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Zirconium carbide (ZrC) is considered to be a potential candidate for ultra high temperature applications due to its high melting point, good chemical inertness, and ablation resistance, but the monolithic form suffers from low fracture toughness and hence poor thermal shock resistance. Reinforcing it using continuous carbon fibers (Cf) to create an ultra high temperature ceramic matrix composite is an obvious solution, however densifying ZrC requires the use of very high temperatures combined with significant pressure, such as obtained by using hot pressing or spark plasma sintering, which risks damaging fibers. In the present work, radio frequency‐assisted chemical vapor infiltration (RF‐CVI) has been investigated with a view to forming Cf/ZrC composites. These initial experiments revealed the ability to deposit pure, nano‐grained, and near stoichiometric ZrC with deposition occurring preferentially from the center of the sample due to the nature of the inverse temperature profile developed. The deposited ZrC grains were in the range of 4–9 nm in size and had a lattice parameter of 0.4750 nm. The work also showed that the use of RF‐CVI enabled the minimization of early pore sealing, a common problem for conventional CVI. [ABSTRACT FROM AUTHOR]

This study selected ZrB2–hBN ceramics as the matrix for low‐cost second‐generation SiC fibers, due to their low modulus and ability to be sintered at relatively low temperatures. The resulting composites, which contained up to 30 wt.% short‐chopped SiCf, were consolidated using reactive spark plasma sintering at 1550 and 1700°C under 50 MPa for 5 min. Without needing to prepare interfaces on the SiCf surfaces, fiber pullout, strengthening, and toughening during the fracture process were realized. By constructing the volatility phase diagram for the fiber, the microstructural changes that occurred on the fiber surfaces during sintering were successfully illustrated. Mechanical properties of ZrB2–hBN ceramics with 10 wt.% SiCf sintered at 1550°C still showed considerable improvements, including an elastic modulus of 187 GPa, a flexural strength of 337 ± 16 MPa, and a fracture toughness of 4.12 ± 0.25 MPa m1/2, increases of 12.6%, 61.2%, and 118%, respectively, compared to the counterparts without adding chopped fibers. Variations in these properties were linked to the matrix porosity and SiCf pullout behaviors, which were subsequently analyzed using the He–Hutchinson model. [ABSTRACT FROM AUTHOR]

Ceramics were susceptible to thermal shock failure. In this work, aluminum nitride‐based ceramics with excellent thermal shock resistance were developed. Thermal shock resistance was evaluated in the temperature range of 1200–1600°C by thermal cycling of 10, 20, 30, and 50 times. The phase composition, microstructure, and mechanical properties of the ceramics before and after the thermal shock were investigated. After 50 thermal cycles, the retention rates of the flexural strength and fracture toughness were 81.7% and 113.1%, respectively. The excellent thermal shock resistance was attributed to the combined effects of amorphous grain boundaries that passivate the propagation of microcracks and in‐situ precipitation of nanocrystalline particles during thermal cycles, both of which provide useful guidelines for developing ceramics with better thermal shock resistances. [ABSTRACT FROM AUTHOR]

High entropy monoborides powders are mainly prepared from metal and boron powders which can be costly. In this study, a novel approach starting from mixed oxides, boron carbide, and graphite was first developed to synthesize quaternary monoboride powders by combining multiple boro/carbothermal reductions (MBCTR). As an exemplar, (Mo0.25W0.25Cr0.25Ta0.25)B powders with purity up to 94.3 wt.% were successfully synthesized by heating the mixtures in vacuum at 1650°C for three times, as‐synthesized powders exhibit fine particle size (<1 µm) and uniform elemental distribution. MBCTR not only increased the purity of monoborides but resulted in abundant twins and stacking faults in the final powders. This work has proposed a new and low‐cost strategy of synthesizing high‐purity and fine monoboride powders. [ABSTRACT FROM AUTHOR]

