ICMCTF2006 Session A3-2: Thermal Barrier Coatings
Wednesday, May 3, 2006 1:30 PM in Sunrise
A3-2-1 Erosion and FOD of TBCs: Mechanisms, Maps and Models
J.R. Nicholls, R.G. Wellman (Cranfield University, United Kingdom)
Thermal barrier coatings have been used in gas turbine engines for a number of decades and have been the subject of numerous research papers over the years. These have generally concentrated on production methods, oxidation/cyclic life, TGO growth, thermal conductivity and erosion. Until recently most of the papers on erosion of TBCs concentrated on the effects of particle velocity and impact angle and compare the performance of EB-PVD to PS TBCs. Most of these tests were conducted on samples in the as-received condition. More recently the effects of aging and column morphology on erosion rates has also been investigated, different erosion mechanisms have been identified and various erosion models have been produced. The effect of 3rd element dopants on the erosion of TBCs has also been studied to a limited extent.@paragraph@ This papers aims to provide a review of research that has been undertaken on the erosion of thermal barrier coatings. The different erosion mechanisms that have been identified will be presented as well the merits of the various maps and models that have developed over the years. The effect of TBC morphology along with the effect of dopants and aging an erosion will also be discussed. The important of the size of the contact footprint relative to microstructural size of the TBCs (the D/d ratio) is considered an important characteristic dimensionless parameter for erosion and FOD of TBCs and its application also will be discussed.
A3-2-3 Experiences with Improving Toughness in Low @kappa@ Thermal Barrier Oxides Based on ZrO@sub 2@-GdO@sub 1.5@
R.M. Leckie, S. Krämer, C. Levi (University of California, Santa Barbara)
Next generation thermal barrier oxides with lower thermal conductivity and slower sintering kinetics are often based on rare earth (RE) oxide co-doping of yttria stabilized zirconia (YSZ) and/or RE zirconates. Of particular interest are GdO@sub 1.5@ additions, and some experience already exists with coatings based on Gd@sub 2@Zr@sub 2@O@sub 7@. There is anecdotal evidence, however, that the advantages offered by these compositions typically carry concomitant penalties in cyclic life and/or erosion resistance. While the details of the mechanisms are not fully understood, research on YSZ has suggested that a decline in cyclic life with increasing stabilizer content is associated with the operation of toughening mechanisms in the tetragonal form of zirconia (7YSZ) that are not accessible in the cubic form (e.g. 20YSZ). This presentation reports on research aimed to translate the lessons learned in 7YSZ to potential toughening of Gd-doped thermal barrier oxides by suitable ternary additions that promote tetragonality in the parent phase, or the evolution of a tetragonal phase-dispersion within a cubic matrix. Opportunities presented by the phase equilibria in the ternary systems based on ZrO@sub 2@-GdO@sub 1.5@, and the connection between mechanisms and microstructure are discussed.
A3-2-4 Thermal Cycling Behavior of Lanthanum Zirconate as EB-PVD TBC on Modified NiAl Alloy
K. Bobzin, E. Lugscheider, N. Bagcivan (RWTH Aachen University, Germany)
Good thermal cycling behavior of thermal barrier coatings (TBC) is essential for the increase of efficiency of gas turbines. Traditionally Yttria partially stabilized Zirconia (YPSZ) coatings are used as TBCs on turbine blades. The thermal cycling behaviour of the newly developed Thermal Barrier Coating (TBC) based on Lanthanum Zirconate is investigated in comparison to state of the art Yttria Stabilized Zirconia (YSZ). Investigations on Lanthanum Zirconate with pyrochlor structure show a promising performance for application as TBC. Both TBCs are deposited on modified intermetallic NiAl alloy substrates using Electron Beam Physical Vapour Deposition (EB-PVD). Due to the superior corrosion and oxidation behaviour of NiAl substrates no MCrAlY as adhesion layer is used. TGO growth during the thermal cycling test is examined by Scanning Electron Microscopy (SEM). Phase structures of TBCs are analyzed by X-Ray Diffraction (XRD). Energy-Dispersive X-ray diffraction (EDX) is applied to investigate chemical changes due to thermally induced diffusion between TBCs and substrate. Cycling is stopped when a visible spallation of TBCs occurs. The long-term aim is the development of new material concepts within the collaborative research center (SFB) 561 "Thermally highly loaded, porous and cooled multilayer systems for combined cycle power plants" for the next generation of EB-PVD-TBCs.
