ICMCTF2000 Session A3: Thermal Barrier Coatings

Tuesday, April 11, 2000 8:30 AM in Room Royal Palm Salon 1-3

Tuesday Morning

Start	Invited?	Item
8:30 AM	Invited	A3-1 Laser Rig Testing of Thermal Barrier Coatings R.A.. Miller, D. Zhu (NASA Glenn Research Center) CO2 lasers provide an inexpensive yet controlled way to deliver energy to the surface of a thermal barrier coating.. They are adaptable to a wide range of test requirements and can yield certain materials properties along with durability. For aero applications, turbine section coatings can be exposed to realistic heat fluxes with realistic temperature gradients across simple furnace-type test specimens. The test gives coating durability in the high heat flux environment along with real-time measurement of overall coating thermal conductivity. Sintering rates may also be assessed indirectly as the change in overall conductivity vs time or, under certain test conditions, directly by measuring crack opening displacement. For diesel engine applications, coatings may be tested im pulse mode with a pulse frequency representing, for example, 1300RPM diesel engine operation. The laser test may also be combined with a mechanical bend test and the combined mechanical and thermal fatigue response of the coating may be evaluated with relevance to aero combustor or diesel engine test conditions. Specific examples of the above will be presented.
9:10 AM	Invited	A3-3 Two-source Jumping Beam Evaporation for Advanced EB-PVD TBC Systems U. Schulz, K. Fritscher, C. Leyens (DLR-German Aerospace Center, Germany) Thermal barrier coatings (TBCs) are currently used for lifetime improvement of highly loaded turbine blades and vanes and for increase of efficiency in aero-engines and land-based gas turbines. They typically comprise a ceramic top coating that reduces the metal temperature and lowers the temperature peaks during the transient stage and a metallic bond coat on a superalloy substrate. For the next generation of TBCs a further increase in the turbine inlet gas temperature is expected up to a level where the common yttria partially stabilized zirconia material for TBCs is above its temperature capability. In the present overview coatings are presented that have been produced by the electron beam-physical vapor deposition (EB-PVD) process, a rapidly growing technology for TBC manufacture. They offer the advantage of a superior strain and thermoshock tolerance due to their columnar microstructure. Advanced coating design concepts will be addressed with main focus on processing of novel TBCs like ceria stabilized zirconia, mullite, and zircon that aim at increasing the maximum application temperature and reducing cost of the coating material, graded alumina-zirconia coatings to reduce internal stresses and to prolong lifetime, multilayers to reduce thermal conductivity, and new bond coat alloys to minimize oxidation and enhance adhesion of the ceramic TBCs. Since material properties are closely linked to processing conditions, the paper addresses this relationship with special emphasis on dual source evaporation carried out by one e-beam gun and jumping beam technology. Material properties like vapor pressure and evaporation behavior as well as processing conditions like jumping frequency of the beam have been studied in order to utilize the full potential of advanced EB-PVD processing for novel coatings.
9:50 AM		A3-5 Two New Candidates for Thermal Barrier Coatings R. Vaßen, X. Cao, V. Verlotzki, H. Lehmann, M. Dietrich (Forschungszentrum Jülich GmbH, Germany); D. Stöver (Forschungszentrum Jülich GmbH, Germany) Two new types of thermal barrier coating systems are presented, one based on a ceramic and one on a glass-metal composite. In the ceramic system the conventional 7-8 wt.% yttria-stabilised zirconia (YSZ) is replaced by a La₂Zr₂O₇ - based ceramic. The two major advantages of this material are its low thermal conductivity which is even lower than that of YSZ and its high phase stability. Results of long term annealing experiments at 1400 °C underline this statement. Additionally, results of sintering experiments at elevated temperatures and thermal cycling tests will be presented. The outcome of the experiments suggest directions for a further development and improvement of this type of material. In the second type of TBC system the composition of the metal-glass composite is chosen in such a way that the thermal expansion coefficient of the composite is close to the one of the substrate. This leads to reduced thermal stresses and hence improved thermal cycling life times. Another advantage of the gas tight composite coatings is their ability to protect the bondcoat from severe oxidation. Correspondingly, longer life times have been found for these TBCs in oxidation tests.
