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Hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 in V2O5 molten salt at 700–1000 °C
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Hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 in V2O5 molten salt at 700–1000 °C ⁎
Chenglong Zhanga,b, Mingzhu Lia,b, Yuchen Zhanga, Lei Guoa,b,c,d, , Junxiu Donga, Fuxing Yea,b,c, Linwei Lid, Vincent Jid a
School of Materials Science and Engineering, Tianjin University, China Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, China c Key Lab of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China d ICMMO/SP2M, UMR CNRS 8182, Université Paris-Sud, 91405 Orsay Cédex, France b
A R T I C L E I N F O
A BS T RAC T
Keywords: Thermal barrier coating (Gd0.9Sc0.1)2Zr2O7 V2O5 Hot corrosion
Hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 ceramic exposed to V2O5 molten salt at 700–1000 °C was investigated, providing better understanding of its corrosion resistance as a promising thermal barrier coating. Obvious corrosion reaction occurred between (Gd0.9Sc0.1)2Zr2O7 and V2O5 molten salt after 4 h heat treatment, corrosion products being temperature dependent. At 700 °C, large amount of Sc2O3 doped ZrV2O7 and GdVO4, together with a minor amount of Sc2O3-stabilized tetragonal ZrO2 (t-ZrO2), formed on the sample surfaces. With the increase of the test temperature, Sc2O3 doped ZrV2O7 turned to decompose, leading to the formation of more t-ZrO2. At 900 °C and 1000 °C, the corrosion products were composed of GdVO4 and t-ZrO2. The mechanism by which the corrosion reaction occurs is proposed based on phase diagrams and Lewis acid-base rule.
1. Introduction Thermal barrier coatings (TBCs) are extensively applied to hotsection metallic components in gas-turbine engines to provide insulation and corrosion protection, improving engine efficiency and performance [1,2]. A typical TBC system usually consists of a ceramic topcoat as thermal insulation layer and a metallic bond coat. The bond coat is designed to provide oxidation and corrosion resistance as well as to improve the bonding between the topcoat and the substrate [3,4]. The widely used top coat is made of 7 wt% Y2O3-stabilized metastable tetragonal ZrO2 (t′-YSZ). However, the accepted upper limit for YSZ TBC use is 1200 °C. Higher temperatures cause the destabilization of t′ phase into Y-lean and Y-rich phases. The former transforms to monoclinic (m) phase on cooling accompanied with a large volume increase [1,5–8]. When low grade fuels are used, hot corrosion becomes a life‐ limiting factor for TBCs application. The impurities in low quality fuels commonly contain vanadium and sodium. In a temperature range of 600–1050 °C, molten sodium salts of vanadium and sulfur condensed onto the TBCs are extremely corrosive [9–11]. They can leach out the stabilizer yttria in YSZ and destabilize the t′ structure, which causes the spallation of the coating much more quickly than if the molten salts are
⁎
absent. Extensive efforts have been conducted to enhance the hot corrosion resistance of YSZ against sulfate-vanadate molten salts. Research has indicated that titania stabilized zirconia coating reveals superior hot corrosion resistance compared with YSZ coating [12]. Omar et al. have enhanced the corrosion resistance of YSZ coating by doping MgO [13]. Loghman-Estarki and Liu et al. have found that the addition of Sc2O3 in YSZ can improve its hot corrosion resistance [14– 16]. It has been reported that nanostructured coatings exhibit better hot corrosion resistance than their conventional counterparts [15–17]. The power output and efficiency of gas engines scale with their maximum operation temperature. Hence, there is a great need to increase engine-operating temperature. However, YSZ TBCs are unlikely to meet the long-term requirements for advanced engines even when the molten salt is not a concern. Additionally, in anticipation of better thermal insulation, there is a practical requirement for TBCs with even lower thermal conductivity. The search is thus underway for TBC materials that have even better phase stability, higher sintering resistance and lower thermal conductivity. Several high-temperature ceramics with lower thermal conductivity (for example, Gd2Zr2O7, LaPO4, La2Ce2O7 and LaMgAl11O19) are being pursued, some of which have been proposed as novel TBC candidates [1,18–23]. Many reports indicate enormous threat imposed to high-temperature ceramics by
Corresponding author at: School of Materials Science and Engineering, Tianjin University, China. E-mail address: [email protected] (L. Guo).
http://dx.doi.org/10.1016/j.ceramint.2017.04.048 Received 22 March 2017; Received in revised form 5 April 2017; Accepted 8 April 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhang, C., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.04.048
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molten salts [11,12,23–26]. Thus, investigation on the hot corrosion behavior of new TBC materials in molten slats is essential for developing alternative TBCs that have the potential capability operating above 1200 °C. (Gd0.9Sc0.1)2Zr2O7 keeps phase stability up to 1600 °C, has thermal expansion coefficient comparable to YSZ and reveals much lower thermal conductivity than YSZ [27,28]. Specially, compared with Gd2Zr2O7, a widely investigated TBC material, its toughness is significantly improved [27,28]. Thus, (Gd0.9Sc0.1)2Zr2O7 has been considered as a promising TBC material. However, its phase and microstructure evolution upon high temperature exposure to molten vanadium oxide is limited in open literature. In this study, the hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 ceramic in V2O5 salt at 700 °C, 800 °C, 900 °C and 1000 °C is investigated. The emphasis of this work is placed on analyzing the corrosion products resulted from the reaction between (Gd0.9Sc0.1)2Zr2O7 and V2O5 salt by using dense pellets and identifying the corrosion mechanisms.
