Ultra-high temperature ceramic (UHTC) structural materials have emerged as critical candidates in the fields of aerospace, defense equipment, energy and power sectors due to their outstanding oxidation/ablation resistance, high-temperature strength retention, and thermal shock resistance in oxidative environments exceeding 1600 ℃. In recent years, extensive research has been achieved in both fundamental research and technological applications focusing on compositional control, structural design, fabrication techniques, and performance optimization of these materials. UHTC systems, characterized by carbides, borides and nitrides, are currently facing increasingly stringent demands for enhanced thermal performance in more complex environments. To further advance development of UHTC structural materials for such conditions, this paper systematically reviews the latest research progress in this field. Firstly, synthesis techniques of UHTC powders are elaborated. Subsequently, systems, densification methods and structural regulation strategies of UHTCs are presented. Furthermore, fabrication techniques and performance enhancement strategies of UHTC matrix composites (UHTCMCs), UHTCs modified carbon/carbon composites (UHTCs-C/C), and UHTC coatings are examined, with particular emphasis on the latest breakthroughs in oxidation/ablation resistance. Additionally, primary technical challenges related to the long-term stability and reliability of UHTC structural materials under extreme conditions are identified, and a forward-looking perspective on future development trends is provided.

In recent years, ZrB₂, as a representative material of ultra-high temperature ceramics (UHTCs), has become an important candidate material system for components of new generation aerospace vehicles. However, its practical application is limited by difficulties in material preparation and processing of complex components. This study aims to optimize sintering process of ZrB₂-based UHTCs by introducing HfSi₂ as a sintering aid, specifically addressing the challenge of densification caused by low intrinsic diffusion coefficient of traditional ZrB₂ ceramics. The research focuses on elucidating formation mechanism of core-rim structured borides and their role in enhancing densification of ZrB₂-HfSi₂ ceramics. Dense ZrB₂-HfSi₂ ceramics were successfully fabricated via hot-press sintering at 1600 ℃. The results reveal that softening of HfSi₂ phase during sintering effectively fills interparticle gaps, thereby facilitating low-temperature densification. Furthermore, during the holding stage, interdiffusion of Hf and Zr atoms through a dissolution-reprecipitation mechanism facilitates formation of a core-rim structured ZrB₂/(Zr,Hf)B₂ composite. This core-rim structure consists of ZrB₂ core encased by a (Zr,Hf)B₂ rim, characterized by a fully coherent interface (hexagonal P6/mmm symmetry) with a low lattice mismatch (<5%), ensuring interfacial stability. The ZrB₂-HfSi₂ ceramic exhibits a compressive strength of (1333±83) MPa, a Vickers hardness of (15.86±0.72) GPa, and a fracture toughness of (2.01±0.36) MPa·m^1/2. The ZrB₂-HfSi₂ ceramic demonstrates typical intergranular fracture behavior, with only a limited number of cleavage planes displaying core-rim structural features. These findings provide critical insights into low-temperature sintering of UHTCs and underscore potential of core-rim structures in advancing the preparation of high-performance ceramics.

Ultra-high temperature oxide ceramics, known for their outstanding high-temperature strength, microstructural stability, oxidation and corrosion resistance, are anticipated to serve as the next generation of ultra-high temperature structural materials, suitable for prolonged use in high-temperature oxidizing environments, and are expected to have broad application potential in the aerospace sector. In recent years, laser additive manufacturing (LAM) technology has emerged as a prominent method for the preparation of ultra-high temperature oxide ceramics, characterized by advantages such as rapid near-net shaping, mold-free production, and high flexibility for fabricating complex-shaped parts, thereby establishing itself as a significant research hotspot. However, ceramics are highly prone to pore defects during LAM process, which not only hinders the subsequent deposition of samples but also leads to deterioration in the surface quality and mechanical properties of formed parts. This review firstly provides an overview of the basic principles and process characteristics of three LAM techniques, including selective laser sintering (SLS), laser powder bed fusion (LPBF), and laser directed energy deposition (LDED). It focuses on characteristics of pore defects, flow characteristics of molten pool, and formation mechanism of pore defects in the LAM of ultra-high temperature oxide ceramics. Furthermore, their research progress in suppressing pore defects is detailed from three aspects: optimization of process parameters, outfield assistance, and second-phase doping. Finally, their challenges associated with achieving practical engineering applications are summarized, along with prospective development trends and breakthrough points in the field, focusing on suppression of forming defects, powder characteristics and subsequent heat treatment.

