ICMCTF2003 Session A3-1: Thermal Barrier Coatings
Tuesday, April 29, 2003 8:30 AM in Room Sunset
A3-1-1 Importance of TBC's for Industrial Gas Turbines and Challenges Ahead
M. Oechsner (Siemens Power Generation, Germany); J. Goedjen (Siemens Westinghouse Power Corporation); W. Stamm (Siemens Power Generation, Germany); R. Subramanian (Siemens Westinghouse Power Corporation)
Gas turbine engine manufacturer for power generation systems are challenged to improve performance, efficiency, and emissions of their products by simultaneously keeping the reliability, availability as well as maintainability (RAM) at highest levels. The increasing demand for higher firing temperatures and for reduced coolant mass flow resulting from the performance, efficiency, and emission objectives require the application of thermal barrier coatings (TBC) on components in the hot gas path of advanced gas turbine engines. In order to satisfy RAM and thus, as a prerequisite for the application of a TBC system, gas turbine designers depend on the reliability and predictability of performance of the applied coatings. This confidence comes to a great extent through the use of advanced life prediction methodologies. The time / temperature kinetics of bond coat degradation in service, such as oxidation and bond coat depletion, is well understood and predictable. The push to higher firing temperatures and reduced cooling flows emphasizes TBC degradation, primarily due to the effects of sintering and phase transformation. The kinetic processes and associated failure modes of the above bond coat and TBC degrading mechanisms become important for coating systems in advanced industrial gas turbines. The paper will highlight the challenges for thermal barrier coating systems regarding advanced industrial gas turbine engines, will focus on the analysis of those bond coat and TBC factors and discuss the methods required to assess TBC reliability.
A3-1-3 On the Effect of Ageing on the Erosion of EB-PVD TBCs
R.G. Wellman, J.R. Nicholls (Cranfield University, United Kingdom)
Thermal barrier coatings have been used in gas turbine engines for four decades, and the erosion resistance of these coatings has been well documented over the years by various different groups. However, as far as can be ascertained all of the laboratory research on erosion resistance of the coatings has been conducted on the coatings in the as received condition. The effect of service conditions on the erosion rate of the coatings needs to be determined. This paper looks at the effects of thermal aging on the erosion rate of electron beam physical vapour deposited thermal barrier coatings (EB-PVD TBCs) by testing the erosion resistance of coatings that have received various different heat treatments and comparing them to those in the as received condition. Initially two different aging heat treatments were used 1500°C (sintering) for 24hrs and 1100°C (ageing) for 100hrs. It was found that both of the heat treatments resulted in a significant increase in the erosion rates when compared to the as received samples. In the sintered samples the increase in the erosion rate was attributed to the fact that the columns were partially sintering together which enabled cracks to propagate into neighbouring columns, as opposed to the column boundaries inhibiting crack propagation, as occurs in samples that have not been heat treated. This results in an increase in erosion rate due to the fact that more material is removed per impact event.
A3-1-4 Effects of Sintering on the Thermophysical Properties of ZircoatTM Vertically Macrocracked Thermal Barrier Coating
A. Bolcavage (Praxair Surface Technologies); R. Subramanian (Siemens Westinghouse Power Corporation)
Vertically segmented thermal barrier coatings (TBCs), whether deposited by electron beam PVD or by air plasma spray, owe their extended lifetimes under thermo-mechanical cycling to their strain compliant microstructures. Due to this longer service life, sintering of the key microstructural features in the yttria-stabilized zirconia ceramic becomes a concern, especially as operating temperatures in the hot sections of industrial gas turbine (IGT) engines become higher. Among other detrimental effects, the thermal conductivity of the TBC system increases, providing a lesser amount of thermal protection to the underlying metallic components. In this study, free-standing samples of Zircoat, a dense, vertically macrocracked air plasma-sprayed TBC, are sintered at temperatures up to 1400°C. The effect of prolonged high temperature exposure on the thermophysical properties of the Zircoat specimens is then assessed using the laser flash technique to determine thermal diffusivity. From this measurement, the thermal conductivity is then calculated. The resulting values are then correlated to changes in the TBC microstructure at each exposure temperature.
