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Journal ArticleDOI

On stresses induced in a thermal barrier coating due to indentation testing

01 Feb 2009-Computational Materials Science (Elsevier)-Vol. 44, Iss: 4, pp 1178-1191

AbstractInstrumented indentation has been suggested as a method to determine interfacial fracture toughness of thermal barrier coatings. However, in a previous experimental study we showed that the results are ambiguous. In this work, we investigate the experimental results by numerical simulations incorporating the material microstructure. In the numerical simulations, based on finite element analyses, the stress fields that are associated with the loading and unloading of the indenter are investigated. By comparing these stress fields to the damage observed in the experimental study, including crack path and interfacial delaminations, we explain key findings from the experimental observations. Our results suggest that indentation testing of multilayered coated structures might not induce the delamination in the overall weakest interface and therefore the test results must be evaluated with care.

Topics: Indentation (58%), Delamination (53%), Fracture toughness (52%), Thermal barrier coating (51%), Stress (mechanics) (51%)

Summary (2 min read)

1. Introduction

  • Thermal barrier coatings (TBes) are multilayered coatings that are frequently used in gas turbine applications to protect structural components from the intrinsic high temperatures.
  • Even though there are several possible scenarios that eventually can lead to the failure of a TBe.
  • The challenges associated with designing and tes ting TBCs comes from the multilayered structure of the coating, where the properties evolve as the system is used.
  • The TGO commonly also has other oxidation prod ucts that may affect the overall interfacial st rength.
  • In the following, the authors first summarize the experimental results, before discussing the finite element models and the results.

2.1. Specimens and experimental procedures

  • Flat specimens of IN 625 and a limited number of CMSX-4 were coated by electron beam physical vapor deposition (EB-PVD), first with a NiCoCrAlY bond coat (100 lm) followed by a partially stabi lized YSZ (7–8 wt% yttria, 280 lm).
  • The samples were kept at high temperature for 23 h and at room temperature for 1 h, until the specified ‘‘time-at-temperature” was reached.
  • Spontaneous spallation occurred in the samples aged to 400 h; consequently, these were not used in the indentation testing.
  • During the indentation testing, the indentation displacement and force were recorded continuously.
  • Based on these curves, it appears that there is one type of response for lower indentation forces and another for higher indentation forces, where the lower maximum indentation forces result in a higher slope (of the delamination–indentation force curve) than for the higher maximum indentation forces.

2.2. Experimental observations

  • The heat treatment of the samples causes changes in the micro structures, including sintering of the YSZ and growth of the TGO, as illustrated in Fig. 1 [6].
  • Aging of the system is simulated by changing three classes of parameters: (i) Increasing the width of the columns in the top coat and decreasing the distance (ICS) between the columns.
  • The selected geometry is presented in Table 1. (ii) Increasing the thickness of the TGO, combined with decreas ing the thickness of bond coat.
  • Results and discussion of-plane” stress (associated with mode I at the interface), r22, shown in Fig. 6, and ‘‘shear stress” (associated with mode II at As mentioned previously, the authors will conduct a qualitative assess- the interface), r12, as shown in Fig.
  • For the cases of higher maximum indentation force, the unloaded stress state shows that the stress level decreases in the interface under the indenter and vanishes at the higher indentation forces (Fig. 8E and F).

5. Concluding remarks

  • The response from using Rockwell indentation as a means of establishing the interfacial fracture toughness in thermal barrier coatings (TBCs) was explored by numerical simulations.
  • In addition, for a given top coat column width, different maximum indentation forces (or depths) lead to different bending deforma tion of top coat columns, thus causing distinct influence zones via columnar interactions.
  • Thus, the authors believe that the experimentally observed discrepancy is due to the toughness change of the TGO-system due to ageing.
  • This model did not include the crack propagation and was therefore not able to capture this behavior.

