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Beyond Nickel-Based Superalloys II Proceedings

7-17-2016 Improving the oxidation resistance of via aluminum diffusion coatings and halogen effect A.S. Ulrich DECHEMA-Forschungsinstitut, Theodor-Heuss-Allee 25, Germany, [email protected]

M.C. Galetz DECHEMA-Forschungsinstitut, Theodor-Heuss-Allee 25, Germany, [email protected]

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Recommended Citation A.S. Ulrich and M.C. Galetz, "Improving the oxidation resistance of refractory metals via aluminum diffusion coatings and halogen effect" in "Beyond Nickel-Based Superalloys II", Chair: Dr Howard J. Stone, University of Cambridge, United Kingdom Co-Chairs: Prof Bernard P. Bewlay, General Electric Global Research, USA Prof Lesley A. Cornish, University of the Witwatersrand, South Africa Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/superalloys_ii/55

This Abstract and Presentation is brought to you for free and open access by the Proceedings at ECI Digital Archives. It has been accepted for inclusion in Beyond Nickel-Based Superalloys II by an authorized administrator of ECI Digital Archives. For more information, please contact [email protected]. Improving the Oxidation Resistance of Refractory Metals via Aluminum Diffusion Coatings and Halogen Effect

A. S. Ulrich, M. C. Galetz DECHEMA‐Forschungsinstitut, Theodor‐Heuss‐Allee 25, 60486 Frankfurt a. M. e‐mail: [email protected] Introduction Research on high temperature applications such as gas turbine engines, reformer systems, turbochargers or various power plant components [1] focuses on increasing the efficiency which is achieved by increasing the temperature. Refractory metals offer higher working temperatures than Ni‐based superalloys due to their high melting points and comparable mechanical properties. However, refractory metals suffer from low oxidation resistance at elevated temperatures. Meeting this challenge, aluminum diffusion layers

were manufactured on the refractory metals , niobium, and using pack cementation process with the aim to form protectiveAl2O3 scales during oxidation. The oxidation kinetics of the coated samples are investigated via thermogravimetric analysis (TGA) for up to 100 h at 1300°C in synthetic air. The halogen effect is used

to enhance the protective Al2O3 layer formation and thereby increase the materials resistance against high temperature oxidation. The importance of optimization of the halogen treatment depending on the substrate material is demonstrated on tantalum. Coating System The Fluorine Treatment on Tantalum T = 1300°C TGO: Oxidation of aluminized and additional fluorine treated tantalum substrates for 40 h @ • oxidation protection 1300°C in synthetic air Al2O3 • diffusion barrier Al2O3 11 µm Al reservoir AlTa2/AlTa/Ta BC: 39 µm • Al reservoir to form AlTa2 refractory metal and maintain the Mo, Nb, Ta, W oxide layer Ta substrate Schematic of the target protective layer system for refractory metal

single F‐treatment double F‐treatment Pack Cementation Process • competition between Al2O3 and TaxOy formation Ni plating • similar to TiAl alloys the amount of fluorine is a critical factor [2] • Diffusion coating process • Substrate is embedded in Halogen effect: powder mixture (1 wt.% Al, 1 • Halogen treatment enhances wt.% NH4Cl, 98 wt.% Al2O3) protective Al2O3 scale formation • Heated for 8h @ 1000°C during oxidation  Homogeneous intermetallic • Good oxidation protection by layers with high aluminum Al2O3 because of its very low contents are formed permeability for O, N, H, and metal ions Fluorine treatment process • Fluorine superior to other halogens (+F) Cross sections after pack cementation Al‐rich coati Effect of Aluminizing and Halogen Treatment on Oxidation Isothermal Oxidation for 100 h • Uncoated substrates show destructive oxidation • Aluminized and double fluorine treated Ta: 12 h @ 1300°C 100 h @ 1300°C . • Substrates with an Al‐diffusion surface layer: in synthetic air shows sub‐parabolic oxidation kinetics in synthetic air . formation of protective Al2O3 surface layer on Mo . forms a dense and protective Al2O3 layer . mixed oxide formation on W, Ta, Nb . a high aluminum reservoir remains in the subsurface region • Coated samples with halogen treatment: • Aluminized Mo: . low mass gains for all substrates . shows paralinear oxidation kinetic due to the volatility of its oxides

. formation of protective Al2O3 layer on Mo and Ta . local oxide spallation causes destructive oxidation  volatile MoO3 is formed Al2O3 Ni plating  Volatile MoO3 themselves promote Al2O3 formation, no additional fluorine • Aluminized and fluorine treated Nb and W: 6µm Al69Ta39 treatment is needed . develop no protective Al2O3 layer 143 µm AlTa

 The fluorine treatment highly AlTa2 improves oxidation resistance of aluminized Ta, W and Nb Ta substrate

Ni plating Al O 18 µm 2 3 Al8Mo3 mixed 176 µm oxide+ pores

AlMo3 Oxide growth mechanism analysis via TGA for 100 h Mo substrate

Mass change curves of uncoated, aluminized, and additional fluorine treated substrates Cross sections after oxidation Acknowledgement The authors would like to thank Prof. Dr. Ralf Riedel and Dr. Ionescu of the Dispersive Solid of [1] J. R. Nicholls, K. A. Long, N. J. Simms, Sheir‘s Corrosion, ed. By T. J. A. Richardson, (Elsevier, Oxford, 2010), Material‐ and Geosciences at TU Darmstadt for supporting this work. In addition thanks goes 2532 ‐ 2555 to PLANSEE Group for providing the samples. [2] A. Donchev, B. Gleeson, M. Schütze, Intermetallics 11 (2003)