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Hot corrosion behavior of calcium magnesium aluminosilicate (CMAS) on the Yb2SiO5-8YSZ composite as a candidate for environmental barrier coatings
Journal Pre-proof Hot corrosion behavior of calcium magnesium aluminosilicate (CMAS) on Yb2SiO5-8YSZ composite as a candidate for environmental barrier coatings Ali Abedi Nieai, Majid Mohammadi, Maryam Shojaie-Bahaabad PII:
S0254-0584(19)31406-3
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122596
Reference:
MAC 122596
To appear in:
Materials Chemistry and Physics
Received Date: 8 September 2019 Revised Date:
9 December 2019
Accepted Date: 30 December 2019
Please cite this article as: A.A. Nieai, M. Mohammadi, M. Shojaie-Bahaabad, Hot corrosion behavior of calcium magnesium aluminosilicate (CMAS) on Yb2SiO5-8YSZ composite as a candidate for environmental barrier coatings, Materials Chemistry and Physics (2020), doi: https://doi.org/10.1016/ j.matchemphys.2019.122596. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Hot corrosion behavior of calcium magnesium aluminosilicate (CMAS) on Yb2SiO58YSZ composite as a candidate for environmental barrier coatings Ali Abedi Nieai1, Majid Mohammadi1*, Maryam Shojaie-Bahaabad1 Faculty of Chemical and Materials Engineering, Shahrood University of Technology, Shahrood, Iran. * [email protected]
Abstract Dense Yb2SiO5 and three types of Yb2SiO5-YSZ composite specimens, as an environmental barrier coating, were prepared using pressureless sintering process at 1600 ºC for 10 h. Hot corrosion behaviors of the sintered specimens were investigated in the presence of calcium–magnesiumaluminosilicate (CMAS) at 1400 ºC for 4, 8, 24 and 48 h. Phase change identification, cross-sectional microstructure, reaction layer thickness, and elemental distribution, of the different samples during hot corrosion was investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) equipped by EDS technique. Mechanical properties of synthesized samples include elastic modulus, hardness and microhardness were measured by the nanoindentation method. The results showed that the reaction layer of all composite samples was smaller than that of the Yb2SiO5 sample and the formation of dense phases such as Al5Yb3O12 and Al5Y3O12 phases in the reaction layer can reduce the infiltration of molten CMAS into the samples, which mean better hot corrosion resistance of composite samples. Yb2SiO5-30wt.%YSZ composite sample showed better performance in contrast to the composite samples because of the high concentration of dissolved Y3+ ion in the glassy CMAS which promotes the formation of the barrier oxide phases (Ca2Al2SiO7, Ca3ZrSi2O9) and protective garnet phases (Al5Yb3O12) in the reaction layer.
Keywords: Hot Corrosion, Environmental barrier coatings, Yb2SiO5, YSZ, calcium magnesium aluminosilicate.
1. Introduction High-temperature components experience harsh conditions such as hot corrosion, oxidation and thermal shock during services. Nickel and cobalt-base superalloys coated with thermal barrier
coatings (TBC) were used as effective materials in gas turbine components for decades [1-5]. The demands for higher operating temperatures and efficiency, reduced the emissions of greenhouse gases (CO2 and NOx), and the need for more axial loads, led to the development of new materials. In fact, superalloys had reached their highest thermal efficiency at high temperatures, but the new generation of gas turbines required a higher working temperature
[6, 7]. Ceramic matrix composites (CMCs) such as SiC/Cf, SiC and Si3N4 appeared as a good candidate to develop as a novel class of high-temperature materials [8-10]. These materials are used in radomes, rocket motors, motor nose, central wing boxes, airplane wings, pressure vessels, engine chambers, floor beams, long cones, etc. CMCs are attractive materials in the aerospace industry, because of their ability at high temperatures. The major advantage of CMC is its lightweight. The lightweight of CMCs decreases fuel consumption increases speed and reduces greenhouse emissions. It has been reported that 30% of the total weight of the turbine blades can be reduced by using ceramic materials. Despite all these benefits, the main problem with the use of CMCs in gas turbines components is the surface recession due to the reaction of silica and water vapor at high temperature in the combustion chamber. The SiC oxidation process is shown in reaction [11]: SiC + 1.5 O2 (g) → SiO2 + CO(g)
(1)
In the existence of water vapor and oxygen in the working condition, SiC will oxidize based on the reactions (1) and (2): SiC + 3H2O (g) → SiO2 + 3H2(g) + CO(g)
(2)
The surface recession of CMCs depends on the gas temperature, gas velocity and as well as working pressure, at the constant gas velocity the rate of recession will increase sharply with increasing temperature and pressure [12, 13]. Protection of CMCs against water vapor corrosion or recession was done by applying environmental barrier coatings (EBCs) [11]. Protective EBCs are used to reduce the oxidation of SiC and limit the formation of SiO2 at the substrate/ coating interface [14]. The materials used as EBC should have low thermal conductivity, limited oxygen diffusivity, high melting point, chemical stability, suitable coefficient of thermal expansion (CTE), and good compatibility with the CMCs substrate [15]. RE-silicates (RE = rare earth elements: Gd, Yb, Y, La) are used widely as EBC materials. These materials exhibit low volatilities and good phase stability in ultra-high temperature combustion environments. Generally, Ytterbium monosilicate (Yb2SiO5) is the most used material in EBC systems; this compound possesses a unique combination of properties such as good corrosion resistance, low volatility, excellent phase stability, superior chemical compatibility and closes CTE to the SiC and Si3N4 substrate [16]. Richards and Wadley applied multifunctional EBC on the CMC substrates. They used Yb2SiO5 as the top layer to protect the substrate against hot corrosion [17]. They showed the formation of cracks and pores in the multifunctional coating facilitate the oxygen transfer
from the top layer to the substrate and reduce the corrosion resistance of the coating system [17]. Jang et al. [18] were investigated the high-temperature corrosion behavior of sintered ytterbium monosilicate in the presence of calcium-magnesium-aluminosilicate (CMAS) at 1400 ˚C for 2, 12 and 48 hours. The results confirmed the weak corrosion resistance of Yb2SiO5 against CMAS. Lee et al. [16], investigated the recession of SiC, mullite, and 8YSZ in a high-pressure burner rig test. The porous alumina layer crumbled and spalled readily, but 8YSZ did not show any weight loss which indicates the superior recession resistance and excellent stability of 8YSZ in water vapor. Poerschke et al. [19] investigated the reaction of yttrium disilicate environmental barrier coatings with molten calcium-magnesium-iron alumino-silicate (CMFAS). The results showed weak corrosion resistance of Y2Si2O7 coatings against CMFAS [19]. In the present study, theYb2SiO5 and Yb2SiO5-YSZ ceramic composites were fabricated via the pressureless sintering process with different compositions of YSZ. The hot corrosion behavior and phase stability of the synthesized Yb2SiO5 and ceramic composite was investigated in the presence of CMAS at 1400 ºC.
2. Experimental Commercial powders of Yb2O3, (75 µm, Kojundo Chemical Laboratory Co., Ltd. Japan), SiO2, (10-20 nm, Evonik, Germany) and 8YSZ (10 µm, Merck, Germany) were used as starting materials. The synthesizing process of ytterbium monosilicate powder (Yb2SiO5) was described in our previous study [20]. In order to investigate the effect of 8YSZ on the hot corrosion performance of Yb2SiO5, three different types of Yb2SiO5/8YSZ composite with the 10, 20 and 30 wt.% of YSZ were prepared. The powder was mixed for 5 hours in the polyethylene vial with Al2O3 balls. The mixed powder was granulated by adding PVA and then was pressed at 300 MPa in 20 mm disc-shaped samples. Pressed samples were sintered in an atmospheric environment at 1600 °C for 10 hours with a heating rate of 5 °C/min in order to produce porous ceramic pellets. The pellets samples were named as Yb2SiO5, Yb2SiO5-10YSZ, Yb2SiO5-20YSZ, and Yb2SiO5-30YSZ, respectively. The corrosion performance of Yb2SiO5 and composite samples were investigated in the presence of the molten CMAS at 1400°C with various duration times of 4, 12, 24 and 48 h. The heating rate was set as 5 °C/min, and after hot corrosion, the specimens were cooled to room temperature in an ambient atmosphere. The effect of molten CMAS on the sample
microstructure was investigated using a scanning electron microscope (SEM, Mira3 Tescan) equipped with energy dispersive spectroscopy. Phase identification of the synthesized powders and hot corroded samples was carried out by the X-ray diffractometer analyzer (D8Advanced, Bruker), using monochromatic Cu–Kα radiation operating at 40 kV and 80 mA in the range of 20° to 80˚. Crystalline phases were determined by the Xpert High Score Plus software, utilizing the ICDD (International Centre for Diffraction Data) database. Mechanical properties (hardness and Young modulus) of the Yb2SiO5 and three types of Yb2SiO5-YSZ samples were measured by the microhardness and nano-indentation methods (Anton Paar NHT3).
