Spark Plasma Sintering of Superhard B4C–ZrB2 Ceramics by Carbide Boronizing

A carbide boronizing method was first developed to produce dense boron carbide‐ zirconium diboride (“B₄C”–ZrB₂) composites from zirconium carbide (ZrC) and amorphous boron powders (B) by Spark Plasma Sintering at 1800°C–2000°C. The stoichiometry of “B₄C” 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. B₈C–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 B₈C–ZrB₂ ceramics was considered to be the formation of nano‐sized ZrB₂ grains.     

Electrochemical corrosion behavior and surface modification of ZrB2 in hydrofluoric acid aqueous solution

The electrochemical corrosion behavior in an open system of zirconium diboride (ZrB₂) was studied in hydrofluoric acid aqueous solution at room temperature. Based on analysis of the intermediate products of corrosion, a corrosion mechanism for ZrB₂ is suggested. The morphology evolution of anodic ZrB₂ is investigated. The density of the hexagonal‐pyramid grain array is dictated by current density, and the time dictates the length of the hexagonal pyramid grains. The microstructure of the surface can be controlled by the current density, which demonstrates that the surface wettability of ZrB₂ can be modified by tailoring the current density.     

Volatility diagram of ZrB2‐SiC‐ZrC system and experimental validation

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.     

Mechanical Behavior and Applications of Plasma Arc Welded Ceramics

Zirconium diboride and zirconium carbide‐based ceramics were joined by plasma arc welding to demonstrate the versatility of this technique. A parent material composition consisting of ZrB₂ with 20 vol% ZrC was hot pressed to near full density, sectioned to produce specimens for welding, and welded together to produce billets for mechanical property studies. The four‐point flexure strength of the parent material was ~660 MPa, while the strength of the welded specimens ranged from ~140 to ~250 MPa. Microstructural analysis revealed that decreased strength in the welded specimens was caused by volume flaws, microcracking of large ZrB₂ grains (up to 1 mm in length), and residual tensile stresses that developed at the surface of weld pools during cooling. The versatility of plasma arc welding was demonstrated by joining of ZrC‐based ceramics and fabricating three ZrB₂–ZrC components for potential applications, including a high‐temperature electrical contact, an ultra‐high‐temperature thermocouple, and a wedge that was a notional wing leading edge. These three applications demonstrated the ability to join ceramics to a refractory metal, fabricate a chemically inert high‐temperature thermocouple, and produce complex shapes for aerospace applications.     

Effect of tantalum solid solution additions on the mechanical behavior of ZrB2

Mechanical properties and microstructure were compared for zirconium diboride and two zirconium diboride solid solutions containing 3 and 6 at% tantalum diboride. X-ray diffraction indicated that the ceramics were nearly phase-pure and that tantalum dissolved into the ZrB₂ lattice to form (Zr,Ta)B₂ solid solutions. Microstructural analysis indicated that samples achieved nearly full relative density with average grain sizes that ranged from 3?5 μm. The three compositions had similar values of Young’s modulus (510?531 GPa), shear modulus (225?228 GPa), Vickers hardness (15.2–16.4 GPa), and flexural strength (391?452 MPa). Fracture toughness ranged from 2.6 to 3.7 MPa m^1/2 and with increasing tantalum content, the fracture mode changed from predominantly intergranular to predominantly transgranular. Diboride solid solution materials had comparable properties to the single metal diboride, but differences in microstructure, secondary phases, and strain state among the three ceramics partially obscured the actual effects of the solid solution on fracture behavior.     

