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The ablation behavior of (Hf 0.25 Zr 0.25 Ti 0.25 Ta 0.25)C was investigated using a plasma flame gun over a wide temperature range of 2070–2600℃. The structural transformation during ablation and the effect of temperature on the failure mechanism were discussed. Results showed a sharp increase in mass and linear ablation rates from 0.23 mg·cm−2·s−1 and 4.6 μm·s−1 at 2070℃ to 4.6 mg·cm−2·s−1 and 37 μm·s−1 at 2600℃. This variation was attributed to the temperature's impact on on the microstructure and composition of the oxide layer. During ablation, Hf/Zr-rich oxycarbides preferentially nucleated within the amorphous Hf-Zr-Ti-Ta-C-O layer, leading to phase separation of the oxide layer. With increasing ablation temperature, the filler and binder oxides with low melting point (e.g., (Hf,Zr)TiO 4 , TaZr 2.75 O 8) were rapidly consumed. The skeleton oxides (e.g., (Hf,Zr)O 2) failed to bond and peeled off. The transiently formed (Hf,Zr,Ti,Ta)O x layer was repeatedly destroyed and reconstructed, resulting in the continuous damage and eventual failure of the matrix. • Carbides experience a structural transition from amorphous to crystalline oxycarbides during the initial stage of ablation. • Phase separation occurs in the oxide and the oxycarbide layer. • The failure behavior of high-entropy carbides is primarily determined by the consumption of low-melting-point oxides. [ABSTRACT FROM AUTHOR]

The thermal properties of ultra-high temperature ceramics (UHTCs) in the MB2-WC (M = Ti, Zr, Hf) system and tungsten were studied for potential application as plasma-facing materials in fusion power plants. The sintered UHTC and tungsten samples were subjected to deuterium plasma or protons irradiation. Thermal diffusivity was measured using the laser flash method, and superficial thermal conductivity was analyzed through atomic force microscopy. Results showed that the thermal properties did not degrade when exposed to relevant environments and remained stable over a range of temperatures, unlike the reference tungsten material. Thermal conductivity ranged from 61 to 68 W m−1 K−1 for TiB2-2(WC-6Co), from 53 to 63 W m−1 K−1 for ZrB2-6WC, from 67 to 75 W m−1 K−1 for HfB2-6WC, and from 180 to 119 W m−1 K−1 for tungsten across the temperature range from room temperature to 1200 °C. The increasing trend of thermal effusivity, over 19000 J s−0.5 m−2 K−1 at 1200 °C, justifies further testing and of UHTC materials for fusion applications.

In this study, to overcome the difficulties associated with the preparation of PyC-ceramic interface and controllable deposition of the structure, we initially used a two-step molten salt infiltration method to prepare a C/C-Me-Zr-Hf (Me = Ta, Si) composite with excellent oxyacetylene ablation performance. The microstructure and the behavior of ablative were addressed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as well as by X-ray diffraction（XRD）. The results reveals that C/C-ZrHfC-SiC composite prepared by two-step molten salt infiltration method have a controllable ultra-high temperature ceramic structure and better ablation resistance. [ABSTRACT FROM AUTHOR]

Based on the self-made Zr0.5Hf0.5C precursor and commercial liquid polycarbosilane, C/Zr0.5Hf0.5C-SiC composite was successfully prepared by the precursor impregnation and pyrolysis(PIP) process. The influence of the thickness of pyrolytic C coating on the structure and bending properties of composite materials was studied. The results show that the self-made Zr0.5Hf0.5C precursor can be converted into Zr0.5Hf0.5C solid solution at a relatively low temperature of 1400 ℃. Because of its good permeability, the transformed Zr0.5Hf0.5C matrix exists in both the inter-bundle and intra-bundle regions of the C/Zr0.5Hf0.5C-SiC composite, which presents as a layered structure on SiC matrix. The phase composition of C/Zr0.5Hf0.5C-SiC composite mainly includes C, SiC and Zr0.5Hf0.5C. The densities of three groups of composites with different thicknesses of pyrolytic C coating (0.67, 0.84, 1.36 μm) are 2.07, 1.99, 1.98 g/cm3, respectively. SiC content in the composite decreases with the increase of the thickness of pyrolytic C coating. The three groups of composites with different thicknesses show pseudoplastic fracture mode during bending loading tests, bending strength, bending modulus and fracture toughness are above 410 MPa, 60 GPa and 15.6 MPa·m1/2, respectively. Good interface bonding and pre-introduced SiC matrix are the keys to obtaining excellent bending properties of C/Zr0.5Hf0.5C-SiC composites.

Copyright of Journal of Materials Engineering / Cailiao Gongcheng is the property of Journal of Materials Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Cf/SiC composites are considered as one of the most important candidates for aerospace thermal protection systems because of their low density, high specific strength, good thermal shock, oxidation and ablation resistance, and excellent high temperature strength retention. However, Cf/SiC composites are prone to oxidize at a temperature above 500 ℃ due to inevitable fabrication defects. So it is necessary to carry out effective oxidation protection for the composites. Oxidation resistant coating is an efficient technology to realize long-term oxidation protection. Based on harsh requirement of thermal protection systems, the research progress of anti-oxidation coatings for Cf/SiC composites was summarized, mainly focusing on the coating material systems and their preparation technologies. Improving the service temperature (≥1800 ℃) and bonding strength of the coatings is an important issue to be solved at present.The preparation of multi-functional coating with longer service time and higher service temperature, as well as oxidation resistance, water vapor corrosion resistance and even good heat insulation performance is an important direction for future development.

