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Successful synthesis of novel high entropy ceramic (HEC) for ultra-high temperature application classes, namely, borides, carbides, and nitrides, has been experiencing a bottleneck in having a suitable design and successful synthesis strategy. Producing high-entropy ultra-high-temperature ceramics from their oxides offers a major processing benefit, while employing a design approach using machine learning enhances the efficiency of the formation of single-phase HECs. In this regard, we propose a generalized strategy to generate a semi-synthetic database for each of these classes using literature data and atomic environment mapping-based structure plots, which can further be used to build machine learning models. The imbalance of the dataset was addressed using adaptive synthetic sampling and the edited nearest neighbors technique. The trained models are able to accurately predict over 90% of the single-phase chemistry for each of the classes. Furthermore, a few compositions representing these classes were successfully synthesized from the corresponding oxide mixture to validate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

To address the requirements for reusable thermal protection materials, ZrB 2 -SiC-YSi 2 ceramics were developed for the first time using the reactive melt infiltration (RMI) technique. This study systematically examines the influence of carbon content on the densification and microstructural of ZrB 2 -SiC-YSi 2 ceramics. The results demonstrate that with increasing carbon content, the bulk density initially increases and then decreases, while the open porosity decreases initially before rising. The maximum density of the ceramics is 3.80 g/cm³ and the open porosity is as low as 0.17 % when the graphite carbon content is 20 wt%. Further analysis of densification behavior and phase morphology showed that the residual Si ratio of ZSY-2 specimen decreased from 10 % to about 4.2 %, while the SiC ratio increased from 24.6 % to 29.8 %. The primary phases present in the ZrB 2 -SiC-YSi 2 ceramics synthesized via the RMI process include ZrB 2 , YSi 2 , and SiC, with minor phases of Y 5 Si 3 C and free Si detected. Finally, the oxidation behavior of ZrB 2 -SiC-YSi 2 ceramics at an oxidation temperature of 1550 °C was verified. Its main oxidation products are ZrO 2 , Y 2 O 3 , SiO 2 , ZrSiO 4 , Y 2 Si 2 O 7 , and yttrium silicate phase with excellent oxidation resistance is determined to be generated. These advantages demonstrate the significant potential of the RMI process in advancing the fabrication of ultra-high-temperature ceramics. [ABSTRACT FROM AUTHOR]

Rare earth strontium aluminates have attracted much attention due to their excellent properties and widely functional applications. In this work, the bonding characteristics, mechanical/thermal properties, and phonon behavior of La2SrAl2O7 and Nd2SrAl2O7 with layered structure are investigated using first‐principles calculations. The weak chemical bonds within the rock‐salt layer lead to the anisotropy of elastic moduli, tensile, and shear strength, benefiting their damage tolerance. It is also found that the low‐frequency phonons present much lower scattering rates, making themselves vital contributors to heat conduction. Owing to the enhanced anharmonicity, Nd2SrAl2O7 exhibits lower thermal conductivity than La2SrAl2O7. Moreover, the lower thermal conductivities are observed along the z direction, which is attributed to the anisotropic chemical bonding. These results clarify the role of the weak bonds within layered structures in modulating their mechanical and thermal performance, which is expected to shield light on the development of perovskites with layered Ruddlesden–Popper structures. [ABSTRACT FROM AUTHOR]

In this work, a novel polymer precursor for yttrium modified hafnium carbide (HfC) was prepared by blending polyhafnium carboxane, yttrium acetylacetonate with xylene. The pyrolysis behavior and structural evolution of the precursor were comprehensively investigated, along with a thorough examination of the microstructure and composition of the synthesized HfC particles. The results showed that Y element was introduced into the ceramics to form c‐HfO2, with Hf, Y, C and O elements well distributed. The pyrolysis of the polymer precursor at 1600°C produced HfC nanocrystallites with an average grain size of 43 nm, encapsulated by an amorphous carbon shell. The synthesized HfC ceramics exhibited distinct nanostructures that varied from amorphous structure to almost spherical morphology under different pyrolysis temperatures. The underlying formation mechanisms were also discussed. [ABSTRACT FROM AUTHOR]

The fabrication of ultra-high-temperature ceramics using the sintering method requires maintaining high temperatures of around 1500 °C for several hours. In contrast, this study demonstrated an alternative method for uniform formation of the corresponding microphases, which could reduce production costs in the future. The essence of the reactive hot pressing method lies in initiating a chemical reaction at an adiabatic temperature, which constitutes 60-80 % of the precursors' melting temperature, with the application of external pressure. The combination of these conditions significantly accelerates the densification process of the powder batch. The HfB2-SiC-C heteromodulus ceramics with different content of carbon platelets were manufactured via the reactive hot pressing of HfC-B4C-Si precursors at 1850 °C and 30 MPa for 4 minutes. Thus, the microhardness of the synthesized ceramics with specific chemical compositions reached 17.3 GPa, while the fracture toughness was 6.9 MPa/m². The reactively pressed materials were compared to non-reactively pressed ones with the same compositions. X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) have been used for the composite characterization. Carbon inclusions were shown affecting the HfB2-SiC hardness while improving thermal shock resistance. The stratification of reactively pressed materials has been identified with silicon-depleted inner areas of the samples. [ABSTRACT FROM AUTHOR]

This study presents the fabrication possibilities of ultra-high-temperature ceramics of ZrB2-30 vol.%SiC and (ZrB2-HfB2)-30 vol.% SiC composition using the reaction spark plasma sintering of composite powders ZrB2(HfB2)-(SiO2-C) under two-stage heating conditions. The phase composition and microstructure of the obtained ceramic materials have been subjected to detailed analysis, their electrical conductivity has been evaluated using the four-contact method, and the electron work function has been determined using Kelvin probe force microscopy. The thermal analysis in the air, as well as the calcination of the samples at temperatures of 800, 1000, and 1200 °C in the air, demonstrated a comparable behavior of the materials in general. However, based on the XRD data and mapping of the distribution of elements on the oxidized surface (EDX), a slightly higher oxidation resistance of the ceramics (ZrB2-HfB2)-30 vol.% SiC was observed. The I-V curves of the sample surfaces recorded with atomic force microscopy demonstrated that following oxidation in the air at 1200 °C, the surfaces of the materials exhibited a marked reduction in current conductivity due to the formation of a dielectric layer. However, data obtained from Kelvin probe force microscopy indicated that (ZrB2-HfB2)-30 vol.% SiC ceramics also demonstrated enhanced resistance to oxidation. [ABSTRACT FROM AUTHOR]

