K. M. DOLEKER et al.: COMPARISON OF MICROSTRUCTURES AND OXIDATION BEHAVIORS OF Y2O3- ...
315–322
COMPARISON OF MICROSTRUCTURES AND OXIDATION
BEHAVIORS OF YTRIA AND MAGNESIA STABILIZED
ZIRCONIA THERMAL BARRIER COATINGS (TBC)
PRIMERJAVA MED MIKROSTRUKTURAMI IN OKSIDACIJSKIMI
OBNA[ANJI Z ITRIJEM IN MAGNEZIJEM STABILIZIRANIH
CIRKONSKIH PREVLEK ZA TERMI^NO ZA[^ITO (TBC)
Kadir Mert Doleker
1
, Yasin Ozgurluk
1
, Dervis Ozkan
2
, Nihal Mesekiran
1
,
Abdulah Cahit Karaoglanli
1
1
Bartin University, Department of Metallurgical and Materials Engineering, 74110 Bartin, Turkey
2
Bartin University, Department of Mechanical Engineering, 74110 Bartin, Turkey
kmdoleker@bartin.edu.tr
Prejem rokopisa – received: 2017-09-18; sprejem za objavo – accepted for publication: 2017-11-14
doi:10.17222/mit.2017.150
Thermal barrier coatings (TBCs) provide protection to minimize aggressive environmental conditions such as oxidation,
corrosion and thermal shocks occurring at high temperatures. In this study, a CoNiCrAlY metallic-bond coat was deposited on
an Inconel 718 superalloy substrate with a thickness about 100 μm using the atmospheric-plasma-spray (APS) method. The
production of TBCs was accomplished by depositing Y2O3 partially stabilized zirconia (YSZ) and MgO-stabilized zirconia
(MSZ) as two different ceramic top-coating materials, having the total thickness of 300 μm. The specimens were subjected to a
metallographic investigation before the oxidation tests; their surface roughness, porosity, hardness and microstructural
properties were investigated and then compared with the results obtained after the oxidation tests. The TBC systems were
subjected to isothermal-oxidation tests at 900 °C and 1000 °C for (8, 24, 50 and 100) h. After the oxidation tests, XRD, SEM
and EDX elemental analyses were carried out and the changes in the thickness and structure of the thermally grown oxide
(TGO) layer were investigated. A remarkable change occurred between the top and the bond coat in the TBC systems depending
on the increasing time and temperature. The TGO layer thickness showed an increase. At the interface, Al2O3 and other
mixed-oxide layers occurred. Before and after the oxidation, XRD analyses showed that YSZ had a tetragonal phase and a bit of
a monoclinic phase. The MSZ coating included tetragonal, monoclinic and cubic phases at the end of the oxidation. This phase
transformation causes a large volume increase in an MSZ lattice. Due to this volume increase, MSZ coatings suffer more
damage than YSZ coatings.
Keywords: atmospheric plasma spray, thermal barrier coatings, oxidation, ZrO 2-Y
2O 3, ZrO
2-MgO
Prevleke kot termi~ne prepreke (TBCs; angl.: Thermal Barrier Coatings) zagotavljajo za{~ito pred agresivnim okoljem in
zmanj{ajo oksidacijo, korozijo in termi~ni {ok zaradi obratovanja pri visokih temperaturah. V {tudiji avtorji obravnavajo
kovinsko difuzijsko vezano prevleko na osnovi CoNiCrAlY, nane{eno na podlago iz superzlitine Inconel 718 debeline okoli 100
μm, ki je izdelana z metodo atmosferskega plazemskega napr{evanja (APS; angl: Atmospheric Plasma Spray). Dve razli~ni
kerami~ni TBCs, celotnih debelin 300 μm, so izdelali z nana{anjem z Y2O3 delno stabiliziranega cirkonijevega oksida ZrO2
(YSZ, angl.: Yttria partially Stabilized Zirconia) in z MgO-stabiliziranega cirkonijevega oksida (MSZ; Magnesia Stabilized
Zirconia). Na izdelanih vzorcih so izvedli primerjalne metalografske preiskave, pred in po oksidacijskih testih, in sicer:
hrapavost povr{ine, poroznost, trdoto in mikrostrukturne lastnosti. TBC-sistema sta bila izpostavljena testom izotermalne
oksidacije pri 900 °C in 1000 °C za (8, 24, 50 in 100) ur. Po oksidacijskih testih so izvedli XRD, SEM, EDX-elementne analize,
kakor tudi ugotovili spremembe debeline in strukture termi~no zrasle oksidne plasti (TGO; angl.: Thermally Grown Oxide).