3D printing of technical ceramics using direct ink writing (DIW) of multiphase colloidal inks has the unique ability to create structures with hierarchical features. To facilitate the application of 3D printed hierarchical porous ultra-high temperature ceramics (UHTCs), additional limiting factors such as strength and the effect of 3D printed internal lattice structure need to be better understood. This study reports on the strength dependence of common DIW print parameters including internal lattice structure shape, nozzle diameter and spacings between adjacent filaments. The present study applies Weibull statistics to the experimental array that considers macro features introduced through print parameters as flaw types, which shows strength of 3D printed hierarchical ZrB 2 is highly dependent on the introduced 3D printed structure, size and the stressed volume. This work provides essential information that can be used in the initial stages of design when considering implementation of additively manufactured hierarchical porous UHTCs. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Materials Engineering / Cailiao Gongcheng is the property of Journal of Materials Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Aging strengthening of traditional alloys is of great practical significance in industry, and the nano second phase precipitated from supersaturated solid solution alloys effectively prevent the movement of grain boundaries and dislocations, so as to improve the hardness of alloys. However, the concept is difficult to be realized in ultra-high temperature ceramics (UHTCs) by the existing sintering process, it is still a challenge to obtain large-scale high concentration supersaturated UHTCs. To achieve this, we prepared a supersaturated Ta-Hf-W-C solid solution ceramic coating by a step-by-step reactive plasma spraying, inducing a high-density precipitation of W by spark plasma sintering and finally obtained an ultra-high temperature Ta-Hf-W-C nano-composite ceramic coating with grid interlocking structure. The average hardness and elastic modulus of the Ta-Hf-W-C composite ceramic coating exceeded 47 GPa and 580 GPa due to fine grain strengthening, solid solution strengthening and ordered grain boundary strengthening. [ABSTRACT FROM AUTHOR]

Copyright of Chinese Journal of High Pressure Physics is the property of Chinese Journal of High Pressure Physics Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

ZrB 2 -SiC ceramics are one of the most promising materials for high temperature applications, and their mechanical properties are key indicators that determine whether they can long-time serve safely. In this work, the mechanical properties of ZrB 2 -SiC ceramics are determined by nanoindentation under six different strain rates. The relative density of ZrB 2 -SiC ceramics prepared by spark plasma sintering reaches 99.1%. The effects of strain rate on the elastic modulus, hardness, creep and pop-in behaviors are discussed in detail. Results show that the effect of strain rate on the elastic modulus can be ignored. The hardness increases with the increase of strain rate, and the increase gradually slows down. The relationship between the natural logarithms of hardness and strain rate can be accurately described as a linear function. The creep strain rate under indentation declines rapidly in the initial holding stage, and then gradually transitions into a constant in the steady-state creep stage under different strain rates. The pop-in load and displacement also showed obvious strain rate-dependence, based on which the shear strength of ZrB 2 gains was estimated and compared with the theoretical value. [ABSTRACT FROM AUTHOR]

ZrC and ZrB2 are both typical ultra-high temperature ceramics, which can be used in the hyperthermal environment. In this study, a method for preparing ultrafine ZrC-ZrB2 composite powder was provided, by using the raw materials of nano ZrO2, carbon black, B4C, and metallic Ca. It is worth pointing out that ZrC-ZrB2 composite powder with any proportion of ZrC to ZrB2 could be synthesized by this method. Firstly, a mixture of ZrC and C was prepared by carbothermal reduction of ZrO2. By adjusting the addition amount of B4C, ZrC was boronized by B4C to generate ZrC-ZrB2 composite powder with different compositions. Using this method, five composite powders with different molar ratios of ZrC and ZrB2 (100ZrC, 75ZrC-25ZrB2, 50ZrC-50ZrB2, 25ZrC-75ZrB2, and 100ZrB2) were prepared. When the temperature of boronization and decarburization process was 1473 K, the particle size of product was only tens of nanometres. Finally, the oxidation characteristics of different composite powders were investigated through oxidation experiments. The oxidation resistance of ZrC-ZrB2 composite powder continued to increase as the content of ZrB2 increased. [ABSTRACT FROM AUTHOR]

In this work, a novel and facile technique based on using KCl as space holders, along with partial sintering (at 1900 °C for 30 min), was explored to prepare porous ZrB 2 –SiC ceramics with controllable pore structure, tunable compressive strength and thermal conductivity. The as-prepared porous ZrB 2 –SiC samples possess high porosity of 45–67%, low average pore size of 3–7 μm, high compressive strength of 32–106 MPa, and low room temperature thermal conductivity of 13–34 W m−1 K−1. The porosity, pore structure, compressive strength and thermal conductivity of porous ZrB 2 –SiC ceramics can be tuned simply by changing KCl content and its particle size. The effect of porosity and pore structure on the thermal conductivity of as-prepared porous ZrB 2 –SiC ceramics was examined and found to be consistent with the classical model for porous materials. The poring mechanism of porous ZrB 2 –SiC samples via adding pore-forming agent combined with partial sintering was also preliminary illustrated. [ABSTRACT FROM AUTHOR]