Laminated Ta/Ta_0.5Hf_0.5C cermets, characterized by high strength, high toughness, and high-temperature resistance, are excellent candidate materials for structural applications in aerospace field. To further investigate ablation performance of Ta/Ta_0.5Hf_0.5C cermets under high-temperature environment, a high-frequency plasma wind tunnel was utilized to evaluate their ablation resistance at nearly 3000 ℃. Their phase composition and microstructure before and after ablation were characterized and analyzed. Results revealed that the laminated Ta/Ta_0.5Hf_0.5C cermets demonstrated remarkable ablation resistance, with a mass ablation rate of 0.061 g/s and a linear ablation rate of 0.019 mm/s. During the ablation process, distinctive ridge-groove surface morphologies and internal cracks were produced along the layered structure direction. These features were attributed to inconsistent ablation rates and thermal expansion coefficients of Ta metal layer and Ta_0.5Hf_0.5C ceramic layer. Specifically, the ridge region primarily consisted of Hf₆Ta₂O₁₇ formed by oxidation of Ta_0.5Hf_0.5C ceramic layer. This compound could stably exist at high temperatures to protect the interior of ceramic layer from further oxidation. In contrast, the groove region primarily comprised Ta₂O₅, which was formed by oxidation of Ta metal layer. Yet Ta₂O₅ had a tendency to melt and vaporize at elevated temperatures, potentially leading to ejection or loss toward the ablation edge. The cracks formed within the layered structure during cooling process after ablation were mainly generated by thermal stress acting on the Ta_0.5Hf_0.5C ceramic layer due to differences in thermal expansion coefficients between the layers. Additionally, the Ta₂C interfaces between metal and ceramic layers played a crucial role in branching, deflecting, and initiating micro-cracks, which endowed the material with good thermal shock resistance.

Achieving complete densification of ultrafine-grained tungsten carbide (WC) without inducing grain growth has long been a challenge, which limits industrial applications of WC. The dynamic sinter forging process, which involves forging of incompletely dense materials under oscillatory pressure, facilitates densification while suppressing grain growth. This study explores effects of oscillatory pressure amplitude during dynamic sinter forging on microstructure and tribological properties of WC. The results show that increasing pressure amplitude leads to higher relative density of WC, accompanied by a reduction in grain size, an increase in proportion of low angle grain boundaries and special grain boundaries Σ2, as well as an enhancement of dislocation density. At a pressure amplitude of 20 MPa, the relative density, average grain size and dislocation density of WC reach 99.6%, 203 nm and 1.68×10¹⁵ m^-2, respectively. With an increase in pressure amplitude, both the friction coefficient and the wear rate gradually decrease. Under this condition, the adhesive wear and ploughing were identified as the dominant wear mechanisms. The reduction in wear rate is attributed to complete densification, finer grains and higher dislocation density, which result from the increased pressure amplitude. Grain refinement and high dislocation density enhance plastic deformation capacity and hardening ability during the wear, thereby increasing hardness of worn surface while mitigating crack initiation and propagation. Furthermore, special grain boundaries Σ2 also effectively impede motion of dislocation, thereby improving strain hardening capability and enhancing hardness of worn surface.