3. Results & discussion 3.1. Characterization of sintered samples The XRD patterns of the sintered Yb2SiO5, Yb2SiO5-10YSZ, Yb2SiO5-20YSZ, and Yb2SiO530YSZ composite samples are presented in Fig. 1. The results indicate that the synthesized powders consisted of stoichiometric chemical components. In Fig. 1(a) only diffraction peaks corresponding to Yb2SiO5 (JCPDS 070-1918), Yb2Si2O7 (JCPDS 030-1441) and SiO2 (JCPDS 046-1242) were detected, indicating that crystalline silicate powders without the presence of impure phases were obtained. The monoclinic Yb2SiO5 was the main phase in the synthesized powder. The XRD pattern of the synthesized Yb2SiO5 was the same as the diffraction pattern obtained by Lu et al. [21]. In the case of sintered composite samples, Y2SiO5, Y2Si2O7, and Yb4Zr3O12 phases were detected. From the peak in Fig. 1(b, c, and d), it is clear that the Yb2SiO5 and ytterbium zirconium oxide (Yb4Zr3O12) are the main phases in the Yb2SiO5-10YSZ sample and by increasing the YSZ amount in the composite samples the intensity of Yb4Zr3O12 peak increased to the higher amount. According to the JCPDS file number 071-1023, the Yb4Zr3O12 spectrum can be indexed as the rhombohedral structure. The weak peaks of Y2SiO5 and Y2SiO7 were regarded as a side reaction of Y and Si during the sintering process at 1600˚C. In order to determine the composition of each sample, an EDS point analysis carried out. The results of the chemical composition of powders are shown in Table 1. Formation of the Yb4Zr3O12 phase can be proved by the atomic percentages of Yb, Zr and O. Also, the atomic percent of the elements in the EDS analysis confirmed the formation of Yb2SiO5.
3.2. Corrosion behavior of sintered Yb2SiO5 by CMAS
Fig. 2 shows the XRD patterns of the reaction layer of sintered Yb2SiO5 samples after hot corrosion at various time intervals. After 4, 12 and 24 h of the test, Yb2SiO5, Yb2Si2O7 and aluminum ytterbium oxide (Al5Yb3O12) were detected. The reaction of sintered Yb2SiO5 with the SiO2 and Al2O3 exist in the CMAS result in the formation of the ytterbium disilicate (Yb2Si2O7) and Al5Yb3O12 phases. Formation of these phases which is not undesirable for EBC was reported during hot corrosion investigation of Yb2SiO5 coatings by Lakiza et al. [22]. There are several kinds of literature using Yb2Si2O7 as the topcoat in EBC systems [2325]. Al5Yb3O12 is an intermediate phase with a garnet structure. As reported by Klemm and Fritsch [26], the materials with garnet structure exhibit a similar behavior like the rare earth silicates, and nearly dense Al5Yb3O12 layer protect the material from further corrosion. XRD pattern investigation of the corrosion scales formed on the Yb2SiO5 sample in Fig. 2d showed that after 48 h of hot corrosion, Ca3Al2(SiO4)3 and Ca2Mg(Si2O7) phases were formed in the reaction layer and the protective Yb2SiO5 and Yb2Si2O7 phases vanished completely. Exposure of Yb2SiO5 to a high concentration of corrosive components, in terms of Ca, for a long time at elevated temperature led to consuming of beneficial phases and formation of non-protective phases in the reaction layer. Fig. 3 shows the FE-SEM cross-sectional microstructures and the corresponding EDS analysis of the Yb2SiO5 samples after hot corrosion at 1400 °C for 4, 12, 24 and 48 h. It is clear from fig. 3 (a-b) that there is not any significant change in the microstructure in the short time corrosion and the reaction layer was observed after 24 h (Fig. 3d) of hot corrosion test due to the reaction of molten CMAS and Yb2SiO5. The elemental distribution of the Yb2SiO5 sample after 48 h of corrosion in molten CMAS is shown in Fig. 4. cross-sectional and The results show that the typical structures were saturated with Yb, Si, Al, Mg, O, and Ca. High Ca concentration on the reaction layer can confirm the degradation of Yb2SiO5 by the molten CMAS. The EDS analysis results of the reaction layer obtained in Fig 3a are summarized in table 2. The high concentration of Ca, Mg and Al in the point A can confirm the degradation process of the Yb2SiO5 at the top of the sample, EDS analysis of the point C showed no Ca, Mg and Al after 48 h of hot corrosion. The thickness of the reaction layer after hot corrosion for 48 h with CMAS was about 68 µm. The high thickness of the corrosion layer representing the severe consumption of sintered Yb2SiO5 by CMAS attack. 3.3. Corrosion behavior of Yb2SiO5-10YSZ
Fig. 5, shows the XRD patterns of the Yb2SiO5-10YSZ composite samples after hot corrosion at 1400°C in a presence of CMAS for 4, 12, 24 and 48 h. After 4 h of corrosion test in Fig. 5a, the Zr3Yb4O12 phase was obtained as the same as-sintered sample and Al5Yb3O12 and Ca3ZrSi2O9 phases were also detected due to the reaction between molten CMAS and solid phases in 10YSZ-Yb2SiO5 composite sample. By increasing the corrosion time (Fig. 5b-d) the relative intensities of the Zr3Yb4O12, Al5Yb3O12, and Ca3ZrSi2O9 peaks remain unchanged and small amount of Al5Y3O12, SiO2 and Yb2SiO7 phases have been observed. A comparison of XRD patterns of the sintered Yb2SiO5-10YSZ in Fig 1a and hot corroded samples in Fig. 6 shows that all of the Yb2SiO5 phases vanished during hot corrosion test and no reaction happened between molten CMAS and Yb4Zr3O12 phase and it was stable in all corrosion times. Therefore, it can be concluded that the CMAS corrosive agent cannot consume all of the Yb in the Yb2SiO5-10YSZ samples, whereas in the sintered Yb2SiO5 all Yb was consumed after 48 h of corrosion test. The cross-sectional backscatter micrographs of the Yb2SiO5-10YSZ composite samples during the corrosion test are shown in Fig. 6. It is clear that the reaction layer appeared after 4 h of hot corrosion test (fig. 6a). According to XRD results, it can be concluded the reaction of Yb2SiO5 and molten CMAS and formation of the Al5Yb3O12 and Ca3ZrSi2O9 is the main reason for the formation of the reaction layer. The reaction layer thickness increases rapidly at the first stage of the test and reaches about 40 µm after 12 h of corrosion, but at the higher time the growth rate of the reaction layer decrease and it reaches 62 µm after 48 h of the test. The corrosion product at the top of the composite samples can act as a barrier layer and prevents the infiltration of the molten CMAS into the beneath of the sample. As shown in Fig. 6c and 6d, the reaction layer becomes denser which means that less corrosion may happen for a long period. Fig. 7 shows the elemental map distribution of the Yb2SiO5-10YSZ composite sample after 42 h of corrosion at 1400°C. The result shows that the typical structures are saturated with Yb, Zr, Si, Al, Mg, Y, O, and Ca. The high concentration of Al and Ca at the top of the sample can confirm the existence of the reaction layer due to the formation of Ca3ZrSi2O9 and Al5Yb3O12 phases. The EDS analysis of the reaction layer selected in Fig. 7 (box A) are summarized in table 3. High Ca, Al in the reaction layer are in a good agreement with the XRD analysis of the reaction layer. The concentration of both Al and Ca in the Yb2SiO5-10YSZ composite sample is less than the Yb2SiO5 sample which obtained in table 3. On the other hand, it can be
concluded that the composite Yb2SiO5-10YSZ sample may show better corrosion performance in the presence of molten CMAS. 3.4. Corrosion behavior of Yb2SiO5-20YSZ The XRD patterns of the Yb2SiO5-20YSZ after hot corrosion at 1400°C for 4, 12, 24 and 48 h are shown in Fig. 8. Similar to the Yb2SiO5-10YSZ sample Zr3Yb4O12, Ca3ZrSi2O9 and Al5Yb3O12 are the main phases in all of the corrosion times. Comparison of XRD analysis in Fig 1c and Fig 9 clearly show the YbSiO5 phase completely vanished in the corrosion condition and there are not any significant changes in the Zr3Yb4O12 phases. Yb2Si2O7 was detected up to 24 h after 48 h Yb2Si2O7 peak was disappeared and changed to Zr3Yb4O12 and Ca3ZrSi2O9. The main peaks of sintered Yb2SiO5 in Fig. 1, was changed completely and compounds consist of Yb severely attacked by CMAS, whereas in Yb2SiO5-20YSZ the main peak (which corresponds to Zr3Yb4O12) remained unchanged after hot corrosion at 1400˚C for 48 h. Fig. 9 shows the cross-sectional microstructure of the Yb2SiO5-20YSZ composite samples after hot corrosion in the presence of the CMAS at 1400°C for 4, 12, 24 and 48 h. As shown in Fig. 9 (a) and (b), no recognizable reaction layer appeared after 4 h and 12 h of hot corrosion test, according to the XRD results it can be concluded that phase change after these times of hot corrosion may happen in a small area of the surface. The obvious reaction layer formed on the surface after 24 h (Fig. 9 c) and its thickness were increased after 48 h of the test. This dense layer can act as a barrier and play an important role to reduce the penetrating of molten CMAS into the sample. The barrier layer became denser after 48 h of corrosion test. When the corrosion operation stopped, a layer of Ca3ZrSi2O9, Zr3Yb4O12, and Al5Yb3O12 were formed which prevents corrosion of the lower parts of the Yb2SiO5-20YSZ composite samples. The elemental mapping analysis of the Yb2SiO5-20YSZ composite samples after 48 h of hot corrosion shows in Fig. 10. The resulting elemental maps demonstrate the uniform distributions for aluminum and magnesium and a gradient distribution for the calcium. The high concentration of Ca at the top of the sample can confirm the formation of the reaction layer and existence of the Ca3ZrSi2O9 phase, which detected by XRD analysis. The presence of the reaction layer by the formation of the phases contains a high amount of Ca which detected in the XRD results. Uniform distribution of the Yb, Si, Zr, Y, and O as the main elements of the sample can be observed in the maps. The thickness of the reaction layer was about 55 µm.
