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Hot-Corrosion Behavior of Gd2O3–Yb2O3 Co-Doped YSZ Thermal Barrier Coatings in the Presence of V2O5 Molten Salt

School of Mechanical & Electrical Engineering, Hunan Applied Technology University, Changde 415000, China
Author to whom correspondence should be addressed.
Coatings 2023, 13(5), 886;
Submission received: 22 March 2023 / Revised: 30 April 2023 / Accepted: 2 May 2023 / Published: 8 May 2023


In this study, thermal barrier coatings (TBC) consisting of 3.5 mol% Yb2O3-stabilized ZrO2 co-doped with 1 mol% Gd2O3 and 1 mol% Yb2O3 (referred to as GdYb-YSZ) were fabricated by means of air plasma spraying. The as-fabricated coatings exhibited a metastable tetragonal (t′) structure. The hot-corrosion behavior of the GdYb–YSZ TBCs was investigated at 700, 800, 900, and 1000 °C for 10 h in the presence of V2O5 molten salt. During the corrosion tests, the t′ phase transformed into a monoclinic (m) phase; nevertheless, it was still detected on the corroded surfaces. The amount of t′ phase decreased with increasing corrosion temperature. The corrosion products formed on the GdYb-YSZ TBCs in V2O5 comprised Yb, Gd-doped YVO4, and m-ZrO2, irrespective of the temperature of corrosion. However, higher temperatures changed the morphologies of the Yb- and Gd-doped YVO4 corrosion products. The GdYb–YSZ TBCs exhibited improved corrosion resistance to V2O5 molten salt when compared to YSZ TBCs, and the related mechanism is discussed in detail in this paper.

1. Introduction

Thermal barrier coatings (TBCs) are widely applied on components of advanced gas-turbine engines, which are subjected to high temperatures, to provide thermal protection, thereby improving the efficiency and performance of the engines [1,2,3,4,5]. A common thermal barrier coating (TBC) consists of a ceramic topcoat and a bond coat. The ceramic topcoat typically contains 7–8 wt% Y2O3-stabilized ZrO2 (YSZ), which possesses several desirable characteristics for TBC applications, including a high melting point, low thermal conductivity, thermal expansion coefficient similar to that of the substrate, and high toughness [1,6]. The most commonly used TBC fabrication technologies are air plasma spraying (APS), electron-beam physical vapor deposition, and plasma-spray physical vapor deposition [7,8,9,10,11,12]. The YSZ TBCs fabricated in their original state exhibit a desirable metastable tetragonal prime phase (t′), owing to their high toughness and suitability for TBC applications.
However, YSZ TBCs entail some limitations when used in gas-turbine engines [1,13,14,15]. At temperatures exceeding 1200 °C on the surface of TBCs, the t′ phase becomes unstable and breaks down into tetragonal (t) and cubic (c) phases. During cooling, the tetragonal phase undergoes a phase transformation to a monoclinic (m) phase. This is accompanied by a large volume expansion, which can crack and delaminate the coatings. Moreover, a high operation temperature accelerates the sintering of YSZ coatings, which is detrimental to the thermal insulation and thermal cycling behavior of TBCs. To achieve better thermal insulation, TBCs with low thermal conductivities are required. Several high-temperature ceramics, such as ZrO2 doped/co-doped with rare-earth oxides, rare-earth zirconates, and rare-earth phosphates, which exhibit lower thermal conductivities than YSZ [16,17,18,19,20,21], have been proposed as potential candidates for TBC materials.
YSZ TBCs undergo severe degradation when operated in corrosive environments [22,23,24,25,26,27,28]. Impurities, such as vanadium, condense on the coating surface and melt at temperatures exceeding 700 °C, followed by their penetration into the coating. The Y in YSZ reacts with molten salt, resulting in a depletion of the stabilizer in the coating. As a result, an undesired phase transformation from the t′ phase to the m phase occurs in the coating. Researchers have made significant efforts to improve the corrosion resistance of YSZ coatings against vanadium. Studies have shown that doping YSZ with MgO and Sc2 O3 can enhance hot-corrosion resistance [29,30,31,32]. Furthermore, it has been reported that titania- or ceria-stabilized ZrO2 exhibits better resistance to hot corrosion compared to YSZ [33,34]. Apart from YSZ, newly developed TBC candidates, such as rare-earth phosphates (REPO4), have demonstrated excellent resistance to hot corrosion. In the presence of a molten salt (V2O5 or V2O5+Na2SO4), a solid solution of RE(P,V)O4 can be formed, which is beneficial for maintaining the microstructural integrity of REPO4 TBCs and enhancing the hot-corrosion resistance of these coatings [35].
Doping YSZ with rare-earth oxides can also improve hot-corrosion resistance. Guo et al. reported that Gd2O3–Yb2O3-co-doped YSZ (GdYb–YSZ) ceramics exhibited excellent resistance to molten-salt corrosion [36]. GdYb–YSZ has several desirable properties, such as low thermal conductivity, good phase stability, and high thermal expansion coefficient, making it a promising candidate for TBC applications [37]. However, previous research on molten-salt corrosion had focused only on ceramic pellets; the behavior of GdYb–YSZ TBCs in the presence of molten salt has not been studied so far.
The present study involves air plasma spraying (APS) of GdYb-YSZ TBCs and an investigation into their hot-corrosion characteristics when exposed to molten V2O5 at temperatures ranging from 700 to 1000 °C. The focus is on identifying the resulting corrosion products and tracking their microstructural evolution as the corrosion temperature increases. Additionally, the mechanism behind the formation of these corrosion products is discussed.

