Effect of microstructure evolution and crystal structure on thermal properties for plasma-sprayed RE2SiO5 (RE = Gd, Y, Er) environmental barrier coatings

doi:10.1016/j.jmst.2020.11.077

Abstract

Abstract:

In this work, the microstructure evolution, thermal expansion, thermal conductivity, and thermal shock resistance properties of the plasma-sprayed X1-Gd₂SiO₅, X2-Y₂SiO₅, and X2-Er₂SiO₅ coatings were evaluated and compared by experimental measurement and theoretical exploration. Results showed that significant microstructure evolution such as crystallization of amorphous phase, grain growth, and defects reduction was observed in the RE₂SiO₅ coatings after thermal aging at 1400 °C. The X1-Gd₂SiO₅ coating exhibited higher CTE values than the X2-Y₂SiO₅ and X2-Er₂SiO₅ coatings, which was related to their crystal structure. The thermal conductivity of thermal-aged RE₂SiO₅ coating was much higher than that of the as-sprayed RE₂SiO₅ coating, and thermal conductivity was determined not only by crystal structure but also mainly by the microstructure of the coatings. The X2-Y₂SiO₅ and X2-Er₂SiO₅ coatings with lower thermal mismatch stresses presented much better thermal shock resistance than the X1-Gd₂SiO₅ coating.

Key words: Rare-earth monosilicates, Environmental barrier coating, Microstructure evolution, Thermal properties, Crystal structure

Xin Zhong, Tao Zhu, Yaran Niu, Haijun Zhou, Le Zhang, Xiangyu Zhang, Qilian Li, Xuebin Zheng. Effect of microstructure evolution and crystal structure on thermal properties for plasma-sprayed RE₂SiO₅ (RE = Gd, Y, Er) environmental barrier coatings[J]. J. Mater. Sci. Technol., 2021, 85: 141-151.

Figures/Tables 20

Fig. 1. Crystal structures of X1-Gd2SiO5 (a) and X2-RE2SiO5 (b).

Table 1 Operating parameters used for atmospheric plasma spray.

APS Layer	Power(kW)	Primary Ar(slpm)	Secondary H₂ (slpm)	Carrier Ar(slpm)	Spray distance(mm)
RE₂SiO₅^a	43	38	12	3	120
Yb₂Si₂O₇	43	38	12	3	120
Si	32	42	8	4	120

Fig. 2. XRD patterns of Gd2SiO5, Y2SiO5, Er2SiO5 powders (a?c) and as-sprayed coatings (d?f).

Fig. 3. Surface morphologies (a?c) and fracture section morphologies (d?f) of as-sprayed RE2SiO5 coatings: Gd2SiO5 (a, d), Y2SiO5 (b, e), and Er2SiO5 (c, f).

Fig. 4. Cross-sectional morphologies of as-sprayed RE2SiO5 coatings: (a) Gd2SiO5, (b) Y2SiO5, and (c) Er2SiO5.

Fig. 5. Porosity of as-sprayed Gd2SiO5, Y2SiO5, and Er2SiO5 coatings.

Fig. 6. XRD patterns of RE2SiO5 coatings after thermal aging at 1400?°C for 50?h: (a) Gd2SiO5, (b) Y2SiO5, and (c) Er2SiO5.

Fig. 7. Surface morphologies (a?c) and fracture section morphologies (d?f) of RE2SiO5 coatings after thermal aging at 1400?°C for 50?h: Gd2SiO5 (a, d), Y2SiO5 (b, e), and Er2SiO5 (c, f).

Fig. 8. Cross-sectional morphologies of RE2SiO5 coatings after thermal aging at 1400?°C for 50?h: (a) Gd2SiO5, (b) Y2SiO5, and (c) Er2SiO5.

Table 2 Summary of EDS results for areas in Fig. 8.

Spectrum location	Gd(at.%)	Y (at.%)	Er(at.%)	Yb (at.%)	Si (at.%)	O (at.%)
Point a1	27.21	—	—	—	12.27	59.64
Point a2	39.61	—	—	—	1.04	54.18
Point b1	—	28.09	—	—	13.06	59.73
Point b2	—	44.78	—	—	1.47	58.92
Point c1	—	—	27.41	—	12.11	60.48
Point c2	—	—	42.61	—	1.78	55.61
Point d1	—	—	—	26.43	12.89	60.68
Point d2	—	—	—	39.34	1.53	59.13

Fig. 9. TG-DTA curves of as-sprayed RE2SiO5 coatings: (a) Gd2SiO5, (b) Y2SiO5, and (c) Er2SiO5.

