M. KAPLAN et al.: THERMAL-CYCLING BEHAVIOR OF CoNiCrAlY BONDS COATED WITH THERMAL BARRIER ...
897–901
THERMAL-CYCLING BEHAVIOR OF CoNiCrAlY BONDS
COATED WITH THERMAL BARRIER COATINGS (TBCs)
PRODUCED WITH ATMOSPHERIC PLASMA SPRAYING (APS)
OBNA[ANJE CoNiCrAlY PREVLEKE MED TERMI^NIM
UTRUJANJEM
Mustafa Kaplan1, Mesut Uyaner2, Abdullah Cahit Karaoglanli3
1Selcuk University, Department of Metallurgical and Materials Engineering, Graduate School of Natural and Applied Sciences, Konya, Turkey
2Necmettin Erbakan University, Faculty of Aeronautics and Astronautics, Department of Aeronautical Engineering, Konya, Turkey
3Bartin University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Bartin, Turkey
mkaplan5442@gmail.com
Prejem rokopisa – received: 2017-01-23; sprejem za objavo – accepted for publication: 2017-05-12
doi:10.17222/mit.2017.024
Thermal barrier coatings (TBCs) are commonly applied as a thermal insulation in order to protect against environmental
influences in the components of high-temperature gas turbines and jet engines. Gas-turbine components are affected by
aggressive conditions of the environment during the service, being subjected to failures such as corrosion, thermal shock and
oxidation. Plasma-spray technology is used to produce metallic bonds and ceramic top coats as a cost-effective method to
prolong the lifetime of TBCs. In the present research, CoNiCrAlY bond-coat and YSZ top-coat powders included in the TBCs
were deposited onto Inconel 718 superalloy substrates using atmospheric plasma spraying (APS). The TBCs were exposed to a
furnace cycling test at 1150 °C and one-hour cycles. The presence of porosity and cracks facilitating the diffusion of oxygen in
the top-coating structure led to the formation of thermally grown oxides (TGOs) at the interface. In addition, the most effective
factors of failures were the formation and growth of mixed oxides at the bond/top-coat interface and the presence of
imperfections. According to the test results, an increasing number of thermal cycles resulted in a decrease in the lifetime of the
TBCs related to the sintering of the top coating due to the effect of high temperature.
Keywords: thermal barrier coatings (TBCs), thermal cycling, Inconel 718 superalloy, atmospheric plasma spraying (APS),
CoNiCrAlY, ytrria-stabilized zirconia (YSZ)
Prevleke s toplotno pregrado (angl. TBCs) se obi~ajno uporabljajo kot toplotna izolacija, ki {~iti komponente visoko
temperaturnih plinskih turbin ali reaktivnih motorjev. Na komponente plinskih turbin vplivajo agresivni vplivi okolja, katerim so
izpostavljene med obratovanjem, zato pride do okvar, kot so: korozija, termi~ni {ok in oksidacija. Tehnologija plazemskega
napr{evanja je cenovno ugodna tehnologija za izdelavo kovinskih in kerami~nih prevlek. V ~lanku avtorji predstavljajo
raziskavo v kateri sta bila na podlago iz zlitine Inconel 718 s tehnologijo plazemskega napr{evanja v zra~ni atmosferi nane{ena
vezivna prevleka CoNiCrAlY in prekrivni sloj iz YSZ prahu. Izdelani prevleki sta bili nato izpostavljeni termi~nem utrujanju v
pe~i pri 1150 °C z ve~imi enournimi cikli. Prisotnost poroznosti in razpok olaj{a difuzijo kisika v zgornjo plast prevleke kar
vodi do nastanka termi~no induciranega oksida (TGO) na povr{ini prevleke. Poleg tega sta bila najpomembnej{a dejavnika za
nastanek napak tvorba me{anih oksidov na meji med vezno in vrhno plastjo ter prisotnost drugih nepravilnosti. Rezultati
preizkusov so pokazali, da pove~anje {tevila termi~nih ciklov zmanj{uje dobo trajanja TBC prevlek, zaradi sintranja zgornje
plasti pri povi{anih temperaturah.
