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DOI: 10.1002/adem.201000212Testing of Thermal Barrier Coatings by Laser Excitation**
By Daniel Nies, Robert Pulz, Steffen Glaubitz, Monika Finn, Birgit Rehmer* andBirgit Skrotzki
Thermal barrier coatings (TBCs) are used to increase the operating temperature of land-, sea-, orair-based turbines. As failure of the coating may result in serious damage of the turbine, reliableestimation of the lifetime is essential. Most experiments to assess the lifetime or to determineparameters for simulations of the behavior of TBCs are done by burner-rig-tests, where the operatingconditions are simulated by cyclic heating of the surface and cooling of the backside of a coated sample.In this work a possibility is presented to do comparable experiments by heating the surface with laserirradiation instead of a burner. For this purpose a Nd:YAG-laser with a maximum output power of1 kW and a wavelength of 1064 nm is used. The laser spot can be moved by integrated optics across thesample surface to achieve homogeneous heating of the coating. Cooling of the backside is done by air.The temperature of the sample surface is determined by an infrared-camera which also enables thepossibility to detect failures in the coating via thermography. Additionally, acoustic sensors attached tothe sample holder are used to detect failures in the sample. The investigated ceramic material (yttriastabilized zirconia) has a very low absorption coefficient at the used laser wavelength. Therefore, apre-treatment of the samples was needed to increase the absorption coefficient to be able to heat up thesamples.In this paper, the experimental setup and first experimental results are presented.
[*] Dr. Birgit Rehmer, D. Nies, R. Pulz, S. Glaubitz, M. Finn,Dr. B. SkrotzkiBAM Federal Institute for Materials Research and Testing,12205 Berlin, GermanyE-mail: [email protected]
[**] We would like to thank the group of Prof. Vaßen of theForschungszentrum Julich for providing us with samples ofthermal barrier coatings.
1224 wileyonlinelibrary.com � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2010, 12, No. 12
The improvement of land-, sea-, or air-based turbines is
driven by the goal of higher efficiency and lower fuel burn.
One way to achieve this goal is to increase the operating
temperatures. This was very successfully done in the past few
years by improvements in the fabrication of the components,
but has now become limited due to themelting temperature of
the traditional materials like nickel base superalloys used for
turbine blades. To overcome this problem thermal barrier
coatings (TBCs) can be applied onto turbine parts. First TBCs
were used in the mid 1970s, but the concept has become
increasingly interesting in the past few years.[1–3] The idea
behind TBCs is to apply additional thin layers on the turbine
parts that act as insulation without adding toomuchweight to
the part itself. By the insulation of the coating, the component
is exposed to a lower surface temperature than the gas
temperature at the coating surface. This allows reducing the
maintenance intervals while maintaining the operating
temperature due to lower degradation or increasing the
operating temperatures beyond the melting temperature of
the metallic component. As failure of the coating in the latter
case may result in serious damage of the turbine, reliable
estimation of the lifetime is essential to estimate maintenance
intervals with good balance between safety and cost-efficiency.
The TBC itself consists mainly of a ceramic material of low
heat conductivity and a thinner layer (bond coat) between the
metallic substrate and the ceramic top coat for better adhesion.
The layers are applied onto the components by different
techniques. Mostly used are physical vapor deposition
methods or plasma spraying methods. Different ceramic
materials were studied but the best results yet were achieved
with yttria stabilized zirconia (YSZ) and therefore it is the
most commonly used coating.[4]
The optimal ceramic material for the top coat needs a low
thermal conductivity and a comparable thermal expansion as
the underlying substrate and bond coat. A high thermal shock
resistance is also desirable as thermally induced stresses due
to thermal cycling and thermal shock are themain load during
operation. Themismatch of the thermal expansion coefficients
of the metallic substrate and the coating as well as the
temperature gradient during heating and cooling are the main
reasons for failures of TBCs.
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Fig. 1. Photo (left) and schematic (right) of the thermoshock facility with high-frequency infrared camera (1),laser (2), CCD-camera (3), sample holder (4), and vacuum sealed testing chamber (5).
Many experiments to estimate the lifetime
or to determine parameters for simulation of
the behavior of TBCs are done by burner rig
tests. In these tests the operating conditions are
simulated by cyclic heating the surface of a
small sample by a burner and simultaneously
cooling the backside of the sample. The lifetime
of samples is determined by the number of
cycles before spallation of parts of the coating
occurs. An overview of the activities on the
field of burner rig tests is given in the
literature.[5]
Additional techniques used to investigate
TBCs are thermography and acoustic emission.
Thermography is used to detect damages and
failures in components that are not visible to
the naked eye because they might be located
below the surface.[6,7] In thermography a
sample or component is excited with optical,
electric, magnetic, or other sources. Damages in
the component disturb the normal heat flow or are generating
heat. These disturbances or additional heat can be detected by
infrared (IR) detection systems.
