Effect of ZrB2 powders on densification,microstructure, mechanical properties and thermal conductivity of ZrB2-SiC ceramics

With the view to improve the densification behaviour and mechanical properties of ZrB₂-SiC ceramics, three synthesis routes were investigated for the production of ZrB₂, prior to the fabrication of ZrB₂-20 vol. % SiC via spark plasma sintering (SPS). Two borothermal reduction routes, modified with a water-washing stage (BRW) and partial solid solution of Ti (BRS), were utilised, alongside a boro/carbothermal mechanism (BRCR) were utilised to synthesise ZrB₂, as a precursor material for the production of ZrB₂-SiC. It was determined that reduction in the primary ZrB₂ particle size, alongside a diminished oxygen content, was capable of improving densification. ZrB₂-SiC ceramics, with ZrB₂ derived from BRW synthesis, exhibited a favorable combination of high relative density (98.6%), promoting a marked increase in Vickers hardness (21.4 ± 1.7 GPa) and improved thermal conductivity (68.7 W·m^-1K^-1).     

Synthesis of rod-like ZrB2 powders

Rod-like ZrB₂ powders were synthesised at 1500°C in vacuum by boro/carbothermal reduction using ZrO₂, B₄C and graphite as the starting materials. During the heating process, the ZrB₂ grains primarily grow along the c axis to form a rod-like morphology without any heterogeneous catalyst. The final products are pure rod-like ZrB₂ particles, which are thought to be promising starting powders to prepare high performance ultrahigh temperature ceramics with unique microstructures such as textured one through tape casting process.     

Selection principle of the synthetic route for fabrication of HfB2 and HfB2-SiC ceramics

Densification, microstructure, and mechanical properties of spark plasma sintered HfB₂ and HfB₂-SiC ceramics using HfB₂ powders from borothermal reduction and boro/carbothermal reduction were investigated and compared. It was found that HfB₂ceramics obtained by boro/carbothermal reduction exhibited a significantly higher sinterability compared to that by borothermal reduction. Inversely, HfB₂-SiC ceramics obtained by borothermal reduction exhibited a refined microstructure and better mechanical properties (Vickers hardness: 23.60 ± 2.43 GPa; fracture toughness: 5.89 ± 0.30 MPa.m^1/2) than that by boro/carbothermal reduction. These results indicated that optimal fabrication of HfB₂-based ceramics could be achieved by the selection of synthetic route of HfB₂ powders.     

Controllable preparation of porous ZrB2–SiC ceramics via using KCl space holders

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 ZrB₂–SiC ceramics with controllable pore structure, tunable compressive strength and thermal conductivity. The as-prepared porous ZrB₂–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 ZrB₂–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 ZrB₂–SiC ceramics was examined and found to be consistent with the classical model for porous materials. The poring mechanism of porous ZrB₂–SiC samples via adding pore-forming agent combined with partial sintering was also preliminary illustrated.     

In-situ synthesis of ultra-fine ZrB2–ZrC–SiC nanopowders by sol-gel method

ZrB₂–ZrC–SiC nanopowders with uniform phase distribution were prepared from cost-effective ZrOCl₂·8H₂O by a simple sol-gel method. The synthesis route, ceramization mechanism and morphology evolution of the nanopowders were investigated. ZrB₂–ZrC–SiC ceramic precursor can be successfully obtained through hydrolysis and condensation reactions between the raw materials. Pyrolysis of the precursor was completed at 650 °C, and it produced ZrO₂, SiO₂, B₂O₃ and amorphous carbon with a yield of 39% at 1300 °C. By heat-treated at 1500 °C for 2 h, highly crystallized ZrB₂–ZrC–SiC ceramics with narrow size distribution were obtained. With the holding time of 2 h, both the crystal size and the particle size can be refined. Further prolonging the holding time can lead to serious particles coarsening. Studies on the microstructure evolution of the generated carbon during the ceramic conversion demonstrates the negative effect of the ceramic formation on the structure order improvement of the carbon, due to the large amount of defects generated in it by the boro/carbothermal reduction reactions.     

Oxidation resistance of ZrB2 and ZrB2-SiC ultrafine powders synthesized by a combined sol-gel and boro/carbothermal reduction method

ZrB₂ and ZrB₂-SiC powders were prepared by a combined sol-gel and boro/carbothermal reduction method, and their oxidation kinetics was studied by using a non-isothermal thermogravimetric technique. The results showed that the Mample power law (n=1) was the most probable mechanism function, and the incorporation of SiC into ZrB₂ greatly enhance the latter's oxidation resistance. The oxidation activation energy values of phase pure ZrB₂ and ZrB₂-SiC powders were respectively 249 and 308 kJ/mol.     

