Search

Core‒rim structures were identified as a common feature in hot-pressed ZrB2 ‒SiC‒MC ceramics (M = Nb, Hf, Ta and W) by a combination of X-ray diffraction, scanning and transmission electron microscopies. Quantitative analyses associate them with the bi-solubility of M in ZrB2 phase, in which transition of solubility across the core/rim boundary is abrupted, signifying their creation via dissolution‒reprecipitation process facilitated by transient liquid-phase. The cores were retained from starting powder after surface melting and the rims were grown from the liquid-phase to incorporate more solutes, leaving the residual liquid to turn into ZrC phase with higher solubility of M. We propose g-point scheme in the ZrB2‒MB2 diagrams to combine the bi-solubility and the core‒rim structures into an intra-phase relationship created by sintering, leading further to a hierarchical phase relationship. The temperature dependence of flexural strength in the ZrB2 ‒SiC‒MC ceramics varies with MC additions, which can be respectively strengthened by the strain energy created in the core‒rim structures and metal segregation to grain boundaries.

The object of research is the effect of sintering under pressure (10MPa–4.1GPa) on the formation of the structure and properties of ZrB2, HfB2, and composites on their bases. It has been found that high pressure consolidation results in an improvement of mechanical characteristics. In particular, the hardness and fracture toughness of the materials sintered under 4.1GPa pressure are higher than those of the materials obtained under hot pressing conditions at 20–30MPa and spark-plasma sintering at 50MPa. High-pressure sintered HfB2demonstrated hardness HV(9.8N)=21.3±0.8GPa, HV(49N)=19.3±1.3GPa, and HV(98N)=19.2±0.5GPa and fracture toughness K1C(49N)=7.2MPa·m0.5and K1C(98N)=5.7MPa·m0.5. The HfB2sintered by hot pressing at 1850°C and 30MPa demonstrated hardness: HV(9.8N)=19.0GPa, HV(49N)=18.7GPa, and HV(98N)=18.1GPa, K1C(9.8N)=7.7MPa·m0.5, K1C(49N)=6.6MPa·m0.5and K1C(98N)=5.3MPa·m0.5. High pressure sintered ZrB2(a=0.3167nm, c=0.3528nm, γ=6.2g/cm3) demonstrated HV(9.8N)=17.7±0.6GPa, HV(49N)=15.4±1.2GPa, and HV(98N)=15.3±0.36GPa and K1C(9.8N)=4.3MPa·m0.5, K1C(49N)=4.2MPa·m0.5and K1C(98N)=4.0MPa·m0.5. Addition of 20wt.% of SiC to ZrB2and sintering under high pressure (4.1GPa) allowed essential increase of hardness to HV(9.8N)=24.2±0.7GPa, HV(49N)=16.7±0.5GPa, and HV(98N)=17.6±0.4GPa and fracture toughness to K1C(49N)=7.1MPa·m0.5, K1C(98N)=6.2MPa·m0.5; the material density was γ=5.03g/cm3. Additions of SiC and Si3N4to ZrB2lead to some increase in fracture toughness (up to K1C(98N)=9.2MPa·m0.5). The developed ZrB2- and HfB2-based materials and composites can be used for aerospace applications, in cutting and refractory industries, etc.

The in-situ exothermic reactions between ZrC0.8, B4C and Si have assisted densification and allowed to obtain fully dense ZrB2-31 wt.%SiC ultra-high temperature ceramics within 6 min at 1750 °C. The use of zirconium carbide instead of metallic zirconium in the green body obviated the possibility of in-situ SHS process and allowed to apply the pressure at low temperatures. The latter provided a first densification stage just above 1050 °C. A slight carbon excess was created in the green body to preserve the carbon nanotubes. The developed reactive hot pressing route (1830 °C, 3 min, 30 MPa) has been successfully used to obtain ZrB2-SiC ceramics containing 8 vol.% of multi-wall carbon nanotubes (MW-CNT). The carbon nanotubes survived the thermal cycle and could be clearly observed in the sintered ceramics. The CNT addition improved the fracture toughness of the composite from 4.3 MPa m1/2 for ZrB2-31 wt.%SiC to 6.8 MPa m1/2 for ZrB2-29 wt.%SiC-CNT.

