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Hot corrosion evaluation of Gd2O3-Yb2O3 co-doped Y2O3 stabilized ZrO2 thermal barrier oxides exposed to Na2SO4+V2O5 molten salt
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Hot corrosion evaluation of Gd2O3-Yb2O3 co-doped Y2O3 stabilized ZrO2 thermal barrier oxides exposed to Na2SO4+V2O5 molten salt ⁎
Lei Guoa,b,c, , Chenglong Zhanga,b, Mingzhu Lia,b, Wei Suna, Zhaoyang Zhanga, Fuxing Yea,b,c a b c
School of Materials Science and Engineering, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China Key Lab of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Thermal barrier coatings (TBCs) Doped YSZ Hot corrosion Molten salt
1 mol% Gd2O3 and 1 mol% Yb2O3 co-doped 3.5 mol% Y2O3 stabilized ZrO2 (GdYb-YSZ) ceramics in metastable tetragonal (t′) structure were fabricated, and their hot corrosion behaviors were evaluated. The corrosion tests were performed at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h in the presence of Na2SO4+V2O5 molten salt. After corrosion, t′ phase could still be detected on the corroded surfaces and its content decreased with increasing the corrosion temperature. The corrosion products were composed of doped YVO4 and m-ZrO2, but some amounts of Na4V2O7 were found at low corrosion temperatures (700 °C and 800 °C). The corrosion resistance characteristic of GdYb-YSZ was compared with that of YSZ. Under an identical corrosion condition, GdYb-YSZ had less monoclinic phase formed, indicative of a better hot corrosion resistance. The hot corrosion mechanism was discussed in detail.
1. Introduction Thermal barrier coatings (TBCs) are strongly required for advanced gas-turbine engine, which can protect hot-components (blades, vanes, combustion chamber) from thermal, corrosion and erosion degradation and thus improve the engine efficiency and performance [1–3]. Due to several desirable attributes such as high melting point, low thermal conductivity, comparative thermal expansion coefficient with the substrate and excellent fracture toughness, 7–8 wt% Y2O3 stabilized ZrO2 (YSZ) has been considered as a typical TBC material [3,4]. YSZ TBCs have been successfully produced by various methods, including air plasma spray (APS), electron beam physical vapor deposition (EBPVD) and plasma spray physical vapor deposition (PS-PVD) [5–8]. The as-fabricated coatings usually consist of non-transformable metastable tetragonal prime phase (t′), which is the most desirable phase for TBC applications. YSZ TBCs have proved to be worked well below 1200 °C, but their further applications are severely limited by even-increasing TBC temperature capability [3,4,9–11]. At higher temperatures, t′ phase lose its stability and suffers from phase transformation. Meanwhile, the sintering of the coating is accelerated with the increase of the operation temperature. Furthermore, in anticipation of better thermal insulation, there is a practical requirement for TBCs with even lower thermal conductivity. Therefore, the search is underway for new TBC materials
⁎
that meet the requirements for the higher operation temperature and better thermal insulation. Several high-temperature ceramics are being pursued, such as different rare-earth oxides doped/co-doped ZrO2, perovskite-structured materials, rare-earth phosphates and rare earth zirconates, some of which have been proposed as promising TBC material candidates [12–17]. Additionally, YSZ coatings are severely degraded when operated in corrosive environments [18–24]. In this case, impurities such as vanadium, sulfate and sodium form sulfate and vanadate salts, which condense on the coating surfaces at a temperature range of 700–1000 °C, followed by melting and penetration into the coating, destroying the coating microstructure. Extensive efforts have been conducted to improve the corrosion resistance of YSZ coatings against sulfate-vanadate molten salts. Titania stabilized zirconia coatings have been reported to reveal superior hot corrosion resistance compared with YSZ coatings [25]. Omar et al. have indicated that MgO doping could successfully suppress V2O5 salt corrosion to YSZ coating [26]. Hot corrosion tests conducted on nanostructured Al2O3/YSZ composite coatings have indicated that this type of coating has excellent hot corrosion resistance [27]. Hajizadeh-Ogha et al. have fabricated nanostructured ceria-yttria co-stabilized zirconia coatings by APS and found that they have better hot corrosion resistance than their conventional counterparts and YSZ coatings [28]. Loghman-Estarki and other researchers have reported that the addition of Sc2O3 in YSZ can cause an enhanced hot corrosion
Corresponding author. E-mail address: [email protected] (L. Guo).
