Papers

ICMCTF2002 Session A3-1: Thermal Barrier Coatings

Tuesday, April 23, 2002 8:30 AM in Room Esquire/Towne

Tuesday Morning

Start	Invited?	Item
8:30 AM		A3-1-1 Factors Affecting The Thermal Conductivity of Oxide Coatings D.R. Clarke (University of California, Santa Barbara) For many years yttria-stabilized zirconia has been the material of choice as a thermal barrier coating of metallic components in the hot sections of gas turbine engines and, increasingly, some diesel engines. The original reasons for selecting zirconia were associated with it being a refractory oxide having a low thermal conductivity and its success, in the form of plasma-sprayed coatings, in various thermal cycling tests to evaluate candidate materials at the time. As increasing demands are now being made for coatings with still lower thermal conductivity than the standard 7YSZ material, it is timely to assess what factors affect the high-temperature thermal conductivity of materials. It is also timely to understand why the thermal conductivity of zirconia is as low as it is and how much scope there is for future improvements in thermal conductivity. This presentation will address these issues, describe some of the modern ideas of the factors that lead to temperature independent and low thermal conductivity, and suggest approaches for the search of still lower thermal conductivity materials.
9:10 AM		A3-1-3 Combined Mode I and Mode II Fracture of Thermal Barrier Coatings S.R. Choi, D. Zhu, R.A. Miller (NASA Glenn Research Center) The mode I, mode II, and combined mode I-mode II fracture behaviors of zirconia-based thermal barrier coatings were determined in asymmetric flexure loading at both ambient and elevated temperatures. Precracks were introduced in test specimens using the single-edge-v-notched beam (SEVNB) method incorporated with final diamond polishing to achieve sharp crack tips. A fracture envelope of KI versus KII was determined for the coating material at ambient and elevated temperatures. Propagation angles of fracture as a function of KI/KII were also determined. The mixed-mode fracture behaviors of the coating material were compared with those of monolithic advanced ceramics determined previously. The mixed-mode fracture behavior of the thermal barrier coatings were predicted in terms of fracture envelope and propagation angle using mixed-mode fracture theories.
9:30 AM		A3-1-4 Structural, Thermal and Elastic Properties of Plasma Sprayed Thermal Barrier Coatings V. Teixeira, F. Macedo, J.A. Ferreira (Universidade do Minho, Portugal); I. Valka (Technical University of Brno, Portugal); W. Fischer, R. Vassen, D Stoever (Forschungszentrum Juelich, Germany) Thermal barrier coatings (TBCs) are used in a number of energy-related applications, such as in protecting aero and land based gas turbine components at very high temperature. Stabilized zirconia based TBCs were produced by vacuum and atmospheric plasma spraying processes and characterised in terms of microstruture, thermal diffusivity, porosity, microhardness, elastic modulus and residual stresses. In this paper we present and discuss the relationships between processing parameters of the deposition of the plasma sprayed zirconia coatings and their structural, thermal and elastic properties. For the determination of coating thermal diffusivity we report to a non-destructive evaluation of layered systems by using photothermal techniques. The degree porosity of coatings obtained by image analysis is related to the thermal diffusivity and to the elastic properties of the coatings using simple models. Some considerations about the thermo-mechanical behaviour of the TBC at high temperature are given and the obtained thermo-physical properties were used in the numerical modelling of the evolution of the residual stresses within the thermal barrier coatings. The stress state in the as deposited and in thermal cycled coatings was verified by x ray diffraction technique. The measurements were in good agreement with residual stress modelled results. It was observed that the residual stresses were dependent on the thermal history of the thermal barrier coating (as deposited and thermal cycled). Thermal cycling allowed the stresses to relax by microcracking and creep mechanisms at high temperature such that on cooling down to room temperature an in-plane biaxial compressive stress will arise on the zirconia top coating due to the difference on the coefficients of thermal expansion between substrate and coating.
9:50 AM		A3-1-5 High Temperature Corrosion Resistance and Microstructural Evaluation of Laser-glazed Plasma-sprayed Zirconia/MCrAlY Thermal Barrier Coatings P.C. Tsai, C.S. Hsu (National Huwei Institute of Technology, Taiwan, ROC) Laser glazing of a plasma-sprayed yttria stabilized zirconia/MCrAlY thermal barrier coatings (TBCs) was carried out using a plused CO₂ laser. The high temperature corrosion resistance of plasma-sprayed and laser-glazed zirconia thermal barrier coatings at 910°C was investigated using coupons deposited with Na₂SO₄ and V₂O₅. The phase transformation and microstructure phenomena of the coatings were studied with X-ray diffractometer and Electron Probe Microanalyzer. The results of this investigation showed that the lifetimes of the plasma-sprayed TBCs were enhanced about fourfold by laser glazing. The failure of the TBCs was initiated and propagated mainly within the top coat. The X-ray diffraction showed that reation between yttria (Y₂O₃) and V₂O₅ produced YVO₄, leaching Y₂O₃ from the YSZ and causing progressive tetragonal to monoclinic destabilizationtransformation. Reducing the penetration of the molten V₂O₅ by laser-glazed layer is the major enhancement mechanism for TBC life extension.