First-principles prediction,fabrication and characterization of (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 high-entropy borides

The microstructure and mechanical properties of (Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)B₂ high-entropy boride (HEB) were first predicted by first-principles calculations combined with virtual crystal approximation (VCA). The results verified the suitability of VCA scheme in HEB studying. Besides, single-phase (Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)B₂ ceramics were successfully fabricated using boro/carbothermal reduction (BCTR) method and subsequent spark plasma sintering (SPS); furthermore, the effects of different amounts of B₄C on microstructure and mechanical properties were evaluated. Due to the addition of B₄C and C, all samples formed single-phase solid solutions after SPS. When the excess amount of B₄C increased to 5 wt%, the sample with fine grains exhibited superior comprehensive properties with the hardness of 18.1 ± 1.0 GPa, flexural strength of 376 ± 25 MPa, and fracture toughness of 4.70 ± 0.27 MPa m^1/2. Nonetheless, 10 wt% excess of B₄C coarsened the grains and decreased the strength of the ceramic. Moreover, the nanohardness (34.5–36.9 GPa) and Young's modulus (519–571 GPa) values with different B₄C contents just showed a slight difference and were within ranges commonly observed in high-entropy diboride ceramics.     

Highly-efficient preparation of anisotropic ZrB2–SiC powders and dense ceramics with outstanding mechanical properties

Highly-dense ZrB₂–SiC ceramics with excellent mechanical properties including Vickers hardness of 24.5 GPa and fracture toughness of 4.8 MPa/m^1/2 were successfully prepared, by spark plasma sintering of the raw powders synthesized by a novel molten-salt and microwave co-assisted boro/carbothermal reduction (MSM-BCTR) method. Compared with the processing conditions required for synthesizing ZrB₂–SiC by conventional reduction method, the present MSM-BCTR method possessed a variety of significant merits including the smaller material cost, lower processing temperature (1200°C), and remarkably higher efficiency (soaking time as short as 20 minutes). More importantly, the ZrB₂–SiC powders, resultant from MSM-BCTR treatment, were verified to have single-crystalline nature and uniform well-grown anisotropic morphologies (rod-like ZrB₂ and sheet-like SiC) as well as great potential in promoting the mechanical properties of their bulk counterparts. This great achievement was mainly ascribed to the specific MSM-BCTR conditions characterized by microwave heating and molten-salt medium.     

Final-stage densification kinetics of direct current–sintered ZrB2

Final-stage sintering was analyzed for nominally phase pure zirconium diboride synthesized by borothermal reduction of high-purity ZrO₂. Analysis was conducted on ZrB₂ ceramics with relative densities greater than 90% using the Nabarro–Herring stress–directed vacancy diffusion model. Temperatures of 1900°C or above and an applied uniaxial pressure of 50 MPa were required to fully densify ZrB₂ ceramics by direct current sintering. Ram travel data were collected and used to determine the relative density of the specimens during sintering. Specimens sintered between 1900 and 2100°C achieved relative densities greater than 97%, whereas specimens sintered below 1900°C failed to reach the final stage of sintering. The average grain size ranged from 1.0 to 14.7 μm. The activation energy was calculated from the slope of an Arrhenius plot that used the Kalish equation. The activation energy was 162 ± 34 kJ/mol, which is consistent with the activation energy for dislocation movement in ZrB₂. The diffusion coefficients for dislocation motion that controls densification were 5.1 × 10⁻⁶ cm²/s at 1900°C and 5.1 × 10⁻⁵ cm²/s at 2100°C, as calculated from activation energy and average grain sizes. This study provides evidence that the dominant mechanism for final-stage sintering of ZrB₂ ceramics is dislocation motion.     

High temperature strength of hot pressed ZrB2–20 vol% SiC ceramics based on ZrB2 starting powders prepared by different carbo/boro-thermal reduction routes

ZrB₂–20 vol% SiC (ZS) ceramics based on ZrB₂ starting powders obtained by different boro/carbo-thermal reductions involving ZrO₂ + B₄C, ZrO₂ + B₄C + C, and ZrO₂ + B, were fully densified by hot pressing at 1900–2000 °C. The flexural strength of these ZS ceramics was measured from room temperature up to 1600 °C. At 1600 °C, the flexural strength of the ceramics is 460 ± 31, 471 ± 32 and 345 ± 11 MPa, respectively. The evolution of the strength as function of temperature is explained in terms of the differences in oxygen content, nature of fracture, grain sizes, grain boundary phases and microstructural defects.     