EDS analysis results carried out at point B in Fig. 10 was summarized in table 4. The results were well matched with that of XRD analysis. The amount of Ca and Mg is similar to the Yb2SiO5-10YSZ sample but lower than the Yb2SiO5 sample. The Al content increase in the Yb2SiO5-20YSZ sample because of the formation of high aluminum phases in the form of Al5Y3O12 and Al5Yb3O12. The reaction of infiltrates molten alumina with Y2O3 and Yb2O3 phases at high temperature lead to the formation of these phases. In composite samples, both Yb2O3 and Y2O3 react with molten Al2O3 and new phases formed in the reaction layer. Al5Y3O12 and Al5Yb3O12 phases can act as a barrier agent and reduce the infiltration of molten CMAS into the sample and the specimen showed more resistant to hot corrosion [22]. 3.5. Corrosion behavior of Yb2SiO5-30YSZ samples The XRD patterns of the Yb2SiO5-30YSZ composite sample after hot corrosion at 1400°C for 4, 12, 24 and 48 h are shown in Fig 11. The evidence of corrosion was observed only after 4 h of the test by the formation of Ca2Al2SiO7 and MgAl2O4 phases which detected in Fig. 11a. The intensity of the Zr3Yb4O12 phase was reduced by increasing the corrosion time to 48 h (Fig. 11d). No Yb2SiO5 peak was detected in all of the corrosion time; this phase changed to a complex phase during synthesis and hot corrosion test. Ca3ZrSi2O9 phase just detected after 3 h of hot corrosion and this phase was disappeared in more time of the test. In the case of Yb2SiO5-10YSZ and Yb2SiO5-20YSZ samples, the Ca3ZrSi2O9 phase was the main product of corrosion in all of the times. In other words, it seems less corrosion product was determined in the Yb2SiO5-30YSZ samples that mean it can select as a candidate for hightemperature application. Fig. 12 shows the cross-sectional microstructure of the Yb2SiO5-30YSZ samples after 4, 12, 24 and 48 h of hot corrosion at 1400°C in the presence of molten CMAS. It is clear from Fig 12a and 12b that no obvious and continuous reaction layer was obtained on the surface and it seems corrosion products formed in a small area of the surface. As the corrosion progresses to 48 h in Fig. 12d, the reaction layer formed on the surface by penetrating the molten CMAS into the sample; it seems the reaction layer thickness is smaller than other composite samples. According to the XRD results, the reaction layer is composed of the Al5Yb3O12 and Al5Y3O12 and Yb2Si2O7 phases. Fig. 13 shows the elemental mapping analysis of the Yb2SiO5-30YSZ composite samples after 48 h of hot corrosion. The results show that the typical structure was saturated with Yb, Zr, Si, Al, Mg, Y, O, and Ca. Uniform distribution of the Al and Mg was observed in the sample but Ca and Si concentrate on the top of the sample due to the formation of Ca3ZrSi2O9
and Ca2Al2SiO7 phases as a reaction layer. Another important distinction in comparison with the Yb2SiO5-20YSZ and Yb2SiO5-10YSZ is the greater distribution of the Si element, which shows the presence of more silicate compounds such as Ca2Al2SiO7 in the sample. The EDS analysis of points A and B in Fig. 13 was summarized in table 5. The amount of Ca, Al and Si in both points is less than Yb2SiO5-10YSZ and Yb2SiO5-20YSZ and Yb2SiO5 samples which confirm the less infiltration of molten species into the sample during hot corrosion. The reaction layer thickness was measured as 10 µm which is 5 orders smaller than other composite samples. The mechanical properties of Yb2SiO5 with various YSZ additions were measured by AntonPaar indentation using a Berkovich indenter. Fig. 14 presents the load-displacement curve and microhardness results of the Yb2SiO5 and three types of composite samples. the average amount of microhardness Vickers, hardness and the Young modulus are listed in table 6. No crack or delamination happened after the microhardness and nanoindentation measurement. it is clear that mechanical properties were improved by adding YSZ to the Yb2SiO5 and high hardness and Young modulus were achieved in the Yb2SiO5-20YSZ samples because of larger slope in loading and less depth of indenter penetration. The indenter displacement in all of the composite sintered samples is lower than the Yb2SiO5 sample and small displacement was achieved in the Yb2SiO5-20YSZ samples, which signifies that this sample is stronger than the other samples. better mechanical properties of composite samples can be attributed to the denser microstructure, and formation of the suitable liquid phase, during the sintering process as compared with that of the Yb2SiO5 sample. 3-6-Corrosion mechanisms of the Yb2SiO5 and Yb2SiO5-YSZ composite samples
the main mechanism of hot corrosion in TBC and EBC at the presence of CMAS is the infiltration of the molten CMAS into the ceramic compound at the temperature above 1200ºC. Besides the molten CMAS, any oxide would be unstable and will dissolve in it according to the oxide solubility and dissolving kinetics. The effective ceramic compound should be able to form a rapid crystallization between molten CMAS and dissolved oxide, in order to form refractory oxide phases to delay further infiltration of molten CMAS into the samples. In the TBCs composed of YSZ, the easy dissolving of Y2O3 in the molten CMAS lead to the destabilization of zirconia and cause to the phase changing of t-ZrO2→m-ZrO2. Large volume change occurs during this transformation leads to spalling and delamination in YSZ samples. in the case of EBCs, reactions which happened between the ceramic/CMAS
lead to the formation of crystalline silicate phases, these phases act as an infiltration barrier for molten CMAS and reduce the degradation mechanism. As reported by J.M. Drexler et al. [27], incorporation of Al and Ti ions and rare earth element such as Y, Ga, Yb, will change the glass composition of CMAS and facilitate the formation of easy crystallize phases such as CaAl2Si2O8
(anorthite),
Ca4Y6(SiO4)6O,
Ca2Gd8(SiO4)6O2
(apatite
phase),
and
Ca4Yb6(SiO4)6O (Yb- apatite) phases. Crystallization tendency of rare earth (RE) elements as apatites phase- in the presence of silicate glasses is a function of the RE ions and it decreases by decreasing the RE ion size [28, 29]. Fig. 15 shows the variation in the reaction layer thickness of Yb2SiO5 and three types of composite samples after 48 h of hot corrosion test in the presence of CMAS at 1400 ºC. It is clear that YSZ addition to the Yb2SiO5 sample reduces the reaction layer thickness, the minimum thickness was achieved in the Yb2SiO5-30YSZ samples. According to the literature [18], the reaction layer thickness of the sintered Yb2SiO5 was about 121 µm after 48 of corrosion in the presence of CMAS which implies Yb2SiO5 had been consumed rapidly by CMAS. The smaller size of the Yb3+ cation relative to Y3+ delay the apatite and barrier phase formation in the Yb2SiO5 sintered sample and higher reaction layer develops in these samples. This is in agreement with the reaction layer thickness which observed in Yb2SiO5 and Yb2SiO5-YSZ composite samples. The main reasons for the better corrosion resistance of the composite samples are the formation of Yb4Zr3O12 and Y2Zr2O7 phases during the sintering process. Easy dissolution of Y in the glassy CMAS promotes the crystallization process of the barrier oxide phases such as Ca2Al2SiO7, Ca3ZrSi2O9, and generation of dense Al5Yb3O12 and Al5Y3O12 garnet phases in the reaction layer, that form a barrier layer and reduce the further infiltration of molten CMAS.