This research presents a study on the fabrication and characterization of tungsten‑zirconium carbide (W-ZrC), an ultra-high temperature ceramic, using an in-situ reaction sintering method. Tungsten carbide (WC) and zirconia (ZrO 2) powders were used as precursors to synthesize composites with three different ratios. The thermodynamic phase stabilities of the precursor materials were analyzed, and the reaction products were determined based on the total pressure within the system. The fabricated composites were characterized by their microstructures, porosities, hardnesses, and thermal conductivities. The results show that the composites exhibited distinct crystal phases, including those of tungsten, tungsten carbide (W 2 C), zirconium carbide (ZrC), and tetragonal zirconia (t-ZrO 2). The reaction pathways were determined based on the composition ratios and thermodynamic stability. The composites demonstrate good densification and interparticle bonding owing to the in-situ reaction sintering process, which simplifies the fabrication and lowers processing costs. The composites also exhibited desirable properties, such as high hardness and thermal conductivity (maximum 36.4 W·m−1·K−1), making them suitable for high-temperature applications. The findings of this study contribute to the understanding of the synthesis process and properties of W-ZrC composites and provide insights for their potential applications to industries such as aerospace, defense, and nuclear energy. • Ultra-high temperature tungsten‑zirconium carbide ceramic fabricated. • New pressureless in-situ reaction sintering method reduced cost and complexity. • Mechanisms of formation and reactions deduced from phase stability diagram. • Analyzed effect of starting composition on final properties. • Had strong mechanical and thermal properties, good for use in extreme environments. [ABSTRACT FROM AUTHOR]

Refractory metal diboride is a kind of excellent ultra-high temperature ceramics and could be applied as thermal shielding material for supersonic aircraft. In this paper, a two-step reaction process for the synthesis of refractory metal diboride powders was proposed using refractory metal oxide, carbon black (C), B 4 C, and calcium Ca as raw materials. Firstly, the carbothermal reduction reaction between oxides and C was taken place at 1773 K; and then, the reduction product was reacted with B 4 C and Ca for the purposes of boronization and decarburization. After acidic leaching, pure diboride was prepared. The experimental results indicated that diborides of Ti, Zr, Hf, V, Nb and Ta with low residual carbon content can be prepared by this method. In addition, the inheritance relationship of particle size between raw oxide and finally prepared diboride was revealed. By this method, ultra-fine TiB 2 (123 nm) and ZrB 2 (76 nm) powders were also successfully prepared, with the raw materials of nano TiO 2 and nano ZrO 2 , respectively. [ABSTRACT FROM AUTHOR]

An ultra-high temperature composite with superior mechanical characteristics was developed using zirconium diboride (ZrB 2 , 60% by volume), silicon carbide (SiC, 30% by volume), and tantalum carbide (TaC, 10% by volume), and its microstructure characteristics and mechanical properties were obtained experimentally by nanoindentation testing, X-ray diffraction, field emission scanning electron microscopy, X-ray fluorescence analysis, and X-ray photoelectron spectroscopy. Oxide contamination was eliminated, and the ceramic was densified to achieve a fully-dense ternary compound. To assess its performance in a typical real-world application, a finite element analysis was performed using the commercially available ABAQUS package for a one-bay one-story steel frame in which the prepared ceramic was used for the beam–column joint reinforcement end-plates; a good agreement was found with the results reported in the literature for comparable structures. The performance of a steel frame with a plate prepared from functionally graded materials located at the beam–column joint was then modeled in ABAQUS and subjected to the conditions recorded during a 1940 earthquake with a magnitude of 7.7. The results show that the utilized ceramic significantly enhanced the structural behavior of the reinforced concrete frame, confirming its potential utility in a wide range of industrial, structural, and medical applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

ZrB 2 based ultra-high temperature ceramic is one of the most promising thermal protection materials that can be used in hypersonic vehicles. Four different ZrB 2 -25 mol.% SiC–Ni ceramic composites are prepared by spark plasma sintering at 1700 οC, in which the content of Ni are 2, 4, 6 and 8 mol.%, respectively. To make a comparison, ZrB 2 –SiC ceramic is also sintered with the same method. The densifications and microstructures are carefully checked. Nanoindentation tests are performed to determine the hardness and elastic modulus of sintered specimens. The fracture toughness of ceramics is evaluated by a series of single-edge notched-beam tests. Results show that densities of the ZrB 2 –SiC–Ni ceramic composites obviously increase with the increase of the content of Ni. The hardness and elastic modulus of the specimens slightly decline, while the fracture toughness significantly increase with the increase of the Ni content. The fracture toughness of ZrB 2 -25 mol.% SiC-8 mol.% Ni reaches 8.3 MPa m1/2, which is about 1.7 times that of the ZrB 2 –SiC composites. The results in this paper show that the introduction of Ni can greatly improve the fracture toughness of ZrB 2 –SiC ceramics. [ABSTRACT FROM AUTHOR]