This permits to detect failures in the sample before there is
a visible damage like spallation of part of the ceramic top coat.
Acoustic emission is a well-established technique to
investigate the behavior of materials and allows locating the
crack source by use of multiple sensors.[8,9] For TBCs acoustic
emission is mainly used to detect cracks and to conclude
information of the crack from the signal.[10–13] Locating the crack
source by sound location is also reported in the literature.[14]
However, combining acoustic emission with burner rig test
is challenging, because the airflow of the burner causes very
much noise in the acoustic signal.
In this work an approach for testing samples of TBCs in a
comparable way to burner rig tests is examined. Heating the
sample by laser radiation applies a controlled reproducible
thermal load to TBC samples while the samples are surveyed
during testing by thermography and acoustic emission. Heating
by laser can be considered as a contact free heating method and
has the advantage to burner rig tests that there is no need to hold
the specimen tight against the hot airflow of the burner.
Therefore, a stress free mounting of the sample is possible. Also
the absence of noise resulting from the airflow of the burner
enables the possibility to use acoustic emission sensors to
observe the experiments. Monitoring the sample by an IR
camera offers the possibility tomeasure the surface temperature
of the whole sample during the experiment and to detect
damage by thermography before they are visible by the naked
eye.
Experimental Procedure and Investigated Materials
Testing Facility
For the experiments an existing laser-thermal shock facility
is used. In Figure 1 photo of the test facility is shown on the left
ADVANCED ENGINEERING MATERIALS 2010, 12, No. 12 � 2010 WILEY-VCH Verl
side together with a schematic of the setup on the right side.
Alongside the test chamber inwhich various atmospheres and
vacuum can be realized, the main parts of the facility are the
laser and an IR camera.
The Nd:YAG laser (continuous wave, 1064 nm) with a
maximum output power of 1 kW (Haas Laser) is used for
heating the samples.
To heat up the sample the laser spot is moved across the
sample surface by a programmable focusing optic (FPO) in an
area of 26� 26mm2 and the power adjusted in steps of 1W
from 10W to the maximum power. For the realized
experiments a power between 50 and 120W was chosen,
with the laser spot moving in different ways across the sample
surface with velocities between 520 and 820mms�1 depend-
ing on the chosen trajectory.
To achieve a homogenous heating of the sample surface,
different movement trajectories of the laser on the sample
surface were tested. One possible trajectory is the one used
during our standard thermal shock test. Here the laser is
moved over the sample in an arithmetic spiral in which the
successive turnings of the spiral have a constant separation
distance as shown schematically on the left side of Figure 2.
Diameter, path distance, and time for the spiral are varied
depending on the area which should be heated. Typical
parameters for the spiral used in the experiments are a
diameter of 17mm, a path separation of 1.4mm and 0.26 s for
the movement of one spiral, resulting in a velocity of the laser
of 522mms�1. The desired surface temperature can be
controlled by the laser power which depends on the used
coating for improving the absorption of the laser.
Another trajectory that seems to be more promising is the
use of a line pattern with a line spacing of 1mm. A schematic
of the first part is shown on the right side of Figure 2. After this
first part the pattern is repeated with a 908 turn and after that
both steps are repeated with a lateral shift of the pattern of
0.5mm. As the heated area of the pattern is greater than that of
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Fig. 2. Schematics of the used laser trajectories and indicated laser spot with a diameter of about 1.5 mm. Left side:Arithmetic spiral with typical parameters: diameter 17 mm, path separation 1.4 mm, and duration 0.26 s. Rightside: First part of the used line pattern after which the pattern was repeated with a 908 turn.
the spiral a higher velocity is necessary to prevent excessive
cooling of local areas between the repetition of the pattern. For
the described pattern the velocity of the laser was about
820mms�1.
The IR camera (Raytheon, model Radiance HS) is used to
measure the temperature of the sample surface. The camera
uses an InSb detector with a focal array of 256� 256 pixels. The
sensitivity range of the camera is 3–5mm and is narrowed by a
peak filter to 4.474mm (70% transmission). This is necessary as
the radiation would otherwise result in too short integration
time not permitted by the camera. The integration time and
recording frequency can be chosen freely with a maximum
frequency of 500Hz.
For thermal shock tests the highest possible recording
frequency of 500Hz is used to maximize the time resolution of
the temperature information detected by the camera. As the
time resolution is not so important for cyclic testing of TBC
samples and would lead to too much data of the IR images,
only a recording frequency of 10Hz is used in this case.
For different temperatures to which the sample should be
heated up during an experiment and different absorption
coatings used, integration times of the camera between 70 and
20ms are used.