4. Conclusions In this work, three different compositions of 8YSZ/Yb2SiO5 were made to investigate the effects of CMAS corrosive agent at 1400 ˚C and the results were compared with sintered Yb2SiO5. Based on the results, adding of YSZ lead to improvement in mechanical properties and Yb2SiO5-30YSZ was selected as an optimal combination because of the lower reaction layer thickness in contrast to other samples and the formation of the garnet phase (Al5Yb3O12) along with Ca3ZrSi2O9 in this sample formed a protective layer that prevented corrosion of
the samples. All of the composite samples showed a smaller reaction layer in contrast to the Yb2SiO5 samples which mean better hot corrosion resistance of composite samples. So they can be used as an alternative to Yb2SiO5 in an environmental barrier coating system. Another source responsible for EBC degradation is water vapor. Investigating the durability of 8YSZ-Yb2SiO5 against water vapor at high temperatures is a future topic to be studied. [1]
[2]
[3]
[4]
[5]
[6] [7]
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[11] [12] [13]
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Table caption: Table 1. EDS analysis of the sintered samples (wt.% and at.%). Table 2. Chemical composition (at.%) of points A, B and C shown in Fig. 4. Table 3. Chemical composition (at.%) of the reaction layer determined as points A in Fig. 7. Table 4. Chemical composition (at.%) of points B has shown in Fig. 11. Table 5. Chemical composition (at.%) of points A and B as shown in Fig. 14. Table 6. Mechanical properties include, Young modulus and hardness of the Yb2SiO5 and three types of composite sample.
Table 1. Yb2SiO5
10YSZ-Yb2SiO5
20YSZ-Yb2SiO5
30YSZ-Yb2SiO5
wt.%
at.%
wt.%
at.%
wt.%
at.%
wt.%
at.%
Yb
64.37
14.97
62.35
16.87
48.71
14.46
40.65
10.76
Si
4.22
6.05
6.51
10.86
3.04
5.21
4.22
6.43
Y
-
-
0.78
0.11
1.46
0.63
2.1
0.88
Zr
-
-
6.23
1.56
23.88
10.79
26.51
11.02
O
31.41
78.99
24.13
70.60
22.91
68.92
26.52
70.91
Total
100%
100%
100%
100%
Table 2. Elements
Ca
Mg
Al
O
Si
Yb
Point A
12.30
0.58
11.39
57.72
5.71
12.3
Point B
3.19
0.18
8.89
67.84
5.28
14.62
Point C
0
0.01
0
76.17
7.04
16.78
Table 3. Elements
Ca
Mg
Al
Si
Y
Zr
Yb
O
Point A
8.57
0.01
8.82
9.81
0.04
1.15
11.18
60.42
Table 4. Elements
Ca
Mg
Al
Si
Y
Zr
Yb
O
Point A
8.19
0.01
16.14
8.28
0.06
1.85
12.07
53.40
Table 5. Elements
Ca
Mg
Al
Si
Y
Zr
Yb
O
Point A
5.46
0.65
7.80
8.00
0.66
6.86
11.34
59.23
Point B
2.99
0.64
8.53
6.36
0.90
10.23
9.03
61.32
Table 6 Sample Yb2SiO5 Yb2SiO5-10YSZ Yb2SiO5-20YSZ Yb2SiO5-30YSZ
Vickers hardness (HV) 252 480 670 335
Young modulus (GPa) 417 495 546 512
Hardness- nano-indentation (GPa) 2.6 6.7 7.8 6.1
Figure Caption: Fig. 1. XRD diffraction patterns of (a)-pure Yb2SiO5 powder, (b)- Yb2SiO5-10YSZ, (c)- Yb2SiO5-20YSZ and (d)- Yb2SiO5-30YSZ composite samples. Fig. 2. XRD diffraction patterns of sintered Yb2SiO5 after hot corrosion at 1400 ˚C for (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h. Fig. 3. Cross-sectional BSE images of the Yb2SiO5 samples after, a)- 4 h, b)- 12 h, c)- 24 h and d)- 48 h of corrosion test in a presence of CMAS at 1400°C Fig. 4. Cross-sectional elemental maps of the Yb2SiO5 samples after 48 h of hot corrosion test. Fig. 5. XRD diffraction patterns of Yb2SiO5-10%YSZ after hot corrosion at 1400 ˚C for (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h Fig. 6. Cross-sectional BSE images of the Yb2SiO5-10%YSZ samples after (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h of hot corrosion test in the presence of CMAS at 1400°C. Fig. 7. Elemental distribution of the Yb2SiO5 -10%YSZ sample after 48 h of hot corrosion test. Fig. 8. XRD diffraction patterns of Yb2SiO5-20%YSZ after (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h of hot corrosion at 1400 ˚C. Fig. 9. Cross-sectional BSE images of the Yb2SiO5-20%YSZ samples after (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h of hot corrosion test in the presence of CMAS at 1400°C. Fig. 10. Cross-sectional elemental distribution of the Yb2SiO5 -20%YSZ sample after 48 h of hot corrosion test. Fig. 11. XRD diffraction patterns of Yb2SiO5-30%YSZ after (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h of hot corrosion at 1400 ˚C. Fig. 12. Cross-sectional BSE images of the Yb2SiO5-30%YSZ samples after (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h of hot corrosion test in the presence of CMAS at 1400°C. Fig. 13. Cross-sectional elemental distribution of the Yb2SiO5 -20%YSZ sample after 48 h of hot corrosion test. Fig. 14. a)- micro-hardness results and b)- force-displacement curve of the Yb2SiO5 and Yb2SiO5/YSZ composite samples obtained by nano-indentation test. Fig. 15. Thickness of the reaction layer of the Yb2SiO5 and Yb2SiO5/YSZ composite samples.
Fig. 1.
Fig. 2.
a
b
c
c Reaction layer
Reaction layer
Fig. 3.
A B
C
Reaction layer
Fig. 4
Fig. 5.
b
a
Reaction layer
Reaction layer
c
d Reaction layer Reaction layer
Fig. 6
A
Reaction layer
Fig. 7
Fig. 8.
a
b
c
d Reaction layer
Reaction layer
Fig. 9.
A
Reaction layer
Fig. 10.
Fig. 11.
a
b Reaction layer
c
d
Fig. 12
A
B
Reaction layer
Fig. 13.
Fig. 14.
Fig. 15.
Highlight •
Yb2SiO5/YSZ composite pellets were prepared using a pressureless sintering process.