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On
stresses induced
in
a thermal barrier coating due to indentation testing
Jin
Yan
a
,
Anette M. Karlsson
J
··,
Marion
Ba
rt
sc
h
b
,
Xi Chen '
Dt>panmelll
of
Mff~aniroJ
Engineering.
Universily of
Delaware.
NfWflrk.
DE
19716.
USA
" /nsalille
of
MOle/lOis
Research.
GemlOl1
Aerosp<lO'
n'mer (DlR
),
Linder
HlH'he.
D-51147
Cologne.
Cennany
' Deparrmenr
of
Civil
Engjn~ring
and
Engineering
Mechanics.
Columbia
University.
New
York.
NY
10027
-
6699,
USA
1.
Introduction
Thermal barrier coatings (
TBes
) are multilayered coatings that
are frequently used in gas turbine applications to protect structural
components from the intrinsic high temperatures.
By
actively cool-
ing the structural component. the
TB
e
can
sustain a temperature
difference
of
up to 150 O( during
use
.
Thus,
the thermal load
on
the structural component
is
reduce
d,
and
it
is
possible to either
operate the turbine at higher temperature
or
increase the life time
of
the structural component. Unfortunately, the coatings
fail
pre-
maturely, preventing the benefits ofTBCs to be fully utilized. Even
though there are several possible scenarios
that
eventually can
lead to
the
failure
of
a
TBe.
a dominating class
of
failure
is
associ-
ated with nucleation
of
damage
at
or
near an interface, followed by
crack growth and coalescence parallel to the interface. resulting
in
that
the coating eventually spalls from
the
substrate.
Th
u
s,
interfa-
cial damage increases with use (i.e
..
the age)
of
the system. Conse-
quently, several
author
s have suggested
that
the interfacial
fracture toughness could be a measure describing how damage
accumulates in theTBC as the system
is
aged [1
~
51
.
Howeve
r.
there
Corresponding
~ulhor.
f~x:
+1
3028313619.
E_mail
addrrss."
k.arl
sw
n@udel.edu (
A.M.
K.1rlsson
).
is
currently no consensus on how to measure the
in
terfacial frac-
ture toughness ofTBCs
[4
.6.
71
.
The challenges associated with designing and testing
TBC
s
comes from
the
multilayered
str
ucture
of
the
coating,
where
the
properties evolve as the system
is
used.
For
the
TBC
systems con-
sidered here, three major
la
yers can be identified, starting from
the
substrate
(Fig.
I
):
(i) a metallic bond coat: (
ii
) a thermally
grown oxide (
TGO
):
and {
iii
)a
ceramic top coa
t.
Cu
rr
ent
ly,
the most
common top coat is yttria stabilized zirconia
(
VZf
). There are two
major groups
of
bond coats: platinum modified aluminide and
MCrA
IY
(where M stands f
or
iron (
Fe
)
or
nickel and cobalt
(NiC
o
)).
The
TGO
is
a reaction product that is formed du
ri
ng high
temper
-
ature exposure. Currently.
the
preferred
TGO
is a-alumina, which
is
formed by
that
the bond coat provides aluminum and from the
oxygen that diffuses through the Y
ZT.
which
is
permeable to oxy-
ge
n.
However, the
TGO
commonly also has
other
oxidation prod-
ucts
that
may affect the overall interfacial st rength. e.g
..
Ref.
[8[.
Several methods have been proposed to measure the interfacial
fracture toughness
of
thermal barrier coating
s,
including "pull-out
techniques
ri
(an extension
of
methods used for testing fibe rs in a
ceramic
or
metal matrix)
[91
. notched coat
in
gs in 4-point bending
1101
. and various indentation techniques
[4
.6
,7J
1.121
. The inden-
tation technique has been proposed by many as the most promis-
ing method, since it
is
easy to perform and involves minimum

5 µm
As-coated
200 hours
5 μm
5 μm
B
D
5 μm
E
Bond coat
TGO
Dense
TGO
A
Top coat
5 μm
C
Top coat
Bond coat
1 μm
Porous
TGO
Fig. 1. Schematic and SEM images of the thermal barrier system, showing as-coated samples and samples aged for 200 h at 1000 C in air. The as-coated sample shows (B) the
intercolumnar spacing (ICS) between YSZ columns near the top coat surface and (C) the intermixed, porous (preexisting) TGO. The aged samples show (D) narrowed ICS and
rigid connections between the YSZ columns and (E) the bi-layered structure of the TGO. For both as-coated and aged samples, the top coat closest to the TGO does not show
expressed ICS between columns.
Annular
Crack
Porous
TGO
Dense
TGO
Compacted
Region
TGO
TGO
Bond
Coat
Center
Crack
Kink band in
columnar
top coat
Annular
Crack
Diameter of Annular Crack
Top
Coat
Bond
Coat
As-coated
Aged
Annular
Crack
Fig. 2. Schematic of the main pattern after indentation, including the kink-bands. The extended annular delamination crack tends to propagate in the TGO and TGO–bond
coat interface for as-coated samples but does not reach the bond coat interface for the aged samples.
sample preparation. However, as was shown in our previous work,
the results may be hard to interpret since they are indirectly asso-
ciated with material toughness (not just interfacial toughness) and
deformation modes, and may even give contradicting results [6,7].
In our experimental work, we investigated Rockwell indenta-
tion of thermal barrier coatings, where the indentation was con-
ducted on the surface of a thermal barrier system so to establish
the interfacial fracture toughness [6]. Two classes of TBCs systems
were investigated: one set was tested in ‘‘as-coated” conditions
and the second set had been subjected to thermal heat treatment.
Based on previous observations, e.g., Refs. [1–4], it was expected
that the heat treated (aged) samples should exhibit lower interfa-
cial toughness than the as-coated and that the delamination size
would increase with increasing maximum indentation load. How-
ever, the results indicated otherwise. These contradictive results
will be explored here by means of finite element simulations. In
the following, we first summarize the experimental results, before
discussing the finite element models and the results.
2. Experimental investigations
The experimental work and results were discussed in Ref. [6],
and will be summarized here for clarity.

