Development of Re-based diffusion barrier coatings on nickel based superalloys T. Narita*, K.Z. Thosin, L. Fengqun, S. Hayashi, H. Murakami, B. Gleeson, and D. Young A diffusion barrier type coating with a duplex layer structure, an inner r-(Re, W, Cr, Ni) as a diffusion barrier and outer Ni-aluminide as an Al reservoir, was formed on a Nickel based, single crystal, superalloy (TMS-82 +) and on Hastelloy X. Oxidation properties of both the alloys with or without the diffusion barrier coating were investigated in air under thermal cycling between room tem- perature and 1423 K for up to 360 ks. The inner r layer with a com- position (at%) of (35 – 40) Re, (15 – 20) W, (15 – 25) Cr and (15 – 25) Ni was produced by electrodeposition of Ni-70Re and Ni-20W films from aqueous solutions followed by Cr-pack cementation at temperatures between 1473 and 1573 K, and the outer Ni-alumi- nides of b-(Ni,Cr)Al + c 0 -(Ni,Cr) 3 Al was formed by electrodeposi- tion of a Ni film, followed by Al pack cementation. After the 360 ks oxidation it was found that the structure and composition of both r layer and alloy substrate were retained with little change. Further- more, there was little Al in the r layer. It could be concluded that the Re-based alloys such as r (Re(W),Cr,Ni) are very promising can- didates as a diffusion barrier between the outer Al-reservoir layer and alloy substrate at temperature of 1423 K. It was found that the Re(W)-Cr-Ni acts as a diffusion barrier for both inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate. 1 Introduction Ni-base superalloy has been used for blades and vanes along with multi-layer thermal barrier coatings (TBC) in gas turbines and jet engines. Thermal barrier coating systems are typically two-layered, consisting of a ceramic topcoat and an underlying metallic bond coat. The bond coat is either va- cuum plasma-sprayed MCrAlY (M = Ni, and/or Co) or an Al diffusion coating such as b-(Ni,Pt)Al [1, 2]. The topcoat is commonly yttria-stablized zirconia (YSZ), which is highly permeable to oxygen, imposing the constraint that the metallic bond coat must be resistant to oxidative attack. Fig. 1 is a schematic representation of a cross-section of the TBC and its degradation modes [3], which can be divided into two groups. One is spallation of the ceramic topcoat due to thermally grown oxide (TGO) and thermal stress, and the other is changes in the microstructure of the alloy substrate due to mutual diffusion with the bond coat, resulting in pre- cipitation of topologically close-packed (TCP) phases. The life span of the TBC is controlled by maintaining a re- servoir of Al as MCrAlY or b-Ni (Pt)Al that forms a protective Al 2 O 3 scale on the surface. Loss of Al from the coatings oc- curs by spallation of the Al 2 O 3 scale due to thermal cycling and also by interdiffusion between coating and substrate. Moreover, the inward diffusion of Al into the alloy and out- ward diffusion of the alloying elements from the substrate could lead to decreases in the mechanical properties of the alloy substrate. * T. Narita, K.Z. Thosin, L. Fengqun, S. Hayashi Research Group of Interface Control, Graduate School of Engineering, Hokkaido University, Kita-13 Nishi-8 Kita-Ku, Sapporo 060-8628 (Japan), E-mail: narita@eng.hokudai.ac.jp H. Murakami Dept. Materials Engineering, School of Engineering, The University of Tokyo, Tokyo 113-8655 (Japan) B. Gleeson Dept. Materials Science and Engineering, Iowa State University, Ames IA 50011 (USA) D. Young School of Materials Science and Engineering, The University of New South Wales, Sydney NSW 2033 (Australia) Fig. 1. Outline of the cross-section of the thermal barrier coating and degradation processes Materials and Corrosion 2005, 56, No. 12 Re-based diffusion barrier coatings 923 F 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/maco.200503924