The emergence of a new class of high entropy ultra-high temperature ceramics (HE-UHTCs) is expected to possess exceptionally superior mechanical, oxidation, and erosion resistance as compared to conventional UHTCs. The improvement in the properties of HE-UHTCs is attributed to the high configurational entropy of the multi-component system stabilized into a single-phase, but with little discussion on entropy contributions. This perspective article presents a viewpoint on the critical scientific question regarding the actual role entropy plays in stabilizing and enhancing the thermo-mechanical properties of a multi-component system. The high throughput first-principle calculations, which can provide the entropy formation ability (EFA) of thermodynamically stable HE-UHTCs, can prove to be valuable toolset to this puzzle. HE-UHTCs show enhancement in the mechanical properties much higher than the predicted rule of the mixture, which cannot traditionally be explained by a mere solid solution formation effect. Isothermal oxidation studies of HE-UHTCs limited to furnace testing state remarkable oxidation resistance which is attributed to the passivity of complex, layered structure, and composition of the oxide scale. However, whether all the constituents in the HE-UHTC system react simultaneously or preferential oxidation takes place needs to be addressed. To address all these issues, an integrated computational and experimental approach has been postulated towards designing a thermal protection system (TPS) for re-entry applications. Succeeding the approach will provide new opportunities in the compositional space of HE-UHTCs with tailored properties.

In reactive-hot-pressed ZrB2–SiC–ZrC ceramics, ZrO2 was found to replace ZrC phase, hence leading to confusion in designing ultra-high-temperature ceramics (UHTCs). We employ high-precision X-ray diffraction and electron microscopies to reassess the phase behavior during entire reaction and densification and to reveal the evolution of multiphase relationship at different stages before reaching the final ZrB2–SiC–ZrO2 composition. Frozen from transient liquid-phase, bulk glassy phase of 15 vol% was found to be constituted of Zr–Si–B–C–O with stable Zr:O ratio, which starts as early as in the intermediate stage to suppress ZrC in favor of SiC nucleation. Inhomogeneity in phase relations and microstructures results from variation in local transient liquid-phase to develop SiC phase in various modes and rates. As inferred from the earlier report of phase formation, competing reactions for ZrC and ZrB2 phases in the initial stage below 1000 °C were mediated via Zr–O–B–C liquid phase. Such liquid phase was moderated by stable B–O components, as initiated from surface oxides of starting powders. This picture under a continuous mother liquid phase can unify the reactions and sintering into a collective melting–nucleation–growth process, which enables and guides the evolution of multiphase relationship through several stages to reach final densification at relatively low temperature with the help of residual oxides.

Dense (Hf0.2,Zr0.2,Ti0.2,Ta0.2,Nb0.2)B2 high-entropy ceramics with high phase purity were produced by two-step spark plasma sintering of precursor powders synthesized by boro/carbothermal reduction of oxides. The reacted powders had low oxygen (0.404 wt%) and carbon (0.034 wt%) contents and a sub-micron average particle size (∼0.3 μm). Powders were synthesized by optimizing the excess B4C content of the reaction mixture and densified by a two-step spark plasma sintering process. The relative density increased from 98.9% to 99.9% as the final sintering temperature increased from 2000 °C to 2200 °C. The resulting ceramics were nominally single-phase (Hf,Zr,Ti,Ta,Nb)B2 with oxygen contents as low as 0.004 wt% and carbon as low as 0.018 wt%. The average grain size increased from 2.3 ± 1.2 μm after densification at 2000 °C to 4.7 ± 1.8 μm after densification at 2100 °C, while significant grain growth occurred during sintering at 2200 °C. The high relative densities, low oxygen and carbon contents, and fine grain sizes achieved in the present study were attributed to the use of synthesized precursor powders with high purity and fine particle size, and the two-step synthesis-densification process. These are the first reported results for dense high-entropy boride ceramics with high purity and fine grain size.

