Surface-Roughness Induced Residual Stresses in Thermal Barrier Coatings: Computer Simulations Page: 1 of 9
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ORNL/CP-99990
Surface-Roughness Induced Residual Stresses R C fV O
in Thermal Barrier Coatings: Computer Simulations NOV 2
C. H. Hsueh', P. F. Becher ',
Edwin R. Fuller, Jr.2, Stephen A. Langer, and W. Craig Carter30 , 1 j
'Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6068, U.S.A.
2National Institute of Standards and Technology, Gaithersburg, Maryland 20899, U.S.A.
3 presently at Massachusetts Institute of Technology, Cambridge, MA 02139-4307, U.S.A.
Keywords: asperities; computer simulations; finite element analysis; microstructure; OOF; plasma
spray; residual stress; spallation; surface roughness; thermal barrier coatings
Abstract
Adherence of plasma-sprayed thermal barrier coatings (TBC's) is strongly dependent on mechanical
interlocking at the interface between the ceramic coating and the underlying metallic bond coat.
Typically, a rough bond-coat surface topology is required to achieve adequate mechanical bonding;
However, the resultant interfacial asperities modify the residual stresses that develop in the coating
system due to thermal expansion differences, and other misfit strains, and generate stresses that can
induce progressive fracture and eventual spallation of the ceramic coating. For a flat interface, the
principal residual stress is parallel to the interface, as the stress normal to the interface is zero. However,
the residual stress normal to the interface becomes non-zero, when the interface has the required
interlocking morphology. In the present study, an actual microstructure of a plasma-sprayed TBC system
was numerically simulated and analyzed with a recently developed, object-oriented finite element analysis
program, 00F, to give an estimate of the localized residual stresses in a TBC system. Additionally,
model TBC microstructures were examined to evaluate the manner in which the topology of interfacial
asperities influences residual stresses. Results are present for several scenarios of modifying interfacial
roughness.
Introduction
Ceramic thermal barrier coatings (TBC's) are commonly used to protect air-cooled superalloy hardware
in hot-sections of gas turbine engines by reducing the surface temperatures of metallic components (1-4].
A typical plasma-sprayed TBC system consists of an oxidation-resistant metallic bond coat overlaid by
a porous, thermally-insulating ceramic coating [1-3}. Since adherence of the plasma-sprayed ceramic
coating to the metallic surface is enhanced by mechanical interlocking, a rough bond-coat surface
topology is required to achieve the level of mechanical adhesion needed for the severe thermal cycles
of a turbine engine [3,5,6]. Plasma-spray deposition of the bond coat provides an inherently rough and
irregular metallic surface, with the resultant asperities providing good bonding along the ceramic/metal
interface. Furthermore, the nature of the bond-coat surface-roughness can be tailored to a limited extent
by control of plasma-spray parameters and/or powder size distributions. Currently, the magnitude and
morphology of bond-coat roughness that provide optimum TBC durability have not been established.
/ Substantial residual stresses exist in TDC systems due to the large mismatch in metal-ceramic thermal
,e n during cooling and to high-temperature oxidation of the bond coat [2-7]. During high-
86c1 temp ture operation, a thin oxide scale (predominantly, a'-A2O,) forms at the irregular bond-coat/
S terrace, resulting in a constrained volumetric increase along the interface [2-7]. The interfacial
oxide'le and ceramic coating have significantly lower coefficients of thermal expansion (CTE) than
Ing metallic components, and thus, are subjected to significant compressive residual stresses
p e interface during cooling [4,8,9]. The residual stress normal to the interface (i.e., the out-t1:9T 6604
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Becher, P.F.; Carter, C.; Fuller, E.R., Jr.; Hsueh, C.H. & Langer, S.A. Surface-Roughness Induced Residual Stresses in Thermal Barrier Coatings: Computer Simulations, article, October 26, 1998; United States. (https://digital.library.unt.edu/ark:/67531/metadc669253/m1/1/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.