2. Experimental Procedures

To produce GdYb-YSZ powder with a chemical composition of 1 mol% Gd2O3 and 1 mol% Yb2O3 co-doped with 3.5 mol% Y2O3-stabilized ZrO2, a chemical co-precipitation and calcination method was employed. The raw materials used were RE2O3 (RE = Gd2O3, Yb2 O3, and Y2O3, purity 99.99%) and ZrOCl2·8H2O (purity 99.95%). The RE2O3 powders were first calcined at 900 °C for 4 h to eliminate moisture and other volatile impurities, and then dissolved in an appropriate amount of nitric acid. ZrOCl2·8H2O was dissolved in deionized water. The two solutions were mixed and stirred to obtain a homogeneous solution, which was then slowly added to excess ammonia water (pH > 12) to yield a precipitate. The precipitate was filtered and washed several times with distilled water and alcohol until the pH reached 7. The resulting precipitate was dried at 120 °C for 20 h and calcined at 800 °C for 5 h to achieve crystallization.
The obtained GdYb–YSZ powder was not suitable for direct thermal spraying because of its poor fluidity and agglomerated into microscopic particles when subjected to spray drying. The agglomerated GdYb–YSZ particles were sprayed onto Ni-based superalloy substrates with NiCoCrAlY as the bond coat through APS (Praxair 7700), obtaining a coating with thickness of about 200 μm. The chemical composition of NiCoCrAlY is provided in Table 1, and the spraying parameters selected from the pre-optimization procedures are listed in Table 2.
Hot-corrosion tests were carried out by subjecting the salt-covered GdYb-YSZ coatings to V2O5 at a concentration of 10 mg/cm2, followed by heating them at temperatures of 700, 800, 900, and 1000 °C for a duration of 10 h. After the completion of heating, the furnace was cooled to room temperature. The phase structures of both the as-fabricated and corroded surfaces of the coatings were determined using X-ray diffraction (XRD) with a Rigaku Diffractometer (Tokyo, Japan), while microstructural and compositional analyses were conducted using a scanning electron microscope (SEM; TDCLS4800, Hitachi Ltd., Tokyo, Japan) equipped with energy-dispersive spectroscopy (EDS, IE 350).

