Diboride Ceramics Research Articles

Dense nine elemental high entropy diboride ceramics with unique high temperature mechanical and physical properties

Due to their huge composition space and superior performance characteristics, high entropy borides have garnered great interest in many fields, particularly where their high-temperature performances at high temperatures are critical. Herein, we first discovered that dense (Ti1/9Zr1/9Hf1/9Nb1/9Ta1/9V1/9Cr1/9Mo1/9W1/9)B2 (HEB9) ceramics prepared at 1850 °C showed an exceptionally low temperature coefficient of electrical resistivity (4.28 × 10−4 K−1), which is ∼38% of that of (Ti1/5Zr1/5Hf1/5Nb1/5Ta1/5)B2 (HEB5) and nearly an order of magnitude lower than those of previously reported ZrB2 and ZrB2-30 vol% SiC ceramics. Their mechanical, thermophysical properties at elevated temperatures were also investigated systematically. Although the thermal conductivity of HEB9 increased with elevated temperatures, it remained at a very low level (∼29 W/(m·K)) at 1273 K, nearly half that of HEB5. Interestingly, it is found that the thermal conduction of HEB9 and HEB5 was mainly contributed by electrons, suggeting their thermal conductivity could be roughly estimated from corresponding electrical conductivity values. Moreover, HEB9 exhibited a geometrically necessary dislocation density over 30 times greater than HEB5, likely contributing to its higher Vickers hardness and unique physical properties. Notably, the flexural strength of HEB9 at 1600 °C was even improved to 650.0 ± 86.3 MPa, compared to the value (559.7 ± 36.7 MPa) at room temperature without degradation, which was comparable to that of HEB5, although thermodynamic calculations indicated a lower melting point of HEB9 and grain boundary softening was occurred in HEB9 at higher temperatures. The excellent mechanical, electrical and thermophysical properties of HEB9 at elevated temperatures make it a competitive candidate for various high-temperature applications.

Powder metallurgy processing of seven/eight component multi-phase (HfTiZr-Mn/Mo/W/Cr/Ta)B2 high entropy diboride ceramics

This study aims to show the possibility of synthesizing seven- and eight-component high entropy diboride (HEB) ceramics using high energy ball milling-assisted spark plasma sintering (SPS). Metal boride powders, synthesized in laboratory conditions from metal oxide-boron oxide-magnesium powder blends, were combined equimolarly as seven and eight components containing systems. Afterwards, hybridized powders were mechanically alloyed (MA) for 6 h and subjected to spark plasma sintering (SPS) at 2000 °C and under 30 MPa. Detailed phase analysis and physical, microstructural, and mechanical characterization of the samples were performed. ın the sintered products, the main phase belongs to the HEB, and also low amounts of Hf/Zr oxides and secondary phases (W or Ti-rich) occurred. The highest hardness was observed at the (HfTiZrMoWCrTa)B2 with ∼25 GPa, and the lowest hardness was seen at the (HfTiZrMnCrMoWTa)B2 with ∼17 GPa. Also, the highest wear resistance was calculated for the (HfTiZrMnCrMoTa)B2 as 6.05 × 10−7 mm3/Nm. Additionally, (HfTiZrMnMoWTa)B2 and (HfTiZrMnMoCrTa)B2 have the highest and lowest Archimedes’ densities, with 7.94 g/cm3 and 6.91 g/cm3, respectively.