In response to the evolving landscape of high-speed aircraft, characterized by an expansive airspace, prolonged flight durations, and increased velocities, the thermal protection requirements for key structures such as the nose cone, leading edge, and engine combustion chamber have become more exacting. This necessitates a concerted focus on the development of high-performance thermal protection materials capable of withstanding extreme conditions. Ultra-high temperature ceramic composites have emerged as noteworthy candidates, showcasing exceptional oxidation and ablation resistance. Despite their commendable properties, the inherent brittleness of these composites poses a significant obstacle to widespread engineering applications. To address this limitation, there is a growing emphasis on toughening through structural modulation. Simultaneously, the imperative to enhance aircraft payload capacity underscores the demand for lightweight ultra-high temperature ceramic composites. This paper provides a systematic overview of the major research advances made in recent years on ultra-high temperature ceramic composites, including preparation methods such as pressure sintering, slurry infiltration, precursor impregnation and pyrolysis, reactive melt infiltration, chemical vapor infiltration/deposition, and “solid-liquid” combination process, toughening methods such as particles, whiskers, soft-phase materials, short-cut fibers, and continuous fibers, as well as oxidation ablation resistant properities and mechanisms, and lightweighting design. The relationship between the components, microstructures and properties of ultra-high temperature ceramic composites is discussed in depth, and the current challenges as well as the future development trends of ultra-high temperature ceramic composites are presented.

The development of high-speed flight technology has put forward an urgent demand for high- performance thermal structure materials. High-entropy carbides (HECs) ceramics are a fast-emerging family of materials that combine the excellent properties of high-entropy ceramics and ultra-high temperature ceramics. HECs have a broad application prospect in extreme service environments, which has received extensive attention from scholars in recent years. Compared with traditional ultra-high temperature carbides containing only one or two transition metal elements, HECs have a greater potential for development because of their improved comprehensive performance and greater designability of composition and properties. After successive exploration of HECs in recent years, researchers have obtained many interesting results, developed a variety of preparation methods, and gained comprehensive understanding of microstructure and properties. The basic theories and the laws on HECs obtained from experimental process are reviewed in this paper. Preparation methods of HECs including powders, blocks, coatings and films, as well as fiber-reinforced HECs-based composites are summarized. Research progress on the properties of HECs, such as the mechanical properties, thermal properties, and especially the oxidation and ablation resistance related to high-temperature applications, is reviewed and discussed. Finally, the scientific issues that need to be further explored in this area are emphasized, and the prospects are proposed.

Owing to the high strength/toughness and excellent anti-oxidation ability, continuous fiber reinforced ceramic matrix composites have become the preferred candidates for high temperature structural materials in aerospace field. Reactive melt infiltration can achieve the large-scale, short-cycle and low-cost production of ceramic matrix composites, which has been widely considered to be one of the most promising technologies from a commercial perspective. However, the mechanical and anti-oxidation/ablation properties of obtained composites prepared by conventional reactive melt infiltration are not satisfactory due to the existence of residual carbon and corroded fibers. In order to address the problems, relevant researchers constructed porous carbon matrix to replace conventional densified structure to promote its ceramic transformation and the consumption of reactive melt, thus achieving the improved performance of ceramic matrix composites. This paper reviewed the research progress about the preparation of SiC ceramics, SiC/SiC composites, C/SiC composites, and ultra-high temperature ceramic matrix composites by porous carbon ceramization strategy. Besides, the superiority of the method was verified compared to conventional reactive melt infiltration. The development of preparation methods for porous carbon matrix was also summarized. Finally, in term of the requirements of basic theory and technology for advanced ceramic matrix composites, the prospect for the future development of improved reactive melt infiltration to prepared ceramic matrix composites was discussed.

Oxide ceramics, known for their outstanding strength and excellent oxidation and corrosion resistance, are prime candidates for high-temperature structural materials of aero-engines. These materials hold vast potential for application in high-end equipment fields of the aerospace industry. Compared with traditional ceramic preparation methods, laser additive manufacturing (LAM) can directly realize the integrated forming from raw powders to high-performance components in one step. LAM stands out for its high forming efficiency and good flexibility, enabling rapid production of large complex structural components with high performance and high precision. Recently, research on LAM for melt-grown oxide ceramics, which involves liquid-solid phase transition, has surged as a hot topic. This paper begins by outlining the basic principles of LAM technology, with an emphasis on the process characteristics of two typical LAM technologies: selective laser melting and laser directed energy deposition. On this basis, the paper summarizes the microstructure characteristics of several different oxide ceramics prepared by LAM and examines how process parameters influence these microstructures. The differences in mechanical properties of laser additive manufactured oxide ceramics with different systems are also summarized. Finally, the existing problems in this field are sorted out and analyzed, and the future development trend is prospected.

