Effects of TaB2 and TiB2 on the grain growth behavior and kinetics of HfB2 ceramics during pressureless sintering

HfB₂, Hf_0.95Ta_0.05B₂, and Hf_0.95Ti_0.05B₂ powders were self-synthesized and the grain sizes were 2.04, 0.36, and 1.15 μm, respectively. The three powders were pressureless sintered from 1700℃ to 2000℃ to study and compare effects of the introduction of TaB₂ and TiB₂ on the grain growth behavior and kinetics of HfB₂. The results revealed that HfB₂ showed moderately slow grain growth in the whole process. However, significant grain growth consistently happened in Hf_0.95Ti_0.05B₂ and Hf_0.95Ta_0.05B₂ at 1900℃ or above. Eventually, the grain size of Hf_0.95Ti_0.05B₂ increased to almost the same as HfB₂, but Hf_0.95Ta_0.05B₂ still possessed smaller grains due to the finest original powders. The grain growth exponent was determined to be ~3, and the dominant growth rate-controlling mechanism was volume diffusion. The average activation energy of HfB₂, Hf_0.95Ta_0.05B₂, and Hf_0.95Ti_0.05B₂ for grain growth at 1700℃-2000℃ was 191 ± 34, 678 ± 73, and 321 ± 61 kJ/mol, respectively.     

Densification,microstructure, and mechanical properties of V-substituted HfB2-based ceramics

This study firstly developed Hf_1-xV_xB₂ (x = 0, 0.01, 0.02, 0.05) powders, which were derived from borothermal reduction of HfO₂ and V₂O₅ with boron. The results revealed that significantly refined Hf_1-xV_xB₂ powders (0.51 μm) could be obtained by solid solution of VB₂, and x ≥ 0.05 was a premise. However, as the content of V-substitution for Hf increased, Hf_1-xV_xB₂ ceramics sintered by spark plasma sintering at 2000 °C only displayed a slight densification improvement, which was attributed to the grain coarsening effect induced by the solid solution of VB₂. By incorporating 20 vol% SiC, fully dense Hf_1-xV_xB₂-SiC ceramics were successfully fabricated using the same sintering parameters. Compared with HfB₂-SiC ceramics, Hf_0.95V_0.05B₂-20 vol% SiC ceramics exhibited an elevated and comparable value of Vickers hardness (23.64 GPa), but lower fracture toughness (4.09 MPa m^1/2).     

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.     

Low-temperature synthesis of high-entropy (Hf0.2Ti0.2Mo0.2Ta0.2Nb0.2)B2 powders combined with theoretical forecast of its elastic and thermal properties

A theoretical calculation combined with experiment was used to study high-entropy (Hf_0.2Ti_0.2Mo_0.2Ta_0.2Nb_0.2)B₂ (HEB-HfTiMoTaNb). The theoretical calculation suggested HEB-HfTiMoTaNb could be stable over a wide temperature range. Then, a novel solvothermal/molten salt-assisted borothermal reduction method was proposed to efficiently pre-disperse transitional metal atoms in a precursor and synthesize (Hf_0.2Ti_0.2Mo_0.2Ta_0.2Nb_0.2)B₂ nanoscale powders at 1573 K for 6 h, which is nearly 300 K lower than previous reports. The characterization results indicated that the as-synthesized nanoscale HEB-HfTiMoTaNb powder was hexagonal single-phase with homogeneous elements distribution and uniform size, and the oxygen content of the particles is 0.97 wt%. Simultaneously, the mechanical properties, anisotropic nature, and thermal properties of HEB-HfTiMoTaNb were investigated by density functional theory (DFT) calculations. The Cannikin's law was adopted to explain the improvement of comprehensive mechanical properties. In addition, a significant reduction of thermal conductivity was observed for HEB-HfTiMoTaNb and it only was 1/15 of the value of HfB₂. This work suggests a reliable technique for synthesis of nanosized HEB powders and discovery of high-entropy materials under the guidance of first-principle theory.     