Avtorji so ugotovili znatne spremembe, ki so nastale na vrhu prevlek in v prevlekah glede na nara{~ajo~i ~as in temperaturo
oksidacijskega testa. Debelina TGO- plasti se je pove~evala na meji med Al2O3 in drugimi med oksidacijo nastalimi me{animi
oksidi. XRD-analize, pred in po oksidaciji, so pokazale, da vsebuje YSZ prete`no tetragonalno in malo monoklinske faze. MSZ
pa vsebuje tetragonalno, monoklinsko in na koncu oksidacije kubi~no fazo. Ta fazna transformacija povzro~a velike volumske
spremembe v MSZ-mre`i. Zaradi pove~anja volumna so po{kodbe pri enakih pogojih oksidacije ve~je pri MSZ-prevlekah kot
pri YSZ-prevlekah.
Klju~ne besede: atmosfersko plazemsko napr{evanje, prevleke za termi~no za{~ito, oksidacija, ZrO2-Y 2O3, ZrO2-MgO
1 INTRODUCTION
Thermal-barrier-coating systems (TBCs) are gene-
rally used to provide thermal insulation for aircraft
engines and stationary, hot gas-turbine components.
1–3
The TBC systems used at high temperatures increase the
efficiency and life time of engines and turbine compo-
nents.
4–7
The TBCs consist of a heat-resistant ceramic
top coating, a metallic bond coat on superalloy substrates
and a thermally grown oxide (TGO) layer formed at the
interface of the metallic bond and ceramic top coating
during a high-temperature oxidation.
3,8–10
The bond coat
generally protects superalloy substrates from excessive
oxidation and corrosion and also provides an adhesive
surface for the ceramic top coat in these systems. The
ceramic top coat is mainly used for thermal insulation
and some additional corrosion and oxidation resistance
in TBC systems. For the deposition of TBCs, the APS
Materiali in tehnologije / Materials and technology 52 (2018) 3, 315–322 315
UDK 620.1:621.793.7:621.793:620.197 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(3)315(2018)

technique is generally used due to the need for high
melting temperatures. Since the APS coatings exhibit a
lamellar microstructure, lower thermal conductivities are
obtained compared to the other methods.
4,11–13
In the production of TBCs, nickel-based superalloys
are used as the substrate material, MCrAlY powders are
chosen as metallic bond coats and stabilized zirconia is
widely used as the top-coat material.
9,14,15
ZrO
2
has a
monoclinic phase at room temperature and tetragonal
and cubic structures at high temperature. The monoclinic
tetragonal transition occurs with a high volume change.
Thus, stabilizing oxides such as MgO (15–24 %) and
Y
2
O
3
(7–8 %) are added to zirconia to overcome this
problem. YSZ is a conventional state-of-the-art material
for TBC systems due to its high fracture toughness, low
thermal conductivity and high thermal-expansion
coefficient compatible with that of the substrate. While
YSZ with a fully stable tetragonal phase preserves its
stability in a range from room temperature to about
1200 °C, MSZ has a partially stable tetragonal phase in a
range from room temperature to about 950 °C.
12,16
In
addittion, at elevated temperatures up to around 1250 °C,
it will decompose into a monoclinic system and MgO.
However, MSZ is cheaper than YSZ. In jet engines,
exhaust-nozzle materials running at low temperatures
can be coated with MSZ to provide thermal insulation.
17
Regarding the thermal properties of both TBC systems,
the thermal conductivity of MSZ is relatively lower than
that of YSZ. On the other hand, the thermal-expansion
coefficient of MSZ is close to that of YSZ.