Effect of grain size on mechanical properties and tribological behavior of size-tunable high entropy diboride ceramics obtained by two-step SPS sintering

Effect of grain size on mechanical properties and tribological behavior of size-tunable high entropy diboride ceramics obtained by two-step SPS sintering

Strength comparison for fully dense zirconium diboride ceramics tested by different methods

AbstractThe strength of zirconium diboride ceramics was tested by three different methods, 3‐point flexure, 4‐point flexure, and compression. Nearly full‐density ceramics were obtained by hot pressing commercial ZrB2 powder with the addition of .5 wt.% carbon as a sintering aid. The thermal properties and hardness were studied for ZrB2 milled with ZrB2 and WC media. Based on phase purity and higher thermal conductivity, ZrB2 ceramics prepared from powders milled with ZrB2 media were selected for mechanical property studies. The strength in 3‐point flexure was 546 ± 55 MPa. The flexure strength was 476 ± 41 MPa in 4‐point bending, which was ∼20% higher than the previously reported value of 398 MPa for ZrB2 with similar grain sizes due to higher relative density and lower impurity contents. Compression testing was performed at room temperature, and the strength was 1110 ± 358 MPa. Finally, the fracture toughness of pure ZrB2 ceramics was determined by the chevron‐notched beam method to be 3.6 ± .7 MPa m1/2. The strength and fracture toughness values are higher than those previously published for ZrB2 ceramics and can be attributed to higher density and lower grain size. The strength‐limiting flaw sizes were comparable to the grain size, suggesting that porosity and impurity phases did not play a significant role in the strength of these ceramics.

WB2 ceramics prepared by combination of boro/carbothermal reduction process and spark plasma sintering: Effect of C content and sintering rate

Tungsten diboride ceramics with a relative density higher than 90 % without addition of sintering aid can be prepared by boron/carbothermal reduction method combined with spark plasma sintering. Among the raw materials,WO3, B4C, and C, C content is of vital importance to obtain WB2 bulk samples with high density and good mechanical properties. In the present work, we investigated the effects of C content in powder synthesis, as well as SPS sintering rate on phase composition, microstructure, and Vickers hardness of WB2 bulk samples.WB2 bulk sample with a relative density as high as ∼94 % was obtained with the optimal raw material ratio of WO3:B4C:C=2:1.075:4.8, which was then increased to ∼96 % with the decrease of sintering rate from 100 °C/min to 50 ℃/min. This represents the highest relative density of the pure WB2 ceramic publicly reported in the literatures.

Microstructure and mechanical properties of high-entropy diboride-based ceramic assisted by Ti3AlC2 additive

Ti3AlC2 was used as the additive to fabricate a novel (Zr0·2Ta0·2Nb0.2Hf0.2Mo0.2)B2-based (HEBTAC) ceramic using spark plasma sintering at 1900 °C. The as-prepared HEBTAC ceramic sample consists of (Ti, Zr, Hf, Ta, Nb, Mo)B2, multiscale (Zr, Hf, Ta, Nb, Mo)C, and Al2O3 phases, which are produced by the decomposition of Ti3AlC2 and subsequent exchange reaction & solid solution reactions during the sintering process. The relative density, hardness, fracture toughness, and flexural strength of the HEBTAC ceramic sample are 99.43 %, 27.92 ± 1.87 GPa, 5.84 ± 0.14 MPa m1/2, and 629 ± 8.7 MPa, respectively, which are even better than the same class of high-entropy diboride ceramics previously published. Solid-solution strengthening, dislocation strengthening and nano-HEC strengthening existed in the ceramic sample contribute to its excellent mechanical properties. The in situ formed Al2O3 and (Zr, Hf, Ta, Nb, Mo)C effectively enhances the toughness of the HEBTAC ceramic sample through the synergistic effect of intrinsic and extrinsic toughening. This study presents a new approach for reactive sintering densification of high-entropy diborides with both high strength and high toughness.