Klju~ne besede: prevleke s toplotno pregrado, toplotni (termi~ni) cikel, superzlitina Inconel 718, plazemsko napr{evanje,
CoNiCrAlY, cirkon stabiliziran z itrijem
1 INTRODUCTION
Atmospheric plasma spraying (APS) is one of the
most important methods in the thermal-spray family. The
APS technique, which is a type of the plasma spray tech-
nique, provide many advantages, such as an economic,
quick and easy production; besides, almost all materials
can be sprayed with this method. Plasma-sprayed part-
icles quickly cool down from very high temperatures to
low temperatures. Plasma-spray coatings consist of
microstructured, layered and porous structures.1,2
Thermal barrier coatings (TBCs) are widely em-
ployed in gas turbines and its components operating at
high-temperature service conditions. The main function
of TBCs is to provide a protective thermal shield to
protect the main material components, especially turbine
blades and vanes, from the negative effects caused by
high temperatures in the environment. There are three
main layers in TBC systems, including a nickel-based
superalloy substrate, an MCrAlY metallic bond coat and
partially yttria-stabilized zirconia (YSZ), acting as the
ceramic top coat.3,4 The bond coat, which consist of
MCrAlY (M = Ni and/or Co), is extensively applied to
minimize the thermal mismatch between the superalloy
substrate and the ceramic top coat.5,6
TBCs are subject to sudden temperature changes and
thermal stress during service conditions. In high-tempe-
rature applications, TBCs are adversely affected and tend
to suffer because of failures such as oxidation, hot corro-
sion and thermal cycling/shock so that they lose struc-
tural integrity. Residual stresses due to the thermal-
expansion mismatch during the cooling and sintering of
Materiali in tehnologije / Materials and technology 51 (2017) 6, 897–901 897
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 621.793.7:620.192.42 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(6)897(2017)
the top coat, and oxidation of the bond coating cause the
coating to become damaged during thermal cycling.7,8
In this study, the TBC system consists of a
CoNiCrAlY metallic bond and YSZ ceramic top coat
deposited with the APS technique on an Inconel 718
superalloy substrate. The effects of oxidation and
TGO-growth behavior on the thermal durability were
investigated through a furnace thermal cyclic test. The
microstructure evolution and thermal durability were
evaluated before and after the furnace thermal-cycling
tests.
2 EXPERIMENTAL PART
2.1 Material
The Inconel 718 nickel-based superalloy was used as
the substrate material. Superalloy disk-shaped samples
with a diameter of 25.4 mm and thicknesses of 4 mm
were used as substrates. The chemical composition of
the nickel-based superalloy substrate is given in Table 1.
Table 1: Chemical composition of Inconel 718 Ni-based superalloy9
Elements, Chemical Composition, in mass fractions (w/%)
Ni Cr Nb Ti Mo Al Co Si
53.53 18.45 5.31 0.95 3.02 0.48 0.27 0.09
Cu Mn C Ta P B S Fe
0.04 0.06 0.03 0.005 0.007 0.005 0.001 Balance
2.2 Spraying processes
Commercial CoNiCrAlY metallic bond-coat powder
(Amdry 9951, Sulzer-Metco) with a 5–37 μm particle
size and ZrO2 + 8 %Y2O3 (YSZ) ceramic top-coat
powder (Metco 204NS, Sulzer Metco) with a -45+20 μm
particle size were used as the feedstocks. As previously
mentioned, the APS technique was used for depositing
both the metallic bond and ceramic top coatings. The top
layer of YSZ with a thickness of 300 μm was applied on
the CoNiCrAlY layer with a thickness of 100 μm on the
superalloy substrates.
2.3 Furnace thermal-cycling test
The lifetime of the TBCs tested with the furnace
thermal-cycling test using rapid temperature furnace
1710 BL was as follows: The samples were heated to
1150 °C in 7 min, held at this temperature for 45 min
and then cooled to 20 °C in air within 8 min. The ther-
mal-loading diagram of the samples coated during
thermal cycling is given in Figure 1. During isothermal-
oxidation cycles, the temperature changes in the working
furnace did not exceed 5 °C. The coating lifetime was
determined with the thermal cycling carried out until the
cracking point of the outer ceramic layer. This was con-
sidered to be cracked when 25 % of the sample surface
was blistering. The surface failure was visually deter-
mined based on the percentage of the separation of the
outer ceramic layer from the sample surface. At the end
of the thermal-cycling tests, the samples were investi-
gated from both the top surface and cross-sectional area
via a scanning-electron-microscopy (SEM) analysis.
