Thermo-mechanical Fatigue Failure of Thermal Barrier Coated Superalloy Specimen
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INTRODUCTION
COMPONENTS in the hot parts of gas turbine engines, such as turbine blade, undergo severe cyclic thermal and mechanical loads during startup and shutdown operation.[1,2] Nickel-based superalloys have proven to be suitable materials for turbine blades, because of their superior high-temperature properties.[3] However, under prolonged exposure to combustion gases at these high operating temperatures, these alloys are susceptible to hot corrosion and oxidation, impairing its performance. Therefore, thermal barrier coatings (TBCs) are applied for oxidation resistance and thermal insulation.[4–14] The TBCs typically consist of three layers: a metal substrate, a metallic bond coat, and a ceramic top coat. The bond coat, which is deposited on the superalloy substrate, has two main functions: (i) protection of the base material from oxidation and corrosion attack during high-temperature exposure and (ii) providing good adhesion to the ceramic top coat.[4–7] The latter (ceramic top coat), protects the base material from heat of the flowing combustion gases; thermal insulation. Nevertheless, premature failures, such as RAJIVGANDHI SUBRAMANIAN, PhD Student, SATOSHI YAMAGISHI, Assistant Professor, MASAKAZU OKAZAKI, Professor, are with the Department of Mechanical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka-machi, Nagaoka, Niigata 9402188, Japan. Contact email: [email protected] YUZURU MORI, Sales Representative, is with the Department of Mechanical Engineering, Nagaoka University of Technology, and also with the Nippon Chuzo Co., Ltd., Tsurumi-ku, Yokohama, Kanagawa Prefecture, Japan. Manuscript submitted October 12, 2014. Article published online June 23, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
spallation of the TBC from the substrate, still limit the use of TBCs as a prime-reliant material.[4–7] In fact, it has widely been observed that the bond coat contributors significantly to the TBC failure through spallation.[4–19] However, the failure process from crack initiation and subsequent propagation defining the precise failure mechanisms related to the DBTT of the bond coat has not yet been clearly understood.[15–24] Hence, in order to obtain accurate lifetime expectancy and performances of the TBC system, it is necessary to have a reliable understanding of the mechanical properties and failure mechanisms of the bond coat. In this paper, failure evolutions relating to mechanical behaviors of the bond coat at various temperatures will be discussed. At first, tensile tests were performed on plate specimens of the bond coat alloy, and then fatigue tests (TMF and LCF) were carried out using cylindrical specimens of the substrate material with TBCs, hereafter referred to as TBC specimens. The failed specimens were cross sectioned to observe the damage evolution by means of an optical microscope and scanning electron microscope (SEM). Based on the observations, a life prediction model is explored for the LCF and TMF failures of the TBC specimen.
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EXPERIMENTAL PROCEDURE
A. Specimen Prepar
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