2.1. Specimens and experimental procedures
Flat specimens of IN 625 and a limited number of CMSX-4 were
coated by electron beam physical vapor deposition (EB-PVD), first
with a NiCoCrAlY bond coat (100
l
m) followed by a partially stabi-
lized YSZ (7–8 wt% yttria, 280
l
m). Before indentation testing,
thermal aging was conducted, where a set of samples was sub-
jected to 1000 C in air for 50, 100, 200, and 400 h, respectively.
The samples were kept at high temperature for 23 h and at room
temperature for 1 h, until the specified ‘‘time-at-temperature”
was reached. Spontaneous spallation occurred in the samples aged
to 400 h; consequently, these were not used in the indentation
testing. The samples aged for 200 h were indented, but showed de-
layed spontaneous spallation after indentation (‘‘desk-top failure”).
Thus, only limited evaluation could be done for the 200 h samples.
The behavior of all aged specimens was compared to specimens
that were not heat treated, i.e., tested in ‘‘as-coated” conditions.
An electromechanical testing machine was used to indent the
coated surface with a Rockwell brale C indenter [6]. During the
indentation testing, the indentation displacement and force were
recorded continuously. In some cases, the pre-selected maximum
indentation force was not exactly achieved since the equipment
yields, resulting in permanent deformations not only in the top coat,
but also of the underlying layers [6].
To investigate if the indentation technique indeed can be used
as a test method for determining interfacial fracture toughness in
a thermal barrier coating, the diameter of the delamination crack
was measured in the SEM (Fig. 3). For smaller loads and aged sam-
ples, the delamination size sometimes coincides with the cone-
crack diameter. Even though some scatter is observed, two distinct
regions are identified: a bifurcating in the behavior can be seen for
indentation forces around 175 N. By using a linear curve fit based
on linear regression, an estimate of the delamination diameter as
a function of maximum indentation force is obtained, Fig. 3. Based
on these curves, it appears that there is one type of response for
lower indentation forces and another for higher indentation forces,
where the lower maximum indentation forces result in a higher
slope (of the delamination–indentation force curve) than for the
higher maximum indentation forces. For the lower loads, a minor
but distinct difference can be seen between the as-coated and
the aged samples (Fig 3B). For the higher maximum indentation
forces, the as-coated specimens result in significantly larger
was manually controlled. Several indentations could be made on
each sample, where each indentation imprint was separated with
at least 10 mm to avoid interference between the stress fields
generated.
A key part of the experimental investigations was to investigate
the damage in the TBC after the indentation, thus careful sample
preparation for microscopy was conducted. A detailed description
of the procedure is presented in Ref. [6]. The specimens were ana-
lyzed by both an optical microscope and a scanning electron
microscope.
2.2. Experimental observations
The heat treatment of the samples causes changes in the micro-
structures, including sintering of the YSZ and growth of the TGO, as
illustrated in Fig. 1 [6]. The sintering of the top coat is associated
1.0
2.0
3.0
4.0
0
Delamination Diameter [mm]
All loads
A
with that the featherlike structure of the columnar YSZ and the
0 200 400 600 800 1000
pores gets coarser, along with the formation of rigid contacts be-
Maximum Indentation Force [N]
tween the columns. The TGO grows from a single intermixed oxide
layer in the as-coated samples to a bi-layered TGO, which includes
3.0
the preexisting TGO along with a newly grown, dense TGO, Fig. 1.
The intermixed layer consists of both aluminum oxide and zirco-
nia, in accordance with previous observations of the selected mate-
rial system [8].
The microstructural imaging of cross-sections of the indented
regions indicated that there are three major classes of damage in-
duced by the Rockwell indentation,
1
Fig. 2: (i) crushing of the top
coat adjacent to the indenter tip; (ii) cone shaped shear bands;
and (iii) interfacial debonding cracks. The interfacial debonding
cracks are found in the vicinity of the TGO, but not necessarily at a
particular interface. An ‘‘overall debonding crack” typically starts
from the cone-crack, kinks when it reaches the interface, and be-
Delamination Diameter [mm]
2.5
2.0
1.5
1.0
Smaller loads
B
0 50 100 150 200
comes parallel to the TGO, propagating in the YSZ. As the crack
grows further from the center of the indentation, the crack propa-
gates into the TGO. In the as-coated samples, the annular debonding
crack eventually propagates in the TGO and the TGO–bond coat
interface. However, for the aged samples, the TGO cracks were not
able to propagate through the dense (and new) TGO, Fig. 2. For
indentation forces larger than 200 N, the substrate and bond coat
Due to the scale of the indentation, the radius of the tip of the Rockwell indenter
has to be considered.
0.5
0
Maximum Indentation Force [N]
Fig. 3. The diameter of the annular delamination cracks as a function of the
maximum indentation force. Linear regressions for as-coated samples are shown
with solid lines and for aged samples with dashed lines (there is no statistical
difference between 50 h and 100 h samples). The delamination diameter show two
distinct responses: one for small maximum indentation forces and one for larger
maximum indentation forces, with a bifurcation around 175 N. (The diameter is
measured after unloading.) (A) All loads and (B) enlargement for smaller loads.
1