High entropy materials, which include high entropy alloys, carbides, and borides, are a topic of substantial research interest due to the possibility of a large number of new material compositions that could fill gaps in application needs. There is a current need for materials exhibiting high temperature stability, particularly oxidation resistance. A systematic understanding of the oxidation behavior in high entropy materials is therefore required. Prior work notes large differences in the thermodynamic favorability between oxides formed upon oxidation of high entropy materials. This work uses both analytical and computational thermodynamic approaches to investigate and quantify the effects of this large variation and the resulting potential for preferential component oxidation in refractory high entropy materials including group IV-, V- and VI-element based alloys and ceramics. Thermodynamic calculations show that a large tendency towards preferential oxidation is expected in these materials, even for elements whose oxides exhibit a small difference in thermodynamic favorability. The effect is reduced in carbides, compared to their alloy counterparts. Further, preferential oxidation in high entropy refractory materials could result in possible destabilization of the solid solution or formation of other, competing phases, with corresponding changes in bulk material properties., Acta Materialia. (2020). Also see Part II of this work: https://doi.org/10.1016/j.actamat.2020.07.004

Refractory high entropy materials have garnered significant research interest due to their potential ability to fill a need in high temperature structural applications. However, challenges remain with respect to designing for oxidation resistance. A knowledge gap exists with respect to a rigorous understanding of the mechanisms driving oxidation processes unique to high entropy materials. This work provides an experimental complement to a companion publication, which outlines analytical and computational thermodynamic approaches that are envisioned to aid the design of refractory high entropy materials containing group IV (Hf, Zr, Ti) and group V (Ta, Nb) constituents. In this work, (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2) carbide and diboride specimens were exposed at 1700°C in 1% O2 for 5 min. Experimental results show good agreement with the computational predictions for the same temperature, despite differences in the overall morphology of the oxidized regions. The carbide formed porous oxides, while the diboride formed a denser external scale. Oxidation products are dominated by group IV oxides, depleting the underlying materials, which were found to consist of primarily group V carbides and borides respectively. The results provide a first look at the oxidation of high entropy UHTCs at ultra-high temperatures and validate the preferential nature of high entropy material oxidation predicted by the computational approach developed for the study of this new class of materials.

ZrB2‐ZrCx composites were produced using Zr:B4C powder mixtures in the molar ratios of 3:1, 3.5:1, 4:1, and 5:1 by reactive hot pressing (RHP) at 4‐7 MPa, 1200°C for 60 minutes. X‐ray diffraction analyses confirmed the formation of nonstoichiometric zirconium carbide (ZrCx) with different lattice parameters and enhanced carbide formation by increasing the Zr mole fraction. An increase in applied pressure from 4 to 7 MPa was responsible for the improved relative density (RD) of 4Zr:B4C composition from 86% to 99%. Microstructural studies on Zr‐rich composites showed a reduction in unreacted B4C particles and enriched elongated ZrB2 platelets. Reaction and densification mechanism in 4Zr:B4C composition were studied as a function of temperature increased from 600 to 1200°C at an applied constant pressure of 7 MPa. After 1000°C

Spark plasma sintering is a novel sintering technique in the manufacturing of ultra high temperature ceramics. Temperature distribution during the sintering process controls microstructure and consequent thermomechanical properties of manufactured samples. Therefore, the temperature distribution in the spark plasma sintering of TiB2 is investigated numerically in this work. A pulsed direct electrical current was applied during the sintering process, and current density distribution as well as generated heat, as a result of the Joule heating effect, were obtained at each point. The thermoelectrical governing equations were solved by the finite element method. The obtained temperature contours showed that the heat generation rate is higher at the sample camped to the die, so the heat flow is from the sample center toward the die and finally, the surroundings. The obtained results showed a uniform temperature distribution in the sample, so that a maximum temperate difference concerning the sample center was 75 °C at the sample/die interface at the sintering temperature of 2200 °C. This uniformity is one of the advantages of spark plasma sintering ,which results in a uniform microstructure in the as-sintered sample.

