Comparison of CMAS corrosion and sintering induced microstructural characteristics of APS thermal barrier coatings

doi:10.1016/j.jmst.2018.09.046

Abstract

Abstract:

The microstructural features of high-temperature sintered and CaO-MgO-Al₂O₃-SiO₂ (CMAS) corroded air plasma sprayed Y₂O₃ stabilized ZrO₂ (YSZ) thermal barrier coatings (TBCs) under the thermal gradient condition were comparatively studied. As-sprayed YSZ has a lamellar structure and the lamellae are composed of closely aligned columnar crystals. The sintered and the CMAS corroded YSZ coatings maintain the t’-ZrO2 phase as the as-sprayed YSZ coating. The sintered YSZ remains the lamellar structure with reduced interlamellar gaps and grains coarsening. After the CMAS corrosion, the top layer of the YSZ coating keeps its lamellar structure consisting of some columnar grains with the CMAS infiltration into the intergrain gaps and the formation of striped Zr₂Y₂O₇. The typical lamellar structure transforms into more equiaxed grains in the middle and bottom layers of the ceramic coating along with significant infiltration of amorphous CMAS and anorthite formation in the bottom layer owing to the high contents of Ca and Al.

Key words: YSZ thermal barrier coating, APS, CMAS corrosion, Anorthite

Yiyou Wu, Hua Luo, Canying Cai, Yanguo Wang, Yichun Zhou, Li Yang, Guangwen Zhou. Comparison of CMAS corrosion and sintering induced microstructural characteristics of APS thermal barrier coatings[J]. J. Mater. Sci. Technol., 2019, 35(3): 440-447.

Figures/Tables 12

Fig. 1. (a) Particle size distribution of CMAS powder and (b) DSC profile of CMAS powder ranging from 500?°C to 1300?°C. The sigmoidal glass transition occurs at approximately 766.1?°C and the melting starts around at 1178.0?°C.

Fig. 2. XRD patterns of top ceramic layer of as-sprayed, thermal gradient sintered, thermal gradient CMAS corroded, and YSZ/BC interface region of CMAS corroded samples (210?μm down from the top surface).

Fig. 3. Cross-sectional SEM morphology images: (a) as-sprayed coating; (b) thermal gradient sintered coating; (c) zoom-in SEM image from region indicated by yellow box in (b); (d) CMAS-corroded YSZ; (e) zoom-in SEM image from region marked with a yellow box region in upper layer of YSZ coating shown in (d); (f) zoom-in SEM image from region marked with a yellow box in YSZ/BC interface region shown in (d).

Table 1 EDS obtained from lamellar gap and YSZ/BC interface shown in Fig. 3 (mol.%).

Region	O	Mg	Al	Si	Ca	Ni	Cr	Y	Zr
1	38.81	1.94	4.25	16.49	14.90	0	0	5.61	18
2	38.31	2.43	29.15	9.36	4.19	12.58	3.98	0	0
3	35.45	0	46.64	0	0	13.36	4.55	0	0

Fig. 4. Cross-sectional SEM morphologies of as-sprayed (a), thermal gradient sintered (b) and thermal gradient CMAS corroded (c) samples.

Fig. 5. BF TEM images of as-sprayed sample at low (a) and high (b) magnification, where the inset SAED pattern taken from the region marked with the yellow box confirms that the as-sprayed YSZ has a tetragonal structure and TEM image of YSZ coating exposed to 1250?°C for 8?h under thermal gradient condition (c).

Fig. 6. (a) TEM image from top surface region of CMAS-corroded YSZ coating, where the inset SAED was obtained from the CMAS deposit marked with the yellow arrow and confirms that the unreacted CMAS depositions remain amorphous, (b) TEM image from upper layer region of CMAS-corroded YSZ coatings, where the inset SAED was obtained from the strip-like region marked by the yellow square, (c) high-resolution image of strip-like region shown in (b), and (d) SAED pattern taken from strip-like region as marked by yellow circle confirming formation of Zr2Y2O7.

Table 2 EDS obtained from top surface region, middle region, and YSZ/BC interface of CMAS-corroded YSZ in Fig. 6, Fig. 7, Fig. 8 (mol.%).

Region	SiO₂	AlO_1.5	CaO	MgO
A	53.79	21.17	8.78	16.26
B	46.30	28.71	20.31	4.68
C	36.31	37.62	26.07	0

Fig. 7. (a) BF image from middle region of CMAS-corroded YSZ coating and (b) SAED pattern from CMAS deposit in (a).

Fig. 8. (a) BF images from interface region between ceramic layer and bond coat, where the lamellar structure of the ceramic layer completely disappeared and the YSZ developed into a granulated structure and (b) SAED pattern from gray substance was indicated by the yellow circle in (a), which confirms the anorthite formation.

Table 3 EDS obtained from YSZ/BC interface region of CMAS-corroded YSZ coating in Fig. 8a (mol.%).

Region	O	Mg	Al	Si	Ca	Ni	Cr	Cu	Zr
1	34.93	0	43.64	0	0	13.64	6.09	1.70	0
2	38.81	1.24	30.49	6.54	3.22	13.07	4.09	2.54	0
3	19.41	1.19	1.55	2.62	1.96	0	0	4.32	68.95

Fig. 9. Schematics of microstructure features of as-sprayed sample (a), thermal gradient sintered sample (b), and CMAS corroded sample at 1250?°C for 8?h (c).

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