Measuring the Properties of Diffusion Aluminide Bond Coat Layers with Microsample Tensile Testing
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MEASURING THE PROPERTIES OF DIFFUSION ALUMINIDE BOND COAT LAYERS WITH MICROSAMPLE TENSILE TESTING DENG PAN, P.K. WRIGHT* and K.J. HEMKER Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, MD 21218 * GE-Aircraft Engines, Materials and Process Engineering Department, Cincinnati, OH 45215 ABSTRACT The bond coat layer plays an important role in the TBC system by providing oxidation resistance and a foundation for the ceramic top coat. The thermal cyclic durability of a TBC is thought to be strongly dependent on the physical and mechanical properties of the bond coat layer. Attempts to measure these properties for as-deposited and thermally cycled diffusion aluminide bond coats have been greatly inhibited by the limited thickness (~60 µm) of these coatings. In the present study, novel high temperature microsample tensile testing technique is employed to measure the Young’s modulus (E), coefficient of thermal expansion (CTE), yield strength and stress relaxation behavior of an as-deposited (Ni, Pt)Al bond coat in the temperature range of 25 o C to 1150 oC. Preliminary results for these material parameters are reported here. They are being used as material inputs for an independent finite element analysis (FEA) study of the development of stresses in the TBC layers during thermal cycling.
1. INTRODUCTION Thermal barrier coatings (TBC) are commonly used in commercial gas turbine engines to provide a thermal insulation between the hot gas stream and engine components. The interest in TBCs has grown continuously over the last 20 years, thanks in large part to their positive effect on the performance and durability of the turbine engines [1-3]. A detailed understanding of the micro-mechanisms that cause TBC spallation and failure has not, to date, been well elucidated. One of the key components in a TBC system is the bond coat, which provides the oxidation resistance and acts as a foundation for the ceramic top coat. The mechanical properties of the bond coat have been shown to play an important role in the study of failure mechanism and life prediction of the TBC system [4]. To date, no investigations of the properties of diffusion aluminide bond coats, which have thickness of less than 60 µm, have been reported. Characterization of the mechanical behavior of the bond coat layer is, however, needed to provide a more comprehensive understanding of the development of stresses in the TBC system during thermal cycling. The present study is motivated by a desire to measure the Young’s modulus (E), coefficient of thermal expansion (CTE), yield stress (σy) and stress relaxation behavior of a single phase diffusion aluminide bond coat layer as a function of temperature. The development of a “microsample” tensile testing at JHU has greatly facilitated the mechanical testing of materials that are only available in small quantities or limited dimensions, as free-standing microsample specimens with a thickness of 25-500 µm and gage widths of 100800 µm are routinely tested by this technique. Detailed descripti
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