A3-2-5 Parameters Affecting TGO Growth and Adherence on MCrAlY-Bond Coats for TBCTs
J. Toscano, R. Vassen (Forschungszentrum Juelich, Germany); A. Gil (AGH University of Science and Technology, Poland); D. Naumenko, L. Singheiser, W.J. Quadakkers (Forschungszentrum Juelich, Germany)
In industrial gas turbines coatings of the MCrAlY type are commonly used as overlay and bond coatings (BCTs) for ceramic thermal barrier coatings (TBCTs). It is generally accepted that the TBC life crucially depends on the properties of the aluminium based, thermally grown oxide (TGO) which forms during high temperature service at the TBC/BC interface. In the present study a number of parameters known to affect the growth kinetics and adherence of the TGO were investigated. For this purpose different types of test specimens were prepared for oxidation studies and TBC life time tests. First, free-standing, rectangular specimens were prepared by vacuum plasma spraying from three different BCTs. Additionally, coatings of these three BCTs were applied to a conventional nickel base superalloy. Finally, a thermal barrier coating was applied to part of the coated super alloy specimens by EB-PVD: All specimens were subjected to oxidation treatments in the temperature range 1000 to 1100°C with exposure times between 1 and 1000 hours. Thereby, two different atmospheres were used, i.e. air and an Ar/H@sub 2@/H@sub 2@O mixture. The used approach allowed to identify the effect of bond coat composition, presence of the super alloy and presence of the TBC on TGO formation as function of time and temperature. The use of two test gases with large differences in oxygen partial pressure allowed to quantitatively estimate the effect of TGO growth rate on TBC life without altering a further parameter such as e.g. oxidation temperature. After oxidation the specimens were analysed by optical metallography, scanning electron microscopy, fluorescence spectroscopy and, in selected cases, sputtered neutrals mass spectrometry.
A3-2-7 Oxidation Resistant Coatings in Combination with Thermal Barrier Coatings on @gamma@-TiAl Alloys for High Temperature Applications
M. Froehlich, R. Braun (DLR-German Aerospace Center, Germany); C. Leyens (Technical University of Brandenburg at Cottbus, Germany)
Providing attractive properties like low density, high specific stiffness, high specific yield strength and a good creep resistance up to high temperatures @gamma@-TiAl alloys are considered for applications in automotive, aircraft, aerospace as well as in power generation industry with the potential to replace the presently used heavy steels and nickel-based superalloys. To extend the oxidation resistance granted up to 750°C of bare material protective coatings are commonly used. A novel approach is the use of thermal barrier coatings (TBCs) on @gamma@-TiAl, typically applied to nickel-based superalloys. This allows to reduce the metal surface temperatures in combination with component cooling for use in high temperature range. @paragraph@ The paper is focussed on both development of oxidation resistant coatings as well as thermal barrier coatings for gamma titanium aluminides. TiAlCrY nitride and metallic overlay coatings based on TiAlCr were produced by magnetron sputtering techniques. Both coatings provide excellent oxidation protection for @gamma@-TiAl alloys in air at 750°C. EB-PVD partially-yttria-stabilized zirconia TBCs were deposited on Ti-45Al-8Nb, either pre-oxidized or coated with various "bond coats" mentioned above. @paragraph@ The specimens were examined under cyclic conditions at 900°C and 950°C, respectively. In comparison with pure material oxidation the nitride coating reveals poor oxidation resistance even exposed at 900°C for 420 cycles whereas the TiAlCr layer shows moderate oxidation behaviour under these terms of examination up to 1100 cycles. Furthermore the effect of several deposition temperatures, 820°C and 900°C, for TBCs in combination with oxidation resistant coatings will be examined. @paragraph@ Discussing appeared effects due to diffusion processes post-oxidation SEM/EDX analysis in combination with oxidation behaviour and lifetime of the coating systems will be presented.