A physics-and-data co-driven material design strategy for multicomponent diboride ceramics

The vast chemical composition space of multicomponent ceramics provides opportunity for regulating their properties and performances. Other than trial-and-error and screening, a physics-and-data co-driven material design strategy was reported to efficiently explore chemical composition space through establishing prediction model between physical properties and chemical composition via physics-guided data mining method. Firstly, causal and correlative relationships were discriminated according to physicochemical knowledge. Secondly, ensemble features such as chemical formula and chemical bond were digitized by constructing descriptors for crystalline solids with atomic features including atomic weight and radius contained in the Periodic Table of elements. Finally, quantitative causal relations between material structure/properties and ensemble descriptors were mined out of small dataset using a least square regression method, on which physical prediction model established therefore for forward predicting structure/properties and inverse designing chemical composition. Choosing diboride ceramics as a case, those causal analytic relations of composition-structure and composition-property were successfully derived and illustrated to predict the value of structure and properties accompanying with quantitative relation between chemical pressure and chemical bonding ensemble descriptors. This method bridges the connection between atomic features and macroscopic properties via ensemble descriptors, which significantly accelerates the discovery of new materials.

Pressureless sintered high-entropy transition metal diboride ceramics doped with yttrium tetraboride

High entropy diborides (HEBs) are new candidates of ultra-high temperature ceramics. However, it is difficult to obtain dense HEB ceramics via pressureless sintering. In this study, yttrium tetraboride (YB4) was introduced to consolidate (Ti0.25Zr0.25Hf0.25Ta0.25)B2 (4TMB2) ceramics and its effects on the microstructures and mechanical properties were studied. YB4 addition facilitates the densification and grain elongation of 4TMB2 ceramics by forming eutectic liquid phases and eliminating oxide impurities. Relative density of 4TMB2 ceramics containing 5 vol% YB4 reaches to 94.4 % after pressureless sintered at 2000 °C for 3 h. The hardness and toughness determined by indention method are 26.9±1.5 GPa and 5.73±1.65 MPa∙m1/2. This ceramics also exhibit excellent four-point bending creep resistance with the strain rate of 1.48×10−7 s−1 at 1900 °C under 20 MPa. The observed creep behavior is attributed to grain boundary sliding, plastic deformation, grain refinement and enhanced atomic diffusion of Y element, all driven by high-temperature stresses.

Review on the Development of Titanium Diboride Ceramics

Titanium diboride (TiB<sub>2</sub>) materials have garnered significant attention due to their remarkable comprehensive properties. They offer potential applications in high-temperature structural materials, cutting tools, armor, electrodes for metal smelting, and wear-resistant parts. However, due to the low self-diffusion coefficient, the TiB<sub>2</sub> exhibits poor sinterability, excessive grain growth at elevated temperatures, and inadequate oxidation resistance, limiting its wide application. Therefore, many research works are devoted to processing TiB<sub>2</sub> at a lower sintering temperature and improving the properties through various sintering additives and more advanced techniques. This article comprehensively reviews the multiple synthesis methods and sintering technologies of TiB<sub>2</sub>, and at the same time, critically discusses the impacts of sintering additives and reinforcing agents on densification, microstructure, and various properties, including those at high temperatures, and finally predicts the future development of TiB<sub>2</sub> composite materials.

Toughened bulk high-entropy diborides with high hardness and enhanced oxidation resistance via SiC whiskers

The fracture toughness and oxidation resistance are crucial factors affecting the practical applications of high-entropy diboride ceramics (HEBs). In this study, silicon carbide whiskers (SiCw) were introduced as a reinforcement for HEBs for the first time. To achieve a uniform dispersion of SiCw within the HEB powders, a combination of simultaneous ultrasonic dispersion, mechanical agitation, and freeze-drying technology was employed. Consequently, a series of composites, (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2-nSiCw (n = 0–20 vol%), were densified by spark plasma sintering. The effect of SiCw on the microstructure and mechanical properties of the HEBs were studied, as well as the toughening mechanism involved. The results revealed that the introduction of SiCw led to grain-size refinement in (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2, along with significant improvements in relative density, hardness, fracture toughness, and oxidation resistance. The relative density, hardness and fracture-toughness of (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2-SiCw composites exhibited an initial increase followed by a decrease with increasing SiCw content. Specifically, the (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2–15 vol% SiCw composite simultaneously obtained the highest hardness of 25.6 GPa and fracture-toughness of 4.8 MPa·m1/2, representing a 26% improvement compared to (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2 ceramics.