•
Hot corrosion of samples was investigated in the presence of the CMAS at 1400 ºC.
•
The reaction layer thickness was reduced by increasing the amout of YZS.
•
Ca2Al2SiO7, Ca3ZrSi2O9, and garnet phases were formed on the composite samples.
Conflict of Interest:
The authors whose names are listed immediately below declare that there is no conflict of interest regarding the publication of this article. Author names: Ali Abedi Nieai Majid Mohammadi Maryam Shojaie-Bahaabad
S0254-0584(19)31406-3
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122596
Reference:
MAC 122596
To appear in:
Materials Chemistry and Physics
Received Date: 8 September 2019 Revised Date:
9 December 2019
Accepted Date: 30 December 2019
Please cite this article as: A.A. Nieai, M. Mohammadi, M. Shojaie-Bahaabad, Hot corrosion behavior of calcium magnesium aluminosilicate (CMAS) on Yb2SiO5-8YSZ composite as a candidate for environmental barrier coatings, Materials Chemistry and Physics (2020), doi: https://doi.org/10.1016/ j.matchemphys.2019.122596. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Hot corrosion behavior of calcium magnesium aluminosilicate (CMAS) on Yb2SiO58YSZ composite as a candidate for environmental barrier coatings Ali Abedi Nieai1, Majid Mohammadi1*, Maryam Shojaie-Bahaabad1 Faculty of Chemical and Materials Engineering, Shahrood University of Technology, Shahrood, Iran. * [email protected]
Abstract Dense Yb2SiO5 and three types of Yb2SiO5-YSZ composite specimens, as an environmental barrier coating, were prepared using pressureless sintering process at 1600 ºC for 10 h. Hot corrosion behaviors of the sintered specimens were investigated in the presence of calcium–magnesiumaluminosilicate (CMAS) at 1400 ºC for 4, 8, 24 and 48 h. Phase change identification, cross-sectional microstructure, reaction layer thickness, and elemental distribution, of the different samples during hot corrosion was investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) equipped by EDS technique. Mechanical properties of synthesized samples include elastic modulus, hardness and microhardness were measured by the nanoindentation method. The results showed that the reaction layer of all composite samples was smaller than that of the Yb2SiO5 sample and the formation of dense phases such as Al5Yb3O12 and Al5Y3O12 phases in the reaction layer can reduce the infiltration of molten CMAS into the samples, which mean better hot corrosion resistance of composite samples. Yb2SiO5-30wt.%YSZ composite sample showed better performance in contrast to the composite samples because of the high concentration of dissolved Y3+ ion in the glassy CMAS which promotes the formation of the barrier oxide phases (Ca2Al2SiO7, Ca3ZrSi2O9) and protective garnet phases (Al5Yb3O12) in the reaction layer.
Keywords: Hot Corrosion, Environmental barrier coatings, Yb2SiO5, YSZ, calcium magnesium aluminosilicate.
1. Introduction High-temperature components experience harsh conditions such as hot corrosion, oxidation and thermal shock during services. Nickel and cobalt-base superalloys coated with thermal barrier
coatings (TBC) were used as effective materials in gas turbine components for decades [1-5]. The demands for higher operating temperatures and efficiency, reduced the emissions of greenhouse gases (CO2 and NOx), and the need for more axial loads, led to the development of new materials. In fact, superalloys had reached their highest thermal efficiency at high temperatures, but the new generation of gas turbines required a higher working temperature
[6, 7]. Ceramic matrix composites (CMCs) such as SiC/Cf, SiC and Si3N4 appeared as a good candidate to develop as a novel class of high-temperature materials [8-10]. These materials are used in radomes, rocket motors, motor nose, central wing boxes, airplane wings, pressure vessels, engine chambers, floor beams, long cones, etc. CMCs are attractive materials in the aerospace industry, because of their ability at high temperatures. The major advantage of CMC is its lightweight. The lightweight of CMCs decreases fuel consumption increases speed and reduces greenhouse emissions. It has been reported that 30% of the total weight of the turbine blades can be reduced by using ceramic materials. Despite all these benefits, the main problem with the use of CMCs in gas turbines components is the surface recession due to the reaction of silica and water vapor at high temperature in the combustion chamber. The SiC oxidation process is shown in reaction [11]: SiC + 1.5 O2 (g) → SiO2 + CO(g)
(1)
In the existence of water vapor and oxygen in the working condition, SiC will oxidize based on the reactions (1) and (2): SiC + 3H2O (g) → SiO2 + 3H2(g) + CO(g)
(2)
The surface recession of CMCs depends on the gas temperature, gas velocity and as well as working pressure, at the constant gas velocity the rate of recession will increase sharply with increasing temperature and pressure [12, 13]. Protection of CMCs against water vapor corrosion or recession was done by applying environmental barrier coatings (EBCs) [11]. Protective EBCs are used to reduce the oxidation of SiC and limit the formation of SiO2 at the substrate/ coating interface [14]. The materials used as EBC should have low thermal conductivity, limited oxygen diffusivity, high melting point, chemical stability, suitable coefficient of thermal expansion (CTE), and good compatibility with the CMCs substrate [15]. RE-silicates (RE = rare earth elements: Gd, Yb, Y, La) are used widely as EBC materials. These materials exhibit low volatilities and good phase stability in ultra-high temperature combustion environments. Generally, Ytterbium monosilicate (Yb2SiO5) is the most used material in EBC systems; this compound possesses a unique combination of properties such as good corrosion resistance, low volatility, excellent phase stability, superior chemical compatibility and closes CTE to the SiC and Si3N4 substrate [16]. Richards and Wadley applied multifunctional EBC on the CMC substrates. They used Yb2SiO5 as the top layer to protect the substrate against hot corrosion [17]. They showed the formation of cracks and pores in the multifunctional coating facilitate the oxygen transfer
from the top layer to the substrate and reduce the corrosion resistance of the coating system [17]. Jang et al. [18] were investigated the high-temperature corrosion behavior of sintered ytterbium monosilicate in the presence of calcium-magnesium-aluminosilicate (CMAS) at 1400 ˚C for 2, 12 and 48 hours. The results confirmed the weak corrosion resistance of Yb2SiO5 against CMAS. Lee et al. [16], investigated the recession of SiC, mullite, and 8YSZ in a high-pressure burner rig test. The porous alumina layer crumbled and spalled readily, but 8YSZ did not show any weight loss which indicates the superior recession resistance and excellent stability of 8YSZ in water vapor. Poerschke et al. [19] investigated the reaction of yttrium disilicate environmental barrier coatings with molten calcium-magnesium-iron alumino-silicate (CMFAS). The results showed weak corrosion resistance of Y2Si2O7 coatings against CMFAS [19]. In the present study, theYb2SiO5 and Yb2SiO5-YSZ ceramic composites were fabricated via the pressureless sintering process with different compositions of YSZ. The hot corrosion behavior and phase stability of the synthesized Yb2SiO5 and ceramic composite was investigated in the presence of CMAS at 1400 ºC.
2. Experimental Commercial powders of Yb2O3, (75 µm, Kojundo Chemical Laboratory Co., Ltd. Japan), SiO2, (10-20 nm, Evonik, Germany) and 8YSZ (10 µm, Merck, Germany) were used as starting materials. The synthesizing process of ytterbium monosilicate powder (Yb2SiO5) was described in our previous study [20]. In order to investigate the effect of 8YSZ on the hot corrosion performance of Yb2SiO5, three different types of Yb2SiO5/8YSZ composite with the 10, 20 and 30 wt.% of YSZ were prepared. The powder was mixed for 5 hours in the polyethylene vial with Al2O3 balls. The mixed powder was granulated by adding PVA and then was pressed at 300 MPa in 20 mm disc-shaped samples. Pressed samples were sintered in an atmospheric environment at 1600 °C for 10 hours with a heating rate of 5 °C/min in order to produce porous ceramic pellets. The pellets samples were named as Yb2SiO5, Yb2SiO5-10YSZ, Yb2SiO5-20YSZ, and Yb2SiO5-30YSZ, respectively. The corrosion performance of Yb2SiO5 and composite samples were investigated in the presence of the molten CMAS at 1400°C with various duration times of 4, 12, 24 and 48 h. The heating rate was set as 5 °C/min, and after hot corrosion, the specimens were cooled to room temperature in an ambient atmosphere. The effect of molten CMAS on the sample
microstructure was investigated using a scanning electron microscope (SEM, Mira3 Tescan) equipped with energy dispersive spectroscopy. Phase identification of the synthesized powders and hot corroded samples was carried out by the X-ray diffractometer analyzer (D8Advanced, Bruker), using monochromatic Cu–Kα radiation operating at 40 kV and 80 mA in the range of 20° to 80˚. Crystalline phases were determined by the Xpert High Score Plus software, utilizing the ICDD (International Centre for Diffraction Data) database. Mechanical properties (hardness and Young modulus) of the Yb2SiO5 and three types of Yb2SiO5-YSZ samples were measured by the microhardness and nano-indentation methods (Anton Paar NHT3).