A3-2-9 Effect of Impurities on the Properties of Thermal Barrier Coatings
L. Xie, M. Dorfman, J. Doesburg (Sulzer Metco (US) Inc.); S. Tsipas, I. Golosnoy, B. Clyne (University of Cambridge)
Thermal barrier coatings (TBCs) are used in advanced gas turbines to protect metallic components against high temperatures. In most commercial applications, zirconia partially stabilized with yttria, 7~8 wt.% Y@sub 2@O@sub 3@-ZrO@sub 2@ (7YSZ), is the material of choice due to its low thermal conductivity and high thermal expansion coefficient. In this study, thermal barrier coatings were sprayed from various 7YSZ feedstock materials that contain different amount of impurities. Processing conditions are the same for all coatings and the as-sprayed coatings exhibit similar microstructure. Coating properties like thermal conductivity, sintering resistance, elastic modulus and thermal cyclic life were investigated relative to coating chemistry.
A3-2-10 Buckling Delamination in Compressed Multilayers on Curved Substrates with Accompanying Ridge Cracks
S. Faulhaber, C. Mercer (University of California Santa Barbara); M.-W. Moon, J.W. Hutchinson (Harvard University); A.G. Evans (University of California Santa Barbara)
Residually compressed films and coatings are susceptible to buckle delamination. The buckles often have linear or telephone cord morphology. When the films are brittle, such buckles are susceptible to the formation of ridge cracks that extend along their length, terminating close to the propagating front. The ridge-cracked buckles are invariably straight-sided (not telephone cord) and differ in width. Buckle delaminations of this type occur on flat and curved substrates, having greatest technological relevance in the latter. They occur not only in single layer films but also in multilayers, such as thermal barrier systems. Establishing the mechanics of ridge-cracked buckle delaminations for multilayers on curved substrates serves two purposes. (a) It allows the prediction of buckle delamination and spalling for technologically important systems. (b) It provides a test protocol for measuring properties such as the stresses in the layers and the delamination toughness of the interface. The latter has been identified as a key parameter in the failure of thermal barrier systems with NiCoCrAlY bondcoats that show failure by interface delamination rather than by ratcheting. Implementation of an inverse algorithm for the calculation of these properties is demonstrated for diamond-like carbon films on planar glass substrates and a thermal barrier multilayer on a curved superalloy substrate.
A3-2-11 Thermal Cycling Lifetime and Microstructural Development of Thermal Barrier Coatings with Air Plasma Sprayed NiCoCrAlY Bond Coats
B. Jayaraj (Mitsubish Power Systems); Y.H. Sohn (University of Central Florida)
Air plasma sprayed (APS) ZrO@sub 2@Y@sub 2@O@sub 3@ (YSZ) thermal barrier coatings (TBCs) with an APS NiCoCrAlY bond coat were examined for the evaluation of thermal cycling lifetime and microstructural development. The TBCs consisted of 600 µm-thick APS YSZ, 175 µm-thick APS NiCoCrAlY and Haynes 230 substrate, and the 1-, 10- and 50 hour thermal cycling consisted of 10-minute heat-up to 1121-C, dwell at 1121-C and 10-minute forced-air-quench. Despite the significant internal oxidation of APS NiCoCrAlY bond coat, these TBCs exhibited excellent thermal cycling lifetime. Microstructural evolution of TBCs with an emphasis on the development of thermally grown oxide (TGO) including internal oxidation was examined by scanning electron microscopy, transmission electron microscopy (TEM) and scanning TEM. Oxides such as Al@sub 2@O@sub 3@, (Al,Cr)@sub 2@O@sub 3@ and (Ni,Co)(Al,Cr)@sub 2@O@sub 4@ within the TGO were identified by electron diffraction, and quantitative microscopy were carried out elucidate the superior performance these TBCs with APS NiCoCrAlY bond coats.