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Zirconium carbide (ZrC) ceramics is an important candidate material for structural applications both at high temperature and intensive irradiation. However, its sensitivity to defects and cracks often causes low reliability and limited the application of ZrC ceramics. In this work, graphite nanoplatelets (GNPs) doped zirconium carbide ceramics were prepared by spark plasma sintering (SPS). The results show that the addition of GNPs not only improved the densification of ZrC ceramics but also inhibited grain growth of ZrC matrix phase. The GNPs were mainly located at the grain boundaries, and the c-axis of the disk-like GNPs preferred to align with the applied pressing direction during SPS. The preferred orientation of GNPs in the sintered ceramics resulted in anisotropic microstructure and fracture toughness of the material. Compared with the monolithic ZrC ceramics with a fracture toughness of 2.27 MPa m1/2, GNPs doped ceramics shows higher fracture toughness. The fracture toughness of ZrC ceramics with 0.5, 1.5, 3 vol% GNPs are 2.70 MPa m1/2, 3.03 MPa m1/2 and 3.24 MPa m1/2 measured on the surface perpendicular to the applied pressing direction and 3.43 MPa m1/2, 3.66 MPa m1/2 and 4.55 MPa m1/2 measured on the surface parallel to the applied pressing direction, respectively. The crack deflection and re-initiation by GNPs are the main toughening mechanisms for the GNPs doped ZrC ceramics.

Wood gains much attention owing to its unique characters such as cellular pore structures, low density etc. Wood-derived carbides have great potential applications as the high-temperature filters for gas or liquid, catalyst carrier, fluid/gas reservoir devices, biocatalyst supports, etc. In this paper, we review recent progress in comprehensively understanding the processing techniques, properties and applications of wood-derived carbides. The key techniques for producing wood-derived carbides involves the infiltration techniques which are categorized into six parts (slurry infiltration, polymer precursor infiltration, melting infiltration, molten salt infiltration, sol-gel infiltration, and chemical or physical vapor infiltration). The advantages and disadvantages of these techniques are discussed in details, and the according solutions for solving the problems of each technique are further proposed. The infiltration kinetics of processing carbides are also discussed in details. The investigated properties of wood-derived carbides are summarized, which includes the mechanical properties and thermal properties. The potential applications of wood-derived carbides are explored, along with an overview of the existing challenges and practical limitations. In the end, we provide future perspectives to highlight the future directions of research in this growing area.

We report a simple liquid precursor method for the synthesis of single-phase high-entropy carbide (HEC) nanopowders. Transition metal halides (TiCl4, ZrCl4, HfCl4, NbCl5, TaCl5) and furfuryl alcohol are used as metal sources and carbon source. The liquid precursor can be converted into mixtures of five binary carbides via carbothermal reduction at 1400 °C in vacuum, and can be further transformed into single-phase HEC nanopowders through solid solution formation by heat treatment at 2000 °C. The as-synthesized HEC nanopowders possess an average particle size of 132 ± 5 nm, a specific surface area of 24.5 m2/g, and an oxygen content of 0.22 wt%, respectively.

High entropy ceramics have attracted much attention in recent years, however, few open literature on the composites in which high entropy phases were used as matrix can be found. In this work, high entropy diboride powder was firstly synthesized, and then monolithic high entropy diboride ceramics of (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 (sample HEB) and high entropy particulate composites of (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2-20 vol% SiC (sample HEB-SiC) were prepared by SPS at different temperatures. As-synthesized high entropy diboride powders demonstrated good sinterability in the temperature range from 1600 °C to 1900 °C. The grain growth rate of the high entropy phase was comparable to that in the ZrB2 system, but the addition of SiC second phase particles inhibited the grain growth of the high entropy diboride phase. Due to the grain growth inhibition by the introduced SiC particles, the hardness for the particulate composites increased. Moreover, the SiC particles improved the fracture toughness through crack deflection and branching.

Porous ZrB2/ZrC/SiC ceramics modified with SiC whiskers were successfully prepared via a sol-gel method and catalyst-assisted pyrolysis. The high porosity (85.4%) and large pore size (19.1 μm) of the porous structures enabled the in-situ growth of SiC whiskers during pyrolysis with iron as catalyst. Short and tangled SiC whiskers were observed in the interior part of the porous ZrB2/ZrC/SiC ceramics, while long and straight SiC whiskers were grown on their exterior surface. The growth of SiC whiskers followed the tip-growth model and Vapor-Liquid-Solid mechanism.

