Neutron Pair Distribution Function Analysis to the Local Structure of High-Entropy RE2SiO5 Environmental Barrier Coating

doi:10.11900/0412.1961.2024.00115

Acta Metall Sin

2024, Vol. 60

Issue (8): 1130-1140 DOI: 10.11900/0412.1961.2024.00115

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Neutron Pair Distribution Function Analysis to the Local Structure of High-Entropy RE₂SiO₅ Environmental Barrier Coating

WANG Haoyu¹, LÜ Xirui¹, CHEN Qi¹^,², XIONG Ying³, LUO Zhixin¹, ZHANG Jie¹(

), WANG Jingyang¹

1 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2 School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230000, China
3 AECC Shenyang Liming AERO-ENGINE Science and Technology Co. Ltd., Shenyang 110043, China

Cite this article:

WANG Haoyu, LÜ Xirui, CHEN Qi, XIONG Ying, LUO Zhixin, ZHANG Jie, WANG Jingyang. Neutron Pair Distribution Function Analysis to the Local Structure of High-Entropy RE₂SiO₅ Environmental Barrier Coating. Acta Metall Sin, 2024, 60(8): 1130-1140.

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Abstract

Environmental barrier coatings (EBCs) enable SiC_f/SiC ceramic matrix composites (CMC) to operate under high-temperature combustion conditions. They reduce the oxidation rate of SiC_f/SiC, the volatilization of the composites due to reaction with water vapor, and the surface temperature of the composites. Rare-earth monosilicates (RE₂SiO₅), owing to their excellent high-temperature durability, low thermal conductivity, and good phase stability, are used as the top layer of EBCs. However, they exhibit a high coefficient of thermal expansion (CTE), leading to thermal mismatch and inducing tensile residual stress (with a magnitude of several hundred MPa) in the coating, resulting in the formation of vertical cracks, which act as extremely-high-diffusivity paths for oxidation species transportation and silica volatilization. Therefore, regulating the CTE of RE₂SiO₅ EBCs and minimizing the CTE mismatch among constituent RE₂SiO₅ layers with SiC_f/SiC CMCs are critical for multilayered EBCs. Through atmospheric plasma spraying, a typical Yb₂SiO₅/Yb₂Si₂O₇/Si coating system and a tri-layer structured multicomponent (Y_1/4Ho_1/4Er_1/4Yb_1/4)₂SiO₅/Yb₂Si₂O₇/Si coating system with better matched CTEs were manufactured. Both the coatings remained adhered to the substrate during deposition and after annealing, and no mud cracks that would compromise the coating gas-tightness quality and delamination cracks were observed at any of the coating interfaces. In thermal cycling tests, (Y_1/4Ho_1/4Er_1/4Yb_1/4)₂SiO₅/Yb₂Si₂O₇/Si coatings showed a lifetime that is three times longer than that of conventional Yb₂SiO₅/Yb₂Si₂O₇/Si coatings. The failure mechanisms in thermal cycling were investigated via the finite element simulation of stress. It was found that the stress in the substrate was low, and the residual thermal stress was mainly concentrated on the top, inter, and bond layers and increased with an increase in temperature. Compared with that of the (Y_1/4Ho_1/4Er_1/4Yb_1/4)₂SiO₅ top coat, the Yb₂SiO₅ top coat showed obviously higher residual tensile stress, which contributed to a higher tendency for mud-crack formation and higher energy release rate, substantially reducing the coating's thermal cycling lifetime. Through neutron powder diffraction and pair distribution function (PDF) analysis, the average and local structures of RE₂SiO₅ were studied. Overall, the average and local structures did not differ significantly, both of which can be described using the C2/c structure. Nevertheless, the PDF results demonstrated some differences in the disorder degree of Si—O and RE—O coordination environments. In particular, Rietveld refinement results of the PDF showed lower local distortion degree of [ORE₄] tetrahedrons when compared with that of the average structure. It is effective to reduce the distortion degree of [ORE₄] tetrahedrons by introducing Y³⁺, Ho³⁺, and Er³⁺ into the Yb³⁺ sites of Yb₂SiO₅, and smaller distortion degrees lead to lower CTE values. Coordinative local disturbances introduced by strategic high-entropy design have been proposed as the key method for CTE regulation.

