Thermal Properties and Durability of a Rare-Earth-Doped YSZ Bilayer Coating System
(GyeWon Lee)
12
(YongSeok Choi)
1
(HakBeom Jeon)
12
(JongIl Kim)
1
(ChangWoo Lee)
3
(JangHyeok Pyeon)
4
(SeungCheol Yang)
4
(InHwan Lee)
2
(YoonSuk Oh)
1*
Copyright © 2025 The Korean Institute of Metals and Materials
Key words(Korean)
Thermal barrier coating (TBC), Thermal conductivity, Bilayer coating, Thermal cycling durability
1. INTRODUCTION
Turbines generate energy from combustion gases generated in a combustion chamber.
The key components of gas turbines include compressors, combustors, and turbines.
To develop high-efficiency gas turbines, researchers have focused on increasing the
turbine inlet temperature (TIT), and recent trends require materials capable of stable
operation in extreme environments exceeding 1100 °C. For example, in such extreme
thermal environments, nickel-based superalloys are commonly used for turbine components.
To protect these high-temperature components, thermal barrier coating (TBC) technology
is essential [1-5].
TBCs can be classified according to the function of each layer. A typical TBC system
consists of a nickel-based superalloy substrate, a bond coat that enhances the oxidation
resistance and the adhesion between the substrate and the top coat, and a ceramic
top coat with low thermal conductivity and high chemical stability [1,2,4,6]. Yttria-stabilized zirconia (YSZ), widely used as a material for ceramic top coats,
is characterized by a high melting point, low thermal conductivity, excellent thermal
shock resistance, and a high thermal expansion coefficient, and accordingly is suitable
TBC applications.
YSZ is generally deposited onto nickel-based gas turbine component surfaces using
two key methods: atmospheric plasma spraying (APS) and electron beam physical vapor
deposition (EB-PVD). In APS, spherical ceramic materials are sprayed using a high-temperature
plasma flame under atmospheric conditions. TBC coating layers fabricated using APS
tend to exhibit lamellar structures with low thermal conductivities. In contrast,
TBC layers deposited via EB-PVD exhibit columnar microstructures with higher thermal
conductivities than APS-deposited coatings; however, EB-PVD coatings exhibit superior
mechanical properties, such as good strain tolerance [3]. When exposed to high-temperature environments (>1200 °C) for extended periods,
the YSZ present in these coatings undergoes a phase transformation that reduces the
thermal cycling durability. This issue arises from the phase transformation characteristics
of zirconia at different temperatures. For example, zirconia exists in the monoclinic
phase at room temperature, transforms into the tetragonal phase at ~1173 °C, and undergoes
a transformation into the cubic phase at ~2370 °C. The phase transformation between
the monoclinic and tetragonal phases induces a 3–5% volume change owing to crystal
structure modification. This change generates stress on the coating layer during gasturbine
operation and subsequent cooling cycles, potentially causing cracking and delamination
[7-11]. To address the issues raised by the phase transformation, extensive research has
been conducted to stabilize the zirconia phases by incorporating various rare earth
elements. On the basis of these studies, YSZ containing 6–8 wt% yttria has been commercialized
for use as a TBC material. Notably, 6–8 wt% YSZ exhibits optimal coating durability
through the formation of a non-transformable tetragonal (t') phase, which does not
transform into a monoclinic phase even under applied stress or upon cooling to room
temperature [12,13]. The t' phase forms when the YSZ powder momentarily melts during the APS coating
process and undergoes rapid quenching upon deposition on the substrate [13,14]. However, when the t' phase is exposed to high-temperature environments (i.e., >1200
°C) for extended periods, the tetragonal phase transforms into monoclinic and cubic
phases due to the diffusion of yttria [10,11].
To mitigate the detrimental tetragonal-to-monoclinic phase transformation of YSZ under
extremely high-temperature operating conditions, various investigations to incorporate
different rare earth elements into zirconia beyond conventional yttria doping as well
as substitute zirconia-based systems with alternative structures, such as perovskite-type
compounds or complex zirconates, are currently underway. As part of these efforts,
our group designed various compositions by co-doping YSZ with Sc2O3, Gd2O3, Dy2O3,
and TiO2. We specifically selected elements with atomic masses and ionic radii that
differ significantly from those of yttrium and zirconium to systematically investigate
their influence on the phase stability [7,9,14-28]. Using the designed compositions, thermal conductivity and phase stability analyses
were performed and the results confirmed that co-doping led to lower thermal conductivity
and superior inhibition of monoclinic phase formation compared to YSZ alone [8].
