ICMCTF2005 Session A3-3: Thermal Barrier Coatings

Wednesday, May 4, 2005 8:30 AM in Room Sunrise

Wednesday Morning

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8:30 AM A3-3-1 Characterisation of Thermal Barrier Coatings using Impedance Spectroscopy
P. Xiao (University of Manchester, United Kingdom)
Impedance spectroscopy has been used as a non-destructive tool for evaluation of TBCs degradation. Both plasma spray and EBPVD TBCs have been studied in this work. The first part of this work is to summarise our study on characterisation of TGO growth, TGO continuity and TGO composition using impedance spectroscopy. Meanwhile, crack formation along TGO/YSZ interface has been monitored. The second part of this work is to examine sintering and phase transformation of YSZ top coatings using impedance spectroscopy. The third part of this work is to examine how electrode size and electrode position affect impedance spectra of TBCs. Finally, impedance spectroscopy will be used to study TBCs with special microstructural features, e.g with a controlled porosity of YSZ and YSZ/TGO interface. Meanwhile various electrical responses relating to electrode size, thickness and continuity and purity of TGO, YSZ coating thickness, porosity, phase composition and crack propagation will be explained using equivalent circuit modelling.
9:10 AM A3-3-3 Defect Evolution in Thermal Barrier Coating Systems Under Multi-Axial Thermomechanical Loading
B. Baufeld, B. Bartsch (German Aerospace Center, Germany); S. Dalkilic (Anadolu University, Turkey); M. Heinzelmann (University of Applied Science, Rheinbach, Germany)

In service, gas turbine components with thermal barrier coatings experience high cyclic mechanical and thermal loading. Important, but not yet considered sufficiently, are the multi-axial stresses arising from thermal gradients.

Multi-axial stresses were simulated in laboratory experiments using a specially designed test rig. Cyclic thermomechanical fatigue experiments with a radial thermal gradient (TGMF) were performed on tubular specimens consisting of a directionally densified super alloy substrate, a NiCoCrAlY bond coat, and an EB-PVD ceramic thermal barrier coating (TBC). The test set-up enables surface temperatures of 1000°C, thermal gradients over the whole wall thickness of the specimen of about 200 K, and heating and cooling between 100 and 1000°C within 20 s. For comparison, on the same material system isothermal low cycle fatigue tests (LCF) were carried out at 950°C.

The resultant defect evolution in TGMF tests was substantially different to that in LCF tests or in conventional thermal mechanical fatigue tests, the latter reported in literature. Typical in TGMF were cracks parallel to the bond coat/TBC interface, located within the bond coat and close to the interface. Weakening thus this interface, these defects implicate a connection with early TBC spallation. LCF tests, on the other hand, resulted in oxidized cracks within the bond coat, propagating into the substrate perpendicular to the interface, but no spallation.

Finite element analyses of the transient stress distribution during testing have demonstrated the importance of thermal gradients for the different defect evolution in TGMF and LCF tests.

