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Optimal rare-earth disilicates as top coat in multilayer environmental barrier coatings
Accepted Manuscript Optimal rare-earth disilicates as top coat in multilayer environmental barrier coatings Yonghong Lu, Yejie Cao, Xing Zhao PII:
S0925-8388(18)32957-8
DOI:
10.1016/j.jallcom.2018.08.084
Reference:
JALCOM 47172
To appear in:
Journal of Alloys and Compounds
Received Date: 24 May 2018 Revised Date:
6 August 2018
Accepted Date: 9 August 2018
Please cite this article as: Y. Lu, Y. Cao, X. Zhao, Optimal rare-earth disilicates as top coat in multilayer environmental barrier coatings, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.08.084. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Optimal Rare-Earth Disilicates as Top Coat in Multilayer Environmental Barrier Coatings Yonghong Lu, Yejie Cao*, Xing Zhao
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Science and Technology on Thermostructure Composites Laboratory, Northwestern Polytechnical University, Xi’an, Shanxi 710072, P. R. China Abstract: In last decade, rare-earth disilicates have been considered as candidate
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materials for top coat in multilayer environmental barrier coatings (EBCs). However, optimal coating material has not yet been found. Previous studies revealed that
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parabolic rate constant of thermally grown silica scale (silica-TGO) beneath EBCs played a key role in the durability of SiC matrix ceramic composites coated with EBCs. To determine optimal coating material, in this study, Lu2Si2O7 and Sc2Si2O7 coatings were first corroded under quasi-static water-vapor corrosion conditions. The
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evolution of silica-TGO beneath coatings was then examined in detail. Accordingly, parabolic rate constants of silica-TGO growth were calculated. Compared with rate
constants
of
silica-TGO
growth
beneath
barium-strontium
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parabolic
aluminosilicate (BSAS), Yb2Si2O7, and Y2Si2O7-BSAS coatings in previous studies,
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Y2Si2O7-BSAS coating shows the lowest parabolic rate constant and growth rate of silica-TGO. Crystal structure of rare-earth disilicates was also studied. Results showed that bond lengths of Y–O and Si–O in Y2Si2O7 were short, indicating that it possessed low ionic oxygen permeability. This resulted in its low parabolic rate constant of silica-TGO growth. Therefore, Y2Si2O7 is the optimal material as top coat *
Corresponding authors at: Science and Technology on Thermostructure Composites Laboratory, Northwestern Polytechnical University, Xi’an, Shanxi 710072, P. R. China. Tel.: +86 29 88494914; Fax: +86 29 88494620; e-mail addresses: [email protected].
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Keywords: Environmental barrier coatings; Thermally grown silica oxide; Durability; SiC matrix ceramic composites Introduction
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1.
SiC matrix ceramic composites (CMC-SiC) coated with environmental barrier
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coatings (EBCs) are often used as hot-section components of turbine engines instead of super alloys due to their low density and high strength at elevated temperatures [1–
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6]. In the presence of high-velocity hot-gas steam, EBCs not only serve as a layer for insulating the destructive gas, but also serve as a barrier layer retarding the diffusion of oxidants (ionic oxygen and water vapor) to the EBC/bond coat [6–9]. Thus, EBCs significantly improve the service time of CMC-SiC [10].
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With ever-increasing demand for EBC-coated CMC-SiC in advanced gas turbine components, its durability has become a concern, as it determines the
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successful application of these materials in turbine engines. To improve the durability of EBC-coated CMC-SiC, several EBCs have been developed, including
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mullite/yttria-stabilized zirconium (YSZ) [6,11,12], barium-strontium aluminosilicate (BSAS) [6,7,10,13–15], and rare-earth silicates [8,16–27]. In the past decade, rare-earth silicates have been widely studied as EBC materials owing to their excellent water-vapor resistance, good phase stabilization, high operation temperature (~1500 °C), and low volatilization in high-velocity combustion [8]. Rare-earth disilicates (RE2Si2O7, RE = Y, Yb, Sc, Lu, and Gd) are considered as candidate materials for the top coat in art-three-layer EBCs. To determine suitable coating 2
ACCEPTED MANUSCRIPT materials, most studies focused on their water-vapor resistance [8,21,22], hot-corrosion behavior [25,26], phase stabilization [8,17], and failure mechanism [16– 18,20,27].
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However, the durability of RE2Si2O7-coated CMC-SiC has been rarely studied. Recent studies found that the durability of EBC-coated CMC-SiC was strongly related to the thermally grown silica scale (silica-TGO) beneath EBCs [17,18,20,27].
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Richards [18] studied the failure mechanisms of Yb2SiO5/mullite/Si-coated SiC
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during thermal cycling in water-vapor containing environments. The results indicated that β-cristobalite was formed during high-temperature exposure and transformed to α-cristobalite during cooling. This phase transformation was accompanied by considerable volumetric contraction, followed by severe fracturing of silica-TGO.
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This ultimately resulted in coating delamination. It was suggested that the formation of cristobalite silica-TGO contributed to the poor steam cycling durability of Yb2SiO5/mullite/Si-coated SiC at 1316 °C with 1 h cycle in 90%H2O-10%O2. Thus,
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the silica-TGO layer plays a key role in the durability of EBC-coated CMC-SiC. Lu et
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al. [27] reported that the EBC-coated composites failed when the silica-TGO grew to a certain thickness, even though the coatings were well preserved without erosion under the water-vapor flow. Also, the top coat in bilayer SiC bond coat/EBC system slightly affected the failure thickness of silica-TGO [27]. Thus, the durability of RE2Si2O7-coated CMC-SiC can be evaluated by examining the growth rate of silica-TGO beneath EBCs. To determine the optimal material for the top coat of multilayer EBCs, the 3
ACCEPTED MANUSCRIPT growth rate of silica-TGO beneath RE2Si2O7 (RE = Lu and Sc) coated C/SiC composites was investigated in detail in this study. The Lu and Sc based coatings were corroded under quasi-static water-vapor corrosion conditions, and the evolution of
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silica-TGO layer beneath EBCs was monitored. The parabolic rate constants of silica-TGO were calculated. Then, the parabolic rate constants of silica-TGO growth beneath BSAS, Yb2Si2O7, and Y2Si2O7-BSAS coatings from previous studies were
Experimental
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2.
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compared to determine the best material for top coat in multilayer EBCs.
Two-dimensional (2D) C/SiC composites were prepared by the chemical vapor infiltration (CVI) technique as described previously [28]. Bar-shaped samples with dimensions of 40 mm × 5 mm × 3.5 mm were cut from the as-prepared 2D C/SiC
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composites. In order to rule out the influence of the sample edges, the bars were beveled at 45° by 0.2 mm. The same deposition conditions employed for the SiC matrix were used to deposit the SiC bond coat on the surface of bars. The total
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deposition time was 80 h, and the coating thickness was about ~40 µm. The resulting
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C/SiC bars were polished and cleaned with alcohol. RE2Si2O7 (RE = Lu and Sc) and Ba0.5Sr0.5Al2Si2O8 powders were synthesized by
a sol–gel process described previously [14,25]. The obtained powders were ball-milled in a shock-type high-energy ball-milling machine (QM-3A High Speed Vibrating Ball Mill, Nanjing T-Bota Scietech Instruments & Equipment, Nanjing, China) at 120 rpm for 30 min. RE2Si2O7 coatings were fabricated on cleaned C/SiC bars by dip-coating process. Liquid polysilazane (Institute of Chemistry, Chinese 4
ACCEPTED MANUSCRIPT Academy of Science, Beijing, China) was first dissolved in ethanol to form a 50 wt% solution. Then, 85 wt% ball-milled powders were added into the polysilazane solution. A small amount of Ba0.5Sr0.5Al2Si2O8 was added to the solution as the sintering aid to
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decrease the sintering temperature of RE2Si2O7 coating. The mass ratio of RE2Si2O7 (RE = Lu and Sc) to BSAS was 9. The mixture was ball-milled using a QM-3A machine at 120 rpm for 1 h. The obtained slurry was uniformly coated on the surface
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of C/SiC bars by dip-coating, followed by heat treatment at 70 °C for 5 h. To obtain
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the RE2Si2O7 coatings, the resulting coatings were heat-treated at 400 °C for 1 h under argon to allow the crosslinking of polysilazane, followed by pyrolysis at 900 °C for 2 h, and heat treatment at a high temperature for 3 h under argon. During the heat treatment, part of the polysilazane in the coating converted to SiCN ceramic [29].