2. Experimental procedure (Gd0.9Sc0.1)2Zr2O7 powders were produced by a chemical co-precipitation and calcination method, using RE2O3 (RE=Gd and Sc, purity 99.99%) and ZrOCl2·8H2O (purity 99.95%) as raw materials. Before fabrication process, Gd2O3 and Sc2O3 powders were calcined at 900 °C for 4 h to remove moisture and other volatile impurities. Appropriate amounts of RE2O3 and ZrOCl2·8H2O were dissolved in nitric acid and deionized water, respectively. The obtained solutions were mixed and stirred to yield homogeneous solution. Then, the mixed solution was slowly added to excess ammonia water (pH > 12) to get precipitate, followed by filtering and washing with deionized water and alcohol several times until a pH 7 was reached. The resultant precipitate was dried at 120 °C for 10 h and then calcined at 900 °C for 5 h for crystallization. In order to obtain bulks for hot corrosion tests, the powders were cold pressed at ~250 MPa and then sintered at 1600 °C for 15 h. Theoretical density of (Gd0.9Sc0.1)2Zr2O7 was calculated by using its lattice parameter available in the literature [27], and the bulk density was measured by Archimedes method. The theoretical density and the measured bulk density are 6.62 g/cm3 and 5.08 g/cm3, respectively. Thus, the relative density of the sintered samples are ~76.7% and the porosity is ~23.3%, which is close to that of the coating produced by air plasma spraying (APS). Hot corrosion tests were conducted according to our previous study and other researchers’ experiments [12,14,16,17,26]. Prior to the tests, the sintered samples were ground by 800 grit sandpaper, followed by ultrasonic cleaning in ethanol and drying at 120 °C. Subsequently, V2O5 powders with average particle size of ~10 µm were uniformly spread on the sample surfaces by using a very fine glass rod. An analytical balance was used to determine the weight of the specimens before and after V2O5 coverage. Finally, 0.0178 ± 0.005 g of V2O5 powders were coated on the sample surfaces, and the salt concentration was calculated to be ~10 mg/cm2. Then, the specimens were isothermally heated at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h, followed by cooling down to room temperature with furnace. Phase constitution of the samples was characterized by X-ray diffraction (XRD; Rigaku Diffractometer, Tokyo, Japan). Raman spectrum was recorded by a microscopic confocal Raman spectrometer (RM2000; Renishaw, Gloucestershire, UK) using 532 nm excitation from an argon ion laser. The spectral resolution was ~1 cm−1, and the signal was collected at a rate of 600 cm−1/30 s. Microstructure and composition analysis were carried out using a scanning electron microscope (SEM; FEI, Eindhoven, Holland) equipped with energy dispersive spectroscopy (EDS, IE 350).
Fig. 1. XRD patterns of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion tests in V2O5 salt at 700–1000 °C for 4 h. The standard PDF cards of GdVO4, ZrV2O7 and t-ZrO2 are also presented.
3. Results and discussion XRD measurements were carried out on the corroded surfaces of (Gd0.9Sc0.1)2Zr2O7 samples, and the results are presented in Fig. 1. In the XRD pattern of the sample after 4 h hot corrosion at 700 °C, strong peaks ascribed to GdVO4 (PDF#17–0260) and close to ZrV2O7 (PDF#16-0422) can be clearly observed. The detection of weak (Gd0.9Sc0.1)2Zr2O7 peaks indicates that the reaction layer on the sample surface is quite thin, thus X ray can penetrate through it. It is worthwhile to note that there is a weak peak appearing at 2θ≈30.5°, suggesting the presence of another phase. However, only based on this single diffraction peak, the phase cannot be determined. In order to investigate the influence of temperature on the corrosion products, (Gd0.9Sc0.1)2Zr2O7 bulks were exposed to 800 °C, 900 °C and 1000 °C for 4 h in V2O5 salt, and their XRD patterns are also included in Fig. 1. GdVO4 (PDF#17-0260) and (Gd0.9Sc0.1)2Zr2O7 peaks are evident in all the patterns. Notice that the peak at 2θ ≈30.5° still exists, the intensity of which increases with the increase of the corrosion temperature. Meanwhile, another two peaks appear at 2θ ≈34° and 50°. Comparing with the standard PDF card of t-ZrO2 (PDF#42-1164) shown in Fig. 1, one could ascribe these unidentified peaks to t-ZrO2 phase. Based on the aforementioned XRD analysis, it is possible to find that the corrosion products of (Gd0.9Sc0.1)2Zr2O7 in V2O5 molten salt are temperature dependent. At 700 °C, they contain GdVO4 and a phase close to ZrV2O7, together with a minor amount of tZrO2 phase, while those at 800 °C, 900 °C and 1000 °C mainly consist of GdVO4 and t-ZrO2. Raman spectroscopy is an extremely valuable tool for charactering the crystal structure of compounds [29,30]. To confirm the above conclusions and further analyze the structure of the corrosion products, Raman spectrum measurements were conducted. As shown in Fig. 2a, Raman modes appearing at ~434 cm−1, ~474 cm−1, ~570 cm−1, ~690 cm−1, ~720 cm−1 and ~885 cm−1 could be ascribed to GdVO4. To confirm this argument and exclude the presence of ScVO4, we produced GdVO4 and ScVO4 bulks and measured their Raman spectra. 2
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the literature [31,32] and our study are similar including no extreme temperature and pressure, the influence resulted from temperature and pressure variation can be ruled out. Hence, some doped ZrV2O7 compounds is possible to form in this study, which has broader Raman band resulted from the structural disorder due to composition variation. Since Gd3+ reacts with V2O5 to form GdVO4 and no evidence is found for ScVO4, considering Sc3+ ion acting as the dopant in ZrV2O7 is reasonable. As can be seen in Fig. 2a, Sc2O3 doped ZrV2O7 and GdVO4 Raman bands are found in the (Gd0.9Sc0.1)2Zr2O7 sample corroded at 800 °C, while only GdVO4 Raman bands are detected in the samples after 900 °C and 1000 °C corrosion. By comparison, it is possible to find that GdVO4 Raman bands in the samples corroded at different temperatures have no obvious Raman shifts, indicating that no structural disorder or distortion is produced in the GdVO4 lattice. Thus, Sc3+ ions doping in GdVO4 could be precluded. With the increase of the corrosion temperature, Sc2O3 doped ZrV2O7 turns to decompose. Since the formed Sc3+ ions have no possibility to take part in the formation of GdVO4, they remain in ZrO2 acting as a dopant. This leads to the formation of t-ZrO2 phase, which is detected by XRD. However, the Raman bands of t-ZrO2 phase are not obvious in Fig. 2a. It has been reported that t-ZrO2 has a Raman band at ~470 cm−1. In combination with the XRD results and the above analysis, the Raman mode at ~474 cm−1 in Fig. 2a could be assigned to both t-ZrO2 and GdVO4 phases. Fig. 3a shows a typical surface image of (Gd0.9Sc0.1)2Zr2O7 ceramic after hot corrosion at 700 °C for 4 h. Obvious new crystals are observed on the sample surface, which exhibit two different morphologies, i.e.
Fig. 2. Raman spectra of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion tests in V2O5 salt at 700–1000 °C for 4 h (a). Fig. 2b shows the Raman spectra of the fabricated GdVO4 and ScVO4 bulks.