The intrinsic brittleness and poor damage tolerance of ultrahigh-temperature ceramics are the key obstacles to their engineering applications as nonablative thermal protection materials. Biomimetic layered or "brick-and-mortar" hybrid composites composed of alternative strong/weak interfaces exhibit excellent strength and high toughness; however, the commonly used interfacial materials are weak and have poor thermal stability and ablation resistance, which strictly limit their use in high-temperature and oxidative environments. In this work, ZrB2-based "brick-and-mortar" hybrid ceramics were constructed with a hierarchical biomimetic design to improve the fracture resistance and damage tolerance. ZrB2-20vol %SiC ceramics containing 30 vol % reduced graphene oxide nanosheets were used as the weak interface to increase crack growth resistance without destroying the excellent ablation resistance. Finally, the ZrB2-based "brick-and-mortar" composites achieve the synergy of superior damage tolerance and ablation resistance.

Grain coarsening normally occurs at the final stage of sintering, resulting in trapped pores within grains, which deteriorates the density and the performance of ceramics, especially for ultra-high temperature ceramics (UHTCs). Here, we propose to sinter this class of ceramics in a specific temperature range and coupled with a relatively high pressure. The retarded grain boundary migration and pressure-enhanced diffusion ensure the proceeding of densification even at final stage. A highly dense TaC ceramic (98.6 %) with the average grain size of 1.48 μm was prepared under 250 MPa via high pressure spark plasma sintering using a Cf/C die at 1850 °C. It was suggested that the final-stage densification is mainly attributed to grain boundary plastic deformation-involved mechanisms. Compared to the usual sintering route using a high temperature (>2000 °C) and normal pressure (

Ablation behavior of Ta0.8Hf0.2C and Ta0.8Hf0.2C-10 vol%SiC ceramics was investigated by plasma flame ablation test in air. Linear ablation rate was used for the characterization of ablation resistance and grazing-incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) were used for characterization. Ta0.8Hf0.2C-10 vol%SiC exhibited significantly higher ablation resistance, with ablation rate of 0.7 μm/sec in comparison to the monolithic system with 3.5 μm/sec. An unstable single oxide layer of monolithic Ta0.8Hf0.2C with thickness approximately 20 μm was observed after ablation. Oxidation reactions happened individually as TaC and HfC while ‘self-sacrificial’ volatilization was mainly controlled by active oxidation of TaC. Hf6Ta2O17 was generated by the combination of HfO2 and TaO. A more stable double reaction-layer was observed after ablation of Ta0.8Hf0.2C-10 vol%SiC ceramic. Passive oxidation of TaC was dominant. Textured Ta2O5 skeleton, filled with Hf-containing TaO6, stopped consumption of SiC and hampered oxygen further diffusion.