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According to previous studies, SiCN affects the water-vapor resistance of coating only slightly due to its small content [9,23,27]. The RE2Si2O7-coated C/SiC bars were corroded at 1250 °C in a high purity
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alumina tube furnace under 50%H2O-50%O2 environment with a total pressure of 1
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atm. The flow rate of the destructive gas was about 8.5×10−4 m·s−1 at room temperature. According to the Belton and Richardson method [30], water vapor was introduced into the tube by oxygen carrier gas bubbling through distilled water. To obtain the experimental environment of 50%H2O-50%O2, distilled water was heated at 81.7 °C. To prevent water vapor condensation on the entrance side of furnace, a heating tape was used to maintain the tube at a temperature of 110 °C. Water vapor condensed on the exit side of the tube was collected to verify the experimental 5
ACCEPTED MANUSCRIPT condition of 50%H2O-50%O2. The samples were placed on an alumina boat crucible located at mid tube. Microstructure of the samples was observed using a scanning electron
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microscope (SEM) in the back-scattered electron (BSE) mode (JEOL-6700F, Tokyo, Japan). Elemental analysis was performed by energy-dispersive X-ray spectroscopy (EDS, EDAX, USA). To obtain the average thickness of silica-TGO layer beneath
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Sc2Si2O7 and Lu2Si2O7 coatings, at least 30 different areas with similar thickness were
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measured for each sample. The pores in the coating affected the diffusion of the oxidants in the coatings and promoted the growth of the silica layer. To rule out the influence of such pores, pore-free areas were selected for measuring the thickness of silica-TGO layer. X-ray diffraction (XRD) analysis was carried out using a Rigaku
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D/max-2400 X-ray diffractometer (Tokyo, Japan) with CuKα radiation. Data were digitally recorded in a continuous scan in the 2θ range of 10–70° at a scanning rate of 0.02°/s.
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To calculate the RE2Si2O7 (RE = Y, Yb, Lu, and Sc) crystal structures,
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CASTEP code was used in this study [31]. Based on density functional theory (DFT), CA-PZ local density approximation (LDA) and Broyden–Fletcher–Goldfarb–Shanno (BFGS), the crystal structures were optimized [32]. The energy cut-off for the basis set was 320 eV, and integrations over the Brillouin zone were carried out using the Monkhorst-Pack scheme in the relevant irreducible wedge [33]. The structures were optimized by relaxing both the internal coordinates and lattice constants by calculating the ab initio forces on ions within the Born–Oppenheimer approximation 6
ACCEPTED MANUSCRIPT until the absolute values of forces were converged to less than 0.05 eV/Å. After the geometric optimization, the bond lengths of RE–O and Si–O were obtained. 3.
Results and Discussion
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3.1 Growth rate of silica-TGO beneath Lu2Si2O7 coating Fig. 1 shows the XRD profiles and typical morphology of the as-prepared Lu2Si2O7 coating. The main phase of the coating is Lu2Si2O7 according to the XRD
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profiles in Fig. 1(a). No obvious patterns of BSAS phase can be observed in the
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as-prepared coating. This is because part of the polysilazane converts into SiO2 during the fabrication process [34]. The derived SiO2 then reacts with BSAS to form a low melting-point glass phase during the heat-treatment process [7, 15]. Moreover, the concentration of BSAS in the coating is quite low (10 wt%). Therefore, the BSAS
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peaks cannot be observed in the XRD patterns.
The morphology in Fig. 1(b) indicates that its surface is uniform and smooth, and few defects such as pores and cracks are observed. Fig. 1(c) shows that the inner
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coating is rather dense without cracks. Some pores were observed at the inner coating.
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The formation of these pores was likely due to the pyrolysis of polysilazane during the heat treatment. Based on previous studies, such pores do not decrease the water-vapor resistance of coated composites [14,23,24,27]. The thickness of the as-prepared coating was around 40–60 µm. Fig. 1(c) also shows that the as-prepared coating included particulates and a continuous phase. The EDS patterns of these particulates in Fig. 1(d) show that they contain lutetium, silicon, and oxygen, indicating that the particulates are Lu2Si2O7 based on the XRD results. Fig. 1(e) 7
ACCEPTED MANUSCRIPT shows that the continuous phase is composed of barium, strontium, aluminum, silicon, and oxygen, indicating that this phase is BSAS. At the interface of SiC bond coat/Lu2Si2O7 coating, a dark layer is present. The EDS results show that this layer
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contains silicon and oxygen, indicating that it is silica. As shown in Fig. 1(c), the initial silica layer is irregular, and its thickness is about 0.8 µm.
The as-prepared Lu2Si2O7 coating was then corroded in 50%O2-50%H2O at
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1250 °C. The crystal phase and cross-sectional morphology of Lu2Si2O7-coated
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composites after water-vapor corrosion were studied by XRD and SEM analyses. As shown in Fig. 2(a), the main phase of the coating after corrosion test for 200 h is still Lu2Si2O7, indicating that the Lu2Si2O7 phase is well preserved even after 200 h in this corrosive environment. Typical cross-sectional morphology of the coating after water
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vapor corrosion is shown in Fig. 2(b). From the BSE morphology, it is clear that no other phase is observed at the inner coating after water-vapor corrosion compared with the as-prepared coating. The inner coat is still dense and tightly adhered onto the
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C/SiC composites, and its thickness is still 40–60 µm. These results show that the
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coating maintains the same structure and thickness after corrosion. Therefore, the Lu2Si2O7 coating is unaffected during the quasi-static water-vapor corrosion test. After the corrosion test, the silica layer beneath Lu2Si2O7 coating becomes regular. It is quite dense without cracks and tightly adhered onto the SiC bond coat. The evolution of silica-TGO layer beneath Lu2Si2O7 coating was monitored after water-vapor corrosion for different corrosion times. The growth showed the typical trend of increasing thickness with the increase in corrosion time, but at a 8
ACCEPTED MANUSCRIPT decreasing rate, as shown in Fig. 3. The square of silica-TGO thickness as a function of corrosion time was also plotted, as shown in the inset of Fig. 3. A linear relationship was observed between the square of silica-TGO thickness and corrosion
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time, indicating that the growth of silica-TGO does follow a parabolic rate law [35]. The intercept of the fitting line is not zero. This is likely due to the initial oxide layer, according to the classical Deal-Grove model [35].
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The parabolic rate constant can be obtained from the slope by using the following equation [35]:
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h 2 = k pt
(1)
where h is the thickness of silica-TGO, kp is the parabolic rate constant, and t is the corrosion time. The parabolic rate constant is about 5.59 × 10−2 µm2·h−1.
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3.2 Growth of silica-TGO beneath Sc2Si2O7 coating
The XRD profiles of the as-prepared Sc2Si2O7 coating in Fig. 4(a) indicate that the main phase of the coating is Sc2Si2O7. Similar to the Lu2Si2O7 coating, the BSAS
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phase cannot be observed in the XRD patterns of the as-prepared Sc2Si2O7 coating.