Joining technique of ultra-high temperature ceramics is of great importance for their applications. In this work, ZrB 2 -20 vol% SiC composites were joined with Ni powders as brazed fillers by spark plasma sintering (SPS) and hot pressing (HP) at 1200 οC. Joining mechanisms and microstructure evaluation of the joints were discussed. In addition, a quartz lamp thermal shock testing platform was established, based on which thermal shock tests (20, 30, 40 and 50 cycles) with a rapid heating rate of 50 οC/s were conducted to investigate cyclic thermal shock resistance of the ceramics joints. Results showed that the joints were composed of the residual Ni layer, transition layer and initial ceramics layer. The average grains size of ZrB 2 in the joints using SPS was lower than that in the joints using HP. The microstructures analysis indicated that joining using SPS could obviously improve the diffusion ability of different elements. Experimental results showed that cracks were prone to occur near the residual Ni layer and the interface between transition layer and initial ceramic layer, due to the joining defects and high thermal mismatching stress. Compared with the joints using HP, joints using SPS had better cyclic thermal shock resistance. • The ZrB 2 composites were successfully joined by SPS and HP at 1200 °C, with Ni powders as brazed fillers. • A quartz lamp testing platform was firstly established to test the cyclic thermal shock resistances of the joints. • Joining mechanisms and microstructure evaluation of the ceramics joints were detailed discussed. • Cyclic thermal shock behaviours of the joints for different cycles were comparatively studied firstly. [ABSTRACT FROM AUTHOR]

Abstract ZrB 2 based ultra-high temperature ceramic (UHTC) is one of the most promising materials for thermal protection systems. This paper focuses on the oxidation behaviors of ZrB 2 under compressive stress. ZrB 2 UHTC is fabricated with nano-sized powders at 1500 °C, which is a relative low temperature. Nanoindentation tests are conducted to measure the basic mechanical properties of sintered samples. Results of nanoindentation show that ZrB 2 is prepared with good mechanical properties. Oxidation tests are performed under different stress states and temperatures, and oxidation mechanisms and microstructure evolution are discussed. As the oxidation proceeds, ZrO 2 grains grow slowly at 1000 °C, but fast at 1200 °C. When oxidized over 1400 °C, ZrO 2 grains fuse and micro-sized pores form on the surface due to the release of gaseous oxidation products. The thickness of oxide layer on the samples almost keeps unchanged under various compressive loads. Compared with the stress state, microstructure has a more obvious effect on the oxidation rate. The results in this paper provide a better understanding on the oxidation mechanism of ZrB 2 based UHTCs. [ABSTRACT FROM AUTHOR]

The review has been made of recent publications on modification of ZrB2/HfB2-SiC ultra-hightemperature ceramic composite materials (UHTC) by carbon components: amorphous carbon, graphite, graphene, fibers, and nanotubes. Available data have been presented on some aspects of oxidation of such materials at temperatures ≥1500°C and both at the atmospheric pressure and at the reduced oxygen partial pressure; structural features of the formed multilayer oxidized regions have been noted. It has been considered how the type and content of the carbon component and the conditions (first of all, temperature) of UHTC production affect the density, flexural strength, hardness, fracture toughness, and thermal and oxidation resistance of the modified ceramic composites. [ABSTRACT FROM AUTHOR]

Zirconium diboride (ZrB 2 ) has many potentially beneficial properties for fusion plasma-facing component application, but almost no data exist on the response of ZrB 2 to ion irradiation. In this work, ZrB 2 samples were irradiated with 30 keV He + to fluences of 8.4 × 10 21 and 5.0 × 10 22 He/m 2 at temperatures of 920, 1020, and 1120 K in the Materials Irradiation Experiment (MITE-E) at the University of Wisconsin Inertial Electrostatic Confinement (UW-IEC) Laboratory to simulate some of the conditions of plasma-facing components in fusion reactors. The samples irradiated to the higher fluence developed surface morphology changes in the ion irradiated zone including rough, porous, and ripple structures. The ZrB 2 had similar mass loss as W irradiated to similar conditions. Additionally, the ZrB 2 samples did not exhibit flaking as did the SiC samples previously irradiated to similar conditions. This first look at ZrB 2 behavior under ion irradiation is promising and justifies further testing of this emerging ultra-high temperature ceramic material for fusion applications. [ABSTRACT FROM AUTHOR]

The oxidation behavior of four ZrB 2 -SiC-ZrC compositions with varying ZrC contents (20, 34, 50, and 64 vol.%) was compared to that of ZrB 2 -SiC. The ceramics were oxidized at 1700 °C in an oxygen-hydrogen torch environment. The liquid oxide on the ZrB 2 -SiC sample came off from the surface under such an environment. In contrast, the all ZrB 2 -SiC-ZrC samples maintained the convex oxide on the surface, which consisted of ZrO 2 and SiO 2 . The convex oxide of ZSZ with higher ZrC content was thicker, with the exception of ZrB 2 -SiC-64vol.%ZrC sample. The ZrB 2 -SiC-64vol.%ZrC sample formed a ZrO 2 -rich layer, which was clearly denser than the ZrO 2 -SiO 2 . This densification was caused by ZrO 2 -sintering, and it was specific behavior under the dynamic pressure. [ABSTRACT FROM AUTHOR]