Design of a ductile carbon nanofiber/ZrB2 nanohybrid film with entanglement structure fabricated by electrostatic spinning

Polyacrylonitrile (PAN), a kind of multi-purpose man-made polymer material, has been widely used in various products, including carbon fiber precursor fiber manufacturing. Organic/inorganic nanocomposites can provide precursor material with unique properties due to optimal structural design. Herein, PAN based carbon nanofiber (CNF) coated zirconium borate (ZrB₂) particles fiber film was prepared via electrostatic spinning strategy. Crosslinking network between carbon atoms formed at 280 °C due to long chain PAN molecules, which underwent pyrolysis at 800–1200 °C. Scanning electron microscope analysis showed that ductile CNF/ZrB₂ hybrid material with entanglement structure was successfully fabricated. Phase composition of the materials was analyzed by X-ray diffractometer, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy, which confirmed the presence of carbon atoms in the materials. Entanglement structure between CNFs and ZrB₂ enhanced tensile performance of nanohybrid film, in which CNF film with 25% ZrB₂ content exhibited optimal mechanical properties. The design of nanohybrid structure provides facile and universal approach for exploration of organic/inorganic nanocomposites with controlled structures and excellent mechanical properties.     

Structural Influence on the Thermal Conversion of Self‐Catalyzed HfB2/ZrB2 Sol–Gel Precursors by Rapid Ultrasonication of Oxychloride Hydrates

Sol–gel precursors to HfB₂ and ZrB₂ are processed by high‐energy ultrasonication of Hf,Zr oxychloride hydrates, triethyl borate, and phenolic resin to form precipitate‐free sols that turn into stable gels with no catalyst addition. Both precursor concentration and structure (a sol or a gel) are found to influence the synthesis of the diboride phase at high temperature. Decreasing sol concentration increases powder surface area from 3.6 to 6.8 m²/g, whereas heat‐treating a gel leads to residual oxides and carbides. Particles are either fine spherical particles, unique elongated rods, and/or platelets, indicating particle growth with directional coarsening. Investigation of the conversion process to ZrB₂ indicates that a multistep reaction is likely taking place with: (1) ZrC formation, (2) ZrC reacts with B₂O₃ or ZrC reacts with B₂O₃ and C to form ZrB₂. At low temperatures, ZrC formation is limiting, while at higher temperatures the reaction of ZrC to ZrB₂ becomes rate limiting. ZrC is found to be a direct reducing agent for B₂O₃ at low temperature (~1200°C) to form ZrB₂ and ZrO₂, whereas at high temperatures (~1500°C) it reacts with B₂O₃ and C to form pure ZrB₂.     

Potential-current characteristics in SiC/ZrB2 composite ceramics

ZrB₂/SiC composite ceramics were fabricated to improve the electrical conductive properties of SiC matrix. The debinding and sintering temperatures were determined by computation of Gibbs free energy. As a result, all the samples have the relative density above 99%, and have excellent mechanical and electrical properties. The effects of ZrB₂ content on the microstructure, mechanical and electrical properties were systematically studied. With increasing ZrB₂ content, as-prepared composites show great improvement in their mechanical properties. Importantly, the introduction of ZrB₂ weakened varistor nonlinear characteristic of composite and reduced its resistivity. The reason is the evolution of grain boundary in conductive paths. The sharp decrease of resistivity indicates the formation of percolation paths. The percolation threshold at 1?mA?cm^?2 obtained via percolation model is 10.7963?vol% (19.7098?wt%) ZrB₂. This value is much less than conventional composites, because the percolation path originates from grain boundary breakdown other than continuous conductor chains.     

Fabrication and properties of CNT/Ni/Y/ZrB2 nanocomposites reinforced in situ

Carbon nanotubes (CNTs) were synthesized in situ by chemical vapor deposition of methane over nano‐ZrB₂ matrix using Ni/Y catalysts. Well‐grown CNTs with tangled and long bodies and mainly composed of well‐crystallized graphite were obtained when the Ni content reaches 10 wt%. The CNT/ZrB₂ nanocomposites obtained by spark plasma sintering at 1400°C exhibited full density and optimal mechanical properties. The flexural strength and fracture toughness of the nanocomposites were 1184 ± 52 MPa and 10.8 ± 0.3 MPa·m^1/2, respectively. Results indicated that the dispersion of CNTs in situ can improve composite performance, rendering the mechanical properties of the CNT/ZrB₂ nanocomposites synthesized in situ considerably superior to those of monolithic ZrB₂ nanoceramics and CNT/ZrB₂ nanocomposites synthesized using the traditional method. The toughening mechanisms included crack deflection, crack bridging, CNT debonding, pull‐out, and fracture.     