Multi-component ultra-high-temperature ceramics (MC-UHTCs) are promising for high-temperature applications due to exceptional thermo-mechanical properties, yet their wear characteristics remain unexplored. Herein, the wear behavior of binary (Ta, Nb)C, ternary (Ta, Nb, Hf)C, and quaternary (Ta, Nb, Hf, Ti)C UHTCs synthesized via spark plasma sintering (SPS) is investigated. Gradual addition of equimolar UHTC components improves the wear resistance of MC-UHTCs, respectively, by ~29% in ternary UHTCs and ~49% in quaternary UHTCs when compared to binary UHTCs. Similarly, the penetration depth decreased from 115.14 mm in binary UHTCs to 73.48 mm in ternary UHTCs and 44.41 mm in quaternary UHTCs. This has been attributed to the complete solid solutioning, near-full densification and higher hardness (~up to 30%) in quaternary UHTCs. Analysis of the worn-out surface suggests pull-out, radial, and edge micro-cracking and delamination as the dominant wear mechanisms in binary and ternary UHTCs. However, grain deformation and minor delamination are the dominant wear mechanisms in quaternary UHTCs. This study underscores the potential of MC-UHTCs for tribological applications where material experiences removal and inelastic deformation under high mechanical loading. [ABSTRACT FROM AUTHOR]

To achieve the repeatability of aerospace thermal components, C/TaC‒SiC composites were fabricated. Cycle ablation and bending tests were carried out. After 3 × 60 s of ablation beyond 2100°C, the mechanical property retention rate was 80.9%. Interestingly, a reaction similar to "ouroboros ring," in which the cyclic reactions of "TaC being oxidized to Ta2O5 and Ta2O5 being reduced to TaC," occurred in the central ablation region of C/TaC‒SiC composites. On the one hand, the continuous generation of TaC could prevent liquid state Ta2O5 from being blown off central ablation region, playing a similar role in "water and soil conservation." On the other hand, liquid Ta2O5 covered the surface of C/TaC‒SiC composites during ablation process, contributing to block the inward permeation of oxidized gases. In addition, novel "Grotto" structures were detected in the transitional ablation region of C/TaC‒SiC composites. The formation reason of the "Grotto" structure has also been discussed. [ABSTRACT FROM AUTHOR]

Ultra-high temperature ceramics (UHTCs) are candidates that can be used in extremely high temperature environments relative to the Si-based ceramic due to their excellent high-temperature performance. However, there are few researches on the electromagnetic wave absorption of the UHTCs themselves. In this work, rod-like ZrB 2 and its composites were prepared by polymer derived method at a lower temperature. The phase composition and electromagnetic wave absorption properties of the ceramics at different pyrolyzed temperatures were analyzed. The rod-like ZrB 2 pyrolyzed at 1500 °C shows a RL min of −54.77 dB at a thickness of 4.7 mm. The rod-like ZrB 2 /SiC has an absorption bandwidth of 5.68 GHz (RL <-10 dB, 7.6–13.28 GHz) at a thickness of 4.25 mm, exceeding the X-band. An absorption bandwidth of 2.24 GHz for RL <-20 dB (99% absorbed) was obtained by the fabricated composites. The strong absorption of rod-like ZrB 2 /SiC composites is due to the strong conductive loss and multiple scattering of electromagnetic wave by the rod-like network structure. The formation of core-shell ZrB 2 /SiC increases the polarization and enhance the electromagnetic wave absorbing properties of the ceramics. The results of this study show that the polymer derived UHTCs have the strong ability to absorb electromagnetic wave. [Display omitted] [ABSTRACT FROM AUTHOR]

High costs and time consuming associated with experimental trial-and-error result in low efficiency, creating an urgent need for a more effective strategy for ultra-high temperature ceramic matrix composites (UHTCMCs) development. Inspired by the exceptional performance of machine learning (ML) algorithms across various domains, this work employs ML algorithms to construct models and conduct in-depth analysis of the key factors and their patterns influencing the ablation resistance of UHTCMCs. A set of 26 dimensional features that could potentially impact the ablation resistance of UHTCMCs were established based on domain knowledge. Eight typical ML models were used to build and predict the linear ablation rate (LAR) of UHTCMCs. Results show that the random forest model has optimal generalization performance, with mean absolute error (MAE), mean squared error (MSE), and coefficient of determination (R 2) being 2.75 μm s−1 and 7.3 (μm s−1)2, and 0.71 respectively. The Shapley additive explanations values based on the random forest model reveal that the key features affecting the LAR of UHTCMCs are ranked as average melting point of ceramics (AMPC) > thermal conductivity of material (TCM) > thermal expansion coefficient of oxides (TECO) > fabrication temperature of material (FTM), all showing a negative influence on the LAR. Symbolic regression further indicates that AMPC, TCM, and TECO have an exponential negative correlation with LAR. These data-driven conclusions have been thoroughly validated through the use of C f /(TiZrHfNbTa)C composites. The established model can accelerate the discovery of material knowledge and provide reliable guidance for UHTCMC development. [ABSTRACT FROM AUTHOR]