Building on these previous works, the current study was performed to investigate the
performance of TBC materials fabricated using the previously developed multi-oxide-doped
YSZ compositions containing Sc2O3, Gd2O3, Dy2O3, and TiO2. Before evaluating these
novel compositions, bilayer TBC structures consisting of highly durable conventional
YSZ as a base layer combined with gadolinium-ytterbium-co-doped YSZ (GdYb-YSZ) as
the top layer were characterized. Additionally, the thermal conductivities of these
structures were compared with that of a standard YSZ coating. Furthermore, the influence
of the coating-layer thickness ratio on the thermal conductivity and thermal cycling
durability of the coating was evaluated. Based on the obtained results, bilayer-structured
coating specimens with the optimal thickness ratio were prepared using the developed
compositions containing Sc2O3, Gd2O3, Dy2O3, and TiO2 (designated as ScGdTi and ScDyTi)
and their coating properties were analyzed. Through an evaluation of the properties
of the optimal bilayer-structured TBC, a TBC coating design methodology to enhance
both the thermal conductivity and the thermal cycling durability compared to that
of a conventional single-layer YSZ coating is proposed.
2. EXPERIMENTAL
The coating specimens were fabricated using commercial YSZ (8YSZ), GdYb-YSZ, and rare-earth-doped
YSZ compositions (ScGdTi and ScDyTi) with the addition of Sc2O3, Gd2O3, Dy2O3, and
TiO2, and the respective thermal conductivity and thermal cycling durability of the
resulting bilayer TBC materials were evaluated. More specifically, the changes in
the thermal conductivity and thermal cycling durability were comparatively analyzed
for various coating thickness ratios, and the structural characteristics and phase
transformation behaviors of the TBC layers were examined. The powder compositions
of the coatings are listed in Table 1. Commercial YSZ (8YSZ) and GdYb-YSZ were obtained from Oerlikon Metco (Switzerland),
whereas the rare-earth-doped YSZ compositions (ScGdTi and ScDyTi) were synthesized
by HanKyung TEC (Republic of Korea).
While YSZ maintains its tetragonal phase and exhibits excellent thermal cycling durability,
GdYb-YSZ is characterized by reduced thermal conductivity due to additional rare earth
element doping. ScGdTi and ScDyTi meanwhile are known to exhibit lower thermal conductivities
and superior phase stabilities than the conventional YSZ [8]. The properties of bilayer-structured TBCs fabricated using these materials were
investigated. Commercial YSZ and GdYb-YSZ powders were used to determine the optimal
bilayer thickness ratio (see Table 2) to maximize the thermal conductivity and durability of the bilayer-structured TBC.
The bilayer-coated structures were fabricated by combining a bottom layer (YSZ) and
a top layer (GdYb-YSZ), where the GdYb-YSZ layer was designed to withstand direct
exposure to a high-temperature environment. Additionally, ScGdTiand ScDyTi-based bilayer
coatings were fabricated at a thickness ratio of 3:1, which was determined to be the
optimal ratio for maximizing the coating performance. The coating properties of these
bilayers were comparatively analyzed against those of a single-layer YSZ coating.
TBC specimens were deposited using APS (KK-90, AMT AG, Switzerland) according to the
coating process parameters summarized in Table 3. YSZ and GdYb-YSZ were deposited on Ni-based superalloy substrates (CM939, 25.4 mm
diameter), where MCrAlY (Amdry 9951, Oerlikon Metco, Switzerland) was applied as a
bond coat (250 μm thickness) using a vacuum plasma spraying process. Following application
of the bond coat, the top coating layer was deposited, resulting in a thickness of
~550–600 μm. The obtained specimens were used to evaluate the thermal cycling durability
of the coating. Additionally, free-standing coating specimens for the thermal conductivity
analysis were prepared by APS deposition of the coating layers (1000 μm thickness)
on aluminum substrates without a bond coat, followed by mechanical separation of the
coating layer.
