Corrosion Behavior and Mechanism of Aluminum-rich CMAS on Rare-earth Silicate Environmental Barrier Coatings:

doi:10.15541/jim20220532

Abstract

Abstract:

Environmental barrier coatings (EBCs) are expected to be applied to the hot-section components of a new generation of high thrust-to-weight ratio aero-engines. Rare-earth silicates have been acknowledged as promising alternatives to EBC materials due to their superior high-temperature phase stability, suitable coefficient of thermal expansion, and long durability in water vapor. However, the calcium-magnesium-alumino-silicates (CMAS) molten salt corrosion under service conditions has become a bottleneck that limits the application of rare-earth silicates in EBCs. Factors such as the composition of CMAS and the crystal structures of rare-earth silicates have a significant impact on their corrosion behavior. In this paper, X1-Gd₂SiO₅ and X2-RE₂SiO₅ (RE=Y, Er) coatings with different crystal structures, were prepared by atmospheric plasma spraying (APS) technique. Their corrosion behaviors and mechanisms of the three kinds of coatings under CMAS melt environment at 1400 ℃ were explored. Results showed that the corrosion resistance of X2-RE₂SiO₅ coatings were better than that of X1-Gd₂SiO₅ coating due to their phase compositions and stability of crystal structure. After corrosion by CMAS, X1-Gd₂SiO₅ coating dissolved in CMAS melt and formed apatite phase, while the X2-RE₂SiO₅ coatings not only formed apatite phase, but also formed garnet phase from reaction of the RE₂O₃ in the coatings with Al₂O₃ in CMAS. Formation of generate garnet phase could increase relative content of CaO and SiO₂ in CMAS, and promote formation of dense apatite layer, thereby improving corrosion resistance. This study provides a strategy for designing EBC systems with excellent CMAS corrosion resistance.

Key words: environmental barrier coating, rare-earth silicate, CMAS corrosion, corrosion mechanism

CLC Number:

TQ174

FAN Dong, ZHONG Xin, WANG Yawen, ZHANG Zhenzhong, NIU Yaran, LI Qilian, ZHANG Le, ZHENG Xuebin. Corrosion Behavior and Mechanism of Aluminum-rich CMAS on Rare-earth Silicate Environmental Barrier Coatings:[J]. Journal of Inorganic Materials, 2023, 38(5): 544-552.

Figures/Tables 13

Table 1 Technical parameters used for plasma spraying

Parameter	RE₂SiO₅(RE=Gd, Y, Er)
Primary Ar/(L·min^-1)	43
Secondary H₂/(L·min^-1)	12
Carrier Ar/(L·min^-1)	2.3
Spray distance/mm	230

Fig. S1 XRD pattern of CMAS powders

Table 2 Chemical compositions of CMAS powders

XRF/(%，in mol)	CaO	MgO	AlO_1.5	SiO₂
CMAS	27.87	8.79	26.22	38.52

Fig. 1 XRD patterns of powders and as-sprayed coatings (a) X1-Gd2SiO5; (b) X2-Y2SiO5; (c) X2-Er2SiO5

Fig. 2 XRD patterns of coatings after CMAS corrosion for 4 and 25 h (a) X1-Gd2SiO5; (b) X2-Y2SiO5; (c) X2-Er2SiO5

Fig. S2 Surface and cross-sectional microstructures of as-sprayed coatings (a-b) X1-Gd2SiO5; (c-d) X2-Y2SiO5; (e-f) X2-Er2SiO5

Fig. 3 Surface microstructures of coatings after corrosion for 4 and 25 h (a-b) X1-Gd2SiO5; (c-d) X2-Y2SiO5; (e-f) X2-Er2SiO5

Table 3 EDS elemental compositions of the marked regions in Fig. 3

EDS/ (%, in atom)	Gd	Y	Er	Si	O	Ca	Al	Mg
Point 1	20.60	—	—	19.62	51.73	8.02	—	—
Point 2	—	26.04	—	17.55	50.57	5.83	—	—
Point 3	—	—	26.95	15.16	50.78	7.10	—	—
Point 4	—	14.03	—	5.72	52.27	4.01	20.31	3.66

Fig. 4 Cross-sectional microstructures of coatings after CMAS corrosion for 4 h (a, b) X1-Gd2SiO5; (c, d) X2-Y2SiO5; (e, f) X2-Er2SiO5

Table 4 EDS elemental compositions of the marked regions in Fig. 4

EDS/(%, in atom)	Gd	Y	Er	Si	O	Ca	Al	Mg
Point 1	19.21	—	—-	11.70	60.65	8.44	—	—
Point 2	1.16	—	—	13.06	59.37	13.78	10.18	2.45
Point 3	—	15.29	—	16.00	61.82	6.89	—	—
Point 4	—	7.18	—	15.50	50.17	9.58	12.14	5.43
Point 5	—	0.96	—	13.56	59.62	13.03	10.40	2.42
Point 6	—	—	20.27	12.11	61.12	6.50	—	—
Point 7	—	—	8.70	10.95	59.15	8.09	10.95	4.65
Point 8	—	—	0.92	3.29	58.74	19.61	3.29	2.07

Fig. 5 Cross-sectional microstructures of the X1-Gd2SiO5 coating after corrosion for 25 h (a) X1-Gd2SiO5; (b) X2-Y2SiO5; (c) X2-Er2SiO5

Table 5 EDS elemental compositions of the marked regions in Fig. 5

EDS/ (%, in atom)	Gd	Y	Er	Si	O	Ca	Al	Mg
Point 1	24.48	—	—	16.22	51.61	7.68	—	—
Point 2	1.05	—	—	12.62	59.63	14.25	9.78	2.66
Point 3	—	26.70	—	15.78	50.12	7.40	—	—
Point 4	—	17.10	—	8.23	51.25	2.92	16.41	4.09
Point 5	—	0.94	—	14.23	57.36	13.94	11.98	1.54
Point 6	—	—	28.80	12.77	52.22	6.22	—	—
Point 7	—	—	7.12	13.05	49.26	7.08	18.26	5.24

Fig. 6 Schematic diagrams of different coatings under CMAS molten salt corrosion at 1400 ℃ (a) Reaction process; (b) X1-Gd2SiO5; (c) X2-Y2SiO5; (d) X2-Er2SiO5

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