The effect of 3.6 vol% WC addition to ZrB2–20 vol% SiC (ZSW) on oxidation resistance is studied over a broad range of oxidation temperatures, 1000–1800 °C. Non‐WC‐containing samples (ZS) show significant surface damage and degradation during oxidation, losing protective B2O3 and SiO2‐based surface layers and exposing a porous ZrO2 layer and base material for further oxidation. ZSW samples preserve their surface protective layers during oxidation up to 1800 °C while the underlying ZrO2 scale remains dense. The appearance of convection cells on the surface of ZSW samples during oxidation above 1600 °C is reported. This confirms the presence of boron‐rich phases, suppressing oxygen permeation into the material and enhancing oxidation resistance of ZSW samples. During exposure of the samples to 1800 °C for 15 min, ZS and ZSW samples gain 11.1 ± 1.5 and 7.8 ± 0.3 mg cm−2, respectively, due to oxidation. Exposure of the composites for 5 h at 1600 °C results in weight gains of 10.5 and 7.0 mg cm−2 for ZS and ZSW samples, respectively. Cross sections of oxidized samples at 1800 °C show a tight zirconia layer below a glassy surface in ZSW and complete loss of surface glass in ZS, demonstrating the effectiveness of WC addition. [ABSTRACT FROM AUTHOR]

C/C-ZiC-ZrC-SiC composites were prepared by a combination of PIP (polymer infiltration & pyrolysis) and RMI (reactive melt infiltration) processes on porous C/C composites. The PIP is used for preparing a ZrC-SiC interlayer (denoted as ZiC) between the C/C composites and ZrC-SiC matrix prepared by RMI. Mechanical properties and microstructure of the composites with and without ZiC were characterized. The results show that, the increase of both flexural and tensile strength of the composites after RMI can be optimized by adjusting the ZiC and achieves 50.54% and 56.76%, respectively. The mechanisms can be attributed to the facts that the ZiC effectively protects carbon fibers and PyC from serious erosion by high-temperature melt and weakens the interfacial bond between ceramic matrix and carbon fibers, ensuring a series of stress release under external load. Interestingly, the composites with ZiC also exhibit enhanced ablation performance at ca. 2200 °C due to the improved ZrC content. [ABSTRACT FROM AUTHOR]

In order to investigate the most extreme conditions in which materials potentially applicable in reusable thermal shields can be operated, ultra-high-temperature ZrB2 ceramics with 20 vol.% MoSi2 were prepared using two different techniques, cold isostatic pressing (CIP) and robocasting (RC, an additive manufacturing technique), followed by consolidation using pressureless spark plasma sintering (SPS). The oxidation behavior of the resulting materials was analyzed in low-pressure dissociated air at three different temperatures, namely 1800, 2000 and 2200 K. Using XRD and surface and cross-section SEM (coupled with EDS), zirconia was found to form at all three temperatures, while silica was only present at 1800 K, with gaseous SiO forming at a higher temperature. The elaboration technique influences the density of the ceramic, and less dense materials undergo deeper oxidation. This investigation suggests that 2000 K is already beyond the maximum temperature threshold at which damage to ceramics is limited by the formation of protective silica. This study confirms that the selected material is a promising candidate for thermal protection applications. [ABSTRACT FROM AUTHOR]

Functionally graded composites with fibre gradients were prepared with three different ceramic matrices: ZrB 2 -SiC-Y 2 O 3 , ZrB 2 -MoSi 2 and SiC-Y 2 O 3 , with short carbon fibres in amounts ranging from 0–50%. Short term oxidation tests at 1650 °C in air were carried out to study the influence of the graded architecture and fibre content on the structural stability and oxidation resistance. The composite layers were perfectly joined at the interface, with no visible defects. Even after oxidation testing, no spallation phenomena were observed. ZrB 2 -MoSi 2 matrix proved to be the most effective in protecting the composite from oxidation thanks to the formation of a compact ZrO 2 -SiO 2 scale. • Fibre gradient, ultra-refractory ceramic composites were produced by sintering. • Bulk ceramic layers and fibre reinforced layers were joined without cracks or spallation. • Oxidation tests at 1650 °C did not affect the structural integrity of the graded composites. • Out of the three ceramic matrices tested, ZrB 2 /MoSi 2 proved to be the most performing. [ABSTRACT FROM AUTHOR]

Surface structure and relevant oxygen vacancy play an important role in the application of RE2Si2O7 for environmental barrier coatings, in which the oxygen vacancies in RE2Si2O7 may influence their thermal and optical properties. In this work, the structure and thermodynamics of (0 0 1) and (1 1 0) surfaces of RE2Si2O7 (RE = Yb, Lu) are studied via first‐principles calculations to reveal the underlying mechanism of the surface formation and the associated oxygen vacancy behavior. The (1 1 0) surface is preferred energetically, being in good agreement with the experiential results. It is uncovered that the weak chemical bond broken dominates the decrease of the surface energy, together with the contribution from the polyhedral distortion. Furthermore, the [O3Si–O–SiO3] site is found to be the preferred site for oxygen vacancies on (1 1 0) surface. The formation energies of oxygen vacancies on the (1 1 0) surfaces are lower than those in the bulk, suggesting their segregation on the surfaces. These findings provide essential insights into the surface excitation and oxygen vacancy behaviors of RE2Si2O7, which could shield light on the experimental improvement of the thermal, mechanical, and corrosion properties for RE2Si2O7‐based environmental barrier coating materials. [ABSTRACT FROM AUTHOR]

Ultra high-temperature ceramic (UHTC) composite materials are physically and chemically stable at high temperatures (more than 2000°C) and in reactive environments. Generally, the family group of diboride, dicarbide, and dinitride are used as ultra-high temperature ceramic due to their high thermal conductivity and high melting point. The main aim of this paper is the development of zirconium diboride (ZrB2)-based UHTC and characterisation of their density, hardness, porosity, and work considered, three powder constituents B4C (35%), ZrB2 (55%), and Cr (10%) of particle size 30 μm, 15 μm, and 4 μm, respectively, the conventional sintering and spark plasma sintering technique is used for the development of composite. The results revealed that the relative density of conventionally sintered composite at 1600°C and 1700°C are 72% and 80%, respectively, and spark plasma sintered composite at 1900°C is 99.99%. The bulk density and apparent porosity of SPS composite at 1900°C are 4.73 g/cm3 and 4%, respectively. Similarly, bulk density and apparent porosity of conventionally sintered composite, at 1600°C are 2.49 g/cm3 and 30%, respectively, and at 1700°C is 2.89 g/cm3 and 27%, respectively. Furthermore, the hardness of the SPS composite at 1900°C is 2830 HV, while the hardness of conventionally sintered composite at 1700°C and 1800°C were 280 HV and 520 HV, respectively. Additionally, scanning electron microscopy was performed to observe the bonding and porosity in the microstructure of the UHTC. [ABSTRACT FROM AUTHOR]