Reactive Hot Pressing and Properties of Zr1−xTixB2–ZrC Composites

Reactive hot pressing was used to prepare Zr_1?xTi_xB₂–ZrC composites with advantageous microstructure and mechanical properties from ZrB₂–TiC powders. The reaction mechanisms and the effects of different levels of TiC on the physical and mechanical properties of the resulting composite were explored in detail and compared to conventionally hot‐pressed ZrB₂ and ZrB₂–ZrC. Incorporation of 10 to 30 vol% TiC enabled full densification and restrained grain growth, reducing the final average grain size from 5.6 μm in pure ZrB₂ to a minimum of 1.4 μm in samples with 30 vol% TiC. The flexural strengths and hardnesses of the composites sintered with TiC were consequently greater than the conventionally processed ZrB₂–ZrC materials, increasing from 440 MPa and 17.4 GPa to a maximum of 670 MPa and 24.2 GPa at 10 vol% TiC. However, despite a decrease in the total average grain size, the flexural strength at higher TiC levels was limited by an increase in ZrC grain growth, which was observed to determine the flexural strength of the reaction sintered composites similar to the case of ZrB₂–SiC.     

In situ synthesis,physical and mechanical properties of ZrB2–ZrC–WB composites

In this study, two composition ZrB₂–ZrC–WB composites were synthesized by reactive hot-pressing of Zr + B₄C + WC powder mixtures at 1900 °C. The microstructure of the resulting composites was characterized by a combination of scanning electron microscopy and X-ray diffraction. It is seen that highly-dense ZrB₂–ZrC–WB composites with a homogenous fine-microstructure were obtained after the sintering. The mechanical behavior of the composites was evaluated using by testing under four-point bend testing at room and high temperatures. The results show that the high-temperature strength of the ZrB₂–ZrC–WB composites was substantially improved, compared to ZrB₂–ZrC-based composites without WB. In addition, the elastic properties, electrical conductivity, hardness and fracture toughness of the composites were measured at room temperature. The results reveal that these properties were comparable to those of ZrB₂–ZrC-based composites without WB.     

Combined role of SiC particles and SiC whiskers on the characteristics of spark plasma sintered ZrB2 ceramics

In this research work, the effects of silicon carbide (SiC) as the most important reinforcement phase on the densification percentage and mechanical characteristics of zirconium diboride (ZrB₂)-matrix composites were studied. In this way, a monolithic ZrB₂ ceramic (as the baseline) and three ZrB₂ matrix specimens each of which contains 25 vol% SiC as reinforcement in various morphologies (SiC particulates, SiC whiskers, and a mixture of SiC particulates/SiC whiskers), have been processed through spark plasma sintering (SPS) technology. The sintering parameters were 1900 °C as sintering temperature, 7 min as the dwell time, and 40 MPa as external pressure in vacuum conditions. After spark plasma sintering, a relative density of ~96% was obtained (using the Archimedes principles and mixture rule for evaluation of relative density) for the unreinforced ZrB₂ specimen, but the porosity of composites containing SiC approached zero. Also, the assessment of sintered materials mechanical properties has shown that the existence of silicon carbide in ZrB₂ matrix ceramics results in fracture toughness and microhardness improvement, compared to those measured for the monolithic one. The simultaneous addition of silicon carbide particulates (SiC_p) and whiskers (SiC_w) showed a synergistic effect on the enhancement of mechanical performance of ZrB₂-based composites.     