Ultrastrong and High Thermal Insulating Porous High-Entropy Ceramics up to 2000°C.

High mechanical load-carrying capability and thermal insulating performance are crucial to thermal-insulation materials under extreme conditions. However, these features are often difficult to achieve simultaneously in conventional porous ceramics. Here, for the first time, it is reported a multiscale structure design and fast fabrication of 9-cation porous high-entropy diboride ceramics via an ultrafast high-temperature synthesis technique that can lead to exceptional mechanical load-bearing capability and high thermal insulation performance. With the construction of multiscale structures involving ultrafine pores at the microscale, high-quality interfaces between building blocks at the nanoscale, and severe lattice distortion at the atomic scale, the materials with an ≈50% porosity exhibit an ultrahigh compressive strength of up to ≈337MPa at room temperature and a thermal conductivity as low as ≈0.76W m-1 K-1. More importantly, they demonstrate exceptional thermal stability, with merely ≈2.4% volume shrinkage after 2000°C annealing. They also show an ultrahigh compressive strength of ≈690MPa up to 2000°C, displaying a ductile compressive behavior. The excellent mechanical and thermal insulating properties offer an attractive material for reliable thermal insulation under extreme conditions.

A simulative approach to obtain higher temperatures during spark plasma sintering of ZrB2 ceramics by geometry optimization

This study provides a detailed analysis of the Spark Plasma Sintering (SPS) process for Zirconium Diboride (ZrB2) ceramics, utilizing the finite element method in COMSOL Multiphysics. The focus is on understanding the temperature distribution during the SPS of a ZrB2 sample in a graphite die. Heat diffusion equations, augmented with Joule heating considerations, are utilized to simulate temperature variations within the system over time. Critical boundary conditions at the system's extremities are modeled as convection cooling. The Analysis of Variance (ANOVA) reveals that the diameter of the sample is the most significant factor influencing the peak temperature at the center of the ZrB2 sample. It is found that the sample diameter's variance accounts for a predominant impact on temperature, markedly more than other factors such as the die's outer diameter and sample thickness. Notably, the standard deviation of the temperature in the axial direction across all samples is less than 4 °C, a value that is statistically minor in comparison to the sintering temperatures, which are around 2000 °C. These findings are instrumental in providing an in-depth understanding of the SPS process, which is essential for the optimization of sintering parameters for ZrB2 ceramics.

High-entropy diboride ceramics with graphite addition

Graphite is a widely used oxygen impurity-removing additive and a property-tuning second phase for diboride and carbide-based ultrahigh temperature ceramics (UHTCs). In recent years, high-entropy diboride (HEB) ceramics have attracted much attention for the development of better UHTCs. However, oxide impurity generally inhibits their densification and harms their properties. Here, (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 ceramics added with different contents of graphite (HEB-xC, x = 0, 1, 2.5, 5, 10, 20 vol%) were prepared by spark plasma sintering. It was found that the added graphite acted as an oxygen-removing agent and the oxide impurity phase t-(Zr, Hf)O2 was converted to the (Hf, Ta)C-based solid solution carbide. With the increase in graphite content, the relative density increased from 89.4% for HEB-0 C to 100% for HEB-10 C. Based on the evaluated properties of the prepared specimens, including Young’s modulus, hardness, toughness, and thermal conductivity, the added graphite with an optimized content is important for a given started powder and processing to achieve good and balanced material properties.