Graphite is a widely used oxygen impurity-removing additive and a property-tuning second phase for diboride and carbide-based ultrahigh temperature ceramics (UHTCs). In recent years, high-entropy diboride (HEB) ceramics have attracted much attention for the development of better UHTCs. However, oxide impurity generally inhibits their densification and harms their properties. Here, (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)B 2 ceramics added with different contents of graphite (HEB- x C, x = 0, 1, 2.5, 5, 10, 20 vol%) were prepared by spark plasma sintering. It was found that the added graphite acted as an oxygen-removing agent and the oxide impurity phase t -(Zr, Hf)O 2 was converted to the (Hf, Ta)C-based solid solution carbide. With the increase in graphite content, the relative density increased from 89.4% for HEB-0 C to 100% for HEB-10 C. Based on the evaluated properties of the prepared specimens, including Young's modulus, hardness, toughness, and thermal conductivity, the added graphite with an optimized content is important for a given started powder and processing to achieve good and balanced material properties. [ABSTRACT FROM AUTHOR]

The ZrC nanoparticles were successfully fabricated using a novel amorphous zirconium metal-organic frameworks (aZMOFs) by one-step thermal decomposition at 1500 °C. The effects of different metal/linker ratios and solvent types on the synthesized ZrC nanoparticles were investigated. The property analysis of the particle in terms of phase purity, particle size, TG-DTA analysis and morphology indicated the as-synthesized ZrC nanoparticles with high purity, spherical shape and uniform particle size distribution (100 ± 21 nm). Notably, the partial distilled water substitution of DMF solvent amplifies the coordination between zirconium and linker, which leads to improving the consumption of excess carbon via carbothermal reduction. The current research provides a potential cost-effective synthesis method of preparing ultra-high temperature ceramic (UHTC) nanoparticles as matrices for fiber reinforced UHTC composites. [ABSTRACT FROM AUTHOR]

In this study, functionally graded ZrB2–B4C–SiC–LaB6 composite materials (FGMs) with potential applications in hypersonic aircraft thermal protection systems were fabricated using spark plasma sintering. A systematic study of the thermal conductivity of the FGM, the conductivity of respective FGM layers, and the equivalent non-graded composites, was performed from room temperature up to 450 °C. The results suggest that the thermal conductivity of the FGMs (in the through-thickness direction) and their equivalent non-graded composites ranged between 25 and 34.9 W/mK, which was ∼60 % less than ZrB2. While the overall thermal conductivity of the FGM and equivalent non-graded composites were similar, in the FGM, the topmost layer with high ZrB2-content displayed up to 245 % higher thermal conductivity than the bottom layer with high B4C content. A systematic comparison between experimentally determined conductivity and relevant thermal conductivity models was conducted.

Double-interface fibrous ZrB 2 -SiC ceramics (DZS w) were prepared by wet-spinning and hot-pressing. The introduction of double-interface layers improves the interface bonding and crack tolerance making the bending strength and toughness of DZS w increased to 607 MPa and 6.3 MPa m1/2, respectively. After ablation testing in 9.34 MW/m2 plasma flame, DZS w remained intact, with linear and mass ablation rates of − 0.3 µm s-1 and − 0.11 mg s-1, respectively. However, single-interface fibrous ZrB 2 -SiC ceramic (SZS w) produced penetrating cracks, so DZS w presented high ablation resistance than SZS w. The introduction of a dense ZrB 2 -SiC outer interface layer in DZS w interrupts the three-dimensional connected structure of the porous SiC w interface layer in SZS w and consequently reduces the oxygen diffusion rate of the ceramics. Moreover, the high emissivity and thermal conductivity enable DZS w to serve in the high heat flux ablation environment caused by faster flight speed. • An effective way of toughening: double-interface layer fibrous structure. • The ceramic remained intact after the 9.34 MW/m2 flame ablation for 300 s • Double-interface has better ablation resistance than single-interface. [ABSTRACT FROM AUTHOR]

Hf-based carbides are highly desirable candidate materials for oxidizing environments above 2000 °C. However, the static oxidation behavior at their potential service temperatures remains unclear. To fill this gap, the static oxidation behavior of (Hf, Ti)C and the effect of Ti substitutions were investigated in air at 2500 °C under an oxygen partial pressure of 4.2 kPa. After oxidation for 2000 s, the thickness of the oxide layer on the surface of (Hf, Ti)C bulk ceramic is reduced by 62.29 % compared with that on the HfC monocarbide surface. The dramatic improvement in oxidation resistance is attributed to the unique oxide layer structure consisting of various crystalline oxycarbides, HfO2, and carbon. The Ti-rich oxycarbide ((Ti, Hf)CxOy) dispersed within HfO2 formed the major structure of the oxide layer. A coherent boundary with lattice distortion existed at the HfO2/(Ti, Hf)CxOy interface along the (111) crystal plane direction, which served as an effective oxygen diffusion barrier. The Hfrich oxycarbide ((Hf, Ti)CxOy) together with (Ti, Hf)CxOy, HfO2, and precipitated carbon constituted a dense transition layer, ensuring favorable bonding between the oxide layer and the matrix. The Ti content affects the oxidation resistance of (Hf, Ti)C by determining the oxide layer's phase distribution and integrity. [ABSTRACT FROM AUTHOR]