International audience; Ultra-High Temperature Ceramics (UHTC) are good candidates to fulfil the harsh requirements of hypersonic applications. For more than a decade, ONERA has been actively involved in several programs to develop such materials for different applications (hypersonic flights, propulsion systems ...). In our laboratories, monolithic and composite materials have been investigated as well as several processing methods. In this paper, we present for example the ZrB2-SiC and HfB2-SiC compositions with TaSi2 or Y2O3 additions which have been especially studied in the European Projects ATLLAS and ATLLAS 2. Assessments of several prototypes in realistic environment are also described. Furthermore, based on these material developments, a specific study on the oxidation behaviour of such monoliths from 1200°C to 2400°C with a dedicated test bench using a 2 kW CO2 laser has been carried out (oxidation under air and water vapour atmospheres). Recently, some work on the manufacturing of Ultra-High Temperature Ceramic Matrix Composites (UHTCMC) has been initiated using slurry infiltration and pyrolysis. The behaviour and properties of these materials are encouraging.; Les céramiques ultraréfractaires (UHTC) sont de bons candidats pour répondre aux besoins sévères des applications liées à l'hypersonique. Depuis plus de dix ans, l'ONERA est impliquée activement dans plusieurs programme de recherche pour développer ce type de matériaux pour différentes applications (vols hypersonique, systèmes de propulsion ...). Au sein de nos laboratoires, des matériaux monolithiques et composites ont été étudiés ainsi que plusieurs méthodes d'élaboration. Dans cet article, nous présentons par exemple les compositions de type ZrB2-SiC et HfB2-SiC contenant des ajouts de TaSi2 ou Y2O3 qui ont été spécifiquement étudiées dans les projets européens ATLLAS et ATLLAS 2. L'évaluation de plusieurs prototypes dans des conditions réalistes est également présentée. Par ailleurs, en se basant sur ces développements matériaux, une étude spécifique sur le comportement à l'oxydation de tels monolithes entre 1200°C et 2300°C a été effectuée avec un moyen d'essai utilisant un laser CO2 de 2 kW (oxydation sous air et sous vapeur d'eau). Récemment, des travaux sur l'élaboration de composites à matrice ultraréfractaire (UHTCMC) ont été engagés en employant l'infiltration de suspension et leur pyrolyse. Le comportement et les propriétés de ces matériaux sont encourageants.

Highly disperse and reactive (HfB2–SiC)@(Ta2O5–HfO2–C) composite powders were manufactured by sol–gel technology where Ta2O5–HfO2–C amorphous components were nanostructured and distributed in each other as uniformly as possible. The reactive sintering of the prepared composite powders at a relatively low temperature (1800°С) with an exposure time of 30 min and the pressure 30 MPa yielded (HfB2–30 vol % SiC)–xTa4HfC5 ultra-high temperature ceramics (UHTCs), where x = 5, 10, and 15 vol %, with a relative density of 75–78%. X-ray powder diffraction proved the complete conversion of tantalum and hafnium oxides to complex carbide Ta4HfC5. The average grain size as determined by scanning electron microscopy did not exceed 2–3 µm in HfB2 and 30–60 nm for the Ta4HfC5 phase. Thermal analysis in flowing air showed that, in the temperature range 20–1400°С, the increasing percentage of tantalum–hafnium carbide (the least oxidation resistant phase) leads to a systematic increase in oxidation-induced weight gain; however, a tendency to saturation is observed. The microstructural specifics of the oxidized surface are noticed depending on the composition of HfB2–SiC–Ta4HfC5 ceramics.

This work was supported by the Korean Government (Defense Acquisition Program Administration, DAPA) through research institute (Agency for Defense Development, ADD).

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.

Pomocí výpočtů ab initio zkoumáme tvrdý a elektricky vodivý multikomponentní diborid Ti0,25Zr0,25Hf0,25Ta0,25B2 o vysoké tepelné stabilitě. Soustředíme se na vliv poruch (buď vakancí, nebo atomů C, tedy poruch relevantních pro četné experimenty včetně našeho vlastního) na charakteristiky materiálu. Byly prozkoumány různé druhy, koncentrace i rozložení poruch a byla rozpoznána uspořádání vedoucí na nejnižší formovací energie. Prokazujeme, že náhrada atomů B atomy C je méně výhodná než tvorba bórových vakancí. Ukazujeme, že vakance upřednostňují shlukování do rozsáhlejší plošné oblasti bez atomů, minimalizujíce počet přerušených vazeb B–B a objem na atom, zatímco uhlíkové substituce na bórových pozicích shlukování neupřednostňují a mají sklon minimalizovat počet vazeb C–C. Odhalujeme vliv vakancí na mechanické a elektronické vlastnosti a získané výsledky používáme k vysvětlení experimentálních údajů. We study the hard and electrically conductive multicomponent diboride Ti0.25Zr0.25Hf0.25Ta0.25B2 with high thermal stability by ab-initio calculations. We focus on the effect of defects (either vacancies or C atoms, both relevant for numerous experiments including our own) on material characteristics. Different types, concentrations and distributions of defects were investigated, and the configurations leading to the lowest formation energies were identified. We show that the replacement of B by C is more unfavorable than the formation of B vacancies. We show that vacancies prefer to coalesce into a larger planar void, minimizing the number of broken B–B bonds and the volume per atom, while carbon substitutions at boron sites do not prefer coalescence and tend to minimize the number of C–C bonds. We show the effect of vacancies on mechanical and electronic properties, and use the results to explain experimental data.

