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

Abstract

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. The chemical composition of NiCoCrAlY bond coat.

Element	Ni	Co	Cr	Al	Y
Content	Bal.	20–22	24	20	1.5

, and the spraying parameters selected from the pre-optimization procedures are listed in

Table 2. Air plasma spraying parameters for GdYb–YSZ TBCs.

Parameter	NiCoCrAlY	GYb–YSZ
Current (A)	530	600
Voltage (V)	50	56
Primary gas, Ar (L/min)	40	40
Secondary gas, H2 (L/min)	10	10
Feedstock giving rate (g/min)	30	30
Spray distance (mm)	100	100

.

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:

$$\frac{M_m}{M_{t’}}=0.82\frac{I_m^{}(\overline{1}11)+I_m(111)}{I_{t’}(111)}$$

(1)

$$M_m+M_{\mathrm{t}’}=1$$

(2)

In these equations, the variables used in the calculation are defined as follows: M_m and M_t′ 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. Chemical compositions of regions A–J in Figure 4, Figure 5, Figure 6 and Figure 7 (in at.%).

	Y	Yb	Gd	Zr	V	O
A	16.3	1.1	1.2	—	21.1	60.3
B	2.0	—	0.6	46.6	—	51.8
C	2.3	1.1	0.8	46.4	—	49.4
D	16.4	1.1	1.4	—	20.7	60.4
E	1.9	—	0.6	47.9	—	49.6
F	2.5	0.9	0.8	45.4	—	50.4
G	16.0	0.9	1.0	—	19.8	62.3
H	2.1	—	0.7	49.3	—	47.9
I	17.1	1.4	2.0	—	22.6	56.9
J	1.9	—	0.8	48.4	—	48.9

. 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:

$${\mathrm{V}}_2{\mathrm{O}}_5\left(\mathrm{l}\right)+\mathrm{GdYb}-\mathrm{YSZ}\left(\mathrm{s}\right)\to (\mathrm{Y}, \mathrm{Yb}, \mathrm{Gd})\mathrm{V}{\mathrm{O}}_4\left(\mathrm{s}\right)+\mathrm{m}-\mathrm{Zr}{\mathrm{O}}_2\left(\mathrm{s}\right)$$

(3)

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:

(1)

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.

(2)

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.

(3)

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.