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The surface of the as-prepared coating in Fig. 4(b) is uniform and smooth as well as intact and has no defects such as cracks and pores. The inner coating shown in Fig. 4(c) is fairly dense. A few pores formed during the heat treatment are also present in the as-prepared coating. Fortunately, they would not decrease the water-vapor resistance of Sc2Si2O7 coating [14,23,24,27]. The as-prepared coating is tightly adhered onto the SiC bond coat along the Sc2Si2O7 coating/SiC bond coat interface. Its thickness is about 30–50 µm. An enlarged view of the morphology of Sc2Si2O7 9
ACCEPTED MANUSCRIPT coating is shown in Fig. 4(d). As seen from Fig. 4(d), the Sc2Si2O7 coating contains both particles and continuous phase. According to the EDS results in Fig. 4(e) and 4(f), the particles are Sc2Si2O7 and the continuous phase is BSAS. Similar to the Lu2Si2O7
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coating, an irregular silica layer with a thickness of about 0.6 µm is formed at the Sc2Si2O7 coating/SiC bond coating interface during the heat-treatment.
The Sc2Si2O7-coated C/SiC samples were then corroded in 50%H2O-50%O2 at
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1250 °C. According to the XRD results shown in Fig. 5(a), the main phase of samples
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after corrosion for 200 h is still Sc2Si2O7, indicating that Sc2Si2O7 coating undergoes no obvious phase change during the corrosion test compared with the as-prepared samples. Figure 5(b) shows the typical morphology of Sc2Si2O7 coatings after corrosion. Similar to the as-prepared coatings, its thickness remains in the range of
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30–50 µm, indicating that its thickness does not decrease during the corrosion test. The inner coating is still dense, and it is tightly adhered onto the composite along the Sc2Si2O7 coating/SiC bond coat interface. Also, no other phase is formed after
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corrosion, according to the BES morphology. These results reveal that the Sc2Si2O7
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coating is well preserved after long-term corrosion in this corrosive environment. A silica-TGO scale is also observed in Sc2Si2O7-coated C/SiC after water-vapor corrosion, as shown in Fig. 5(b). Compared to the initial oxide layer, the silica-TGO layer after corrosion test becomes regular and dense. It is tightly adhered onto the SiC bond coat. After corrosion test for different times, the bars were analyzed by SEM in order to monitor the evolution of silica-TGO layer beneath Lu2Si2O7 coating. Fig. 6 presents 10
ACCEPTED MANUSCRIPT the relationship between the thickness and corrosion time. As shown in Fig. 6, the thickness of silica-TGO scale in Sc2Si2O7-coated C/SiC grows with the increase in corrosion time, but at a decreasing rate. The square of silica-TGO thickness as a
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function of corrosion time is plotted in the inset of Fig. 6, indicating that the growth of silica-TGO in Sc2Si2O7-coated C/SiC also follows a parabolic rate law. According to Eq. (1), the kp of silica-TGO beneath Sc2Si2O7 coating is about 4.26 × 10−2 µm2·h−1.
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3.3 Optimal RE2Si2O7 materials as top coat in multilayer EBCs
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Table 1 shows the parabolic rate constants of silica-TGO beneath various coatings at 1250 °C in 50%H2O-50%O2 environment. As shown in Table 1, the Y2Si2O7-BSAS coating had the lowest parabolic rate constant of silica-TGO, indicating that it was a suitable material for the top coat in multilayer EBCs. Both the
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RE2Si2O7 coatings in Table 1 have a similar structure, which includes the continuous BSAS phase and the RE2Si2O7 particles. Due to the continuous BSAS phase, the path of oxidant’s permeation has an influence on the growth rate of silica-TGO [24, 27].
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However, the BSAS concentration in the coating is low, and thus, the flux of oxidants
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through the BSAS phase is limited. Therefore, most of the oxidant molecules reached the SiC bond coat by diffusion through the RE2Si2O7 particles. The parabolic rate constant of silica-TGO beneath BSAS coating is relatively
high. This indicates that the BSAS can improve the permeation of oxidants in the RE2Si2O7 coating because it is the continuous phase, followed by increase in parabolic rate constant of silica-TGO [24,27]. Moreover, the extent of this influence depends on the concentration of BSAS in the RE2Si2O7 coating. The RE2Si2O7 11
ACCEPTED MANUSCRIPT (RE=Lu, Sc and Yb) coatings contain only 10wt% of BSAS phase, while the Y2Si2O7-BSAS coating contains 30wt% BSAS phase. Although the content of BSAS phase in Y2Si2O7-BSAS coating is higher than that in RE2Si2O7 coating, the growth
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kinetics of silica-TGO beneath Y2Si2O7-BSAS coating is still lower than that of silica-TGO beneath RE2Si2O7 coating. This result indicates that Y2Si2O7 strongly retards the diffusion of oxidants and its permeability for oxidants is lower than that of
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RE2Si2O7.
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Previous studies have showed that ionic oxygen is the main oxidant for the formation and growth of TGO beneath Yb2Si2O7 and Y2Si2O7 coatings in 50%H2O-50%O2 environment [9]. Furthermore, the Lu2Si2O7 and Sc2Si2O7 materials have a similar structure as Yb2Si2O7 and Y2Si2O7 [21]. Therefore, it is reasonable to
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assume that ionic oxygen is the main oxidant involved in the formation and growth of silica-TGO beneath Lu2Si2O7 and Sc2Si2O7 coatings. The above mentioned results indicate that the growth of silica-TGO beneath rare-earth disilicate coatings follows a
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common mechanism [9]: (1) the growth of silica-TGO follows a parabolic rate law, (2)
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ionic oxygen is the main oxidant for the formation and growth of silica-TGO scale, (3) the diffusion of ionic oxygen through silica-TGO scale is the rate controlling process. Therefore, the lower the ionic oxygen permeability of the coating materials, the lower the oxygen pressure at the EBCs/silica-TGO interface for the same coating thickness. As a result, the silica-TGO beneath the coating with low oxygen permeability has a small parabolic rate constant and growth rate.
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ACCEPTED MANUSCRIPT To further understand the ionic oxygen permeability in RE2Si2O7, crystal structures of the coating materials were studied. The crystal structure of RE2Si2O7 was constructed using the parameters obtained from literature, followed by geometric
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optimization [37–40]. The optimized structures are shown in Fig. 7. Table 2 shows the lattice parameters after optimization and the corresponding data in XRD standard diffraction card. A slight difference indicates that the geometric optimization is
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reasonable. As shown in Fig. 7(a), Liu et al. reported that the bridge-oxygen site was
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the most possible oxygen vacancy in Y2Si2O7 due to the lowest defect formation energy for oxygen vacancy [41]. Due to the similar structures for different RE2Si2O7 materials, the bridge-oxygen site is the most probable oxygen vacancy in RE2Si2O7. Therefore, the RE–O (RE = Y, Yb, Lu, and Sc) and Si–O bonds in which the O atom
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is located at the bridge-oxygen site and the RE and Si atoms located at the nearest neighbor atomic site are very important for the ionic oxygen permeability of RE2Si2O7 [41]. The related RE–O and Si–O bond lengths are shown in Table 3. It can be seen
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that the Y–O bond in Y2Si2O7 has the lowest bond length, and the Si–O bond length
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of Y2Si2O7 is also very short, indicating that the ionic oxygen permeability in Y2Si2O7 is the most difficult. Therefore, Y2Si2O7 has the lowest ionic oxygen permeability compared to the other RE2Si2O7 coating materials (RE = Yb, Lu, and Sc).