18
The aim of this paper is to investigate the oxidation
behavior and make a comparison between YSZ and MSZ
coatings. This will enable us to understand the oxidation
mechanism of TBC systems. A cross-sectional micro-
structural analysis was carried out to examine the oxi-
dation-product formation and degradation mechanisms.
Significant results of our study are presented in this
paper.
2 MATERIALS AND METHODS
2.1 Characterization
The cross-sectional microstructures of TBCs were
evaluated using a scanning electron microscope (SEM,
Tescan, MAIA3 XMU, Czech Republic) equipped with
energy dispersive spectroscopy (EDS, Oxford Xmax 50,
UK). TBCs were subjected to an X-ray diffraction
(Rigaku Dmax 2200 PC, Cu-K
 radiation, Japan)
analysis from their top surfaces to observe the phase
changes indicated by the results obtained before and
after the oxidation.
2.2 Preparation of TBC samples
In this study, the Inconel 718 superalloy with a
25.4-mm diameter and 4-mm thickness was selected as
the substrate material for its high strength and creep
resistance. This material is commonly used for the
applications that require high mechanical capabilities.
Sample surfaces were completely grit blasted before the
spraying process. The CoNiCrAlY (Co-32Ni-21Cr-8Al-
0.5Y, Sulzer Metco, Amdry 9951, 5.5–38 μm) powder
with a thickness of approximately 100 μm was sprayed
on Inconel 718 as the bond-coat layer using the APS tech-
nique. After the deposition of the bond coat, ZrO
2
+8%
Y
2
O
3
(GTV, -45+20 μm) and ZrO
2
-24 % MgO (Metco
210, -90+11 μm) powders were sprayed on the
CoNiCrAlY bond coat as the top coating layer with a
300-μm thickness using the APS technique.
2.3 Microstructural characterization of the powder ma-
terial
The characterization of the powder materials was
carried out with SEM using a 600× magnification to
analyze their shapes and sizes. After the SEM investiga-
K. M. DOLEKER et al.: COMPARISON OF MICROSTRUCTURES AND OXIDATION BEHAVIORS OF Y2O3- ...
316 Materiali in tehnologije / Materials and technology 52 (2018) 3, 315–322
Figure 1: SEM images of coating powders: a) CoNiCrAlY,
b) ZrO
2
-%8Y
2
O
3
,c)ZrO
2
-MgO

tions, powder-particle sizes were observed. Figure 1
presents SEM images of coating powders. Figure 1a
shows the metallic bond-coat powder with a large major-
ity of spheroidal granules.
In Figure 1b, the YSZ powder is a mixture of
spheroidal granules and planar-angular particles when
compared with the CoNiCrAlY bond-coat powder
particles. The morphology of the MSZ powder particles
is given in Figure 1c. These particles have a more angu-
lar structure compared to the YSZ and bond-coat powder
particles.
2.4 Chemical composition of the substrate material
The chemical composition of the commercially avail-
able Inconel 718 superalloy is given in Table 1. As seen
from this table, the high corrosion resistance of this
material can be attributed to its high Ni and Cr contents.
Table 1: Chemical composition of Ni-based superalloy Inconel 718
Inconel 718 superalloy chemical composition, in mass
fraction (w/%)
Ni
53.55
Cr
18.0
Nb
5.31
Mo
3.03
Ti
0.96
Al
0.56
Co
0.27
Si
0.09
Cu
0.06
Mn
0.06
C
0.03
Ta
0.01
P
0.007
B
0.004
S
0.001
Fe
Balance
2.5 Porosity measurement of bond and top coats
The structural changes of the as-sprayed specimens
were examined after the porosity measurements. The
porosity measurements were conducted using the Image
J software package through the identification of the
differences in the matrix and porosity colors. Five
images of different regions of the coatings were taken at
a 1000× magnification and the porosity value for each
image was evaluated. The mean value of all the measure-
ments was included in the experimental data.
2.6 Measurements of the surface-roughness values of
the bond and top coatings
A SJ-310 Mitutoyo (Japan) test device was used for
the surface-roughness measurements in accordance with
DIN EN ISO 3274 standards. Each value was obtained
after taking measurements in five different regions. The
average value of these measurements was assumed as the
surface-roughness value.