Fig. 10. Thermal expansion curves of RE2SiO5 coatings: (a) Gd2SiO5, (b) Y2SiO5, and (c) Er2SiO5.

Table 3 The average bond length (Å), calculated cation field strength (CFS) (Å-2) as well as the Grüneisen parameter γ for RE2SiO5 (RE?=?Gd, Y and Er).

RE₂SiO₅	Crystal structure	Average bond length (Å)	CFS (Å^-2)
X1-Gd₂SiO₅	[SiO₄]	1.633 [14]	3.41 1.94 [33]
	[GdO₇]	2.393 [14]
	[GdO₉]	2.488 [14]
X2-Y₂SiO₅	[SiO₄]	1.618 [34]	3.70 1.49 [16]
	[YO₆]	2.260 [34]
	[YO₇]	2.359 [34]
X2-Er₂SiO₅	[SiO₄]	1.644 [35]	3.79 1.53 [16]
	[ErO₆]	2.258 [35]
	[ErO₇]	2.359 [35]

Fig. 11. Thermal diffusivities (a) and thermal conductivities (b) of as-sprayed and thermal-aged Gd2SiO5, Y2SiO5, and Er2SiO5 coatings.

Table 4 Theoretical parameters for calculating lattice thermal conductivity based on Slack’s model.

RE₂SiO₅	$\bar{M}$ (g mol^-1)	δ (Å)	n	Θ_D (K)	A(γ)/γ²
X1-Gd₂SiO₅	52.82	2.49 [33]	32	419 [33]	8.12E-09
X2-Y₂SiO₅	35.74	2.35 [16]	32	513 [16]	1.44E-08
X2-Er₂SiO₅	55.33	2.37 [16]	32	423 [16]	1.36E-08

Fig. 12. Macro-photographs of RE2SiO5/Yb2Si2O7/Si system on C/SiC substrates before and after thermal shock tests.

Fig. 13. Cross-sectional micrographs of Gd2SiO5/Yb2Si2O7/Si coatings before (a) and after 40 thermal cycles (b?d): as-sprayed coatings (a), Gd2SiO5/Yb2Si2O7/Si coatings with 40 cycles (b), high magnification views of Gd2SiO5-Yb2Si2O7 (c) and Yb2Si2O7-Si (d) interfaces.

Fig. 14. Cross-sectional micrographs of Y2SiO5/Yb2Si2O7/Si coatings before (a) and after 40 thermal cycles (b?d): as-sprayed coatings (a), Y2SiO5/Yb2Si2O7/Si coatings with 40 cycles (b), high magnification views of Y2SiO5-Yb2Si2O7 (c) and Yb2Si2O7-Si (d) interfaces.

Fig. 15. Cross-sectional micrographs of Er2SiO5/Yb2Si2O7/Si coatings before (a) and after 40 thermal cycles (b?d): as-sprayed coatings (a), Er2SiO5/Yb2Si2O7/Si coatings with 40 cycles (b), high magnification views of Er2SiO5-Yb2Si2O7 (c) and Yb2Si2O7-Si (d) interfaces.

Table 5 Thermal mismatch stress calculations of EBC system components.

Coating layer	CTE, α (×10^-6  K^-1)	E (GPa)	Poisson ratio ν	Thermal stress (MPa)
Gd₂SiO₅	9.94^a	75.15^b	0.30 [33]	1098 (tensile)
Y₂SiO₅	7.11^a	72.94^b	0.20 [16]	578 (tensile)
Er₂SiO₅	7.42^a	74.51^b	0.22 [16]	646 (tensile)
Yb₂Si₂O₇	4.25 [20]	67.49 [20]	0.30 [43]	232 (tensile)
Si	4.10 [44]	105.30 ^b	0.22 [44]	297 (tensile)

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Effect of microstructure evolution and crystal structure on thermal properties for plasma-sprayed RE₂SiO₅ (RE = Gd, Y, Er) environmental barrier coatings

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