http://dx.doi.org/10.1016/j.ceramint.2016.11.109 Received 3 November 2016; Received in revised form 14 November 2016; Accepted 16 November 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Guo, L., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.11.109
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resistance to Na2SO4+V2O5 molten salt [29–31]. These indicate that doping rare earth oxides or other oxides have positive effects on improving the hot corrosion resistance of YSZ. In our previous study, we have investigated a series of RE2O3-Yb2O3 co-doped YSZ (RE=La, Nd, Gd, Yb, REYb-YSZ) ceramics, and found that they all exhibit better phase stability and lower thermal conductivity than YSZ [32]. Specially, GdYb-YSZ has the best comprehensive properties, and it might be a promising TBC candidate [32]. For the application of this material in the future, it is thus necessary to evaluate its hot corrosion behavior in molten salts. In this study, GdYb-YSZ bulk samples were produced, and their hot corrosion behaviors at 700 °C, 800 °C, 900 °C and 1000 °C in the presence of Na2SO4+V2O5 salt were investigated. The emphasis was focused on analyzing the phase constitution, composition and structural evolution of the corrosion products of the samples corroded at various temperatures. The related mechanisms were discussed in detail. The hot corrosion resistance of GdYb-YSZ was evaluated by comparing with that of YSZ. 2. Experimental procedure 1 mol% Gd2O3 and 1 mol% Yb2O3 co-doped 3.5 mol% Y2O3 stabilized ZrO2 (GdYb-YSZ) powders were fabricated by a chemical coprecipitation and calcination method, using RE2O3 (RE=Gd, Yb and Y, purity 99.99%) and ZrOCl2·8H2O (purity 99.95%) as raw materials. RE2O3 powders were first calcined at 900 °C for 4 h to remove moisture and other volatile impurities. Appropriate amounts of RE2O3 powders and ZrOCl2·8H2O were dissolved in nitric acid and deionized water, respectively. Then, the solutions were mixed together and stirred to yield homogeneous solution. The obtained mixed solution was slowly added to excess ammonia water (pH > 12) to get precipitate, followed by filtering and washing with distilled water and alcohol several times until a pH 7 was reached. The resultant precipitate was dried at 120 °C for 20 h and then calcined at 800 °C for 5 h for crystallization. In order to produce bulks for hot corrosion tests, the powders were cold pressed at ~ 250 MPa and then sintered at 1500 °C for 10 h. For comparison, YSZ bulks were also prepared by similar procedure. Hot corrosion tests were conducted according to our previous study and other researchers’ experiments [23–25,28,30]. The salt used in this study was a mixture of 50 wt% Na2SO4+50 wt% V2O5 (Na2SO4+V2O5) and the corrosion time was 4 h. In order to ensure the uniform coverage of the salt on the sample surface, the salt was spread carefully by using a very fine glass rod at a concentration of 10 mg/cm2. The salt covered specimens were isothermally heated at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h, and then the furnace was cooled down to room temperature. The phase constitution of the as-fabricated and corroded surfaces of YSZ and GdYb-YSZ samples was characterized by X-ray diffraction (XRD; Rigaku Diffractometer, Tokyo, Japan). Microstructure and composition analysis were conducted using a scanning electron microscope (SEM; FEI, Eindhoven, Holland) equipped with energy dispersive spectroscopy (EDS, IE 350).
Fig. 1. XRD patterns of the as-fabricated and corroded GdYb-YSZ samples exposed to Na2SO4+V2O5 molten salt at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h. The standard PDF cards of m-ZrO2, t′-ZrO2, YVO4 and Na4V2O7 are also presented.
appear in the pattern, but they slightly shift to the lower angles, implying dissolution of some other atoms. Besides, some diffraction peaks ascribed to Na4V2O7 (PDF#28-1179) could be detected, suggesting that the molten salts are not completely consumed at this temperature. In order to study the influence of temperature on the corrosion products, GdYb-YSZ samples covered with Na2SO4 + V2O5 salt were exposed to higher temperatures. The XRD patterns of the samples after hot corrosion at 800 °C, 900 °C and 1000 °C for 4 h are also included in Fig. 1. In all the patterns, t′ phase is clearly observed, but the peak intensity of m phase increases with increasing corrosion temperature, indicating an increased amount of m phase. A doped YVO4 phase could also be detected on the corroded surfaces of the three samples. Note that with the increase of the corrosion temperature, diffraction peaks resulted from Na4V2O7 (PDF#28-1179) become weaker and disappear in the XRD pattern of the sample after 1000 °C hot corrosion. Based on the aforementioned XRD analysis, it could be concluded that temperature has little effect on the type of the corrosion products of GdYb-YSZ, but affecting the amounts of the m phase content and the remained molten slats, i.e. the corrosion products are composed of m-ZrO2 and doped YVO4, but some amounts of Na4V2O7 remain at low corrosion temperatures (700 °C and 800 °C). Since the molten salts used in our study are excess, the presence of t′ phase in the corroded surfaces of the samples may suggest that GdYb-YSZ could resist hot corrosion in some extent. As it is known, t′ phase is desirable for ZrO2-based ceramics for TBC applications. The failure of TBCs caused by hot corrosion is mainly due to the destabilization of t′ phase [25,29,33,34]. Under molten salts attack, the stabilizer in t′ phase is consumed, leading to the formation of m phase. This phase transformation has destructive effects on the coating lifetime. In order to evaluate the hot corrosion behavior of GdYb-YSZ at different temperatures, the decomposition extent of t′ phase was estimated by calculating the m phase content of the samples after hot corrosion according to the following equations [10]:
3. Results and discussion XRD measurements were performed on the surfaces of the asfabricated and corroded GdYb-YSZ samples, and the results are showed in Fig. 1. It is possible to observe that the as-fabricated sample is composed of t′ phase, without of any evidence for the presence of other phases. After hot corrosion at 700 °C for 4 h, the peaks representing t′ phase (PDF#71-1282) still exist in the pattern, but the intensities of which are relatively lower and some peaks become broader. It is worthwhile to note that representative diffraction peaks of m phase (PDF#72-0597), (−111) and (111) peaks, could be clearly observed at 2θ≈28° and 31°, indicating that some amounts of t′ phase experience phase transformation during corrosion test. Additionally, it could be found that some strong peaks close to those of YVO4 (PDF#17-0341) 2
Mm I (111) + Im (111) = 0.82 m Mt ′ It ′ (111)
(1)
Mm + Mt′ = 1
(2)
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Table 1 Chemical compositions of regions A, B and C in Fig. 3 (in at%).
A B C
Y
Yb
Gd
Zr
V
O
15.82 2.12 2.65
0.95 – 0.96
1.08 0.43 0.78
– 45.23 42.25
20.59 – –
61.56 52.22 53.36
Fig. 2. The calculated concentrations of m and t′ phases in GdYb-YSZ samples after hot corrosion at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h.
