Papers

ICMCTF2006 Session A1-1: Coatings to Resist High Temperature Corrosion and Wear

Tuesday, May 2, 2006 8:30 AM in Room Sunrise

Tuesday Morning

Start	Invited?	Item
8:30 AM		A1-1-1 The Evolution, Flexibility and Potential of the MCrAlY Coatings F.S. Pettit (The University of Pittsburgh) The MCrAlY coatings are now widely used for high temperature applications in oxidizing and deposit-influenced corrosion situations. This paper will discuss the development and evolution of the MCrAlY coatings. It will be shown that the MCrAlY coatings initially appeared as FeCrAlY coatings but were changed to NiCrAlY due to diffusion stability factors. The NiCrAlY coatings were displaced, temporarily, by CoCrAlY coatings due to hot corrosion conditions in gas turbines. Eventually a wide variety of MCrAlY coatings appeared to respond to a variety of high temperature corrosion conditions and mechanical property requirements. This paper will consider the various factors that must be considered in using the flexibility of the MCrAlY coatings to respond to the various corrosion and mechanical property requirements of high temperature metallic coatings.
9:10 AM		A1-1-3 Thermal Expansion of Tribomet TM MCrAlY Coatings T.A. Taylor, J. Foster (Praxair Surface Technologies) A wide range of Tribomet electroplated MCrAlY coatings have been tested for thermal expansion up to 1100°C in argon. The free-standing coatings were all thermally stabilized prior to the thermal expansion test. Equations for the expansion vs. temperature were obtained for each coating. At two fixed temperatures, the expansions of the coatings were correlated to their chemical composition.
9:30 AM		A1-1-4 The Effect of Surface Condition on the Oxidation Kinetics of MCrAlY Coatings A. Gil (AGH University of Science and Technology, Poland); V. Shemet, R. Vassen, J. Toscano, M. Subanovic, D. Naumenko, L. Singheiser, W.J. Quadakkers (Forschungszentrum Juelich, Germany) In industrial gas turbines coatings of the MCrAlY type are commonly used as overlay coatings and as bond coatings (BC's) for ceramic thermal barrier coatings (TBC's). During high temperature service the MCrAlY coatings form aluminium based surface oxide scales. The technologically most relevant properties of the oxide scales, growth rate and adherence, not only depend on the exact MCrAlY composition but also on the surface condition after coating manufacturing. The surface condition may differ, depending on the coating manufacturing process (e.g. VPS, LPPS or HVOF), subsequent surface treatments (e.g. shot peening, grinding) and heat treatments (e.g. diffusion annealing, TBC deposition method). In the present study the oxidation behaviour of a number of MCrAlY coated superalloys was studied in the temperature range 900 to 1100°C. Thereby main emphasis was put on the effect of different coating manufacturing processes and the subsequent mechanical surface treatments on oxide scale formation. The exposure times for the oxidation studies ranged from 100 to several thousands of hours. After exposure the surface scale composition and morphology were analysed by common light and electron optical analysis techniques as well as by fluorescence spectroscopy, XRD and SNMS.
9:50 AM		A1-1-5 Advanced Burner-Rig Test for Oxidation-Corrosion Resistance Evaluation of MCrAlY/Superalloys Systems A. Raffaitin, F. Crabos (TURBOMECA-SAFRAN, France); E. Andrieu, D. Monceau (CIRIMAT, France) Protective coatings are used on gas turbine components for survival in the operating conditions of engines. Cyclic oxidation testing is one of the fundamental ways to assess high temperature environmental resistance of materials, but laboratory tests are not able to fully simulate engine environment. This study presents a recently developed cyclic burner-rig test that is used to simulate Turbomeca's engines conditions and to assess the oxidation and hot corrosion behavior of MCrAlY coatings on nickel-base superalloys. A dilute sea-salt solution is atomised into the burner-rig to simulate hot-corrosion. Specimens are ramped from 900°C to oxidising conditions (1000°C, 5min) similar to those present during take off and climb, then dropped to 950°C (25min dwell). One cycle thus corresponds to 1h at varying temperatures in the range of 900°C to 1000°C followed by 15min cooling to room temperature. Specimens are tested up to 1000 such cycles. Three different MCrAlY coatings thicknesses have been used to determine the influence of the Al reservoir on the time of life of the coated MC2 superalloy. The evolving microstructural features have been identified using high resolution scanning electron microscopy and energy dispersive spectroscopy. Elemental profiles of the coatings are compared to a numerical model predictions. Influence of coating thickness and test atmosphere are discussed, and compared with isothermal testing in pure oxidising conditions. Metallographic evaluation revealed performance results somewhat different from those obtained by laboratory testing. This type of advanced burner rig test cycle is successful at reproducing the accelerated combined hot-corrosion/oxidation attack. Nevertheless, laboratory tests in simplified conditions are necessary to understand the elementary mechanisms of degradation. They are complementary to such complex burner-rig tests in which oxidation, corrosion and interdiffusion interact.