Powder characteristics,sinterability, and mechanical properties of TiB2 prepared by three reduction methods

Reaction processes, powder characteristics, sinterability, and mechanical properties of TiB₂ prepared by borothermal reduction, B₄C reduction, and boro/carbothermal reduction were compared. Results showed that TiBO₃ and Ti₂O₃ as the intermediate phases existed in the three reduction processes. The temperature where TiB₂ became major phase was lowest during borothermal reduction, resulting in the finest TiB₂ particle size, in contrast to the other two methods, especially boro/carbothermal reduction. However, TiB₂ powders prepared by B₄C reduction and boro/carbothermal reduction after pressureless sintering at 1800°C for 2 hours showed the relatively higher sinterability, due to the lower oxygen content and higher carbon content. Finally, using 5 wt% Ni as sintering additive, hot-pressed TiB₂ ceramics from B₄C reduction demonstrated higher densification, more fine-grained microstructure and higher mechanical properties, due to the better balance of oxygen/carbon content.     

Preparation and characterization of porous,biomorphic SiC ceramic with hybrid pore structure

Bio-carbon template (charcoal) was prepared by carbonizing pine wood at 1200 °C under vacuum, and was impregnated with phenolic resin/SiO₂ sol mixture by vacuum/pressure processing. Porous SiC ceramics with hybrid pore structure, a combination of tubular pores and network SiC struts in the tubular pores, were fabricated via sol–gel conversion, carbonization and carbothermal reduction reaction at elevated temperatures in Ar atmosphere. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) were employed to characterize the phase identification and microstructural changes during the C/SiO₂ composites-to-porous SiC ceramic conversion. Experimental results show that the density of C/SiO₂ composite increases with the number of impregnation procedure, and increases from 0.32 g cm⁻³ of pine-derived charcoal to 1.5 g cm⁻³ of C/SiO₂ composite after the sixth impregnation. The conversion degree of charcoal to porous SiC ceramic increases as reaction time is lengthened. The resulting SiC ceramic consists of β-SiC with a small amount of α-SiC. The conversion from pine charcoal to porous SiC ceramic with hybrid pore structure improves bending strength from 16.4 to 42.2 MPa, and decreases porosity from 76.1% to 48.3%.     

Low‐Temperature Rapid Synthesis of Rod‐Like ZrB2 Powders by Molten‐Salt and Microwave Co‐Assisted Carbothermal Reduction

A novel molten‐salt and microwave coassisted carbothermal reduction (termed as MSM‐CTR) method was developed to prepare ZrB₂ powders from raw materials of ZrO₂, B₄C, and amorphous carbon. The results indicated that the carbothermal reduction reaction for synthesizing ZrB₂ was initiated at the temperature as low as 1150°C, and phase pure ZrB₂ powders were obtained after only 20 min at 1200°C, which were significantly milder than that of the conventional CTR method as well as the modified CTR method even using active metal as additional reducing agents. More interestingly, the as‐obtained ZrB₂ powders consisted of well‐defined single‐crystalline nanorods, which had diameters of 40–80 nm and high aspect ratios of >10. These results demonstrated that the MSM‐CTR is a simple and efficient route for preparation of high‐quality ZrB₂ powders.     

Effect of SiC addition on the formation of ZrB2 through reaction of ZrO2, boric acid (H3BO3) and carbon black

ZrB₂-SiC powders with different amounts of SiC (10–30 wt%) were in-situ synthesized at 1600 °C for 90 min in Ar atmosphere. Effects of SiC addition on the formation of ZrB₂ via carbothermal reduction of ZrO₂, H₃BO₃ and carbon black were investigated. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and transmission electron microscope (TEM). The grain size of ZrB₂ in final powders decreased with adding SiC. Columnar ZrB₂ and granular SiC were combined interactively when the SiC content was 25 wt%. Layer-like hexagonal SiC was obtained in the product containing 30 wt% SiC, whereas the ZrB₂ grain growth was strongly inhibited. Furthermore, the growth mechanisms of ZrB₂ and SiC were studied.     

Oxidation of ZrB2 Nanoparticles at High Temperature under Low Oxygen Pressure

The oxidation of ZrB₂ nanoparticles was observed at high temperature of 1500°C under low oxygen partial pressure of 5 × 10^?2 Pa by an environmental transmission electron microscope. The results demonstrate that the oxidation starts on the surface of ZrB₂ nanoparticles with decomposition of ZrB₂ into ZrO₂ and B₂O₃. The nucleation and growth of ZrO₂ on the surface of ZrB₂ proceed with B₂O₃ being evaporated.     