Here, Mm and Mt′ are the mole fractions of m and t′ phases, respectively, and I refers to the integral intensity corresponding to the peaks concerned. The calculated values are presented in Fig. 2. It is possible to observe that with the increase of the corrosion temperature, the content of m phase increases while that of t′ phase decreases. The m phase content increases from 35.11 to 58.63 mol% as the hot corrosion temperature increases from 700 °C to 1000 °C. Note that about 41.37 mol% t′ phase remains in the GdYb-YSZ sample after 1000 °C hot corrosion. Fig. 3a shows the typical surface morphology of GdYb-YSZ after hot corrosion at 700 °C for 4 h. Some corrosion products could be clearly observed on the surface. When analyzing these products at a higher
Fig. 4. Surface morphologies of GdYb-YSZ ceramic after hot corrosion in Na2SO4+V2O5 molten salt at 800 °C for 4 h.
SEM magnification (Fig. 3b), it is possible to find three different shaped compounds, which are marked as A (rod shaped), B (bulk shaped) and C (particle shaped). Their compositions were identified by EDS analysis and the results are presented in Table 1. It is demonstrated that product A consists of Y, Gd, Yb, V and O, while B and C both contain Zr, O and some rare earth elements, but C has more rare earth elements. In combination with the EDS and above XRD results, further analysis confirms that A is Yb, Gd doped YVO4, B and C are ZrO2 with different phase structure and could be considered as m-ZrO2 and t′-ZrO2 phases, respectively, by taking into account their rare earth contents. Fig. 4 shows the SEM images of the GdYb-YSZ sample after hot corrosion at 800 °C for 4 h. Obvious corrosion products could be found on the surface. In the enlarged image as shown in Fig. 4b, the compounds with three different shapes could be clearly observed, which are denoted as A, B and C, respectively. EDS results in Table 2 combined with the XRD analysis indicate that these compounds are Yb, Gd doped YVO4, m-ZrO2 and t′-ZrO2 phases. The corroded surface images of the samples after thermal exposure to 900 °C and 1000 °C are shown in Fig. 5. It is possible to find more rod shaped compounds in these images, indicating that higher temperatures accelerate the corrosion behavior. Note that the rod shaped compounds resulted from 1000 °C corrosion have larger size than those due to 900 °C corrosion. In Fig. 5b and d, three different shaped corrosion products could be
Fig. 3. Surface morphologies of GdYb-YSZ ceramic after hot corrosion in Na2SO4+V2O5 molten salt at 700 °C for 4 h.
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Table 2 Chemical compositions of regions A, B and C in Fig. 4 (in at%).
A B C
Table 3 Chemical compositions of regions A, B, C, D, E and F in Fig. 5 (in at%).
Y
Yb
Gd
Zr
V
O
17.27 1.86 2.34
0.98 – 0.91
1.13 0.45 0.75
– 41.63 40.64
22.04 – –
58.58 56.06 55.36
A B C D E F
observed. The compositions of the marked regions A, B, C, D, E and F are determined by EDS, and the results are listed in Table 3. In combination with the EDS and XRD results, it could be found that the corrosion products of the samples after 900 °C and 1000 °C corrosion are the same as those of the samples after 700 °C and 800 °C corrosion, i.e. consisting of Yb, Gd doped YVO4 and m-ZrO2, and there still exists t′-ZrO2 phase remaining in the samples after corrosion. Research has indicated that YSZ ceramic and coatings exhibit poor resistance to molten salts corrosion. The main reason is that Y2O3 stabilizer has large tendency to react with molten salt, resulting in the depletion of the stabilizer in the coatings. Thus, large amount of m phase is formed, which is harmful to the thermal cycling performance of the coatings. According to the results of this study, GdYb-YSZ is also vulnerable to the attack from molten salt. However, we find that there is still large amount of t′ phase remaining in the ceramic after hot corrosion, implying that it might perform better than YSZ against molten salt corrosion. To compare the hot corrosion behaviors of GdYb-YSZ and YSZ ceramics under an identical corrosion condition, we also carry out the corrosion experiments of YSZ in the presence of Na2SO4+V2O5 salt at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h. Fig. 6 Shows the XRD patterns of YSZ samples after corrosion at various temperatures for 4 h. It is possible to find that the corrosion products of the samples after corrosion all consist of YVO4 (PDF#17-0341) and
Y
Yb
Gd
Zr
V
O
16.57 1.95 2.42 18.46 1.73 2.01
1.03 – 0.88 1.12 – 0.85
1.86 0.52 0.72 1.37 0.48 0.69
– 43.56 41.24 – 44.36 40.89
21.37 – – 20.63 – –
59.17 53.97 54.74 58.42 53.43 55.56
Fig. 6. XRD patterns of YSZ ceramics after hot corrosion in Na2SO4+V2O5 molten salts at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h.
m-ZrO2 (PDF#72-0597). Note that Na4V2O7 (PDF#28-1179) remained
Fig. 5. Surface morphologies of GdYb-YSZ ceramics after hot corrosion in Na2SO4+V2O5 molten salts at 900 °C (a, b) and 1000 °C (c, d) for 4 h.
