Papers

ICMCTF2009 Session A1-2: Coatings to Resist High Temperature Oxidation

Tuesday, April 28, 2009 1:30 PM in Room Royal Palm 1-3

Tuesday Afternoon

Start	Invited?	Item
1:30 PM		A1-2-1 Compositional, Geometrical and Manufacturing Related Parameters Affecting the Oxidation Resistance of MCrAlY-Coatings D. Naumenko, J. Toscano, M. Subanovic, L. Singheiser, W.J. Quadakkers (Forschungszentrum Jülich GmbH, Germany) MCrAlY (M = Ni,Co) overlay coatings and bondcoats (BC) in combination with thermal barrier coatings (TBC) are commonly used to protect gas-turbine components from high-temperature oxidation and corrosion. The oxidation behavior of MCrAlY coatings is, therefore, a crucial factor for the component lifetime. In the present paper a number of factors affecting the oxidation resistance of state of the art MCrAlY-coatings are reviewed. It is shown that the scale formation on the MCrAlY-coatings and TBC-lifetime are affected by the coating major chemical composition, i.e. the Co/Cr/Al-contents. Furthermore, the composition of the formed oxide scale is influenced by the coating geometrical parameters, i.e. surface roughness, profile and thickness. These parameters affect the depletion of elements, such as Y and Al in the coating sub-surface regions resulting in formation of inhomogeneous oxide scales underneath rough, as-sprayed MCrAlY-surfaces. Moreover, the coating thickness not only determines the reservoir of the scale forming element Al, but also that of Y. It is shown that not only the Y-content but also the Y-reservoir has a significant effect on the growth rate and adherence of the alumina scale. Finally, the effects of various processing parameters on the oxidation resistance of MCrAlY overlay coatings and bondcoats are considered. These parameters include vacuum quality during plasma spraying, temperature regime of the vacuum heat-treatment as well as smoothening treatment prior to electron-beam physical vapor deposition (EB-PVD) of the TBC. High temperature and high vacuum quality during heat-treatment promote selective oxidation of reactive elements (RE) such as Y at the coating surface. This results in RE-depletion from the coating and favors accelerated scale growth and high growth stresses during subsequent oxidation exposure. It is shown that minor variations in processing parameters can result in significant variations in the oxidation behavior and lifetime of nominally the same MCrAlY-coatings and bondcoats.
2:10 PM		A1-2-3 Microstructure and Oxidation Resistance of Nanocrystalline 304SS-Al Coatings N.S. Cheruvu, R. Wei (Southwest Research Institute); R. Govindaraju (Karta Technologies); D.W. Gandy (Electric Power Research Institute) The long-term oxidation behavior of nanocrystalline Fe18Cr8NiAlx (where x = 0 to 10) coatings has been investigated. The coatings were deposited on 304SS samples by sputtering a 304 stainless steel target and an Al target using two magnetrons. Cyclic oxidation tests were conducted on the coated and un-coated samples at a peak temperature of 750°C for up to 1000 one-hour thermal cycles between peak and room temperatures. Optical, transmission and scanning electron microscopy and x-ray diffraction have been used to assess the microstructure, chemical composition of the coating and the oxide scale on the exposed samples. The crystal structure of the coatings, irrespective of Al content, in the as-deposited condition was found to be nanocrystalline BCC. The microstructure of the as-deposited Fe18Cr8Ni coating showed the presence of sigma phase. The addition of Al to the Fe18Cr8Ni coating stabilized the BCC structure and prevented the formation of the sigma phase. The re sults showed that the external scale (Cr₂O₃ on Fe18Cr8Ni and A₂O₃ aluminum containing coatings) formed during cyclic oxidation testing on the nano crystalline coated samples exhibited good spallation resistance compared to the scale on the un-coated samples. The addition of Al to the coating further increased the oxide spallation resistance. After exposure to approximately 1000 thermal cycles, the coating containing 10% Al was in good condition showed no evidence of internal oxidation. Inward diffusion of Al into the substrate during thermal cycling resulted in precipitation of iron-aluminde particles in the interdiffusion zone. The interdiffusion zone width increased with increasing Al content in the coating. Due to inward and outward diffusion of Al during thermal cycling, the Al content in the coating dropped from 10.5 to 3.7 wt.%. The improvement in oxide scale spallation resistance and accelerated depletion of aluminum are believed to be related to the fine grai n structure of the coating.