The effect of chemical element on hardness in high-entropy transition metal diboride ceramics

The relationship between the chemical elements of high-entropy boride (HEB) ceramics and their hardness is important for the prediction of high-hardness HEB ceramics. In this work, by designing four HEB ceramics with different chemical elements, the effect of lattice parameter difference factor (δ, represents the difference-degree in lattice parameters among the five individual constitute diborides) on phase composition and lattice-distortion, and the effect of rule of mixture (ROM) average hardness and lattice-distortion on HEB ceramics hardness were studied. The results indicated that, as δ value increases, more severe lattice-distortion occurs inside the HEB ceramics, and a single solid-solution is difficult to be formed. Furthermore, lattice-distortion and the ROM average hardness codetermine the HEB ceramics hardness. The greater lattice-distortion brings about significantly higher hardness for HEB ceramics than their ROM average hardness. Among the four HEBs, (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2 exhibits the high hardness of 26 GPa measured using a load of 9.8 N.

Core-rim microstructure formation and mechanical properties of TiB2 ceramics with TiC, B4C, Co, and Mo sintering aids

This research investigates titanium diboride (TiB2) ceramics, renowned for their exceptional properties: high hardness, chemical inertness, and thermal stability. The challenge lies in achieving dense structures through pressureless sintering, given TiB2 ceramics' high melting point and low self-diffusion coefficient. To address this, a combined method of liquid phase and reactive sintering is employed, focusing on the influence of metallic cobalt (Co) and molybdenum (Mo) as sintering aids to enhance microstructural and mechanical properties. Vacuum sintering at 1900 °C facilitates the reaction between TiC and B4C, forming TiB2, with Co improving wettability and Mo inhibiting high-temperature grain growth. The matrix of 90TiB2+10(TiC + B4C + Co + Mo) recipe achieved 94.7 % relative density and a hardness of 31.97 GPa. Moreover, adding polyvinyl butyral (PVB) assists in creating gas evacuation channels during the sintering process. The research also reveals the reaction between TiC and B4C, forming (TiMoB)C with molybdenum, while the eutectic Co–Mo alloy dissolves into borides, promoting densification and forms dense core-rim structures.

(HfTiZrMnCr)B2 high entropy diboride ceramics: Synthesis mechanism, microstructural, mechanical and thermal characterization

This study explains the synthesis mechanism of (HfTiZrMnCr)B2 high entropy diboride ceramics prepared via a combined route consisting of ball milling and spark plasma sintering. Firstly, metal borides were in-house synthesized using mechanochemical synthesis and leaching processes under optimum conditions from metal oxides, B2O3 and Mg precursors. The (HfTiZr)B2 composition, chosen as the main composition, was hybridized with two different methods: planetary and vibratory ball milling. As a result of milling experiments, 6 h vibratory milling was determined as the optimum duration, and all compositions were produced by spark plasma sintering method (2000 °C, 30 MPa) following this optimum condition. Different compositions were used to see which one of the HfB2, TiB2, ZrB2 compounds acted as a host material. Based on the phase analyses, single-phase HEB structures, Ti-rich phases and Hf, Ti, Zr-oxides were observed in the microstructure. Detailed physical, microstructural, mechanical and termal characterizations were performed: the highest hardness values were observed in the (HfTiMnCr)B2 and (HfTiZrMnCr)B2 samples as ∼ 27 GPa, the lowest wear rate was recorded for the (HfTiZrMnCr)B2 sample as ∼2 × 10−6 Nm/mm3, and the highest oxidation resistance was achieved at the (HfZrMnCr)B2 and (HfTiMnCr)B2 samples as weight gains ∼ 1.90%.

Strength retention of single‐phase high‐entropy diboride ceramics up to 2000°C

AbstractThe mechanical properties of single‐phase (Hf0.2,Nb0.2,Ta0.2,Ti0.2,Zr0.2)B2 ceramics with high purity were investigated. The resulting ceramics had relative density greater than 99%, and an average grain size of 4.3 ± 1.6 μm. At room temperature (RT), the Vickers hardness was 25.2 ± 0.6 GPa at a load of 0.49 N, Young's modulus was 551 ± 7 GPa, fracture toughness was 4.5 ± 0.4 MPa m1/2, and flexural strength was 507 ± 10 MPa. Flexural strength increased by more than 50% from 507 ± 10 MPa at RT to 776 ± 26 MPa at 1400°C. Strength remained above 750 MPa up to 2000°C, but decreased to 672 ± 18 MPa at 2100°C and the bars deformed during testing. No significant changes in residual porosity, average grain size, or oxide impurity levels were observed after testing at elevated temperatures. The increase in strength at elevated temperatures was attributed to healing of microcracks due to thermal expansion at high temperatures, and dislocation formation. The retention of strength up to 2000°C is presumably due to the lack of oxide impurities in the HEBs. This is the first reported study on the flexural strength up to 2100°C of dense and pure HEB ceramics.