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2NaVO3 (l ) + N a2O (s ) → Na4 V2 O7 (l )
When adding Eq. (8) to Eq. (5) or Eq. (7), we could obtain Eq. (4) or Eq. (6). This indicates that considering the reaction between NaVO3 and GdYb-YSZ or YSZ takes place first might be more reasonable. According to the XRD analysis results, Na4V2O7 rather than NaVO3 is detected on the corroded surfaces of the samples after corrosion at 700 °C and 800 °C. This indicates that Na4V2O7 remains on the ceramic surfaces while NaVO3 is consumed. Therefore, it could be concluded that in the presence of Na2SO4+V2O5 molten salt, the formed NaVO3 first reacts with GdYb-YSZ or YSZ. When the hot corrosion temperature is enhanced or the corrosion duration is prolonged, the corrosion reactions follow Eq. (5) or Eq. (7). The different hot corrosion resistance of GdYb-YSZ and YSZ ceramics could be understood based on Lewis acid-base mechanism, which indicates that acid vanadium compound reacts more readily with the ceramic oxides that have higher basicity [35]. Compared with YSZ, GdYb-YSZ contains more rare earth elements. Additionally, Yb2O3 has a lower basicity than Y2O3 and Gd2O3 [36,37]. Thus, Y2O3 and Gd2O3 have larger tendency to react with molten salt, leaving Yb2O3 as a stabilizer for ZrO2. Therefore, it is more possible for GdYb-YSZ to keep t′ phase stability than YSZ when molten salts are presented. However, the hot corrosion resistance of GdYb-YSZ is not good enough, so its degradation resulted from molten salt attack needs to attract attention. In our previous study, we have indicated that GdYb-YSZ has better phase stability and lower thermal conductivity than YSZ, and this study indicates its relatively better resistance to molten salt corrosion. Therefore, it could be concluded that GdYb-YSZ might be a promising material for TBC applications.
Fig. 7. Comparison of m phase contents in YSZ and GdYb-YSZ ceramics after hot corrosion in Na2SO4+V2O5 molten salt at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h.
in the samples corroded at low temperatures (700 °C and 800 °C). In combination with the aforementioned analysis, we could conclude that in the presence of Na2SO4 + V2O5 molten salt, GdYb-YSZ and YSZ have similar corrosion products when the corrosion condition is identical. However, closer observation on the diffraction peaks of t′ and m phases in the XRD patterns indicates that after corrosion, m phase in YSZ has higher peak intensity compared with that in GdYbYSZ, suggesting that more m phase is formed in YSZ ceramic. We calculated the m phase contents of YSZ ceramics after corrosion at various temperatures using Eqs. (1) and (2), and the results are shown in Fig. 7. For comparison, the m phase contents of GdYb-YSZ after hot corrosion are also included. It could be clearly observed that at an identical corrosion temperature, less m phase is formed in GdYb-YSZ. Since the m phase is generated due to the decomposition of t′ phase, the amount of which could be used to characterize the t′ phase stability, i.e. the less the m phase forms, the higher the t′ phase stability is. Therefore, it could be concluded that in the presence of Na2SO4+V2O5 molten salts, GdYb-YSZ in t′ structure is more stable than YSZ. In other words, GdYb-YSZ has better corrosion resistance to molten salt attack than YSZ. Based on the aforementioned analysis of the corrosion products of GdYb-YSZ and YSZ, it is possible to conclude that the two ceramics have close hot corrosion mechanism. At high temperatures, Na2SO4 reacts with V2O5. Since the melting temperatures of Na2SO4 and V2O5 are about 884 °C and 690 °C, respectively, the reaction between them depends on the temperature, which could be expressed as follows:
2V2 O5 (s) + 3Na2 SO4 (s ) → 2NaVO3 (l ) + N a 4V2 O7 (l ) + 3SO3 (g)
(3-1)
2V2 O5 (l ) + 3Na2 SO4 (l ) → 2NaVO3 (l ) + N a 4V2 O7 (l ) + 3SO3 (g)
(3-2)
4. Conclusions GdYb-YSZ ceramics were fabricated by a chemical co-precipitation and calcination method, and their hot corrosion behavior in Na2SO4+V2O5 molten salt at temperatures of 700–1000 °C were evaluated and compared with YSZ. Although GdYb-YSZ was vulnerable to molten salt attack, it exhibited better hot corrosion resistance than YSZ. After hot corrosion, large amount of t′ phase could still be detected on the corroded surfaces, and the corrosion products consisted of doped YVO4 and m-ZrO2, but some amounts of Na4V2O7 existed at low corrosion temperatures (700 °C and 800 °C). With the increase of the corrosion temperature, the content of t′ phase decreased while that of m phase increased. Compared with YSZ, less m phase was formed on the corroded surface of GdYb-YSZ, which could be attributed to the more rare earth content and the lower basicity of Yb2O3 based on the consideration from Lewis acid-base mechanism. 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).