Carbide ultra-high temperature ceramics (UHTCs) have emerged as ideal coating materials for the thermal protection systems of hypersonic vehicles due to their high melting point (>3000 ℃), high hardness, low thermal conductivity, excellent heat resistance, and good chemical stability. This review provides a comprehensive overview of structure and properties of carbide UHTCs, namely TiC, ZrC, HfC, NbC, and TaC. Furthermore, it summarizes recent developments in preparation of carbide UHTC coatings using various methods, including chemical vapor deposition, plasma spraying, and solid-phase reaction. Effects of coating microstructure, composition, structural design, and heat flux on the ablation behavior are analyzed. Data from recent literature corroborate that the added second phase can facilitate formation of complex oxides, generate an oxidation layer during ablation to undergo moderate sintering, protect structural integrity, and enhance oxygen barrier properties. Multi-layer structural designs utilize gradient layering and multi-functional structures, which effectively alleviate thermal stress within the coating, suppress crack propagation, and facilitate synergistic enhancing effects among different layers. Finally, the challenges and opportunities in development of carbide UHTC anti-ablation coatings are prospected.

ZrB₂-based ceramics typically necessitate high temperature and pressure for sintering, whereas ZrB₂-SiC ceramics can be fabricated at 1500 ℃ using the process of reactive melt infiltration with Si. In comparison to the conventional preparation method, reactive synthesis allows for the more facile production of ultra-high temperature ceramics with fine particle size and homogeneous composition. In this work, ZrSi₂, B₄C, and C were used as raw materials to prepare ZrB₂-SiC via combination of tape casting and reactive melt infiltration herein referred to as ZBC ceramics. Control sample of ZrB₂-SiC was also prepared using ZrB₂ and SiC as raw materials through an identical process designated as ZS ceramics. Microscopic analysis of both ceramic groups revealed smaller and more uniformly distributed particles of the ZrB₂ phase in ZBC ceramics compared to the larger particles in ZS ceramics. Both sets of ceramics underwent cyclic oxidation testing in the air at 1600 ℃ for a cumulative duration of 5 cycles, each cycle lasting 2 h. Analysis of the oxidation behavior showed that both ZBC ceramics and ZS ceramics developed a glassy SiO₂-ZrO₂ oxide layer on their surfaces during the oxidation. This layer severed as a barrier against oxygen. In ZBC ceramics, ZrO₂ is finely distributed in SiO₂, whereas in ZS ceramics, larger ZrO₂ particles coexist with glassy SiO₂. The surface oxide layer of ZBC ceramics maintains a dense structure because the well-dispersed ZrO₂ increases the viscosity of glassy SiO₂, preventing its crystallization during the cooling. Conversely, some SiO₂ in the oxide layer of ZS ceramics may crystallize and form a eutectic with ZrO₂, leading to the formation of ZrSiO₄. This leads to cracking of the oxide layer due to differences in thermal expansion coefficients, weakening its barrier effect. An analysis of the oxidation resistance shows that ZBC ceramics exhibit less increase in oxide layer thickness and mass compared to ZS ceramics, suggesting superior oxidation resistance of ZBC ceramics.

Hf_xTa_1-xC is a very promising candidate for thermal protection materials above 2000 ℃ due to its excellent properties such as high melting point, high hardness, high strength, high electrical conductivity, and high thermal conductivity. However, the rules of its mechanical properties and melting temperature varying with the composition remain elusive. Firstly, the mechanism of the variation of mechanical properties of Hf_xTa_1-xC system solid solutions with its components was systematically investigated from the microscopic point of view of covalent bond strength and valence electron concentration (VEC) based on the special quasirandom structures (SQS) method and first-principles calculations. It revealed that among the five components of solid solutions (i.e., HfC, Hf_0.75Ta_0.25C, Hf_0.5Ta_0.5C, Hf_0.25Ta_0.75C and TaC), the Hf_0.25Ta_0.75C solid solution possessed the largest elastic modulus and shear modulus. It was mainly attributed to two reasons: (1) the component possessing the strongest covalent bonding strength among the above ternary compounds; (2) the special bonding states between the p-orbital from C and the d-orbital from Hf or Ta strongly resisting the deformation and being completely filled near VEC=8.75 (for Hf_0.25Ta_0.75C). Secondly, the melting curves of the Hf_xTa_1-xC system solid solutions were calculated using the ab initio molecular dynamics (AIMD)-based molecular dynamics Z method. It showed that there existed indeed the phenomenon for anomalous increase in the melting temprature of Hf_xTa_1-xC system solid solutions, and the highest melting temperature of 4270 K was predicted on Hf_0.5Ta_0.5C, which was mainly attributed to the synergistic effect of the conformational entropy and the strength of the covalent bond. The results provide a theoretical guidance for the experimental selection of the optimal components of high melting temprature and high mechanical properties for Hf_xTa_1-xC system solid solutions in the thermal barrier coating applications, as well as a reference for the study of other transition metal carbides.