3. Results and Discussion

Figure 1 shows the SEM image of the YSZ agglomerated particles after thermal spraying. The particles exhibit a spherical shape with sizes ranging from 30 to 50 μm. The XRD patterns of the as-sprayed and corroded GdYb–YSZ coatings are presented in Figure 2. The as-sprayed coating has a t′ phase structure, which is desirable for TBC applications owing to its high toughness. Following 10 h of hot corrosion at 700 °C, the XRD pattern of the sample still reveals the presence of the t′ phase peaks, albeit with reduced intensity and broadening. Notably, the appearance of the (111) and (−111) peaks corresponding to the m phase at approximately 2θ ≈ 28 and 31° indicates t′-phase decomposition during the corrosion process. Additionally, a few strong peaks resembling YVO4 (PDF#17-0341) with slight angle shifts are evident, implying the dissolution of other elements. At higher corrosion temperatures of 800, 900, and 1000 °C, the XRD patterns of all samples exhibit similar characteristics, with discernible peaks corresponding to the t′, m, and doped-YVO4 phases. Notably, the relative intensity of the m phase increases with higher temperatures, signifying an increase in the formation of the m phase. The XRD analysis indicates that corrosion temperature has a minimal effect on the type of corrosion products formed; however, it does affect their relative abundance. Even though our study used an excessive amount of molten salt, the presence of the t′ phase in the corroded coating surfaces suggests that the GdYb-YSZ coating may maintain its phase stability and demonstrate some resistance to hot corrosion.
Phase instability is one of the key factors causing the failure of ZrO2-based TBCs [33,38]. According to the XRD analysis, the GdYb–YSZ coatings exhibit phase decomposition during the corrosion tests. In order to assess the hot-corrosion resistance of the coatings, we calculated the amount of m phase present in the samples after hot corrosion, which allowed us to estimate the degree of t′-phase decomposition. The equations used for this calculation are as follows:
M m M t = 0.82 I m ( 1 ¯ 11 ) + I m ( 111 ) I t ( 111 )
M m + M t = 1
In these equations, the variables used in the calculation are defined as follows: Mm and Mt represent the mole fractions of the m and t′ phases, respectively, and I corresponds to the integral intensity of the diffraction peaks. The results of the calculations are presented in Figure 3, where it is evident that higher corrosion temperatures cause an increase in the m-phase content and a decrease in the t′-phase content. According to the analysis, as the hot-corrosion temperature increases from 700 to 1000 °C, the mole fraction of the m-phase content in the coating increases from 68.87 to 85.32 mol%.
After 10 h of exposure to hot corrosion at 700 °C, the GdYb-YSZ coating exhibits a typical surface morphology, as shown in Figure 4a. Compared to the as-sprayed coating, the corroded coating exhibits a distinct surface morphology. The surface of the coating appears to be entirely covered by the corrosion products. At a higher magnification using SEM (Figure 4b), it can be observed that the products possess bulk shape (A) and spherical shape (B). Table 2 lists the results of the EDS analysis used to determine the compositions of the products. Product A was found to contain Y, Gd, Yb, V, and O, while product B consists of Zr, O, and some rare-earth elements. Additional analysis reveals that A is composed of Yb- and Gd-doped YVO4, whereas B contains ZrO2 with some rare-earth elements. This is confirmed by the earlier XRD results, which identify the m-ZrO2 phase from the rare-earth content. Region C was also characterized using EDS and was found to contain more rare-earth elements than region B. Combined with the XRD results, it can be confirmed that region C corresponds to the sprayed coating, which has suffered little attack from the molten salt.