9:30 AM A3-3-4 Damage Evolution In Plasma-Sprayed TBCs Under Predominant Oxidation Loading
R. Herzog, E. Trunova, P. Bednarz, W.J. Quadakkers, F. Schubert, L. Singheiser (Research Centre Jülich, Germany)
Cyclic or isothermal furnace tests on plasma-sprayed thermal barrier coatings lead generally to a failure mode which is sometimes called black-and-white failure. The black and white appearance after spallation of macroscopic parts of the ceramic top coat is due to delamination cracks which are located partly in the TGO and partly in the TBC. This final appearance of damage is the result of a continuous damage evolution, which starts in the first 10% of life. This presentation summarizes results from interrupted cyclic and isothermal oxidation tests with regard to crack formation, crack growth, crack linking, statistical and kinetic aspects. The crack growth vs. time curves always allowed a separation into two stages. The first stage was characterized by discrete micro cracks in the TGO distributed along the metal/ceramic interface. The length of TGO cracks remained below 50µm during the first stage. The second stage of damage evolution was characterized by crack linking and further crack growth up to macroscopic values. The crack length data allowed to interpret the first stage as an damage incubation phase with small and restricted crack length values and the second stage as actual crack growth phase. The end of the incubation phase coincides with a TGO thickness between 6 and 9µm. FE simulations support the view that during the incubation phase alternating local compressive stress regions restrict the crack length to values below one roughness wavelength maintaining the macroscopic mechanical integrity. Above a certain TGO thickness, stress redistributions, which are related to e.g. TGO growth and visco-plastic coating properties, lead to a long range and continuous zone of out-of-plane tensile stresses during cooling. The formation of a long range tensile stress zone along the interface is regarded as the formation of a mixed TGO/TBC crack path close to the interface, along which crack linking and macroscopic delamination can occur.
9:50 AM A3-3-5 Rumpling of a PtAl Bond Coat: Comparison Between Experiment and Simulation
A.W. Davis (Princeton University); A.G. Evans (University of California, Santa Barbara)
Codes that simulate oxide elongation and distortion around surface imperfections in certain alloys have been developed. For their application to the rumpling of PtAl coatings under cyclic oxidation, these codes must include such factors as oxide thickening and lengthening, bond coat phase transformations, bond coat swelling, and temperature dependent strengths: requiring many input parameters. Some of these can be independently measured. In the present study, cyclic oxidation experiments were conducted and the resulting distortions around intentionally introduced imperfections were recorded. Comparison between these results and a series of simulations validates the use of the codes and facilitates the determination of unknown parameters.
10:10 AM A3-3-6 Observations and Analyses of Failure Mechanisms in Thermal Barrier Systems with Two Phase Bond Coats
S. Faulhaber, C. Mercer, T. Xu, A.G. Evans (University of California, Santa Barbara)
An investigation into the failure mechanisms operative in a thermal barrier system with a NiCoCrAlY bond coat has been conducted by examining test specimens subjected to thermal cycling in a burner rig. Earlier findings (substantiated by observations made on an actual turbine blade) show that the dominant delamination extends primarily along the interface between the thermally grown oxide (TGO) and the bond coat. The morphology and microstructure of the TGO have been characterized (emphasizing heterogeneities), as well as the TGO thickness at failure. Calculations of the delamination energy release rate, upon comparison with the interface toughness, reveal a critical TGO thickness comparable to that found experimentally. Studies of the interface toughness are described. Analyses of the principal stresses and of the corresponding energy release rates show that, for the delamination to propagate, vertical separations are needed in the thermal barrier coating. An investigation of burner rig specimens at different life fractions has been used to characterize these separations and their mode of propagation through the TGO.
10:30 AM A3-3-7 Effects of Phase Constituents/Microstructure of Thermally Grown Oxide on the Thermal Cycling Lifetime and Failure of EB-PVD Thermal Barrier Coatings with MCrAlY Bond Coats
J. Liu, J.W. Byeon, Y.H. Sohn (University of Central Florida)
A correlation between thermal cycling lifetime, bond coat surface preparation, phase constituents and microstructure of thermally grown oxide (TGO) was examined for electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) consisting of ZrO2-7wt.%Y2O3 (YSZ) ceramic top coat, NiCoCrAlY bond coats and CMSX-4 superalloy. Variation in the bond coat surface in this study is characterized based on surface roughness modification and pre-oxidation (1100°C at PO2 of 10-8 atm up to 4 hours) carried out prior to YSZ deposition by EB-PVD. TBC specimens with pre-oxidized bond coat exhibited longer lifetime than those without pre-oxidation, especially for metallographically-polished bond coat. For NiCoCrAlY bond coats with the same surface roughness, TBC lifetime was observed to increase with an increase in the amount of Al2O3, observed by photostimulated luminescence (PL) during the initial formation of the thermally grown oxide (TGO) scale. Using focused ion beam in-situ lift out technique, site-specific preparation of thermally cycled TBC specimens for transmission electron microscopy (TEM) was successfully carried out. Phase identification based on electron diffraction and imaging based on bright, dark, and high angle annular dark fields were carried out to provide detailed microstructural understanding for high temperature oxidation of NiCoCrAlY bond coats in TBCs.
10:50 AM A3-3-8 High Temperature Aging of YSZ Coatings and Subsequent Transformation at Low Temperature
V. Lughi, D.R. Clarke (University of California Santa Barbara)