Titanium boride (TiB2) is a material classified as an ultra-high-temperature ceramic. The TiB2 structure is dominated by covalent bonds, which gives the materials based on TiB2 very good mechanical and thermal properties, making them difficult to sinter at the same time. Obtaining dense TiB2 polycrystals requires a chemical or physical sintering activation. Carbon and molybdenum disilicide (MoSi2) were chosen as sintering activation additives. Three series of samples were made, the first one with carbon additives: 0 to 4 wt.%; the second used 2.5, 5 and 10 wt.% MoSi2; and the third with both additions of 2 wt.% carbon and 2.5, 5 and 10 wt.% MoSi2. On the basis of the dilatometric sintering analysis, all additives were found to have a favourable effect on the sinterability of TiB2, and it was determined that sintering TiB2 with the addition of carbon can be carried at 2100 °C and with MoSi2 and both additives at 1800 °C. The polycrystals were sintered using the hot-pressing technique. On the basis of the studies conducted in this work, it was found that the addition of 1 wt.% of carbon allows single-phase TiB2 polycrystals of high density (>90%) to be obtained. The minimum MoSi2 addition, required to obtain dense sinters with a cermet-like microstructure, was 5 wt.%. High density was also achieved by the materials containing both additives. The samples with higher MoSi2 content, i.e., 5 and 10%, showed densities close to 100%. The mechanical properties, such as Young's modulus, hardness and fracture toughness (KIc), of the polycrystals and composites were similar for samples with densities exceeding 95%. The Vickers hardness was 23 to 27 GPa, fracture toughness (KIC) was 4 to 6 MPa·m0.5 and the Young's modulus was 480 to 540 GPa. The resulting TiB2-based materials showed potential in high-temperature applications. [ABSTRACT FROM AUTHOR]

SiHfBCN ceramics are the most promising candidates as ultrahigh temperature ceramics for aerospace applications. In this work, PHSNB precursors were synthesized by hydrosilylation reaction of polyborosilazane (PSNB) and polyhafnocenecarbosilane (PHCS), and then the SiHfBCN ceramics can be obtained from the PHSNB precursors through the precursor-derived ceramics (PDC) strategy. The structure, composition, pyrolysis process, and phase evolution behavior at high temperatures of the SiHfBCN ceramics were systematically investigated. The results show that the optimum SiHfBCN ceramic prepared in this study can remain stable until 1780 °C, and the weight loss at 1800 °C is only 0.8 wt%, demonstrating the excellent high temperature thermal stability of the SiHfBCN ceramics. The outstanding high temperature thermal stability of the SiHfBCN ceramics can be attributed to two reasons. Firstly, the formation of HfC x N 1-x solid solution at high temperatures can greatly inhibit the formation of SiNx, so the carbothermal reduction reaction and thermal decomposition reaction of SiNx can be greatly inhibited. Secondly, the generated crystal phases at high temperatures are surrounded by the chaotic layered structures of B–C–N phase, which greatly restricts the decomposition process of the SiHfBCN ceramics. This work presents a convenient strategy in preparing multiphase ceramics (not only SiHfBCN) that can be applicable in various harsh environmental conditions. [ABSTRACT FROM AUTHOR]

Mo-Si-B alloys and ultra-high-temperature ceramics including ZrB2-based ceramics are potential candidates for hypersonic applications. In Mo-Si-B base systems and with SiC addition to ZrB2, a protective SiO2 rich glassy scale is formed at the surface. Diffusion through the scale and scale viscosity play an important role in understanding the oxidation behavior of the system. In the present work, the diffusion coefficient and activation energies of the diffusing species have been calculated using molecular dynamics, as these govern the scale growth and surface coverage. The effects of Hf and Al additions to the SiO2 have also been studied. Si4+ ions substitution with Hf4+ was found to increase the activation energy of both Si4+ and O2- ions with the increase in their concentration. On the other hand, Al3+ ions reduced the activation energy with a reversal in trend at higher content. However, in both cases, the diffusivity values of the added cations were highest and that of Si4+ ions were lowest. [ABSTRACT FROM AUTHOR]

To inhibit the negative influence of impact on ablation performance of HfC coating reinforced by SiC nanowires (SiCnws/HfC coatings), BN interphases were fabricated and introduced into SiCnws/HfC coatings by chemical vapor deposition. The effect on BN interphase thickness upon SiCnws@BN/HfC coatings performance under impact‐ablation environment was investigated. The interphase thickness was adjusted by changing the deposition time of BN. Impact and ablation tests were carried out by drop hammer and oxyacetylene ablation devices. Among the prepared coatings, the one with 150 ± 20 nm thickness interphase possessed a good performance under impact test, of which the crack propagation path exhibited a significant deflection. After ablation, the coating could still maintain intact surface, because the interphase thickness of 150 ± 20 nm was appropriate to ensure the toughness of the coating and to avoid the porous structure formation. [ABSTRACT FROM AUTHOR]

Experiments on heat transfer in supersonic underexpanded jets of high-enthalpy nitrogen with ceramic samples based on HfB2–SiC are carried out at the induction RF plasmatron VGU-4 (Ishlinsky Institute for Problems in Mechanics of the Russian Academy of Sciences) at a pressure in the pressure chamber of 8.5 hPa, a gas flow rate through the discharge channel of 3.6 g/s, and an RF power of the plasma torch generator for anode supply of 64 kW. Three heat transfer modes are implemented using water-cooled conical nozzles with outlet diameters of 30, 40, and 50 mm. For the experimental conditions in supersonic modes, using a numerical method within the framework of the Navier–Stokes equations and simplified Maxwell equations, we simulate nitrogen plasma flows in a plasmatron discharge channel and the flow of dissociated nitrogen underexpanded jets around a cylindrical holder with a ceramic sample. From a comparison of the experimental and calculated data on heat fluxes to the surface of three samples, the effective coefficient of heterogeneous recombination of nitrogen atoms on the surface of ultra-high-temperature ceramics (UHTCs) at temperatures of 2273–2843 K is determined. [ABSTRACT FROM AUTHOR]

