Ultra-high temperature ceramics (UHTCs) are generally referred to the carbides, nitrides, and borides of the transition metals, with the Group IVB compounds (Zr & Hf) and TaC as the main focus. The UHTCs are endowed with ultra-high melting points, excellent mechanical properties, and ablation resistance at elevated temperatures. These unique combinations of properties make them promising materials for extremely environmental structural applications in rocket and hypersonic vehicles, particularly nozzles, leading edges, and engine components, etc. In addition to bulk UHTCs, UHTC coatings and fiber reinforced UHTC composites are extensively developed and applied to avoid the intrinsic brittleness and poor thermal shock resistance of bulk ceramics. Recently, highentropy UHTCs are developed rapidly and attract a lot of attention as an emerging direction for ultra-high temperature materials. This review presents the state of the art of processing approaches, microstructure design and properties of UHTCs from bulk materials to composites and coatings, as well as the future directions.

High-entropy pyrochlore-type structures based on rare-earth zirconates are successfully produced by conventional solid-state reaction method. Six rare-earth oxides (La₂O₃, Nd₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, and Y₂O₃) and ZrO₂ are used as the raw powders. Five out of the six rare-earth oxides with equimolar ratio and ZrO₂ are mixed and sintered at different temperatures for investigating the reaction process. The results demonstrate that the high-entropy pyrochlores (5RE_1/5)₂Zr₂O₇ have been formed after heated at 1000°C. The (5RE_1/5)₂Zr₂O₇ are highly sintering resistant and possess excellent thermal stability. The thermal conductivities of the (5RE_1/5)₂Zr₂O₇ high-entropy ceramics are below 1 W·m^–1·K^–1 in the temperature range of 300–1200°C. The (5RE_1/5)₂Zr₂O₇ can be potential thermal barrier coating materials.

A high-entropy silicide (HES), (Ti_0.2Zr_0.2Nb_0.2Mo_0.2W_0.2)Si₂ with close-packed hexagonal structure is successfully manufactured through reactive spark plasma sintering at 1300 °C for 15 min. The elements in this HES are uniformly distributed in the specimen based on the energy dispersive spectrometer analysis except a small amount of zirconium that is combined with oxygen as impurity particles. The Young’s modulus, Poisson’s ratio, and Vickers hardness of the obtained (Ti_0.2Zr_0.2Nb_0.2Mo_0.2W_0.2)Si₂ are also measured.

High-entropy ceramics attract more and more attention in recent years. However, mechanical properties especially strength and fracture toughness for high-entropy ceramics and their composites have not been comprehensively reported. In this work, high-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta _0.2)B₂ (HEB) monolithic and its composite containing 20 vol% SiC (HEB–20SiC) are prepared by hot pressing. The addition of SiC not only accelerates the densification process but also refines the microstructure of HEB, resulting in improved mechanical properties. The obtained dense HEB and HEB–20SiC ceramics hot pressed at 1800 ℃ exhibit four-point flexural strength of 339±17 MPa and 447±45 MPa, and fracture toughness of 3.81±0.40 MPa·m^1/2 and 4.85±0.33 MPa·m^1/2 measured by single-edge notched beam (SENB) technique. Crack deflection and branching by SiC particles is considered to be the main toughening mechanisms for the HEB–20SiC composite. The hardness Hv_0.2 of the sintered HEB and HEB–20SiC ceramics is 23.7±0.7 GPa and 24.8±1.2 GPa, respectively. With the increase of indentation load, the hardness of the sintered ceramics decreases rapidly until the load reaches about 49 N, due to the indentation size effect. Based on the current experimental investigation it can be seen that the room temperature bending strength and fracture toughness of the high-entropy diboride ceramics are within ranges commonly observed in structure ceramics.

Ultra-high temperature ceramics (UHTCs) are considered as a family of nonmetallic and inorganic materials that have melting point over 3000 °C. Chemically, nearly all UHTCs are borides, carbides, and nitrides of early transition metals (e.g., Zr, Hf, Nb, Ta). Within the last two decades, except for the great achievements in the densification, microstructure tailoring, and mechanical property improvements of UHTCs, many methods have been established for the preparation of porous UHTCs, aiming to develop high-temperature resistant, sintering resistant, and lightweight materials that will withstand temperatures as high as 2000 °C for long periods of time. Amongst the synthesis methods for porous UHTCs, sol-gel methods enable the preparation of porous UHTCs with pore sizes from 1 to 500 urn and porosity within the range of 60%-95% at relatively low temperature. In this article, we review the currently available sol-gel methods for the preparation of porous UHTCs. Templating, foaming, and solvent evaporation methods are described and compared in terms of processing-microstructure relations. The properties and high temperature resistance of sol-gel derived porous UHTCs are discussed. Finally, directions to future investigations on the processing and applications of porous UHTCs are proposed.