A large amount of multi-walled carbon nanotubes (MWCNTs) (10 vol%) were uniformly and intimately dispersed within ZrB 2 ceramic matrix using an aqueous hetero-coagulation approach followed by spark plasma sintering (SPS) at 1700 °C. Addition of MWCNTs inhibited grain growth, increased the relative density from 89% to 97%, and resulted in a composite with a fine, rather pore-free and uniform microstructure. Structural evolution of MWCNTs in the ZrB 2 matrix was further studied after SPS sintering at 1900 °C by XRD, Raman, scanning electron microscopy and TEM. When sintered at 1700 °C, the MWCNTs remained intact with their tubular one-dimensional structure though experiencing an expected radial elastic compression/buckling due to thermal misfit residual stresses. When sintered at 1900 °C, their unzipping into graphite nanoribbons was observed. For practical applications, MWCNTs content should however be lowered to prevent their agglomeration during grain growth.

Textured and untextured M n+1 AX n compounds, Ti 2 AlC and Ti 3 AlC 2 , namely MAX phases have been synthesized and examined with respect to their corrosion resistance in static supercritical water at 500 °C. The textured ceramics were obtained by hot forging process at high temperatures. Both X-ray diffraction and SEM analysis revealed well alignment of c -plane of MAX phases parallel to the hot-forging surface. Better corrosion resistance on the surface perpendicular to the hot-forged direction was verified by SEM. On the other hand, the side surfaces of the samples showed thick oxidation layers and abundant cracks. The (00 l ) faces consist of strongly bonded Ti 3 C 2 and Ti 2 C layers in Ti 3 AlC 2 and Ti 2 AlC, respectively, hence exhibit higher resistance to water corrosion. On the contrary, the side surfaces where most of weakly bonded interlayers of these hexagonal phases were exposed tend to be easily corroded especially through Al-layers. The corrosion process involved a phase transition of oxidized product, i.e. TiO 2 from anatase to rutile phase, which gave rise to the formation of cracks due to accompanied volume changes.

Corrosion behavior of self-sintered, ternary-layered titanium silicon carbide (Ti 3 SiC 2 ) and titanium aluminum carbide (Ti 3 AlC 2 ) fabricated by an in-situ solid-liquid reaction/hot pressing process was investigated by potentiodynamic polarization, potentiostatic polarization and electrochemical impedance spectroscopy in a 3.5% NaCl solution. Commercially pure titanium (Ti) was selected for comparison through XRD, XPS, SEM and EDS examinations for elucidating both the passivation behavior and corrosion mechanism of the alloys. Both Ti 3 SiC 2 and Ti 3 AlC 2 exhibited significantly superior passivation characteristics compared to Ti; Ti 3 SiC 2 also showed better corrosion resistance. The silicon/aluminum site is prone to attack, and the difference in the diffusion rate between the A-site atoms and titanium decreases the passivation ability of the MAX phase. CP titanium exhibited a lower passivation current density and did not undergo breakdown in the test potential region while two MAX phases are destroyed. Nevertheless, the corrosion resistances of Ti 3 SiC 2 and Ti 3 AlC 2 are comparable to that of CP titanium.

A series of ZrB2-ZrC-SiC composites with various SiC content from 0 to 20 vol% were prepared by reactive hot-pressing using Zr, B4C and SiC as raw materials. Self-propagating high-temperature synthesis (SHS) occurred, and ZrC grains connected each other to form a layered structure when the SiC content is below 20 vol%. The evolution of microstructure has been discussed via reaction processes. The composite with 10 vol% SiC presents the most excellent mechanical properties (four-point bending strength: 828.6±49.9 MPa, Vickers hardness: 19.9±0.2 GPa) and finest grain size (ZrB2: 1.52 µm, ZrC: 1.07 µm, SiC: 0.79 µm) among ZrB2-ZrC-SiC composites with various SiC content from 0 to 20 vol%.

SiC and HfC were used to tailor microstructures and improve properties of hot pressed HfB 2 -based ceramics. SiC and HfC particle, as the second phase, could hinder grain growth of HfB 2 . Compared with HfC, SiC grains grew slower, which was more effective to hinder grain growth of HfB 2 . The ternary ceramics HfB 2 –20vol%HfC–20vol%SiC had the finest microstructure, indicating the effect of SiC and HfC on grain growth. Several SiC clusters were found on related fracture surfaces. Large SiC clusters can cause high levels of residual stress and stress concentration, which are harmful to mechanical properties of boride-SiC ceramics. The predominant fracture mode of HfB 2 –20vol%SiC was transgranular, however, in HfB 2 –10vol%HfC–10vol%SiC and HfB 2 –20vol%HfC–20vol%SiC ceramics, the dominating fracture mode shift from transgranular to intergranular. With the incorporation of HfC, the fracture toughness of HH10S10 and HH20S20 increased to 4.34 MPa m 1/2 and 5.05 MPa m 1/2 , which were 17.3% and 26.7% higher than HS20 (3.72 MPa m 1/2 ), respectively. The flexural strengths of HfB 2 –HfC–SiC ceramics were improved to ~750 MPa, which were about 1.3 times higher than that of HfB 2 –20SiC.