Aqueous Gelcasting and Pressureless Sintering of Zirconium Diboride Ceramics

Dense (97.3%) zirconium diboride (ZrB₂) ceramics were obtained via gelcasting and pressureless sintering. Four wt% B₄C was used as sintering aid. ZrB₂, SiC, and B₄C can codisperse well in the alkaline region, using a polyacrylate dispersant. Compared with monolithic ZrB₂ (Z), the mechanical properties of ZrB₂‐SiC (ZS) were enhanced. The Vickers hardness and fracture toughness of ZS were (13.1 ± 0.6) GPa and (2.5 ± 0.4) MPa m^1/2, respectively.     

Synthesis of ZrB2–ZrC hybrid powders via boro-carbothermal reduction of ZrO2 by B4C and carbon black

ZrB₂–ZrC hybrid powders were synthesized by a novel two-step reduction on basis of ZrO₂ + B₄C + C→ ZrC + ZrB₂ + CO reaction in Ar atmosphere, using ZrO₂, B₄C, and carbon black powders as starting materials. Thermodynamics of relevant reactions were evaluated. Effects of excess additions of B₄C and C on phase constituents were investigated. Morphology and chemistry of the powder products were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and transmission electron microscopy (TEM). The results showed that ZrB₂–ZrC hybrid powders with no obvious impurity content could be obtained after heating at 1350 °C for 1 h followed by further reaction at 1700 °C for 1 h with 16 wt% B₄C + 8 wt% C excess addition. Relative contents of the ZrB₂: ZrC phase in the product powders could be conveniently regulated by varying the B₄C and C content in the starting compositions. The resultant powders had good oxidation resistance with an oxidation activation energy value of 433 kJ/mol. Good sinterability of the powder products was demonstrated by hot pressing at 1950 °C for 60min under 30 MPa pressure, which resulted in fully dense ZrB₂–ZrC composite ceramics with Vickers hardness value larger than 18.3 ± 0.6 GPa.     

Synthesis of ZrB2-SiC powders in one-step reduction process

ZrB₂-SiC composite powders were synthesized through one-step reduction process of ZrO₂, B₄C, carbon black, silicon or silica under flowing argon. Effects of B₄C contents, calcination temperatures and different silicon sources on the phase composition and morphology were investigated. Combining the X-ray diffraction (XRD) results and scanning electron microscope (SEM) images, the spherical ZrB₂-SiC powders ranging from 100?nm to 300?nm would be prepared with silicon at 1500?°C for 60?min when n(B)/n(Zr) was at 2.4. As using silica as the raw material, the obtained ZrB₂ and SiC particles in the powders exhibited different shapes and sizes. The SiC grains were uniformly formed among the ZrB₂ grains.     

Oxidation behavior of ZrB2-SiC-ZrC at 1700 °C

The oxidation behaviors of four compositions of ZrB₂-SiC-ZrC and one composition of ZrB₂-SiC were studied at 1700 °C in air and under low oxygen partial pressure. Volatility diagrams for ZrB₂-SiC-ZrC and ZrB₂-SiC were used to thermodynamically elucidate the oxidation mechanisms. SiO₂ and ZrO₂ layers formed on the surfaces of ZrB₂-SiC-ZrC and ZrB₂-SiC oxidized at 1700 °C. A SiC-depleted layer only formed on the surface of the ZrB₂-SiC oxidized under low oxygen partial pressure. The oxide layer thickened with increasing ZrC volume content during oxidation in air and under low oxygen partial pressure. The ZrB₂-SiC-ZrC oxide surface exploded in air when the ZrC volume content was more than 50%. Under low oxygen partial pressure, the oxide surfaces of all the ZrB₂-SiC-ZrC specimens bubbled.     

ZrB2-based solid solution ceramics and their mechanical and thermal properties

Zirconium diboride ceramics as one of the main members of ultrahigh-temperature ceramics are capable of being used as structural components at ultrahigh temperatures. Entropy adjusting is a newly developed approach to improving the properties of ceramics. In this work, a series of ZrB₂-based solid solution ceramics with different mixing entropies, formulated (Zr_xTi_yNb_yTa_y)B₂ (x = .25, .85, .925, .9625, 1; x + 3y = 1), were prepared by adjusting the content of other diborides. Diboride solid solution powders were synthesized by boro/carbothermal reduction process and then densified by spark plasma sintering. The results show that the formation of a single-phase solid solution is independent of the mixing entropy in (Zr_xTi_yNb_yTa_y)B₂ system. The addition of other diborides into ZrB₂ is beneficial to reduce the particle size of the synthesized powder and promote the densification process. The dense sintered samples with higher mixing entropy have finer grain size, higher hardness, and modulus. The (Zr_0.25Ti_0.25Nb_0.25Ta_0.25)B₂ ceramic has the highest hardness of 31 GPa and a modulus of 682 GPa. Severe lattice distortion in samples with higher mixing entropy will result in increased phonon scattering and lower thermal conductivity.