A cross-sectional analysis of the fabricated coating specimens was performed using
scanning electron microscopy (SEM, JSM-6390, JEOL, Japan) to confirm the deposition
thickness and characterize the morphological features of the coating microstructure.
X-ray diffraction (XRD, RINT-2500HF, Rigaku, Japan) was performed on the prepared
coating specimens to investigate their monoclinic- and cubic-phase formation behaviors.
To identify the formation of a monoclinic phase, the analysis was conducted in the
2θ range of 10–80° with parameters of 40 kV and 100 mA, along with a scan rate of
5°/min. To analyze the tetragonal and cubic phase formation behaviors of the materials,
scanning was performed in the 2θ range of 72–76° at a scan rate of 0.2°/min with parameters
of 40 kV and 100 mA.
To analyze the thermal conductivities of the TBC samples, free-standing samples machined
to a diameter of 12.7 mm and a thickness of 1 mm were used for the high-temperature
thermal conductivity measurements. These measurements were performed using laser flash
analysis (LFA; LFA 467 HT, NETZCH, Germany) at intervals of 100 °C in the temperature
range of 500–800 °C. Five measurements were conducted at each temperature and the
thermal diffusivity and specific heat were obtained for each temperature and sample.
The thermal conductivity of the coating layer (λ) was calculated using Equation (1), where α is the thermal diffusivity, Cp is the specific heat, and ρ is the density:
The densities of the specimens employed for the thermal conductivity measurements
were subsequently determined for the free-standing TBC coating specimens using the
Archimedes method.
To analyze the thermal cycling durability of the coating specimens, the JETS (Jet
Engine Thermal Shock) test was employed. The test procedure involved exposing the
top coat surface to a high-temperature and high-pressure flame at 1400 °C for 20 s,
followed by rapid cooling with N2 for 20 s to induce thermal stress. A cross-sectional
analysis of the coating layer was conducted after the JETS test to evaluate the occurrence
of delamination and crack formation. The failure criterion was set at 20% delamination
of the total surface area. The coating lifetime was expressed as a relative comparison
between the single-layer and bilayer-structured specimens, with the cycle at which
delamination occurred in the YSZ single-layer specimen serving as the reference standard.
After various bilayer-structured TBC coating specimens were prepared using the APS
process, their optimal thickness ratios and performances were analyzed through thermal
conductivity and thermal cycling durability evaluations.
3. RESULTS AND DISCUSSION
3.1 Analysis of the TBC Bilayer Structure Using Commercial Materials
A comparative analysis of the thermal conductivity and thermal cycling durability
of the single-layer and bilayer structures (i.e., YSZ and GdYb-YSZ) was conducted.
XRD analysis was conducted to investigate the phase formation behaviors of the YSZ
and GdYb-YSZ coating specimens deposited via the APS process, with the results presented
in Figure 1.
The XRD results of the YSZ coating presented in Figure 1(a) reveal that only the tetragonal phase—identified as the t' phase—was formed. Additionally,
the XRD results for the GdYb-YSZ coating shown in Figure 1(b) confirm the formation of a cubic phase without any tetragonal phase. This phase transformation
behavior was attributed to an increase in lattice distortion and oxygen vacancies
resulting from the increased concentration of trivalent rare earth elements added
to the zirconia, which promoted transformation from the tetragonal to the cubic phase
of zirconia [29,30]. Furthermore, the compositions containing additional rare earth elements, such as
in the case of GdYb-YSZ, exhibited lower thermal conductivities and reduced monoclinic
phase formation behaviors than conventional YSZ [20]. This phase transformation behavior and lower thermal conductivity characteristics
stem from the increased lattice distortion and oxygen vacancy formation caused by
the increased concentrations of rare earth elements. Generally, as the yttria content
increases and YSZ transitions to the cubic phase, both the thermal conductivity and
mechanical strength tend to decrease. Consequently, when GdYb-YSZ is fabricated as
a single-layer coating, the thermal conductivity is enhanced, while the thermal cycling
durability of the coating is expected to weaken. However, when fabricated as a bilayer
coating, obtaining both low thermal conductivity and low thermal cycling durability
is possible.
