Ferroelastic domain identification and toughening mechanism for yttrium tantalate-zirconium oxide

doi:10.1016/j.jmst.2022.04.007

Abstract

Abstract:

Yttrium tantalate (YTaO₄) is the next generation of higher service temperature thermal barrier coatings (TBCs) materials due to its smaller volume effect in phase change, lower thermal conductivity and unique ferroelastic domain structure. However, the low fracture toughness limits its application. We first characterized the diffraction patterns of variants, and two variants (M₁ and M₂) observed in transmission electron microscopy (TEM) results were determined from four possible variants by mechanical derivation. The role of Zr⁴⁺ doping in ferroelastic toughening was explained in detail. With the increase of Zr⁴⁺ doping concentration, the monoclinic angle β and the domain rotation angle α decrease, respectively. The spontaneous strain component and the principal strain in the main space also have a similar decreasing trend. The decrease of the ferroelastic domain inversion energy barrier is beneficial to the improvement of fracture toughness. Combining the results of Vickers indentation, we found that Zr⁴⁺ could be enriched at the domain boundary to inhibit the generation of cracks. An appropriate amount of Zr⁴⁺ is conducive to the improvement of fracture toughness, and the excessive Zr⁴⁺ will reduce the fracture toughness due to the generation of by-product t-ZrO₂. So, the optimal composition is Y_0.44Ta_0.44Zr_0.12O₂ and the best fracture toughness (2.9-3.8MPa m^1/2) is equivalent to the commercial 8YSZ. This result will promote the application of a new generation of TBCs.

Key words: Ferroelastic domain, Variants, Domain switch, TBCs, Fracture toughness

Cheng Luo, Cong Li, Ke Cao, Junbao Li, Junhui Luo, Qinghua Zhang, QianQian Zhou, Fan Zhang, Lin Gu, Li Yang, Yichun Zhou. Ferroelastic domain identification and toughening mechanism for yttrium tantalate-zirconium oxide[J]. J. Mater. Sci. Technol., 2022, 127: 78-88.

Figures/Tables 13

Fig. 1. TEM images of YTaO4, (a) bright field image, (b) electron diffraction (SAED) image, (c, d) dark field images corresponding to the diffraction spots in the insets of variant M1 and variant M2.

Fig. 2. (a) Four possible M-phase variants in YTaO4 during the T→M phase transition, (b) lattice projections of four M-phase variants on the (010) plane of YTaO4 after the T→M phase transition.

Table 1. The ratio of atomic layers corresponding to different combinations of variants.

Combinations	Atomic layer ratio
(M₁, M₂)	${{n}_{{{\text{M}}_{1}}}}:{{n}_{{{\text{M}}_{2}}}}={{c}_{\text{M}}}:{{a}_{\text{M}}}$
(M₁, M₃)	${{n}_{{{\text{M}}_{1}}}}:{{n}_{{{\text{M}}_{3}}}}=1:1$
(M₂, M₄)	${{n}_{{{\text{M}}_{2}}}}:{{n}_{{{\text{M}}_{4}}}}=1:1$
(M₃, M₄)	${{n}_{{{\text{M}}_{3}}}}:{{n}_{{{\text{M}}_{4}}}}={{c}_{\text{M}}}:{{a}_{\text{M}}}$

Fig. 3. Variant combinations (M1, M2) and (M3, M4) show mirror symmetry.

Fig. 4. XRD patterns of Y0.5-x/2Ta0.5-x/2ZrxO2 (x = 0, 0.04, 0.08, 0.12, 0.16, 0.20, 0.24 and 0.28) ceramic after maintaining temperature at 1600 °C for 10 h. (a) Partial enlarged view of characteristic t-ZrO2 peak, (b) percentage of M-YTaO4 phase and t-ZrO2 phase, volume unit cell, and (c) monoclinic angle β of M-phase Y0.5-x/2Ta0.5-x/2ZrxO2 with different ZrO2 concentrations,TEM image of Y0.5-x/2Ta0.5-x/2ZrxO2, (d) bright field image, (e) electron diffraction (SAED) image, (f and g) dark field images corresponding to the diffraction spots in the inserts of variant M1 and variant M2.

Table 2. The lattice constant of Y0.5-x/2Ta0.5-x/2ZrxO2.

ZrO₂ content (x)	b/2_M	a_M	c_M	_β
0	5.32128	5.46291	5.0538	95.48669
0.04	5.31435	5.45629	5.0541	95.45977
0.08	5.30785	5.4495	5.05335	95.38817
0.12	5.30039	5.44383	5.05682	95.30099
0.16	5.29383	5.43746	5.0571	95.20142
0.20	5.29021	5.43565	5.06251	95.10217
0.24	5.27696	5.42452	5.06264	94.98067
0.28	5.26192	5.41265	5.06347	94.78799

Fig. 5. The morphology of pure YTaO4 (a) and Y0.44Ta0.44Zr0.12O2 (c) after small-load dimensional indentation (0.01 kg), (b, d) local morphology in the vicinity of the dimensional indentation, (e) cross-section morphology of the pure YTaO4 after Vickers indentation, (f) microscopic morphology of the complete domain switch zone and the domain switching area of pure YTaO4, (g) uneven stripe area near Vickers indentation, (h) high-resolution imaging of the light-dark fringe interface in the uneven fringe area near the Vickers indentation, (i) schematic of the mutual switching between variants M1 and M2, (j) schematic of the mutual switch process between two variants.

Fig. 6. Spontaneous strain component in the T phase space of the conversion of M1 and M2 combination to (a) variant M1 and (b) variant M2, the magnitude of the principal strain component generated in the normal strain space when the combination of variants (M1 and M2) switches to (c) variant M1 and (d) variant M2.

Fig. 7. Schematic diagram of (a) monoclinic angle β and (b) rotation angle α of the ferroelastic domain switch decreased after doping with Zr4+ ions, (c) the energy barrier of the ferroelastic domain switching decreased with Zr4+ ions increasing.

Fig. 8. Three-dimensional indentation morphology of Y0.5-x/2Ta0.5-x/2ZrxO2 ceramic samples under 15 kg load, (a) x = 0, (b) x = 0.04, (c) x = 0.08, (d) x = 0.12, (e) x = 0.16, (f) x = 0.20, (g) x = 0.24, (h) x = 0.28, and (i) ideal dimensional indentation morphology of high toughness ceramic materials.

Fig. 9. Schematic diagram of two calculation models for calculating fracture toughness through Vickers indentation and assumption of crack surface, (a) Law Evans maishall elastic / plastic indentation fracture model, (b) Laugier Niihara radial crack model;

Fig. 10. Average dimensional (a) hardness, (b) Young's modulus and (c) fracture toughness of Y0.5-x/2Ta0.5-x/2ZrxO2 ceramic samples.

Fig. 11. (a) The strain component in the T phase space of the switch of variant M1 to variant M2, (b) the magnitude of the principal strain component generated in the normal strain space when the variant M1 converts to the variant M2, (c) schematic representation and (d) microscopic image of the crack in low-concentration doped Zr4+ Y0.5-x/2Ta0.5-x/2ZrxO2 during the M1 to M2 conversion. Schematic representation of Zr4+ (e) gathered at the interface of the variants and (f) migrated into the Y0.5-x/2Ta0.5-x/2ZrxO2 domain switch area.

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