TiB2 Powders Synthesis by Borothermal Reduction in TiO2 Under Vacuum

TiB₂ powders were synthesized by borothermal reduction in nanoscale TiO₂ with boron under vacuum. Reaction processes were investigated, and the effect of by‐product B₂O₃ was evaluated. Results showed that TiO₂ was firstly reduced by boron to form TiBO₃ and Ti₂O₃, and then to produce TiB₂ and B₂O₃ with increasing temperature. The reaction processes of TiB₂ powders synthesis included two‐step reduction in TiO₂ by boron and the removal of B₂O₃. The presence of B₂O₃, which was previously reported as the most important factor in promoting the coarsening of ZrB₂ and HfB₂ powders by borothermal reduction, did not lead to significant coarsening of TiB₂ powders. Due to the minor effect of B₂O₃, TiB₂ powders with small particle size and low oxygen content could be prepared by direct heat treatment of TiO₂ and boron at 1550°C under vacuum for 1 h. The particle size and oxygen content of synthesized TiB₂ powders were ~0.9 μm and ~1.7 wt%, respectively.     

Synthesis of TaB2 powders by borothermal reduction

Borothermal reduction processes of Ta₂O₅ with boron under vacuum were investigated. Ta₂O₅ reacted with boron to form various borides (TaB₂, Ta₃B₄, and TaB), depending on the boron/Ta₂O₅ molar ratio and temperature. In order to prepare pure TaB₂ powders, two routes were developed. The first route was one‐step heat treatment at 1550°C. With boron/Ta₂O₅ molar ratio of 9.0, pure TaB₂ powders with strong agglomeration were synthesized by the first route, and the particle size and oxygen content were 0.7 μm and 0.9 wt%, respectively. The second route consisted of two‐step heat treatment at 800°C and 1550°C plus intermediate water washing. With lower boron/Ta₂O₅ molar ratio of 8.2, pure TaB₂ powders with less agglomeration and more uniform distribution were synthesized by the second route, and the particle size and oxygen content were 0.8 μm and 0.8 wt%, respectively. Moreover, the particle size similarity of TaB₂ powders by the two routes suggested that byproduct boron oxides, which were previously reported as the most important factor in promoting the coarsening of ZrB₂ powders by borothermal reduction, did not lead to the significant coarsening of TaB₂ powders.     

Low temperature sintering of Hf0.95Nb0.05B2-based ceramics with submicron-scaled grains and enhanced mechanical properties

Hf_0.95Nb_0.05B₂ ceramics and their composites containing 20 vol% SiC were prepared via high-pressure spark plasma sintering in the study. The densification, microstructures, and mechanical properties of the prepared materials were then investigated. It is challenging to achieve full densification of HfB₂ ceramics, even with markedly refined Hf_0.95Nb_0.05B₂ solid solution powder under the sintering conditions of 2000 °C/30 MPa. However, under the sintering conditions of 1700 °C/200 MPa, a dense microstructure of Hf_0.95Nb_0.05B₂ ceramics was achieved. Moreover, the Hf_0.95Nb_0.05B₂-20 vol% SiC composite was densified at a lower temperature (1500 °C) and exhibited ultrafine grains (300 nm) and high-density defects, including stacking faults, Lomer-Cottrell locks, and twins, thus resulting in exceptional comprehensive mechanical properties, such as ultra-high hardness (32 GPa) and significantly improved fracture toughness (5.2 MPa.m^1/2).     