10:10 AM		A1-1-6 Hot Corrosion and Oxidation Behavior of Novel Pt+Hf Modified y-Ni₃Al + y-Ni-Based Aluminide Coating V. Deodeshmukh, B. Gleeson (Iowa State University) This study compares and assesses the hot-corrosion resistance of CoCrAlY and Pt-modified CoAl coatings, together with Pt-modified β-NiAl coatings. A laboratory-scale Dean rig was used to simulate hot corrosion conditions. Coating performances were determined on the basis of cross-sectional imaging of the corroded samples. It was found, for instance, that the CoCrAlY coating had better low-temperature (Type II) hot corrosion resistance than the Pt-modified CoAl coating for the range of times tested (up to 200 h), while Pt-modified β-NiAl coating had the best Type II hot corrosion resistance. The inferred reasons for these findings will be presented.
10:30 AM		A1-1-7 Characterization of Chemical and Microstructural Evolution of a NiPtAl Bondcoat During High Temperature Oxidation R. Molins (Ecole Des Mines, France); P.Y. Hou (Lawrence Berkley National Laboratory) Bondcoats used to protect turbine blades, such as platinum modified NiAl alloys, are designed to develop a protective alumina scale during exposure conditions at high temperatures. However, during high temperature oxidation, the system is subjected to chemical and microstructural changes that arise from the consumption of aluminium to ensure alumina growth and interdiffusion between the underlying nickel-based superalloy and the bondcoat. The aim of the present work is to report experimental results concerning the chemical and microstructural evolutions of a NiPtAl bondcoat, deposited on a single crystal nickel-based superalloy, during isothermal oxidation tests at 1100°C, up to 50 hours. Analytical STEM studies were carried out, in correlation with Auger experiments, in order to follow the various changes that occur in the bondcoat (β into γ prime phase transformation, TCP and α-Cr precipitation, S diffusion pathways) and at the alumina/bond coat interface. Efforts were concentrated on the effect of interfacial sulfur segregation (at both intact interface and void surface) as a function of the oxidation time and the initial S content in the superalloy, as well as its dependence on phase transformations in the external layer of the bondcoat. Strong S segregation at the bondcoat NiPtAl/alumina interface is due to a co-segregation with Cr, which came from the substrate. This indicates the importance of elemental diffusion from the substrate on interface composition. S was also present on all interfacial void surfaces. Its content increased with oxidation time to a saturation level. The extent of voids at the interface and scale spallation concomitantly increased with oxidation time. The presence of S at the interface is believed to enhance pore formation and reduce scale adhesion.
10:50 AM		A1-1-8 Effect of Pt and Al Contents on High Temperature Oxidation Behaviour and Interdiffusion of a Pt Modified Aluminide Coating Deposited on Ni-Base Superalloys N. Vialas, D. Monceau (CIRIMAT-INPT, France) The present work is devoted to the effect of Al and Pt content on the oxidation behaviour and interdiffusion of the industrial NiPtAl coating RT22(TM) deposited on two different Ni-base superalloys: SCB(TM) and IN792(TM). Some specimens of RT22/SCB experienced a defective aluminization resulting in one side with a lower Al content, and some specimens of RT22/IN792 with less platinum than the specification. Detailed investigations showed the effect of both elements Pt and Al on the initial microstructure of the coating. Isothermal oxidation tests for 100 h and long term cyclic oxidation/interdiffusion tests at 1050°C were performed (up to 6, 17, 35 and 51 cycles of 300h each) on defective coatings as well as on standard coatings. Comparison of their oxidation kinetics, of the nature of oxides formed and of their microstructural and chemical evolution, by using thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning-electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), highlighted the importance of a good aluminization and help to better understand the beneficial effect of the Pt content on high temperature resistance.