A3-1-5 Microstructure of As-coated Thermal Barrier Coatings with Varying Lifetimes
A.J. Burns, R. Subramanian (Siemens Westinghouse Power Corporation); B.W. Kempshall, Y.H. Sohn (The University of Central Florida)
Phase constituents and microstructure of thermally grown oxide (TGO) in as-coated thermal barrier coatings were examined by high resolution scanning transmission electron microscopy (HR-STEM) as a function of processing-dependent lifetime. As-coated TBCs examined in this investigation was selected based on a statistically-significant lifetime variation (i.e., infant failure to long-lasting) during furnace thermal cycling test among several groups of TBCs. The lifetime variation can be specifically related to the processing parameters of bond coat and/or YSZ. Site specific specimen preparation was carried out by in-situ liftout focused ion beam (FIB) technique. Differences in phase constituents and microstructure of TGO were observed with emphasis on the presence of embedded oxide, interfacial roughness, constituents and thickness of mixed oxide layer and continuous oxide layer.
A3-1-7 Diffusional Interactions in Layered TBC Systems Based on Gd2Zr2O7
R.M. Leckie, A.S. Gandhi, C.G. Levi (University of California); S. Krämer, M. Rühle (Consultant)
Pyrochlore zirconates (e.g. Gd2Zr2O7) have emerged as potential next generation EB-PVD TBC materials due to their low thermal conductivity and potentially higher sintering resistance as compared with 7-wt% YSZ. However, Gd2Zr2O7 itself is not thermodynamically stable in contact with Al2O3. This presentation will discuss the interactions between the gadolinium zirconate TBC and the underlying thermally grown oxide (TGO). This interaction involves interphase formation and physical restructuring of the interface. Multilayered architectures involving an intermediate YSZ layer between the Gd2Zr2O7 and the TGO have been proposed. The stability of such structures will be discussed. Key issues are the diffusion of cations between the YSZ and the Gd2Zr2O7 layers which can degrade the phase stability of the system, and the possibility of gadolinium transport down the columns to react with the TGO.
A3-1-8 Environmental and Thermal Protection of Gamma Titanium Aluminides
C. Leyens, R. Braun (DLR-German Aerospace Center, Germany)
The current development of new generation gamma titanium aluminides is expected to result in alloy chemistries and microstructures capable of resisting temperatures well in excess of 850°C. Under these conditions, environmental protection becomes a concern since oxidation and wear/erosion might eventually limit the maximum service temperatures achievable. Furthermore, thermal protection might become of interest, especially for aeroengine applications. Therefore, protective coatings must be available in order to fully use the high temperature capabilities of the materials. After a short introduction to latest development of oxidation resistant coatings, EB-PVD thermal barrier coatings applied to gamma titanium aluminides with and without a bond coat will be addressed. Unlike nickel-base alloys, gamma titanium aluminides are prone to microstructure changes and heavy oxidation in the temperature range where typically EB-PVD TBCs are fabricated. Therefore TBCs deposited at different temperatures and thus having different morphologies were included in this study. The TBCs survived at least 1000 h at 850 and 900°C without failure before the tests were terminated. Microstructure analysis will emphasize the outstanding performance of the coatings. As common for TBCs on nickel-base alloys and coatings, failure of the ceramic top coating by spallation was closely related to failure of the thermally grown oxide (TGO) layer, either within the layer itself or at the TGO/metal interface.