2:30 PM		A1-2-5 Oxidation Resistance of a Zr-Doped NiAl Bondcoat Thermochemically Deposited on a Nickel-Based Superalloy S. Hamadi, M.P. Bacos, M. Poulain (ONERA, France); V. Maurice, P. Marcus (ENSCP/CNRS, France) Turbine blades are exposed to oxidation at high temperatures (>1100°C) and are thus protected by coatings such as nickel aluminide. In industry, Pt-modified or unmodified NiAl are used, but investigations are carried out to replace them. The beneficial effects of adding reactive elements to NiAl in order to improve thermally grown alumina adhesion on top of NiAl are highlighted in the literature. Our study is based on a coating developed by Onera and Snecma that includes Zr in NiAl by a vapor phase thermochemical co-deposition of Al and Zr on a nickel-based superalloy. In previous experiments (SEM, spectral maps, GDMS), we pointed out that our aluminizing process led to a β-NiAl coating with 300 at. ppm of Zr located at the interface between the superalloy substrate and the coating layer. However, as soon as it was annealed or short-time oxidized, Zr migrated through the coating to the interface oxide/NiAl mostly as a metal (ToF-SIMS, XPS). In this study, we focus our effort on the oxidation resistance of this system. It appeared that doping NiAl with hundreds at. ppm of Zr highly increased the lifetime of the system under cyclic oxidation (1h 1100°C cycles under ambient air). Promising results were obtained when compared to AM1/(Ni,Pt)Al system. We investigated the influence of Zr on the morphology of the alumina developed and on the oxide/NiAl interface. Furthermore, the coating evolution during these oxidation tests was periodically characterized. Isothermal oxidation tests in a thermobalance were performed and revealed that Zr, initially present deep within the coating, played a role during the transient oxidation stages and lowered the parabolic constant of α-alumina in the steady-state stage.
2:50 PM		A1-2-6 Reactive Element-Modified Aluminide and Platinum Aluminide Coatings on Ni-Base Superalloys M.S. Priest, Y. Zhang (Tennessee Technological University); B.A. Pint, J.A. Haynes (Oak Ridge National Laboratory); B.T. Hazel, B.A. Nagaraj (GE Aircraft Engines) A pack cementation process was developed for synthesizing reactive element (RE)-doped aluminide and platinum aluminide coatings on Ni-base superalloys. Three RE dopants, including Hf, Zr and Y, were investigated in this study by incorporating various RE-containing sources in the pack mixtures. All coatings were deposited by utilizing a non-contact arrangement where the substrates were physically separated from the powder mixture, which produced clean coating surfaces comparable to that made by chemical vapor deposition (CVD) or above-the-pack. The effect of aluminizing parameters, particularly the amount of RE-containing powder in the pack, was studied. Different responses of the three dopants in the aluminizing process and the resultant coating microstructures were discussed. The cyclic oxidation behavior of RE-doped aluminide coatings was investigated at 1150°C in air, and compared with RE-free aluminide coatings.
3:10 PM		A1-2-7 NiW Diffusion Barrier Influence on the Oxidation Behaviour of β-(Ni,Pt)Al Coated Fourth Generation Ni-Base Superalloy E. Cavaletti, S. Naveos, S. Mercier, P. Josso, M.P. Bacos (ONERA, France); D. Monceau (ENSIACET-INPT, France) For long-term life at high temperature of gas turbine blades, interdiffusion between the Ni-base superalloy and its protective coating degrades the complete system. Firstly, diffusion of some substrate alloying elements into the coating damages the protective oxide scale adhesion. Secondly, oxidation and diffusion cause aluminium depletion in the coating which leads to phase transformations and affects the lifetime of the system. Finally, in Re-rich Ni-base superalloys coated with β-(Ni,Pt)Al, discontinuous precipitation leads to the formation of detrimental Secondary Reaction Zones for which interdiffusion is supposed to be a driving force. To limit interdiffusion phenomenon, diffusion barriers (DB) have been developed. Fabrication of DB layers based only on refractory metals is complex and often requires a Ni deposition before aluminisation. To simplify the process, a DB layer based on a Ni-W electrolytic coating was developed to limit interdiffusion between a Ni -base fourth generation superalloy (MCNG) and a β-NiAl coating. The DB layer is created thanks to the precipitation of α-W phase that forms definite compounds with Re from the alloy during high temperature exposure. In this paper, the Ni-W DB was applied on a (Ni,Pt)Al coated MCNG alloy. The cyclic and isotherm kinetics were determined at 1100°C in air. The structures and composition changes of both coating and superalloy were measured with image analysis, X-ray diffraction, SEM observation and EDS analysis. It was found that the DB acts at the onset of oxidation on the oxide formation and on the β to γ’ transformation kinetics, e.g. after 300 h oxidation at 1100°C the γ’ fraction is reduced from 70 to 40%.
