During the last decade a number of ceramic materials, mostly oxides have been suggested as new thermal barrier coating (TBC) materials. These new compositions have to compete with the state-of-the-art TBC material yttria stabilized zirconia (YSZ) which turns out to be difficult due to its unique properties. On the other hand YSZ has certain shortcomings especially its limited temperature capability above 1200 °C which necessitates its substitution in advanced gas turbines. In the paper an overview is tried on different new materials covering especially doped zirconia, pyrochlores, perovskites, and aluminates. Literature results and also results from our own investigations will be presented and compared to the requirements. Finally, the double-layer concept, a method to overcome the limited toughness of new TBC materials, will be discussed.

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Overview on advanced thermal barrier coatings

Progress in additive manufacturing on new materials: A review

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.

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

High temperature oxidation behavior and research status of modifications on improving high temperature oxidation resistance of titanium alloys and titanium aluminides: A review

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.

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

Bulk material and coatings of Lanthanum–Cerium Oxide (La2Ce2O7) with a fluorite structure were studied as a candidate material for thermal barrier coating (TBC). It has been showed that such material has the properties of low thermal conductivity about four times lower than YSZ, the difference in the thermal expansion coefficient between La2Ce2O7 and bond coat is smaller than that of YSZ in TBC systems, high phase stability between room temperature and 1673 K, about 300 K higher than that of the YSZ. The coating prepared by electron beam physical vapor deposition (EB–PVD) showed that it has good thermal cycling behavior, implying that such material can be a promising thermal barrier coating material. The deviation of coating composition from ingot can be overcome by the addition of excess La2O3 during ingot preparation and/or by adjusting the process parameters.

The thermal cycling behavior of Lanthanum–Cerium Oxide thermal barrier coating prepared by EB–PVD

(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

Thermal shock resistance and mechanical properties of La2Ce2O7 thermal barrier coatings with segmented structure

On improving the phase stability and thermal expansion coefficients of lanthanum cerium oxide solid solutions

Compressor blades used in gas turbine engines for marine applications must work in harsh environment such as erosion and corrosion Multilayer nitride coatings produced by physical vapour deposition (PVD) have exhibited promising potential in improving the corrosion resistance In this paper, three types of Ti/TiN multilayer coatings with different thickness ratios (M1: 10 nm/40 nm, M2: 20 nm/80 nm, M3: 30 nm/80 nm) were deposited onto 1Cr11Ni2W2MoV stainless steel substrates by plasma activated electron beam physical vapour deposition (EB-PVD) The microstructure, hardness and corrosion behaviour of the Ti/TiN multilayer coatings and TiN single layer coating were comparatively studied The columnar grain growth of the TiN single layer coating was suppressed by the introduction of the Ti metallic sublayers With the increase of the Ti sublayer thickness, the characteristics of the layered microstructure became more apparent The substrates coated with Ti/TiN multilayer coatings exhibited better corrosion resistance (maximum 87% protection efficiency for M3) and higher hardness value (maximum 341 GPa for M1) than those coated with TiN single layer

Shengkai Gong

Papers

The thermal cycling behavior of Lanthanum–Cerium Oxide thermal barrier coating prepared by EB–PVD

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

Thermal shock resistance and mechanical properties of La2Ce2O7 thermal barrier coatings with segmented structure

On improving the phase stability and thermal expansion coefficients of lanthanum cerium oxide solid solutions

Microstructure, hardness and corrosion behaviour of Ti/TiN multilayer coatings produced by plasma activated EB-PVD