delamination than the aged samples (Fig. 3A). This is a contradic-
3. Numerical model
tion to what is observed in durability experiments and in field tests
of thermal barrier coatings [5,13].
Numerical simulations using finite element analysis (FEA) is
This study attempts to explain some of these observations
employed to investigate the micro-mechanical response in the
through numerical simulations.
TBC due to indentation. We will limit the discussion to the stress
200 columns
New
TGO
Old
TGO
Transition
zone
w
d
60
ο
TGO/bond coat
Top Coat/TGO
old/new TGO
Interface
R radius
Substrate
Bond coat
R = 200 μm
Indenter
Axisymmetric
axis
Top
coat
Fig. 4. Schematic of the axi-symmetric model with columnar top coat including the intercolumnar spacing (ICS), d. The radius of the indenter tip is included in the model.
(The standard Rockwell brale C indenter as used in the test has a tip radius of 0.2 mm.)
0
100
200
300
400
500
600
A As-coated
Force (N)
FEM
Experiment
0
100
200
300
400
500
600
B 50 hours
Force (N)
FEM
Experiment
0
50 100 150 200 250
0
50 100 150 200 250
Displacement (μm)
Displacement (μm)
600
Force (N)
500
400
300
200
100
0
C 100 hours
FEM
Experiment
0
50 100 150 200 250
Displacement (μm)
Fig. 5. Force-displacement curves from experiments and numerical simulation of the columnar structure: (A) as-coated, (B) 50 h aged, (C) 100 h aged.

Figures (15)
Citations
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Reference EntryDOI
31 Oct 2001
Abstract: The American Society for Testing and Materials (ASTM) is an independent organization devoted to the development of standards.

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Abstract: In this paper, the residual stress of double-ceramic-layer (DCL) La2Zr2O7/8YSZ thermal barrier coatings (TBCs) fabricated by atmospheric plasma spraying (APS) was calculated by finite element simulation using birth and death element technique. The residual stress was composed of two parts, i.e. the quenching stress and the thermal stress. The simulation results indicated that the surface and the edge of interface are often the positions of stress concentration. The DCL La2Zr2O7/8YSZ has lower residual stress compared with that of the single-ceramic-layer (SCL) 8YSZ TBCs with the same thickness. In addition, the influence of defects on the residual stress has been calculated and discussed using finite element method combined with Computational Micro-Mechanics (CMM). As the DCL TBCs has better thermal insulation effect, sintering resistance ability and lower residual stress compared with that of the SCL 8YSZ at the same time, it was expected to be an ideal candidate material for the application in the future.

79 citations


Journal ArticleDOI
Abstract: The evolution of microhardness, fracture toughness and residual stress of an air plasma-sprayed thermal barrier coating system under thermal cycles was investigated by a modified Vickers indentation instrument coupled with three kinds of indentation models. The results show that fracture toughness on the top coating surface after thermal cycles changes from 0.64 to 3.67 MPa m 1/2 , and the corresponding residual stress near the indented region varies from − 36.8 to − 243 MPa. For the interface region of coating and bond coat, fracture toughness in the coating close to interface ranges from 0.11 to 0.81 MPa m 1/2 , and residual stress varies from − 5 to − 30 MPa, which are consistent with available data. For the lateral region of coating, fracture toughness and residual stress display strong gradient characteristics along the thickness direction due to the special layered structure.