Furthermore, the compositions containing additional rare earth elements, such as in
the case of GdYb-YSZ, exhibited lower thermal conductivities and reduced monoclinic
phase formation behaviors relative to the conventional YSZ [20]. The phase transformation behavior and lower thermal conductivity characteristics
stem from increased lattice distortion and oxygen vacancy formation caused by the
increased concentrations of rare earth elements. Generally, as the yttria content
increases and YSZ transitions to the cubic phase, both the thermal conductivity and
mechanical strength tend to decrease. Consequently, when GdYb-YSZ is fabricated as
a single-layer coating, the thermal conductivity is enhanced whereas the thermal cycling
durability of the coating is expected to weaken. However, when fabricated as a bilayer
coating, both low thermal conductivity and low thermal cycling durability can be obtained.
The XRD results for the ScGdTi and ScDyTi coating specimens, presented in Figures 1(c) and 1(d), respectively, confirm that the monoclinic phase is not formed. Notably, the XRD
analysis conducted in the 72–76° range revealed a mixed tetragonal–cubic phase structure,
possibly resulting from partial transformation of the tetragonal phase to the cubic
phase upon adding Sc2O3, Gd2O3, Dy2O3, and TiO2, which led to lower thermal conductivity
and superior phase stability through lattice distortion. Considering the superior
mechanical properties of the tetragonal phase compared to those of the cubic phase,
the coexisting tetragonal and cubic phases were expected to contribute to superior
thermal cycling durability for the coating layer. Furthermore, increasing the content
of Sc2O3, Gd2O3, Dy2O3, and TiO2 would further reduce the thermal conductivity and
increase the high-temperature phase stability of the coating layer compared to YSZ
due to complete transformation of the zirconia phase to the cubic phase, as in the
case of GdYb-YSZ; however, this process would likely negatively impact the mechanical
properties of the coating.
A thermal conductivity analysis was thus performed using the GdYb-YSZ bilayer-coated
specimens and the effect of the bilayer thickness was analyzed. For these analyses,
the coating specimens were processed as free-standing samples with thicknesses of
~1 mm, wherein the coating layer was separately delaminated, as described previously.
To analyze the thermal conductivity behaviors of the coating specimens upon exposure
to high-temperature environments, each specimen was heated from 500 to 800 °C and
their thermal conductivities were measured, as presented in Figure 2.
The obtained results revealed that the single-layer YSZ coating specimen exhibited
a high-temperature thermal conductivity of ~1.0–1.1 W/(m·K), while the single-layer
GdYb-YSZ coating specimen demonstrated a value of ~0.8–0.9 W/(m·K). This difference
was attributed to the high contents of Gd2O3, Yb2O3, and Y2O3 incorporated into the
zirconia, which induced lattice distortion, generated oxygen vacancies, and reduced
the thermal conductivity of the material [20]. Upon applying YSZ and GdYb-YSZ coatings with various thickness ratios, thermal
conductivity measurements were conducted once again to evaluate the effect of the
thickness ratio. The specimen with a GdYb-YSZ/YSZ thickness ratio of 1:3 exhibited
the lowest thermal conductivity, whereas the specimen with a GdYb-YSZ/YSZ thickness
ratio of 1:1 exhibited the highest thermal conductivity. Notably, the 1:3 thickness
ratio led to a low thermal conductivity of ~0.7–0.8 W/(m·K), confirming that the bilayer
structure leads to lower thermal conductivity than that achieved using single-layer
coatings.
Subsequently, to evaluate the thermal cycling durability of the TBC coating layers
prepared using different GdYb-YSZ/YSZ thickness layers, the JETS test was conducted
using one-inch coupon specimens with a top coat thickness of ~550–600 μm and a bond
coat layer thickness of ~250 μm.