Then, NaVO3 or Na4V2O7 reacts with GdYb-YSZ or YSZ. According to the previous analysis results, the possible reactions could be expressed as following equations:
NaVO3 (l ) + GdYb−YSZ (s ) → (Y , Yb , Gd ) VO4 (s ) + m−ZrO2 (s ) + Na2 O (s )
References
(4)
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Na4 V2 O7 (l ) + G dYb−YSZ (s ) → (Y , Yb , Gd ) VO4 (s ) + m−ZrO2 (s ) + Na2 O (s )
(5)
NaVO3 (l ) + YSZ (s ) → YVO4 (s ) + m−ZrO2 (s ) + Na2 O (s )
(6)
Na4 V2 O7 (l ) + YSZ (s ) → YVO4 (s ) + m−ZrO2 (s ) + Na2 O (s )
(7)
(8)
For NaVO3 and Na4V2O7, it needs to determine which of them has priority to react with the ceramics. Assuming that Na4V2O7 first reacts with GdYb-YSZ or YSZ, the reactions follow Eq. (5) or Eq. (7). Then, the formed Na2O would react with NaVO3 as follow: 5
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Hot corrosion evaluation of Gd2O3-Yb2O3 co-doped Y2O3 stabilized ZrO2 thermal barrier oxides exposed to Na2SO4+V2O5 molten salt ⁎
Lei Guoa,b,c, , Chenglong Zhanga,b, Mingzhu Lia,b, Wei Suna, Zhaoyang Zhanga, Fuxing Yea,b,c a b c
School of Materials Science and Engineering, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China Key Lab of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Thermal barrier coatings (TBCs) Doped YSZ Hot corrosion Molten salt
1 mol% Gd2O3 and 1 mol% Yb2O3 co-doped 3.5 mol% Y2O3 stabilized ZrO2 (GdYb-YSZ) ceramics in metastable tetragonal (t′) structure were fabricated, and their hot corrosion behaviors were evaluated. The corrosion tests were performed at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h in the presence of Na2SO4+V2O5 molten salt. After corrosion, t′ phase could still be detected on the corroded surfaces and its content decreased with increasing the corrosion temperature. The corrosion products were composed of doped YVO4 and m-ZrO2, but some amounts of Na4V2O7 were found at low corrosion temperatures (700 °C and 800 °C). The corrosion resistance characteristic of GdYb-YSZ was compared with that of YSZ. Under an identical corrosion condition, GdYb-YSZ had less monoclinic phase formed, indicative of a better hot corrosion resistance. The hot corrosion mechanism was discussed in detail.
1. Introduction Thermal barrier coatings (TBCs) are strongly required for advanced gas-turbine engine, which can protect hot-components (blades, vanes, combustion chamber) from thermal, corrosion and erosion degradation and thus improve the engine efficiency and performance [1–3]. Due to several desirable attributes such as high melting point, low thermal conductivity, comparative thermal expansion coefficient with the substrate and excellent fracture toughness, 7–8 wt% Y2O3 stabilized ZrO2 (YSZ) has been considered as a typical TBC material [3,4]. YSZ TBCs have been successfully produced by various methods, including air plasma spray (APS), electron beam physical vapor deposition (EBPVD) and plasma spray physical vapor deposition (PS-PVD) [5–8]. The as-fabricated coatings usually consist of non-transformable metastable tetragonal prime phase (t′), which is the most desirable phase for TBC applications. YSZ TBCs have proved to be worked well below 1200 °C, but their further applications are severely limited by even-increasing TBC temperature capability [3,4,9–11]. At higher temperatures, t′ phase lose its stability and suffers from phase transformation. Meanwhile, the sintering of the coating is accelerated with the increase of the operation temperature. Furthermore, in anticipation of better thermal insulation, there is a practical requirement for TBCs with even lower thermal conductivity. Therefore, the search is underway for new TBC materials
⁎
that meet the requirements for the higher operation temperature and better thermal insulation. Several high-temperature ceramics are being pursued, such as different rare-earth oxides doped/co-doped ZrO2, perovskite-structured materials, rare-earth phosphates and rare earth zirconates, some of which have been proposed as promising TBC material candidates [12–17]. Additionally, YSZ coatings are severely degraded when operated in corrosive environments [18–24]. In this case, impurities such as vanadium, sulfate and sodium form sulfate and vanadate salts, which condense on the coating surfaces at a temperature range of 700–1000 °C, followed by melting and penetration into the coating, destroying the coating microstructure. Extensive efforts have been conducted to improve the corrosion resistance of YSZ coatings against sulfate-vanadate molten salts. Titania stabilized zirconia coatings have been reported to reveal superior hot corrosion resistance compared with YSZ coatings [25]. Omar et al. have indicated that MgO doping could successfully suppress V2O5 salt corrosion to YSZ coating [26]. Hot corrosion tests conducted on nanostructured Al2O3/YSZ composite coatings have indicated that this type of coating has excellent hot corrosion resistance [27]. Hajizadeh-Ogha et al. have fabricated nanostructured ceria-yttria co-stabilized zirconia coatings by APS and found that they have better hot corrosion resistance than their conventional counterparts and YSZ coatings [28]. Loghman-Estarki and other researchers have reported that the addition of Sc2O3 in YSZ can cause an enhanced hot corrosion
Corresponding author. E-mail address: [email protected] (L. Guo).