Elevated temperature thermal properties of ZrB2-B4C ceramics

The elevated temperature thermal properties of zirconium diboride ceramics containing boron carbide additions of up to 15 vol% were investigated using a combined experimental and modeling approach. The addition of B₄C led to a decrease in the ZrB₂ grain size from 22 µm for nominally pure ZrB₂ to 5.4 µm for ZrB₂ containing 15 vol% B₄C. The measured room temperature thermal conductivity decreased from 93 W/m·K for nominally pure ZrB₂ to 80 W/m·K for ZrB₂ containing 15 vol% B₄C. The thermal conductivity also decreased as temperature increased. For nominally pure ZrB₂, the thermal conductivity was 67 W/m·K at 2000 °C compared to 55 W/m·K for ZrB₂ containing 15 vol% B₄C. A model was developed to describe the effects of grain size and the second phase additions on thermal conductivity from room temperature to 2000 °C. Differences between model predictions and measured values were less than 2 W/m·K at 25 °C for nominally pure ZrB₂ and less than 6 W/m·K when 15 vol% B₄C was added.     

Establishing microstructure-mechanical property correlation in ZrB2-based ultra-high temperature ceramic composites

The current work focuses on enhancing the flexural strength and fracture toughness of zirconium diboride (ZrB₂) reinforced with silicon carbide (SiC) and carbon nanotubes (CNT). The flexural strength has shown to increase by ~ 1.2 times from 322.8 MPa (for ZrB₂) to 390.7 MPa and fracture toughness up to 3 times from 3.2 MPam^0.5 (for ZrB₂) to 9.5 MPam^0.5 with the synergistic addition of both SiC and CNT in ZrB₂ matrix through energy dissipating mechanisms such as deflection, branching and strong interfacial bonding evidenced from the transmission electron microscopy (TEM). A modified fractal model is used to evaluate the fracture toughness and delineate the contribution of residual stresses, and reinforcements (SiC and CNT) in enhancing the fracture toughness. Interfacial bonding, in terms of a debonding factor, was also evaluated by theoretically predicting the elastic modulus and then correlated with the microstructure along with other mechanical properties of ZrB₂-SiC-CNT composites.     

Grain boundary strengthening in ZrB2 by segregation of W: Atomistic simulations with deep learning potential

Interaction between grain boundaries and impurities usually leads to significant altering of material properties. Understanding the composition-structure-property relationship of grain boundaries is a key avenue for tailoring and designing high performance materials. In this work, we studied segregation of W into ZrB₂ grain boundaries by a hybrid method combining Monte Carlo (MC) and molecular dynamics (MD), and examined the effects of segregation on grain boundary strengths by MD tensile testing with a fitted machine learning potential. It is found that W prefers grain boundary sites with local compression strains due to its smaller size compared to Zr. Rich segregation patterns (including monolayer, off-center bilayer, and other complex patterns); segregation induced grain boundary structure reconstruction; and order-disorder like segregation pattern transformation are discovered. Strong segregation tendency of W into ZrB₂ grain boundaries and significant improvements on grain boundary strengths are certified, which guarantees outstanding high temperature performance of ZrB₂-based UHTCs.     

Synthesis of ZrB2 Powders from ZrO2, BN,and C

Submicrometer (200–800 nm) ZrB₂ powders have been successfully prepared via a new ZrO₂–C–BN precursor powder system at a relatively low temperature (1550°C) for 1.5 h. As a moderate cost boron source, BN has a well‐defined stoichiometry and low impurity. Both thermodynamic and experimental results indicated that ZrC was formed below 1300°C, a temperature required for ZrB₂ formation. Moreover, the reaction of ZrC–BN mixture at 1300°C indicated that the ZrC acted as an effective direct reducing reagent for BN to form ZrB₂, indicating that the pathway involving the formation of intermediate phase ZrC determined the formation mechanism.     