As shown in Fig. 2b, the characteristic Raman modes for GdVO4 are at ~434 cm−1, ~474 cm−1, ~690 cm−1, ~720 cm−1 and ~885 cm−1, while those for ScVO4 are at ~355 cm−1, ~818 cm−1, ~857 cm−1 and ~914 cm−1. This provide strong indication for the formation of GdVO4 rather than ScVO4 after molten salt corrosion. The results suggest that in (Gd0.9Sc0.1)2Zr2O7, Sc3+ ions have smaller tendency to react with V2O5 than Gd3+. Thus, it is necessary to determine the role of Sc3+ ions during corrosion reaction. Besides GdVO4 Raman modes, it is possible to find some modes at ~405 cm−1, ~510 cm−1, ~775 cm−1 and ~987 cm−1 in the Raman spectrum of the (Gd0.9Sc0.1)2Zr2O7 sample corroded at 700 °C. These Raman modes are close to those of ZrV2O7 in the literature [31,32], but exhibiting much broader appearance. For ZrV2O7 compound, the Raman modes at ~987 cm−1 and ~775 cm−1 are assigned to the symmetric stretching and the asymmetric stretching of VO4 tetrahedra, respectively. The modes at ~510 cm−1 and ~474 cm−1 are due to the ZrO4 octahedral stretching and the VO4 asymmetric bending, respectively, and that at ~405 cm−1 is assigned to the symmetric bending of VO4 tetrahedra [29,33]. Notice that compared with the Raman spectrum of ZrV2O7 in the literature [31,32], some Raman modes are absence in this study. It has been reported that structural disorder can broader the Raman bands as well as preclude some Raman modes. The ordering degree of ZrV2O7 structure depends on temperature, pressure and composition [34,35]. Since the sample fabrication conditions in
Fig. 3. Surface morphologies of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion in V2O5 salt at 700 °C for 4 h. Fig. 3b shows an enlarge image.
3
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Table 1 Chemical compositions of different regions in Figs. 3–6 (in at%).
A B C D E F H G
Gd
Sc
Zr
V
O
16.3 – 18.1 – 18.3 – 18.5 –
– 1.4 – 2.6 – 2.9 – 2.5
– 15.4 – 28.3 – 31.7 – 27.9
18.1 31.1 18.8 – 17.8 – 17.3 –
65.6 52.1 63.1 69.1 63.9 65.4 64.2 69.6
Fig. 5. Surface morphologies of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion in V2O5 salt at 900 °C for 4 h. Fig. 5b shows an enlarge image.
Fig. 5 shows the SEM images of (Gd0.9Sc0.1)2Zr2O7 ceramic after hot corrosion at 900 °C for 4 h. Corrosion products with two different morphologies can be clearly observed. From the EDS results in Table 1 obtained from regions E and F in Fig. 5b, it could be confirmed the formation of GdVO4 (region E) and t-ZrO2 (region F), which supports the above XRD and Raman results. The representative surface morphologies of the sample corroded at 1000 °C for 4 h are shown in Fig. 6. It is clear that there are two different regions in the images. EDS results obtained from regions G and H in Fig. 6b are listed in Table 1. The elements identified agree great well with the presence of reaction products GdVO4 (region H) and t-ZrO2 (region G). This is consistent with the above XRD and Raman results. Based on the aforementioned observation and analysis, it is possible to find that hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 in V2O5 molten salt is temperature dependent. At 700 °C, the corrosion products consist of large amounts of Sc2O3 doped ZrV2O7 and GdVO4, together with a minor amount of t-ZrO2. With the increase of the corrosion temperature, Sc2O3 doped ZrV2O7 turns to decompose, causing the formation of more t-ZrO2. At 900 °C and 1000 °C, the corrosion products are GdVO4 and t-ZrO2. The related mechanisms by which the corrosion reaction occurs could be discussed based on phase diagram and Lewis acid-base rule. Since V2O5-(Gd0.9Sc0.1)2Zr2O7 binary phase diagram is not available in open literature and (Gd0.9Sc0.1)2Zr2O7 could be viewed as a compound that is composed of Gd2O3, Sc2O3 and ZrO2 at an appropriate ratio. V2O5-RE2O3 (RE=Gd and Sc) and V2O5-ZrO2 phase diagrams could be used to analyze the corrosion reaction between (Gd0.9Sc0.1)2Zr2O7 and V2O5
Fig. 4. Surface morphologies of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion in V2O5 salt at 800 °C for 4 h. Fig. 4b shows an enlarge image.
cubic-shaped and particle-shaped. In an enlarged image shown in Fig. 3b, the two types of the crystals are marked as A and B. EDS results in Table 1 indicate that compound A is composed of Gd, V and O, while compound B contains Zr, V, Sc and O. In combination with the above XRD and Raman results, further analysis confirms that A and B are GdVO4 and Sc2O3 doped ZrV2O7, respectively. A SEM micrograph obtained from the corroded surface of (Gd0.9Sc0.1)2Zr2O7 ceramic exposed to 800 °C for 4 h is shown in Fig. 4a. The crystals with two different shapes are denoted as C and D in Fig. 4b. EDS analysis reveals that crystal C consists of Gd, V and O, while crystal D is composed of Zr, Sc and O, as presented in Table 1. Further analysis demonstrates that C and D are GdVO4 and t-ZrO2, respectively. It is possible to find that in Figs. 3 and 4, GdVO4 crystals have quite similar morphology, while Sc2O3 doped ZrV2O7 and t-ZrO2 exhibit different morphologies and sizes. 4
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Fig. 7. Gibbs free energy of the reaction between oxides (Gd2O3, Sc2O3, ZrO2) and V2O5.
700 °C. Hence, the reaction mechanism between (Gd0.9Sc0.1)2Zr2O7 and V2O5 molten salt at 700 °C could be expressed as follow:
(Gd 0.9Sc0.1)2 Zr2O7(s) + V2O5(l ) → GdVO4(s ) + ZrV2O7(Sc2O3)(s )
According to V2O5-ZrV2O7 phase diagram [40], ZrV2O7 dissolves in V2O5-rich liquid above 747 °C, and the dissolution process is affected by local composition. Considering the fact that Gd2O3 have stronger basicity than ZrO2, it reacts more readily with V2O5, reducing the V2O5 content in V2O5-rich liquid. As a result, V2O5-ZrV2O7 system loses its equilibrium and more ZrV2O7 is dissolved. After the complete consumption of V2O5 by Gd2O3, pure ZrO2 is precipitated. In this study, there exists Sc2O3 in ZrV2O7 solid solution as a dopant. When all V2O5 salts are consumed by Gd2O3, the precipitate is ZrO2 together with Sc2O3 rather than pure ZrO2. Based on the aforementioned analysis, the reaction mechanism between (Gd0.9Sc0.1)2Zr2O7 and V2O5 at 800 °C, 900 °C and 1000 °C could be represented by the following expression:
Fig. 6. Surface morphologies of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion in V2O5 salt at 1000 °C for 4 h. Fig. 6b shows an enlarge image.