10:30 AM		A3-7 EB-PVD-Thermal Barrier Coatings on Laser Drilled Surfaces for Transpiration Cooling E. Lugscheider, K. Bobzin, A. Etzkorn (University of Technology Aachen, Materials Science Institute, Germany); A. Horn, R. Weichenhain, E.W. Kreutz, R. Poprawe (University of Technology Aachen, Lehrstuhl für Lasertechnik, Germany) Transpiration cooling for gas turbine applications shows a high potential of reducing the amount of cooling air which would cause an increase of efficiency. The idea for the transpiration cooling in this case is to produce a dense net of laser drilled holes with a diameter of 200 µm with distances of 600 µm crosswise and 1200 µm along the gas flow. Depending on the area on the gas turbine blades geometry the holes have to be tilted. Using the whole capacity of transpiration cooled airfoils means that the use of thermal barrier coatings is needed. By depositing a thermal barrier coating onto a laser drilled surface the holes and specially the tilted ones will be closed. To fulfill the main requirement beside a long lifetime and a low thermal conductivity to thermal barrier coatings on transpiration cooled surfaces the permeability for the cooling fluid has to be ensured. For the present examinations samples with laser drilled holes (diameter 200 µm) and tilting angles of 0°, 15°, 30°, 45° and 60° were produced. As substrate material VPS-MCrAlY-coated nickelbased superalloy was used. The zirconia thermal barrier coatings were deposited by the EB-PVD technique. Examined was to what extend, specially the tilted holes, were closed by the deposition process. With an increase of the tilting angle the holes were closed by the deposition of the thermal barrier coating with a decrease of coating thickness. To avoid the closing of the holes and on top of that to reach a streamline contour at the outlet additional deposition processes were performed. During these tests a gas flew through the holes at several partial pressures. As a result a reduction of deposition rate was determined. The dependencies between gas pressure, tilting angle of the holes and the outlet geometry were analysed.
10:50 AM		A3-8 Oxide Scale Growth on MCrAlY Bond Coatings After Pulsed Electron Beam Treatment and Deposition of EBPVD-TBC. N. Czech, W. Stamm (Siemens AG, Power Generation Group (KWU), Germany); G. Müller, G. Schumacher (Forschungszentrum Karlsruhe, Germany); W.J. Quadakkers (Forschungszentrum Jülich, Germany); D. Strauss (Forschungszentrum Karlsruhe, Germany) Thin layers restructured by surface melting of about 30 µm depth on MCrAlY coatings were created using the large area pulsed electron beam facility GESA. With a beam diameter of up to 10 cm it is possible to treat samples like turbine blades with just a few electron beam pulses. The high cooling rates lead to nanocrystalline structures at the sample surface, their oxidation properties were studied at 950 °C in air. As the surface treatment leads to smooth surfaces even on former rough LPPS-samples with an R_A<1.5mu m, after surface modification the samples can directly be coated with EBPVD Thermal Barrier Coatings. The growth of the TGO on such samples was studied also at 950 °C in air up to 5000 h and compared to the samples without the TBC. The treated samples appeared to have a strongly enhanced oxidation resistance without spinel formation in the α-Al₂O₃oxide scale. Keywords: MCrAlY, bond coat, TBC, surface treatment
11:10 AM		A3-9 Spalling Failure of EB-PVD Thermal Barrier Coatings C. Leyens (DLR-German Aerospace Center, Germany); J.M. McNaney, P.Y. Hou (Lawrence Berkeley National Laboratory) Significant improvement of spallation resistance is a major requirement for development of advanced thermal barrier coating (TBC) systems. For electron physical vapor deposited (EB-PVD) TBCs, spallation of the ceramic top coat is closely related to the fracture resistance of the bond coat-ceramic interface but it is experimentally difficult to quantitatively determine adhesion. In the present study, 100 and 200µm thick EB-PVD TBCs deposited on NiCoCrAlY-coated nickel superalloy IN 617 were isothermally exposed at 1050 and 1100°C for up to 1000h and subsequently tested to failure in a four-point-bending arrangement. Experimental limitations predominantly controlled by the specific columnar microstructure of the coatings precluded the measurement of strain energy release rates desirable to quantify fracture resistance of the interface. Alternatively, a critical strain range for failure for each exposure condition was determined to semi-quantitatively evaluate the spallation behavior, which in turn was correlated with results of post-testing microstructural and analytical examination of the bond coat-top coat region.