3. Results & discussion 3.1. Characterization of sintered samples The XRD patterns of the sintered Yb2SiO5, Yb2SiO5-10YSZ, Yb2SiO5-20YSZ, and Yb2SiO530YSZ composite samples are presented in Fig. 1. The results indicate that the synthesized powders consisted of stoichiometric chemical components. In Fig. 1(a) only diffraction peaks corresponding to Yb2SiO5 (JCPDS 070-1918), Yb2Si2O7 (JCPDS 030-1441) and SiO2 (JCPDS 046-1242) were detected, indicating that crystalline silicate powders without the presence of impure phases were obtained. The monoclinic Yb2SiO5 was the main phase in the synthesized powder. The XRD pattern of the synthesized Yb2SiO5 was the same as the diffraction pattern obtained by Lu et al. [21]. In the case of sintered composite samples, Y2SiO5, Y2Si2O7, and Yb4Zr3O12 phases were detected. From the peak in Fig. 1(b, c, and d), it is clear that the Yb2SiO5 and ytterbium zirconium oxide (Yb4Zr3O12) are the main phases in the Yb2SiO5-10YSZ sample and by increasing the YSZ amount in the composite samples the intensity of Yb4Zr3O12 peak increased to the higher amount. According to the JCPDS file number 071-1023, the Yb4Zr3O12 spectrum can be indexed as the rhombohedral structure. The weak peaks of Y2SiO5 and Y2SiO7 were regarded as a side reaction of Y and Si during the sintering process at 1600˚C. In order to determine the composition of each sample, an EDS point analysis carried out. The results of the chemical composition of powders are shown in Table 1. Formation of the Yb4Zr3O12 phase can be proved by the atomic percentages of Yb, Zr and O. Also, the atomic percent of the elements in the EDS analysis confirmed the formation of Yb2SiO5.
3.2. Corrosion behavior of sintered Yb2SiO5 by CMAS
Fig. 2 shows the XRD patterns of the reaction layer of sintered Yb2SiO5 samples after hot corrosion at various time intervals. After 4, 12 and 24 h of the test, Yb2SiO5, Yb2Si2O7 and aluminum ytterbium oxide (Al5Yb3O12) were detected. The reaction of sintered Yb2SiO5 with the SiO2 and Al2O3 exist in the CMAS result in the formation of the ytterbium disilicate (Yb2Si2O7) and Al5Yb3O12 phases. Formation of these phases which is not undesirable for EBC was reported during hot corrosion investigation of Yb2SiO5 coatings by Lakiza et al. [22]. There are several kinds of literature using Yb2Si2O7 as the topcoat in EBC systems [2325]. Al5Yb3O12 is an intermediate phase with a garnet structure. As reported by Klemm and Fritsch [26], the materials with garnet structure exhibit a similar behavior like the rare earth silicates, and nearly dense Al5Yb3O12 layer protect the material from further corrosion. XRD pattern investigation of the corrosion scales formed on the Yb2SiO5 sample in Fig. 2d showed that after 48 h of hot corrosion, Ca3Al2(SiO4)3 and Ca2Mg(Si2O7) phases were formed in the reaction layer and the protective Yb2SiO5 and Yb2Si2O7 phases vanished completely. Exposure of Yb2SiO5 to a high concentration of corrosive components, in terms of Ca, for a long time at elevated temperature led to consuming of beneficial phases and formation of non-protective phases in the reaction layer. Fig. 3 shows the FE-SEM cross-sectional microstructures and the corresponding EDS analysis of the Yb2SiO5 samples after hot corrosion at 1400 °C for 4, 12, 24 and 48 h. It is clear from fig. 3 (a-b) that there is not any significant change in the microstructure in the short time corrosion and the reaction layer was observed after 24 h (Fig. 3d) of hot corrosion test due to the reaction of molten CMAS and Yb2SiO5. The elemental distribution of the Yb2SiO5 sample after 48 h of corrosion in molten CMAS is shown in Fig. 4. cross-sectional and The results show that the typical structures were saturated with Yb, Si, Al, Mg, O, and Ca. High Ca concentration on the reaction layer can confirm the degradation of Yb2SiO5 by the molten CMAS. The EDS analysis results of the reaction layer obtained in Fig 3a are summarized in table 2. The high concentration of Ca, Mg and Al in the point A can confirm the degradation process of the Yb2SiO5 at the top of the sample, EDS analysis of the point C showed no Ca, Mg and Al after 48 h of hot corrosion. The thickness of the reaction layer after hot corrosion for 48 h with CMAS was about 68 µm. The high thickness of the corrosion layer representing the severe consumption of sintered Yb2SiO5 by CMAS attack. 3.3. Corrosion behavior of Yb2SiO5-10YSZ
Fig. 5, shows the XRD patterns of the Yb2SiO5-10YSZ composite samples after hot corrosion at 1400°C in a presence of CMAS for 4, 12, 24 and 48 h. After 4 h of corrosion test in Fig. 5a, the Zr3Yb4O12 phase was obtained as the same as-sintered sample and Al5Yb3O12 and Ca3ZrSi2O9 phases were also detected due to the reaction between molten CMAS and solid phases in 10YSZ-Yb2SiO5 composite sample. By increasing the corrosion time (Fig. 5b-d) the relative intensities of the Zr3Yb4O12, Al5Yb3O12, and Ca3ZrSi2O9 peaks remain unchanged and small amount of Al5Y3O12, SiO2 and Yb2SiO7 phases have been observed. A comparison of XRD patterns of the sintered Yb2SiO5-10YSZ in Fig 1a and hot corroded samples in Fig. 6 shows that all of the Yb2SiO5 phases vanished during hot corrosion test and no reaction happened between molten CMAS and Yb4Zr3O12 phase and it was stable in all corrosion times. Therefore, it can be concluded that the CMAS corrosive agent cannot consume all of the Yb in the Yb2SiO5-10YSZ samples, whereas in the sintered Yb2SiO5 all Yb was consumed after 48 h of corrosion test. The cross-sectional backscatter micrographs of the Yb2SiO5-10YSZ composite samples during the corrosion test are shown in Fig. 6. It is clear that the reaction layer appeared after 4 h of hot corrosion test (fig. 6a). According to XRD results, it can be concluded the reaction of Yb2SiO5 and molten CMAS and formation of the Al5Yb3O12 and Ca3ZrSi2O9 is the main reason for the formation of the reaction layer. The reaction layer thickness increases rapidly at the first stage of the test and reaches about 40 µm after 12 h of corrosion, but at the higher time the growth rate of the reaction layer decrease and it reaches 62 µm after 48 h of the test. The corrosion product at the top of the composite samples can act as a barrier layer and prevents the infiltration of the molten CMAS into the beneath of the sample. As shown in Fig. 6c and 6d, the reaction layer becomes denser which means that less corrosion may happen for a long period. Fig. 7 shows the elemental map distribution of the Yb2SiO5-10YSZ composite sample after 42 h of corrosion at 1400°C. The result shows that the typical structures are saturated with Yb, Zr, Si, Al, Mg, Y, O, and Ca. The high concentration of Al and Ca at the top of the sample can confirm the existence of the reaction layer due to the formation of Ca3ZrSi2O9 and Al5Yb3O12 phases. The EDS analysis of the reaction layer selected in Fig. 7 (box A) are summarized in table 3. High Ca, Al in the reaction layer are in a good agreement with the XRD analysis of the reaction layer. The concentration of both Al and Ca in the Yb2SiO5-10YSZ composite sample is less than the Yb2SiO5 sample which obtained in table 3. On the other hand, it can be
concluded that the composite Yb2SiO5-10YSZ sample may show better corrosion performance in the presence of molten CMAS. 3.4. Corrosion behavior of Yb2SiO5-20YSZ The XRD patterns of the Yb2SiO5-20YSZ after hot corrosion at 1400°C for 4, 12, 24 and 48 h are shown in Fig. 8. Similar to the Yb2SiO5-10YSZ sample Zr3Yb4O12, Ca3ZrSi2O9 and Al5Yb3O12 are the main phases in all of the corrosion times. Comparison of XRD analysis in Fig 1c and Fig 9 clearly show the YbSiO5 phase completely vanished in the corrosion condition and there are not any significant changes in the Zr3Yb4O12 phases. Yb2Si2O7 was detected up to 24 h after 48 h Yb2Si2O7 peak was disappeared and changed to Zr3Yb4O12 and Ca3ZrSi2O9. The main peaks of sintered Yb2SiO5 in Fig. 1, was changed completely and compounds consist of Yb severely attacked by CMAS, whereas in Yb2SiO5-20YSZ the main peak (which corresponds to Zr3Yb4O12) remained unchanged after hot corrosion at 1400˚C for 48 h. Fig. 9 shows the cross-sectional microstructure of the Yb2SiO5-20YSZ composite samples after hot corrosion in the presence of the CMAS at 1400°C for 4, 12, 24 and 48 h. As shown in Fig. 9 (a) and (b), no recognizable reaction layer appeared after 4 h and 12 h of hot corrosion test, according to the XRD results it can be concluded that phase change after these times of hot corrosion may happen in a small area of the surface. The obvious reaction layer formed on the surface after 24 h (Fig. 9 c) and its thickness were increased after 48 h of the test. This dense layer can act as a barrier and play an important role to reduce the penetrating of molten CMAS into the sample. The barrier layer became denser after 48 h of corrosion test. When the corrosion operation stopped, a layer of Ca3ZrSi2O9, Zr3Yb4O12, and Al5Yb3O12 were formed which prevents corrosion of the lower parts of the Yb2SiO5-20YSZ composite samples. The elemental mapping analysis of the Yb2SiO5-20YSZ composite samples after 48 h of hot corrosion shows in Fig. 10. The resulting elemental maps demonstrate the uniform distributions for aluminum and magnesium and a gradient distribution for the calcium. The high concentration of Ca at the top of the sample can confirm the formation of the reaction layer and existence of the Ca3ZrSi2O9 phase, which detected by XRD analysis. The presence of the reaction layer by the formation of the phases contains a high amount of Ca which detected in the XRD results. Uniform distribution of the Yb, Si, Zr, Y, and O as the main elements of the sample can be observed in the maps. The thickness of the reaction layer was about 55 µm.