In this study, the oxidation stability up to 1000 °C in air of the Hf(C,N)-MoSi2 composites was explored under non-isothermal and isothermal conditions. Composites with 1, 5, 10, and 20% volume fractions were produced by low-energy ball milling and subsequent spark plasma sintering. Differential scanning calorimetry (DSC) and thermogravimetric (TG) coupled with mass spectrometry were used to reveal the staging of the oxidation process depending on the additive content. It was found that samples containing 1 and 5 vol% MoSi2 had the lowest weight gain and the best oxidation behavior. The results of this study were supported by microstructural and phase analyses of the samples after isothermal treatment in a furnace. The samples with the lowest molybdenum disilicide content had a dense and thin protective oxide film on the surface, consisting of hafnium orthosilicate and monoclinic HfO2. The increase in the amount of MoSi2 contributed to the formation of a loose and porous oxide layer due to the increase in the concentration of volatile MoO3. However, all samples exhibited higher oxidation resistance compared to the pure Hf(C,N).

ZrB2 is of particular interest among ultra-high temperature ceramics because it exhibits excellent thermal resistance at high temperature, as well as chemical stability, high hardness, low cost, and good electrical and thermal conductivity, which meet the requirements of high-temperature components of hyper-sonic aircraft in extreme environments. As raw materials and basic units of ultra-high temperature ceramics and their composites, ZrB2 powders provide an important way for researchers to improve material properties and explore new properties by way of synthesis design and innovation. In recent years, the development of ZrB2 powders’ synthesis method has broken through the classification of traditional solid-phase method, liquid-phase method, and gas-phase method, and there is a trend of integration of them. The present review covers the most important methods used in ZrB2 nanopowder synthesis, focusing on the solid-phase synthesis and its improved process, including modified self-propagating high-temperature synthesis, solution-derived precursor method, and plasma-enhanced exothermic reaction. Specific examples and strategies in synthesis of ZrB2 nano powders are introduced, followed by challenges and the perspectives on future directions. The integration of various synthesis methods, the combination of different material components, and the connection between synthesis and its subsequent application process is the trend of development in the future.

Joining is crucial for ultra-high temperature ceramics (UHTCs) to be used in demanding environments due to the difficulty in manufacturing large and complex ceramic components. In this study, ZrB2-SiC composite UHTCs parts were joined via Ni foil as filler, and the mechanical properties and oxidation behaviour of the fabricated ZrB2-SiC/Ni/ZrB2-SiC (ZS/Ni/ZS) joint were investigated. Firstly, dense ZrB2-SiC composites were prepared from nano-sized powders by spark plasma sintering (SPS). The ZrB2-SiC parts were then joined using SPS. Furthermore, the elastic modulus, hardness, shear strength and high temperature oxidation behaviour of the ZS/Ni/ZS joint were examined to evaluate its properties and performance. The experimental results showed that the ZrB2-SiC parts were effectively joined via Ni foil using SPS and the resultant microstructures were free from any marked defects or residual metallic layers in the joint. Although the elastic modulus and hardness in the joining zone were lower than those in the base ZrB2-SiC ceramics, the shear strength of the joint reached ∼161 MPa, demonstrating satisfactory mechanical properties. Oxidation tests revealed that the ZS/Ni/ZS joint possesses good oxidation resistance for a wide range of elevated temperatures (800–1600 oC), paving the way for its employment in extreme environments.