http://dx.doi.org/10.1016/j.ceramint.2016.11.109 Received 3 November 2016; Received in revised form 14 November 2016; Accepted 16 November 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Guo, L., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.11.109
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resistance to Na2SO4+V2O5 molten salt [29–31]. These indicate that doping rare earth oxides or other oxides have positive effects on improving the hot corrosion resistance of YSZ. In our previous study, we have investigated a series of RE2O3-Yb2O3 co-doped YSZ (RE=La, Nd, Gd, Yb, REYb-YSZ) ceramics, and found that they all exhibit better phase stability and lower thermal conductivity than YSZ [32]. Specially, GdYb-YSZ has the best comprehensive properties, and it might be a promising TBC candidate [32]. For the application of this material in the future, it is thus necessary to evaluate its hot corrosion behavior in molten salts. In this study, GdYb-YSZ bulk samples were produced, and their hot corrosion behaviors at 700 °C, 800 °C, 900 °C and 1000 °C in the presence of Na2SO4+V2O5 salt were investigated. The emphasis was focused on analyzing the phase constitution, composition and structural evolution of the corrosion products of the samples corroded at various temperatures. The related mechanisms were discussed in detail. The hot corrosion resistance of GdYb-YSZ was evaluated by comparing with that of YSZ. 2. Experimental procedure 1 mol% Gd2O3 and 1 mol% Yb2O3 co-doped 3.5 mol% Y2O3 stabilized ZrO2 (GdYb-YSZ) powders were fabricated by a chemical coprecipitation and calcination method, using RE2O3 (RE=Gd, Yb and Y, purity 99.99%) and ZrOCl2·8H2O (purity 99.95%) as raw materials. RE2O3 powders were first calcined at 900 °C for 4 h to remove moisture and other volatile impurities. Appropriate amounts of RE2O3 powders and ZrOCl2·8H2O were dissolved in nitric acid and deionized water, respectively. Then, the solutions were mixed together and stirred to yield homogeneous solution. The obtained mixed solution was slowly added to excess ammonia water (pH > 12) to get precipitate, followed by filtering and washing with distilled water and alcohol several times until a pH 7 was reached. The resultant precipitate was dried at 120 °C for 20 h and then calcined at 800 °C for 5 h for crystallization. In order to produce bulks for hot corrosion tests, the powders were cold pressed at ~ 250 MPa and then sintered at 1500 °C for 10 h. For comparison, YSZ bulks were also prepared by similar procedure. Hot corrosion tests were conducted according to our previous study and other researchers’ experiments [23–25,28,30]. The salt used in this study was a mixture of 50 wt% Na2SO4+50 wt% V2O5 (Na2SO4+V2O5) and the corrosion time was 4 h. In order to ensure the uniform coverage of the salt on the sample surface, the salt was spread carefully by using a very fine glass rod at a concentration of 10 mg/cm2. The salt covered specimens were isothermally heated at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h, and then the furnace was cooled down to room temperature. The phase constitution of the as-fabricated and corroded surfaces of YSZ and GdYb-YSZ samples was characterized by X-ray diffraction (XRD; Rigaku Diffractometer, Tokyo, Japan). Microstructure and composition analysis were conducted using a scanning electron microscope (SEM; FEI, Eindhoven, Holland) equipped with energy dispersive spectroscopy (EDS, IE 350).
Fig. 1. XRD patterns of the as-fabricated and corroded GdYb-YSZ samples exposed to Na2SO4+V2O5 molten salt at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h. The standard PDF cards of m-ZrO2, t′-ZrO2, YVO4 and Na4V2O7 are also presented.
appear in the pattern, but they slightly shift to the lower angles, implying dissolution of some other atoms. Besides, some diffraction peaks ascribed to Na4V2O7 (PDF#28-1179) could be detected, suggesting that the molten salts are not completely consumed at this temperature. In order to study the influence of temperature on the corrosion products, GdYb-YSZ samples covered with Na2SO4 + V2O5 salt were exposed to higher temperatures. The XRD patterns of the samples after hot corrosion at 800 °C, 900 °C and 1000 °C for 4 h are also included in Fig. 1. In all the patterns, t′ phase is clearly observed, but the peak intensity of m phase increases with increasing corrosion temperature, indicating an increased amount of m phase. A doped YVO4 phase could also be detected on the corroded surfaces of the three samples. Note that with the increase of the corrosion temperature, diffraction peaks resulted from Na4V2O7 (PDF#28-1179) become weaker and disappear in the XRD pattern of the sample after 1000 °C hot corrosion. Based on the aforementioned XRD analysis, it could be concluded that temperature has little effect on the type of the corrosion products of GdYb-YSZ, but affecting the amounts of the m phase content and the remained molten slats, i.e. the corrosion products are composed of m-ZrO2 and doped YVO4, but some amounts of Na4V2O7 remain at low corrosion temperatures (700 °C and 800 °C). Since the molten salts used in our study are excess, the presence of t′ phase in the corroded surfaces of the samples may suggest that GdYb-YSZ could resist hot corrosion in some extent. As it is known, t′ phase is desirable for ZrO2-based ceramics for TBC applications. The failure of TBCs caused by hot corrosion is mainly due to the destabilization of t′ phase [25,29,33,34]. Under molten salts attack, the stabilizer in t′ phase is consumed, leading to the formation of m phase. This phase transformation has destructive effects on the coating lifetime. In order to evaluate the hot corrosion behavior of GdYb-YSZ at different temperatures, the decomposition extent of t′ phase was estimated by calculating the m phase content of the samples after hot corrosion according to the following equations [10]:
3. Results and discussion XRD measurements were performed on the surfaces of the asfabricated and corroded GdYb-YSZ samples, and the results are showed in Fig. 1. It is possible to observe that the as-fabricated sample is composed of t′ phase, without of any evidence for the presence of other phases. After hot corrosion at 700 °C for 4 h, the peaks representing t′ phase (PDF#71-1282) still exist in the pattern, but the intensities of which are relatively lower and some peaks become broader. It is worthwhile to note that representative diffraction peaks of m phase (PDF#72-0597), (−111) and (111) peaks, could be clearly observed at 2θ≈28° and 31°, indicating that some amounts of t′ phase experience phase transformation during corrosion test. Additionally, it could be found that some strong peaks close to those of YVO4 (PDF#17-0341) 2
Mm I (111) + Im (111) = 0.82 m Mt ′ It ′ (111)
(1)
Mm + Mt′ = 1
(2)
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Table 1 Chemical compositions of regions A, B and C in Fig. 3 (in at%).
A B C
Y
Yb
Gd
Zr
V
O
15.82 2.12 2.65
0.95 – 0.96
1.08 0.43 0.78
– 45.23 42.25
20.59 – –
61.56 52.22 53.36
Fig. 2. The calculated concentrations of m and t′ phases in GdYb-YSZ samples after hot corrosion at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h.
