OS2-9
RE2O3-SiO2 ガラスへの熱処理による亀裂修復の観察
Observation of crack healing by heat treatment on
RE2O3-SiO2 glass
大川采久 1, Nguyen Thanh Son2, 長谷川拓哉 1, 中山忠親 3, 石川毅彦 4, 下西里奈 4, 小山千尋 4, 織田裕久
4, 増野敦信 5 , 殷澍 1, 6Ayahisa OKAWA1, Nguyen Thanh SON2, Takuya HASEGAWA1, Tadachika NAKAYAMA3, Takehiko
ISHIKAWA4, Rina Shimonishi4, Chihiro KOYAMA4, Hirohisa ODA4, Atsunobu MASUNO5, and Shu
YIN1,6
東北大学 多元物質科学研究所, Institute of Multidisciplinary Research for Advanced Material (IMRAM), Tohoku
University
2 釧路工業高等専門学校 創造工学科, Department of Creative Engineering, National Institute of Technology, Kushiro
College
3 長岡技術科学大学 極限エネルギー密度工学研究センター，Extreme Energy-Density Research Institute, Nagaoka
University of Technology
4 宇宙航空研究開発機構, Japan Aerospace Explanation Agency, Tsukuba
5 京都大学 大学院工学研究科, Graduate School of Engineering, Kyoto University
6 東北大学 材料科学高等研究所, Advanced Institute for Materials Research (WPI-AIMR), Tohoku University
1

1. Introduction
SiC fiber reinforced SiC matrix composites (SiCf/SiCm) show promise for future gas turbine blade
applications. However, SiC suffers from recession in high-temperature, water vapor rich environments,
necessitating an environmental barrier coating (EBC). Typical EBC systems comprise a Si bond coat foroxidation resistance and adhesion improvement, topped with rare earth (RE) silicates like RE2Si2O7. Highentropy RE silicates, containing five or more equimolar RE elements, have recently gained interest forenhancing EBC performance. The β- RE2Si2O7 structure (with RE 6-coordination under 0.885 Å) 1) is consideredmost promising due to its lack of polymorphism during operation temperature range.
To effectively protect SiC from water vapor recession, EBCs must be dense. However, conventional
thermal spraying processes result in rapid cooling and amorphization of coatings due to temperature
differences with the substrate. While subsequent annealing crystallizes the coating, it also leads to residual
cracks and pores, compromising EBC effectiveness. On the other hand, the potential for crack healing throughirreversible phase transitions during annealing (from metastable to stable phases with volume expansion) 2) isappealing, but the thermal evolution of coating microstructure and phases remains poorly understood. This
analysis is further complicated by SiO volatilization during thermal spraying, localized element segregationin coatings3), and the multi-element of REs4). Moreover, the crucial thermophysical properties5) of RE silicates
for thermal spraying processes have not been reported. This study aims to elucidate the thermal evolution of
microstructure and phases in RE silicate glasses without local element segregation, demonstrating crack
healing potential, and to determine their thermophysical properties.
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2.

Experimental Procedures
To form β-RE2Si2O7, the rare earth elements Lu, Yb, Er, Y, and Ho were selected. The term "3RE" will be

used to denote RE sites with equal molar ratios of Lu, Yb, and Er, while "5RE" will represent the inclusion of
Ho and Y. Separate aqueous solutions of RE nitrate (RE(NO 3)3・nH2O) and sodium silicate (Na2SiO3・9H2O)
were prepared using 30 mL of distilled water each, maintaining a stoichiometric RE:Si ratio of 1.0. The Na 2SiO3
solution was treated with 2M HCl, while dilute ammonia was added dropwise to the RE(NO 3)3 solutions.
These solutions were then combined and sealed in an autoclave, which was held at 220°C for 3 h. The resulting
powder was washed via centrifugation and subsequently heat-treated at 1600°C for 10 h in air. The powder
was then subjected to CO2 laser irradiation using a gas-levitation furnace. X-ray diffraction was employed toanalyze the crystallographic phases of the powders prior to CO 2 laser irradiation. Raman spectroscopy wasused to examine the laser-irradiated samples. The density of the glass was determined using a pycnometer.
Vickers-indented samples were heat-treated in air: 10 h at 1000°C, then 10 h at 1025°C.
3.