Effect of ZrB2 content on the densification,microstructure, and mechanical properties of ZrC-SiC ceramics

ZrC ceramics containing 30 vol% SiC-ZrB₂ were produced by high-energy ball milling and reactive hot pressing. The effects of ZrB₂ content on the densification, microstructure, and mechanical properties of ceramics were investigated. Fully dense ceramics were achieved as ZrB₂ content increased to 10 and 15 vol%. The addition of ZrB₂ suppressed grain growth and promoted dispersion of the SiC particles, resulting in fine and homogeneous microstructures. Vickers hardness increased from 23.0 ± 0.5 GPa to 23.9 ± 0.5 GPa and Young’s modulus increased from 430 ± 3 GPa to 455 ± 3 GPa as ZrB₂ content increased from 0 to 15 vol%. The increases were attributed to a combination of the higher relative density of ceramics with higher ZrB₂ content and the higher Young’s modulus and hardness of ZrB₂ compared to ZrC. Indentation fracture toughness increased from 2.6 ± 0.2 MPa⋅m^1/2 to 3.3 ± 0.1 MPa⋅m^1/2 as ZrB₂ content increased from 0 to 15 vol% due to the increase in crack deflection by the uniformly dispersed SiC particles. Compared to binary ZrC-SiC ceramics, ternary ZrC-SiC-ZrB₂ ceramics with finer microstructure and higher relative densities were achieved by the addition of ZrB₂ particles.     

Characterization investigations of ZrB2/ZrC ceramic powders synthesized by mechanical alloying of elemental Zr,B and C blends

ZrB₂/ZrC ceramic powders were fabricated by mechanical alloying (MA) of zirconium (Zr), amorphous boron (B) and graphite (C) powder blends prepared in the mole ratios of Zr/B/C: 1/1/1, 1/2/1, 1/1/2, 1/2/2 and 2/2/1. MA runs were carried out in a vibratory ball mill using hardened steel vial/balls. The effects of Zr/B/C mole ratios and milling duration on the formation and microstructure of ZrB₂/ZrC ceramic powders were examined. Gibbs free energy change-temperature relations of the reactions and moles of the products were interpreted by thermochemical software. Zr/B/C: 1/1/1, 1/2/1, 1/1/2 and 1/2/2 powder blends MA’d for 2 and 3 h contain unreacted Zr and C, ZrB₂, ZrC and B₄C particles. Synthesis of ZrB₂/ZrC ceramic powders was completely accomplished after MA of Zr/B/C: 2/2/1 powder blend for 2 h. ZrC and ZrB₂ particles were obtained ranging in size between 50 and 250 nm in the presence of FeB contamination (<1 wt.%).     

Microstructure and Anisotropic Properties of Textured ZrB2 and ZrB2–MoSi2 Ceramics Prepared by Strong Magnetic Field Alignment

Dense, highly textured ZrB₂ and ZrB₂–MoSi₂ ceramics were fabricated via a strong magnetic field alignment method followed by spark plasma sintering. Unlike with the previous studies, which only focused on the alignment of single‐phase particles, both ZrB₂ and MoSi₂, which exhibited a magnetic anisotropy, have been aligned in this study. The alignment of MoSi₂ in the same direction of ZrB₂ enhanced the degree of orientation of ZrB₂, decreased the grain size, but increased the aspect ratio of the platelet ZrB₂ grains. The microstructure and anisotropic mechanical properties as well as the oxidation resistance in different directions were discussed.     

Preparation of highly porous ZrB2/ZrC/SiC composite monoliths using liquid precursors via direct drying process

We developed a simple liquid precursor method for the syntheses of porous ZrB₂/ZrC/SiC composite monoliths. Furfuryl alcohol (FA), zirconium n-butoxide, tetraethyl orthosilicate and boric acid are used as the raw materials. By combining the polymerization of FA and gelation of inorganic sols, porous hybrid monoliths are prepared by direct drying the wet gels. The inorganic and organic polymers possibly form interpenetrated network which provides the robustness for the wet gel to withstand the severe changes during dessication. When heat-treated at 1600 °C, hybrid gels are converted into porous ZrB₂/ZrC/SiC monoliths. The microstructure of the ZrB₂/ZrC/SiC monoliths can be easily tailored by controlling the synthesis conditions. The porosities of the ZrB₂/ZrC/SiC monoliths can be tuned around 74.3–81.6%, while the average pore diameters can be tuned ranging from 1.0 to 8.5 μm with pretty narrow distribution. The compressive strengths of such highly porous ceramics are in the range of 1.2–1.9 MPa.