(Gd 0.9Sc0.1)2Zr2O7(s) + V2O5(l ) → GdVO4(s ) + t −ZrO2(Sc2O3)(s ) [36]. According to V2O5-RE2O3 binary phase diagram [37–39], V2O5 reacts with RE2O3 to form REVO4, which is stable to 1200 °C. The reaction equation could be given as follow:
V2O5 + RE2O3 → 2REVO4
(4)
In V2O5 molten salt, the major corrosion product of (Gd0.9Sc0.1)2Zr2O7 is GdVO4, which is like the case of Gd2Zr2O7. For Gd2Zr2O7 case, the other corrosion product is m-ZrO2, while it is tZrO2 for (Gd0.9Sc0.1)2Zr2O7 in this study [42,43]. It is known that mZrO2 undergoes phase transformation during thermal cycling, accompanied with volume change resulting in cracks in the coatings. While the t phase stabilized by Sc2O3 has been reported to have excellent resistance to molten salt corrosion, as well as good phase stability, low thermal conductivity and high toughness [14–16,44]. Therefore, (Gd0.9Sc0.1)2Zr2O7 might perform better in V2O5 molten salt than Gd2Zr2O7 by considering the corrosion products.
(1)
In V2O5-ZrO2 system [40], there exists an intermediate compound of ZrV2O7 below 747 °C, the formation of which could be expressed as follow:
V2O5 + Z rO2 → ZrV2O7
(3)
(2)
According to the phase diagram, ZrV2O7 melts incongruently to produce a mixture of ZrO2 and V2O5-rich liquid when the temperature is above 747 °C. It is well known that the reaction between V2O5 and ceramic oxides follows the Lewis acid-base rule, where the acid V2O5 is more prone to react with ceramic oxides that have stronger basicity [14,23–26]. According to the literature [41], Gd2O3 has stronger basicity than ZrO2 and Sc2O3. Thus, it has larger tendency to react with molten V2O5. As a result, GdVO4 is the major reaction product between (Gd0.9Sc0.1)2Zr2O7 and V2O5 molten salt. During hot corrosion tests, V2O5 could also react with ZrO2 or Sc2O3. Since ZrO2 is more basic than Sc2O3, it prefers to react with V2O5 to form ZrV2O7, leaving Sc2O3 as a dopant in the solid solution. To confirm the above argument, the reaction Gibbs free energy of Eqs. (1) and (2) is calculated and the results are shown in Fig. 7. It is evident that Gd2O3 can readily react with V2O5 to form GdVO4, while Sc2O3 has the smallest tendency to react with V2O5. According to the phase diagram, ZrV2O7 is stable at
4. Conclusions Hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 ceramic in V2O5 molten salt at 700 °C, 800 °C, 900 °C and 1000 °C was investigated. During thermal exposure for 4 h, obvious corrosion reaction between (Gd0.9Sc0.1)2Zr2O7 and V2O5 salt occurred, corrosion products being temperature dependent. At 700 °C, molten salt corrosion led to the formation of Sc2O3 doped ZrV2O7, GdVO4 and a minor amount of Sc2O3-stabilized tetragonal ZrO2 (t-ZrO2). With the increase of the corrosion temperature, Sc2O3 doped ZrV2O7 decomposed causing more t-ZrO2. At 900 °C and 1000 °C, the corrosion products were composed of GdVO4 and t-ZrO2. The corrosion reaction mechanism was discussed according to phase diagrams and Lewis acid-base rule. (Gd0.9Sc0.1)2Zr2O7 might perform better in V2O5 molten salt than Gd2Zr2O7 by considering the corrosion products. 5
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Acknowledgments This research is sponsored by the National Natural Science Foundation of China (Grant Nos. 51501127 and 51375332) and the Natural Science Foundation of Tianjin (No. 16JCQNJC02900 and 16JCYBJC18700). References [1] R. Vassen, M.O. Jarligo, T. Steinke, D.E. Mack, D. Stöver, Overview on advanced thermal barrier coatings, Surf. Coat. Technol. 205 (2010) 938–942. [2] H.B. Guo, S.K. Gong, C.G. Zhou, H.B. Xu, Investigation on hot-fatigue behaviors of gradient thermal barrier coatings by EB-PVD, Surf. Coat. Technol. 148 (2001) 110–116. [3] M. Qiao, C.G. Zhou, Hot corrosion behavior of Co modified NiAl coating on nickel base superalloys, Corros. Sci. 63 (2012) 239–245. [4] F. Jia, H. Peng, L. Zheng, H.B. Guo, S.K. Gong, H.B. Xu, Effect of different B contents on the mechanical properties and cyclic oxidation behavior of β-NiAlDy coatings, J. Alloy. Compd. 623 (2015) 83–88. [5] N.P. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gas-turbine engine applications, Science 296 (2002) 280–284. [6] L. Wang, Y. Wang, X.G. Sun, J.Q. He, Z.Y. Pan, C.H. Wan, Thermal shock behavior of 8YSZ and double-ceramic-layer La2Zr2O7/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying, Ceram. Int. 38 (2012) 3595–3606. [7] J. Wu, H.B. Guo, Y.Z. Gao, S.K. Gong, Microstructure and thermo-physical properties of yttria stabilized zirconia coatings with CMAS deposits, J. Eur. Ceram. Soc. 31 (2011) 1881–1888. [8] Y.X. Wang, C.G. Zhou, Microstructure and thermal properties of nanostructured gadolinia doped yttria-stabilized zirconia thermal barrier coatings produced by air plasma spraying, Ceram. Int. 42 (2016) 13047–13052. [9] L. Wang, D.C. Li, J.S. Yang, F. Shao, X.H. Zhong, H.Y. Zhao, K. Yang, S.Y. Tao, Y. Wang, Modeling of thermal properties and failure of thermal barrier coatings with the use of finite element methods: a review, J. Eur. Ceram. Soc. 36 (2016) 1313–1331. [10] H. Huang, C. Liu, L.Y. Ni, C.G. Zhou, Evaluation of microstructural evolution of thermal barrier coatings exposed to Na2SO4 using impedance spectroscopy, Corros. Sci. 53 (2011) 1369–1374. [11] M.H. Habibi, L. Wang, J.D. Liang, S.M. Guo, An investigation on hot corrosion behavior of YSZ-Ta2O5 in Na2SO4+V2O5 salt at 1100 °C, Corros. Sci. 75 (2013) 409–414. [12] M.H. Habibi, S.M. Guo, The hot corrosion behavior of plasma sprayed zirconia coatings stabilized with yttria, ceria, and titania in sodium sulfate and vanadium oxide, Mater. Corros. 