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

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Fundamentals Of Powder Diffraction And Structural Characterization Of Materials

Response of ytterbium disilicate-silicon environmental barrier coatings to thermal cycling in water vapor

Modern gas turbines rely on ceramic coatings to protect structural components along the hot gas path. These coatings are susceptible to accelerated degradation caused by silicate deposits formed when ingested environmental debris (dust, sand, ash) adheres to the coatings. This article reviews the current understanding of the deposit-induced failure mechanisms for zirconia-based thermal barrier coatings and silicate environmental barrier coatings. Details of the debris melting and crystallization behavior, the nature of the chemical reactions occurring between the deposits and coatings, and the implications for the thermocyclic durability of the coatings are described. Given the challenges posed in understanding how prospective coating materials and architectures will respond to a broad range of deposit compositions, it is proposed to develop an integrated framework linking thermochemical and thermomechanical models to predict coating durability. Initial progress toward developing this framework, and the r...

Silicate Deposit Degradation of Engineered Coatings in Gas Turbines: Progress Toward Models and Materials Solutions

Fracture mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal exposure

An air plasma spray process has been used to deposit tri-layer environmental barrier coatings consisting of a silicon bond coat, a mullite inter-diffusion barrier, and a Yb 2 SiO 5 top coat on SiC substrates. Solidified droplets in as-deposited Yb 2 SiO 5 and mullite layers were discovered to be depleted in silicon. This led to the formation of an Yb 2 SiO 5  + Yb 2 O 3 two-phase top coat and 2:1 mullite (2Al 2 O 3 *SiO 2 ) coat deposited from 3:2 mullite powder. The compositions were consistent with preferential silicon evaporation during transient plasma heating; a consequence of the high vapor pressure of silicon species at plasma temperatures. Annealing at 1300 °C resulted in internal bond coat oxidation of pore and splat surfaces, precipitation of Yb 2 O 3 in the top coat, and transformation of 2:1 mullite to 3:2 mullite + Al 2 O 3 . Mud-cracks were found in the Yb 2 SiO 5 layer and in precipitated Al 2 O 3 due to the thermal expansion mismatch between these coating phases and the substrate.

Plasma spray deposition of tri-layer environmental barrier coatings

An air plasma spray process has been used to apply a model tri-layer Yb2SiO5/Al6Si2O13/Si environmental barrier coating system on SiC test coupons. Significant differences in the thermal expansion of the component layers resulted in periodically spaced mud cracks in the Yb2SiO5 and Al6Si2O13 layers. Upon thermal cycling between 1316°C and 110°C in a 90% H2O/10% O2 environment flowing at 4.4 cm/s, it was found that partial delamination occurred with the fracture plane located within a thermally grown oxide (TGO) at the Al6Si2O13–Si interface. Delamination initiated at test coupon edges where the gaseous environment preferentially oxidized the exposed Si bond coat to form β-cristobalite. Simultaneous ingress of the gaseous environment through mud cracks initiated local formation of β-cristobalite (SiO2), the thickness of which was greatest directly below mud cracks. Upon cooling, cristobalite transformed from the β to α phase with a large, constrained volume contraction that resulted in severe microfracture of the TGO. Continued thermal cycling eventually propagated delamination cracks and caused partial spallation of the coatings. Formation of the cristobalite TGO appears to be the delamination life-determining factor in protective coating systems utilizing a Si bond coat.

Mechanisms of Ytterbium Monosilicate/Mullite/Silicon Coating Failure During Thermal Cycling in Water Vapor

Environmental barrier coatings (EBCs) are needed to protect SiC structures exposed to high temperatures in water vapor-rich environments. Recent studies of a tri-layer EBC system consisting of a silicon layer attached to the SiC, a mullite diffusion barrier and a low-steam volatility ytterbium silicate topcoat have shown some promise for use at temperatures up to 1316 °C. However, the performance of the coating system appeared to be dependent upon the manner of its deposition. Here, an air plasma spray method has been used to deposit this tri-layer EBC on α-SiC substrates, and the effects of the plasma arc current and hydrogen content upon the structure, composition, and defects in ytterbium monosilicate (Yb2SiO5) and disilicate (Yb2Si2O7) topcoats are investigated. Modification of spray parameters enabled the loss of SiO from the injected powder to be reduced, leading to partial control of coating stoichiometry and phase content. It also enabled significant control of the morphology of solidified droplets, the porosity, and the microcracking behavior within the coatings. Differences between the Yb2SiO5 and Yb2Si2O7 are discussed in the context of their EBC application.

Bradley T. Richards

Papers

Response of ytterbium disilicate-silicon environmental barrier coatings to thermal cycling in water vapor

Plasma spray deposition of tri-layer environmental barrier coatings

Fracture mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal exposure

Mechanisms of Ytterbium Monosilicate/Mullite/Silicon Coating Failure During Thermal Cycling in Water Vapor

Structure, composition, and defect control during plasma spray deposition of ytterbium silicate coatings