3 RESULTS AND DISCUSSION
3.1 As-sprayed TBC
A SEM microstructure image of an as-deposited TBC
is shown in Figure 2. The APS-applied CoNiCrAlY
metallic bond and the YSZ top coat exhibit a porous
structure and some cracks and open discontinuities due
to the deposition process. The metallic-bond and top-
coat structures show typically characteristic properties of
the plasma-spray technique.
In Figure 3, the top surface of the TBC is shown.
There are some micro-cracks due to heating and rapid
cooling of the sprayed particles during the deposition
process as well as melted and unmelted particles.
M. KAPLAN et al.: THERMAL-CYCLING BEHAVIOR OF CoNiCrAlY BONDS COATED WITH THERMAL BARRIER ...
898 Materiali in tehnologije / Materials and technology 51 (2017) 6, 897–901
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: As-deposited TBC cross-sectional SEM microstructureFigure 1: Thermal-cycling diagram during the furnace cycling test
3.2 Thermal-cycling test of the coated specimens
At high temperatures, oxygen can easily diffuse from
the top coat to the bond coat due to the ionic conduc-
tivity of YSZ. Also, the coating includes many imper-
fections such as cracks and porosity, which accelerate
the penetration of oxygen to the metallic bond coat. As a
result, the bond coat oxidizes easily at high temperatures
in spite of the ceramic coat. According to the Ellingham
diagram, Y and Al are the elements that enter the first
reaction with oxygen in the CoNiCrAlY bond coat due
to low-standard free energies. With the effect of high
temperature, the particles are still oxidized in flight and
during the deposition on the substrate material.10
Using the APS method, there are many internal oxide
formations in the coating because of the excessively high
temperatures due to plasma during the deposition of the
coating. Since the concentration of the Y element is
much lower than those of the other elements, Y can be
entirely depleted. Thus, after the deposition of a TBC, Al
is oxidized at the interface and Al2O3 is formed.11–13
In the early stages of the cyclic oxidation, the TGO
layer mainly occurred as the Al2O3 phase. This oxidation
causes thermal stresses at the interface between the bond
and the top coat due to both the phase transformations of
Al2O3 (, ,  and ) and a lower thermal-expansion
coefficient compared to YSZ. Al2O3 (especially in the
-Al2O3 form) is thermodynamically a very stable phase
compared to the other phases such as Cr2O3, (Co, Ni)O
or (Co,Ni)(Al,Cr)2O4.13 In addition, it provides a better
resistance against the oxygen expansion into the bond
coat. Depending on the time, the Al concentration de-
creases to a critical level and Cr2O3 and (Co, Ni)O phases
form at the interface. Afterwards, these oxides react with
Al2O3 and spinel phases occur. The thermal-expansion
coefficient of the spinel phases is relatively lower than
that of Al2O3, which means it leads to a higher stress.14,15
In addition, the spinel phases grow faster than Al2O3 and
show larger volume changes.16
In Figure 4, an elemental mapping microstructure
belonging to the YSZ TBC after a 40-h thermal-cycling
test is shown. The coating structure almost preserved its
uniformity. At the interface, the Al2O3 phase is dominant
and a mixture of oxide formations like Cr2O3, (Co, Ni)O
and spinels can be observed. In the bond coat, dark
stringers represent the Al2O3 phase. In addition, the top
coat was sintered due to the effect of high temperature
M. KAPLAN et al.: THERMAL-CYCLING BEHAVIOR OF CoNiCrAlY BONDS COATED WITH THERMAL BARRIER ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 897–901 899
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 4: Elemental mapping microstructure belonging to the YSZ
TBC after a 40-h thermal-cycling test
Figure 3: Top-coat SEM image of the as-sprayed TBC
Figure 5: Top-surface SEM image of the TBC after the thermal-
cycling test
the top coat, and oxidation of the bond coating cause the
coating to become damaged during thermal cycling.7,8
In this study, the TBC system consists of a
CoNiCrAlY metallic bond and YSZ ceramic top coat
deposited with the APS technique on an Inconel 718
superalloy substrate. The effects of oxidation and
TGO-growth behavior on the thermal durability were
investigated through a furnace thermal cyclic test. The
microstructure evolution and thermal durability were
evaluated before and after the furnace thermal-cycling
tests.