66 (2015) 270–277. [13] P. Bajpai, A. Das, P. Bhattacharya, S. Madayi, K. Kulkarni, S. Omar, Hot corrosion of stabilized zirconia thermal barrier coatings and the role of Mg inhibitor, J. Am. Ceram. Soc. 98 (2015) 2655–2661. [14] H.F. Liu, X. Xiong, X.B. Li, Y.L. Wang, Hot corrosion behavior of Sc2O3-Y2O3-ZrO2 thermal barrier coatings in presence of Na2SO4+V2O5 molten salt, Corros. Sci. 85 (2014) 87–93. [15] M.R. Loghman-Estarki, M. Nejati, H. Edris, R.S. Razavi, H. Jamali, A.H. Pakseresht, Evaluation of hot corrosion behavior of plasma sprayed scandia and yttria co-stabilized nanostructured thermal barrier coatings in the presence of molten sulfate and vanadate salt, J. Eur. Ceram. Soc. 35 (2015) 693–702. [16] M.R. Loghman-Estarki, R.S. Razavi, H. Edris, S.R. Bakhshi, M. Nejati, H. Jamali, Comparison of hot corrosion behavior of nanostructured ScYSZ and YSZ thermal barrier coatings, Ceram. Int. 42 (2016) 7432–7439. [17] L. Guo, M.Z. Li, S.X. He, C.L. Zhang, Q. Wang, F.X. Ye, Preparation and hot corrosion behavior of plasma sprayed nanostructured Gd2Zr2O7-LaPO4 thermal barrier coatings, J. Alloy. Compd. 698 (2017) 13–19. [18] M.Z. Li, L. Guo, F.X. Ye, Phase structure and thermal conductivities of Er2O3 stabilized ZrO2 toughened Gd2Zr2O7 ceramics for thermal barrier coatings, Ceram. Int. 42 (2016) 16584–16588. [19] Z.G. Liu, W.H. Zhang, J.H. Ouyang, Y. Zhou, Novel thermal barrier coatings based on rare-earth zirconates/YSZ double-ceramic-layer system deposited by plasma spraying, J. Alloy. Compd. 647 (2015) 438–444.
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Hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 in V2O5 molten salt at 700–1000 °C ⁎
Chenglong Zhanga,b, Mingzhu Lia,b, Yuchen Zhanga, Lei Guoa,b,c,d, , Junxiu Donga, Fuxing Yea,b,c, Linwei Lid, Vincent Jid a
School of Materials Science and Engineering, Tianjin University, China Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, China c Key Lab of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China d ICMMO/SP2M, UMR CNRS 8182, Université Paris-Sud, 91405 Orsay Cédex, France b
A R T I C L E I N F O
A BS T RAC T
Keywords: Thermal barrier coating (Gd0.9Sc0.1)2Zr2O7 V2O5 Hot corrosion
Hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 ceramic exposed to V2O5 molten salt at 700–1000 °C was investigated, providing better understanding of its corrosion resistance as a promising thermal barrier coating. Obvious corrosion reaction occurred between (Gd0.9Sc0.1)2Zr2O7 and V2O5 molten salt after 4 h heat treatment, corrosion products being temperature dependent. At 700 °C, large amount of Sc2O3 doped ZrV2O7 and GdVO4, together with a minor amount of Sc2O3-stabilized tetragonal ZrO2 (t-ZrO2), formed on the sample surfaces. With the increase of the test temperature, Sc2O3 doped ZrV2O7 turned to decompose, leading to the formation of more t-ZrO2. At 900 °C and 1000 °C, the corrosion products were composed of GdVO4 and t-ZrO2. The mechanism by which the corrosion reaction occurs is proposed based on phase diagrams and Lewis acid-base rule.
1. Introduction Thermal barrier coatings (TBCs) are extensively applied to hotsection metallic components in gas-turbine engines to provide insulation and corrosion protection, improving engine efficiency and performance [1,2]. A typical TBC system usually consists of a ceramic topcoat as thermal insulation layer and a metallic bond coat. The bond coat is designed to provide oxidation and corrosion resistance as well as to improve the bonding between the topcoat and the substrate [3,4]. The widely used top coat is made of 7 wt% Y2O3-stabilized metastable tetragonal ZrO2 (t′-YSZ). However, the accepted upper limit for YSZ TBC use is 1200 °C. Higher temperatures cause the destabilization of t′ phase into Y-lean and Y-rich phases. The former transforms to monoclinic (m) phase on cooling accompanied with a large volume increase [1,5–8]. When low grade fuels are used, hot corrosion becomes a life‐ limiting factor for TBCs application. The impurities in low quality fuels commonly contain vanadium and sodium. In a temperature range of 600–1050 °C, molten sodium salts of vanadium and sulfur condensed onto the TBCs are extremely corrosive [9–11]. They can leach out the stabilizer yttria in YSZ and destabilize the t′ structure, which causes the spallation of the coating much more quickly than if the molten salts are
⁎
absent. Extensive efforts have been conducted to enhance the hot corrosion resistance of YSZ against sulfate-vanadate molten salts. Research has indicated that titania stabilized zirconia coating reveals superior hot corrosion resistance compared with YSZ coating [12]. Omar et al. have enhanced the corrosion resistance of YSZ coating by doping MgO [13]. Loghman-Estarki and Liu et al. have found that the addition of Sc2O3 in YSZ can improve its hot corrosion resistance [14– 16]. It has been reported that nanostructured coatings exhibit better hot corrosion resistance than their conventional counterparts [15–17]. The power output and efficiency of gas engines scale with their maximum operation temperature. Hence, there is a great need to increase engine-operating temperature. However, YSZ TBCs are unlikely to meet the long-term requirements for advanced engines even when the molten salt is not a concern. Additionally, in anticipation of better thermal insulation, there is a practical requirement for TBCs with even lower thermal conductivity. The search is thus underway for TBC materials that have even better phase stability, higher sintering resistance and lower thermal conductivity. Several high-temperature ceramics with lower thermal conductivity (for example, Gd2Zr2O7, LaPO4, La2Ce2O7 and LaMgAl11O19) are being pursued, some of which have been proposed as novel TBC candidates [1,18–23]. Many reports indicate enormous threat imposed to high-temperature ceramics by
Corresponding author at: School of Materials Science and Engineering, Tianjin University, China. E-mail address: [email protected] (L. Guo).
