High-Temperature Mechanical Behavior of End-of-Life Cryomilled NiCrAlY Bond Coat Materials
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INTRODUCTION
HIGH performance thermal barrier coating (TBC) systems are often used to increase gas turbine efficiency.[1,2] TBCs typically consist of three layers: a ceramic top coat made of yttria stabilized zirconia (YSZ), a bond coat often made of MCrAlY (where M = Ni, Co, Fe), and a superalloy substrate. A fourth layer, called the thermally grown oxide (TGO), forms between the YSZ and the bond coat as a result of oxidation. In gas turbines, the TBC materials are exposed to thermal cycling between room temperature and up to 1473 K (1200 C).[1] Due to the difference in thermal expansion coefficients relative to the substrate material, each layer of the TBC undergoes cyclic stress when exposed to service conditions. Furthermore, the TGO layer imposes compressive growth stresses in addition to the cyclically induced stresses. These stresses eventually lead to plastic deformation at low temperatures and creep deformation at high temperatures and ultimately result in the spallation of the ceramic top coat. Several models have been developed to describe the stress distributions and the failure mechanisms in select TBC systems.[3,4] M. FUNK, PhD Student, and C. EBERL, Group Leader, are with the Institute for Reliability of Components and Systems (izbs), Karlsruhe Institute of Technology, Karlsruhe 76131, Germany. Contact e-mail: [email protected] K. MA, PhD Student, and J.M. SCHOENUNG, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, CA 95616. M. GO¨KEN, Professor, is with the Materials Science and Engineering Department, University Erlangen–Nu¨rnberg, Erlangen 91058, Germany. K.J. HEMKER, Professor, is with the Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218. Manuscript submitted April 6, 2010. Article published online March 23, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
Critical key components are the creep properties as well as the resistance to oxidation of the bond coat at high temperature.[5] Commercially used bond coats can be either diffusion aluminide or thermal spray overlay bond coats.[2] The most common diffusion aluminide bond coats contain a small amount of Pt, which is electrodeposited prior to aluminization. These bond coats have superior mechanical properties at high temperatures and strong oxidation resistance compared to MCrAlY bond coats.[6] Unfortunately, the lifetime of TBCs containing Pt modified bond coats can be limited due to the high-temperature martensitic phase transformation developing during service or thermal cycling.[7] The large strain cycles can lead to the formation of interface undulations, and as a result, the TBC can undergo spallation. MCrAlY overlay bond coats can be applied to complicated geometries such as airfoils, e.g., by air plasma spraying (APS), vacuum plasma spraying, and low pressure plasma spraying (LPPS).[1,2] MCrAlY coatings are chemically and thermomechanically compatible to the superalloy substrates and form a protective alumina layer at the top coat
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