2 EXPERIMENTAL PART
2.1 Material
The Inconel 718 nickel-based superalloy was used as
the substrate material. Superalloy disk-shaped samples
with a diameter of 25.4 mm and thicknesses of 4 mm
were used as substrates. The chemical composition of
the nickel-based superalloy substrate is given in Table 1.
Table 1: Chemical composition of Inconel 718 Ni-based superalloy9
Elements, Chemical Composition, in mass fractions (w/%)
Ni Cr Nb Ti Mo Al Co Si
53.53 18.45 5.31 0.95 3.02 0.48 0.27 0.09
Cu Mn C Ta P B S Fe
0.04 0.06 0.03 0.005 0.007 0.005 0.001 Balance
2.2 Spraying processes
Commercial CoNiCrAlY metallic bond-coat powder
(Amdry 9951, Sulzer-Metco) with a 5–37 μm particle
size and ZrO2 + 8 %Y2O3 (YSZ) ceramic top-coat
powder (Metco 204NS, Sulzer Metco) with a -45+20 μm
particle size were used as the feedstocks. As previously
mentioned, the APS technique was used for depositing
both the metallic bond and ceramic top coatings. The top
layer of YSZ with a thickness of 300 μm was applied on
the CoNiCrAlY layer with a thickness of 100 μm on the
superalloy substrates.
2.3 Furnace thermal-cycling test
The lifetime of the TBCs tested with the furnace
thermal-cycling test using rapid temperature furnace
1710 BL was as follows: The samples were heated to
1150 °C in 7 min, held at this temperature for 45 min
and then cooled to 20 °C in air within 8 min. The ther-
mal-loading diagram of the samples coated during
thermal cycling is given in Figure 1. During isothermal-
oxidation cycles, the temperature changes in the working
furnace did not exceed 5 °C. The coating lifetime was
determined with the thermal cycling carried out until the
cracking point of the outer ceramic layer. This was con-
sidered to be cracked when 25 % of the sample surface
was blistering. The surface failure was visually deter-
mined based on the percentage of the separation of the
outer ceramic layer from the sample surface. At the end
of the thermal-cycling tests, the samples were investi-
gated from both the top surface and cross-sectional area
via a scanning-electron-microscopy (SEM) analysis.
3 RESULTS AND DISCUSSION
3.1 As-sprayed TBC
A SEM microstructure image of an as-deposited TBC
is shown in Figure 2. The APS-applied CoNiCrAlY
metallic bond and the YSZ top coat exhibit a porous
structure and some cracks and open discontinuities due
to the deposition process. The metallic-bond and top-
coat structures show typically characteristic properties of
the plasma-spray technique.
In Figure 3, the top surface of the TBC is shown.
There are some micro-cracks due to heating and rapid
cooling of the sprayed particles during the deposition
process as well as melted and unmelted particles.
