Intermetallic B2 Bond Coats: Systems Compatibility and Platinum-Group Metal Additions
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Intermetallic B2 Bond Coats: Systems Compatibility and Platinum-Group Metal Additions Tresa Pollock, Dan Widrevitz, Russell Pong, Fang Cao and Bryan Tryon Department of Materials Science and Engineering, 2300 Hayward St., University of Michigan Ann Arbor, MI 48109 ABSTRACT Intermetallic interlayers are key to the performance of thermal barrier coating systems. The role of platinum group metal (PGM) additions to B2 bond coat interlayers has been investigated with emphasis on diffusional aspects of coating structure evolution and properties. Additions of Ru and Ir to NiAl reduce interdiffusion and the resultant thickness of the coating and interdiffusion zone. Relative to Pt and Pd, Ru and Ir exhibit slower interdiffusion coefficients. Ru additions to NiAl increase creep resistance but degrade oxidation kinetics. INTRODUCTION The temperature capability of turbine airfoil systems can be substantially increased with the use of multilayered thermal barrier coating (TBC) systems [1 - 3]. However, maintaining adhesion of the yttria-stabilized zirconia TBC coating to the nickel-base superalloy substrate remains a major challenge. Intermetallic interlayers with properties intermediate to the ceramic top coat and the metallic substrate can improve the durability of these systems if a balanced set of properties of can be achieved. Properties of the intermetallic that strongly influence the failure process include diffusion, high temperature strength, coefficient of thermal expansion (CTE) and oxidation resistance [3, 4]. Importantly, the intermetallic bond coat must serve as an environmental barrier, forming a thermally grown oxide (TGO) upon high temperature exposure. Coatings based on the B2 intermetallic NiAl with the CsCl structure have been effective interlayers for thermal barrier systems [1, 2, 5]. Platinum group metals, with significant solubility in the B2 NiAl have the potential to alter the properties of the intermetallic interlayer and thus improve adhesion and system life. This study focuses on additions of Ru, Ir, Pd and/or Pt to NiAl-based systems and their influence on coating structure and properties. The influence of PGM additions on interdiffusion has been investigated via diffusion couples. The structure and properties of PGM-containing coatings fabricated by vapor deposition techniques have also been investigated. EXPERIMENTAL APPROACH Diffusion couples were formed by joining stoichiometric single crystal NiAl to (Ni,PGM)Al polycrystalline alloys. Additions of Ir, Os, Ru, Pd and Pt were investigated with 10% of the PGM substituted for Ni. PGM-containing alloys were prepared by melting in an induction levitation melting system or by arc melting. The joining procedure for the diffusion couples, outlined in
detail by Kulkarni et. al [6], consisted of loading the two halves of the couples under 10 - 20MPa compressive loads at 1000°C or 1050˚C for 13 – 30 hours in a Centorr vacuum furnace operatiang at ~10-7 torr. Bonded specimens were sealed in Ar-backfilled quartz tubes and annealed for either
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