In response to the urgent demand for ultra-high temperature ceramic matrix composites with integrated thermal protection and load-bearing capabilities for high-speed aircrafts, this study prepared stable ceramic slurry from submicron HfC ceramic powder, and utilized the slurry pressure impregnation-assisted precursor infiltration pyrolysis (PIP) process to fabricate C/HfC-SiC composites with uniformly distributed HfC matrix to overcome the shortcomings of the existing reaction-derived HfC precursor, such as high cost, low efficiency, and poor densification effect. The influence of HfC content on the microstructure, mechanical properties, and ablation resistance of composites was investigated. Results showed that the composites had density of 2.20-2.58 g·cm^-3 and open porosity of approximately 5% when the actual volume fraction of HfC was in range of 13.1%-20.3%. Utilizing a single layer of carbon cloth to impregnate the ceramic slurry with pressure, HfC particles were able to disperse into the interior of the fiber bundle and distributed relatively evenly in the composites. Increasing the HfC content resulted in reducted fiber content, and decreased mechanical properties of composites. Specifically, when HfC volume fraction was 20.3%, the composites exhibited density, tensile strength and fracture toughness of 2.58 g·cm^-3, 147 MPa and 9.3 MPa·m^1/2, respectively. Following 60 s of ablation under an oxygen acetylene flame, the composites demonstrated linear ablation rate of 0.0062 mm/s and mass ablation rate of 0.005 g/s. The molten phase Hf_xSi_yO_z formed during the ablation process could effectively cover the composites surface and provide protection.

Continuous silicon carbide fiber reinforced silicon carbide composite (SiC_f/SiC) is a critical structural material for the development of next-generation aircraft engines. The interfacial property is one of the important factors determining the material mechanical properties. Therefore, this study characterized the interfacial mechanical properties of domestic third-generation 2.5D SiC_f/SiC and investigated its relationship with tensile properties. The residual stress of the 2.5D SiC_f/SiC constituents and interfacial sliding stress (IFSS) were quantitatively analyzed by hysteresis characteristics during the cyclic tension loading/unloading test. Statistical distributions of the in-situ fiber strength $({{\sigma }_{\text{fu}}})$ were obtained based on the fracture mirror radius of pull-out fibers. Interfacial shear strength (ISS) and interfacial debonding energy (G_i) were obtained through the push-in method. Results show that combination of macroscopic and microscopic methods can comprehensively describe the interfacial mechanical performance of 2.5D SiC_f/SiC from crack initiation to final debonding. The IFSS, ISS, and G_i of 2.5D SiC_f/SiC are 56 MPa, (28 ± 5) MPa, and (2.7 ± 0.6) J/m², respectively. Values of ISS and G_i indicate weak interface bonding, causing it susceptible to cracking under shear stress, while the large IFSS suggests that relative fiber sliding is inhibited after interface debonding, hindering fiber pull-out. The obtained interfacial properties can predict the proportional limit stress (${{\sigma }_{\text{PLS}}}$) accurately according to the ACK model. Based on the interfacial properties and the in-situ fiber strength (${{\sigma }_{\text{fu}}}$), the tensile strength of 2.5D SiC_f/SiC is predicted to be higher than the experimental value, which is related to the interfacial radial compressive residual stress and residual tensile stress endured by the fiber.