4. Conclusions Lu2Si2O7 and Sc2Si2O7 coatings were fabricated on 2D C/SiC composites using a slurry method and then corroded under 50%H2O-50%O2 gas flow at 1250 °C. The 13
ACCEPTED MANUSCRIPT growth behaviors of silica-TGO beneath Lu2Si2O7 and Sc2Si2O7 coated C/SiC composites after water-vapor corrosion were investigated in detail. The thickness of silica-TGO scale grew with the increase in corrosion time, but at a decreasing rate,
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indicating that the growth of silica-TGO beneath Lu2Si2O7 and Sc2Si2O7 coatings followed a parabolic rate law. The calculated parabolic rate constants for Lu2Si2O7 and Sc2Si2O7 coatings were 5.59 × 10−2 and 4.26 × 10−2 µm2·h−1, respectively.
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Compared with the parabolic rate constants of silica-TGO growth rate beneath BSAS,
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Yb2Si2O7, and Y2Si2O7-BSAS coatings, Y2Si2O7-BSAS coating had the lowest parabolic rate constant and growth rate of silica-TGO. Therefore, the optimal rare-earth disilicate as top coat in multilayer EBCs was found to be Y2Si2O7. In the crystal structure of RE2Si2O7 (RE = Y, Yb, Lu, and Sc), the Y–O bond in Y2Si2O7 has
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the lowest bond length, and the Si–O bond length of Y2Si2O7 is also very short, indicating that Y2Si2O7 possesses the lowest ionic oxygen permeability.
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Acknowledgements
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This work was financially supported by the Chinese Natural Science Foundation (Grant Nos. 51032006 and 51172181), Research Fund of State Key Laboratory of Solidification Processing (Grant No. 56-TZ-2010), project “111” (B08040), and China Postdoctoral Science Foundation (Grant No. 2014M560656).
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[20] S. Ramasamy, S.N. Tewari, K.N. Lee, R.T. Bhatt, D.S. Fox, Mullite-gadolinium silicate environmental barrier coatings for melt infiltrated SiC/SiC composites, Surf. Coat. Technol. 205 (2011)3578-3581.
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[21] Y.G. Wang, J.L. Liu, First-principles investigation on the corrosion resistance of
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rare earth disilicates in water vapor, J. Eur. Ceram. Soc. 29 (2009)2163-2167. [22] Z.L. Hong, L.F. Cheng, L.T. Zhang, Y.G. Wang, Water vapor corrosion behavior of scandium silicates at 1400°C, J. Am. Ceram. Soc. 92 (2009)193-196.
[23] J. Liu, L.T. Zhang, J. Yang, L.F. Cheng, Y.G. Wang, Fabrication of SiCN-Sc2Si2O7 coatings on C/SiC composites at low temperatures, J. Eur. Ceram. Soc. 32 (2012)705-710.
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ACCEPTED MANUSCRIPT [24] J. Liu, L.T. Zhang, F. Hu, J. Yang, L.F. Cheng, Y.G. Wang, Polymer-derived yttrium silicate coatings on 2D C/SiC composites, J. Eur. Ceram. Soc. 33 (2013)433-439. Liu,
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[25] J.
Calcium-magnesium-aluminosilicate corrosion behaviors of rare-earth disilicates at 1400°C,J. Eur. Ceram. Soc. 33(2013)3419-3428.
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influence of calcium-magnesium-aluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer environmental barrier coatings, Acta Mater. 105(2016) 189-198.
[27] Y.H. Lu, L. Luo, J. Liu, Z. Chao, Y.G. Wang, Failure mechanism associated with
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the thermally grown silica scale in environmental barrier coated C/SiC composites, J. Am. Ceram. Soc. 99(2016)2713-2719. [28] L.F. Cheng, Y.D. Xu, L.T. Zhang, R. Gao, Effect of glass sealing on the
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[30] G.R. Belton, F.D. Richardson, A volatile iron hydroxide, Trans. Faraday Soc. 50(1962)1562-1572.
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ACCEPTED MANUSCRIPT [31] M. D. Segall, P. L. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clark, M. C. Payne, First-principles simulation: ideas, illustrations and the CASTEP code, J. Phys. Condens. Matter, 14 (2002)2717–2744.
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[32] M.C. Payne, M.P. Teter, D.C. Allan, T.A. Arias, J.D. Joannopoulos, Iterative minimization techniques for ab initio total-energy calculations: molecular dynamics and conjugate gradients. Rev. Mod. Phys. 64(1992)1045-1097.
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8(1973)5747-5753.
[34] D. Bahloul, M. Pereira, P Goursat, N.S. Choong Yive, R.J.P. Corriu, Preparation of silicon carbonitrides from an organosilicon polymer: I, thermal decomposition of the cross-linked polysilazane, J. Am. Ceram. Soc. 76(1993)1156-1162.
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[36] Y.H. Lu, Y.G. Wang, Effects of oxygen and water vapor on the formation and
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growth of silica layer beneath barium strontium aluminosilicate coatings, Ceram.
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Int. 44(2018)8570-8575.
[37] G.J. Redhammer, G. Roth, β-Y2Si2O7, a new thortveitite-type compound, determined at 100 and 280 K, Acta Crystallographica Section C: Crystal Structure
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[38] Y. I. Smolin, Y. F. Shepelev, The crystal structures of the rare earth pyrosilicates, Acta. Cryst. B. 26(1970) 484-492.
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ACCEPTED MANUSCRIPT [39] Y. I. Smolin, Y. F. Shepelev, A.P. Titov, Refinement of the crystal structure of thortveitite Sc2Si2O7, Kristallografiya,17(1972)857-858. [40] F. Soetebier, W. Urland, Crystal structure of lutetium disilicate, Lu2Si2O7,
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Zeitschrift für Kristallographie-New Crystal Structures, 217(2002) 22-22. [41] B. Liu, J.M. Wang, F.Z. Li, L.C. Sun, J.Y. Wang, Y.C. Zhou, Investigation of native point defects and nonstoichiometry mechanisms of two yttrium silicates by
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first-principles calculations, J. Am. Ceram. Soc. 96(2013)3304-3311.
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ACCEPTED MANUSCRIPT Figure captions Figure 1: XRD profiles and typical morphologies of Lu2Si2O7 coating after heat treatment at 1400 °C under argon for 3 h: (a) XRD profiles, (b) typical
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surface morphology, (c) typical cross-section morphology, (d) EDS pattern of particulates, and (e) EDS pattern of continuous phase.
Figure 2: XRD patterns and typical cross-section morphology of Lu2Si2O7-coated
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C/SiC composites after corrosion in 50%H2O-50%O2 at 1250 °C: (a) XRD
after corrosion for 25 h.
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pattern after corrosion for 200 h and (b) typical cross-section morphology
Figure 3: Thickness of silica-TGO beneath Lu2Si2O7 coating vs. corrosion time in 50%H2O-50%O2 at 1250 °C.
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Figure 4: XRD profiles and typical cross-section morphologies of Sc2Si2O7 coating after heat treatment at 1370 °C under argon for 3 h: (a) XRD profiles, (b) typical surface morphology, (c) typical cross-section morphology, (d) the
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enlarged view of cross-section morphology, (e) EDS pattern of particulates,
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and (f) EDS pattern of continuous phase. Figure 5: XRD patterns and cross-section morphologies of Sc2Si2O7-coated C/SiC composites after corrosion in 50%H2O-50%O2 at 1250 °C: (a) XRD patterns after corrosion for 200 h, and (b) typical cross-section morphology after corrosion for 200 h. Figure 6: Thickness of silica-TGO beneath Sc2Si2O7 coating as a function of corrosion time in 50%H2O-50%O2 at 1250 °C. 21
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and (d) Sc2Si2O7.
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ACCEPTED MANUSCRIPT Tables Table 1. Oxidation kinetics of EBC-coated C/SiC composites and CVD-SiC in destructive environments containing water vapor and oxygen ([9,27]) Temp Coating material
Coating thickness
kp value
(µm)
(×10−2 µm2/h)
Oxidant mixture
Lu2Si2O7-BSAS (10wt%)
50%O2-50%H2O, 1 atm
1250
Sc2Si2O7-BSAS (10wt%)
50%O2-50%H2O, 1 atm
1250
Yb2Si2O7-BSAS (10wt%)
50%O2-50%H2O, 1 atm
1250
Y2Si2O7-BSAS (30wt%)
50%O2-50%H2O, 1 atm
1250
BSAS
40–60
5.59
30–50
4.26
40–60
5.39 [9]
30-40
2.34 [27]
70-100
5.00 [35]
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(°C)
50%O2-50%H2O, 1 atm
Table 2. RE2Si2O7 lattice parameters after optimization and the corresponding data in XRD standard diffraction card Standard diffraction card No.
Y2Si2O7
PDF#8-0440
Lu2Si2O7
6.875 ⅹ 8.97 ⅹ 4.721
6.869 ⅹ 8.96 ⅹ 4.717
< 90.0 ⅹ 101.74 ⅹ 90.0 >
< 90.0 ⅹ 101.73ⅹ 90.0 >
6.802 ⅹ 8.875 ⅹ 4.703
6.802 ⅹ 8.875 ⅹ 4.703
< 90.0 ⅹ 102.12 ⅹ 90.0 >
< 90.0 ⅹ 102.12 ⅹ 90.0 >
6.508 ⅹ 8.506ⅹ 4.677
6.560 ⅹ 8.580ⅹ 4.740
< 90.0 ⅹ 102.7 ⅹ 90.0 >
< 90.0 ⅹ 103.1 ⅹ 90.0 >
6.7787 ⅹ 8.8481 ⅹ 4.7245
6.762 ⅹ 8.835 ⅹ 4.7113
< 90.0 ⅹ 102.0 ⅹ 90.0 >
< 90.0 ⅹ 101.99 ⅹ 90.0 >
PDF#20-1037
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Sc2Si2O7
Lattice parameters after optimization
PDF#25-1345
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Yb2Si2O7
Lattice parameters in standard diffraction card
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Material
PDF#35-0326
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ACCEPTED MANUSCRIPT Table 3. RE–O and Si–O bond lengths in RE2Si2O7 M–O bond length(Å)
Si–O bond length(Å)
Y2Si2O7
3.538
1.629
Yb2Si2O7
3.573
1.632
Sc2Si2O7
3.558
1.642
Lu2Si2O7
3.557
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Highlights The growth of silica-TGO beneath coatings followed a parabolic behavior. The Y2Si2O7 possess the lowest ionic oxygen permeability.
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The Y2Si2O7 was the optimal rare-earth disilicate in multilayer EBCs.
S0925-8388(18)32957-8
DOI:
10.1016/j.jallcom.2018.08.084
Reference:
JALCOM 47172
To appear in:
Journal of Alloys and Compounds
Received Date: 24 May 2018 Revised Date:
6 August 2018
Accepted Date: 9 August 2018
Please cite this article as: Y. Lu, Y. Cao, X. Zhao, Optimal rare-earth disilicates as top coat in multilayer environmental barrier coatings, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.08.084. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Optimal Rare-Earth Disilicates as Top Coat in Multilayer Environmental Barrier Coatings Yonghong Lu, Yejie Cao*, Xing Zhao
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Science and Technology on Thermostructure Composites Laboratory, Northwestern Polytechnical University, Xi’an, Shanxi 710072, P. R. China Abstract: In last decade, rare-earth disilicates have been considered as candidate
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materials for top coat in multilayer environmental barrier coatings (EBCs). However, optimal coating material has not yet been found. Previous studies revealed that
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parabolic rate constant of thermally grown silica scale (silica-TGO) beneath EBCs played a key role in the durability of SiC matrix ceramic composites coated with EBCs. To determine optimal coating material, in this study, Lu2Si2O7 and Sc2Si2O7 coatings were first corroded under quasi-static water-vapor corrosion conditions. The
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evolution of silica-TGO beneath coatings was then examined in detail. Accordingly, parabolic rate constants of silica-TGO growth were calculated. Compared with rate
constants
of
silica-TGO
growth
beneath
barium-strontium
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parabolic
aluminosilicate (BSAS), Yb2Si2O7, and Y2Si2O7-BSAS coatings in previous studies,
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Y2Si2O7-BSAS coating shows the lowest parabolic rate constant and growth rate of silica-TGO. Crystal structure of rare-earth disilicates was also studied. Results showed that bond lengths of Y–O and Si–O in Y2Si2O7 were short, indicating that it possessed low ionic oxygen permeability. This resulted in its low parabolic rate constant of silica-TGO growth. Therefore, Y2Si2O7 is the optimal material as top coat *
Corresponding authors at: Science and Technology on Thermostructure Composites Laboratory, Northwestern Polytechnical University, Xi’an, Shanxi 710072, P. R. China. Tel.: +86 29 88494914; Fax: +86 29 88494620; e-mail addresses: [email protected].
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Keywords: Environmental barrier coatings; Thermally grown silica oxide; Durability; SiC matrix ceramic composites Introduction
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1.
SiC matrix ceramic composites (CMC-SiC) coated with environmental barrier
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coatings (EBCs) are often used as hot-section components of turbine engines instead of super alloys due to their low density and high strength at elevated temperatures [1–
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6]. In the presence of high-velocity hot-gas steam, EBCs not only serve as a layer for insulating the destructive gas, but also serve as a barrier layer retarding the diffusion of oxidants (ionic oxygen and water vapor) to the EBC/bond coat [6–9]. Thus, EBCs significantly improve the service time of CMC-SiC [10].
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With ever-increasing demand for EBC-coated CMC-SiC in advanced gas turbine components, its durability has become a concern, as it determines the
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successful application of these materials in turbine engines. To improve the durability of EBC-coated CMC-SiC, several EBCs have been developed, including
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mullite/yttria-stabilized zirconium (YSZ) [6,11,12], barium-strontium aluminosilicate (BSAS) [6,7,10,13–15], and rare-earth silicates [8,16–27]. In the past decade, rare-earth silicates have been widely studied as EBC materials owing to their excellent water-vapor resistance, good phase stabilization, high operation temperature (~1500 °C), and low volatilization in high-velocity combustion [8]. Rare-earth disilicates (RE2Si2O7, RE = Y, Yb, Sc, Lu, and Gd) are considered as candidate materials for the top coat in art-three-layer EBCs. To determine suitable coating 2
ACCEPTED MANUSCRIPT materials, most studies focused on their water-vapor resistance [8,21,22], hot-corrosion behavior [25,26], phase stabilization [8,17], and failure mechanism [16– 18,20,27].
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However, the durability of RE2Si2O7-coated CMC-SiC has been rarely studied. Recent studies found that the durability of EBC-coated CMC-SiC was strongly related to the thermally grown silica scale (silica-TGO) beneath EBCs [17,18,20,27].
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Richards [18] studied the failure mechanisms of Yb2SiO5/mullite/Si-coated SiC
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during thermal cycling in water-vapor containing environments. The results indicated that β-cristobalite was formed during high-temperature exposure and transformed to α-cristobalite during cooling. This phase transformation was accompanied by considerable volumetric contraction, followed by severe fracturing of silica-TGO.
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This ultimately resulted in coating delamination. It was suggested that the formation of cristobalite silica-TGO contributed to the poor steam cycling durability of Yb2SiO5/mullite/Si-coated SiC at 1316 °C with 1 h cycle in 90%H2O-10%O2. Thus,
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the silica-TGO layer plays a key role in the durability of EBC-coated CMC-SiC. Lu et
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al. [27] reported that the EBC-coated composites failed when the silica-TGO grew to a certain thickness, even though the coatings were well preserved without erosion under the water-vapor flow. Also, the top coat in bilayer SiC bond coat/EBC system slightly affected the failure thickness of silica-TGO [27]. Thus, the durability of RE2Si2O7-coated CMC-SiC can be evaluated by examining the growth rate of silica-TGO beneath EBCs. To determine the optimal material for the top coat of multilayer EBCs, the 3
ACCEPTED MANUSCRIPT growth rate of silica-TGO beneath RE2Si2O7 (RE = Lu and Sc) coated C/SiC composites was investigated in detail in this study. The Lu and Sc based coatings were corroded under quasi-static water-vapor corrosion conditions, and the evolution of
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silica-TGO layer beneath EBCs was monitored. The parabolic rate constants of silica-TGO were calculated. Then, the parabolic rate constants of silica-TGO growth beneath BSAS, Yb2Si2O7, and Y2Si2O7-BSAS coatings from previous studies were
Experimental
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2.
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compared to determine the best material for top coat in multilayer EBCs.
Two-dimensional (2D) C/SiC composites were prepared by the chemical vapor infiltration (CVI) technique as described previously [28]. Bar-shaped samples with dimensions of 40 mm × 5 mm × 3.5 mm were cut from the as-prepared 2D C/SiC
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composites. In order to rule out the influence of the sample edges, the bars were beveled at 45° by 0.2 mm. The same deposition conditions employed for the SiC matrix were used to deposit the SiC bond coat on the surface of bars. The total
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deposition time was 80 h, and the coating thickness was about ~40 µm. The resulting
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C/SiC bars were polished and cleaned with alcohol. RE2Si2O7 (RE = Lu and Sc) and Ba0.5Sr0.5Al2Si2O8 powders were synthesized by
a sol–gel process described previously [14,25]. The obtained powders were ball-milled in a shock-type high-energy ball-milling machine (QM-3A High Speed Vibrating Ball Mill, Nanjing T-Bota Scietech Instruments & Equipment, Nanjing, China) at 120 rpm for 30 min. RE2Si2O7 coatings were fabricated on cleaned C/SiC bars by dip-coating process. Liquid polysilazane (Institute of Chemistry, Chinese 4
ACCEPTED MANUSCRIPT Academy of Science, Beijing, China) was first dissolved in ethanol to form a 50 wt% solution. Then, 85 wt% ball-milled powders were added into the polysilazane solution. A small amount of Ba0.5Sr0.5Al2Si2O8 was added to the solution as the sintering aid to
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decrease the sintering temperature of RE2Si2O7 coating. The mass ratio of RE2Si2O7 (RE = Lu and Sc) to BSAS was 9. The mixture was ball-milled using a QM-3A machine at 120 rpm for 1 h. The obtained slurry was uniformly coated on the surface
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of C/SiC bars by dip-coating, followed by heat treatment at 70 °C for 5 h. To obtain
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the RE2Si2O7 coatings, the resulting coatings were heat-treated at 400 °C for 1 h under argon to allow the crosslinking of polysilazane, followed by pyrolysis at 900 °C for 2 h, and heat treatment at a high temperature for 3 h under argon. During the heat treatment, part of the polysilazane in the coating converted to SiCN ceramic [29].
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According to previous studies, SiCN affects the water-vapor resistance of coating only slightly due to its small content [9,23,27]. The RE2Si2O7-coated C/SiC bars were corroded at 1250 °C in a high purity
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alumina tube furnace under 50%H2O-50%O2 environment with a total pressure of 1
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atm. The flow rate of the destructive gas was about 8.5×10−4 m·s−1 at room temperature. According to the Belton and Richardson method [30], water vapor was introduced into the tube by oxygen carrier gas bubbling through distilled water. To obtain the experimental environment of 50%H2O-50%O2, distilled water was heated at 81.7 °C. To prevent water vapor condensation on the entrance side of furnace, a heating tape was used to maintain the tube at a temperature of 110 °C. Water vapor condensed on the exit side of the tube was collected to verify the experimental 5
ACCEPTED MANUSCRIPT condition of 50%H2O-50%O2. The samples were placed on an alumina boat crucible located at mid tube. Microstructure of the samples was observed using a scanning electron
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microscope (SEM) in the back-scattered electron (BSE) mode (JEOL-6700F, Tokyo, Japan). Elemental analysis was performed by energy-dispersive X-ray spectroscopy (EDS, EDAX, USA). To obtain the average thickness of silica-TGO layer beneath
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Sc2Si2O7 and Lu2Si2O7 coatings, at least 30 different areas with similar thickness were
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measured for each sample. The pores in the coating affected the diffusion of the oxidants in the coatings and promoted the growth of the silica layer. To rule out the influence of such pores, pore-free areas were selected for measuring the thickness of silica-TGO layer. X-ray diffraction (XRD) analysis was carried out using a Rigaku
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D/max-2400 X-ray diffractometer (Tokyo, Japan) with CuKα radiation. Data were digitally recorded in a continuous scan in the 2θ range of 10–70° at a scanning rate of 0.02°/s.
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To calculate the RE2Si2O7 (RE = Y, Yb, Lu, and Sc) crystal structures,
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CASTEP code was used in this study [31]. Based on density functional theory (DFT), CA-PZ local density approximation (LDA) and Broyden–Fletcher–Goldfarb–Shanno (BFGS), the crystal structures were optimized [32]. The energy cut-off for the basis set was 320 eV, and integrations over the Brillouin zone were carried out using the Monkhorst-Pack scheme in the relevant irreducible wedge [33]. The structures were optimized by relaxing both the internal coordinates and lattice constants by calculating the ab initio forces on ions within the Born–Oppenheimer approximation 6
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Results and Discussion
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3.1 Growth rate of silica-TGO beneath Lu2Si2O7 coating Fig. 1 shows the XRD profiles and typical morphology of the as-prepared Lu2Si2O7 coating. The main phase of the coating is Lu2Si2O7 according to the XRD
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profiles in Fig. 1(a). No obvious patterns of BSAS phase can be observed in the
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as-prepared coating. This is because part of the polysilazane converts into SiO2 during the fabrication process [34]. The derived SiO2 then reacts with BSAS to form a low melting-point glass phase during the heat-treatment process [7, 15]. Moreover, the concentration of BSAS in the coating is quite low (10 wt%). Therefore, the BSAS
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peaks cannot be observed in the XRD patterns.
The morphology in Fig. 1(b) indicates that its surface is uniform and smooth, and few defects such as pores and cracks are observed. Fig. 1(c) shows that the inner
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The formation of these pores was likely due to the pyrolysis of polysilazane during the heat treatment. Based on previous studies, such pores do not decrease the water-vapor resistance of coated composites [14,23,24,27]. The thickness of the as-prepared coating was around 40–60 µm. Fig. 1(c) also shows that the as-prepared coating included particulates and a continuous phase. The EDS patterns of these particulates in Fig. 1(d) show that they contain lutetium, silicon, and oxygen, indicating that the particulates are Lu2Si2O7 based on the XRD results. Fig. 1(e) 7
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contains silicon and oxygen, indicating that it is silica. As shown in Fig. 1(c), the initial silica layer is irregular, and its thickness is about 0.8 µm.
The as-prepared Lu2Si2O7 coating was then corroded in 50%O2-50%H2O at
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1250 °C. The crystal phase and cross-sectional morphology of Lu2Si2O7-coated
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composites after water-vapor corrosion were studied by XRD and SEM analyses. As shown in Fig. 2(a), the main phase of the coating after corrosion test for 200 h is still Lu2Si2O7, indicating that the Lu2Si2O7 phase is well preserved even after 200 h in this corrosive environment. Typical cross-sectional morphology of the coating after water
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vapor corrosion is shown in Fig. 2(b). From the BSE morphology, it is clear that no other phase is observed at the inner coating after water-vapor corrosion compared with the as-prepared coating. The inner coat is still dense and tightly adhered onto the
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C/SiC composites, and its thickness is still 40–60 µm. These results show that the
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coating maintains the same structure and thickness after corrosion. Therefore, the Lu2Si2O7 coating is unaffected during the quasi-static water-vapor corrosion test. After the corrosion test, the silica layer beneath Lu2Si2O7 coating becomes regular. It is quite dense without cracks and tightly adhered onto the SiC bond coat. The evolution of silica-TGO layer beneath Lu2Si2O7 coating was monitored after water-vapor corrosion for different corrosion times. The growth showed the typical trend of increasing thickness with the increase in corrosion time, but at a 8
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time, indicating that the growth of silica-TGO does follow a parabolic rate law [35]. The intercept of the fitting line is not zero. This is likely due to the initial oxide layer, according to the classical Deal-Grove model [35].
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The parabolic rate constant can be obtained from the slope by using the following equation [35]:
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h 2 = k pt
(1)
where h is the thickness of silica-TGO, kp is the parabolic rate constant, and t is the corrosion time. The parabolic rate constant is about 5.59 × 10−2 µm2·h−1.
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3.2 Growth of silica-TGO beneath Sc2Si2O7 coating
The XRD profiles of the as-prepared Sc2Si2O7 coating in Fig. 4(a) indicate that the main phase of the coating is Sc2Si2O7. Similar to the Lu2Si2O7 coating, the BSAS
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phase cannot be observed in the XRD patterns of the as-prepared Sc2Si2O7 coating.
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The surface of the as-prepared coating in Fig. 4(b) is uniform and smooth as well as intact and has no defects such as cracks and pores. The inner coating shown in Fig. 4(c) is fairly dense. A few pores formed during the heat treatment are also present in the as-prepared coating. Fortunately, they would not decrease the water-vapor resistance of Sc2Si2O7 coating [14,23,24,27]. The as-prepared coating is tightly adhered onto the SiC bond coat along the Sc2Si2O7 coating/SiC bond coat interface. Its thickness is about 30–50 µm. An enlarged view of the morphology of Sc2Si2O7 9
ACCEPTED MANUSCRIPT coating is shown in Fig. 4(d). As seen from Fig. 4(d), the Sc2Si2O7 coating contains both particles and continuous phase. According to the EDS results in Fig. 4(e) and 4(f), the particles are Sc2Si2O7 and the continuous phase is BSAS. Similar to the Lu2Si2O7
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coating, an irregular silica layer with a thickness of about 0.6 µm is formed at the Sc2Si2O7 coating/SiC bond coating interface during the heat-treatment.
The Sc2Si2O7-coated C/SiC samples were then corroded in 50%H2O-50%O2 at
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1250 °C. According to the XRD results shown in Fig. 5(a), the main phase of samples
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after corrosion for 200 h is still Sc2Si2O7, indicating that Sc2Si2O7 coating undergoes no obvious phase change during the corrosion test compared with the as-prepared samples. Figure 5(b) shows the typical morphology of Sc2Si2O7 coatings after corrosion. Similar to the as-prepared coatings, its thickness remains in the range of
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30–50 µm, indicating that its thickness does not decrease during the corrosion test. The inner coating is still dense, and it is tightly adhered onto the composite along the Sc2Si2O7 coating/SiC bond coat interface. Also, no other phase is formed after
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corrosion, according to the BES morphology. These results reveal that the Sc2Si2O7
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coating is well preserved after long-term corrosion in this corrosive environment. A silica-TGO scale is also observed in Sc2Si2O7-coated C/SiC after water-vapor corrosion, as shown in Fig. 5(b). Compared to the initial oxide layer, the silica-TGO layer after corrosion test becomes regular and dense. It is tightly adhered onto the SiC bond coat. After corrosion test for different times, the bars were analyzed by SEM in order to monitor the evolution of silica-TGO layer beneath Lu2Si2O7 coating. Fig. 6 presents 10
ACCEPTED MANUSCRIPT the relationship between the thickness and corrosion time. As shown in Fig. 6, the thickness of silica-TGO scale in Sc2Si2O7-coated C/SiC grows with the increase in corrosion time, but at a decreasing rate. The square of silica-TGO thickness as a
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function of corrosion time is plotted in the inset of Fig. 6, indicating that the growth of silica-TGO in Sc2Si2O7-coated C/SiC also follows a parabolic rate law. According to Eq. (1), the kp of silica-TGO beneath Sc2Si2O7 coating is about 4.26 × 10−2 µm2·h−1.
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3.3 Optimal RE2Si2O7 materials as top coat in multilayer EBCs
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Table 1 shows the parabolic rate constants of silica-TGO beneath various coatings at 1250 °C in 50%H2O-50%O2 environment. As shown in Table 1, the Y2Si2O7-BSAS coating had the lowest parabolic rate constant of silica-TGO, indicating that it was a suitable material for the top coat in multilayer EBCs. Both the
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RE2Si2O7 coatings in Table 1 have a similar structure, which includes the continuous BSAS phase and the RE2Si2O7 particles. Due to the continuous BSAS phase, the path of oxidant’s permeation has an influence on the growth rate of silica-TGO [24, 27].
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However, the BSAS concentration in the coating is low, and thus, the flux of oxidants
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through the BSAS phase is limited. Therefore, most of the oxidant molecules reached the SiC bond coat by diffusion through the RE2Si2O7 particles. The parabolic rate constant of silica-TGO beneath BSAS coating is relatively
high. This indicates that the BSAS can improve the permeation of oxidants in the RE2Si2O7 coating because it is the continuous phase, followed by increase in parabolic rate constant of silica-TGO [24,27]. Moreover, the extent of this influence depends on the concentration of BSAS in the RE2Si2O7 coating. The RE2Si2O7 11
ACCEPTED MANUSCRIPT (RE=Lu, Sc and Yb) coatings contain only 10wt% of BSAS phase, while the Y2Si2O7-BSAS coating contains 30wt% BSAS phase. Although the content of BSAS phase in Y2Si2O7-BSAS coating is higher than that in RE2Si2O7 coating, the growth
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kinetics of silica-TGO beneath Y2Si2O7-BSAS coating is still lower than that of silica-TGO beneath RE2Si2O7 coating. This result indicates that Y2Si2O7 strongly retards the diffusion of oxidants and its permeability for oxidants is lower than that of
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RE2Si2O7.
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Previous studies have showed that ionic oxygen is the main oxidant for the formation and growth of TGO beneath Yb2Si2O7 and Y2Si2O7 coatings in 50%H2O-50%O2 environment [9]. Furthermore, the Lu2Si2O7 and Sc2Si2O7 materials have a similar structure as Yb2Si2O7 and Y2Si2O7 [21]. Therefore, it is reasonable to
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assume that ionic oxygen is the main oxidant involved in the formation and growth of silica-TGO beneath Lu2Si2O7 and Sc2Si2O7 coatings. The above mentioned results indicate that the growth of silica-TGO beneath rare-earth disilicate coatings follows a
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common mechanism [9]: (1) the growth of silica-TGO follows a parabolic rate law, (2)
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ionic oxygen is the main oxidant for the formation and growth of silica-TGO scale, (3) the diffusion of ionic oxygen through silica-TGO scale is the rate controlling process. Therefore, the lower the ionic oxygen permeability of the coating materials, the lower the oxygen pressure at the EBCs/silica-TGO interface for the same coating thickness. As a result, the silica-TGO beneath the coating with low oxygen permeability has a small parabolic rate constant and growth rate.