M. KAPLAN et al.: THERMAL-CYCLING BEHAVIOR OF CoNiCrAlY BONDS COATED WITH THERMAL BARRIER ...
898 Materiali in tehnologije / Materials and technology 51 (2017) 6, 897–901
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: As-deposited TBC cross-sectional SEM microstructureFigure 1: Thermal-cycling diagram during the furnace cycling test
3.2 Thermal-cycling test of the coated specimens
At high temperatures, oxygen can easily diffuse from
the top coat to the bond coat due to the ionic conduc-
tivity of YSZ. Also, the coating includes many imper-
fections such as cracks and porosity, which accelerate
the penetration of oxygen to the metallic bond coat. As a
result, the bond coat oxidizes easily at high temperatures
in spite of the ceramic coat. According to the Ellingham
diagram, Y and Al are the elements that enter the first
reaction with oxygen in the CoNiCrAlY bond coat due
to low-standard free energies. With the effect of high
temperature, the particles are still oxidized in flight and
during the deposition on the substrate material.10
Using the APS method, there are many internal oxide
formations in the coating because of the excessively high
temperatures due to plasma during the deposition of the
coating. Since the concentration of the Y element is
much lower than those of the other elements, Y can be
entirely depleted. Thus, after the deposition of a TBC, Al
is oxidized at the interface and Al2O3 is formed.11–13
In the early stages of the cyclic oxidation, the TGO
layer mainly occurred as the Al2O3 phase. This oxidation
causes thermal stresses at the interface between the bond
and the top coat due to both the phase transformations of
Al2O3 (, ,  and ) and a lower thermal-expansion
coefficient compared to YSZ. Al2O3 (especially in the
-Al2O3 form) is thermodynamically a very stable phase
compared to the other phases such as Cr2O3, (Co, Ni)O
or (Co,Ni)(Al,Cr)2O4.13 In addition, it provides a better
resistance against the oxygen expansion into the bond
coat. Depending on the time, the Al concentration de-
creases to a critical level and Cr2O3 and (Co, Ni)O phases
form at the interface. Afterwards, these oxides react with
Al2O3 and spinel phases occur. The thermal-expansion
coefficient of the spinel phases is relatively lower than
that of Al2O3, which means it leads to a higher stress.14,15
In addition, the spinel phases grow faster than Al2O3 and
show larger volume changes.16
In Figure 4, an elemental mapping microstructure
belonging to the YSZ TBC after a 40-h thermal-cycling
test is shown. The coating structure almost preserved its
uniformity. At the interface, the Al2O3 phase is dominant
and a mixture of oxide formations like Cr2O3, (Co, Ni)O
and spinels can be observed. In the bond coat, dark
stringers represent the Al2O3 phase. In addition, the top
coat was sintered due to the effect of high temperature
M. KAPLAN et al.: THERMAL-CYCLING BEHAVIOR OF CoNiCrAlY BONDS COATED WITH THERMAL BARRIER ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 897–901 899
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 4: Elemental mapping microstructure belonging to the YSZ
TBC after a 40-h thermal-cycling test
Figure 3: Top-coat SEM image of the as-sprayed TBC
Figure 5: Top-surface SEM image of the TBC after the thermal-
cycling test
depending on time. This can be explained with de-
creasing discontinuity openings.
In Figure 5, a top-surface SEM image of the TBC
after the thermal-cycling test is given. After 40 thermal
cycles, the top-surface crack area of the TBC covers
approximately 25 %. In Figure 5, a failed surface of the
TBC is shown. According to Figure 5, there are large
crack formations and sintered areas.
Figure 6 shows that there is only one phase in the
as-sprayed TBC. After the thermal-cycling test, there is
not any phase transformation so the structure is similar to
the as-sprayed TBC. The semi-stable tetragonal ZrO2
phase is durable up to approximately 1200 °C, which
means that the crack propagation does not progress into
the bond coat.8 This can also be understood from a
cross-sectional SEM image of the failed TBC.
J. A. Haynes et al.17 applied furnace thermal-cycling
tests to YSZ TBCs including a NiCrAlY bond coat
produced with the APS method. The tests were carried
out for one hour in a furnace at 1150 °C with periods of
15-min heating and 30-min cooling. The test result
shows that after 100 thermal cycles, the APS TBCs
failed. However, in the present study, the heating and
cooling times of the thermal-cycling tests are shorter.
Thus, the CoNiCrAlY TBC is exposed to damage earlier
than in the above study. R. Chen et al. used the APS
technique to produce a CoNiCrAlY bond coated with the
YSZ TBC on an Inconel 625 substrate. Their ther-
mal-cycling test at 1050 °C showed a parabolic TGO
growth. The growth rate of the TGO structure increased
proportionally as the thermal-cycling period increased.
This result supports the present study.
A. El-Turki et al.18 studied the thermal-cycling beha-
vior of a VPS NiCoCrAlY bond coated with APS YSZ at
1050 °C, with 25-min heating, 300-h holding and 30-min
cooling. The TBCs were damaged after 21 cycles. The
damage of the TBCs started at the same time as the
damage to TGO interface areas. The use of the VPS
technique allowed a low porosity and oxidation during
the deposition of the bond coat, and their total holding
time was very long compared to the present study.19
J. Sun et al.20 produced their bond coat using the HVOF
technique; afterwards they carried out the deposition of
APS YSZ. Their thermal cycling was performed at 1100
°C for one hour, with 10-min cooling. After 30 thermal
cycles, the TBC underwent a significant failure. In
contrast, the present-study condition is more destructive,
but the thermal-cycling lifetime is longer. As a result, the
used bond-coat powder type, production technique and
thermal-cycling conditions can cause significant
differences in the lifetime of TBCs.20 In the present
study, the APS TBC separated from the bond-coat sur-
face of the ceramic top coat after 40 h of the thermal
cycling and was spalled and damaged. Furthermore, a
TGO structure formed during the thermal cycling,
causing failure of the TBC systems. The growth of the
oxide layer formed the TGO structure and an increase in
the stresses increased the failure formation.
