High-entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near-equal atomic ratios of multi-principal elements. Although in the infant stage, the emerging of this new family of materials has brought new opportunities for material design and property tailoring. Distinct from metals, the diversity in crystal structure and electronic structure of ceramics provides huge space for properties tuning through band structure engineering and phonon engineering. Aside from strengthening, hardening, and low thermal conductivity that have already been found in high-entropy alloys, new properties like colossal dielectric constant, super ionic conductivity, severe anisotropic thermal expansion coefficient, strong electromagnetic wave absorption, etc., have been discovered in HECs. As a response to the rapid development in this nascent field, this article gives a comprehensive review on the structure features, theoretical methods for stability and property prediction, processing routes, novel properties, and prospective applications of HECs. The challenges on processing, characterization, and property predictions are also emphasized. Finally, future directions for new material exploration, novel processing, fundamental understanding, in-depth characterization, and database assessments are given.

/pdf/high-entropy-ceramics-present-status-challenges-and-a-look-1orrn70tj7.pdf

High-entropy ceramics: Present status, challenges, and a look forward

The ceramic top coat has a major influence on the performance of the thermal barrier coating systems (TBCs). Yttria-partially-stabilized zirconia (YSZ) is the top coat material frequently used, and the major deposition processes of the YSZ top coat are atmospheric plasma spraying and electron beam physical vapor deposition. Recently, also new thermal spray processes such as suspension plasma spraying or plasma spray-physical vapor deposition have been intensively investigated for TBC top coat deposition. These new processes and particularly the different coating microstructures that can be deposited with them will be reviewed in this article. Furthermore, the properties and the intrinsic–extrinsic degradation mechanisms of the YSZ will be discussed. Following the TBC deposition processes and standard YSZ material, alternative ceramic materials such as perovskites and hexaaluminates will be summarized, while properties of pyrochlores with regard to their crystal structure will be discussed more in detail. The merits of the pyrochlores such as good CMAS resistance as well as their weaknesses, e.g., low fracture toughness, processability issues, will be outlined.

/pdf/ceramic-top-coats-of-plasma-sprayed-thermal-barrier-coatings-v7t8fvkike.pdf

Ceramic Top Coats of Plasma-Sprayed Thermal Barrier Coatings: Materials, Processes, and Properties

Low thermal conductivity is the key property dominating the heat insulation ability of thermal barrier coatings (TBC). Reducing the intrinsic thermal conductivity is the major topic for developing advanced TBCs. Defect engineering has attracted much attention in seeking better TBC materials since lattice defects play a crucial role in phonon scattering and thermal conductivity reduction. Oxygen vacancies and substitutions are proven to be the most effective, while the accompanying lattice distortion is also of great importance. In this paper, recent advances of reducing the thermal conductivity of potential thermal barrier coating materials by defect engineering are comprehensively reviewed. Effects of the mass and size mismatch between the defects and the host lattice are quantitatively estimated and unconventional thermal conductivity reduction caused by the lattice distortions is also discussed. Finally, challenges and potential opportunities are briefly assessed to further minimize the thermal conductivity of TBC materials in the future.

Defect engineering in development of low thermal conductivity materials: A review

Progress update on failure mechanisms of advanced thermal barrier coatings: A review

Advances on strategies for searching for next generation thermal barrier coating materials

(Gd1−xYbx)2Zr2O7 compounds were synthesized by solid reaction. Yb2O3 doped Gd2Zr2O7 exhibited lower thermal conductivities and higher thermal expansion coefficients (TECs) than Gd2Zr2O7. The TECs of (Gd1−xYbx)2Zr2O7 ceramics increased with increasing Yb2O3 contents. (Gd0.9Yb0.1)2Zr2O7 (GYbZ) ceramic exhibited the lowest thermal conductivity among all the ceramics studied, within the range of 0.8–1.1 W/mK (20–1600 °C). The Young's modulus of GYbZ bulk is 265.6 ± 11 GPa. GYbZ/YSZ double-ceramic-layer thermal barrier coatings (TBCs) were prepared by electron beam physical vapor deposition (EB-PVD). The coatings had an average life of more than 3700 cycles during flame shock test with a coating surface temperature of ∼1350 °C. Spallation failure of the TBC occurred by delamination cracking within GYbZ layer, which was a result of high temperature gradient in the GYbZ layer and low fracture toughness of GYbZ material.

Thermophysical properties of Yb2O3 doped Gd2Zr2O7 and thermal cycling durability of (Gd0.9Yb0.1)2Zr2O7/YSZ thermal barrier coatings

Phase stability and thermal conductivity of RE2O3 (RE=La, Nd, Gd, Yb) and Yb2O3 co-doped Y2O3 stabilized ZrO2 ceramics

Enhanced thermal expansion and fracture toughness of Sc2O3-doped Gd2Zr2O7 ceramics

Gd 2 Zr 2 O 7 has been considered as a promising thermal barrier coating candidate for operating above 1300 °C, but the relatively poor fracture toughness limits its application. In this study, effort was made to toughen Gd 2 Zr 2 O 7 with the addition of metastable tetragonal (t′) phase. For 3.5 mol% RE 2 O 3 (RE=Dy, Y, Er, Yb) stabilized ZrO 2 (RESZ) ceramics, the t′ phase stability increased with the decrease of RE 3+ ionic radius. Among these compounds, YbSZ exhibited the best phase stability and had excellent toughness, thus it was selected as the toughening agent for Gd 2 Zr 2 O 7 . The phase structure and mechanical properties of YbSZ doped Gd 2 Zr 2 O 7 were investigated. YbSZ first dissolved in Gd 2 Zr 2 O 7 lattice, then it presented as a second phase when its content reached to 15 mol%. With YbSZ content increasing, the fracture toughness of the compounds increased, and the related mechanisms were discussed.

Lei Guo

Papers

Thermophysical properties of Yb2O3 doped Gd2Zr2O7 and thermal cycling durability of (Gd0.9Yb0.1)2Zr2O7/YSZ thermal barrier coatings

Phase stability and thermal conductivity of RE2O3 (RE=La, Nd, Gd, Yb) and Yb2O3 co-doped Y2O3 stabilized ZrO2 ceramics

Enhanced thermal expansion and fracture toughness of Sc2O3-doped Gd2Zr2O7 ceramics

Toughening effect of Yb2O3 stabilized ZrO2 doped in Gd2Zr2O7 ceramic for thermal barrier coatings

Degradation of EB-PVD thermal barrier coatings caused by CMAS deposits