EDS analysis results carried out at point B in Fig. 10 was summarized in table 4. The results were well matched with that of XRD analysis. The amount of Ca and Mg is similar to the Yb2SiO5-10YSZ sample but lower than the Yb2SiO5 sample. The Al content increase in the Yb2SiO5-20YSZ sample because of the formation of high aluminum phases in the form of Al5Y3O12 and Al5Yb3O12. The reaction of infiltrates molten alumina with Y2O3 and Yb2O3 phases at high temperature lead to the formation of these phases. In composite samples, both Yb2O3 and Y2O3 react with molten Al2O3 and new phases formed in the reaction layer. Al5Y3O12 and Al5Yb3O12 phases can act as a barrier agent and reduce the infiltration of molten CMAS into the sample and the specimen showed more resistant to hot corrosion [22]. 3.5. Corrosion behavior of Yb2SiO5-30YSZ samples The XRD patterns of the Yb2SiO5-30YSZ composite sample after hot corrosion at 1400°C for 4, 12, 24 and 48 h are shown in Fig 11. The evidence of corrosion was observed only after 4 h of the test by the formation of Ca2Al2SiO7 and MgAl2O4 phases which detected in Fig. 11a. The intensity of the Zr3Yb4O12 phase was reduced by increasing the corrosion time to 48 h (Fig. 11d). No Yb2SiO5 peak was detected in all of the corrosion time; this phase changed to a complex phase during synthesis and hot corrosion test. Ca3ZrSi2O9 phase just detected after 3 h of hot corrosion and this phase was disappeared in more time of the test. In the case of Yb2SiO5-10YSZ and Yb2SiO5-20YSZ samples, the Ca3ZrSi2O9 phase was the main product of corrosion in all of the times. In other words, it seems less corrosion product was determined in the Yb2SiO5-30YSZ samples that mean it can select as a candidate for hightemperature application. Fig. 12 shows the cross-sectional microstructure of the Yb2SiO5-30YSZ samples after 4, 12, 24 and 48 h of hot corrosion at 1400°C in the presence of molten CMAS. It is clear from Fig 12a and 12b that no obvious and continuous reaction layer was obtained on the surface and it seems corrosion products formed in a small area of the surface. As the corrosion progresses to 48 h in Fig. 12d, the reaction layer formed on the surface by penetrating the molten CMAS into the sample; it seems the reaction layer thickness is smaller than other composite samples. According to the XRD results, the reaction layer is composed of the Al5Yb3O12 and Al5Y3O12 and Yb2Si2O7 phases. Fig. 13 shows the elemental mapping analysis of the Yb2SiO5-30YSZ composite samples after 48 h of hot corrosion. The results show that the typical structure was saturated with Yb, Zr, Si, Al, Mg, Y, O, and Ca. Uniform distribution of the Al and Mg was observed in the sample but Ca and Si concentrate on the top of the sample due to the formation of Ca3ZrSi2O9
and Ca2Al2SiO7 phases as a reaction layer. Another important distinction in comparison with the Yb2SiO5-20YSZ and Yb2SiO5-10YSZ is the greater distribution of the Si element, which shows the presence of more silicate compounds such as Ca2Al2SiO7 in the sample. The EDS analysis of points A and B in Fig. 13 was summarized in table 5. The amount of Ca, Al and Si in both points is less than Yb2SiO5-10YSZ and Yb2SiO5-20YSZ and Yb2SiO5 samples which confirm the less infiltration of molten species into the sample during hot corrosion. The reaction layer thickness was measured as 10 µm which is 5 orders smaller than other composite samples. The mechanical properties of Yb2SiO5 with various YSZ additions were measured by AntonPaar indentation using a Berkovich indenter. Fig. 14 presents the load-displacement curve and microhardness results of the Yb2SiO5 and three types of composite samples. the average amount of microhardness Vickers, hardness and the Young modulus are listed in table 6. No crack or delamination happened after the microhardness and nanoindentation measurement. it is clear that mechanical properties were improved by adding YSZ to the Yb2SiO5 and high hardness and Young modulus were achieved in the Yb2SiO5-20YSZ samples because of larger slope in loading and less depth of indenter penetration. The indenter displacement in all of the composite sintered samples is lower than the Yb2SiO5 sample and small displacement was achieved in the Yb2SiO5-20YSZ samples, which signifies that this sample is stronger than the other samples. better mechanical properties of composite samples can be attributed to the denser microstructure, and formation of the suitable liquid phase, during the sintering process as compared with that of the Yb2SiO5 sample. 3-6-Corrosion mechanisms of the Yb2SiO5 and Yb2SiO5-YSZ composite samples
the main mechanism of hot corrosion in TBC and EBC at the presence of CMAS is the infiltration of the molten CMAS into the ceramic compound at the temperature above 1200ºC. Besides the molten CMAS, any oxide would be unstable and will dissolve in it according to the oxide solubility and dissolving kinetics. The effective ceramic compound should be able to form a rapid crystallization between molten CMAS and dissolved oxide, in order to form refractory oxide phases to delay further infiltration of molten CMAS into the samples. In the TBCs composed of YSZ, the easy dissolving of Y2O3 in the molten CMAS lead to the destabilization of zirconia and cause to the phase changing of t-ZrO2→m-ZrO2. Large volume change occurs during this transformation leads to spalling and delamination in YSZ samples. in the case of EBCs, reactions which happened between the ceramic/CMAS
lead to the formation of crystalline silicate phases, these phases act as an infiltration barrier for molten CMAS and reduce the degradation mechanism. As reported by J.M. Drexler et al. [27], incorporation of Al and Ti ions and rare earth element such as Y, Ga, Yb, will change the glass composition of CMAS and facilitate the formation of easy crystallize phases such as CaAl2Si2O8
(anorthite),
Ca4Y6(SiO4)6O,
Ca2Gd8(SiO4)6O2
(apatite
phase),
and
Ca4Yb6(SiO4)6O (Yb- apatite) phases. Crystallization tendency of rare earth (RE) elements as apatites phase- in the presence of silicate glasses is a function of the RE ions and it decreases by decreasing the RE ion size [28, 29]. Fig. 15 shows the variation in the reaction layer thickness of Yb2SiO5 and three types of composite samples after 48 h of hot corrosion test in the presence of CMAS at 1400 ºC. It is clear that YSZ addition to the Yb2SiO5 sample reduces the reaction layer thickness, the minimum thickness was achieved in the Yb2SiO5-30YSZ samples. According to the literature [18], the reaction layer thickness of the sintered Yb2SiO5 was about 121 µm after 48 of corrosion in the presence of CMAS which implies Yb2SiO5 had been consumed rapidly by CMAS. The smaller size of the Yb3+ cation relative to Y3+ delay the apatite and barrier phase formation in the Yb2SiO5 sintered sample and higher reaction layer develops in these samples. This is in agreement with the reaction layer thickness which observed in Yb2SiO5 and Yb2SiO5-YSZ composite samples. The main reasons for the better corrosion resistance of the composite samples are the formation of Yb4Zr3O12 and Y2Zr2O7 phases during the sintering process. Easy dissolution of Y in the glassy CMAS promotes the crystallization process of the barrier oxide phases such as Ca2Al2SiO7, Ca3ZrSi2O9, and generation of dense Al5Yb3O12 and Al5Y3O12 garnet phases in the reaction layer, that form a barrier layer and reduce the further infiltration of molten CMAS.