In order to improve the strength and toughness of the C/C-HfC-ZrB2-SiC composites, uniformly distributed SiC nanowires were in situ grown in C/C preform using a simple precursor infiltration pyrolysis method with ferrocene as catalyst. The SiC nanowires could greatly refine the size of the ultra-high temperature ceramic (HfC-ZrB2-SiC) particles, and make the ceramic particles interconnected with the unitary body. After introducing SiC nanowires, the flexural strength, flexural modulus and fracture toughness of the composites increased by 21%, 29% and 28% respectively, due to interfacial weakening, the deflection of cracks induced by SiC nanowires and the pullout of SiC nanowires. The C/C-HfC-ZrB2-SiC composites with SiC nanowires also displayed good erosion resistance under high velocity Al 2 O 3 particle erosion. [ABSTRACT FROM AUTHOR]

Processing parameters to minimize the residual oxygen content of ZrB2 ceramics prepared by reactive hot-pressing were selected using statistical analysis. Additions of carbon and excess boron were used to react with oxygen present in the starting ZrH2 and B powders as an impurity. A 32 full-factorial experimental design was used to determine the carbon and excess boron contents that minimized residual oxygen content in the final ZrB2 ceramic while also minimizing formation of any impurity phases. Carbon additions were effective at reducing the oxygen content, but resulted in the formation of residual ZrC. Boron additions were also effective at removing oxygen, but to a lesser extent compared to carbon. In addition to being more effective at removing oxygen, carbon additions also improved the densification behavior, whereas boron additions inhibited densification. The stoichiometric reaction resulted in a ZrB2 ceramic with a relative density of 99%, but that contained 6.9 vol% of (Zr1−x,Mgx)O2−x as a residual phase following reactive hot-pressing. The composition with a carbon addition to starting oxygen content having a molar ratio of 1, and a boron to zirconium molar ratio of 2.1, resulted in a 95% dense ceramic with only 0.1 vol% of residual (Zr1−x,Mgx)O2−x and trace amounts of ZrC. [ABSTRACT FROM AUTHOR]

ZrC coatings were deposited on graphite substrates by low pressure chemical vapor deposition (LPCVD) with the Br-Zr-CH-H-Ar system. The effects of deposition time on the microstructures and growth behavior of ZrC coatings were investigated. ZrC coating grew in an island-layer mode. The formation of coating was dominated by the nucleation of ZrC in the initial 20 minutes, and the rapid nucleation generated a fine-grained structure of ZrC coating. When the deposition time was over 30 min, the growth of coating was dominated by that of crystals, giving a column-arranged structure. Energy dispersive X-ray spectroscopy showed that the molar ratio of carbon to zirconium was near 1:1 in ZrC coating, and X-ray photoelectron spectroscopy showed that ZrC was the main phase in coatings, accompanied by about 2.5mol% ZrO minor phase. [ABSTRACT FROM AUTHOR]

Zirconium carbide (ZrC) coatings were deposited on graphite substrates by chemical vapor deposition from the Zr-Br 2 -C 3 H 6 -H 2 -Ar system. The influence of total pressure on the growth of ZrC was investigated in the range of 5–60 kPa. As the total pressure increased, the deposition rate increased evidently, and the preferential orientation of ZrC coatings changed from the (200) plane to the (220) plane. The growth mechanism changed from a mass transport reaction to a surface reaction at the total pressure of 20–40 kPa. At the total pressure below 20 kPa, the deposition was dominated by crystal growth, so the coatings were composed of well-faceted pyramidal-shaped crystals growing along the <001> direction. At the total pressure above 60 kPa, the growth of ZrC coatings was controlled by the nucleation mechanism, so the coatings were cluster-like crystals rapidly growing along the <110> direction. In addition, low pressure was conducive to the formation of near-stoichiometric ZrC without free carbon. These variations of ZrC coatings can mainly be attributed to gas supersaturation and remarkably changed transport diffusion coefficients with increasing total pressure. [ABSTRACT FROM AUTHOR]

VC 1− x compounds, known as ultra-high temperature ceramics, are able to maintain stability in a range of chemical compositions. Herein, using evolutionary algorithm USPEX, we performed a global search for potentially stable VC 1− x compounds. We have discovered two stable compounds ( P 3 1 12-V 6 C 5 and Pnnm -V 2 C) and a number of near-ground-state compounds ( Fm 3 ¯ m -VC, P 4 3 32-V 8 C 7 , P 1 ¯ -V 5 C 4 , I 4 ¯ 3 m -V 4 C 3 , Cmcm -V 3 C 2 , and Cmcm -V 3 C). The stable Pnnm -V 2 C is a new structure with lower total energy than experimentally reported α -V 2 C. Based on these stable and meta-stable VC 1− x compounds, we have systemically investigated their structures, mechanical properties, and chemical bonding. We found that Cmcm -V 3 C 2 and Cmcm -V 3 C display different arrangements of vanadium from other VC 1− x compounds. We suggest that such rearrangement of vanadium are able to enhance mechanical strength. Most of VC 1− x compounds possess very good mechanical properties, indicating that they have potential to be utilized for structural applications. [ABSTRACT FROM AUTHOR]

