A Study of Thermal Fracture in Functionally Graded Thermal Barrier Coatings Using a Cohesive Zone Model

[+] Author and Article Information
Sudarshan Rangaraj, Klod Kokini

School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907-1288

J. Eng. Mater. Technol 126(1), 103-115 (Jan 22, 2004) (13 pages) doi:10.1115/1.1631028 History: Received November 01, 2002; Revised April 01, 2003; Online January 22, 2004
Copyright © 2004 by ASME
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Schematic illustration of the architecture of functionally graded thermal barrier coatings with similar thermal resistance (all dimensions in millimeters, figure NOT drawn to scale): (a) one-layer TBC; (b) three-layer TBC; and (c) nine-layer TBC, each TBC layer and bond coat are 0.22 mm thick.
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Effective traction (σ)-separation (δ) curve for YSZ-BC alloy particulate composite (50 percent YSZ+50 percent BC alloy). S.C.M.-self-consistent model, R.O.M.-rule of mixtures
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Schematic illustration of the thermal/structural boundary conditions during the laser thermal shock experiment and resulting fracture in the TBC
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Finite element mesh for the three-layer TBC with two surface cracks in the symmetric model
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Deformed shape (displacements magnified 10X) of the mesh in the region around the center and side surface cracks at various instances during the 4 seconds heating-ambient cooling (10 sec) thermal shock cycle with maximum surface temperature of 1100°C
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Surface thermal fracture in the one-layer (non-graded) YSZ TBC. (Open symbols denote experimental data, details related to the cohesive parameters and crack-tip location criteria used for the above six cases are shown in Table 3.)
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Surface crack (SC) morphologies resulting from laser thermal shock in the three-layer TBC, SC1-longest surface crack, SC3-shortest surface crack and SC2-surface crack of intermediate length: (a) micrograph of a three-layer specimen subjected to laser thermal shock with maximum surface temperature of 1000°C; and (b) schematic illustration of surface cracks typically observed on the three-layer TBC.
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Surface thermal fracture data for the graded three-layer TBC. (Open symbols denote experimental data, Average (* ) denotes average of the longest two surface cracks (SC1 and SC2) measured on the tested specimens, SCM-self consistent model, ROM-rule of mixtures, F.E. model-finite element model.)
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Work of separation (fracture toughness) for YSZ-BC alloy composites estimated from the self-consistent model, ΓC-YSZ=0.15 J/m2
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Comparison of the surface crack lengths predicted from the finite element (F.E.) model with those measured from laser thermal shock experiments (10 specimens) for the nine-layer functionally graded TBC.




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