Key Takeaways & Executive Findings
- •• Co-doping YTaO4 with Ti, Zr, or Hf significantly reduces thermal conductivity to 1.23 W·m−1·K−1 at 900°C, outperforming conventional YSZ. • The medium-entropy ceramics achieve high thermal expansion coefficients (10.4 × 10−6 K−1 at 1400°C), matching nickel-based superalloys. • Fracture toughness is enhanced to 4.1 ± 0.5 MPa·m1/2 via ferroelastic domain switching and microcrack toughening. • The simultaneous improvement in thermal and mechanical properties makes these MECs promising candidates for next-generation TBC top coats.
Abstract
Thermal and mechanical properties of yttrium tantalate (YTaO4), a top coat ceramic of thermal barrier coatings (TBCs) for aeroengines, are enhanced by synthesizing Y1−xTa1−xM2xO4 (M = Ti, Zr, Hf; x = 0.06, 0.12, 0.18, 0.24) medium-entropy ceramics (MECs) using a two-step sintering method. In addition, the thermal conductivity, thermal expansion coefficients (TECs), and fracture toughness of MECs were investigated. An X-ray diffraction study revealed that the Y1−xTa1−xM2xO4 MECs were monoclinic, and the Ti, Zr, and Hf doping elements replaced Y and Ta. The variations in atomic weights and ionic radii led to disturbed atomic arrangements and severe lattice distortions, resulting in improving the phonon scattering and reduced thermal conductivity, with Y1−xTa1−xM2xO4 MECs (x = 0.24) exhibiting the lowest thermal conductivity of 1.23 W·m−1·K−1 at 900°C. The introduction of MO2 increased the configurational entropy and weakened the ionic bonding energy, obtaining high TECs (10.4 × 10−6 K−1 at 1400°C). The reduction in the monoclinic angle β lowered the ferroelastic domain inversion energy barrier. Moreover, microcracks and crack extension toughening endowed Y1−xTa1−xM2xO4 MECs (x = 0.24) with the highest fracture toughness of (4.1 ± 0.5) MPa·m1/2. The simultaneous improvement of the thermal and mechanical properties of the MO2 (M = Ti, Zr, Hf) co-doped YTaO4 MECs can be extended to other materials.
1. Introduction
With the development of the aerospace industry, aeroengines gradually shift toward high operating efficiency and inlet temperature. Nickel-based alloys cannot withstand the inlet temperature of the most advanced aeroengines, which has surpassed 1700°C [1–2]. Therefore, thermal barrier coatings (TBCs) have attracted considerable attention because of their excellent thermal insulation property, oxidation and corrosion resistance, and mechanical and chemical stability in complex engine environments [3–4]. TBCs for aircraft and supersonic engines comprise super-alloy substrates, bond coat (BC), and top coat (TC) ceramics, preventing the effects of extremely high temperatures on the engine components and increasing the working temperature of the engine [5–7]. To provide thermal protection to nickel-based alloys [8], the TC ceramic should comprise an oxide with low thermal conductivity, superior high-temperature phase stability [4], and thermal expansion coefficients (TECs) compatible with nickel-based alloys [7]. As a predominant TBC material, 6wt%–8wt% yttria-stabilized zirconia (6–8YSZ) exhibits remarkable properties, such as a substantial hardness of approximately 14 GPa, high TECs of nearly 10.9 × 10−6 K−1 at 1200°C [9], large Young’s modulus of approximately 250 GPa [10], and high fracture toughness of 3–4 MPa·m1/2, owing to its unique ferroelastic toughening mechanism [11]. However, YSZ suffers from relatively high thermal conductivity (2.5 W·m−1·K−1, at 900°C), limited resistance to CaO–MgO–AlO1.5–SiO2 (CMAS) corrosion, susceptibility to sintering [12], and relatively low operating temperatures (≤1200°C) attributed to phase transitions [13]. At temperatures above 1200°C, the metastable tetragonal (t′) phase of YSZ transitions into tetragonal (t) and cubic (c) phases, whereas the t phase of YSZ transitions into a monoclinic (m) phase during cooling [5,14]. The substantial differences in lattice volume caused by these phase transitions create large internal stresses, leading to coating spalling and failure [15]. Consequently, the development of a new type of TBCs that can withstand high temperatures while maintaining excellent performance has become a crucial task.
Some novel TBCs have been developed over the previous decades, including rare-earth zirconates (RE2Zr2O7, RE = rare earth) [16–17], rare-earth silicates (RE2Si2O7 and RE2SiO5) [18–19], and rare-earth phosphates (REPO4) [20–21]. However, RE2Zr2O7 exhibits low fracture toughness [22], and RE2Si2O7 presents low TECs [23], hindering their use as replacements for YSZ. In this context, yttrium tantalate (YTaO4) has emerged as a promising material. Feng et al. [24] demonstrated that YTaO4 has a ferroelastic phase transition comparable to that of YSZ, according to first-principles calculations. Moreover, the fracture toughness and thermal conductivity of YTaO4 could be further improved by doping with MO2 (M = Ti, Zr, Hf) to form medium-entropy ceramics, as explored in this study.
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Xunlei Chen, Lin Chen, Jiang Tian, Cheng Xu, Jiaxin Liao, Tianyu Li, Jiankun Wang, Jing Feng (2025). Thermal and mechanical properties of MO2 (M = Ti, Zr, Hf) co-doped YTaO4 medium-entropy ceramics. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-3005-6
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Frequently Asked Questions
What is the main advantage of MO2 co-doped YTaO4 medium-entropy ceramics over conventional YSZ?
The MO2 co-doped YTaO4 medium-entropy ceramics exhibit significantly lower thermal conductivity (1.23 W·m−1·K−1 at 900°C) compared to YSZ (2.5 W·m−1·K−1), while maintaining high thermal expansion coefficients and improved fracture toughness, making them more suitable for advanced thermal barrier coatings.
How does doping with Ti, Zr, or Hf affect the thermal conductivity of YTaO4?
Doping with Ti, Zr, or Hf introduces atomic weight and ionic radius variations, causing lattice distortions and enhanced phonon scattering. This reduces thermal conductivity, with the lowest value achieved at x = 0.24 doping concentration.
What is the fracture toughness of the best-performing MO2 co-doped YTaO4 ceramic?
The highest fracture toughness achieved is (4.1 ± 0.5) MPa·m1/2 for Y1−xTa1−xM2xO4 with x = 0.24, attributed to ferroelastic domain switching and microcrack toughening mechanisms.
Are these medium-entropy ceramics suitable for high-temperature applications?
Yes, they exhibit high thermal expansion coefficients (10.4 × 10−6 K−1 at 1400°C) compatible with nickel-based superalloys, and their thermal and mechanical properties are improved simultaneously, making them promising for next-generation TBCs.
What is the significance of the monoclinic angle β in these ceramics?
The reduction in the monoclinic angle β lowers the energy barrier for ferroelastic domain inversion, which enhances the toughening effect and contributes to higher fracture toughness.
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