Prestress strategy is effective to inhibit crack advancement and toughen structural materials such as concrete and glass. However, it is difficult to be utilized in ceramic because of the solid‐state processing. In this work, nanocrystalline alumina ceramic with high compressive prestress and coherently aligned nanograins were successfully prepared by the ultra‐high pressure sintering technology. The as‐prepared ceramics exhibited remarkably enhanced mechanical properties, including hardness of 25.9 GPa and fracture toughness of 3.2 MPa·m1/2, which were respectively 36% and 51% higher than corresponding parameters of sapphire. Ultra‐high pressure generated compressive prestress as large as several hundred megapascal, and coherent nanograins in the dimensions of 10–15 nm, which markedly contributed to the simultaneously enhancement in toughness and hardness for ceramic materials. The prestressing strategy provides a new opportunity for fabricating advanced ceramics with high failure resistance. [ABSTRACT FROM AUTHOR]

Dense and light B4C–(Ti0.9Cr0.1)B2 composites with excellent mechanical properties were designed and reactively densified from boron, TiC, and Cr3C2 powder mixtures by spark plasma sintering in this work. Due to solid solution effects, the as‐obtained B4C–(Ti0.9Cr0.1)B2 composite exhibited obviously enhanced hardness (43.2 ± 3.0 GPa at 9.8 N) and higher specific hardness (12.82 GPa cm3 g−1) together with improved flexural strength (663 ± 39 MPa) and fracture toughness (KIC, 5.40 ± 0.25 MPa m1/2), compared to the counterparts such as B4C–TiB2 composite and B4C. Toughening contributions in the as‐sintered ceramics were quantitatively analyzed, and higher KIC in B4C–(Ti0.9Cr0.1)B2 was mainly due to their larger initial fracture toughness and compressive stress toughening. The combination of these properties makes B4C–(Ti0.9Cr0.1)B2 composites exhibit great potentials in the application as lightweight structural materials. This work provided an inspiration to achieve lightweight materials with high performance through doping minor‐amount atoms into the matrix. [ABSTRACT FROM AUTHOR]

Effects of SiC whiskers (SiCw) on the mechanical properties of composites largely depend on their thermal stability at high temperature. In this study, pure SiCw and Ti3SiC2 coated SiCw were thermal treated at 1600–1800°C for 1 h. Their phase assemblage, morphology, and structural evolution were investigated. Oxygen partial pressures in the graphite furnace resulted in the breakdown of SiCw into particles at 1600°C, and the degradation became more pronounced with temperature increasing. The thermal stability of SiC whiskers at 1600–1700°C was significantly improved by a thin Ti3SiC2 coating on them, as both thermodynamic calculations and experimental observations suggest Ti3SiC2 coating could be preferentially oxidized/decomposed, prior to the active oxidation of SiC. At 1800°C, the protective role of the coating on the whiskers became weakened. SiC was converted into gaseous SiO and CO, with the remaining of interconnected TiC micro‐rods and amorphous carbon. [ABSTRACT FROM AUTHOR]

Fully dense oriented monolithic zirconium diboride (ZrB2) ceramics with <0001> fiber texture were fabricated using high‐pressure spark plasma sintering. The <0001> fiber texture is proposed to be triggered by grain rotation, as (0001) <112¯0$11\bar{2}0$> basal ⟨a⟩$\langle a\rangle $ slip of ZrB2 was activated during sintering. The high temperature, uniaxial high pressure, and uniform starting particles play a vital role in the texture formation of ZrB2 ceramics. The result provides visible evidence for the occurrence of plastic deformation during high‐pressure‐assisted sintering. [ABSTRACT FROM AUTHOR]

Abstract: A volatility diagram of zirconium carbide (ZrC) at 1600, 1930, and 2200°C was calculated in this work. Combining it with the existing volatility diagrams of ZrB2 and SiC, the volatility diagram of a ternary ZrB2‐SiC‐ZrC (ZSZ) system was constructed in order to interpret the oxidation behavior of ZSZ ceramics. Applying this diagram, the formation of ZrC‐corroded and SiC‐depleted layers and the oxidation sequence of each component in ZSZ during oxidation and ablation could be well understood. Most of the predictions from the diagrams are consistent with the experimental observations on the oxidation scale of dense ZrB2‐SiC‐ZrC ceramics/coatings after oxidation at 1600°C or ablation at 1930 and 2200°C. The reasons for the discrepancy are also briefly discussed. [ABSTRACT FROM AUTHOR]