Ultrahigh temperature ceramic matrix composites (UHTCMCs) are critical for the development of high Mach reusable hypersonic vehicles. Although various materials are utilized as the thermal components of hypersonic vehicles, it is still challenging to meet the ultrahigh temperature ablation‐resistant and reusability. Herein, the Y2O3 reinforced Cf/ZrB2‐SiC composites are designed, which demonstrates near‐zero damage under long‐term ablation at temperatures up to 2500 °C for ten cycles. Notably, the linear ablation rate of the composites (0.33 µm s−1) is over 24 times better than that of the conventional Cf/C‐ZrC at 2500 °C (8.0 µm s−1). Moreover, the long‐term multi‐cycle ablation mechanisms of the composites are investigated with the assistance of DFT calculations. Especially, the size effect and the content of the Zr‐based crystals in the oxide layer fundamentally affect the stability of the oxide layer and the ablation properties. The ideal component and structure of the oxide layer for multi‐cycle ablation condition are put forward, which can be obtained by controlling the Y2O3/ZrB2 mole ratio and establishing Y‐Si‐O – t‐Zr0.9Y0.1O1.95 core‐shell nano structure. This work proposes a new strategy for improving the long‐term multi‐cycle ablation resistance of UHTCMCs. [ABSTRACT FROM AUTHOR]

Despite its attractive properties for applications, including in high-temperature environments, the wide use of TiB-TiB 2 ceramics is hindered by its high densification temperature of >1600 °C. Herein, we clarify the effects of Ti-based sintering aids fabricated by mechanical milling on the density and strength of TiB-TiB 2 by spark plasma sintering at 1300 °C, corresponding to low-temperature sintering (LTS). The particle and crystallite sizes of the aids are significant parameters for LTS and can be adjusted by mechanical milling, influencing the initial densification behavior. TiB-TiB 2 fabricated with 6 wt % of milled Ti aid with particle and crystallite sizes of ∼15 μm and ∼4 nm exhibited 98.9% of the theoretical density and flexural strength of 566.8 ± 170.0 MPa. These values are comparable to those typically fabricated at temperatures >1650 °C. Notably, the TiB generated during sintering enhances the bond strength of the TiB 2 grains, resulting in the fracture surface changing from intergranular to transgranular patterns. Hence, the mechanically milled aid improved the mechanical performance of the TiB-TiB 2 ceramics through the combined effects of flaw elimination by densification and bond strengthening by TiB formation. [Display omitted] • Mechanically milled-Ti aid significantly improved the sinterability of TiB 2. • The refined particle & crystallite of the aid enhanced its dispersion to the matrix. • Spark plasma sintering at 1300 °C achieved full-dense TiB-TiB 2 composites. • TiB was formed between TiB 2 and composites' strengths increased with TiB content. [ABSTRACT FROM AUTHOR]

Ultra-high-temperature ceramics (UHTCs) are materials capable of withstanding temperatures above 2000 °C while maintaining exceptional properties, making them ideal for aerospace, automotive, and energy applications. However, they face challenges such as brittleness and limited oxidation resistance. This review highlights the potential benefits of modifying UHTCs with graphene, a material known for its excellent mechanical, thermal, and electrical properties. Incorporating graphene into UHTCs can enhance their mechanical properties, improve the oxidation resistance, increase the thermal conductivity, and tailor the electrical properties, while also improving the processability. By addressing the inherent limitations of UHTCs and enhancing their properties, graphene-modified UHTCs show promise for a wide range of high-temperature applications. The review covers recent research achievements in graphene/UHTCs composites, focusing on the synthesis methods, microstructures, macroscopic mechanical properties, oxidation resistance, thermal shock resistance, and the underlying mechanisms.

The atomic structural features and the mechanical properties of amorphous silicoboron carbonitride ceramics with 13 different compositions in the Si–BN–C phase diagram are investigated employing ab-initio calculations. Both chemical bonds and local structures within the amorphous network relate to the elemental composition. The distribution of nine types of chemical bonds is composition-dependent, where the B–C, Si–N, Si–C, and B–N bonds hold a large proportion for all compositions. Si prefers to be tetrahedrally coordinated, while B and N prefer sp2-like trigonal coordination. In the case of C, the tetrahedral coordination is predominant at relatively low C contents, while the trigonal coordination is found to be the main feature with the increasing C content. Such local structural characteristics greatly influence the mechanical properties of SiBCN ceramics. Among the studied amorphous ceramics, SiB2C3N2 and SiB3C2N3 with low Si contents and moderate C and/or BN contents have high elastic moduli, high tensile/shear strengths, and good debonding capability. The increment of Si, C, and BN contents on this basis results in the decrease of mechanical properties. The increasing Si content leads to the increment of Si-contained bonds that reduce the bond strength of SiBCN ceramics, while the latter two cases are attributed to the raise of sp2-like trigonal configuration of C and BN. These discoveries are expected to guide the composition-tailored optimization of SiBCN ceramics.

Nanostructured single-phase hafnium-zirconium carbonitride powders were synthesized using a simple and fast mechanochemical synthesis approach. The critical milling duration, after which a (Hf,Zr)(C,N) solid solution formation inside a jar occurred via mechanically induced self-sustained reaction (MSR), was 10 min. After 30 min of treatment, a solid-gas reaction was completed, and as a result, a homogeneous (Hf,Zr)(C,N) powder consisting of 10–500 nm submicron particles was obtained. The phase and structure evolution of the powders after different treatment durations allowed for the establishment of possible reaction mechanisms, which included the formation of Hf/Zr/C-layered composite particles, their interaction via MSR, and further grinding and nitridization. Spark plasma sintering (SPS) was used to produce bulk hafnium-zirconium carbonitride ceramics from nanostructured powder. The sample had higher values of relative density, hardness, and fracture toughness than those for binary compounds of a similar composition.