Results and discussionXRD analysis revealed that the main phases of the synthesized powders were β-RE2Si2O7. A slight shift

in diffraction peaks towards lower angles was observed as the average RE ionic radius increased beyond thatof Yb, indicating lattice expansion with the incorporation of larger RE elements. These irradiated samples
exhibited only broad bands in their spectra, suggesting an amorphous structure similar to as-sprayed RE2Si2O7
reported in previous studies6). EDS analysis confirmed that the stoichiometric ratios in all irradiated samples
generally aligned with the initial ratios, and rare earth elements were uniformly distributed throughout thesamples. Glass densities were measured at 6.41 g/cm3, 6.34 g/cm3, and 5.80 g/cm3 for Yb, 3RE, and 5RE samples,respectively. The theoretical density of β-Yb2Si2O7 is 6.15 g/cm3, this suggests the possibility of volume
expansion during crystallization. Upon exposure to 1000°C, the Yb sample exhibited partial devitrification,
while the 3RE and 5RE samples retained their transparency. Subsequent heat treatment at 1025°C induced
complete devitrification in all samples, suggesting crystallization had occurred. New surface cracks wereobserved in all samples following heat treatment at 1000°C. The cracks formed by pre-Vickers indentation
showedpartialhealing

(Fig.

1).

This

microstructural evolution could be attributed
to various factors such as viscous flow,
crystallization,

orvolume

expansionassociatedwithphasetransitions

metastable

tostablephases.

fromelucidation of these mechanisms warrants
additional

investigation

in

10 μm

5 μm

Further

subsequent

Figure 1. Surface images of the Yb sample
after annealing at 1025 °C for 10 h in air.

studies.
References
1) Luo, Y., Sun, L., Wang, J., Du, T., Zhou, C., Zhang, J., Wang, J.: Phase formation capability andcompositional design of β-phase multiple rare-earth principal component disilicates. Nat. Commun., 14(2023) 1275, https://doi.org/10.1038/s41467-023-36947-6
2 of 32) Bakan, E., & Vaßen, R.: Crack healing mechanisms in atmospheric plasma sprayed Yb-silicate coatings
during post-process heat treatment, J Euro. Ceram. Soc., 43 (2023) 3684-3693,
https://doi.org/10.1016/j.jeurceramsoc.2023.02.0053) Vaßen, R., Bakan, E., Gatzen, C., Kim, S., Mack, D. E., Guillon, O.: Environmental barrier coatings madeby different thermal spray technologies, Coatings, 9 (2019) 784, https://doi.org/10.3390/coatings9120784
4) Peng, Y., Luo, Z., Wang, H., Du, T., Zhang, J., Sun, L., Wang, J.: Crystallization and phase evolution innovel (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6) 2Si2O7 environmental barrier coating. Int. J. Appl. Ceram., 20(2023) 896, https://doi.org/10.1111/ijac.14210
5) Nasiri, M. M., Dolatabadi, A., & Moreau, C.: Modeling of liquid detachment and fragmentation during theimpact of plasma spray particles on a cold substrate, Int. J. Heat Mass Transf., 189 (2022) 122718,
https://doi.org/10.1016/j.ijheatmasstransfer.2022.122718
6) Garcia, E., Sotelo-Mazon, O., Poblano-Salas, C. A., Trapaga, G., & Sampath, S.: Characterization ofYb2Si2O7–Yb2SiO5 composite environmental barrier coatings resultant from in situ plasma sprayprocessing. Ceram. Int., 46 (2020) 21328, https://doi.org/10.1016/j.ceramint.2020.05.228© 2024 by the authors. Submitted for possible open access publication under theterms and conditions of the Creative Commons Attribution (CC BY) license(http://creativecommons.org/li censes/by/4.0/).

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