4 CONCLUSIONS
In this study, TBCs including CoNiCrAlY bonds and
YSZ were deposited using the APS technique. Furnace
thermal-cycling tests were performed to predict the
thermal-cycling lifetime of the TBCs and to observe
their microstructural changes. The results obtained at the
end of the observations are given below.
TBCs were damaged, with their cracks covering
25 % of the top coat after 40 cycles.
The most effective factor of failure was the formation
of mixed oxides and spinels at the interface of the bond
and the top coat, according to the cross-sectional inve-
stigations.
The porosity and cracks of the top coat caused the
formation of TGOs at the interface.
The top coat was sintered as a result of high tempera-
ture depending on time.
At the interface, the Al2O3 phase is dominant, but a
mixture of oxide formations like Cr2O3, (Co, Ni)O and
spinels is also observed. In the bond coat, dark stringers
represent the Al2O3 phase. The mixture of oxides formed
at the interface was mainly responsible for the failure of
the TBCs after the thermal cycling.
Acknowledgements
The present study was derived from the Ph.D. thesis
of Mustafa Kaplan at the Selcuk University Graduate
School of Natural and Applied Sciences, Konya, Turkey.
The study was also supported by Selcuk University
Scientific Research Projects under the Grant Number of
15201071.
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M. KAPLAN et al.: THERMAL-CYCLING BEHAVIOR OF CoNiCrAlY BONDS COATED WITH THERMAL BARRIER ...
900 Materiali in tehnologije / Materials and technology 51 (2017) 6, 897–901
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 6: XRD analysis of the TBC before and after the thermal-
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mal barrier coatings exposed to NaCl vapor by finite element
method, Materials Science and Engineering A, 490 (2008) 6,
351–358, doi:10.1016/j.msea.2008.01.050
16 S. Laxman, B. Franke, B. W. Kempshall, Y. H. Sohn, L. A. Gian-
nuzzi, K. S. Murphy, Phase transformations of thermally grown
oxide on (Ni,Pt)Al bondcoat during electron beam physical vapor
deposition and subsequent oxidation, Surface and Coatings
Technology, 177 (2004), 121–130, doi:10.1016/j.surfcoat.2003.
08.072
17 J. A. Haynes, M. K. Ferber, W. D. Porter, Thermal cycling behavior
of plasma-sprayed thermal barrier coatings with various MCrAlX
bond coats, Journal of Thermal Spray Technology, 9 (2000) 38,
38–48, doi:10.1361/105996300770350041
18 R. Chen, X. Wu, D. Dudzinski, Influence of thermal cycle frequency
on the TGO growth and cracking behaviors of an APS-TBC, Journal
of Thermal Spray Technology, 21 (2012) 6, 1294–1299, doi:10.1007/
s11666-012-9824-4
19 A. El-Turki, G. C. Allen, C. M. Younes, J. C. C. Day, An investi-
gation of the effect of thermal cycling on plasma-sprayed zirco-
nia/NiCoCrAlY thermal barrier coating, Materials and Corrosion, 55
(2004) 1, 24–29, doi:10.1002/maco.200303709
20 J. Sun, L. L. Zhang, D. Zhao, Microstructure and thermal cycling
behavior of nanostructured yttria partially stabilized zirconia (YSZ)
thermal barrier coatings, Journal of Rare Earths, 28 (2010) 1,
198–201, doi:10.1016/S1002-0721(10)60
M. KAPLAN et al.: THERMAL-CYCLING BEHAVIOR OF CoNiCrAlY BONDS COATED WITH THERMAL BARRIER ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 897–901 901
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
depending on time. This can be explained with de-
creasing discontinuity openings.