New inorganic nanostructured matrices for fiber-reinforced composites with enhanced high-temperature stability were developed from alkali aluminosilicate polymers doped with different ultra-high-temperature ceramic (UHTC) particles. The alkali aluminosilicate matrices were synthesized at room temperature with a high SiO2:Al2O3 ratio and then further functionalized by doping with 4–5 wt % of micrometric SiC, ZrB2, ZrC, and HfC powders and finally thermally stabilized as glass–ceramics at 750 °C. The different UHTC-doped matrices were characterized according to their dimensional and microstructural changes after thermal cycling in air flux at 1000 °C. The first results showed that carbide-based UHTC powders improved the thermal stability of the matrices, preventing the excessive swelling of the material and the formation of detrimental voids that might result in the lack of adhesion with reinforcing fibers. Contrarily, the addition of ZrB2 resulted in an excessive matrix swelling at high temperature, thus proving no efficacy compared to the undoped matrix. Impregnation tests carried out on C-fiber fabrics showed good processability, adhesion to the fibers, and fracture pull-out, especially for carbide-based matrices. [ABSTRACT FROM AUTHOR]

The development of high-temperature organic adhesive for bonding ultra-high-temperature ceramics with excellent thermal shock resistance has important significance to thermal protection systems for high-temperature environment application. In this study, high-temperature organic adhesive (HTOA) with carbon-fiber-SiC nanowires (CF-SiCNWs) binary phase enhancement structure was prepared. The method is that the SiCNWs grow on the chopped carbon-fiber surface and in the matrix of modified HTOA during high-temperature heat treatment with the help of a catalyst by a tip-growth way and with a vapor–liquid–solid (V-L-S) growth pattern. The results showed that the CF-SiCNWs binary phase enhancement structure plays a significant role in improving thermal shock resistance of high-temperature organic adhesive. The retention rate of the joint bond strength for the bonding samples after 20 cycles of thermal shock testing reaches 39.19%, which is higher than for the ones without CF, whose retain rate is only 6.78%. The shear strength of the samples with the CF-SiCNWs binary phase enhancement structure was about 10% higher than for those without the enhancement structure after 20 cycles of thermal shock. [ABSTRACT FROM AUTHOR]

The features of oxidation of ultra-high-temperature ceramic material HfB2-30 vol.%SiC modified with 1 vol.% graphene as a result of supersonic flow of dissociated CO2 (generated with the use of high-frequency induction plasmatron), as well as under the influence of combined heating by high-speed CO2 jets and ytterbium laser radiation, were studied for the first time. It was found that the addition of laser radiation leads to local heating of the central region from ~1750 to ~2000–2200 °C; the observed temperature difference between the central region and the periphery of ~300–550 °C did not lead to cracking and destruction of the sample. Oxidized surfaces and cross sections of HfB2-SiC-CG ceramics with and without laser heating were investigated using X-ray phase analysis, Raman spectroscopy and scanning electron microscopy with local elemental analysis. During oxidation by supersonic flow of dissociated CO2, a multilayer near-surface region similar to that formed under the influence of high-speed dissociated air flows was formed. An increase in surface temperature with the addition of laser heating from 1750–1790 to 2000–2200 °C (short term, within 2 min) led to a two to threefold increase in the thickness of the degraded near-surface area of ceramics from 165 to 380 microns. The experimental results indicate promising applications of ceramic materials based on HfB2-SiC as part of high-speed flying vehicles in planetary atmospheres predominantly composed of CO2 (e.g., Venus and Mars). [ABSTRACT FROM AUTHOR]

To improve the ablation resistance of carbon fibre (Cf), alleviate the interdiffusion between Cf and the matrix, and improve the complex preparation process of double-layer carbide coating, a high-quality SiC-HfC double interpenetrating coating was prepared on the surface of Cf using a molten salt synthesis (MSS) method. The microstructure, formation mechanism, and ablation behaviour of the coating were studied. The results show that the coating is composed of interpenetrating HfC and SiC phases. The interpenetrating structure was formed due to the transport of Hf ions into the interior of porous SiC nanowires facilitated by the molten salt medium, followed by diffusion through carbon. The SiC-HfC double-layer interpenetrating coating can serve as an oxygen barrier to protect Cf from oxidative damage under extreme temperature (2000 °C) and aerobic environment (oxyacetylene flame). • SiC-HfC interpenetrating coating was prepared on the surface of Cf. • Low temperature molten salt synthesis (MSS) method were used. • SiC-HfC interpenetrating coating can protect the Cf from failure for 60 s under an oxyacetylene flame at 2000 ℃. [ABSTRACT FROM AUTHOR]

The mechanical properties of an ultrahigh temperature ceramic (UHTC) made up of ZrB 2 , SiC and graphite flake (ZrB 2 -SiC-G) are explored by ultrahigh-temperature uniaxial tension, combined with in-situ monitoring of the fracture behavior. The results suggest an inconsistent property degradation of ZrB 2 -SiC-G at 1200–2000 °C. Significant degradation in tensile modulus is observed at 1400 °C, which differs from the strength degradation that mainly occurs at 1800 °C. Moreover, ZrB 2 -SiC-G is elastically fractured at 1200 °C, 1400 °C and 1600 °C, which is interrupted at 1800 °C and 2000 °C. At the temperature range, severe plasticity takes place, which reduces the bearing capacity of this material. The brittle-to-ductile transition is well ascertained by in-situ observation of the fracture behavior at 1200–2000 °C. Finally, the constitutive relation of modulus and strength is built by coupling the effect of temperature. It can be integrated in finite element models and is useful for structural design of UHTC based thermal protection systems of hypersonic vehicles. [ABSTRACT FROM AUTHOR]

A liquid‐phase polymer‐to‐ceramic approach is reported for the synthesis of hafnium carbide (HfC)/hafnium oxide (HfO2) composite particles from a commercial precursor. Typically, HfC ceramics have been obtained by sintering of fine powders, which usually results in large particle size and high porosity during densification. In this study a single‐source liquid precursor was first cured at low temperature and then pyrolyzed at varying conditions to achieve HfC ceramics. The chemical structure of the liquid and cured precursors, and the resulting HfC ceramics was studied using various analytical techniques. The nuclear magnetic resonance and Fourier transform infrared spectroscopy indicated the presence of partially hydrated hafnium oxychloride (Hf–O–Cl·nH2O) in the precursor. Scanning electron microscopy of the resulting HfC crystals showed a size distribution in the range of approx. 600–700 nm. The X‐ray diffraction of the pyrolyzed samples confirmed the formation of crystalline HfC along with monoclinic‐HfO2 and free carbon phase. The formation of HfO2 in the ceramics was significantly reduced by controlling the low‐temperature curing temperature. Pyrolysis at various temperatures showed that HfC formation occurred even at 1000°C. These results show that the reported precursor could be promising for the direct synthesis of ultrahigh temperature HfC ceramics and for precursor infiltration pyrolysis of reinforced ceramic matrix composites. [ABSTRACT FROM AUTHOR]