ZrC–SiC composite nanopowder is synthesized via a simple sol–gel process. First, stable precursor sols are prepared from zirconium oxychloride and tetraethoxysilane respectively. Two sols are mixed and sucrose is added as the carbon source. The mixed precursor sol is converted to ZrC–SiC composite nanopowders after ceramic conversion at 1500 °C for 1 h under vacuum. The particle sizes of the as-synthesized ZrC–SiC composite powders are strongly affected by their compositions. It appears that the coexistences of silica and zirconia hinder the particle growth, leading to smaller and more uniform products. Particles of an average diameter at 180 nm are obtained when SiC volume content is around 12%. The powders are systematically characterized by XRD, SEM, TEM, TG-DSC and TEM-EDS.

The suitability of ZrC-SiC-based ceramics was assessed as candidate materials for fluoride. An immersion corrosion with ZrC-SiC composites was performed in molten FLiNaK salts (LiF-NaF-KF: 46.5/11.5/42 mol%) at 850 degrees C. Results indicated that ZrC-SiC-based ceramics were more corrosion resistant than the ZrC ceramic. ZrC-40SiC ceramic exhibited 50% weight-loss to ZrC. This carbide exhibited minimal corrosion attack because of strong resistance to dissolution in molten fluorides. Furthermore, XRD results were consistent with weight-loss and SEM results. The corrosion behavior of ZrC-SiC-based ceramics was affected by impurities, particularly oxygen content. Oxygen and/or oxygen ion participated in corrosion of ceramic. Corrosion of carbide ceramics is mainly due to C4- in ceramics. This ion is oxidized to elemental C by oxidants or impurities in the molten salt. The addition of C may inhibit corrosion of molten salts and thus improve the corrosion resistance of carbide ceramics. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Oxidation behavior of ZrB 2 –SiC ceramics containing 0, 10 and 20 vol% ZrC was studied at 1600 °C in stagnant air. The structures of oxide scales depended on ZrC content. The thickness of the SiO 2 -rich glass layer on the outermost surface decreased steeply with the addition of ZrC. One “ZrC-corroded layer”, containing ZrC x O y , SiC and ZrB 2 , was observed in ZrB 2 –SiC–ZrC. The oxidation kinetics was converted from parabolic to linear for the ceramics with 20 vol% ZrC.

Due to the extremely harsh reaction conditions, it is very difficult to control the particle morphology during the synthesis of ZrC, a well-known ultra high temperature ceramic material. In this paper, carbon encapsulated zirconia microspheres (ZrO 2 @C) are synthesized via two consecutive hydrothermal reactions in one pot. After carbothermal reduction, ZrC particles with controlled spherical morphology can be prepared. The spherical morphology suggests that carbon diffuses into zirconia during carbothermal reduction. The influences of reactant concentrations, reaction time and temperature on the microencapsulation process are also investigated. Meanwhile, as a comparison, zirconia coated carbon microspheres (C@ZrO 2 ) are also prepared, which results in ZrC particles with layered structure.

ZrC nanopowder can be synthesized by a carbothermal reduction via solution-based pathway. For the first time, an additional foaming process is introduced before high temperature pyrolysis. By foaming assisted approach, smaller particles can be synthesized. The foam formation can reduce the mutual contact of the particles by separating them physically, thus to hinder the particle growth and coalescence.

In this work, we present an easy approach to prepare zirconium carbide (ZrC) nanofibers by electrospinning technique. Polyzirconoxane (PZO) is used as the zirconium source. Polyacrylonitrile (PAN) is used as the spinning aid and primary carbon source while sucrose is used as the additional carbon source. The as-spun green fibers are converted to ZrC nanofibers after carbothermal reduction at 1400 °C. Core–shell structured nanofibers are obtained when using PAN only as the carbon source, while more homogeneous nanofibers which are composed of uniformly nano-sized ZrC crystals are obtained when using PAN and sucrose as the carbon source. The carbon source has a strong influence on the fiber morphology by affecting the kinetics of the carbothermal reduction reaction. The obtained ZrC fibers are characterized by XRD, SEM and TEM.