Phase stability, mechanical and thermodynamic properties of (Hf, Zr, Ta, M)B2 (M= Nb, Ti, Cr, W) quaternary high-entropy diboride ceramics via first-principles calculations

As the high-entropy design concept applied to the diboride ceramic system, high-entropy diboride ceramics with a wide range of composition control, is expected to become a new high-performance material for extreme high-temperature environments. Herein, the effects of four transition metal elements (Nb, Ti, Cr, W) on the phase stability and properties of (Hf, Zr, Ta)B2-based high-entropy diboride ceramics are systematically investigated via the first-principles calculations. All components were identified as thermodynamically, mechanically and dynamically stable from enthalpy of formation, elastic and phonon spectrum calculations. Among these, compared with the (Hf, Zr, Ta)B2 ceramics, the addition of Nb and Ti on the metal sublattice is beneficial to improve the mechanical properties of ceramics, including Young's modulus, hardness and fracture toughness, while the introduction of Cr and W weakens the strength of covalently and ionic bonds inside the material, reducing its mechanical properties. The predicted thermophysical properties show that the high-entropy diboride ceramics containing Nb and Ti have better high-temperature comprehensive performance, including higher Debye temperature, thermal conductivity and lower thermal expansion characteristics, which is conducive to the application in extreme high-temperature environments. This research will provide important guidance for the design and development of new high-performance high-entropy diboride ceramics.

Thermal conductivity of equiatomic high‐entropy diboride ceramics with 5–9 cations

AbstractAs a new type of high‐entropy material, most of the current work has focused on the synthesis and characterization of mechanical properties of high‐entropy diboride ceramics (HEBs). In this work, single‐phase HEBs with 5–9 cations were prepared by spark plasma sintering with self‐synthesized HEB powders through the boro/carbothermal reduction method. The distribution of the metal elements in the obtained HEBs was homogeneous. Due to the phonon scattering caused by the inherent lattice distortion in the HEBs, the thermal conductivity of the ceramics decreased with the increased configurational entropy or the number of cation kinds. However, this study reveals that the degree of reduction in thermal conductivity for the HEBs demonstrate a weakened tendency with cation kinds of more than 6.

Grain growth inhibition by sluggish diffusion and Zener pinning in high‐entropy diboride ceramics

AbstractThis work reported the grain growth kinetics of high‐entropy diboride (HEB) and HEB‐SiC ceramics containing 10, 20, and 30 vol% SiC during heat treatment at 1800°C. The coarsening of HEB phase occurred in the four kinds of ceramics during heat treatment, especially in HEB ceramics. The kinetic analysis showed that the grain growth of HEB phase in HEB and HEB‐SiC ceramics is controlled by interface‐controlled kinetics and grain‐boundary pinning, respectively. The growth rate constant of HEB grains is lower than ZrB2, which is related to the low grain‐boundary energy and the sluggish diffusion effect in dynamics of high‐entropy materials. The growth rate of matrix phase in HEB‐SiC ceramics is similar to that in ZrB2–SiC ceramics, indicating that the pinning effect of the SiC second‐phase played the dominant role in inhibiting the grain growth of the high‐entropy matrix phase and disguised the sluggish diffusion effect. This study reveals that the grain growth inhibition through sluggish diffusion effect in a high‐entropy ceramic system may be magnified by the possible existence of segregated second‐phase particles located at the grain boundaries.