As presented in Figure 3, the results of the cross-sectional analysis performed after the JETS test revealed
that delamination occurred at the interface between the bond and top coats in most
specimens. However, in the specimen with a GdYb-YSZ/YSZ thickness ratio of 3:1, delamination
was not observed at the interface between YSZ and the bond coat but rather at the
interface between GdYb-YSZ and YSZ. This delamination behavior appears to be attributable
to the deposition of cubic phase GdYb-YSZ, along with increased residual stress in
the coating layer as the thickness increased. The residual stress in a deposited coating
layer increases with increasing coating thickness [31]. Thus, in specimens bearing thicker GdYb-YSZ coating layers, a combination of relatively
higher residual stress and reduced mechanical properties compared to those of YSZ
likely resulted in delamination at the interface owing to their inability to withstand
the stress generated during the JETS test. However, further research is required to
understand this phenomenon.
The results of the thermal cycling durability evaluations performed for the TBC coating
specimens using the JETS test are presented in Figure 4, wherein the relative durability of each coating specimen is compared to that of
the YSZ single-layer coating specimen as the reference standard. The JETS test of
the GdYb-YSZ single-layer coating specimen revealed a relative lifetime of ~29.28%
compared to that of YSZ, indicating reduced thermal cycling durability. In contrast,
the specimen with a GdYb-YSZ/YSZ thickness ratio of 1:1 demonstrated an average relative
lifetime of ~97.92%, which confirms its thermal cycling durability is comparable to
that of YSZ. Notably, the specimen with a GdYb-YSZ/YSZ thickness ratio of 1:3 exhibited
a relative lifetime of ~103.12%, whereas that with a GdYb-YSZ/YSZ thickness ratio
of 3:1 displayed a relative lifetime of ~107.28%. These results indicate that, although
the singlelayer GdYb-YSZ with a high rare earth element content exhibited reduced
thermal conductivity, its mechanical properties deteriorated owing to the transformation
of the zirconia phase from the mechanically superior tetragonal phase to the cubic
phase. Additionally, the resistance to stress arising from the thermal expansion coefficient
mismatch between GdYb-YSZ (9-10 × 10−6 K−1) and the bond coat layer (15.0 × 10−6 K−1)
[32] appeared to decrease compared to that of YSZ, resulting in a significantly shorter
coating layer lifetime compared to the single-layer YSZ.
These results demonstrate that fabricating a bilayer TBC using a combination of GdYb-YSZ
and YSZ leads to both reduced thermal conductivity and a longer coating layer lifetime.
Based on the thermal conductivity and JETS test results, these properties were optimized
using a GdYb-YSZ/YSZ thickness ratio of 3:1.
3.2 Analysis of the TBC Bilayer Structure Incorporating Doped Rare Earth Materials
Based on the thermal conductivity and thermal cycling durability evaluation results
of the GdYb-YSZ and YSZ coating specimens described above, bilayer structures were
fabricated using the developed compositions (ScGdTi and ScDyTi) at an optimal 3:1
thickness ratio. The thermal conductivity and thermal cycling durability of the resulting
coatings were then compared with those of the single-layer YSZ.
Initially, the high-temperature thermal conductivity values of the developed single
layer and bilayer compositions were evaluated and compared with those of YSZ. These
measurements were performed in the 500–800 °C range and the results are shown in Figure 5. In the case of the YSZ single-layer coating specimen, a thermal conductivity of
~1.1 W/(m·K) was obtained. Additionally, the ScGdTi single-layer coating specimen
demonstrated thermal conductivity values in the range of 0.83–0.92 W/(m·K), while
the ScDyTi composition gave corresponding values of 0.78–0.85 W/(m·K). Notably, both
compositions exhibited lower thermal conductivities than YSZ. This indicates that
enhanced low thermal conductivity can be achieved through lattice distortion by incorporating
trivalent and tetravalent oxides into YSZ.
Following the same methodology used for the GdYb-YSZ and YSZ bilayer-structured specimens,
ScGdTi and ScDyTi compositions were fabricated as bilayer-structured TBC coatings
with a 3:1 thickness ratio. A high-temperature thermal conductivity analysis of these
bilayer-coated specimens revealed that the ScGdTi bilayer-coated specimen exhibited
thermal conductivity values of 0.82–0.9 W/(m·K) while the ScDyTi bilayer-coated specimen
demonstrated thermal conductivity values in the range of 0.89–1.1 W/(m·K). These results
indicate a lower thermal conductivity compared to the single-layer YSZ coating specimen
and follow a similar trend to that observed for the previous GdYb-YSZ bilayer specimens.