2.7 Measurements of the hardness values of the sub-
strate material, bond and top coat
The measurements were conducted at five different
depths from the coating surface. The hardness values of
the coatings were measured using the Vickers inden-
tation test with a Duramin microhardness tester (Qness,
Q10A, Austria). The hardness measurements were taken
via an application of a 0.1 N load for a period of 15 s for
metallic bond coats. For the ceramic top coatings, a
0.25 N load was applied for a period of 5 s. Each test
was repeated five times and the mean values were taken
as the measurement results.
2.8 Oxidation tests of TBC samples
The produced TBC samples were cut into four pieces
with a cutting tool in a laboratory environment. These
specimens were subjected to oxidation tests in an open-
atmosphere furnace at (900 and 1000) °C for (8, 24, 50
and 100) h.
3 RESULTS AND DISCUSSION
3.1 Microstructural analysis of TBCs
3.1.1 Microstructural analysis of TBC samples before
the oxidation test
The average porosity percentages, surface roughness
and hardness values of the as-sprayed TBCs are given in
Table 2.
Table 2: Porosity, surface roughness and hardness of the coatings
before the oxidation test
TBCs
Porosity
(%)
Surface
roughness
(R
a
, μm)
Hardness
value (Hv)
YSZ 7.1±0.55 6.3±0.40 805±35
MSZ 5.1±1.30 5.7±0.35 607±30
According to Table 2, some physical properties of
TBCs can be compared with each other. The porosity
level of the MSZ top coat is lower than that of YSZ,
which is very important for the diffusion of oxygen over
the bond coat. The surface-roughness values of TBCs are
close to each other. The hardness of YSZ TBC is higher
than that of MSZ TBC although the latter exhibits a
lower porosity.
A SEM micrographs of the YSZ coating is given in
Figure 2. Structural differences between the bond coat
and YSZ can be observed. As seen in the figure, the
metallic coating has a dense and non-porous structure. A
SEM micrograph of the interface of the CoNiCrAlY
bond coat and MSZ top coat is given in Figure 3. Both
K. M. DOLEKER et al.: COMPARISON OF MICROSTRUCTURES AND OXIDATION BEHAVIORS OF Y2O3- ...
Materiali in tehnologije / Materials and technology 52 (2018) 3, 315–322 317
Figure 2: SEM image of the as-sprayed YSZ TBC

ceramic top-coating layers exhibit similar structures in
terms of porosity and homogeneity.
3.1.2 Microstructural analysis of TBC samples after
the oxidation test
3.1.2.1 Microstructures of TBCs after oxidation tests at
900 °C for (8, 24, 50 and 100) h
As seen in the SEM images from Figure 4, the TGO
thickness is significantly low for YSZ TBC after an 8-h
oxidation at 900 °C. At the same temperature, the
thickness of the TGO layer visibly increases with the
increasing time. Higher periods of the exposure to
temperature also increase the oxygen permeability of the
structure, thus leading to oxidation as a result of a high
oxygen content.
A uniform TGO growth occurred after the 8-h and
24-h oxidation periods, while an increase in the TGO
thickness was observed after the 50-h oxidation period.
This is related to the aluminum depletion in the structure
after 50 h. Following this depletion, a formation of
mixed oxides occurs. In Figures 4C and 4D, the blurred
gray areas in the upper regions of the TGO layer repre-
sent the mixed oxide formations. The elemental-analysis
micrograph of the TBC system with the YSZ top cera-
mic coating after the 100-h oxidation at 900 °C is given
in Figure 5. In this figure, the Al
2
O
3
formation is visible
in the region where the TGO layer was formed. Regard-
ing the elemental distribution, formations of mixed
oxides and spinels such as (Co, Ni)O, Cr
2
O
3
and (Co, Ni)
(Cr, Al)
2
O
4
are indicated by the overlapping elemental
colors. In the upper region of the TGO layer, dark-gray
regions represent the mixed oxides.
2500× SEM micrographs of MSZ TBC after (8, 24,
50 and 100) h of oxidation at 900 °C are given in Fig-
ure 6. As indicated by the images, the TGO formation
occurs in the form of a thin film layer after the 8-h
oxidation period. After 24 h, the formation of mixed
oxides is observed in the vicinity of the Al
2
O
3
layer, indi-
cated by a dark-gray color.