Strengthening Hf0.95Ta0.05B2 ceramic with ultrafine grains and high-density dislocations

Full densification and fine microstructures are the two key optimization targets of ceramic materials. Although fine Hf_0.95Ta_0.05B₂ powder (∼ 0.36 µm) has been synthesized, it was still difficult to obtain densified Hf_0.95Ta_0.05B₂ ceramics with ultrafine grains (< 1 µm) using conventional high temperature sintering. Increasing sintering pressure could provided higher densification driving force, but it usually negatively promoted grain growth for nanoceramics. Our strategy was to gain the fully dense Hf_0.95Ta_0.05B₂ ceramic under a high pressure at a selected temperature with retarded grain growth. In this work, fully dense Hf_0.95Ta_0.05B₂ ceramic was prepared at 1700 °C under a high pressure of 200 MPa. The limited grain growth was achieved with the average grain size of 0.6 µm. Therefore, the mechanical properties were significantly improved, including Vickers hardness (24.8 GPa) and fracture toughness (4.2 MPa.m^1/2), which were ascribed to Hall-Petch and dislocation strengthening mechanism.     

Hardness and toughness improvement of SiC-based ceramics with the addition of (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2

The influences of different contents ranging 0–15 wt% of high-entropy boride (HEB) (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ on the mechanical properties of SiC-based ceramics using Al₂O₃-Y₂O₃ sintering additives sintered by spark plasma sintering process were investigated in this study. The results showed that the introduction of 5 and 10 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ could facilitate the densification and the grain growth of SiC-based ceramics via the mechanism of liquid phase sintering. However, the grain growth of SiC-based ceramics was inhibited by the grain boundary pinning effect with the addition of 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂. The SiC-based ceramics with 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ showed the enhanced hardness (21.9±0.7 GPa) and high toughness (4.88±0.88 MPa·m^1/2) as compared with high-entropy phase-free SiC-based ceramics, which exhibited a hardness of 16.6 GPa and toughness of 3.10 MPa·m^1/2. The enhancement in mechanical properties was attributed to the addition of higher hardness of HEB phase, crack deflection toughening mechanism, and presence of residual stress due to the mismatch of coefficient of thermal expansion.     

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.     

Microstructures and oxidation mechanisms of (Zr0.2Hf0.2Ta0.2Nb0.2Ti0.2)B2 high-entropy ceramic

A nano dual-phase powder with great sinterability was synthesized by molten-salt assisted borothermal reductions at 1100 °C using B, ZrO₂, HfO₂, Ta₂O₅, Nb₂O₅ and TiO₂ powders as raw materials. Single-phase (Z_r0.2Hf_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ high-entropy ceramic was prepared by spark plasma sintering using the as-synthesized nano dual-phase powder. Oxidation behavior of the (Zr_0.2Hf_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ ceramic was investigated over the range of 30–1400 °C in air and the result indicated that the rapid oxidation of ceramic began at 1300 °C. The phenomenon could be ascribed to the rapid volatilization of B₂O₃ from oxide scale. A layered structure was formed at the cross section of (Zr_0.2Hf_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ ceramic after oxidation. The relationship between partial pressures of gaseous metal oxides and oxygen partial pressures was calculated, which inferred that the formation of layered structure could be ascribed to the active oxidation of (Zr_0.2Hf_0.2Ta_0.2Nb_0.2Ti_0.2)B₂, the generation of gaseous metal oxides, their outward diffusion and further oxidation.     

Enhanced high-temperature strength in textured (Ti1/3Zr1/3Hf1/3)B2 medium-entropy ceramics via strong magnetic field

This study prepared textured (Ti_1/3Zr_1/3Hf_1/3)B₂ medium-entropy ceramics for the first time that maintain enhanced flexural strength up to 1800°C using single-phase (Ti_1/3Zr_1/3Hf_1/3)B₂ powders, slip casting under a strong magnetic field, and hot-pressed sintering methods. Effects of WC additive and strong magnetic field direction on the phase compositions, orientation degree, microstructure evolution, and high-temperature flexural strength of (Ti_1/3Zr_1/3Hf_1/3)B₂ were investigated. (Ti_1/3Zr_1/3Hf_1/3)B₂ grain grows along the a,b-axes, resulting in a platelet-like morphology. Pressure parallel and perpendicular to the magnetic field direction can promote the orientation degree and hinder the texture structure formation, respectively. Reaction products of W(B,C) and (Ti,Zr,Hf)C between (Ti_1/3Zr_1/3Hf_1/3)B₂ and WC additive can efficiently refine the (Ti_1/3Zr_1/3Hf_1/3)B₂ grain size and promote grain orientation. (Ti_1/3Zr_1/3Hf_1/3)B₂ ceramics doped with 5 vol.% WC yielded a Lotgering orientation factor of 0.74 through slip casting under a strong magnetic field (12 T) and hot-pressed sintering at 1900°C. Furthermore, cleaning the boundary by W(B,C) and introducing texture can enhance the grain-boundary strength and improve its high-temperature flexural strength. The four-point flexural strength of textured (Ti_1/3Zr_1/3Hf_1/3)B₂-5 vol.% WC ceramics was 770 ± 59 MPa at 1600°C and 638 ± 117 MPa at 1800°C.     