Hence, fabricating bilayer TBCs using materials with low thermal conductivity leads
to lower thermal conductivity than that exhibited by YSZ.
Subsequently, to evaluate the thermal cycling durability of the bilayer-structured
TBCs, JETS test evaluations were performed on the ScGdTi and ScDyTi coating specimens.
Additionally, JETS tests were performed to evaluate the thermal cycling durability
of the single-layer TBC specimens, as shown in Figure 6. Compared to YSZ, the ScDyTi and ScGdTi coating samples exhibited thermal cycling
durability of 49.3% and 39.5%, respectively. A cross-sectional analysis was conducted
to examine the coating failure behaviors; as shown in Figure 7, failure occurred not within the top coat but in the form of delamination at the
interface between the bond coat and the top coat.
This is consistent with the previously observed behavior of the GdYb-YSZ single layer
and suggests that the thermal cycling durability weakened compared to that of the
YSZ single layer. The decreased thermal cycling durability is attributed to the structure
changing from a purely tetragonal phase to a mixed tetragonal and cubic phase structure
with additional incorporation of rare earth elements into YSZ.
To enhance the thermal cycling durability of the single-layer coatings, bilayer-coated
specimens were fabricated using a 3:1 thickness ratio.
As shown in Figure 8, the thermal cycling durability obtained for the various coatings confirmed that
the bilayer-structured coating specimens had superior durability compared to that
of YSZ. Among these, the ScDyTi and ScGdTi bilayer-coated specimens exhibited three
and two times higher thermal cycling durability than YSZ, respectively. A cross-sectional
analysis also was conducted to investigate the failure behaviors of these bilayer-coated
specimens and the results are presented in Figure 9. It was observed that delamination occurred at the interface between YSZ and the
bond coat, which was located between the developed composition and the bond coat.
These results differ from those of previously reported GdYb-YSZ bilayer-coated specimens.
The primary factor responsible for delamination at the top coat appears to be the
transformation of the zirconia phase from a purely tetragonal structure to a mixture
of cubic and tetragonal phases. This transformation is attributed to the addition
of trivalent and tetravalent oxides to YSZ, which lowers thermal conductivity and
enhances high-temperature phase stability through lattice distortion, as described
earlier. In other words, by using materials where tetragonal and cubic phases coexist
(c.f., a purely cubic phase) it is possible to achieve a level of thermal cycling
durability that protects the coating against the thermal stresses generated in high-temperature
combustion environments. Further research, however, is required to understand this
phenomenon.
4. CONCLUSION
This study analyzed the thermomechanical properties of our previously developed compositions
containing Sc2O3, Gd2O3, Dy2O3, and TiO2 (designated as ScGdTi and ScDyTi) for application
as thermal barrier coatings (TBCs). For this purpose, coating fabrication was performed
using the atmospheric plasma spraying technique and the thermal conductivity and thermal
cycling durability of the resulting coating layers were subsequently evaluated. Additionally,
bilayer structures were prepared using yttria-stabilized zirconia (YSZ) as the bottom
layer and Gd2O3-doped YSZ (GdYb-YSZ) as the top layer with various thickness ratios.
The effects of the thickness ratio on the thermal conductivity and thermal cycling
durability were evaluated. Although the GdYb-YSZ single-layer specimen exhibited lower
thermal conductivity than YSZ, it also displayed reduced thermal cycling durability.
However, the GdYb-YSZ bilayer specimens demonstrated lower thermal conductivity and
similar or higher thermal cycling durability than those exhibited by YSZ. Notably,
using an optimal GdYb-YSZ/YSZ thickness ratio of 3:1 minimized the thermal conductivity
and maximized the thermal cycling durability of the coating. Based on these results,
the developed compositions (i.e., ScGdTi and ScDyTi) were used to prepare coatings
with thickness ratios of 3:1, and their thermal conductivity and thermal cycling durability
were analyzed. The bilayer specimens exhibited lower thermal conductivity and superior
thermal cycling durability than YSZ. Overall, the results obtained from incorporating
the developed compositions into bilayer coating structures highlights their significant
potential for use as TBC materials in gas turbines.