ICMCTF1999 Session A3-2: Thermal Barrier Coatings

Tuesday, April 13, 1999 1:30 PM in Room Council/Chamber/Cabinet

Tuesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	A3-2-1 The Evolution of Thermal Barrier Coatings - Status and Upcoming Solutions for Today's Key Issues W. Beele, G. Marijnissen, A. van lieshout (Interturbine Coating Center, The Netherlands) Thermal barrier coating applications re known for thermally loaded combustion process components for decades. From the early beginning, science and industry worked on solutions about how to combine such different materials properties as from superalloy metals and ceramic thermal insulators. While partially stabilized zirconia became very early the material standard, thermal spraying and electron beam physical vapor deposition were even in the early 90's considered as competing technologies. This paper reviews why EB-PVD is the actual choice for the latest state-of-the-art components and will attempt to summarize the major roadblocks for thermal barrier technology on its way to become a fully prime reliant, designed-in feature. Some potential answers for today's technical issues will be discussed with respect to their ability to shift TBS-applications to the next level of reliability within the various engine environments.
2:10 PM		A3-2-3 Influence of EB-PVD TBC Microstructure on Thermal Barrier Coating System Performance Under Cyclic Oxidation and Hot Corrosion Conditions C. Leyens, B.A. Pint, I.G. Wright (Oak Ridge National Laboratory); U. Schulz (DLR-German Aerospace Center, Germany) The increasing demands placed on current high-temperature Ni-base alloys for gas turbine applications have spurred the development of thermal barrier coating (TBC) systems for both aeroengine and land-based gas turbines. For future advanced turbines in which TBCs will be essential for the functioning of the hot gas path components (prime reliant coatings), reliable adhesion of the ceramic top layer to the component is mandatory. Oxidation and hot corrosion of the bond coat are two life-limiting factors for TBC systems. In the present study, Ni-base single crystal alloys coated with an EB-PVD NiCoCrAlY bond coat and EB-PVD YSZ top layers of three different columnar morphologies were subjected to 1-h cyclic oxidation tests at 1100 and 1150°C. Tests were mainly focused on assessment of the spalling resistance of different TBC microstructures related to oxide scale growth and failure mechanisms. In addition, the influence of bond coat composition for uncoated specimens and TBC microstructure on coated specimens on the hot corrosion behavior of a YSZ coated NiCrAlY cast bond coat alloy was evaluated in 1-h cyclic tests at 950°C. Two kinds of salt mixtures were used to simulate deposits actually found in industrial gas turbines, as well as deposits expected to be formed by using biomass-derived fuels.
2:30 PM		A3-2-4 EB-PVD Process Guidance for Highly Productive Zirconia Thermal Barrier Coating of Turbine Blades E. Reinhold, P. Botzler, C. Deus (Von Ardenne Anlagentechnik GmbH, Germany) Zirconia thermal barrier coatings are well introduced in the turbine manufacturing industry because they ensure extended lifetimes of special components like turbine blades. Compared with other techniques EB-PVD proceses are best suited for the deposition of turbine blades with regard to the layer properties. Therefore EB-PVD coaters for turbine blades are becoming increasingly interesting. The coating costs per component are mainly dependent on a highly productive solution for the deposition task. Thus the EB-PVD process guidance has to be optimized in order to meet the productivity requirements of the manufacturers. This includes the requirement of high deposition rates, large deposition areas, long time stable production cycles as well as a matched duration of preheating, deposition and cooling down per charge. Modern EB-PVD solutions to be introduced allow deposition rates on blades up to 7µm/min. Consequences for the technological process guidance and plant design concerning long time stable coating cycles with high productivity will be discussed.
2:50 PM		A3-2-5 Thermal Cyclic Behaviour and Structure of EB-PVD TBC V.I. Topal, K.Y. Yakovchuk, V.Y. Bratus (Pratt and Whitney-Paton, Ukraine) Some experimental results of thermal cyclic behavior and microstructure investigations of the EB-PVD metal-ceramic two layers thermal barrier coatings for industrial gas turbine blades are discussed. MCrAlY bond coat and YSZ top coat produced using EB high-rate evaporation. The effect of additions of Y, Si and Hf in bond coat was also investigated. Furnace cyclic test in temperature range from 1093⁰C to ambient temperature of coupons and burner rig test of the bars were used to determine of thermal cyclic lifetime of TBC in comparison with diffusion Pt-Al bond coat. The method used for TBC investigations were optical metallography and scanning electron microscopy
3:30 PM		A3-2-7 Improvement of Thermal Barrier Coating Stability by Surface Preparation of the Bond Coat With Large Area Electron Beams. G. Müller (Forschungszentrum Karlsruhe, Germany); N. Czech, W. Stamm (Siemens AG, Germany); G. Schumacher, D. Strauss (Forschungszentrum Karlsruhe, Germany); W.J. Quadakkers (Forschungszentrum Jülich, Germany) The surface of LPPS-MCrAlY-coatings is treated by the large area electron beam of the GESA-facility. The diameter of the beam is 6 - 10 cm in diameter and the energy density high enough to melt a surface layer of 10 - 20 µmm depth by one single pulse of 10 - 40 µms. Structures produced by this treatment are of nanometer size with a preferential orientation perpendicular to the surface because of the rapid cooling of the melt. This structures are expected to form an ideal interface for a Yttria-stabilized zirconia thermal barrier coating, because oxide scales growing on those structures are dense and consist of pure α-alumina. Thermal barrier coatings are deposited by EBPVD onto such prepared MCrAlY-bond coat surfaces. The structure and properties of the obtained coating systems are investigated and the results presented in this paper. Keywords: thermal barrier coatings, bond coat, surface treatment
3:50 PM		A3-2-8 Reactively Magnetron Sputter Deposited Yttria Stabilized Zirconia Coatings: Phase Formation, Crystallographic Texture and Growth Morphology Z. Ji (The University of Alabama at Birmingham); J.M. Rigsbee (North Carolina State University) A fundamental understanding of phase formation and stability in yttria stabilized zirconia (YSZ) coatings may allow improved properties and performance as thermal barrier coatings. In this work, a series of YSZ coatings were produced by reactive-magnetron sputter deposition using a system with multiple sputter sources and a r.f-biased substrate stage. Phase formation, crystal structure and growth morphology of the coatings as functions of yttria contents, applied substrate bias and anneal temperature were investigated by transmission electron microscopy (TEM), convergent beam electron diffraction (CBED) and x-ray diffraction (XRD). The mechanism of phase formation and stability in the YSZ coatings will be discussed.