Synthesis of single-phase high-entropy diboride powders and their spheroidization process

Spherical (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ powders with a uniform particle size distribution are successfully prepared using a novel industrial approach, which combines spray-drying process and thermal plasma sintering technology together. In this, single-phase (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ powders are first synthesized via a borothermal reduction process using a mixture of individual metallic oxides and boron powders as starting materials. The influence of boron powder content on the structure of prepared powders is researched. Then, (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ granules are prepared after wet-grinding and spray-drying process, which exhibit a spherical shape and homogeneous element distribution. RF induction thermal plasma is finally used to sinter the granulated particle, and the apparent density of sintered spherical powders is increased to 2.57 g/cm³ from 1.43 g/cm³. Such powders are in potential demand for additive manufacturing techniques, and the successful synthesis of spherical (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ powders may guide the way toward the preparation of many other spherical high-entropy diboride powders.     

Superhard single-phase (Ti,Cr)B2 ceramics

A nominally pure and dense (Ti_0.9Cr_0.1)B₂ ceramic was produced by spark plasma sintering of powders synthesized by boro/carbothermal reduction of oxides. The synthesized powders were a single phase and had an average particle of 0.4 ± 0.1 μm and an oxygen content of 1.2 wt%. Average Vickers hardness values of the resulting ceramics increased from 25.9 ± 0.8 GPa at a load of 9.81 N, to 46.3 ± 0.8 GPa at a load of 0.49 N. Compared to the nominally pure TiB₂ ceramic obtained under the same processing conditions, the (Ti_0.9Cr_0.1)B₂ ceramic had higher values under the same load due to the finer average grain size (2.4 ± 1.0 μm), higher relative density, and solid solution hardening. The results indicated that the Cr addition promoted densification, suppressed grain growth, and improved the hardness of TiB₂ ceramics. This is the first report for dense and single-phase (Ti,Cr)B₂ ceramics as superhard materials.     

Enthalpy driving force and chemical bond weakening: The solid-solution formation mechanism and densification behavior of high-entropy diborides (Hf1−x/4Zr1−x/4Nb1−x/4Ta1−x/4Scx)B2

(Hf_0.2Zr_0.2Nb_0.2Ta_0.2Sc_0.2)B₂ was designed to improve the densification and solid-solution formation of high-entropy transition metal diborides, and its phase stability was predicted using the energy distribution of the local mixing enthalpy of all possible configurations. It was found that (Hf_0.2Zr_0.2Nb_0.2Ta_0.2Sc_0.2)B₂ are enthalpy-stabilized materials. The two-component metal diborides formed by transition metal diborides (HfB₂, ZrB₂, TaB₂ and NbB₂) with ScB₂ are thermodynamically favorable, based on the mixing enthalpy. Therefore, the introduction of ScB₂ in high-entropy metal diborides is beneficial to reduce the mixing Gibbs free energy during the boro/carbothermal reduction process, which enables the formation of single-phase solid solution at low temperatures. Even high-entropy metal diboride powders with large particle sizes, 25–57 µm, can achieve sintered density up to ~97% due to the introduction of ScB₂ in high-entropy metal diborides, owing to its weakening action on the TM d - B p and the TM dd bonding.     