After 50-h and 100-h oxidation periods, an increased
TGO thickness is observed in the structure. MSZ TBC
exhibited a lower oxidation resistance than YSZ TBC.
K. M. DOLEKER et al.: COMPARISON OF MICROSTRUCTURES AND OXIDATION BEHAVIORS OF Y2O3- ...
318 Materiali in tehnologije / Materials and technology 52 (2018) 3, 315–322
Figure 5: Elemental-analysis images of YSZ TBC after a 100-h oxi-
dation at 900 °C
Figure 3: SEM image of the as-sprayed MSZ TBC
Figure 6: SEM images of MSZ TBC after the oxidation test at
900°C:a)8h,b)24h,c)50h,d)100h
Figure 4: SEM images of APS-CoNiCrA1Y bond and APS-YSZ
ceramic top coatings after the oxidation test at 900 °C: a) 8 h, b) 24 h,
c) 50 h, d) 100 h

Despite having a denser and more planar microstructure,
it became even denser after the 100-h oxidation period.
However, there was no significant difference between the
TGO thicknesses when compared with the YSZ after 50-
and 100-h oxidation periods. The elemental-analysis
micrograph of the MSZ TBC after the 100-h oxidation at
900 °C is given in Figure 7. In the elemental analysis, a
large mixed-oxide content is visible in the upper region
of the TGO structure, impairing the homogeneity of
TGO. Especially the presence of cobalt and chromium
with more dominant colors in the structure is an indi-
cation of the Cr
2
O
3
and (Co) Al
2
O
4
spinel formations.
The regions with aluminum and oxygen contents are
clearly visible in this figure.
3.1.2.2 Microstructures of TBC after oxidation tests at
1000 °C for (8, 24, 50 and 100) h
The SEM micrographs of YSZ TBC after (8, 24, 50
and 100) h oxidation periods at 1000 °C are given in Fig-
ure 8. As seen in the micrographs, after 24 h, mixed-
oxide formations are observed in the upper region of
TGO, indicated by a dark-gray color. An increase in the
mixed-oxide formations was observed in the TGO layer
after 50-h and 100-h oxidation periods. The TGO
thickness increased with the increasing oxidation time.
Elemental-analysis micrographs of YSZ TBC after
the 100-h oxidation period at 1000 °C is given in Fig-
ure 9. As seen in the elemental analysis, there is a very
low mixed-oxide formation and the dominant phase at
the interface is Al
2
O
3
. However, internally oxidized areas
were increased compared to the other oxidation periods.
The SEM micrographs of MSZ TBC after (8, 24, 50
and 100)-h oxidation periods at 1000 °C are given in
Figure 10. The YSZ TBC subjected to the 1000 °C
oxidation test was found to be more durable than the
MSZ TBC. The presence of porosity and gaps as well as
crack formations are visible in the microstructures of the
TBCs. In the SEM images, a TGO formation is observed
after the 8-h oxidation period. However, in this phase, a
higher mixed-oxide content is exhibited compared to
K. M. DOLEKER et al.: COMPARISON OF MICROSTRUCTURES AND OXIDATION BEHAVIORS OF Y2O3- ...
Materiali in tehnologije / Materials and technology 52 (2018) 3, 315–322 319
Figure 9: Elemental-analysis images of APS-YSZ ceramic-coating
top layer after the 100-h oxidation at 1000 °C
Figure 7: Elemental-analysis images of APS-MSZ ceramic top-coat-
ing layer after 100-h oxidation at 900 °C
Figure 10: SEM images of MSZ TBC after the oxidation test at
1000 °C: a) 8 h, b) 24 h, c) 50 h, d) 100 h
Figure 8: SEM images of YSZ TBC after the oxidation tests at
1000 °C: a) 8 h, b) 24 h, c) 50 h, d) 100 h

YSZ. After the 8-h and 24-h oxidation periods, a uni-
form TGO growth can also be seen, while an increase in
the TGO thickness is observed after 50 h. After the 50-h
oxidation period, the MSZ exhibits a higher mixed-oxide
content as compared to the YSZ top coat. A significant
increase in the TGO-layer thickness is observed after the
100-h oxidation period. After the 100-h oxidation period
at 1000 °C, YSZ TBC outperforms MSZ TBC in terms
of the TGO growth rate.