4:10 PM		A3-2-9 CVD Mullite for Environmental Barrier Coating Systems J.A. Haynes, K.M. Cooley, D.P. Stinton, R.A. Lowden (Oak Ridge National Laboratory); W.Y. Lee (Stevens Institute of Technology) Silicon-based composites (e.g., SiC/SiC) are candidate materials for high temperature structural components in gas turbine engines. The environmental resistance of Si-based ceramics is dependent on the formation of an adherent, protective silica scale. However, it has been clearly demonstrated that silica scales can be rapidly volatilized by reaction with high temperature water vapor in combustion environments. Thus, for gas turbine engine applications environmental barrier coatings (EBCs) will be necessary in order to protect silica scales that form on SiC-based composite systems. Mullite (3Al 2O3-2SiO2) coatings are being considered for use in multi-layered EBC systems. In this study, mullite coatings were deposited on various SiC ceramics and composites by chemical vapor deposition (CVD). The advantages of fabricating mullite by CVD include: (1) dense, crystalline mullite can be deposited, (2) lower impurity levels than plasma-spray, (3) no substrate surface roughening is required for adherence, (4) coating surface roughness can be controlled, and (5) it is not a line-of-sight process. The effects of deposition temperature, reactant concentrations and flow conditions on coating composition and morphology will be discussed. Coating response to thermal cycle testing in ambient air and isothermal testing in high pressure steam-containing environments will also be described.}
4:30 PM		A3-2-10 High Temperature Oxidation Behavior of Electron Beam Physical Vapor Deposited Thermal Barrier Coatings M.J. Stiger, N.M. Yanar, F.S. Pettit, G.H. Meier (University of Pittsburgh) Yttria-stabilized Zirconia (YSZ) coatings deposited by electron beam physical vapor deposition on platinum aluminide bond coats on the superalloy N5 have been oxidized at temperatures between 1000 and 1200°C in air. The thermally grown oxide (TGO) that develops between the bond coat and the TBC during oxidation, as well as the bond coat and the TBC adjacent to the TGO, have been examined in detail using optical metallography, scanning electron microscopy (SEM), and cross-sectional transmission electron microscopy (XTEM). The YSZ is observed to undergo significant amounts of sintering. The TGO grows by the inward diffusion of oxygen and the outward diffusion of aluminum. The outward growth component incorporates some of the TBC into the TGO. Failure of the TBCs occurs after fixed amounts of oxidation which result in increasing amounts of elastic energy being stored in the TGO and YSZ as well as degradation of bonding at the TGO-bond coat interface. Spalling of the TBC occurs by fracture predominantly along the TGO-bond coat interface. The fracture surface contains precipitates which are rich in refractory metals. A mechanism for the above-mentioned phenomena will be presented and preliminary results on the effects of water vapor on TBC degradation will be presented.
4:50 PM		A3-2-11 Improved Oxidation Resistance of Thermal Barrier Coatings Kh.G. Schmitt-Thomas, M. Hertter (Technical University of Munich, Germany) In order to improve the engine output and the efficiency of gas turbines, optimized thermal barrier coatings (TBCs) are required to protect the metallic components at high temperatures. In common TBC-systems, consisting of a Ni-base alloy substrate/MCrAlY-bond coat/ZrO₂7wt.% Y₂O₃top coat, an oxide layer grows at the interface bond coat/ceramic under high temperature service, which limits the life of these coatings. In this paper the oxidation resistance of a new triplex TBC-system, consisting of a CoNiCrAlY-bond coat/Pt-modified aluminide coating/ZrO₂7wt.% Y₂O₃top coat is compared with that of a common TBC-system. The as-coated Pt-aluminide coating consists of an outer region of PtAl₂+ (CoNiPt)Al followed by a single phase layer of (CoNiPt)Al. The results of the oxidation tests at 1273K, 1323K and 1373K in air show excellent oxidation resistance of the triplex TBC-systems, which increases with the Pt-aluminide coating thickness. In particular, a 28 µm thick Pt-aluminide coating allows the thickness of the oxide layers to be reduced up to 70% compared to the common TBC after 500h at all examined temperatures. After heat treatment the coating systems were investigated by SEM, EDX and X-ray analysis. Annealing tests with Al₂O₃-powder indicate which mechanism is responsible for the improved oxidation resistance of platinum additions. Platinum is evidently capable of decomposing aluminumoxide into aluminum and oxygen at temperatures above 1173K .