Textured AlN-based ceramics with improved mechanical properties were prepared by hot pressing using Si3N4 and Y2O3 as additives. The introduction of Si3N4-Y2O3 into AlN matrix led to the formation of secondary Y3AlSi2O7N2 and fiber-like 2Hδ AlN-polytypoid phases, the partial texture of all crystalline phases, and the fracture mode change from intergranular to transgranular. Consequently, Vickers hardness, fracture toughness and flexural strength of AlN-based ceramics by the replacement of Y2O3 by Si3N4-Y2O3 increased significantly from 10.4±0.3 GPa, 2.4±0.3 MPa m½ and 333.3±10.3 MPa to 14.2±0.4 GPa, 3.4±0.1 MPa m½ and 389.5±45.5 MPa, respectively. [ABSTRACT FROM AUTHOR]

A carbide boronizing method was first developed to produce dense boron carbide- zirconium diboride (' B4 C'- ZrB2) composites from zirconium carbide ( ZrC) and amorphous boron powders ( B) by Spark Plasma Sintering at 1800°C-2000°C. The stoichiometry of ' B4 C' could be tailored by changing initial boron content, which also has an influence on the processing. The self-propagating high-temperature synthesis could be ignited by 1 mol ZrC and 6 mol B at around 1240°C, whereas it was suppressed at a level of 10 mol B. B8 C- ZrB2 ceramics sintered at 1800°C with 1 mole ZrC and 10 mole B exhibited super high hardness (40.36 GPa at 2.94 N and 33.4 GPa at 9.8 N). The primary reason for the unusual high hardness of B8 C- ZrB2 ceramics was considered to be the formation of nano-sized ZrB2 grains. [ABSTRACT FROM AUTHOR]

Hafnium carbide/tungsten ( HfC/ W) cermets were prepared by an in situ reaction sintering process, using hafnium oxide ( HfO2) and tungsten carbide ( WC) as the raw materials. The reaction path, densification behavior, microstructure development, and mechanical properties of the cermets were comprehensively investigated. It was found that WC decomposed to tungsten semicarbide ( W2 C) and tungsten ( W) in sequence, and meanwhile HfC was formed by carbothermal reduction between HfO2 and as-released carbon from the dissociation of WC. The solid solution formation between HfC and W during sintering was also studied. The obtained cermets (>98% TD) have a Vickers' hardness of 8.16 GPa, a fracture toughness of 14.45 MPa m1/2, and a high flexural strength of 1211 MPa. [ABSTRACT FROM AUTHOR]

High temperature flexural strength of ZrB2-20 vol% SiC ceramics ( ZS) up to 1600°C in high purity argon atmosphere was significantly improved by adding 5 vol% WC, but degraded when 5 vol% ZrC was added. ZrB2-20 SiC-5 WC ceramic ( ZSW) has a very high strength (mean ± SD) of 675 ± 33 MPa at 1600°C, and also an elastic and transgranular fracture mode was observed. According to the analysis of the fracture modes and crack origins in ZSW ceramics, the improvement in strength above 1000°C was attributed to the removal of the oxide impurities from grain boundaries. [ABSTRACT FROM AUTHOR]

Plate-like ZrB2 grains were synthesized at 1550°C by in situ solid/liquid reaction using Zr and B powders mixed with transition metal ( Mo, Nb, Ti, or W) and Si powder. The preferred growth direction of plate grains was along a- or b-axis depending on the initial content of transition metal and silicon in the mixtures. The synthesis mechanism of plate-like grain was possibly related to the catalysis of in situ formed silicides. [ABSTRACT FROM AUTHOR]