3:30 PM		A1-2-8 Formation and Oxidation Performance of Low-Temperature Pack Aluminide Coatings on Ferritic-Martensitic Steels Y. Zhang, B. Bates (Tennessee Technological University); B.A. Pint (Oak Ridge National Laboratory) A pack cementation process was developed to coat commercial 9% Cr ferritic-martensitic steel P91 at temperatures below its tempering temperature to avoid any potential detrimental effect on the mechanical properties of the coated alloy. To prevent the formation of Al-rich intermetallic phases such as Fe₂Al₅ in the coating, the Al activity in the pack cementation process was reduced by substituting the pure Al masteralloy with Cr-Al binary masteralloys containing 25 and 15 wt.% Al. When the Cr-25Al masteralloy was used, a duplex coating was formed at 700ºC, consisting of a thin Fe₂Al₅ outer layer and an inner layer of FeAl. With the Cr-15Al masteralloy, the Fe₂Al₅ phase was eliminated and an FeAl coating of ~12 µm thick was achieved. In addition, an effort was made to combine the coating process with the standard heat treatment for ferritic-martensitic alloys, e.g., 2h austenization at 1050°C and 2h tempering at 750°C. The coatings fabricated at 700°C are being tested in air + 10% H₂O at 650°C to evaluate their long-term oxidation performance. Aluminide coatings synthesized at 1050°C via pack cementation or chemical vapor deposition (CVD) are included in the test for comparison.
3:50 PM		A1-2-9 Pt Modified Nickel Aluminides, MCrAlY-Base Multilayer Coatings and TBC Systems Fabricated by Sparks Plasma Sintering (SPS) for the Protection of Nickel Base Superalloys D. Monceau (ENSIACET-INPT, France); D. Oquab (CIRIMAT Laboratory - Toulouse, France); C. Estournes (CIRIMAT and PNF2 CNRS - Toulouse, France); M. Boidot (CIRIMAT Laboratory - Toulouse, France); Y. Cadoret (SNECMA, France) Pt-modified Ni aluminides and MCrAlY coatings (where M = Co, Ni or Co/Ni) are widely used on turbine blades and vanes for protection against oxidation and corrosion and as bondcoatings in thermal barrier coatings systems. The present work shows the ability of a new fabrication technique, the Spark Plasma Sintering, to develop rapidly new coating compositions and microstructures. This technique allows combining powders and metallic foils on a superalloy substrate in order to obtain multilayered coatings in a single short experiment. Fabrication of MCrAlY overlays with local Pt and/or Al enrichment will be shown, as well as fabrication of coatings made of PtAl₂, PtAl, αβ-AlNiPt₂, martensitic β-NiPtAl or Pt-rich γ/γ' phases. The realization of an entire TBC system with a porous adherent YSZ layer on a γ/γ' low mass bond-coating will be demonstrated. Difficulties of this technique will be reviewed and discussed, such as unexpect ed segregations, risks of carburization, of local over-heating, or difficulty to coat complex shape parts. Finally, some first results of cyclic oxidation will be given including a comparison with industry made coatings and systems.
4:30 PM		A1-2-11 Role of Al Oxide PVD Coatings in the Protection Against Metal Dusting J. Alvarez (ITESM-CCV); B. Bautista, D. Melo (IPN, Mexico); O. Salas (ITESM-CEM, Mexico); R. Reichelt (Wilhelms-Universitaet, Germany); J. Oseguera (ITESM-CEM, Mexico); . López (IPN, Mexico) The presence of some surface oxides in materials exposed to metal dusting have been proven to be an effective method to prevent this type of corrosion due to the very low diffusivity of carbon in oxides. However, the surface oxide films have to be dense and adhere well to the component to be protected. Reactive magnetron sputtering (RMS) is a promising method to produce these type of oxide layers due to its flexibility in terms of process parameters and resulting structures. In the present work, Al oxide/Al layers have been deposited by RMS on HK40 substrates under various conditions in order to develop the most adequate structure for protection against metal dusting. Some coated substrates were subjected to metal dusting conditions in a thermobalance. The microstructure of the coatings before and after metal dusting was characterized by scanning electron microscopy, atomic force microscopy and x-ray diffraction.
4:50 PM		A1-2-12 Observation of High-Temperature Phase Transformation in the Si-Modified Aluminide Coating on Mild Steel Using EBSD W.-J. Cheng, C.-J. Wang (National Taiwan University of Science and Technology, Taiwan) Mild steel was coated by hot-dipping into a molten bath containing Al-10 wt.% Si. The phase transformation in the aluminide layer during diffusion at 750°C in static air was analyzed by Electron Backscatter Diffraction (EBSD). The results showed that the aluminide layer of the as-coated specimen consisted of an outer Al-Si eutectic topcoat and the inner Fe-Al-Si and Fe-Al intermetallic layers. The formation of τ₅-Al₇Fe₂Si and τ₆-Al₄FeSi can be observed with increasing exposure time at 750 °C, while the τ₁-(Al,Si)₅Fe₃ phase precipitated in Fe₂Al₅. After 60 min of exposure, the τ₅-Al₇Fe₂Si and τ₆-Al₄FeSi phases disappeared. The FeAl phase not only formed at the interface between Fe₂Al₅ and the steel substrate, but also transformed from τ₁-(Al,Si)₅Fe₃ after diffusion for 10 h. With prolonged exposure, the growing FeAl phase decreased the thickness of Fe₂Al₅ and forced the formation of FeAl₂ phase. Finally, the aluminide layer composed of FeAl₂ and FeAl.