Here, Mm and Mt′ are the mole fractions of m and t′ phases, respectively, and I refers to the integral intensity corresponding to the peaks concerned. The calculated values are presented in Fig. 2. It is possible to observe that with the increase of the corrosion temperature, the content of m phase increases while that of t′ phase decreases. The m phase content increases from 35.11 to 58.63 mol% as the hot corrosion temperature increases from 700 °C to 1000 °C. Note that about 41.37 mol% t′ phase remains in the GdYb-YSZ sample after 1000 °C hot corrosion. Fig. 3a shows the typical surface morphology of GdYb-YSZ after hot corrosion at 700 °C for 4 h. Some corrosion products could be clearly observed on the surface. When analyzing these products at a higher
Fig. 4. Surface morphologies of GdYb-YSZ ceramic after hot corrosion in Na2SO4+V2O5 molten salt at 800 °C for 4 h.
SEM magnification (Fig. 3b), it is possible to find three different shaped compounds, which are marked as A (rod shaped), B (bulk shaped) and C (particle shaped). Their compositions were identified by EDS analysis and the results are presented in Table 1. It is demonstrated that product A consists of Y, Gd, Yb, V and O, while B and C both contain Zr, O and some rare earth elements, but C has more rare earth elements. In combination with the EDS and above XRD results, further analysis confirms that A is Yb, Gd doped YVO4, B and C are ZrO2 with different phase structure and could be considered as m-ZrO2 and t′-ZrO2 phases, respectively, by taking into account their rare earth contents. Fig. 4 shows the SEM images of the GdYb-YSZ sample after hot corrosion at 800 °C for 4 h. Obvious corrosion products could be found on the surface. In the enlarged image as shown in Fig. 4b, the compounds with three different shapes could be clearly observed, which are denoted as A, B and C, respectively. EDS results in Table 2 combined with the XRD analysis indicate that these compounds are Yb, Gd doped YVO4, m-ZrO2 and t′-ZrO2 phases. The corroded surface images of the samples after thermal exposure to 900 °C and 1000 °C are shown in Fig. 5. It is possible to find more rod shaped compounds in these images, indicating that higher temperatures accelerate the corrosion behavior. Note that the rod shaped compounds resulted from 1000 °C corrosion have larger size than those due to 900 °C corrosion. In Fig. 5b and d, three different shaped corrosion products could be
Fig. 3. Surface morphologies of GdYb-YSZ ceramic after hot corrosion in Na2SO4+V2O5 molten salt at 700 °C for 4 h.
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Table 2 Chemical compositions of regions A, B and C in Fig. 4 (in at%).
A B C
Table 3 Chemical compositions of regions A, B, C, D, E and F in Fig. 5 (in at%).
Y
Yb
Gd
Zr
V
O
17.27 1.86 2.34
0.98 – 0.91
1.13 0.45 0.75
– 41.63 40.64
22.04 – –
58.58 56.06 55.36
A B C D E F
observed. The compositions of the marked regions A, B, C, D, E and F are determined by EDS, and the results are listed in Table 3. In combination with the EDS and XRD results, it could be found that the corrosion products of the samples after 900 °C and 1000 °C corrosion are the same as those of the samples after 700 °C and 800 °C corrosion, i.e. consisting of Yb, Gd doped YVO4 and m-ZrO2, and there still exists t′-ZrO2 phase remaining in the samples after corrosion. Research has indicated that YSZ ceramic and coatings exhibit poor resistance to molten salts corrosion. The main reason is that Y2O3 stabilizer has large tendency to react with molten salt, resulting in the depletion of the stabilizer in the coatings. Thus, large amount of m phase is formed, which is harmful to the thermal cycling performance of the coatings. According to the results of this study, GdYb-YSZ is also vulnerable to the attack from molten salt. However, we find that there is still large amount of t′ phase remaining in the ceramic after hot corrosion, implying that it might perform better than YSZ against molten salt corrosion. To compare the hot corrosion behaviors of GdYb-YSZ and YSZ ceramics under an identical corrosion condition, we also carry out the corrosion experiments of YSZ in the presence of Na2SO4+V2O5 salt at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h. Fig. 6 Shows the XRD patterns of YSZ samples after corrosion at various temperatures for 4 h. It is possible to find that the corrosion products of the samples after corrosion all consist of YVO4 (PDF#17-0341) and
Y
Yb
Gd
Zr
V
O
16.57 1.95 2.42 18.46 1.73 2.01
1.03 – 0.88 1.12 – 0.85
1.86 0.52 0.72 1.37 0.48 0.69
– 43.56 41.24 – 44.36 40.89
21.37 – – 20.63 – –
59.17 53.97 54.74 58.42 53.43 55.56
Fig. 6. XRD patterns of YSZ ceramics after hot corrosion in Na2SO4+V2O5 molten salts at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h.
m-ZrO2 (PDF#72-0597). Note that Na4V2O7 (PDF#28-1179) remained
Fig. 5. Surface morphologies of GdYb-YSZ ceramics after hot corrosion in Na2SO4+V2O5 molten salts at 900 °C (a, b) and 1000 °C (c, d) for 4 h.