In Figure 5, a top-surface SEM image of the TBC
after the thermal-cycling test is given. After 40 thermal
cycles, the top-surface crack area of the TBC covers
approximately 25 %. In Figure 5, a failed surface of the
TBC is shown. According to Figure 5, there are large
crack formations and sintered areas.
Figure 6 shows that there is only one phase in the
as-sprayed TBC. After the thermal-cycling test, there is
not any phase transformation so the structure is similar to
the as-sprayed TBC. The semi-stable tetragonal ZrO2
phase is durable up to approximately 1200 °C, which
means that the crack propagation does not progress into
the bond coat.8 This can also be understood from a
cross-sectional SEM image of the failed TBC.
J. A. Haynes et al.17 applied furnace thermal-cycling
tests to YSZ TBCs including a NiCrAlY bond coat
produced with the APS method. The tests were carried
out for one hour in a furnace at 1150 °C with periods of
15-min heating and 30-min cooling. The test result
shows that after 100 thermal cycles, the APS TBCs
failed. However, in the present study, the heating and
cooling times of the thermal-cycling tests are shorter.
Thus, the CoNiCrAlY TBC is exposed to damage earlier
than in the above study. R. Chen et al. used the APS
technique to produce a CoNiCrAlY bond coated with the
YSZ TBC on an Inconel 625 substrate. Their ther-
mal-cycling test at 1050 °C showed a parabolic TGO
growth. The growth rate of the TGO structure increased
proportionally as the thermal-cycling period increased.
This result supports the present study.
A. El-Turki et al.18 studied the thermal-cycling beha-
vior of a VPS NiCoCrAlY bond coated with APS YSZ at
1050 °C, with 25-min heating, 300-h holding and 30-min
cooling. The TBCs were damaged after 21 cycles. The
damage of the TBCs started at the same time as the
damage to TGO interface areas. The use of the VPS
technique allowed a low porosity and oxidation during
the deposition of the bond coat, and their total holding
time was very long compared to the present study.19
J. Sun et al.20 produced their bond coat using the HVOF
technique; afterwards they carried out the deposition of
APS YSZ. Their thermal cycling was performed at 1100
°C for one hour, with 10-min cooling. After 30 thermal
cycles, the TBC underwent a significant failure. In
contrast, the present-study condition is more destructive,
but the thermal-cycling lifetime is longer. As a result, the
used bond-coat powder type, production technique and
thermal-cycling conditions can cause significant
differences in the lifetime of TBCs.20 In the present
study, the APS TBC separated from the bond-coat sur-
face of the ceramic top coat after 40 h of the thermal
cycling and was spalled and damaged. Furthermore, a
TGO structure formed during the thermal cycling,
causing failure of the TBC systems. The growth of the
oxide layer formed the TGO structure and an increase in
the stresses increased the failure formation.
4 CONCLUSIONS
In this study, TBCs including CoNiCrAlY bonds and
YSZ were deposited using the APS technique. Furnace
thermal-cycling tests were performed to predict the
thermal-cycling lifetime of the TBCs and to observe
their microstructural changes. The results obtained at the
end of the observations are given below.
TBCs were damaged, with their cracks covering
25 % of the top coat after 40 cycles.
The most effective factor of failure was the formation
of mixed oxides and spinels at the interface of the bond
and the top coat, according to the cross-sectional inve-
stigations.
The porosity and cracks of the top coat caused the
formation of TGOs at the interface.
The top coat was sintered as a result of high tempera-
ture depending on time.
At the interface, the Al2O3 phase is dominant, but a
mixture of oxide formations like Cr2O3, (Co, Ni)O and
spinels is also observed. In the bond coat, dark stringers
represent the Al2O3 phase. The mixture of oxides formed
at the interface was mainly responsible for the failure of
the TBCs after the thermal cycling.
Acknowledgements
The present study was derived from the Ph.D. thesis
of Mustafa Kaplan at the Selcuk University Graduate
School of Natural and Applied Sciences, Konya, Turkey.
The study was also supported by Selcuk University
Scientific Research Projects under the Grant Number of
15201071.
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thermal barrier coatings, Journal of Rare Earths, 28 (2010) 1,
198–201, doi:10.1016/S1002-0721(10)60
M. KAPLAN et al.: THERMAL-CYCLING BEHAVIOR OF CoNiCrAlY BONDS COATED WITH THERMAL BARRIER ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 897–901 901
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS