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Diffusion study of rare-earth oxides into silica layer for environmental barrier coating applications
Journal of the European Ceramic Society 39 (2019) 4216–4222
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Diffusion study of rare-earth oxides into silica layer for environmental barrier coating applications E.N. Dayia, N. Al Nasirib, a b
T
⁎
Sabanci University, Department of Materials Science and Nanoengineering, Tuzla, Orta Mahalle, Istanbul 34956, Turkey Centre for Advanced Structural Ceramics, Department of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
ARTICLE INFO
ABSTRACT
Keywords: Environmental barrier coating RE-oxides Diffusion CMCs
The effect of exposure temperature and time on the diffusion rate of rare-earth oxides applied on silicon carbide fiber-reinforced SiC ceramic matrix composites (SiC/SiC CMCs) have been investigated. Knowledge on diffusion mechanism between the deposited rare-earth (RE) slurry and silica layer is necessary to understand the process governing EBCs formation and their properties. SEM/EDS analysis were used to study the effect of microstructure on diffusivity. The diffusion coefficient increases with increasing sintering temperature and time. The measured diffusion coefficients of the RE-coating into silica layer were in the order of 10−15-10-17 m2/s revealing an overall good adhesion on the SiC/SiC CMCs.
1. Introduction Economic and environmental concerns to improve efficiency and reducing emissions, are the main driving forces behind the ever-increasing demand for higher gas turbine engine inlet temperatures. Technology improvements in cooling, materials and coatings are required to achieve higher inlet temperatures [1]. Si-based ceramics, such as silicon carbide (SiC) fiber-reinforced SiC ceramic matrix composites (SiC/SiC CMCs) exhibit superior high-temperature strength and durability, indicating their potential to revolutionize gas turbine technology [2]. They are also light and possess excellent high temperature oxidation resistance in dry air, due to the formation of slow-growing, protective silica scale [3]. However, when the Si-based ceramic is exposed to corrosive environments containing high-pressure steam at elevated temperatures, they are susceptible to hot-corrosion and recession due to the reaction of silica with water vapour to form volatile Si(OH)4 (g) [1]. Environmental barrier coatings (EBCs) are widely used to protect CMCs from such degradation. EBCs must have phase stability and low coefficient of thermal expansion (CTE) with the substrate. Additionally, they must have low permeability to oxygen and chemical compatibility with the silica scale formed from oxidation [4]. Alumina-based coatings such as mullite were attractive choices due to their low thermal expansion coefficients (TEC) (5.1 9 10−6 K −1 from 298 to 1773 K) and good adherence to Si-based ceramics [5].
⁎
Unfortunately, the rapid cooling and heating caused serious problems such as cracking and delamination of the mullite coating [6]. The use of barium strontium aluminosilicate (BSAS) coatings with mullite and silicon in the coatings have shown better performance [7]. BSAS has high temperature stability and low TEC of 4.0–5.15 10−6 K −1 (300–1600 K) [8]. The main drawback of BSAS coatings are their reaction with silica at ˜1300 °C leading to pore formation and coating spallation [7]. Al Nasiri et al. [9] studied the thermal properties of Yb, Er and Lu monosilicates and showed that the low TEC and thermal conductivity values make these RE-monosilicates competitive EBCs for Si-based ceramics. Therefore, further investigation of the properties of these materials is important to understand and improve their performances as promising new generation EBC candidates. Microstructure and durability of such coatings depend on the thickness and the processing methods used [10,11]. The effect of diffusion rate on the thickness of RE-silicates with silica has not been previously investigated. Several studies have been conducted to investigate silver diffusion in coated particles, such as tristructural isotropic (TRISO) coated fuel particle [12–15]. Honorato et al. [13] used a TRISO particle, which consists of a uranium kernel coated with three layers of pyrolytic carbon and a layer of SiC. They developed a new method capable of trapping thin layers of silver between two layers of SiC in these coated fuel particles. This method studied the effect of SiC microstructure on the diffusion of silver without an irradiation effect. Their results confirmed that the diffusion does not occur due to the presence of high excess silicon or formation of defects. Instead, the diffusion process is
Corresponding author. E-mail address: [email protected] (N. Al Nasiri).
https://doi.org/10.1016/j.jeurceramsoc.2019.05.026 Received 5 February 2019; Received in revised form 30 April 2019; Accepted 15 May 2019 Available online 23 May 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Journal of the European Ceramic Society 39 (2019) 4216–4222
E.N. Dayi and N. Al Nasiri
governed by grain boundary diffusion and hence depends on the microstructural effects such as nano and micro porosity of SiC. Other studies revealed correlation between the microstructural changes and diffusion properties into coatings [16–20]. It has been reported that there is a variance in the diffusivity of silver into SiC layer and the scarcity of diffusion coefficients are suggested to be caused by the differences in the properties of the SiC coating. Estimation of diffusion coefficients provides a good insight to the transport mechanisms of the produced coatings. For instance, the estimated diffusion coefficients for Ag through diffusion into SiC layer showed that the grain boundary diffusion mechanism is the dominant diffusion mechanism for Ag transport in SiC and not through bulk diffusion [12,16,21–25]. Grain boundaries of SiC provide high diffusivity paths compared to bulk material, as the free volume is higher at the grain boundaries than within the crystals [21,22] The grain boundary excess free volume is stated to be a determining
factor for grain boundary diffusivity by Lojkowski et al [26]. It has been observed that the changes in the grain boundary structure affects the diffusivity into SiC significantly [17]. The diffusion mechanisms of rare-earth oxides into SiO2 and microstructural effects have not been investigated in detail. Having a high diffusivity will bring an advantage of high chemical compatibility between the Rare-Earth silicate slurry with the thermally grown silica layer (TGO) from oxidation, which is aimed when designing a successful EBC. This paper will investigate the effect of deposition conditions, exposure time and temperature on the microstructure and thickness of the coating and hence on the measured diffusion coefficients. 2. Experimental methods Silicon melt infiltrated (MI) SiC/SiC composites were provided by
Fig. 1. a) Backscattered SEM image of Lu-silicate coated Silica based CMC at 1400 °C for 48 h. S sample that’s slurry deposited by half is shown and b) its corresponding EDS analysis.
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Journal of the European Ceramic Society 39 (2019) 4216–4222
E.N. Dayi and N. Al Nasiri
Fig. 2. XRD of (a) Yb-silicate coating, (b) Er-silicate coating and (c) Lu-silicate coatings on CMC at 1400 °C for 48 h revealing mainly monosilicate with presence of disilicates.
Rolls-Royce High Temperature Composite, California, USA. The samples size is 10 × 10 x 4 mm and cleaned with acetone. Prior to slurry dip coating, oxidation was conducted at 1350 °C for 50 h in a tube furnace (Lenton, U.K) in ambient laboratory air to obtain dense and protective SiO2 layer of 6 ± 0.5 μm thick to ensure reaction with RE-oxides to form RE-silicates [27]. Diffusion experiments were performed using either Er2O3 (Alfa Aesar, Heysham, UK 6 ± 0.2 μm particle size;), Yb2O3 (Abcr, Germany, 4 ± 0.1 μm particle size) or Lu2O3 (Abcr, Germany, 4 ± 0.2 μm particle size). All RE-oxides powders were 99.9% pure. The commercial powders used in this study were confirmed as single phase by Al Nasiri et al. [9] using X-Ray Diffraction (XRD) analysis. Water based slurries were prepared for each RE-oxide separately, making the volume percentage 20 vol % for Yb2O3, 30 vol % for Er2O3 and 30 vol % for Lu2O3. For Yb-oxides 0.7 wt% aurinicarboxylic acid (Aluminon, Sigma Aldrich, Dorset, U.K.) dispersant and 2.0 wt% polyethylene glycol 10,000 binder (Alfa Aesar, Hayshem, U.K.) with water pH of 8 was used. For Lu and Er-oxides 3.2 wt% Dolapix CE64 (Zschimmer and Schwarz, Lahnstein, Germany) dispersant and 2.0 wt% polyethylene glycol 10,000 binder (Alfa Aesar, Hayshem, U.K.) was used with a pH of 9.5 [28]. The slurries were homogeneously mixed using a tubular shaker for 24 h. Dip coating technique was used to apply the RE-oxides slurry on the CMCs. Only half of the CMC’s sample was dip coated to obtain a base for thickness calculation. The dip coated samples were heated in a tube furnace (Lenton, U.K) in ambient laboratory air at 1200 ̊C, 1300 ̊C, 1400 ̊C for 12 and 48 h. A heating and cooling rate of 10 ̊C min−1 and 20 ̊C min−1 respectively was used. Prior to characterisation, sample’s cross section was mounted in epoxy resin and were polished to one-micron diamond suspension. Microstructures and coating thickness of polished samples were examined in backscattered electron imaging mode (BS) using a scanning electron
microscope (SEM) JEOL (JSM 6010LA, Tokyo, Japan) equipped with energy‐dispersive spectrometer (EDS). EDS analysis were used to identify coating’s elements. Coating thickness was measured using Image-J program and the average value is obtained using at least 20 points for each sample. Diffusion coefficients for each specimen were calculated using MATLAB program with respect to the measured coating thickness. 3. Results and discussion Fig. 1 shows backscattered SEM image of Lu-silicate deposited on CMC, after 48 h at 1400 °C and its corresponding EDS analysis. Coating
Fig. 3. Backscattered SEM image of Lu-silicate coated Silica based CMC at 1300 °C for 12 h; showing excess, Yb-silicate coating, Silica layer, SiC fiber and SiC/SiC CMC.
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E.N. Dayi and N. Al Nasiri
Fig. 4. Backscattered SEM images of Yb-silicate coating in 1200 °C for (a)12 h and (b)48 h.
Fig. 5. Backscattered SEM images of Er-silicate coating in 1200 °C for (a)12 h and (b)48 h.
was applied only on one side of each sample (left side in Fig.1a), while the right side shows CMC without coating to be used as baseline for thickness calculations. Fig. 1b shows the EDS analysis of Lu-silicate (point 1) and the excess of the deposited slurry (point 2). Point 1 shows presence of Lu, Si and O which are the 3 elements necessary for Lusilicate, supporting the formation of silicate coating. On the other hand, point 2 reveals the presence of only Lu and O, which is known as the excess of the original RE-oxides that did not react with the silica layer during sintering to form RE-silicates. XRD of the three studied EBCs after sintering at 1400 ̊C for 48 h (Fig. 2) reveals that Yb, Er and Lu coatings consists mainly of monosilicates and the rest is disilicates. Oxidation of samples in air creates a thermall grown SiO2 (TGO) layer, which is observed as the darker grey region under the bright coating (Fig. 3). The porous top layer (dark area) is the coating excess that did not react with silica to form silicates. Because the TGO/EBC assemblage must act as a barrier to the permeation of O2 and H2O, the layers must be dense [29]. Therefore, it is important to evaluate the effect of slurry deposition conditions on microstructure. Fig. 4 through 6 shows SEM images of Yb, Er and Lu-coatings respectively at 1200 °C for 12 and 48 h, where TGO can be clearly observed. Fig. 4 shows Yb-coating at 1200 °C after 12 h with thickness around 6.6 μm and a TGO thickness of 2–3 μm (dark grey area underneath the coating). Fig. 5 reveals a very thin coating of Er (1.8 μm) at 1200 °C after 12 h with presence of a relatively thicker silica layer. It has been observed a distinct silica layer present for all samples tested at this temperature. Similar observations for Lu-coating (Fig. 6) with thin coating of 1.3 μm thick. This indicates that the 1200 °C is considered low for a complete reaction to take place between the RE-oxides and the silica layer from pre-oxidation. Increasing the temperature to 1300 °C for 12 and 48 h shows a thicker and denser coatings for the three RE-oxides used in this work
Fig. 6. Backscattered SEM images of Lu-silicate coating in 1200 °C for (a)12 h and (b)48 h. 4219
Journal of the European Ceramic Society 39 (2019) 4216–4222
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Fig. 7. Backscattered SEM images of Yb-silicate coating in 1300 for (a)12 h and (b)48 h.
Fig. 9. Backscattered SEM images of Lu-silicate coating in 1300 °C for (a)12 h and (b)48 h.
(Fig. 7 through 9 respectively). Yb-coating (Fig. 7) shows better coating properties in terms of density and thickness compared to Er and Lucoatings. TGO layer is observed in all samples tested at 1300 °C for 12 and 48 h, however it is thinner compared to samples tested at 1200 °C for 12 and 48. SEM image of Yb at 1300 °C for 12 h show thicker coating on the left side compared to the right side (Fig. 7(a)). However, the opposite is true for the TGO layer underneath. It is observed that the coating thickness is proportional to the diffusivity of slurry into silica during sintering (Fig. 7(b)). For the samples where no silica layer between the CMC and coating were observed, the reaction was considered good as all silica reacted with the slurry (Fig. 8(b) and 9(b)). The results of samples tested at 1400 °C, 12 h and 48 h are given in Fig. 10 through 12 showing the most dense and thick coatings. TGO layer is only observed in samples tested with Er-coatings 1400 °C for 12 h (Fig. 11a), while TGO layer is absent when tested for 48 h (Fig. 11b). The most dense and thick coating is observed in Fig. 10b for Yb-silicate coating (30 μm) followed by Lu-silicate coating (∼ 18 μm) in Fig. 12. The thickness of coatings was calculated using Image-J programme. For each sample a minimum of 20 different points was obtained to calculate the average thickness value. The diffusion coefficients were estimated using the following equation [30]:
D = X 2t
1
(1)
where D is the diffusion coefficient (m2 / s ), X is the average coating thickness (m) measured using Image-J program, and t is the sintering time (s). Estimated diffusion coefficients (m2/s) and measured coating thickness are presented in Table 1 for Yb, Table 2 for Er and Table 3 for Lu. The diffusion coefficients are in the range of 10 −15 –10 -17(m2/s) similar to the values obtained by Friedland et al. [13]. This is also in agreement within the boundaries estimated by Merwe using the
Fig. 8. Backscattered SEM images of Er-silicate coating in 1300 °C for (a)12 h and (b)48 h. 4220
Journal of the European Ceramic Society 39 (2019) 4216–4222
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Fig. 12. Backscattered SEM images of Lu-silicate coating in 1400 °C for (a)12 h and (b)48 h. Fig. 10. Backscattered SEM images of Yb-silicate coating in 1400 °C for (a)12 h and (b)48 h.
German HTR simulation programme [30]. While calculating the diffusion coefficient for the samples where no silver was detected in SiC wafer, they have given an arbitrary value that’s lower than 1 mm for distance, in order to still obtain a value to be used for comparison with the rest of diffusion coefficients. For samples that had no clear diffusion, they calculated an approximate value of a diffusion coefficient around 10 -18 m2/s. This means that the diffusion coefficient should be greater than this value for diffusion to occur. In this study, no diffusion coefficients were calculated in case of no coating present in the system. The fact that a coating containing all three elements (RE, Si, O) formed is a proof that diffusion took place between RE2O3 and SiO2. Therefore, there is a correlation between the value of diffusion coefficient and the thickness of coating. Having diffusion coefficients greater than 10−18 m2/s confirms the presence of RE-silicate coating in addition to EDS analysis. It is observed that the most dense coating occurs at 1400 °C for 48 h in good agreement with the calculated values of the diffusion coefficients (10-15 m2/s) for the three RE-silicate coatings of this work. 4. Conclusion Three different RE-silicates were investigated as potential EBCs on SiC/SiC CMCs. Diffusivity and thickness of the coatings were studied to understand compatibility between TGO/RE-oxides. A higher value of diffusion coefficient means better reaction between TGO and RE-oxides to form thicker and denser coatings. Better adherence of the RE-silicate coatings to the CMC is an advantage as chemical compatibility between the coating and the silica-based ceramic is an indicator of a successful EBC [4]. There is a strong relationship between the deposition conditions and the microstructure of the samples. Increasing the sintering temperature and time lead to thicker and denser coating and hence higher diffusion coefficient values.
Fig. 11. Backscattered SEM images of Er-silicate coating in 1400 °C for (a)12 h and (b)48 h.
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Journal of the European Ceramic Society 39 (2019) 4216–4222
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Table 1 Coating Thickness and Diffusion Coefficients for Ytterbium-oxides with SiO2. 1200 ˚C Exposure time (h) 12 48
Coating thickness (μm) 6.6 ± 3.4 9.8 ± 6.2
1300 °C 2
Diffusion Coefficient (m / s) 1.0 × 10^-15 5.5 × 10^-16
1400 °C 2
Coating thickness (μm) 20.0 ± 7.4 14.9 ± 4.5
Diffusion Coefficient (m / s) 9.3 × 10^-15 1.3 × 10^-15
Coating thickness (μm) 15.6 ± 11.6 30.4 ± 27.1
Diffusion Coefficient (m2/ s) 5.6 × 10^-15 5.4 × 10^-15
Table 2 Coating Thickness and Diffusion Coefficients for Erbium-oxides with SiO2. 1200 ˚C Exposure time (h) 12 48
Coating thickness (μm) 1.8 ± 0.7 3.8 ± 2.3
1300 °C Diffusion Coefficient (m2/s) 7.4x x10^-17 8.1 × 10^-17
1400 °C
Coating thickness (μm) 10.9 ± 3.7 10.3 ± 10.1
Diffusion Coefficient (m2/s) 2.8 × 10^-16 6.17 × 10^-16
Coating thickness (μm) 8.5 ± 4.5 13.5 ± 6.2
Diffusion Coefficient (m2/s) 1.7 × 10^-15 1.0 × 10^-15
Table 3 Coating Thickness and Diffusion Coefficients for Lutetium-oxides with SiO2. 1200 ˚C Exposure time (h) 12 48
Coating thickness (μm) 1.3 ± 0.9 3.7 ± 3.3
1300 °C 2
Diffusion Coefficient (m / s) 3.9 × 10^-17 7.7 × 10^-17
1400 °C
Coating thickness (μm) 9.4 ± 7.2 9.8 ± 7.3
Acknowledgements
2
Diffusion Coefficient (m / s) 2.0 × 10^-15 5.5 × 10^-16
Coating thickness (μm) 7.5 ± 5.2 17.8 ± 9.0
Diffusion Coefficient (m2/ s) 1.3 × 10^-15 1.8 × 10^-15
coated fuel particles, J. Am. Ceram. Soc. 93 (2010) 3076–3079. [14] H.J. Maclean, Silver Transport in CVD Silicon Carbide, in Dept. of Nuclear Eng. Massachusetts Institute of Technology, 2004. [15] J.B. Malberbe, Diffusion of fission products and radiation damage in SiC, J. Phys. D Appl. Phys. 46 (473001) (2013) 27. [16] López‐Honorato Eddie, Huixing Zhang, Da Xiang Yang, Ping Xiao, Silver diffusion in silicon carbide coatings, J. Am. Ceram. Soc. 94 (9) (2011) 3064–3071. [17] J. Deng, H. Ko, P. Demkowicz, D. Morgan, I. Szlufarska, Grain boundary diffusion of Ag through polycrystalline SiC in TRISO fuel particles, J. of Nuclear Materials 467 (2015) 332–340. [18] P. Demkowicz, J. Hunn, R. Morris, J. Harp, P. Winston, C. Baldwin, F. Montgomery, Preliminary results of post-irradiation examination of the AGR-1 TRISO fuel compacts, Proceedings of the HTR 2012 (2012). [19] C. Baldwin, J. Hunn, R. Morris, F. Montgomery, C. Silva, P. Demkowicz, First elevated-temperature performance testing of coated particle fuel compacts from the AGR-1 irradiation experiment, Nucl. Eng. Des. 271 (2014) 131–141. [20] B.G. Kim, S. Yeo, Y.W. Lee, M.S. Cho, Comparison of diffusion coefficients and activation energies for Ag diffusion in Silicon Carbide, Nucl. Eng. Technol 47 (2015) 608–616. [21] R. Kirschhofer, J.D. Hunn, P.A. Demkowicz, J.I. Cole, B.P. Gorman, Microstructure of TRISO coated particles from the AGR-1 experiment: SiC grain size and grain boundary character, J. of Nuclear Materials 432 (1–3) (2013) 127–134. [22] P. Shewmon, Diffusion in Solids, 2nd ed., TMS, Warrendale, PA, 1989. [23] D. Shrader, S.M. Khalil, T. Gerczak, T.R. Allen, A.J. Heim, I. Szlufarska, D. Morgan, Ag diffusion in cubic silicon carbide, J. of Nuclear Materials 408 (2011) 257–271. [24] J. Rabone, E. Loṕez-Honorato, P.V. Uffelen, Silver and cesium diffusion dynamics at the β‐SiC Σ5 grain boundary investigated with density functional theory molecular dynamics and metadynamics, The Jour. of Phys. Chemistry 118 (2014) 915–926. [25] S. Khalil, N. Swaminathan, D. Shrader, A.J. Heim, D.D. Morgan, I. Szlufarska, Diffusion of Ag along 3 grain boundaries in 3C-SiC, Phys. Rev. B 84 (2011). [26] W. Lojkowski, L.S. Shvindlerman, G. Gottstein, V.A. Ivanov, D.A. Molodov, D. Kolesnikov, Grain boundary excess free volume - direct thermodynamic measurement, J. Mater. Sci. 41 (23) (2006) 7725–7729. [27] N. Al Nasiri, et al., Oxidation behaviour of SiC/SiC ceramic matrix composites in air, J. Eur. Ceram. Soc. 36 (2016) 3293–3302. [28] N. Al Nasiri, et al., Slurry-based Reaction Bonded Environmental Barrier Coating R. Royce, USA Patent Office, Application Number 15/488037. Oct. (2017). [29] D.L. Poerschke, J.H. Shaw, N. Verma, F.W. Zok, C.G. Levi, Interaction of yttrium disilicate environmental barrier coatings with calcium-magnesium-iron aluminosilicate melts, Acta Mater. 145 (2018) 451–461. [30] J.J. van der Merwe, Evaluation of silver transport through SiC during the German HTR fuel program, J. of Nuclear Materials 395 (2009) 99–111.
This project was supported by the Royal Academy of Engineering under the Research Fellowship Scheme. The authors would like to thank Rolls-Royce plc for providing the financial and the technical support for this project. References [1] K.N. Lee, 4.4.2 protective coatings for gas turbines, Gas Turbine Handbook, (2006), pp. 419–435. [2] K.N. Lee, D.S. Fox, N.P. Bansal, Rare earth silicate environmental barrier coatings for SiC/SiC composites and Si3N4 ceramics, J. Eur. Ceram. Soc. 25 (10) (2005) 1705–1715. [3] N.S. Jacobson, Corrosion of silicon‐based ceramics in combustion environments, J. Eur. Ceram. Soc. 76 (1) (1993) 3–28. [4] I. Spitsberg, T. H.B, Thermal / Environmental Barrier Coating System for Siliconcontaining Metals, (2013). [5] J. Federer, Alumina base coatings for protection of SiC ceramics, J. Mater. Eng. 12 (1990) 141–149. [6] K.N. Lee, R. Miller, N. Jacobson, New generation of plasma-sprayed mullite coatings on Silicon Carbide, J. Am. Ceram. Soc. 78 (3) (2014) 705–710. [7] K.N. Lee, Upper temperature limit of environmental barrier coatings based on mullite and BSAS, J. Am. Ceram. Soc. 86 (8) (2003) 1299–1306. [8] C. Cocojaru, S. Kruger, S. Moreau, R. Lima, Elastic Modulus evolution and behavior of Si/Mullite/BSAS-Based environmental barrier coatings exposed to high temperature in water vapor environment, J. Therm. Spray Technol. 20 (1–2) (2010) 92–99. [9] N. Al Nasiri, N. Patra, D. Horlait, D.D. Jayaseelan, W.E. Lee, Thermal properties of rare-earth monosilicates for EBC on Si-Based ceramic composites, J. Am. Ceram. Soc. 99 (2) (2016) 589–596. [10] A. Abdul-Aziz, R.T. Bhatt, Modeling of thermal residual stress in environmental barrier coated Fiber reinforced ceramic matrix composite, J. Compos. Mater. 46 (10) (2012) 1211–1218. [11] E.K. Arthur, E. Ampaw, S.T. Azeko, Y. Danyuo, Design of thermally reliable environmental barrier coating for a SiC/SiC ceramic matrix composites, J. of Composite Mater. 3 (6) (2013) 191–197. [12] E. Friedland, J.B. Malherbe, N.G. Van der Berg, T. Hlatshwayo, A.J. Botha, E. Wendler, E. Wesch, Study of silver diffusion in silicon carbide, J. of Nuclear Materials 389 (2) (2009) 326–331. [13] E. López-Honorato, Y. DaXiang, T. Jun, P.J. Meadows, P. Xiao, Silver diffusion in
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Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Diffusion study of rare-earth oxides into silica layer for environmental barrier coating applications E.N. Dayia, N. Al Nasirib, a b
T
⁎
Sabanci University, Department of Materials Science and Nanoengineering, Tuzla, Orta Mahalle, Istanbul 34956, Turkey Centre for Advanced Structural Ceramics, Department of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
ARTICLE INFO
ABSTRACT
Keywords: Environmental barrier coating RE-oxides Diffusion CMCs
The effect of exposure temperature and time on the diffusion rate of rare-earth oxides applied on silicon carbide fiber-reinforced SiC ceramic matrix composites (SiC/SiC CMCs) have been investigated. Knowledge on diffusion mechanism between the deposited rare-earth (RE) slurry and silica layer is necessary to understand the process governing EBCs formation and their properties. SEM/EDS analysis were used to study the effect of microstructure on diffusivity. The diffusion coefficient increases with increasing sintering temperature and time. The measured diffusion coefficients of the RE-coating into silica layer were in the order of 10−15-10-17 m2/s revealing an overall good adhesion on the SiC/SiC CMCs.
1. Introduction Economic and environmental concerns to improve efficiency and reducing emissions, are the main driving forces behind the ever-increasing demand for higher gas turbine engine inlet temperatures. Technology improvements in cooling, materials and coatings are required to achieve higher inlet temperatures [1]. Si-based ceramics, such as silicon carbide (SiC) fiber-reinforced SiC ceramic matrix composites (SiC/SiC CMCs) exhibit superior high-temperature strength and durability, indicating their potential to revolutionize gas turbine technology [2]. They are also light and possess excellent high temperature oxidation resistance in dry air, due to the formation of slow-growing, protective silica scale [3]. However, when the Si-based ceramic is exposed to corrosive environments containing high-pressure steam at elevated temperatures, they are susceptible to hot-corrosion and recession due to the reaction of silica with water vapour to form volatile Si(OH)4 (g) [1]. Environmental barrier coatings (EBCs) are widely used to protect CMCs from such degradation. EBCs must have phase stability and low coefficient of thermal expansion (CTE) with the substrate. Additionally, they must have low permeability to oxygen and chemical compatibility with the silica scale formed from oxidation [4]. Alumina-based coatings such as mullite were attractive choices due to their low thermal expansion coefficients (TEC) (5.1 9 10−6 K −1 from 298 to 1773 K) and good adherence to Si-based ceramics [5].
⁎
Unfortunately, the rapid cooling and heating caused serious problems such as cracking and delamination of the mullite coating [6]. The use of barium strontium aluminosilicate (BSAS) coatings with mullite and silicon in the coatings have shown better performance [7]. BSAS has high temperature stability and low TEC of 4.0–5.15 10−6 K −1 (300–1600 K) [8]. The main drawback of BSAS coatings are their reaction with silica at ˜1300 °C leading to pore formation and coating spallation [7]. Al Nasiri et al. [9] studied the thermal properties of Yb, Er and Lu monosilicates and showed that the low TEC and thermal conductivity values make these RE-monosilicates competitive EBCs for Si-based ceramics. Therefore, further investigation of the properties of these materials is important to understand and improve their performances as promising new generation EBC candidates. Microstructure and durability of such coatings depend on the thickness and the processing methods used [10,11]. The effect of diffusion rate on the thickness of RE-silicates with silica has not been previously investigated. Several studies have been conducted to investigate silver diffusion in coated particles, such as tristructural isotropic (TRISO) coated fuel particle [12–15]. Honorato et al. [13] used a TRISO particle, which consists of a uranium kernel coated with three layers of pyrolytic carbon and a layer of SiC. They developed a new method capable of trapping thin layers of silver between two layers of SiC in these coated fuel particles. This method studied the effect of SiC microstructure on the diffusion of silver without an irradiation effect. Their results confirmed that the diffusion does not occur due to the presence of high excess silicon or formation of defects. Instead, the diffusion process is
Corresponding author. E-mail address: [email protected] (N. Al Nasiri).
https://doi.org/10.1016/j.jeurceramsoc.2019.05.026 Received 5 February 2019; Received in revised form 30 April 2019; Accepted 15 May 2019 Available online 23 May 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Journal of the European Ceramic Society 39 (2019) 4216–4222
E.N. Dayi and N. Al Nasiri
governed by grain boundary diffusion and hence depends on the microstructural effects such as nano and micro porosity of SiC. Other studies revealed correlation between the microstructural changes and diffusion properties into coatings [16–20]. It has been reported that there is a variance in the diffusivity of silver into SiC layer and the scarcity of diffusion coefficients are suggested to be caused by the differences in the properties of the SiC coating. Estimation of diffusion coefficients provides a good insight to the transport mechanisms of the produced coatings. For instance, the estimated diffusion coefficients for Ag through diffusion into SiC layer showed that the grain boundary diffusion mechanism is the dominant diffusion mechanism for Ag transport in SiC and not through bulk diffusion [12,16,21–25]. Grain boundaries of SiC provide high diffusivity paths compared to bulk material, as the free volume is higher at the grain boundaries than within the crystals [21,22] The grain boundary excess free volume is stated to be a determining
factor for grain boundary diffusivity by Lojkowski et al [26]. It has been observed that the changes in the grain boundary structure affects the diffusivity into SiC significantly [17]. The diffusion mechanisms of rare-earth oxides into SiO2 and microstructural effects have not been investigated in detail. Having a high diffusivity will bring an advantage of high chemical compatibility between the Rare-Earth silicate slurry with the thermally grown silica layer (TGO) from oxidation, which is aimed when designing a successful EBC. This paper will investigate the effect of deposition conditions, exposure time and temperature on the microstructure and thickness of the coating and hence on the measured diffusion coefficients. 2. Experimental methods Silicon melt infiltrated (MI) SiC/SiC composites were provided by
Fig. 1. a) Backscattered SEM image of Lu-silicate coated Silica based CMC at 1400 °C for 48 h. S sample that’s slurry deposited by half is shown and b) its corresponding EDS analysis.
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Fig. 2. XRD of (a) Yb-silicate coating, (b) Er-silicate coating and (c) Lu-silicate coatings on CMC at 1400 °C for 48 h revealing mainly monosilicate with presence of disilicates.
Rolls-Royce High Temperature Composite, California, USA. The samples size is 10 × 10 x 4 mm and cleaned with acetone. Prior to slurry dip coating, oxidation was conducted at 1350 °C for 50 h in a tube furnace (Lenton, U.K) in ambient laboratory air to obtain dense and protective SiO2 layer of 6 ± 0.5 μm thick to ensure reaction with RE-oxides to form RE-silicates [27]. Diffusion experiments were performed using either Er2O3 (Alfa Aesar, Heysham, UK 6 ± 0.2 μm particle size;), Yb2O3 (Abcr, Germany, 4 ± 0.1 μm particle size) or Lu2O3 (Abcr, Germany, 4 ± 0.2 μm particle size). All RE-oxides powders were 99.9% pure. The commercial powders used in this study were confirmed as single phase by Al Nasiri et al. [9] using X-Ray Diffraction (XRD) analysis. Water based slurries were prepared for each RE-oxide separately, making the volume percentage 20 vol % for Yb2O3, 30 vol % for Er2O3 and 30 vol % for Lu2O3. For Yb-oxides 0.7 wt% aurinicarboxylic acid (Aluminon, Sigma Aldrich, Dorset, U.K.) dispersant and 2.0 wt% polyethylene glycol 10,000 binder (Alfa Aesar, Hayshem, U.K.) with water pH of 8 was used. For Lu and Er-oxides 3.2 wt% Dolapix CE64 (Zschimmer and Schwarz, Lahnstein, Germany) dispersant and 2.0 wt% polyethylene glycol 10,000 binder (Alfa Aesar, Hayshem, U.K.) was used with a pH of 9.5 [28]. The slurries were homogeneously mixed using a tubular shaker for 24 h. Dip coating technique was used to apply the RE-oxides slurry on the CMCs. Only half of the CMC’s sample was dip coated to obtain a base for thickness calculation. The dip coated samples were heated in a tube furnace (Lenton, U.K) in ambient laboratory air at 1200 ̊C, 1300 ̊C, 1400 ̊C for 12 and 48 h. A heating and cooling rate of 10 ̊C min−1 and 20 ̊C min−1 respectively was used. Prior to characterisation, sample’s cross section was mounted in epoxy resin and were polished to one-micron diamond suspension. Microstructures and coating thickness of polished samples were examined in backscattered electron imaging mode (BS) using a scanning electron
microscope (SEM) JEOL (JSM 6010LA, Tokyo, Japan) equipped with energy‐dispersive spectrometer (EDS). EDS analysis were used to identify coating’s elements. Coating thickness was measured using Image-J program and the average value is obtained using at least 20 points for each sample. Diffusion coefficients for each specimen were calculated using MATLAB program with respect to the measured coating thickness. 3. Results and discussion Fig. 1 shows backscattered SEM image of Lu-silicate deposited on CMC, after 48 h at 1400 °C and its corresponding EDS analysis. Coating
Fig. 3. Backscattered SEM image of Lu-silicate coated Silica based CMC at 1300 °C for 12 h; showing excess, Yb-silicate coating, Silica layer, SiC fiber and SiC/SiC CMC.
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Fig. 4. Backscattered SEM images of Yb-silicate coating in 1200 °C for (a)12 h and (b)48 h.
Fig. 5. Backscattered SEM images of Er-silicate coating in 1200 °C for (a)12 h and (b)48 h.
was applied only on one side of each sample (left side in Fig.1a), while the right side shows CMC without coating to be used as baseline for thickness calculations. Fig. 1b shows the EDS analysis of Lu-silicate (point 1) and the excess of the deposited slurry (point 2). Point 1 shows presence of Lu, Si and O which are the 3 elements necessary for Lusilicate, supporting the formation of silicate coating. On the other hand, point 2 reveals the presence of only Lu and O, which is known as the excess of the original RE-oxides that did not react with the silica layer during sintering to form RE-silicates. XRD of the three studied EBCs after sintering at 1400 ̊C for 48 h (Fig. 2) reveals that Yb, Er and Lu coatings consists mainly of monosilicates and the rest is disilicates. Oxidation of samples in air creates a thermall grown SiO2 (TGO) layer, which is observed as the darker grey region under the bright coating (Fig. 3). The porous top layer (dark area) is the coating excess that did not react with silica to form silicates. Because the TGO/EBC assemblage must act as a barrier to the permeation of O2 and H2O, the layers must be dense [29]. Therefore, it is important to evaluate the effect of slurry deposition conditions on microstructure. Fig. 4 through 6 shows SEM images of Yb, Er and Lu-coatings respectively at 1200 °C for 12 and 48 h, where TGO can be clearly observed. Fig. 4 shows Yb-coating at 1200 °C after 12 h with thickness around 6.6 μm and a TGO thickness of 2–3 μm (dark grey area underneath the coating). Fig. 5 reveals a very thin coating of Er (1.8 μm) at 1200 °C after 12 h with presence of a relatively thicker silica layer. It has been observed a distinct silica layer present for all samples tested at this temperature. Similar observations for Lu-coating (Fig. 6) with thin coating of 1.3 μm thick. This indicates that the 1200 °C is considered low for a complete reaction to take place between the RE-oxides and the silica layer from pre-oxidation. Increasing the temperature to 1300 °C for 12 and 48 h shows a thicker and denser coatings for the three RE-oxides used in this work
Fig. 6. Backscattered SEM images of Lu-silicate coating in 1200 °C for (a)12 h and (b)48 h. 4219
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Fig. 7. Backscattered SEM images of Yb-silicate coating in 1300 for (a)12 h and (b)48 h.
Fig. 9. Backscattered SEM images of Lu-silicate coating in 1300 °C for (a)12 h and (b)48 h.
(Fig. 7 through 9 respectively). Yb-coating (Fig. 7) shows better coating properties in terms of density and thickness compared to Er and Lucoatings. TGO layer is observed in all samples tested at 1300 °C for 12 and 48 h, however it is thinner compared to samples tested at 1200 °C for 12 and 48. SEM image of Yb at 1300 °C for 12 h show thicker coating on the left side compared to the right side (Fig. 7(a)). However, the opposite is true for the TGO layer underneath. It is observed that the coating thickness is proportional to the diffusivity of slurry into silica during sintering (Fig. 7(b)). For the samples where no silica layer between the CMC and coating were observed, the reaction was considered good as all silica reacted with the slurry (Fig. 8(b) and 9(b)). The results of samples tested at 1400 °C, 12 h and 48 h are given in Fig. 10 through 12 showing the most dense and thick coatings. TGO layer is only observed in samples tested with Er-coatings 1400 °C for 12 h (Fig. 11a), while TGO layer is absent when tested for 48 h (Fig. 11b). The most dense and thick coating is observed in Fig. 10b for Yb-silicate coating (30 μm) followed by Lu-silicate coating (∼ 18 μm) in Fig. 12. The thickness of coatings was calculated using Image-J programme. For each sample a minimum of 20 different points was obtained to calculate the average thickness value. The diffusion coefficients were estimated using the following equation [30]:
D = X 2t
1
(1)
where D is the diffusion coefficient (m2 / s ), X is the average coating thickness (m) measured using Image-J program, and t is the sintering time (s). Estimated diffusion coefficients (m2/s) and measured coating thickness are presented in Table 1 for Yb, Table 2 for Er and Table 3 for Lu. The diffusion coefficients are in the range of 10 −15 –10 -17(m2/s) similar to the values obtained by Friedland et al. [13]. This is also in agreement within the boundaries estimated by Merwe using the
Fig. 8. Backscattered SEM images of Er-silicate coating in 1300 °C for (a)12 h and (b)48 h. 4220
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Fig. 12. Backscattered SEM images of Lu-silicate coating in 1400 °C for (a)12 h and (b)48 h. Fig. 10. Backscattered SEM images of Yb-silicate coating in 1400 °C for (a)12 h and (b)48 h.
German HTR simulation programme [30]. While calculating the diffusion coefficient for the samples where no silver was detected in SiC wafer, they have given an arbitrary value that’s lower than 1 mm for distance, in order to still obtain a value to be used for comparison with the rest of diffusion coefficients. For samples that had no clear diffusion, they calculated an approximate value of a diffusion coefficient around 10 -18 m2/s. This means that the diffusion coefficient should be greater than this value for diffusion to occur. In this study, no diffusion coefficients were calculated in case of no coating present in the system. The fact that a coating containing all three elements (RE, Si, O) formed is a proof that diffusion took place between RE2O3 and SiO2. Therefore, there is a correlation between the value of diffusion coefficient and the thickness of coating. Having diffusion coefficients greater than 10−18 m2/s confirms the presence of RE-silicate coating in addition to EDS analysis. It is observed that the most dense coating occurs at 1400 °C for 48 h in good agreement with the calculated values of the diffusion coefficients (10-15 m2/s) for the three RE-silicate coatings of this work. 4. Conclusion Three different RE-silicates were investigated as potential EBCs on SiC/SiC CMCs. Diffusivity and thickness of the coatings were studied to understand compatibility between TGO/RE-oxides. A higher value of diffusion coefficient means better reaction between TGO and RE-oxides to form thicker and denser coatings. Better adherence of the RE-silicate coatings to the CMC is an advantage as chemical compatibility between the coating and the silica-based ceramic is an indicator of a successful EBC [4]. There is a strong relationship between the deposition conditions and the microstructure of the samples. Increasing the sintering temperature and time lead to thicker and denser coating and hence higher diffusion coefficient values.
Fig. 11. Backscattered SEM images of Er-silicate coating in 1400 °C for (a)12 h and (b)48 h.
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Table 1 Coating Thickness and Diffusion Coefficients for Ytterbium-oxides with SiO2. 1200 ˚C Exposure time (h) 12 48
Coating thickness (μm) 6.6 ± 3.4 9.8 ± 6.2
1300 °C 2
Diffusion Coefficient (m / s) 1.0 × 10^-15 5.5 × 10^-16
1400 °C 2
Coating thickness (μm) 20.0 ± 7.4 14.9 ± 4.5
Diffusion Coefficient (m / s) 9.3 × 10^-15 1.3 × 10^-15
Coating thickness (μm) 15.6 ± 11.6 30.4 ± 27.1
Diffusion Coefficient (m2/ s) 5.6 × 10^-15 5.4 × 10^-15
Table 2 Coating Thickness and Diffusion Coefficients for Erbium-oxides with SiO2. 1200 ˚C Exposure time (h) 12 48
Coating thickness (μm) 1.8 ± 0.7 3.8 ± 2.3
1300 °C Diffusion Coefficient (m2/s) 7.4x x10^-17 8.1 × 10^-17
1400 °C
Coating thickness (μm) 10.9 ± 3.7 10.3 ± 10.1
Diffusion Coefficient (m2/s) 2.8 × 10^-16 6.17 × 10^-16
Coating thickness (μm) 8.5 ± 4.5 13.5 ± 6.2
Diffusion Coefficient (m2/s) 1.7 × 10^-15 1.0 × 10^-15
Table 3 Coating Thickness and Diffusion Coefficients for Lutetium-oxides with SiO2. 1200 ˚C Exposure time (h) 12 48
Coating thickness (μm) 1.3 ± 0.9 3.7 ± 3.3
1300 °C 2
Diffusion Coefficient (m / s) 3.9 × 10^-17 7.7 × 10^-17
1400 °C
Coating thickness (μm) 9.4 ± 7.2 9.8 ± 7.3
Acknowledgements
2
Diffusion Coefficient (m / s) 2.0 × 10^-15 5.5 × 10^-16
Coating thickness (μm) 7.5 ± 5.2 17.8 ± 9.0
Diffusion Coefficient (m2/ s) 1.3 × 10^-15 1.8 × 10^-15
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