4. Conclusions In this work, three different compositions of 8YSZ/Yb2SiO5 were made to investigate the effects of CMAS corrosive agent at 1400 ˚C and the results were compared with sintered Yb2SiO5. Based on the results, adding of YSZ lead to improvement in mechanical properties and Yb2SiO5-30YSZ was selected as an optimal combination because of the lower reaction layer thickness in contrast to other samples and the formation of the garnet phase (Al5Yb3O12) along with Ca3ZrSi2O9 in this sample formed a protective layer that prevented corrosion of
the samples. All of the composite samples showed a smaller reaction layer in contrast to the Yb2SiO5 samples which mean better hot corrosion resistance of composite samples. So they can be used as an alternative to Yb2SiO5 in an environmental barrier coating system. Another source responsible for EBC degradation is water vapor. Investigating the durability of 8YSZ-Yb2SiO5 against water vapor at high temperatures is a future topic to be studied. [1]
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Table caption: Table 1. EDS analysis of the sintered samples (wt.% and at.%). Table 2. Chemical composition (at.%) of points A, B and C shown in Fig. 4. Table 3. Chemical composition (at.%) of the reaction layer determined as points A in Fig. 7. Table 4. Chemical composition (at.%) of points B has shown in Fig. 11. Table 5. Chemical composition (at.%) of points A and B as shown in Fig. 14. Table 6. Mechanical properties include, Young modulus and hardness of the Yb2SiO5 and three types of composite sample.
Table 1. Yb2SiO5
10YSZ-Yb2SiO5
20YSZ-Yb2SiO5
30YSZ-Yb2SiO5
wt.%
at.%
wt.%
at.%
wt.%
at.%
wt.%
at.%
Yb
64.37
14.97
62.35
16.87
48.71
14.46
40.65
10.76
Si
4.22
6.05
6.51
10.86
3.04
5.21
4.22
6.43
Y
-
-
0.78
0.11
1.46
0.63
2.1
0.88
Zr
-
-
6.23
1.56
23.88
10.79
26.51
11.02
O
31.41
78.99
24.13
70.60
22.91
68.92
26.52
70.91
Total
100%
100%
100%
100%
Table 2. Elements
Ca
Mg
Al
O
Si
Yb
Point A
12.30
0.58
11.39
57.72
5.71
12.3
Point B
3.19
0.18
8.89
67.84
5.28
14.62
Point C
0
0.01
0
76.17
7.04
16.78
Table 3. Elements
Ca
Mg
Al
Si
Y
Zr
Yb
O
Point A
8.57
0.01
8.82
9.81
0.04
1.15
11.18
60.42
Table 4. Elements
Ca
Mg
Al
Si
Y
Zr
Yb
O
Point A
8.19
0.01
16.14
8.28
0.06
1.85
12.07
53.40
Table 5. Elements
Ca
Mg
Al
Si
Y
Zr
Yb
O
Point A
5.46
0.65
7.80
8.00
0.66
6.86
11.34
59.23
Point B
2.99
0.64
8.53
6.36
0.90
10.23
9.03
61.32
Table 6 Sample Yb2SiO5 Yb2SiO5-10YSZ Yb2SiO5-20YSZ Yb2SiO5-30YSZ
Vickers hardness (HV) 252 480 670 335
Young modulus (GPa) 417 495 546 512
Hardness- nano-indentation (GPa) 2.6 6.7 7.8 6.1
Figure Caption: Fig. 1. XRD diffraction patterns of (a)-pure Yb2SiO5 powder, (b)- Yb2SiO5-10YSZ, (c)- Yb2SiO5-20YSZ and (d)- Yb2SiO5-30YSZ composite samples. Fig. 2. XRD diffraction patterns of sintered Yb2SiO5 after hot corrosion at 1400 ˚C for (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h. Fig. 3. Cross-sectional BSE images of the Yb2SiO5 samples after, a)- 4 h, b)- 12 h, c)- 24 h and d)- 48 h of corrosion test in a presence of CMAS at 1400°C Fig. 4. Cross-sectional elemental maps of the Yb2SiO5 samples after 48 h of hot corrosion test. Fig. 5. XRD diffraction patterns of Yb2SiO5-10%YSZ after hot corrosion at 1400 ˚C for (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h Fig. 6. Cross-sectional BSE images of the Yb2SiO5-10%YSZ samples after (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h of hot corrosion test in the presence of CMAS at 1400°C. Fig. 7. Elemental distribution of the Yb2SiO5 -10%YSZ sample after 48 h of hot corrosion test. Fig. 8. XRD diffraction patterns of Yb2SiO5-20%YSZ after (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h of hot corrosion at 1400 ˚C. Fig. 9. Cross-sectional BSE images of the Yb2SiO5-20%YSZ samples after (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h of hot corrosion test in the presence of CMAS at 1400°C. Fig. 10. Cross-sectional elemental distribution of the Yb2SiO5 -20%YSZ sample after 48 h of hot corrosion test. Fig. 11. XRD diffraction patterns of Yb2SiO5-30%YSZ after (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h of hot corrosion at 1400 ˚C. Fig. 12. Cross-sectional BSE images of the Yb2SiO5-30%YSZ samples after (a) 4 h, (b) 12 h, (c) 24 h, and (d) 48 h of hot corrosion test in the presence of CMAS at 1400°C. Fig. 13. Cross-sectional elemental distribution of the Yb2SiO5 -20%YSZ sample after 48 h of hot corrosion test. Fig. 14. a)- micro-hardness results and b)- force-displacement curve of the Yb2SiO5 and Yb2SiO5/YSZ composite samples obtained by nano-indentation test. Fig. 15. Thickness of the reaction layer of the Yb2SiO5 and Yb2SiO5/YSZ composite samples.
Fig. 1.
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Fig. 15.
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Yb2SiO5/YSZ composite pellets were prepared using a pressureless sintering process.
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Hot corrosion of samples was investigated in the presence of the CMAS at 1400 ºC.
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The reaction layer thickness was reduced by increasing the amout of YZS.
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Ca2Al2SiO7, Ca3ZrSi2O9, and garnet phases were formed on the composite samples.
Conflict of Interest:
The authors whose names are listed immediately below declare that there is no conflict of interest regarding the publication of this article. Author names: Ali Abedi Nieai Majid Mohammadi Maryam Shojaie-Bahaabad