11:10 AM		A1-1-9 The Effect of Water Vapor on the Oxidation Behavior of Ni-Pt-Al Coatings and Alloys B.A. Pint, J.A. Haynes (Oak Ridge National Laboratory); Y. Zhang (Tennessee Technological University); I.G. Wright (Oak Ridge National Laboratory) Turbines fired with hydrogen or syngas from coal gasification will have significantly higher water vapor contents in the exhaust stream than natural gas fired turbines. This may affect the durability of hot section coatings in the turbine. Current work is examining the effect of up to 50% water vapor on the spallation resistance of single phase (NiPt-40at.%Al) and two phase (NiPt-22Al) coatings and cast alloys in furnace cyclic testing at 1100°C. Initial results will be presented for both simple and Pt-modified CVD aluminide coatings on Y-containing, single-crystal Ni-base superalloys along with Pt diffusion coatings on single crystal and directionally solidified superalloys.
11:30 AM		A1-1-10 Synthesis and Oxidation Behavior of Platinum-Enriched γ+γ' Bond Y. Zhang, D.A. Ballard (Tennessee Technological University); J.A. Haynes, B.A. Pint, I.G. Wright (Oak Ridge National Laboratory) Previous work has indicated a Pt-enriched γ+γ' two-phase coating can be synthesized by electroplating a thin layer of Pt (~7µm) on a superalloy substrate followed by a diffusion treatment in vacuum at 1100-1200°C. However, the Al content in the coating layer prepared by this method is typically in the range of 16-19 at.% for a superalloy with ~13 at.% Al (~6wt.%). The present study focuses on increasing the Al content to ~22 at.% in the as-deposited coating while maintaining the γ+γ' two-phase microstructure without the formation of β-(Ni,Pt)Al. This can be achieved by introducing a short-term aluminizing process via chemical vapor deposition (CVD) or pack cementation prior to or after post-plating diffusion treatment. The effect of the synthesis parameters, such as aluminizing time and temperature of post-plating diffusion treatment, is being investigated. In addition, reactive elements such as Hf can be incorporated into the γ+γ' two-phase coating during the aluminizing process for improved oxidation resistance.
11:50 AM		A1-1-11 Microstructure of Hot-Dip Aluminized Coatings on Ni-Base Superalloy In-718 and their Cyclic Oxidation Behaviors at 1000-1200°C C.-J. Wang, S.-M. Chen (National Taiwan University of Science and Technology, Taiwan) Ni-base superalloy In-718 was coated by hot-dipping into a molten bath containing pure Al, Al-7wt%Si, respectively. The cyclic oxidation tests of aluminized alloy and untreated substrate were studied over the temperature range of 1000-1200°C for 240 hr in static air. After hot-dip treatment, all the coating layers consisted of two phases, where Al, Al₃WNi were detected from the external topcoat to the aluminide/steel substrate. After cyclic oxidation test, a continuous alumina scale was detected on the surface of the aluminide layer and phase transformation reactions also occur in the aluminide layer. These two coatings display excellent cyclic oxidation resistance on In-718 alloy compared with untreated substrate. Cr₂O₃ and Al₂O₃ are found as the primary oxide phase in the oxidation of bare In-718 alloy. However, the inward diffusion of Al in the aluminide layer shows detrimental effect to the cyclic oxidation resistance of superalloy for long-term exposure.
12:10 PM		A1-1-12 A Study on the Microstructure and Cyclic Oxidation Behavior of Pack Aluminized Hastelloy X at 1100°C J.W. Lee, Y.-C. Kuo (Tung Nan Institute of Technology, Taiwan) A pack aluminizing process at 950°C for 9 hours has been employed on the Nickel-base superalloy Hastelloy X to deposit a 75 µm thick NiAl aluminide layer on the surface. A nano scale dendrite structure is observed on the aluminized surface. An interdiffusion zone around 10 µm thick is found between the aluminide layer and the substrate. Lots of fine precipitates with complex phases are also observed in the NiAl layer. The cyclic oxidation tests of aluminized alloy and untreated substrate were conducted at 1100°C for 196 hours. It is observed that the aluminizing process greatly enhances the cyclic oxidation resistance of Hastelloy X at 1100°C due to a dense and protective alumina layer formed on the surface.Complex phase transformation reactions occurred in the aluminide layer. Owing to the oxidation and diffusion reactions at high temperature, the Al content of the NiAl layer was depleted to form some low Al containing grains and a continuous layer between the aluminide layer and substrate. Thermal stress induced transverse cracks in the interdiffusion zone were observed due to the difference of coefficients of thermal expansion among the substrate, aluminide layer and the interdiffusion zone.