For calibrating the camera a 2.6mm thick sample of YSZ
without substrate with a hole for a thermocouple is used. On
this sample the same surface treatment to improve the laser
absorption is applied as on the TBC samples.
The sample is heated up with the laser to different
temperatures. After achieving a steady state of the tempera-
ture measured by the thermocouple, the detected radiation is
determined.
For detecting acoustic emission from the sample a 150 kHz
resonant sensor with a sampling rate of 5MHz (Vallen
Systeme) is integrated in the sample holder.
With this test setup thermal shock test on thin ceramics
disks (about 0.3mm thickness) have already been successfully
carried out to determine the thermal shock resistance of
ceramic materials.[15]
1226 http://www.aem-journal.com � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For a setup comparable to a burner rig test
it is necessary to be able to heat up the sample
surface in cycles while cooling the backside
of the sample by air with the existing test
facility. The heating and cooling times
during the cycles can be chosen indepen-
dently, so the cycle parameters can be chosen
freely. In the used setup and the already
performed experiments no active cooling
was used. The backside of the sample was
just cooled by radiation and convectional
cooling.
The temperature on the sample surface is
determined by analyzing the IR images after
the experiments. After calibrating, as
described above, the detected radiation in
an area of the sample surface inside the
heated zone is averaged to eliminate local
differences of the emission and to reduce the influence on the
temperature in the time the laser is inside the averaging field
resulting in local overheating and overestimating the
temperature.
To determine the temperature gradient between surface
and backside thermocouples (TypeK) are bonded to the
metallic backside.
Investigated Materials
Disks of INCONEL 100 with a thickness of 2mm and a
diameter of 23mm were used as substrate. A NiCoCrAlY-
alloy was applied by plasma spraying as bond coat. The top
coat consists of zirconia (ZrO2) stabilized by 8 wt% of yttria
(Y2O3) and was applied by atmospheric plasma spraying. A
cross-section of a sample is shown in Figure 3. Due to the fact
that YSZ has a low absorption coefficient at the laser
wavelength of 1064 nm it is necessary to increase the
absorption for laser radiation to achieve the desired test
temperatures.[16] For this purpose the samples were first
treated with iron compounds and were then heat treated at
1200 8C for up to 48 h to realize a thin layer of iron oxide.
Results
At high operating temperatures sintering effects occur that
result in change of some properties of YSZ like the Young’s
modulus and the thermal expansion, and the ceramic becomes
more brittle and more prone to thermal shock induced stress
and damages. These changes are time depending processes.
To characterize the influence of the sintering the Young’s
modulus and the thermal expansion coefficient of free
standing YSZ samples without substrate and bond coat
before and after annealing were measured.
Thermal expansion measurements have been performed
by dilatometry and the results are shown on the right side of
Figure 4. Two sample of free standing YSZ were tested, one
annealed for 48 h at 1 200 8C prior to the measurement. The
ADVANCED ENGINEERING MATERIALS 2010, 12, No. 12
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Fig. 3. Microscopic image of a polished cross-section of one of the used TBC samples with substrate, bond coat,and ceramic top coat.
course of the coefficient of thermal expansion, a, as a function
of temperature changes due to the annealing treatment.
The Young’s modulus has been measured at room
temperature by the resonant method. For investigating the
time dependent change measurements of the samples were
performed after different intervals of annealing at 1200 8C. As
shown on the left side in Figure 4 it is clearly visible that the
Young’s modulus almost doubles after an annealing time of
4 h at 1200 8C. After annealing for 48 h only an increase of less
than 10% compared to the previous value is observed and the
Young’smodulus seems to saturate against amaximumvalue.
Annealing at a lower temperature of 900 8C also leads to an
increase of the Young’s modulus with the same time
depending change, but the overall change is only about
50% compared to the annealing at 1 200 8C.
0 20 40 60 80 100
40
50
60
70
80
90
200
8,0
8,5
9,0
9,5
10,0
10,5
11,0
11,5
12,0
Annealing at 900°C Annealing at 1200°C
You
ng´s
Mod
ulus
[GP
a]
Time [h]
Exp
ansi
on C
oeffi
cien
t α [1
0-6 K
-1]
Fig. 4. Left side: Young’s modulus at room temperature after different annealing intervals. Right side: Thermal expa
ADVANCED ENGINEERING MATERIALS 2010, 12, No. 12 � 2010 WILEY-VCH Verlag GmbH & Co. KGaA
The heat treatment of the samples is
needed to improve the laser absorption, but
also fulfils the purpose to achieve a homo-
geneous change of physical properties in the
whole sample.