An elemental analysis of the TBC system with the
MSZ ceramic top coating after the 100-h oxidation at
1000 °C is given in Figure 11. Formations of alumina
and mixed-oxide or spinel phases can be inferred from
the overlapped colors. The homogeneity of the TGO
layer decreased as compared to the samples subjected to
the oxidation tests at 900 °C due to the increase in the
temperature and internal oxidation. When Figures 9 and
11 are observed, mixed-oxide or spinel-phase formations
are more visible according to the elemental distributions
for the TGO layers. Co, Ni and Cr traces can be observed
above the Al
2
O
3
layer.
3.1.2.3 TGO growth behaviors of TBCs after isother-
mal-oxidation tests at 900 and 1000 °C
The TGO-thickness measurements for the specimens
subjected to the oxidation tests were carried out using
the measurement function of the SEM device. SEM
microstructure images were taken at a 2500× magnifica-
tion and measurements were conducted along the TGO
layer at specified intervals.
In terms of TGO-thickness measurements, the speci-
mens with two different top coats exhibited almost
identical TGO-growth behaviors. The TGO-layer thick-
nesses for the specimens with YSZ and MSZ ceramic top
coating measured after (8, 24, 50 and 100) h oxidation
periods at 900 °C and 1000 °C are given in Figures 12
and 13.
As seen from the results, the TGO-thickness values
for both TBCs are close to each other at the initial stages
of oxidation. Afterwards, the TGO-thickness values for
both coatings increased with the increasing time.
3.2 XRD analyses of TBCs
An investigation of the oxide phases present in the
structures of both YSZ and MSZ coatings before and
after the oxidation for 100 hours at 900 °C and 1000 °C
was carried out with the XRD method.
The XRD results for the specimens with the YSZ and
MSZ contents were obtained, for both coatings, before
and after the oxidation tests (Figures 14 and 15). ZrO
2
was completely stabilized in the tetragonal phase, but it
included monoclinic and tetragonal phases after the oxi-
dation tests.
K. M. DOLEKER et al.: COMPARISON OF MICROSTRUCTURES AND OXIDATION BEHAVIORS OF Y2O3- ...
320 Materiali in tehnologije / Materials and technology 52 (2018) 3, 315–322
Figure 13: TGO-growth values for the TBCs with YSZ and MSZ
ceramic top coatings after (8, 24, 50 and 100) h oxidation periods at
1000 °C
Figure 11: Elemental-analysis images of APS-MSZ ceramic top-coat-
ing layer after the 100-h oxidation at 1000 °C
Figure 14: XRD results for YSZ top coating before and after the
oxidation test
Figure 12: TGO-growth values for the TBCs with YSZ and MSZ
ceramic top coatings after (8, 24, 50 and 100) h oxidation periods at
900 °C

For the MSZ TBC, during the 900- and 1000-°C oxi-
dation tests, a transition from the tetragonal to mono-
clinic phase occurred in the structure of the top coating,
which was totally stabilized in the tetragonal phase with
MgO before the oxidation. The overall results of the
XRD analysis clearly indicate that a higher rate of phase
transformation occurred on the MSZ top coating, com-
pared to YSZ TBC.
4 DISCUSSION
In TBC systems, oxidation occurs due to the pene-
tration of oxygen through the existing cracks and
porosities in the top coating. The ionic conductivity of
the top coat also allows progress of oxygen at high tem-
peratures. The oxygen penetrating through the top coat
reacts with the metallic bond coat and an oxidation
occurs.
19,20
After the oxidation of the bond coat, a TGO
layer is formed at the interface. The Y and Al elements
in the CoNiCrAlY bond coat oxidize first due to their
high affinity to oxygen. Since the Y content is very
small, only a very small amount of it is observed in the
TGO layer, while the Al
2
O
3
phase is predominant in the
first stages of oxidation. Depending on the time, the
existing Al-rich phases in the bond are consumed, and
fast-growing mixed oxides and spinels form. As a result,
the fast-growing oxides cause a high stress at the
interface and a spallation of the top coat occurs. The
TGO growth behaviors of the TBC systems with two
different ceramic top coatings generally result in similar
TGO-thickness values.
