ICMCTF1998 Session A3: Thermal Barrier Coatings

Tuesday, April 28, 1998 8:30 AM in Room Council/Chamber/Cabinet

Tuesday Morning

Start	Invited?	Item
8:30 AM	Invited	A3-1 The Contributions of Research Over the Past 25 Years to the Development of Coatings for Gas Turbine Airfoils F. S. Pettit, G.H. Meier (The University of Pittsburgh) With the advent of the gas turbine at the end of World War II there has been, and currently is, a need for high temperature materials to permit gas turbines to generate greater power and with improved efficiency and economy. This paper will describe the major research developments over the past twenty five years that have contributed to progress in this field as well as prospects for future advancements. While other protection schemes were investigated, it will be shown that selective oxidation of aluminum and improved oxide scale adhesion by using reactive elements and low sulfur alloys provided the major improvements for oxidation resistance. Equally important, was the understanding that evolved for the hot corrosion process. In particular, where hot corrosion should be expected and how this type of attack occurred. Finally, the importance of testing procedures to effectively use and improve coatings will be discussed.
9:10 AM	Invited	A3-3 Progress in Coatings for Gas Turbine Airfoils - A Brief History G.W. Goward (Consultant to Turbine Components Corp.) The commercial availability of ever more efficient gas turbines has always been paced by the results of research and development in the concurrent fields of design and materials technology. Improved structural design and airfoil cooling technology applied to higher strength-at-temperature alloys cast by increasingly complex methods, and coat4d with steadily improved protective coating systems, have led to remarkably efficient turbine machinery for aircraft propulsion and power generation. For first state turbine blades, nickel-based superalloys in various wrought and cast forms, and augmented by coatings since the sixties, have been singularly successful materials systems for the past fifty years - and still no real world substitutes are on the horizon. This presentation traces the history of protective coatings for superalloy airfoils beginning with the simple aluminides, followed by these same modified with silicon, chromium and platinum, then MCrAlY overlay coatings, and finally the elegant electron beam vapor deposited ceramic thermal barrier coatings recently introduced to servcie. The publicly available results of several decades of research supporting these advances are acknowledged. The most important of these include generic research on oxidation and hot corrosion mechanisms of superalloys and coatings, the intricacies of protective oxide adherence, mechanisms of low temperature (Type II) hot corrosion, and alumidice coating formation. With no promising practical materials systems beyond coated nickel-base superalloys apparent in the foreseeable future, continued progress will likely be made by further refinement of control of oxide adherence, and by more cost effective manufacturing processes for contemporary types of protective coatings.
9:50 AM		A3-5 Burner Rig Tests on Turbine Airfoil Sections Coated with EB-PVD TBCs from Different Suppliers G.A. Kool, E.F.M. Jansen, J.A.M. Boogers (National Aerospace Laboratory NLR, The Netherlands) Burner Rig Tests on Turbine Airfoil Sections Coated with EB-PVD TBCs from Different Suppliers G.A. Kool, E.F.M. Jansen, J.A.M. Boogers National Aerospace Laboratory NLR This paper presents the results of two burner rig tests on electron-beam physical vapour deposited (EB-PVD) thermal barrier coatings (TBCs). Interturbine Coating Centre (ITC) deposited the TBCs over PtAl- and MCrAlY-bondcoats on leading edge sections of first stage CF6-80 turbine blades. EB-PVD TBCs over PtAl- and MCrAlY-bondcoats from other commercial suppliers were evaluated on identical leading edge sections. The tested TBCs on directionally solidified alloy DSR-142 were of the yttria partially stabilized zirconia type. The TBCs were subjected to service simulated conditions in two tests with different heat-up and cool-down rates. The maximum test temperature was 1130 °C. Sulphur-dioxide and artificial sea salt were injected continuously as pollutants into the hot gas stream. The TBCs degraded by cracking, chipping and finally by spallation. Coating degradation has been evaluated in more detail by metallography and scanning electron microscopy. After these tests rankings were made for the tested coating systems from the different suppliers. The TBCs from ITC demonstrated much better performance than the TBCs from the evaluated commercial suppliers. TBC performance is also affected by the type of bondcoat.
10:10 AM		A3-6 Break
10:30 AM		A3-7 EB-Preheating of Turbine Blades - The Completion of EB-Technology for Thermal Barrier Coating E. Reinhold, C. Deus, B.-D. Wenzel (Von Ardenne Anlagentechnik, GmbH, Germany) EB-PVD circonia thermal barrier coatings have several advantages over coatings deposited with other techniques. Therefore EB-PVD coaters for deposition of turbine blades are becoming increasingly interesting. This applies to pilot coaters as well as to production coaters. Electron beams are not only suited for generating the evaporation process, they are also excellently suitable for preheating the turbine blades. By means of an electron beam preheating process the different parts of the rotating turbine blades (which are partially coverd to prevent coating of this part) can be heated without temperature differences. Moreover, they can be heated faster than by means of the conventional radiation heating. The possibility of unretarded electron beam guidance creates the opportunity to generate very fast alternating heating patterns with matched changes of the power distribution on several parts of the turbine blade. The developed EB-heating technology permits for blades have a weight of more than 7 kg a synchronized temperature increase to 1000°C within less than 15 minutes. This helps to shorten the vacuum cycle time per charge and increases the capacity of the coating plant. A modern conception of an EB-PVD production coater for circonia thermal barrier coating on turbine blades containing the new EB-heating process will be introduced.
10:50 AM		A3-8 Studies of the Bond Coat Oxidation and Phase Structure of TBC's N. Czech (Siemens AG, Power Generation Group (KWU), Germany); M. Juez-Lorenzo, V. Kolarik (Fraunhofer-Institut für Chemische Technologie (ICT), Pfinztal, Germany); W. Stamm (Siemens AG, Power Generation Group (KWU), Germany) To increase the power efficiency of stationary gas turbines the turbine inlet temperature will increase in the future to more than 1400°C. At the same time the cooling air must be reduced avoiding an increase of the NO_x level. In the new designed gas turbines the thermal barrier coatings are an essential tool to reduce the cooling air. The durability of Physical Vapour Deposited (PVD) thermal barrier coatings are influenced by induced stresses and the growth of the TGO (Thermally Grown Oxide) with temperature and time. In situ measurements using high temperature X-ray diffraction were performed on the TBC during heating and cooling and in isothermal experiments. The formation of the TGO at 950°C up to 100 h was studied in situ using samples with a thin TBC layer that is penetrated by the X-ray beam. Post oxidation studies of the TGO scale and the TBC were performed by electron microprobe analysis. Additionally, the TGO formation was studied in long term experiments up to 5200 h.
11:10 AM		A3-9 Relationships Between Residual Stress, Microstructure and Mechanical Properties of EB-PVD TBCs C.A. Johnson, J.A. Ruud (GE Corporate Research and Development); R. Bruce, D. Wortman (General Electric Aircraft Engines) Residual stresses develop in coatings during deposition and can have a large impact on coating mechanical properties and durability. In this study, in-plane residual stresses in EB-PVD TBCs were characterized from the change in substrate curvature upon coating removal. Variations in deposition conditions were observed to produce a large range of stress levels in PVD TBCs. A significant difference in the in-plane residual stress levels was measured along different directions of PVD TBC specimens fabricated by rotation of substrates over the evaporation source. Residual stress levels both along and perpendicular to the axis of rotation were associated with microstructural features of the coating. Two TBC mechanical properties were shown to be affected strongly by residual stress: the in-plane elastic modulus and the strain tolerance. Strain tolerance was measured directly by a new mechanical test that measures the strain at delamination of a coating from an edge-initiated crack from a substrate that is loaded in compression.
11:30 AM		A3-10 Development of a Process Window for NiCoCrAlY Plasma Sprayed Coating A.C. Leger, J. Wigren, M.O. Hansson (Volvo Aero Corporation, Sweden) Some recent research has shown the importance of the substrate temperature during spraying on the properties of thermal barrier coatings. In the future, applications like small combustor cans for stationary gas turbines will be sprayed at high temperatures. For that purpose, the Metco Sulzer Connex Gun which can be used at higher temperatures, has been tried at Volvo Aero. In a first step, which is described in this paper, a NiCoCrAlY bondcoat was studied. A L4 Taguchi matrix was sprayed to improve the microstructure of the coatings. Varying parameters were the current intensity, argon and hydrogen flowrates. The DPV2000 diagnostic system was used to measure the velocity, temperature and diameter of the in-flight particles in order to search for a process window. The microstructures of the coatings, i.e. the number of unmelted particles, the percentage of pores, delaminations and oxides were evaluated and linked both to the spray parameters and to the in-flight properties of the particles. It could be shown that the temperatures of particles are directly related to the amount of unmelted particles, oxides and delaminations in the microstructure. Tensile strength measurements also show a clear correlation to the amount of delaminations in the coating and therefore to the in-flight particle temperature.