Figure 5 shows an example of a possible
thermal cycling experiment comparable to a
burner rig test as temperature versus time for
one cycle. The heating time is 270 s and the
cooling time 150 s. In this experiment a laser
power of 65W is used with the line pattern
described above as trajectory. The tempera-
ture in the center of the sample surface was
determined by averaging over an area of
48� 48 pixels corresponding to an area of
about 12� 12mm2. At the end of the heating
period the average temperature of this area
was determined to be around 700 8C.A radial cut through the center of the
sample almost at the end of the heating period
can be seen in Figure 6. It illustrates that the
temperature in the center of the sample is
higher than the average and lies at about
760 8C. Comparing this value to the tempera-
ture on the backside, for which the thermocouple showed a
temperature of 655 8C, results in a temperature gradient in the
center of the sample of about 100 8C.This value is at the lower end of the aimed gradients
(50–300 8C) mentioned in the literature.[2,3,7] This gradient is
still without active cooling of the sample backside which
should increase the temperature gradient further.
During experiments a crack formed during the heating
phase of one of several tests with a heating duration of 45 s.
Between the tests the samples was allowed to cool down to
room temperature without active cooling. In this experiment
the spiral trajectory was used for heating with the typical
parameters as mentioned above (ؼ 17mm, d¼ 1.4mm,
t¼ 0.26 s) and the laser power was increased between tests
to see at which laser power damage to the sample is caused by
400 600 800 1000
Temperature [°C]
with annealing without annealing
nsion of samples with and without annealing for 48 h at 1 200 8C.
, Weinheim http://www.aem-journal.com 1227
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0 20 40 60 80 100 120400
450
500
550
600
650
700
750
800
850
Tem
pera
ture
[°C
]
Pixel
Fig. 6. Radial cut of the infrared image through the center of the sample to illustrate thetemperature distribution during heating the sample with the line pattern laser trajec-tory.
0 60 120 180 240 300 360 4200
100
200
300
400
500
600
700
800
Surface Backside
Tem
pera
ture
[°C
]
Time [s]
Fig. 5. Example of possible cyclic heating with cooling phase. Heating time 270 s,observed cooling time 150 s. Laser trajectory: Line pattern with 65 W laser power.
Fig. 8. Acoustic emission signal recorded during one experiment.
the laser due to local overheating. The crack occurred during
heating with a laser power of 120W, without showing signs of
damage on the sample due to local overheating, melting, or
similar damages induced by the laser.
Before the crack occurred the surface temperature was
determined to be around 900 8C.
Fig. 7. Microscopic image (left) and Infrared image (right) of the heated TBC sample wit
1228 http://www.aem-journal.com � 2010 WILEY-VCH Verlag GmbH & C
Amicroscope image of the crack can be seen on the left side
of Figure 7. The crack opening is about 40mm and is hardly
visible with the naked eye without magnification. But as
shown on the right side of Figure 7 the crack is clearly visible
in the IR image. (Note the different magnification between left
and right side in Figure 7.) The crack acts as a kind of black
body radiation source and has therefore another emission as
the surrounding surface. Also the heat accumulates at the
crack as the normal heat flow is disturbed. This two reasons
result in a different radiation from the crack that is detected as
an inhomogeneous area on the IR image.
During the heating phase in which the crack was initiated,
an acoustic emission signal could also be detected. In the
existing setup there was only one acoustic sensor allowing
only a time resolved recording of acoustic emission. An
example of a recorded signal without postprocessing is shown
in Figure 8. Additional sensors will be integrated into the
setup during improvement of the test setup to be able to
localize acoustic emission signals.
To achieve testing parameters that are comparable to real
component applications and parameters of burner rig tests, an
increase of the testing temperature to about 1 200 8C or above
is necessary. The required technical modifications are
currently under development together with the integration
of active cooling and sound locating of induced damages.
h induced crack.
o. KGaA, Weinheim
Summary and Outlook to FurtherImprovement of the Test SetUp
In this paper it was shown that the
presented test setup is able to introduce
controlled reproducible thermal loads to
TBC samples by cyclic heating. During first
tests, damage of the samples was achieved.
The induced cracks were clearly identified in
IR images and their development detected
by acoustic emission sensor. Therefore, the
used test setup is a promising possibility to
realize a testing technique comparable to
burner rig tests with advantages due to the
ADVANCED ENGINEERING MATERIALS 2010, 12, No. 12
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D. Nies et al./Testing of Thermal Barrier . . .
contact free heating by a laser. The stress free mounting of the
sample allows acoustic emission detection without the noise
of the airflow of the burner. The IR-camera enables to measure
the surface temperature of the whole sample during the
experiments and damage is detected by thermography before
being visible to the naked eye.
For further improvement a new sample holder is currently
under construction to realize active cooling to adjust defined
temperature gradients.Also, additional acoustic emission sensors
will be integrated. With then four sensors it will be tried to locate
the origin of damages by the delay time between the signals
before they are visible by the naked eye or in the IR image.
Received: July 15, 2010
Final Version: September 1, 2010
Published online: November 9, 2010
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