As-deposited thermal barrier coatings (TBC) made of yttria-stabilized zirconia (7YSZ) have the metastable, tetragonal-prime crystal structure. Even though this is not the stable, equilibrium form of 7YSZ, this crystallographic structure is remarkably resistant to transformation to the equilibrium mixture of cubic and monoclinic phases over thousands of hours at typical TBC use temperatures. Nevertheless, with high-temperature aging, Y3+ ions diffuse to their equilibrium positions and the structure evolves. This evolution can be followed by Raman spectroscopy and forms the basis for a new method of monitoring the aging of TBCs that can, in principle, be implemented using optical probes within an engine. One of the unexpected findings of this characterization is that transformation of aged, electron-beam deposited, coatings can occur to the monoclinic phase after prolonged times at room temperature. The observations and the kinetics of this isothermal transformation will be described.
11:10 AM A3-3-9 Monitoring Damage Evolution in Thermal Barrier Coatings with Thermal Wave Imaging
B. Franke, Y.H. Sohn (University of Central Florida); X. Chen, J.R. Price (Solar Turbines Incorporated)
Development of a robust non-destructive evaluation (NDE) technique is essential for quality control, life assessment and health monitoring of thermal barrier coatings (TBCs) for application, maintenance and prevention of catastrophic failure. In this study, thermal wave imaging (TWI) was employed as a NDE technique to examine as-coated TBCs with varying thickness, and thermally-cycled TBCs for initiation and progression of subcritical-subsurface damage as a function of thermal cycling. TBC specimens examined consisted of air plasma sprayed ZrO2-7wt.%Y2O3, NiCoCrAlY and Haynes 230 superalloy. Thermal cycling was carried out in air with 30-minute heat-up, 10-hour dwell at 1150°C, 30-minute air-quench, and 1-hour hold at room temperature. For every 10 to 20 cycles, thermal wave images were collected, and correlated to the mirostructural characteristics and damage progression of TBCs based on phenomenological expressions of thermal diffusion.
11:30 AM A3-3-10 Microstructure Degradation of TBCs under Molten CMAS
S. Kraemer, J. Yang, C.G. Levi (University of California, Santa Barbara)
Molten silicates (CMAS), which originate mainly from airborne dirt like dust or sand, are increasingly recognized as a serious threat to the durability of thermal barrier coatings (TBC). Investigations on actual turbine blades show that CMAS not only impregnates but also heavily degrades the TBC columnar structure by dissolving the zirconia in the melt and reprecipitating globular structures upon cooling. This paper discusses the chemical de-stabilization of TBC systems by CMAS and explores methods to mitigate its impregnation. Model CMAS deposits with defined composition of the main constituents CaO, MgO, Al2O3 and SiO2 were generated and deposited on 7YSZ TBC which had been prepared by EB-PVD on polycrystalline alumina substrates. The samples were heated for 4h hours at prescribed temperatures between 1200 and 1300C. The CMAS deposit impregnated the TBC extensively, already slightly above its melting temperature. In near-surface regions globular structures developed with a low YO1.5 content (3 mol%) showing martensitic crystal structure. The penetrated CMAS reacted in a complex way with the alumina substrate forming a several micrometer thick layer which contains amorphous CMAS with reduced alumina content, a crystalline phase (presumably anorthite) and globules of zirconia containing 13 mol% YO1.5. The observed variations in the yttria content of the reprecipitated zirconia will be discussed. CMAS was also deposited on TBC systems which had been pre-impregnated with various materials like precursor-derived alumina with the aim to mitigate the impregnation of the TBC columnar structure by CMAS. Preliminary results show that CMAS reacted extensively with the porous alumina and that the chemical degradation of the TBC columns was drastically reduced compared to the pure TBC system. Effects of chemical reactions on the impregnation behavior of CMAS will be discussed.
11:50 AM A3-3-11 Thermal Conductivity Imaging of Thermal Barrier Coatings
X. Zheng, D.G. Cahill (University of Illinois at Urbana-Champaign); J.-C. Zhao (General Electric Global Research)
In thermal barrier coating systems, the microstructure of the ceramic top coating varies with depth. We use time-domain thermoreflectance to measure the depth profile of thermal conductivity for heat flow along both the normal and in-plane directions of an electron beam-physical vapor deposited yttria stabilized zirconia (YSZ) coating. The penetration depth of the thermal waves is about 100 nm and is much smaller than the 4 µm diameter of the laser spot; the lateral resolution of the measurement is about 3 µm. We find the thermal conductivity of the YSZ grains is nealy isotropic (1.84 ± 0.08 W m-1 K-1 in the in-plane direction and 1.7 ± 0.25 W m-1 K-1 in the normal direction) and uniform throughout the entire coating. After 500 engine cycles, the thermal conductivity in the in-plane direction increases to 2.2 ± 0.2 W m-1 K-1.
12:10 PM A3-3-12 Non-destructive Thermal Barrier Coating (TBC) Damage Assessment using Laser-Induced Luminescence and Infrared Radiometry
B. Heeg (MetroLaser, Inc.); D.R. Clarke (University of California Santa Barbara)
We have recently developed a new NDE technique for assessing damage in thermal barrier coating (TBC) systems based on the photo-thermal effect. The method is based on the local heating of the thermally grown oxide (TGO), through the TBC, by a laser pulse and the mid-infrared detection of the subsequent decay in the temperature of the TGO as heat is conducted away from the TGO into the adjacent alloy and TBC coating. Damage decreases the rate at which the laser-induced heating decays away and can be quantified from measurements of the transient temperature changes. The success of the technique is based on selecting a laser wavelength to which the TBC is transparent but the TGO absorbs strongly so that the energy of the laser is converted into heat within the TGO. This method has thus subsurface selectivity to the interfaces of primary interest in determining TBC life. The conditions under which this is favorable will be described together with the optical arrangement used for implementing this technique, as well as combining it with TGO assessment using simultaneous photostimulated luminescence piezospectroscopy. Data will be presented from a series of TBC coatings thermally cycled to different fractions of life, demonstrating that the technique can be used for assessing TBC damage. This work was supported by the Air Force SBIR program at MetroLaser, Inc.
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