5,14,21–27
Another different characteristic of top coatings is
their high oxygen permeability. When Zr
4+
and Mg
2+
are
displaced, two oxygen vacancies occur in the stabiliza-
tion of MgO+ZrO
2
. The formation of these vacancies
accelerates the oxygen penetration from the top coat to
the bond coat at high temperatures.
28
However, the dis-
placement of Zr
4+
and Y
3+
ions causes a formation of
only one oxygen vacancy in YSZ TBC. Besides, YSZ
exhibits a more porous structure as compared to MSZ
TBC. The fact that YSZ is more porous causes more oxi-
dation compared to MSZ. However, due to having a
lower oxygen permeability as well as a higher phase
stability, YSZ TBCs exhibit better resistance against
high-temperature oxidation.
When the thermal-cycling behaviors of MSZ TBC at
1000 °C and 1050 °C were examined, rapid grain
coarsening was observed. The sintering temperature for
YSZ TBC was relatively higher than for MSZ. Khan et
al. investigated the oxidation behavior of MSZ TBCs
with a NiAl bond coat at different time periods at (750,
900 and 1000) °C. They found that MSZ TBCs are not
suitable for 1000 °C and that it is more convenient to use
them at temperatures of up to 750 °C. Their bond-coat
oxidation product mainly consists of nickel oxides.
16
In
the present study, MSZ TBC with the CoNiCrAlY bond
coat can be durable up to 900 °C while YSZ TBC still
preserves its stability at 1000 °C. The most dominant
phase is Al
2
O
3
in the TGO layer due to the use of the
CoNiCrAlY bond coat. Baig et al. studied the stress
distribution in plasma-sprayed MSZ TBCs. Two types of
the TBC system, a duplex one and a functionally graded
one, were produced and their stress distributions in a
range of 100–400 °C were investigate using finite-ele-
ment modelling.
17
The functionally graded MSZ TBCs
exhibit better strength, overcoming the thermal-expan-
sion mismatch.
17
There is no other study about the
oxidation of MSZ TBCs in the literature. However, there
are many studies about the oxidation of YSZ TBCs
under different temperatures and times.
25–27,30
5 CONCLUSIONS
The conducted investigation was focused on the
isothermal-oxidation behavior of the YSZ and MSZ
ceramic top-coating systems. The main conclusions are
listed below.
Porosity measurements of TBCs were conducted
before the other experimental studies on the coatings.
The obtained measurement results indicate that the YSZ
coating exhibits higher porosity compared to the coatings
with the MSZ content.
Based on the conducted analyses, the TGO growth of
the MSZ coating was found to be higher than that of the
YSZ ceramic top coating.
The TGO layer with the Al
2
O
3
content exhibited a
uniform structure at the initial stages of oxidation. With
the increasing oxidation period, depletion of Al in the
coating structure occurred at different temperatures. In
addition, formation of mixed oxides such as NiO, Cr
2
O
3
and CoO was observed due to the presence of Co, Ni and
Cr in the coating.
The phase analyses conducted on the ceramic top
coatings prior to the oxidation indicated that the YSZ
ceramic top coating included t-ZrO
2
and the MSZ
coating included the t-ZrO
2
, m-ZrO
2
and c-MgO phase
structures after 1000 °C and 100-h oxidation, which
means that the MSZ coating was negatively affected by
this oxidation condition. Because of the phase transfor-
K. M. DOLEKER et al.: COMPARISON OF MICROSTRUCTURES AND OXIDATION BEHAVIORS OF Y2O3- ...
Materiali in tehnologije / Materials and technology 52 (2018) 3, 315–322 321
Figure 15: XRD results for MSZ top coating before and after the
oxidation test

mation, volumetric changes in the coating structure
caused cracks.
For lower-temperature applications (<900 °C), the
use of MSZ TBC may be more suitable than YSZ TBC
due to its lower production cost.
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322 Materiali in tehnologije / Materials and technology 52 (2018) 3, 315–322