Figure 5 displays the SEM images of the GdYb-YSZ coating that has undergone hot corrosion for 10 h at 800 °C. The coating surface displays visible corrosion products. Figure 5b presents an enlarged image of the surface, where corrosion products with bulk and spherical shapes are clearly visible and labeled as D and E, respectively. The Yb- and Gd-doped YVO4 and m-ZrO2 compounds were identified through the EDS analysis and XRD findings, as shown in Table 3. As shown in Figure 5b, some as-sprayed coatings can be observed, as marked by F. This indicates that, under this corrosion condition, the coating surface is not completely corroded, suggesting some resistance to molten-salt corrosion.
The corrosion products form a continuous layer on the coating surfaces after 10 h of V2O5 corrosion at 900 and 1000 °C, as shown in Figure 6 and Figure 7. The bulk-shaped crystals (G) are larger and later become rod-shaped (I), as shown in Figure 7. The EDS and XRD results show that compounds G and I are both Yb- and Gd-doped YVO4. Meanwhile, spherical corrosion products are also observed in Figure 6 and Figure 7, which are confirmed as m-ZrO2 based on the above analysis. The study suggests that the type of corrosion products is not significantly affected by an increase in corrosion temperature; however, higher temperatures change the morphologies of the corrosion products of Yb- and Gd-doped YVO4.
Previous studies have shown that YSZ coatings have poor resistance to V2O5 corrosion due to the V2O3 stabilizer’s tendency to react with V2O5, leading to the depletion of Y and the decomposition of the desirable t′ phase. The formation of the m phase due to V2O5 attack could result in large stress and cracks in the coatings, which is detrimental for the thermal cycling performance of the coatings [22,24,25,39]. Despite being susceptible to V2O5 attack, the GdYb-YSZ coating exhibits a certain degree of hot-corrosion resistance, as confirmed by the presence of the t′ phase on the corroded surface even after 10 h of corrosion at 1000 °C. This suggests that the coating could retain its phase stability and may still display some resistance to hot corrosion.
The analysis conducted on the corrosion products of GdYb-YSZ coatings allowed us to describe the corrosion mechanism of the coating using the following equation:
V 2 O 5 ( l ) + GdYb - YSZ ( s ) ( Y ,   Yb ,   Gd ) V O 4 ( s ) + m - Zr O 2 ( s )
The reason behind the superior corrosion resistance of GdYb-YSZ coatings when compared to YSZ coatings can be explained by the Lewis acid–base mechanism. According to this mechanism, acidic V2O5 reacts more easily with oxides having a higher basicity [40]. GdYb–YSZ coatings contain more rare-earth elements than YSZ coatings. Moreover, Yb2O3 has a lower basicity than Y2O3 and Gd2O3 [41,42]. Thus, some rare-earth elements, such as Yb, can remain in the coating as stabilizers when coming in contact with molten salt. Therefore, when molten salt is present, GdYb–YSZ coatings have a higher possibility of maintaining the t′-phase stability than YSZ coatings. However, GdYb–YSZ coatings still have hot-corrosion issues that require special attention.