11:30 AM		A3-10 Effects of Interfacial Oxidation and Residual Stresses on the Failure Mechanisms in Thermal Barrier Coatings V. Teixeira, M. Andritschky (University of Minho, Portugal); D. Stoever (Forschungszentrum Jülich GmbH, Germany) Advanced ceramic multilayered coatings used as protective coatings for engine components to improve performance, e.g. duplex thermal barrier coatings (TBC´s), usually deposited by Plasma Spraying or Physical Vapour Deposition (PVD) techniques, are currently applied on gas turbine blades and diesel engine components. In this contribution, the residual stress and interfacial oxidation are analysed and related to the failure mechanisms observed in TBC's during thermal cycling. Residual stresses in thermal barrier coatings occur due to a mismatch between the coefficient of thermal expansion (CTE) of metallic substrate and ceramic coating, due to transient thermal gradients (e.g. during thermal cycling), and also due to interfacial oxidation. Away from the edges, the in-plane stress is typically compressive in the ceramic layer (due to the smaller CTE of ceramic material) and tensile in the metallic layer. Tensile residual stresses in the ceramic coating cause perpendicular microcracking (through-thickness cracks) while compressive stresses tend to promote microcrack propagation along the interface. Therefore, cracking in the ceramic or interfacial decohesion will affect the thermo-mechanical integrity of the functional coated component. The failure mode observed in TBC's during service are related to a complex mechanism involving microcrack propagation at or near the rough metallic/ceramic interface where an oxide layer is growing up due to the high temperature exposure.
11:50 AM		A3-11 Measurement of Interfacial Toughness Loss in Thermal Barrier Coating and Oxide Systems J.L. Beuth, R.A. Handoko, A. Vasinonta (Carnegie Mellon University); F.S. Pettit, G.H. Meier, M.J. Stiger, L.A. Ortman (University of Pittsburgh) A major concern with thermal barrier coatings (TBCs) is their loss of adhesion during service, leading to spallation. In this talk, an indentation test is introduced for quantifying decreases in interfacial toughness of TBC systems as a function of the duration of high-temperature exposures. Testing techniques proposed in the literature for application to brittle coatings on ductile substrates are adapted for use on standard button-shaped TBC specimens. A detailed description is given of the indentation test, which involves using a standard Rockwell hardness tester to penetrate the TBC and the oxide layer below it, inducing plastic deformation in the underlying metal bond coat and superalloy substrate. This plastic deformation induces a compressive radial stress away from the indent, which drives an axisymmetric delamination of the TBC and oxide layers. Detailed elastic-plastic model results are presented which allow determination of the toughness of the oxide/bond coat interface from a measured radius of delamination. Results from the testing of electron beam physical vapor deposited (EBPVD) TBC systems are presented which demonstrate the consistency of the test. Initial test results tracking the loss of toughness for EBPVD TBC systems as a function of high temperature exposures are also presented. These results indicate a significant loss in interfacial toughness with exposures at 1200° and 1100° C. However, these apparent losses in toughness are correlated with observations of increased oxide thickness and TBC sintering. These changes in the TBC system increase the energy driving delamination and could lead to apparent decreases in toughness even if the oxide/bond coat interface itself is not degraded. Further analyses and tests are discussed which explore the relative importance of oxide thickening and TBC sintering in decreasing apparent TBC system adhesion. This includes indentation testing of oxide scale systems without a TBC on top in an attempt to separate the role of the TBC in contributing to adhesion loss.