AlB2 is usedasasinteringadditivetodensifyZrB2 ceramics. TheAlB2 phase playsapositiveroleinthedensification processofZrB2 ceramics byremovingtheoxygenimpurityandliquidphasesintering.With5vol%AlB2 addition, bothmechanicalpropertiesandoxidation resistance canbeimprovedcomparedwithmonolithicZrB2 ceramics.

An ultra-flexible hexagonal boron nitride (h-BN)/ polymethyl methacrylate (PMMA) composite with extra-large elongation at rupture was prepared by introducing micron-sized surface-modified h-BN platelets into a PMMA matrix. At 32 wt% loading of surface-modified h-BN, the composite can be folded freely and has an elongation at rupture of 68%, which is over 30 times larger than that of PMMA. Such results are due to the lubricant nature and the sliding of h-BN platelets.

ZrB 2 –SiC–ZrN composites were fabricated by reactive hot pressing using Zr, Si 3 N 4 , and B 4 C powders as starting materials. Sintering was conducted at temperatures of 1800 to 2000 °C under a load of 20 MPa in Ar atmosphere. The composite was densified at 2000 °C. The in situ formed BN flakes which distributed uniformly at the grain boundaries were identified by X-ray diffraction and scanning electron microscopy. The formation of h- BN phase and its effect on the mechanical properties of the composite are discussed.

ZrB 2 -20 vol% SiC composites added with 3 vol% Yb 2 O 3 were hot-pressed at 1900 °C, and then heat-treated at 2000 °C in argon. The effect of heat treatment on phase composition, microstructure, and mechanical properties of the composites was investigated. Results showed that the high temperature heat treatment led to the depletion of Yb-containing phases by decomposition or evaporation, the formation of some pores at multiple-grain junctions, significant microstructural coarsening with the increase of ZrB 2 grain size from ∼1.9 μm to ∼4.8 μm, and the change of fracture mode from mixed inter/transgranular to fully transgranular. In addition, Vickers' hardness, fracture toughness and flexural strength were severely degraded after heat treatment.

Fully dense ZrB 2 –20vol% SiC (ZS) ceramics were fabricated by self-made high purity ZrB 2 and SiC powders hot-pressing at 1900 °C/30 MPa. Their properties were compared with other ZS ceramics with the same nominal composition, but prepared by pressureless sintering, using low purity commercially available powders. The hot-pressed composites show superior hardness (17.82 GPa), elastic modulus (504 GPa), flexure strength (462 MPa at 25 °C and 463 MPa at 1300 °C in air), electrical conductivity (7.68×10 −6 Ω −1 m −1 at 25 °C), thermal conductivity (93.7 W/(m K) at 25 °C, 77.2 W/(m K) at 1200 °C) and a lower average thermal expansion coefficient (6.66×10 −6 K −1 from 25 to 2000 °C) compared to the pressureless sintered materials. The improvement in these properties could be primarily explained by the decrease of impurities and second phase levels in hot pressed samples.

Using elemental Zr, B, Mo and Si as raw materials, partially textured ZrB2–MoSi2 composites with in situ platelet ZrB2 grains were fabricated by reactive hot pressing. Synthesized by a combustion reaction, the in situ formed ZrB2 phase had unique characteristics to grow up to platelet grains, on the surroundings of Mo–Si–B ternary liquid phase at high temperature. Mechanical properties were dependent on grain size and aspect ratio of the ZrB2 platelets. The rotation and realignment of the platelet ZrB2 grains under hot pressing led to a partially textured microstructure, which showed anisotropic mechanical properties on different directions of the as-sintered samples. A roadmap of the reaction process, microstructure evolution and texture formation was given to describe the preparation of partially textured ZrB2–MoSi2 composites by reactive hot pressing. # 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

The influence of the pressure loading cycle on spark plasma sintered ZrB2–SiC–Yb2O3 ceramics was investigated. Spark plasma sintering consists of three stages, i.e., a heating-up, isothermal and cooling stage. Applying the load during isothermal stage allowed evaporation of volatile species. Applying the load during the heating-up and isothermal stages resulted in the entrapment of volatile species and the formation of nanometer-sized spherical SiO2 particles. The presence of SiO2 particles led to a large thermal residual stress, and improved fracture toughness in the ZrB2–SiC–Yb2O3 ceramics.