To improve the oxidation protection effect of Si-based coatings containing ZrB 2 or TaB 2 ceramics, ZrO 2 , Ta 2 O 5 , B 2 O 3 , Si and C powders were used as raw materials to prepare the outer Zr x Ta 1− x B 2 –SiC coating by in-situ reaction method on SiC coated C/C composites. Zr x Ta 1− x B 2 phase was obtained by co-reducing ZrO 2 and Ta 2 O 5 using B 2 O 3 and C in the heat-treatment process at 2373 K. Isothermal oxidation test at 1773 K and TGA test from room temperature to 1773 K were used to evaluate the oxidation protection effect of the coated C/C composites. During oxidation, an “inlaid structure” Zr–Ta–Si–O compound glass layer forms on the surface of the Zr x Ta 1− x B 2 –SiC coating. The Zr x Ta 1− x B 2 –SiC/SiC coating not only can protect C/C from oxidation at 1773 K for 1412 h with only 0.1 wt.% loss, but also can provide an effective protection for C/C at a wide range of temperature from room temperature to 1773 K. [ABSTRACT FROM AUTHOR]

To obtain silicon-based ceramic coated C/C composites with excellent mechanical and antioxidative properties, a SiC/SiC-MoSi 2 -ZrB 2 composite ceramic coating was designed and prepared on the surface of C/C composites by a three-step process. Firstly, a pyrolytic carbon (PyC) layer was deposited on the C/C substrate to fill its holes and cracks. Secondly, a SiC porous layer was fabricated on the PyC layer to create many holes subsequently used to distribute massive MoSi 2 -ZrB 2 ceramics. Finally, a double-layer SiC/SiC-MoSi 2 -ZrB 2 composite ceramic coating was formed on C/C composites after a pack cementation (PC) process, which consisted of a dense SiC inner coating (a thickness of ~ 80 µm) and a uniform and rich MoSi 2 -ZrB 2 outer coating (~ 200 µm). The bending strength of the SiC/SiC-MoSi 2 -ZrB 2 coated C/C sample was 21.5 % higher than that of the raw C/C sample. Moreover, the SiC/SiC-MoSi 2 -ZrB 2 coating provided good oxidation protection for C/C composites at high temperature, and its mass loss was only 0.56 % after oxidation for 305 h at 1773 K in air due to the blocking effect of the compound oxide glass (SiO 2 , ZrO 2 and ZrSiO 4) layer on oxygen diffusion into the internal coating and C/C substrate. Therefore, this novel strategy for coating preparation can obtain SiC/SiC-MoSi 2 -ZrB 2 coated C/C composites with excellent mechanical and oxidation resistance properties. [Display omitted] • Preparing the SiC/SiC-MoSi 2 -ZrB 2 coating on C/C composites by a novel strategy. • The coating consisted of a dense SiC inner coating and a uniform and rich MoSi 2 -ZrB 2 outer coating. • The bending strength of coated C/C sample was increased by 21.5 %. • The coating can effectively prevent C/C composites against oxidation for 305 h at 1773 K in air. [ABSTRACT FROM AUTHOR]

Abstract: Zirconium carbide (ZrC) is an important ultra-high temperature ceramic due to its refractory properties. It is commonly synthesized via carbothermal reduction of zirconia above 1657°C according to the reaction ZrO2 (s)+3C (s)→ZrC (s)+2CO (g). Contrary to this reaction, prior research indicates that carbon monoxide (CO) is the responsible species for carburizing ZrO2 to form ZrC. To explore this reaction pathway, investigations were performed by making two mixed phase pellets with 3mol% yttria-stabilized zirconia (YSZ) and graphite. Both had an upper half made of YSZ. The lower half of one sample consisted of finely mixed YSZ and graphite powder whereas the other was pure graphite. Similar experiments were conducted with sintered YSZ pellets on top. After heat treatment at 1800°C, X-ray diffraction analysis revealed higher ZrC conversion for the YSZ pellet face in direct contact with pure graphite. This contradicts the previous work as one would assume higher ZrC yield for YSZ pellet in direct contact with YSZ/graphite mix as they produce more CO upon reaction. Lastly, diffusional experiments showed conversion to be highly localized to a depth of ∼25μm. This is in close agreement with calculations for carbon diffusion in YSZ based on a diffusion coefficient (D)=3×10−14 m2/s, which confirms solid–solid reaction rather than solid–gas reaction. [Copyright &y& Elsevier]

To achieve a ZrO-B-C ternary solid-state reaction, a small amount of carbon was introduced into a binary system of ZrO and boron. Amorphous hydrous ZrO precursor was firstly prepared through precipitation method using zirconyl nitrate and ammonia at room temperature. Then, ZrB powder was successfully synthesized using the as-prepared amorphous hydrous nano-ZrO via borothermal and carbothermal reduction (BCTR) by amorphous boron and nanocarbon powders under flowing argon. Plate-like ZrB particles with a uniform distribution and ca. 1.0 μm in two-dimensions were obtained with a B/Zr molar ratio of 4, and 5 wt.% (total weight of dry hydrous ZrO and boron) nanocarbon at 1550 °C for 2 h. [ABSTRACT FROM AUTHOR]