Dense HfCxN1−x carbonitride ceramics are very promising as potential ultra-high temperature ceramics (UHTCs) for application under extremely harsh environments. However, the thermophysical and mechanical properties of the HfCxN1−x carbonitrides have not been investigated clearly. The present work prepared HfCxN1−x (x = 0.3, 0.4, 0.5, 0.6, 0.7) ceramics at 1950 °C under 30 MPa in flowing Ar atmosphere by using hot pressing sintering method. The relative densities of the samples obtained reached above 96%. Thermal conductivity of the as-prepared HfCxN1−x carbonitrides ranged from 19 to 24 W m−1 K−1 at room temperature. The increased role of electrons in thermal conduction caused by both increasing nitrogen content and increasing temperature, resulted in improved thermal conductivity, varying from 32 to 39 W m−1 K−1. With increasing nitrogen content, the electrical conductivity also increased, ranging from 149 to 213 × 104 Ω−1 m−1. With the increase of nitrogen content, Hf-C covalent bonds are gradually replaced by Hf-N covalent bonds with lower bond strength, resulting in HfC0.7N0.3 exhibiting the highest room-temperature flexural strength and hardness, HfC0.3N0.7 exhibiting the highest fracture toughness. Their mechanical properties are greatly improved over the binary HfC and HfN. The high-temperature flexural strength of the HfC0.7N0.3 decreased from 324 MPa at 1000 °C, to 139 MPa at 1600 °C and 100 MPa at 2000 °C. Meanwhile, it was revealed that the high-temperature flexural strength decreased with increasing nitrogen content for the as-prepared HfCxN1−x carbonitrides, similar to the changing trend of room-temperature flexural strength. The HfC0.3N0.7 possessed high-temperature plasticity at 2000 °C, attributed to the ability of the coarser grain to produce numerous layer dislocations.

Temperature above 2000°C and additional pressure is generally required to achieve the full densification of TaxHf1−xC‐based ceramics. This work proposed a novel method to fabricate dense Ta0.2Hf0.8C ceramics at relatively low temperature. Using a small amount of Si as a sintering aid, Ta0.2Hf0.8C was densified at 1700°C by reactive hot‐pressing (RHP), with SiC formed in situ. Microstructure evolution mechanisms of the ceramics during RHP were investigated. The effect of silicon content on the densification and mechanical properties of the ceramics was revealed. It is indicated that the apparent porosity of the Ta0.2Hf0.8C–SiC ceramics was as low as 0.5%, whereas bending strength and fracture toughness of the ceramics were as high as ∼637 MPa and 6.7 MPa m1/2, respectively, when the silicon content was 8 wt.%. This work provides a new idea for the low‐temperature densification of TaxHf1−xC and other ultrahigh temperature ceramics with high performance. [ABSTRACT FROM AUTHOR]

HfCxN1−x nanoparticles were synthesized using the urea-glass route, employing hafnium chloride, urea, and methanol as raw materials. The synthesis process, polymer-to-ceramic conversion, microstructure, and phase evolution of HfCxN1−x/C nanoparticles were thoroughly investigated across a wide range of molar ratios between the nitrogen source and the hafnium source. Upon annealing at 1600 °C, all precursors demonstrated remarkable translatability to HfCxN1−x ceramics. Under high nitrogen source ratios, the precursor exhibited complete transformation into HfCxN1−x nanoparticles at 1200 °C, with no observed presence of oxidation phases. In comparison to HfO2, the carbothermal reaction of HfN with C significantly reduced the preparation temperature required for HfC. By increasing the urea content in the precursor, the carbon content of the pyrolyzed products increased, leading to a substantial decrease in the electrical conductivity of HfCxN1−x/C nanoparticle powders. Notably, as the urea content in the precursor increased, a significant decrease in average electrical conductivity values was observed for the R4-1600, R8-1600, R12-1600, and R16-1600 nanoparticles measured at a pressure of 18 MPa, yielding values of 225.5, 59.1, 44.8, and 46.0 S·cm−1, respectively. [ABSTRACT FROM AUTHOR]

Ablation resistance of a multi-component carbide (Hf 0.2 Ti 0.2 Zr 0.2 Ta 0.2 Nb 0.2)C (HTZTNC) was investigated using an oxyacetylene flame apparatus. When the surface temperature of the HTZTNC was below 1800 °C, (Nb, Ta) 2 O 5 , (Hf, Zr)TiO 4 , and (Hf, Zr)O 2 were found to be the main oxidation products, while at higher temperature, formation of (Hf, Zr, Ti, Ta, Nb)O x was favored and its content gradually increased with the increase in ablation temperature. Based on the ablation results and thermodynamic simulation analysis, a possible ablation mechanism of HTZTNC was proposed. Active oxidation of TiC and outward diffusion of TiO were demonstrated to occur during the ablation process, which constitute the critical steps for the ablation of HTZTNC. These results can contribute to the design of ablation resistant ultra-high-temperature ceramics. [ABSTRACT FROM AUTHOR]

Nanostructured single-phase hafnium-zirconium carbonitride powders were synthesized using a simple and fast mechanochemical synthesis approach. The critical milling duration, after which a (Hf,Zr)(C,N) solid solution formation inside a jar occurred via mechanically induced self-sustained reaction (MSR), was 10 min. After 30 min of treatment, a solid-gas reaction was completed, and as a result, a homogeneous (Hf,Zr)(C,N) powder consisting of 10–500 nm submicron particles was obtained. The phase and structure evolution of the powders after different treatment durations allowed for the establishment of possible reaction mechanisms, which included the formation of Hf/Zr/C-layered composite particles, their interaction via MSR, and further grinding and nitridization. Spark plasma sintering (SPS) was used to produce bulk hafnium-zirconium carbonitride ceramics from nanostructured powder. The sample had higher values of relative density, hardness, and fracture toughness than those for binary compounds of a similar composition. [ABSTRACT FROM AUTHOR]