http://dx.doi.org/10.1016/j.ceramint.2017.04.048 Received 22 March 2017; Received in revised form 5 April 2017; Accepted 8 April 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhang, C., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.04.048
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molten salts [11,12,23–26]. Thus, investigation on the hot corrosion behavior of new TBC materials in molten slats is essential for developing alternative TBCs that have the potential capability operating above 1200 °C. (Gd0.9Sc0.1)2Zr2O7 keeps phase stability up to 1600 °C, has thermal expansion coefficient comparable to YSZ and reveals much lower thermal conductivity than YSZ [27,28]. Specially, compared with Gd2Zr2O7, a widely investigated TBC material, its toughness is significantly improved [27,28]. Thus, (Gd0.9Sc0.1)2Zr2O7 has been considered as a promising TBC material. However, its phase and microstructure evolution upon high temperature exposure to molten vanadium oxide is limited in open literature. In this study, the hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 ceramic in V2O5 salt at 700 °C, 800 °C, 900 °C and 1000 °C is investigated. The emphasis of this work is placed on analyzing the corrosion products resulted from the reaction between (Gd0.9Sc0.1)2Zr2O7 and V2O5 salt by using dense pellets and identifying the corrosion mechanisms.
2. Experimental procedure (Gd0.9Sc0.1)2Zr2O7 powders were produced by a chemical co-precipitation and calcination method, using RE2O3 (RE=Gd and Sc, purity 99.99%) and ZrOCl2·8H2O (purity 99.95%) as raw materials. Before fabrication process, Gd2O3 and Sc2O3 powders were calcined at 900 °C for 4 h to remove moisture and other volatile impurities. Appropriate amounts of RE2O3 and ZrOCl2·8H2O were dissolved in nitric acid and deionized water, respectively. The obtained solutions were mixed and stirred to yield homogeneous solution. Then, the mixed solution was slowly added to excess ammonia water (pH > 12) to get precipitate, followed by filtering and washing with deionized water and alcohol several times until a pH 7 was reached. The resultant precipitate was dried at 120 °C for 10 h and then calcined at 900 °C for 5 h for crystallization. In order to obtain bulks for hot corrosion tests, the powders were cold pressed at ~250 MPa and then sintered at 1600 °C for 15 h. Theoretical density of (Gd0.9Sc0.1)2Zr2O7 was calculated by using its lattice parameter available in the literature [27], and the bulk density was measured by Archimedes method. The theoretical density and the measured bulk density are 6.62 g/cm3 and 5.08 g/cm3, respectively. Thus, the relative density of the sintered samples are ~76.7% and the porosity is ~23.3%, which is close to that of the coating produced by air plasma spraying (APS). Hot corrosion tests were conducted according to our previous study and other researchers’ experiments [12,14,16,17,26]. Prior to the tests, the sintered samples were ground by 800 grit sandpaper, followed by ultrasonic cleaning in ethanol and drying at 120 °C. Subsequently, V2O5 powders with average particle size of ~10 µm were uniformly spread on the sample surfaces by using a very fine glass rod. An analytical balance was used to determine the weight of the specimens before and after V2O5 coverage. Finally, 0.0178 ± 0.005 g of V2O5 powders were coated on the sample surfaces, and the salt concentration was calculated to be ~10 mg/cm2. Then, the specimens were isothermally heated at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h, followed by cooling down to room temperature with furnace. Phase constitution of the samples was characterized by X-ray diffraction (XRD; Rigaku Diffractometer, Tokyo, Japan). Raman spectrum was recorded by a microscopic confocal Raman spectrometer (RM2000; Renishaw, Gloucestershire, UK) using 532 nm excitation from an argon ion laser. The spectral resolution was ~1 cm−1, and the signal was collected at a rate of 600 cm−1/30 s. Microstructure and composition analysis were carried out using a scanning electron microscope (SEM; FEI, Eindhoven, Holland) equipped with energy dispersive spectroscopy (EDS, IE 350).
Fig. 1. XRD patterns of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion tests in V2O5 salt at 700–1000 °C for 4 h. The standard PDF cards of GdVO4, ZrV2O7 and t-ZrO2 are also presented.
3. Results and discussion XRD measurements were carried out on the corroded surfaces of (Gd0.9Sc0.1)2Zr2O7 samples, and the results are presented in Fig. 1. In the XRD pattern of the sample after 4 h hot corrosion at 700 °C, strong peaks ascribed to GdVO4 (PDF#17–0260) and close to ZrV2O7 (PDF#16-0422) can be clearly observed. The detection of weak (Gd0.9Sc0.1)2Zr2O7 peaks indicates that the reaction layer on the sample surface is quite thin, thus X ray can penetrate through it. It is worthwhile to note that there is a weak peak appearing at 2θ≈30.5°, suggesting the presence of another phase. However, only based on this single diffraction peak, the phase cannot be determined. In order to investigate the influence of temperature on the corrosion products, (Gd0.9Sc0.1)2Zr2O7 bulks were exposed to 800 °C, 900 °C and 1000 °C for 4 h in V2O5 salt, and their XRD patterns are also included in Fig. 1. GdVO4 (PDF#17-0260) and (Gd0.9Sc0.1)2Zr2O7 peaks are evident in all the patterns. Notice that the peak at 2θ ≈30.5° still exists, the intensity of which increases with the increase of the corrosion temperature. Meanwhile, another two peaks appear at 2θ ≈34° and 50°. Comparing with the standard PDF card of t-ZrO2 (PDF#42-1164) shown in Fig. 1, one could ascribe these unidentified peaks to t-ZrO2 phase. Based on the aforementioned XRD analysis, it is possible to find that the corrosion products of (Gd0.9Sc0.1)2Zr2O7 in V2O5 molten salt are temperature dependent. At 700 °C, they contain GdVO4 and a phase close to ZrV2O7, together with a minor amount of tZrO2 phase, while those at 800 °C, 900 °C and 1000 °C mainly consist of GdVO4 and t-ZrO2. Raman spectroscopy is an extremely valuable tool for charactering the crystal structure of compounds [29,30]. To confirm the above conclusions and further analyze the structure of the corrosion products, Raman spectrum measurements were conducted. As shown in Fig. 2a, Raman modes appearing at ~434 cm−1, ~474 cm−1, ~570 cm−1, ~690 cm−1, ~720 cm−1 and ~885 cm−1 could be ascribed to GdVO4. To confirm this argument and exclude the presence of ScVO4, we produced GdVO4 and ScVO4 bulks and measured their Raman spectra. 2