Abstract: Based on the critically assessed B–Si phase diagram, one alloy in the Si-rich side was selected and the temperature of the eutectic reaction L↔(Si)+SiB6 was measured to be 1636K by means of DSC. Using a regular substitutional model for the liquid phase, the B–Si system was reassessed and extrapolated to higher-order systems. Thermodynamic description for the B–C–Si system was developed based on critically reviewed experimental data. The calculated phase diagrams were in good agreement with available experimental data. [Copyright &y& Elsevier]

SiC-ZrC-based composite coatings on carbon/carbon composites via ZrC polymer precursor pyrolysis (PPP) plus gaseous reactive infiltration (GRI) of Si or ZrSi 2 , separately denoted as GSIC and GZSIC, were comparatively studied to explore the feasibility and effectiveness of GRI of ZrSi 2. This composite method by applying ZrSi 2 or Si was proven to similarly produce dense coatings with enhanced coating/substrate interfacial bonding strength for the formed zig-zag transition layer. Furthermore, the GRI of ZrSi 2 instead of Si further increased the UHTC (ZrC) content in the as-deposited coatings resulted from the reaction of infiltrated ZrSi 2 and pre-introduced graphite, enhancing the ablation resistance of the coatings for carbon/carbon composites due to the formed stable Zr-rich Zr-Si-O multiphase oxide. The average mass and linear ablation rates were −0.12 mg/s and 1.56 μm/s for GSIC and −0.3 mg/s and −0.53 μm/s for GZSIC specimens, respectively. The average linear ablation rate of the GZSIC specimen was decreased by ~134% when compared to the GSIC specimen. These results further indicate that the PPP + GRI method with metal silicide MeSi 2 (Me = Zr, Ti, Hf, Cr, etc.) will contribute to the optimal fabrication of dense composite coatings with controllable microstructure, adequate evenly-distributed ultra-high temperature ceramic (UHTC) phases, enhanced interfacial bonding strength and wide circumstance suitability for potential applications. The related work is ongoing in our laboratory. [Display omitted] • SiC-ZrC-based coatings were made by precursor pyrolysis plus gaseous reactive infiltration. • Pre-introduced graphite reacted with infiltrated ZrSi 2 further improved ZrC content in the coatings. • The gaseous infiltrated ZrSi 2 instead of Si enhanced ablation resistance of the coatings. • The coating's linear ablation rate was reduced by ~134% as gaseous infiltrated with ZrSi 2. [ABSTRACT FROM AUTHOR]

This study investigates the capability of a custom-built controlled atmosphere plasma spray (CAPS) system for deposition of zirconium boride (ZrB 2) and hafnium boride (HfB 2) thermal protection system (TPS) coatings. Due to poor oxidation resistance these materials must be deposited in the absence of oxygen to avoid high inclusions of oxides that detrimentally effect TPS coating properties. A low cost, simple CAPS system was developed to allow for the deposition of these coatings within an inert atmosphere, in a typical lab environment. The CAPS system has demonstrated a 46% reduction in oxide inclusions for ZrB 2 coatings with a 40% reduction in elemental oxygen and facilitated the deposition of a HfB 2 coating with low oxide inclusions. Weibull analysis on the variability in the hardness of ZrB 2 samples revealed a tri-modal distribution of hardness that was attributed to regions of differing oxide inclusions. The CAPS deposition of HfB 2 did not achieve a deposited microstructure that was considered effective as a TPS. A Weibull analysis of the hardness of the HfB 2 coating demonstrated a bi-modal distribution that corresponded to low oxide regions and locations composed of a similar mix of boride and monoclinic oxide. The heterogeneous microstructure was attributed to the pre-processing of the HfB 2 powder. • Controlled Atmosphere Plasma Spray (CAPS) achieved a 46% reduction in oxide inclusions and 40% reduction in elemental oxygen. • CAPS deposited zirconium boride coating exhibited less variation in hardness. • Hafnium boride coating exhibited heterogenous microstructure that was attributed to pre-processing of the feedstock. [ABSTRACT FROM AUTHOR]

• Novel ultra-high-temperature-ceramic (UHTC) as well as high-entropy material with superior ablation resistance was proposed. • Special micro-segregation is found in the HECN, where Ta, Hf, Zr, Ti elements aggregated inside of grains and Nb segregated at the grain boundary. • There is the compact multi-oxide scale that involves Hf-Zr-rich grains skeleton and continuous molten-like phase with Ta-Ti-Nb segregation. A novel high-entropy material as well as ultra-high temperature ceramic (Ta 0.2 Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2)C 0.8 N 0.2 was synthesized from ten kinds of transition metal carbides and nitrides, via high-energy ball-milling and spark plasma sintering. Besides, the ablation behavior of high-entropy ceramic was first studied by oxy-acetylene torch system at 2500 K. The results showed that optimized nitrogen substituted in sublattice could remarkably improve the ablation resistance by an impressive 57±1 % and 72 ±1 % in mass ablation rate and linear ablation rate compared to high-entropy carbides, since the highly dense multiple oxides scale with large Hf-Zr-rich grains skeleton surrounded by continuous secondary-phase in the boundaries that was able to retard the diffusion of oxygen. [ABSTRACT FROM AUTHOR]