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ACCEPTED MANUSCRIPT To further understand the ionic oxygen permeability in RE2Si2O7, crystal structures of the coating materials were studied. The crystal structure of RE2Si2O7 was constructed using the parameters obtained from literature, followed by geometric
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optimization [37–40]. The optimized structures are shown in Fig. 7. Table 2 shows the lattice parameters after optimization and the corresponding data in XRD standard diffraction card. A slight difference indicates that the geometric optimization is
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reasonable. As shown in Fig. 7(a), Liu et al. reported that the bridge-oxygen site was
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the most possible oxygen vacancy in Y2Si2O7 due to the lowest defect formation energy for oxygen vacancy [41]. Due to the similar structures for different RE2Si2O7 materials, the bridge-oxygen site is the most probable oxygen vacancy in RE2Si2O7. Therefore, the RE–O (RE = Y, Yb, Lu, and Sc) and Si–O bonds in which the O atom
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is located at the bridge-oxygen site and the RE and Si atoms located at the nearest neighbor atomic site are very important for the ionic oxygen permeability of RE2Si2O7 [41]. The related RE–O and Si–O bond lengths are shown in Table 3. It can be seen
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that the Y–O bond in Y2Si2O7 has the lowest bond length, and the Si–O bond length
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of Y2Si2O7 is also very short, indicating that the ionic oxygen permeability in Y2Si2O7 is the most difficult. Therefore, Y2Si2O7 has the lowest ionic oxygen permeability compared to the other RE2Si2O7 coating materials (RE = Yb, Lu, and Sc).
4. Conclusions Lu2Si2O7 and Sc2Si2O7 coatings were fabricated on 2D C/SiC composites using a slurry method and then corroded under 50%H2O-50%O2 gas flow at 1250 °C. The 13
ACCEPTED MANUSCRIPT growth behaviors of silica-TGO beneath Lu2Si2O7 and Sc2Si2O7 coated C/SiC composites after water-vapor corrosion were investigated in detail. The thickness of silica-TGO scale grew with the increase in corrosion time, but at a decreasing rate,
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indicating that the growth of silica-TGO beneath Lu2Si2O7 and Sc2Si2O7 coatings followed a parabolic rate law. The calculated parabolic rate constants for Lu2Si2O7 and Sc2Si2O7 coatings were 5.59 × 10−2 and 4.26 × 10−2 µm2·h−1, respectively.
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Compared with the parabolic rate constants of silica-TGO growth rate beneath BSAS,
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Yb2Si2O7, and Y2Si2O7-BSAS coatings, Y2Si2O7-BSAS coating had the lowest parabolic rate constant and growth rate of silica-TGO. Therefore, the optimal rare-earth disilicate as top coat in multilayer EBCs was found to be Y2Si2O7. In the crystal structure of RE2Si2O7 (RE = Y, Yb, Lu, and Sc), the Y–O bond in Y2Si2O7 has
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the lowest bond length, and the Si–O bond length of Y2Si2O7 is also very short, indicating that Y2Si2O7 possesses the lowest ionic oxygen permeability.
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Acknowledgements
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This work was financially supported by the Chinese Natural Science Foundation (Grant Nos. 51032006 and 51172181), Research Fund of State Key Laboratory of Solidification Processing (Grant No. 56-TZ-2010), project “111” (B08040), and China Postdoctoral Science Foundation (Grant No. 2014M560656).
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ACCEPTED MANUSCRIPT [39] Y. I. Smolin, Y. F. Shepelev, A.P. Titov, Refinement of the crystal structure of thortveitite Sc2Si2O7, Kristallografiya,17(1972)857-858. [40] F. Soetebier, W. Urland, Crystal structure of lutetium disilicate, Lu2Si2O7,
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ACCEPTED MANUSCRIPT Figure captions Figure 1: XRD profiles and typical morphologies of Lu2Si2O7 coating after heat treatment at 1400 °C under argon for 3 h: (a) XRD profiles, (b) typical
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surface morphology, (c) typical cross-section morphology, (d) EDS pattern of particulates, and (e) EDS pattern of continuous phase.
Figure 2: XRD patterns and typical cross-section morphology of Lu2Si2O7-coated
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C/SiC composites after corrosion in 50%H2O-50%O2 at 1250 °C: (a) XRD
after corrosion for 25 h.
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pattern after corrosion for 200 h and (b) typical cross-section morphology
Figure 3: Thickness of silica-TGO beneath Lu2Si2O7 coating vs. corrosion time in 50%H2O-50%O2 at 1250 °C.
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Figure 4: XRD profiles and typical cross-section morphologies of Sc2Si2O7 coating after heat treatment at 1370 °C under argon for 3 h: (a) XRD profiles, (b) typical surface morphology, (c) typical cross-section morphology, (d) the
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enlarged view of cross-section morphology, (e) EDS pattern of particulates,
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and (f) EDS pattern of continuous phase. Figure 5: XRD patterns and cross-section morphologies of Sc2Si2O7-coated C/SiC composites after corrosion in 50%H2O-50%O2 at 1250 °C: (a) XRD patterns after corrosion for 200 h, and (b) typical cross-section morphology after corrosion for 200 h. Figure 6: Thickness of silica-TGO beneath Sc2Si2O7 coating as a function of corrosion time in 50%H2O-50%O2 at 1250 °C. 21
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and (d) Sc2Si2O7.
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ACCEPTED MANUSCRIPT Tables Table 1. Oxidation kinetics of EBC-coated C/SiC composites and CVD-SiC in destructive environments containing water vapor and oxygen ([9,27]) Temp Coating material
Coating thickness
kp value
(µm)
(×10−2 µm2/h)
Oxidant mixture
Lu2Si2O7-BSAS (10wt%)
50%O2-50%H2O, 1 atm
1250
Sc2Si2O7-BSAS (10wt%)
50%O2-50%H2O, 1 atm
1250
Yb2Si2O7-BSAS (10wt%)
50%O2-50%H2O, 1 atm
1250
Y2Si2O7-BSAS (30wt%)
50%O2-50%H2O, 1 atm
1250
BSAS
40–60
5.59
30–50
4.26
40–60
5.39 [9]
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5.00 [35]
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Table 2. RE2Si2O7 lattice parameters after optimization and the corresponding data in XRD standard diffraction card Standard diffraction card No.
Y2Si2O7
PDF#8-0440
Lu2Si2O7
6.875 ⅹ 8.97 ⅹ 4.721
6.869 ⅹ 8.96 ⅹ 4.717
< 90.0 ⅹ 101.74 ⅹ 90.0 >
< 90.0 ⅹ 101.73ⅹ 90.0 >
6.802 ⅹ 8.875 ⅹ 4.703
6.802 ⅹ 8.875 ⅹ 4.703
< 90.0 ⅹ 102.12 ⅹ 90.0 >
< 90.0 ⅹ 102.12 ⅹ 90.0 >
6.508 ⅹ 8.506ⅹ 4.677
6.560 ⅹ 8.580ⅹ 4.740
< 90.0 ⅹ 102.7 ⅹ 90.0 >
< 90.0 ⅹ 103.1 ⅹ 90.0 >
6.7787 ⅹ 8.8481 ⅹ 4.7245
6.762 ⅹ 8.835 ⅹ 4.7113
< 90.0 ⅹ 102.0 ⅹ 90.0 >
< 90.0 ⅹ 101.99 ⅹ 90.0 >
PDF#20-1037
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Sc2Si2O7
Lattice parameters after optimization
PDF#25-1345
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Yb2Si2O7
Lattice parameters in standard diffraction card
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Material
PDF#35-0326
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ACCEPTED MANUSCRIPT Table 3. RE–O and Si–O bond lengths in RE2Si2O7 M–O bond length(Å)
Si–O bond length(Å)
Y2Si2O7
3.538
1.629
Yb2Si2O7
3.573
1.632
Sc2Si2O7
3.558
1.642
Lu2Si2O7
3.557
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Highlights The growth of silica-TGO beneath coatings followed a parabolic behavior. The Y2Si2O7 possess the lowest ionic oxygen permeability.
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The Y2Si2O7 was the optimal rare-earth disilicate in multilayer EBCs.