvo Aero Corporation, Sweden.) Thermal stresses in the bond coat are thought to play an important role in determining the coating life of thermal barrier coating systems. These stresses can't be predicted unless the properties are known, hence the present work aims to measure the properties, predict the stresses and verify these predictions by measurement. The thermo-mechanical properties (stiffness, strength, creep rate, thermal expansion co-efficient) and microstructural attributes (phase composition, grain size and splat morphology) of free standing air-plasma sprayed NiCoCrAlY bond coats which had undergone 1 thermal cycle, from 25°C to 1000°C, were measured as a function of temperature during two subsequent thermal cycles. The results were used to predict the average in-plane residual stresses in NiCoCrAlY coatings on Hastelloy X substrates during thermal cycling. The predictions agreed well with the actual residual stresses that were measured during cycling using the Stoney method. The in-plane stresses after the first cycle were compressive and gradually decreased in magnitude during heating, until they were close to zero at 1000°C. Little hysteresis or racheting was seen during cooling and further cycling. The effect of oxidation on the bond coat properties and thermal stresses was investigated by oxidising the material at 1050°C for various times prior to testing. The effect on the results was correlated with the microstructural changes occurring.
10:30 AM		A3-1-7 Microstructural Development and Failure Characteristics of EBPVD/MCrAlY/IN738 Thermal Barrier Coatings during Thermal Cyclic Oxidation at 1121°C Y.H. Sohn (University of Connecticut "now with" University of Central Florida); J.H. Kim, E. Jordan, M. Gell (University of Connecticut) Microstructure and failure of a production TBC that consists of electron beam physical vapor deposited ZrO2-7wt.%Y2O3 (7YSZ), thermally grown oxide (TGO), MCrAlY bond coat and IN-738 superalloy substrate were examined as a function of thermal cycles at 1121°C. Thermal cycling for TBC specimens consist of 10-minute heat-up, 40-minute-hold at 1121°C and 10-minute quench, and the characterization was carried out by Cr3+ photo-stimulated luminescence piezo-spectroscopy (CPLPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Change in the interface roughness at the TGO/bond coat interface (i.e., rumpling, wrinkling, racheting) was observed after prolonged exposure to thermal cycling prior to the final TBC spallation. This roughening of the interface led to localized cracking at the 7YSZ/TGO interface after as early as 5 thermal cycles. In addition, development of microstructure in 7YSZ coatings, growth of thermally grown oxide (TGO) and its constituents, phase transformation in the MCrAlY bond coat were carefully examined and quantitatively analyzed as a function of thermal cycles. Spallation of 7YSZ coatings occurred after 400 cycles at the 7YSZ/TGO and TGO/bond coat interfaces and within the TGO. Development of microstructure and the characteristics of the TBC failure are discussed with respect to the failure mechanisms of EBPVD/MCrAlY/IN738 TBC system.
10:50 AM		A3-1-8 Numerical Analysis of the Influence of Coating Porosity and Gradient Profile on the Residual Stresses in High Temperature Graded Coatings V. Teixeira, J Costa, J Martins (University of Minho, Portugal) Thermal processing (e.g. thermal cycling) are currently present in the coating technology. Because of the different thermal expansion and temperature gradients a thermal stress develops in the ceramic-metal structure which can cause delamination at the interface or fracture of the material. The interfacial stresses can be reduced by replacing the sharp interface with an intermediate composite layer within which the structure and mechanical properties are smoothly varied from the ceramic to the metal material. This system is known as a Functionally Gradient Material (FGM) and has many of technological applications such as the functionally graded thermal barrier coatings for use in aero and land based turbine components. In this contribution we present a numerical calculation of residual stress distribution within a multilayer system which consists in a FGM. The structure of the graded system is made out of a ceramic layer and a metallic layer, where between them there is an interlayer which is a graded composite made of a metal-ceramic composite. For numerical modelling of this graded interlayer, an approximation was done as a series of perfectly bonded finite layers, each having slightly different material properties. We present results on the effects on residual stress distribution of the elastic parameters, coating porosity and graded interlayer profile. A thermal stress analysis model enable us to calculate the residual stress, assuming an elastic biaxial model, for different thicknesses of the graded interlayer and for different compositional profiles, and thus, allowing a systematic study of the influence of this parameters on the FGM stress distribution. This modelling gives a potential tool for FGM stress optimization to improve the thermal and structural stability of multilayer graded coatings. The FGM model consists in a Ni-alloy coated with a gradient interlayer based on NiCr and a stabilised zirconia coating which could be used as TBC.
11:10 AM		A3-1-9 On Failure Mechanisms of TBCes with PtNiAl Diffusion Bond Coats I. Spitsberg (General Electric Aicraft Engines) There are many factors involved in the degradation of alumina scales growing between the bond coat and ceramic top coat resulting in the TBC coating failure. Among these factors are chemical composition of the substrate alloy and bond coat, surface topography, phase composition of the thermally grown oxide scale, bond coat grain structure, segregation of sulfur and yttrium to the bond coat/oxide interface, and others. While most of these factors act concurrently, the challenge is to recognize those which are most critical to the performance of a particular alloy/bond coat system and/or cycle profile, and to guide development effort to address those factors.@ @This paper describes major phenomena associated with failure of TBC on PtNiAl bond coat: rumpling of the bond coat/ceramic top coat interface, degradation of the oxide scale adherence, and degradation of the oxide/ceramic interface. This work will summarize observations of TBC failure from PtNiAl diffusion bond coats and review possible contribution of different factors in coating failure. Effect of bond coating processing and surface texture on the failure mode is examined. Possible approaches to control the coating failure mechanism are evaluated.@.
11:30 AM		A3-1-10 Stochastic Features of Microcracking Mechanism in Thermal Barrier During Thermal Cycling M. Lugovy, V Slyunyaev (National Academy of Sciences of Ukraine); V. Teixeira (Universidade do Minho, Portugal) A model of failure is presented which can be applied to simulate cracking of thermal barrier coating during thermal cycling. The authors try to solve a physical problem of the description of failure of a microinhomogeneous solid as stochastic process of the cracking of separate structural elements. The particular modeling corresponds to single-phase ceramic-based coating under tension at high temperature. The above mentioned model is applied for the description of mechanical behaviour of single-phase ceramic-based coating with various statistical distributions of the grains sizes. The proposed model allows the relaxation process of tensile stresses in thermal barrier coatings at high temperature to be calculated.