To investigate the influence of free silicon content in inner SiC-Si coating to thermal cycle ablation behaviour outer CVD-ZrC coating, SiC-Si inner coatings with different free silicon content were prepared by pack cementation on the surface of carbon/carbon (C/C) composites. These three kinds of coatings are named as PC-5, PC-7.5 and PC-10, of which the inner coatings are prepared with three different raw material powders (mass ration Si:C:Al 2 O 3 , PC-5 (5:1:0.5), PC-7.5 (7.5:1:0.5), PC-10 (10:1:0.5)). It can be observed that a gradient distribution tendency exists between the ZrC and SiC layers both in specimens (PC-7.5 and PC-10). Among three kinds of samples, PC-7.5 exhibited the best cyclic ablation resistance. The amount of Zr-Si-O glass on the surface of ablated specimens increases with free silicon content in inner coating. Interestingly, when the Si element in inner coating was excessive, Zr-Si-O glass would exhibit a spherical structure accompanied with the ablation resistance decreasing. Meanwhile convex structure ZrO 2 was formed after the ablation process of ZrC/SiC-Si coating. The amount of convex ZrO 2 structures gradually decreased with the increase of silicon content. The size of ZrO 2 grains increased with the increase of the silicon content or the time of cyclic ablation. • ZrC/SiC-Si coatings with different free silicon contents were fabricated by in situ reaction and chemical vapor deposition. • The PC-7.5 samples possessing gradient distribution tendency at ZrC/SiC interface exhibited best cyclic ablation resistance. • Excessive Si leads to spherical structure Zr-Si-O glass generating under ablation, limiting cyclic ablation resistance. [ABSTRACT FROM AUTHOR]

This study presents a new ultra-high temperature composite fabricated by using zirconium diboride (ZrB2), silicon carbide (SiC), and tantalum carbide (TaC) with the volume ratios of 70%, 20%, and 10%, respectively. To attain this novel composite, an advanced processing technique of spark plasma sintering (SPS) was applied to produce ZrB2–SiC–TaC. The SPS manufacturing process was achieved under pressure of 30 MPa, at 2000 °C for 5 min. The micro/nanostructure and mechanical characteristics of the composite were clarified using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and nano-indentation. For further investigations of the product and its characteristics, X-ray fluorescence (XRF) analysis and X-ray photoelectron spectroscopy (XPS) were undertaken, and the main constituting components were provided. The composite was densified to obtain a fully-dense ternary; the oxide pollutions were wiped out. The mean values of 23,356; 403.5 GPa; and 3100 °C were obtained for the rigidity, elastic modulus, and thermal resistance of the ZrB2–SiC–TaC interface, respectively. To explore the practical application of the composite, the natural frequency of an aircraft wing considering three cases of materials: (i) with a leading edge made of ZrB2–SiC–TaC; (ii) the whole wing made of ZrB2–SiC–TaC; and (iii) the whole wing made of aluminum 2024-T3 were investigated employing a numerical finite element model (FEM) tool ABAQUS and compared with that of a wing of traditional materials. The precision of the method was verified by performing static analysis to obtain the responses of the wing including total deformation, equivalent stress, and strain. A comparison study of the results of this study and published literature clarified the validity of the FEM analysis of the current research. The composite produced in this study significantly can improve the vibrational responses and structural behavior of the aircraft's wings. [ABSTRACT FROM AUTHOR]

Ultra‐high temperature ceramic (UHTC) is one of the most promising materials used in thermal protection system for hypersonic flight vehicles. In this work, ZrB2–0.5 vol% SiC composites are fabricated with nano‐sized ZrB2 and SiC particles by spark plasma sintering (SPS) at 1500 °C. The average density of the composites reaches 5.47 g cm−3, while the average grain size is only 1 μm. Then, single and cyclic ablation tests are conducted using a plasma generator. Ablation mechanisms and microstructure evaluation of the composites are discussed in detail. In addition, a digital optical microscope system is adopted to obtain the surface micrograph after ablation. Results show that ablation center will become protuberant rather than form an ablation crater, due to the relative porous and loose oxide layer and flushing action of ablation flame. Thickness of oxide layer and mass gain of the sample obviously increase during the single ablation, while the mass change rate decreases gradually and is then kept stable with the ablation time increasing. Compared with the single ablation, the cyclic ablation leads to higher mass gain and mass change rate, as many microcracks formed on the surface which will significantly accelerate the oxidation. ZrB2 based ultra‐high temperature ceramic is one of the most promising materials for thermal protection system applications, due to the high melting points and excellent mechanical properties. In this work, single and cyclic ablations under plasma jet are performed to provide a better understanding of the ablation mechanisms of ZrB2 based ceramics. [ABSTRACT FROM AUTHOR]