A3-1-9 Hot Corrosion Behavior of EB-PVD Thermal Barrier Coatings with Thin Al2O3 Layer Coated on Surface
D. Zhang, Z. Wu, S. Gong, H. Xu (Beihang University, PR China)
It is well recognized that an unexpected failure would occur when thermal barrier coatings were exposed to the corrosion atmosphere such as Na2SO4 and V2O5. In the present study, two layered structure TBCs with YSZ top coat of about 150µm thickness and NiCoCrAlY bond coat of 60µm have been prepared by means of electron beam physical vapor deposition (EB-PVD), and a thin Al2O3 layer was also prepared onto the top coat by EB-PVD with different thicknesses. The hot corrosion test was carried out in such a way that the samples were put in Na2SO4 and V2O5 gas atmosphere and cycled from 1223K to room temperature by air forced cooling after being kept at 1223K for 30min. The Na2SO4 and V2O5 gases were evaporated from hot molten salt pool and transported by Oxygen and Nitrogen gases. The ratio of the Na2SO4 and V2O5 gases was controlled by the Oxygen and Nitrogen gas flowing velocities and the temperature of molten salt pool. Thermal cycling test, in which the samples were kept at 1373K for 30min in air and then cooled to room temperature by forced air, was also carried out to investigate the influence of the Al2O3 layer thickness.
All the samples showed excellent thermal cycling behavior in air atmosphere up to 1373K and no spallation occurred after being thermal cycled for 1000 times (500 hrs) when Al2O3 layer thickness was less than 15µm. However, when the thickness was thicker than 20µm, cracks were observed only for no more than 500 times. With only 5wt.% of V2O5 addition to the hot molten salt pool, rapid hot corrosion occurred for the samples without Al2O3 layer and the cycling life was only 40 times. On the other hand, the samples coated with thin Al2O3 layer exhibited excellent hot-corrosion resistant behavior.
A3-1-10 Effect of Thermally Grown Oxide (TGO) Microstructure on Durability of TBCes with PtNiAl Diffusion Bond Coats
I. Spitsberg (General Electric Aircraft Engines); K. More, M. Lance (Oak Ridge National Laboratory)
The mechanism governing the failure of multi-layer thermal barrier system with PtNiAl bond coat and EB-PVD YSZ top coat has been recently proposed. One of the key factors determining the failure is the thermally grown oxide (TGO) growth rate. It has been experimentally shown that the reduced oxide growth rate leads to enhanced TBC system durability. paragraphThis paper discusses effect of the TGO microstructure resulting from different surface pre-treatment on the subsequent TGO growth rate and the TBC performance.
A3-1-11 Characterization of Thermal Barrier Coatings with a Gradient in Porosity
A. Portinha, V. Teixeira, M.F. Costa, J. Martins (University of Minho, Portugal); R. Vassen, D. Stoever (Forscchungszentrum Jülich GmbH, Germany)
A major problem in thermal barrier coatings (TBC) applied to gas turbine components is the spallation of ceramic coating under thermal cycling processes. In order to prevent spallation and improve the thermo-mechanical behaviour of the TBC, graded ceramic coatings can be fabricated. For this purpose we are developing a new concept of Thermal Barrier Coating (TBC) that consist of a conventional NiCoCrAlY bond coat and an atmospheric plasma sprayed ZrO2-8wt%Y2O3 top coat graded in porosity on a Inconel 738 LC substrates. The aim of this work is to produce coatings with low thermal conductivity and a better thermo-mechanical behavior due to the gradient in porosity which reflects a gradient in the elastic properties. Absolute porosity was measured with a mercury porosimetry and by image analysis. The second technique was also used to estimate the porosity variation along the cross section. Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) were used to observed the morphology and coating microstructure. The microhardness was measured with a Vickers indenter and 0.981 N load. The microhardness has been evaluated for coatings in as-sprayed condition and after annealing at 1100°C during 100h. The results show a fast increase of the hardness after annealing. The thermal diffusivity and conductivity were measured by laser flash method, these measurements have been related with porosity variation and sintering effects after thermal shock at 1000°C and annealing at 1100°C in air during 100 hours.