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the literature [31,32] and our study are similar including no extreme temperature and pressure, the influence resulted from temperature and pressure variation can be ruled out. Hence, some doped ZrV2O7 compounds is possible to form in this study, which has broader Raman band resulted from the structural disorder due to composition variation. Since Gd3+ reacts with V2O5 to form GdVO4 and no evidence is found for ScVO4, considering Sc3+ ion acting as the dopant in ZrV2O7 is reasonable. As can be seen in Fig. 2a, Sc2O3 doped ZrV2O7 and GdVO4 Raman bands are found in the (Gd0.9Sc0.1)2Zr2O7 sample corroded at 800 °C, while only GdVO4 Raman bands are detected in the samples after 900 °C and 1000 °C corrosion. By comparison, it is possible to find that GdVO4 Raman bands in the samples corroded at different temperatures have no obvious Raman shifts, indicating that no structural disorder or distortion is produced in the GdVO4 lattice. Thus, Sc3+ ions doping in GdVO4 could be precluded. With the increase of the corrosion temperature, Sc2O3 doped ZrV2O7 turns to decompose. Since the formed Sc3+ ions have no possibility to take part in the formation of GdVO4, they remain in ZrO2 acting as a dopant. This leads to the formation of t-ZrO2 phase, which is detected by XRD. However, the Raman bands of t-ZrO2 phase are not obvious in Fig. 2a. It has been reported that t-ZrO2 has a Raman band at ~470 cm−1. In combination with the XRD results and the above analysis, the Raman mode at ~474 cm−1 in Fig. 2a could be assigned to both t-ZrO2 and GdVO4 phases. Fig. 3a shows a typical surface image of (Gd0.9Sc0.1)2Zr2O7 ceramic after hot corrosion at 700 °C for 4 h. Obvious new crystals are observed on the sample surface, which exhibit two different morphologies, i.e.
Fig. 2. Raman spectra of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion tests in V2O5 salt at 700–1000 °C for 4 h (a). Fig. 2b shows the Raman spectra of the fabricated GdVO4 and ScVO4 bulks.
As shown in Fig. 2b, the characteristic Raman modes for GdVO4 are at ~434 cm−1, ~474 cm−1, ~690 cm−1, ~720 cm−1 and ~885 cm−1, while those for ScVO4 are at ~355 cm−1, ~818 cm−1, ~857 cm−1 and ~914 cm−1. This provide strong indication for the formation of GdVO4 rather than ScVO4 after molten salt corrosion. The results suggest that in (Gd0.9Sc0.1)2Zr2O7, Sc3+ ions have smaller tendency to react with V2O5 than Gd3+. Thus, it is necessary to determine the role of Sc3+ ions during corrosion reaction. Besides GdVO4 Raman modes, it is possible to find some modes at ~405 cm−1, ~510 cm−1, ~775 cm−1 and ~987 cm−1 in the Raman spectrum of the (Gd0.9Sc0.1)2Zr2O7 sample corroded at 700 °C. These Raman modes are close to those of ZrV2O7 in the literature [31,32], but exhibiting much broader appearance. For ZrV2O7 compound, the Raman modes at ~987 cm−1 and ~775 cm−1 are assigned to the symmetric stretching and the asymmetric stretching of VO4 tetrahedra, respectively. The modes at ~510 cm−1 and ~474 cm−1 are due to the ZrO4 octahedral stretching and the VO4 asymmetric bending, respectively, and that at ~405 cm−1 is assigned to the symmetric bending of VO4 tetrahedra [29,33]. Notice that compared with the Raman spectrum of ZrV2O7 in the literature [31,32], some Raman modes are absence in this study. It has been reported that structural disorder can broader the Raman bands as well as preclude some Raman modes. The ordering degree of ZrV2O7 structure depends on temperature, pressure and composition [34,35]. Since the sample fabrication conditions in
Fig. 3. Surface morphologies of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion in V2O5 salt at 700 °C for 4 h. Fig. 3b shows an enlarge image.
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Table 1 Chemical compositions of different regions in Figs. 3–6 (in at%).
A B C D E F H G
Gd
Sc
Zr
V
O
16.3 – 18.1 – 18.3 – 18.5 –
– 1.4 – 2.6 – 2.9 – 2.5
– 15.4 – 28.3 – 31.7 – 27.9
18.1 31.1 18.8 – 17.8 – 17.3 –
65.6 52.1 63.1 69.1 63.9 65.4 64.2 69.6
Fig. 5. Surface morphologies of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion in V2O5 salt at 900 °C for 4 h. Fig. 5b shows an enlarge image.
Fig. 5 shows the SEM images of (Gd0.9Sc0.1)2Zr2O7 ceramic after hot corrosion at 900 °C for 4 h. Corrosion products with two different morphologies can be clearly observed. From the EDS results in Table 1 obtained from regions E and F in Fig. 5b, it could be confirmed the formation of GdVO4 (region E) and t-ZrO2 (region F), which supports the above XRD and Raman results. The representative surface morphologies of the sample corroded at 1000 °C for 4 h are shown in Fig. 6. It is clear that there are two different regions in the images. EDS results obtained from regions G and H in Fig. 6b are listed in Table 1. The elements identified agree great well with the presence of reaction products GdVO4 (region H) and t-ZrO2 (region G). This is consistent with the above XRD and Raman results. Based on the aforementioned observation and analysis, it is possible to find that hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 in V2O5 molten salt is temperature dependent. At 700 °C, the corrosion products consist of large amounts of Sc2O3 doped ZrV2O7 and GdVO4, together with a minor amount of t-ZrO2. With the increase of the corrosion temperature, Sc2O3 doped ZrV2O7 turns to decompose, causing the formation of more t-ZrO2. At 900 °C and 1000 °C, the corrosion products are GdVO4 and t-ZrO2. The related mechanisms by which the corrosion reaction occurs could be discussed based on phase diagram and Lewis acid-base rule. Since V2O5-(Gd0.9Sc0.1)2Zr2O7 binary phase diagram is not available in open literature and (Gd0.9Sc0.1)2Zr2O7 could be viewed as a compound that is composed of Gd2O3, Sc2O3 and ZrO2 at an appropriate ratio. V2O5-RE2O3 (RE=Gd and Sc) and V2O5-ZrO2 phase diagrams could be used to analyze the corrosion reaction between (Gd0.9Sc0.1)2Zr2O7 and V2O5
Fig. 4. Surface morphologies of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion in V2O5 salt at 800 °C for 4 h. Fig. 4b shows an enlarge image.