10:30 AM		A3-1-7 Determination of Mechanical Properties of Electron Beam-physical Vapor Deposition-thermal Barrier Coatings (EB-PVD-TBCs) by Means of Nanoindentation and Impact Testing K.-D. Bouzakis (Laboratory for Machine Tools & Manufacturing Engineering, Greece); A. Lontos, N. Michailidis (Aristoteles University of Thessaloniki, Greece); O. Knotek (Institut fuer Werkstoffkunde B, RWTH, Germany); E. Lugscheider, K. Bobzin, A. Etzkorn (Werkstoffwissenschaften, RWTH, Germany) EB-PVD-zirconia coatings are well known as thermal barrier coating materials for gas turbine applications. By using this material, the gas turbine can work at higher temperatures and hereby the turbine efficiency increases. Due to the fact that not only the turbine efficiency but also its reliability is a very important issue, the life prediction of thermal barrier coatings for their life assessment through simulation and modeling becomes very important. Developing a model of an anisotropic EB-PVD-coating the knowledge of the coating material mechanical behavior is required. In the present paper the mechanical properties of the investigated coatings are determined by means of advanced experimental analytical procedures. The stress-stain curve of the examined coatings, are determined by a continuous FEM supported simulation of the penetration of a carbide ball into the coating surface. Moreover the superficial EB-PVD ZrO₂coating was examined by means of the impact tester, in order to determine its creep behavior under dynamic load. The coating dynamic creep critical stresses are defined by means of a convenient FEM supported algorithm.
10:50 AM		A3-1-8 Morphological Evolution of the Thermal Barrier Coatings Induced by Cyclic Oxidation V.K. Tolpygo, D.R. Clarke (University of California, Santa Barbara) Buckling and spallation of EB-PVD thermal barrier coatings (TBC) deposited on a single-crystal nickel-base superalloy with platinum aluminide bond coat have been studied using optical and scanning electron microscopy. Examination of the TBC morphology in the course of cyclic oxidation at 1150°C reveals that the TBC surface develops undulations, which grow in amplitude with oxidation time and eventually link together leading to TBC failure. The TBC undulations are shown to be associated with local separation near the interface between the TBC and the thermally grown alumina scale. The influence of the morphology of the bond coat surface prior to TBC deposition is discussed. The observations of the pre-failure TBC evolution made by a low-magnification optical microscopy can serve as a very simple and reliable method of non-destructive evaluation of the coating.
11:10 AM		A3-1-9 Round Robin PSLS Analysis of EB-PVD TBCs J.A. Nychka, D.R. Clarke (University of California, Santa Barbara); K.S. Murphy (Howmet Research Corporation); M.J. Lance (Oak Ridge National Laboratory); M. Gell, E. Jordan (University of Connecticut) Photo-stimulated luminescence spectroscopy (PSLS) can be used as a non-destructive tool to gain quantitative information about the stress and strain states of the TGO beneath a TBC. With the increasing popularity of this technique it is paramount that there be some standard for interpreting and extracting from the complex spectra useful and physically meaningful information. This contribution will report the results of a round robin series of PSLS measurements and analysis performed at different labs on the same set of EB-PVD TBC samples, having undergone various oxidation treatments. The goal of this paper is to compare different instruments and conditions that are typically used when performing PSLS measurements in order to elucidate which variables, if any, yield different spectra for the same samples, and to compare the fitting procedures used in the quantitative analysis of PSL spectra.
11:30 AM		A3-1-10 Microanalysis of Hot Corrosion on a Plasma-sprayed Zirconia/MCrAlY Thermal Barrier Coating J.E. Tang, K. Hansson, M. Halvarsson, J.-E. Svensson (Chalmers University of Technology & Göteborg University, Sweden); R. Pompe (Swedish Ceramic Institute (SCI), Sweden) Plasma-sprayed zirconia and MCrAlY thermal barrier coatings often operate in high temperature oxidising environments, some containing potentially corrosive combustion products. The resulting hot corrosion can lead to accelerated deterioration and eventual failure of the coating. This investigation is centered on the effects of molten salts such as sodium chloride and sodium sulfate on the oxidation of the MCrAlY bond coat. Specimens subjected to furnace exposures are subsequently analysed for compositional and microstructural changes in the top coat, bond coat and interfaces using analytical electron microscopy techniques. The aim is to obtain a better understanding of the species involved and the oxidation products that may form during operation of the coating in salt-containing high temperature environments. Chalmers University of Technology & Göteborg University