46 citations


Journal ArticleDOI
Abstract: Determination of interfacial properties of thermal barrier coatings (TBCs) is very important for designing and evaluating the durability of TBCs. A new method combining a simple shear test and an inverse finite element analysis was developed and applied to measure the interfacial properties of two flame-sprayed yttria-stabilized zirconia TBCs. Nanoindentation testing was performed to determine the mechanical properties of different materials of the TBC systems. Variation of the lateral force during the shear test was recorded and analyzed to obtain the nominal ultimate shear strength of TBCs. The interfacial properties, namely fracture energy and stress intensity factor (mode II), of different TBC systems under both as-deposited and heat-treated conditions were determined through inverse finite element analysis.

40 citations


Journal ArticleDOI
Abstract: In a thermal barrier coating (TBC) system with cylindrical geometry, the position of coating plays an important role in the distribution of residual stress. In this paper, the residual stress field in three different types of TBCs with cylindrical geometry has been analyzed. The main focus is on the effects of substrate curvature radius, deposition temperature and coating thickness on the residual stress distribution during a deposition process. The results show that the substrate curvature radius significantly affects the distributions of radial and hoop residual stresses, which are in good agreement with experimental measurements by photo-stimulated luminescence piezospectroscopy (Wang et al., Acta Mater., 2009, 57(1):182–195). The maximum radial residual stress locates closely to the coating/thermal grown oxide interface. However, the maximum hoop residual stress lies in the thermal grown oxide layer, which is much more than other three layers and presents a strong stress singularity along the thickness direction.

35 citations


References
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Reference EntryDOI
31 Oct 2001
Abstract: The American Society for Testing and Materials (ASTM) is an independent organization devoted to the development of standards.

3,708 citations


Journal ArticleDOI
Abstract: The finite–element method is used to perform an accurate numerical study of the normal indentation of an elastic–plastic half–space by a rigid sphere. The effects of elasticity and strain–hardening rate of the half–space are explored, and the role of friction is assessed by analysing the limiting cases of frictionless contact and sticking friction. Indentation maps are constructed with axes of contact radius a (normalized by the indenter radius R and the yield strain of the half–space. Competing regimes of deformation mode are determined and are plotted on the indentation map: (i) elastic Hertzian contact; (ii) elastic–plastic deformation; (iii) plastic similarity regime; (iv) finite–deformation elastic contact; and (v) finite–deformation plastic contact. The locations of the boundaries between deformation regimes change only slightly with the degree of strain–hardening rate and of interfacial friction. It is found that the domain of validity of the rigid–strain–hardening similarity solution is rather restricted: it is relevant only for solids with a yield strain of less than 2 x 10 −4 and a / R

401 citations


Journal ArticleDOI
Abstract: EB-PVD NiCoCrAlY/P-YSZ TBCs on several polycrystalline, directionally solidified, and single crystalline (SX) substrate alloys were thermally cycled at 1100°C. TBC spallation does not correlate solely to TGO thickness, but depends also very much on the substrate alloy. The longest lifetimes are achieved on Hf-containing alloys while SX alloys suffer from early TBC spallation. The formation of the thermally grown oxide was investigated in detail by TEM. A mixed layer of alumina and zirconia exists in the as-coated condition. After initial slight thickening, the thickness of this mixed layer remains constant over a long period of time. During thermal exposure, a continuous layer of pure α-alumina forms and grows underneath the mixed zone by oxygen inward diffusion.

170 citations


Journal ArticleDOI
Abstract: The weight function method is described to analyze the crack growth behavior in functionally graded materials and in particular materials with a rising crack growth resistance curve. Further, failure of graded thermal barrier coatings (TBCs) under cyclic surface heating by laser irradiation is modeled on the basis of fracture mechanics. The damage of both graded and non-graded TBCs is found to develop in several distinct stages: vertical cracking → delamination → blistering → spalling . This sequence can be understood as an effect of progressive shrinkage due to sintering and high-temperature creep during thermal cycling, which increases the energy-release rate for vertical cracks which subsequently turn into delamination cracks. The results of finite element modeling, taking into account the TBC damage mechanisms, are compatible with experimental data. An increase of interface fracture toughness due to grading and a decrease due to ageing have been measured in a four-point bending test modified by a stiffening layer. Correlation with the damage observed in cyclic heating is discussed. It is explained in which way grading is able to reduce the damage.

131 citations


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