cubic-shaped and particle-shaped. In an enlarged image shown in Fig. 3b, the two types of the crystals are marked as A and B. EDS results in Table 1 indicate that compound A is composed of Gd, V and O, while compound B contains Zr, V, Sc and O. In combination with the above XRD and Raman results, further analysis confirms that A and B are GdVO4 and Sc2O3 doped ZrV2O7, respectively. A SEM micrograph obtained from the corroded surface of (Gd0.9Sc0.1)2Zr2O7 ceramic exposed to 800 °C for 4 h is shown in Fig. 4a. The crystals with two different shapes are denoted as C and D in Fig. 4b. EDS analysis reveals that crystal C consists of Gd, V and O, while crystal D is composed of Zr, Sc and O, as presented in Table 1. Further analysis demonstrates that C and D are GdVO4 and t-ZrO2, respectively. It is possible to find that in Figs. 3 and 4, GdVO4 crystals have quite similar morphology, while Sc2O3 doped ZrV2O7 and t-ZrO2 exhibit different morphologies and sizes. 4
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Fig. 7. Gibbs free energy of the reaction between oxides (Gd2O3, Sc2O3, ZrO2) and V2O5.
700 °C. Hence, the reaction mechanism between (Gd0.9Sc0.1)2Zr2O7 and V2O5 molten salt at 700 °C could be expressed as follow:
(Gd 0.9Sc0.1)2 Zr2O7(s) + V2O5(l ) → GdVO4(s ) + ZrV2O7(Sc2O3)(s )
According to V2O5-ZrV2O7 phase diagram [40], ZrV2O7 dissolves in V2O5-rich liquid above 747 °C, and the dissolution process is affected by local composition. Considering the fact that Gd2O3 have stronger basicity than ZrO2, it reacts more readily with V2O5, reducing the V2O5 content in V2O5-rich liquid. As a result, V2O5-ZrV2O7 system loses its equilibrium and more ZrV2O7 is dissolved. After the complete consumption of V2O5 by Gd2O3, pure ZrO2 is precipitated. In this study, there exists Sc2O3 in ZrV2O7 solid solution as a dopant. When all V2O5 salts are consumed by Gd2O3, the precipitate is ZrO2 together with Sc2O3 rather than pure ZrO2. Based on the aforementioned analysis, the reaction mechanism between (Gd0.9Sc0.1)2Zr2O7 and V2O5 at 800 °C, 900 °C and 1000 °C could be represented by the following expression:
Fig. 6. Surface morphologies of (Gd0.9Sc0.1)2Zr2O7 ceramics after hot corrosion in V2O5 salt at 1000 °C for 4 h. Fig. 6b shows an enlarge image.
(Gd 0.9Sc0.1)2Zr2O7(s) + V2O5(l ) → GdVO4(s ) + t −ZrO2(Sc2O3)(s ) [36]. According to V2O5-RE2O3 binary phase diagram [37–39], V2O5 reacts with RE2O3 to form REVO4, which is stable to 1200 °C. The reaction equation could be given as follow:
V2O5 + RE2O3 → 2REVO4
(4)
In V2O5 molten salt, the major corrosion product of (Gd0.9Sc0.1)2Zr2O7 is GdVO4, which is like the case of Gd2Zr2O7. For Gd2Zr2O7 case, the other corrosion product is m-ZrO2, while it is tZrO2 for (Gd0.9Sc0.1)2Zr2O7 in this study [42,43]. It is known that mZrO2 undergoes phase transformation during thermal cycling, accompanied with volume change resulting in cracks in the coatings. While the t phase stabilized by Sc2O3 has been reported to have excellent resistance to molten salt corrosion, as well as good phase stability, low thermal conductivity and high toughness [14–16,44]. Therefore, (Gd0.9Sc0.1)2Zr2O7 might perform better in V2O5 molten salt than Gd2Zr2O7 by considering the corrosion products.
(1)
In V2O5-ZrO2 system [40], there exists an intermediate compound of ZrV2O7 below 747 °C, the formation of which could be expressed as follow:
V2O5 + Z rO2 → ZrV2O7
(3)
(2)
According to the phase diagram, ZrV2O7 melts incongruently to produce a mixture of ZrO2 and V2O5-rich liquid when the temperature is above 747 °C. It is well known that the reaction between V2O5 and ceramic oxides follows the Lewis acid-base rule, where the acid V2O5 is more prone to react with ceramic oxides that have stronger basicity [14,23–26]. According to the literature [41], Gd2O3 has stronger basicity than ZrO2 and Sc2O3. Thus, it has larger tendency to react with molten V2O5. As a result, GdVO4 is the major reaction product between (Gd0.9Sc0.1)2Zr2O7 and V2O5 molten salt. During hot corrosion tests, V2O5 could also react with ZrO2 or Sc2O3. Since ZrO2 is more basic than Sc2O3, it prefers to react with V2O5 to form ZrV2O7, leaving Sc2O3 as a dopant in the solid solution. To confirm the above argument, the reaction Gibbs free energy of Eqs. (1) and (2) is calculated and the results are shown in Fig. 7. It is evident that Gd2O3 can readily react with V2O5 to form GdVO4, while Sc2O3 has the smallest tendency to react with V2O5. According to the phase diagram, ZrV2O7 is stable at
4. Conclusions Hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 ceramic in V2O5 molten salt at 700 °C, 800 °C, 900 °C and 1000 °C was investigated. During thermal exposure for 4 h, obvious corrosion reaction between (Gd0.9Sc0.1)2Zr2O7 and V2O5 salt occurred, corrosion products being temperature dependent. At 700 °C, molten salt corrosion led to the formation of Sc2O3 doped ZrV2O7, GdVO4 and a minor amount of Sc2O3-stabilized tetragonal ZrO2 (t-ZrO2). With the increase of the corrosion temperature, Sc2O3 doped ZrV2O7 decomposed causing more t-ZrO2. At 900 °C and 1000 °C, the corrosion products were composed of GdVO4 and t-ZrO2. The corrosion reaction mechanism was discussed according to phase diagrams and Lewis acid-base rule. (Gd0.9Sc0.1)2Zr2O7 might perform better in V2O5 molten salt than Gd2Zr2O7 by considering the corrosion products. 5
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