Low temperature synthesis of TaB2 nanorods by molten‐salt assisted borothermal reduction

TaB₂ powders were synthesized by a molten‐salt assisted borothermal reduction method at 900°C‐1000°C in flowing argon using Ta₂O₅ and amorphous B as starting materials. The results indicated that the presence of liquid phase, such as B₂O₃ and NaCl/KCl, accelerated the mass transfer of reactant species and resulted in the complete finish of the reaction at low temperatures. The obtained TaB₂ powders exhibited a flow‐like shape assembled from nanorods grow along [001] direction or c‐axis. The morphology of the synthesized TaB₂ powders could be tailored by the amount of B₂O₃ or NaCl/KCl.     

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.     

Synthesis and thermal stability of novel high-entropy metal boron carbonitride ceramic powders

High-entropy metal boron carbonitride ceramic powders including (Ta_0.2Nb_0.2Zr_0.2Hf_0.2W_0.2)BCN, (Ta_0.2Nb_0.2Zr_0.2Hf_0.2Ti_0.2)BCN, and (Ta_0.2Nb_0.2Zr_0.2Ti_0.2W_0.2)BCN, were successfully synthesized via mechanical alloying at room temperature. Results show that for the first step of 10 h milling, the amorphous BCN phases are observed. After 24 h of second step milling, the as-synthesized high-entropy ceramics exhibit a single face-centered cubic solid solution structure with high compositional uniformity from nano-scale to micron-scale. When heated to 1500 °C for 30min in flowing Ar, the as-prepared high-entropy ceramic powders still show relatively high thermal stability; however, some metals oxides like HfO₂ and ZrO₂ are detected due to the pre-existing oxides on sample surfaces. After heat treatment, some amorphous phases are still retained. This work suggests a new processing route on the synthesis of high-entropy metal boron carbonitride ceramics.     

Microstructure and mechanical properties of high-entropy borides derived from boro/carbothermal reduction

Starting from metal oxides, B₄C and graphite, a suite of high-entropy boride ceramics, formulated (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)B₂, (Hf_0.2Zr_0.2Mo_0.2Nb_0.2Ti_0.2)B₂ and (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ derived from boro/carbothermal reduction at 1600 °C were fabricated by spark plasma sintering at 2000 °C. It was found that the synthetic high-entropy boride crystalized in hexagonal structure and the yield of the targeting phase was calculated to be over 93.0 wt% in the sintered ceramics. Benefitting from the nearly full densification (96.3% ˜ 98.5% in relative density) and the refined microstructure, the products exhibited the relatively high Vickers hardness. The indentation fracture toughness was determined to be comparable with the single transition metal-diboride ceramics. It should be noted that the formation of high-entropy boride ceramics were featured with the relatively high hardness at no expense of the fracture toughness.     

Ultra high temperature high-entropy borides: Effect of graphite addition on oxides removal and densification behaviour

The introduction of 0.5–1.0 wt.% graphite to the powders prepared by Self-propagating High-temperature Synthesis (SHS) is found to be highly beneficial for the removal of oxide impurities (from 2.7-8.8 wt.% to 0.2–0.5 wt.%) during spark plasma sintering (1950°C/20 min, 20 MPa) of (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ and (Hf_0.2Mo_0.2Ta_0.2Zr_0.2Ti_0.2)B₂ ceramics. Concurrently, the consolidation level achieved is enhanced from about 92.5% and 88%, respectively, to values exceeding 97%. While a further increase of graphite slightly improves samples densification, final products become progressively richer of the unreacted carbon.It is assumed that graphite plays a double role during SPS, e.g. not only as a reactant during the carbothermal reduction of oxides contaminant, but also as lubricating agent for the powder particles. The latter phenomenon is likely the main responsible for the densification improvement when 3 wt.% or larger amounts of additive are used. Another positive effect is the crystallite size refinement of the high-entropy phases with the progressive abatement of oxides, to confirm that their presence promotes grain coarsening during the sintering process.