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2NaVO3 (l ) + N a2O (s ) → Na4 V2 O7 (l )
When adding Eq. (8) to Eq. (5) or Eq. (7), we could obtain Eq. (4) or Eq. (6). This indicates that considering the reaction between NaVO3 and GdYb-YSZ or YSZ takes place first might be more reasonable. According to the XRD analysis results, Na4V2O7 rather than NaVO3 is detected on the corroded surfaces of the samples after corrosion at 700 °C and 800 °C. This indicates that Na4V2O7 remains on the ceramic surfaces while NaVO3 is consumed. Therefore, it could be concluded that in the presence of Na2SO4+V2O5 molten salt, the formed NaVO3 first reacts with GdYb-YSZ or YSZ. When the hot corrosion temperature is enhanced or the corrosion duration is prolonged, the corrosion reactions follow Eq. (5) or Eq. (7). The different hot corrosion resistance of GdYb-YSZ and YSZ ceramics could be understood based on Lewis acid-base mechanism, which indicates that acid vanadium compound reacts more readily with the ceramic oxides that have higher basicity [35]. Compared with YSZ, GdYb-YSZ contains more rare earth elements. Additionally, Yb2O3 has a lower basicity than Y2O3 and Gd2O3 [36,37]. Thus, Y2O3 and Gd2O3 have larger tendency to react with molten salt, leaving Yb2O3 as a stabilizer for ZrO2. Therefore, it is more possible for GdYb-YSZ to keep t′ phase stability than YSZ when molten salts are presented. However, the hot corrosion resistance of GdYb-YSZ is not good enough, so its degradation resulted from molten salt attack needs to attract attention. In our previous study, we have indicated that GdYb-YSZ has better phase stability and lower thermal conductivity than YSZ, and this study indicates its relatively better resistance to molten salt corrosion. Therefore, it could be concluded that GdYb-YSZ might be a promising material for TBC applications.
Fig. 7. Comparison of m phase contents in YSZ and GdYb-YSZ ceramics after hot corrosion in Na2SO4+V2O5 molten salt at 700 °C, 800 °C, 900 °C and 1000 °C for 4 h.
in the samples corroded at low temperatures (700 °C and 800 °C). In combination with the aforementioned analysis, we could conclude that in the presence of Na2SO4 + V2O5 molten salt, GdYb-YSZ and YSZ have similar corrosion products when the corrosion condition is identical. However, closer observation on the diffraction peaks of t′ and m phases in the XRD patterns indicates that after corrosion, m phase in YSZ has higher peak intensity compared with that in GdYbYSZ, suggesting that more m phase is formed in YSZ ceramic. We calculated the m phase contents of YSZ ceramics after corrosion at various temperatures using Eqs. (1) and (2), and the results are shown in Fig. 7. For comparison, the m phase contents of GdYb-YSZ after hot corrosion are also included. It could be clearly observed that at an identical corrosion temperature, less m phase is formed in GdYb-YSZ. Since the m phase is generated due to the decomposition of t′ phase, the amount of which could be used to characterize the t′ phase stability, i.e. the less the m phase forms, the higher the t′ phase stability is. Therefore, it could be concluded that in the presence of Na2SO4+V2O5 molten salts, GdYb-YSZ in t′ structure is more stable than YSZ. In other words, GdYb-YSZ has better corrosion resistance to molten salt attack than YSZ. Based on the aforementioned analysis of the corrosion products of GdYb-YSZ and YSZ, it is possible to conclude that the two ceramics have close hot corrosion mechanism. At high temperatures, Na2SO4 reacts with V2O5. Since the melting temperatures of Na2SO4 and V2O5 are about 884 °C and 690 °C, respectively, the reaction between them depends on the temperature, which could be expressed as follows:
2V2 O5 (s) + 3Na2 SO4 (s ) → 2NaVO3 (l ) + N a 4V2 O7 (l ) + 3SO3 (g)
(3-1)
2V2 O5 (l ) + 3Na2 SO4 (l ) → 2NaVO3 (l ) + N a 4V2 O7 (l ) + 3SO3 (g)
(3-2)
4. Conclusions GdYb-YSZ ceramics were fabricated by a chemical co-precipitation and calcination method, and their hot corrosion behavior in Na2SO4+V2O5 molten salt at temperatures of 700–1000 °C were evaluated and compared with YSZ. Although GdYb-YSZ was vulnerable to molten salt attack, it exhibited better hot corrosion resistance than YSZ. After hot corrosion, large amount of t′ phase could still be detected on the corroded surfaces, and the corrosion products consisted of doped YVO4 and m-ZrO2, but some amounts of Na4V2O7 existed at low corrosion temperatures (700 °C and 800 °C). With the increase of the corrosion temperature, the content of t′ phase decreased while that of m phase increased. Compared with YSZ, less m phase was formed on the corroded surface of GdYb-YSZ, which could be attributed to the more rare earth content and the lower basicity of Yb2O3 based on the consideration from Lewis acid-base mechanism. 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).
Then, NaVO3 or Na4V2O7 reacts with GdYb-YSZ or YSZ. According to the previous analysis results, the possible reactions could be expressed as following equations:
NaVO3 (l ) + GdYb−YSZ (s ) → (Y , Yb , Gd ) VO4 (s ) + m−ZrO2 (s ) + Na2 O (s )
References
(4)
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Na4 V2 O7 (l ) + G dYb−YSZ (s ) → (Y , Yb , Gd ) VO4 (s ) + m−ZrO2 (s ) + Na2 O (s )
(5)
NaVO3 (l ) + YSZ (s ) → YVO4 (s ) + m−ZrO2 (s ) + Na2 O (s )
(6)
Na4 V2 O7 (l ) + YSZ (s ) → YVO4 (s ) + m−ZrO2 (s ) + Na2 O (s )
(7)
(8)
For NaVO3 and Na4V2O7, it needs to determine which of them has priority to react with the ceramics. Assuming that Na4V2O7 first reacts with GdYb-YSZ or YSZ, the reactions follow Eq. (5) or Eq. (7). Then, the formed Na2O would react with NaVO3 as follow: 5
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