4. Conclusions

In this study, GdYb-YSZ TBCs fabricated using APS were examined for their hot-corrosion behavior when subjected to V2O5 molten salt at 700–1000 °C for 10 h, and the following conclusions could be drawn:
The GdYb-YSZ TBCs experienced some degree of attack by V2O5, but they showed better corrosion resistance compared to YSZ TBCs. This improved resistance can be attributed to the higher rare-earth content and lower basicity of Yb2O3.
The as-fabricated GdYb-YSZ TBCs contained t′ phase, and even after hot corrosion, the t′ phase was still detectable on the corroded surfaces, indicating that the coatings had a certain level of resistance to phase decomposition. As the temperature of corrosion increased, the quantity of t′ phase decreased, while the amount of m phase increased.
In addition to the m phase, Yb- and Gd-doped YVO4 were generated as the corrosion products of the GdYb-YSZ TBCs in V2O5. Higher temperatures had no effect on the type of corrosion products but changed the morphologies of Yb- and Gd-doped YVO4 crystals.

Author Contributions

Conceptualization and supervision, K.L.; validation and writing—original draft preparation, Y.L.; investigation and data curation, Y.S. All authors have read and agreed to the published version of the manuscript.


This study was sponsored by the Hunan Provincial Department of Education General Project, grant number 14C0766.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. SEM micrographs of GdYb–YSZ agglomerated particles.
Figure 1. SEM micrographs of GdYb–YSZ agglomerated particles.
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Figure 2. XRD patterns of the as-sprayed and corroded GdYb–YSZ coatings exposed to V2O5 molten salt at 700, 800, 900, and 1000 °C for 10 h. The standard PDF cards of m-ZrO2, t′-ZrO2, and YVO4 are also presented (bottom figure).
Figure 2. XRD patterns of the as-sprayed and corroded GdYb–YSZ coatings exposed to V2O5 molten salt at 700, 800, 900, and 1000 °C for 10 h. The standard PDF cards of m-ZrO2, t′-ZrO2, and YVO4 are also presented (bottom figure).
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Figure 3. Calculated concentrations of m and t′ phases in GdYb–YSZ coatings after hot corrosion at 700, 800, 900, and 1000 °C for 10 h.
Figure 3. Calculated concentrations of m and t′ phases in GdYb–YSZ coatings after hot corrosion at 700, 800, 900, and 1000 °C for 10 h.
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Figure 4. Surface morphology (a) and its enlarged image (b) of the GdYb–YSZ coating after hot corrosion in V2O5 molten salt at 700 °C for 10 h.
Figure 4. Surface morphology (a) and its enlarged image (b) of the GdYb–YSZ coating after hot corrosion in V2O5 molten salt at 700 °C for 10 h.
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Figure 5. Surface morphology (a) and its enlarged image (b) of the GdYb–YSZ coating after hot corrosion in V2O5 molten salt at 800 °C for 10 h.
Figure 5. Surface morphology (a) and its enlarged image (b) of the GdYb–YSZ coating after hot corrosion in V2O5 molten salt at 800 °C for 10 h.
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Figure 6. Surface morphology (a) and its enlarged image (b) of the GdYb–YSZ coating after hot corrosion in V2O5 molten salt at 900 °C for 10 h.
Figure 6. Surface morphology (a) and its enlarged image (b) of the GdYb–YSZ coating after hot corrosion in V2O5 molten salt at 900 °C for 10 h.
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Figure 7. Surface morphology (a) and its enlarged image (b) of the GdYb–YSZ coating after hot corrosion in V2O5 molten salt at 1000 °C for 10 h.
Figure 7. Surface morphology (a) and its enlarged image (b) of the GdYb–YSZ coating after hot corrosion in V2O5 molten salt at 1000 °C for 10 h.
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Table 1. The chemical composition of NiCoCrAlY bond coat.
Table 1. The chemical composition of NiCoCrAlY bond coat.
Table 2. Air plasma spraying parameters for GdYb–YSZ TBCs.
Table 2. Air plasma spraying parameters for GdYb–YSZ TBCs.
Current (A)530600
Voltage (V)5056
Primary gas, Ar (L/min)4040
Secondary gas, H2 (L/min)1010
Feedstock giving rate (g/min)3030
Spray distance (mm)100100
Table 3. Chemical compositions of regions A–J in Figure 4, Figure 5, Figure 6 and Figure 7 (in at.%).
Table 3. Chemical compositions of regions A–J in Figure 4, Figure 5, Figure 6 and Figure 7 (in at.%).
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Li, Y.; She, Y.; Liao, K. Hot-Corrosion Behavior of Gd2O3–Yb2O3 Co-Doped YSZ Thermal Barrier Coatings in the Presence of V2O5 Molten Salt. Coatings 2023, 13, 886.

AMA Style

Li Y, She Y, Liao K. Hot-Corrosion Behavior of Gd2O3–Yb2O3 Co-Doped YSZ Thermal Barrier Coatings in the Presence of V2O5 Molten Salt. Coatings. 2023; 13(5):886.

Chicago/Turabian Style

Li, Yang, Yajuan She, and Kai Liao. 2023. "Hot-Corrosion Behavior of Gd2O3–Yb2O3 Co-Doped YSZ Thermal Barrier Coatings in the Presence of V2O5 Molten Salt" Coatings 13, no. 5: 886.

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