Key words: neutron diffraction pair distribution function rare-earth silicate environmental barrier coating burner rig test

Received: 23 April 2024

ZTFLH:

TG174.4

Fund: National Natural Science Foundation of China(U21A2063);National Natural Science Foundation of China(52372071);National Natural Science Foundation of China(52302076);National Natural Science Foundation of China(92360304)

Corresponding Authors: ZHANG Jie, professor, Tel: (024)23970490, E-mail: jiezhang@imr.ac.cn

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	WANG Haoyu
	LÜ Xirui
	CHEN Qi
	XIONG Ying
	LUO Zhixin
	ZHANG Jie
	WANG Jingyang

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https://www.ams.org.cn/EN/10.11900/0412.1961.2024.00115 OR https://www.ams.org.cn/EN/Y2024/V60/I8/1130

Table 1 Plasma spray parameters for deposition of RE₂SiO₅ top layer, Yb₂Si₂O₇ intermediate layer, and Si bond layer

Fig.1 Cross-sectional SEM image and finite element mesh model of tri-layer structured environmental barrier coating (EBC)

Table 2 Young's moduli and coefficients of thermal expansion (CTE) of EBC system components^{[10,22,23,25-29]}

Fig.2 XRD spectra of (Y_1/4Ho_1/4Er_1/4Yb_1/4)₂SiO₅/Yb₂-Si₂O₇/Si (a) and Yb₂SiO₅/Yb₂Si₂O₇/Si (b) coatings after annealing

Table 3 Phase fractions in annealed (Y_1/4Ho_1/4Er_1/4Yb_1/4)₂SiO₅ and Yb₂SiO₅ topcoats and corresponding Rietveld refinements factors

Fig.3 Cross-section SEM images of (Y_1/4Ho_1/4Er_1/4Yb_1/4)₂SiO₅/Yb₂Si₂O₇/Si (a, c, e, f) and Yb₂SiO₅/Yb₂Si₂O₇/Si (b, d, g, h) coatings after annealing (a, b) and thermal cycling for 300 cyc (c, e, f) and 83 cyc (d, g, h) (White arrows in Fig.2c indicate the locations of vertical cracks in top layer)

Fig.4 Residual thermal stresses in different layers whiling cooling from different temperatures to room temperature of (Y_1/4Ho_1/4Er_1/4Yb_1/4)₂SiO₅/Yb₂Si₂O₇/Si and Yb₂SiO₅/Yb₂Si₂O₇/Si coatings (Insets in Figs.4b and c show the enlarged details of residual thermal stress in inter layer and bond layer in the range of 1700 K to 1900 K)
(a) top layer (b) inter layer (c) bond layer (d) substrate

Fig.5 Schematics of crystal structure of rare-earth monosilicate RE₂SiO₅ projected along the c axis (a) and b axis (b), and structure of [SiO₄] and [ORE₄] tetrahedrons (c)

Fig.6 Time-of-flight neutron powder diffraction (TOF-NPD) Rietveld refinement patterns of (Y_1/4Ho_1/4Er_1/4Yb_1/4)₂SiO₅ (a) and Yb₂SiO₅ (b)

Table 4 Bond length distortion (λ) and bond angle variance (σ²) of Yb₂SiO₅ and (Y_1/4Ho_1/4Er_1/4Yb_1/4)₂SiO₅

Fig.7 Fit of RE₂SiO₅PDF with structures obtained from the TOF-NPD Rietveld refinements (G(r) is defined as atomic pair distribution funtion, r—distance between atoms in the system)

Fig.8 PDF Rietveld refinement patterns of (Y_1/4Ho_1/4Er_1/4-Yb_1/4)₂SiO₅ (a) and Yb₂SiO₅ (b), and corresponding derived partial PDFs

Fig.9 PDF patterns (a) and Rietveld refinement patterns in the range of 0.15-0.5 nm (b) of Yb₂SiO₅ and (Y_1/4Ho_1/4Er_1/4Yb_1/4)₂SiO₅ (Insets in Fig.8a show the structure of [SiO₄] and [ORE₄] tetrahedrons and bond length of Si—O and RE—O)

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