ZrB2 Grains Research Articles

Microstructure evolution of zirconium boride whiskers prepared by electrospinning and their toughening effect

AbstractZirconium boride (ZrB2) whiskers hold significant potential for toughening ultra‐high temperature ceramics (UHTCs). Yet, their fabrication remains challenging due to limited knowledge of the microstructure evolution. Herein, high‐purity ZrB2 whiskers were successfully prepared by electrospinning combined with carbothermal reduction. Specifically, a precursor solution containing Zr–O–C–B networks stabilized through hydrogen bonds with polyvinyl pyrrolidone was synthesized as a spinnable ZrB2 precursor. High‐purity ZrB2 whiskers with a diameter of ca. 200 nm and a length of 5–10 µm were obtained by precise heat treatment of ZrB2 precursor fibers. In this process, ZrO2 grains formed initially and transformed into ZrB2 grains through carbothermal reduction, which further converted into ZrB2 whiskers through a solid‐liquid‐solid mechanism. To testify to the toughening effect for UHTCs, the ZrB2 whiskers were added into ZrB2 powders and followed by sintering. This strategic addition of 5 wt% ZrB2 whiskers led to a 19% enhancement in the fracture toughness of ZrB2 bulk. This study illuminated the preparation strategy and formation mechanism for ZrB2 whiskers and their application for toughening UHTCs.

Defect-regulated synthesis of ZrB2 powders via carbon thermal reduction and elucidation of its hot-pressing densification mechanism

Through defect modulation, ZrB2 powder was controllably synthesized using the carbon thermal reduction (CTR) method, with boron powder serving as a novel additive. Thermodynamic data for the ZrO2-B2O3-C system in the presence of boron were calculated using HSC thermodynamic software. Additionally, TG-DSC thermal analysis combined with XRD was used to investigate the reaction process. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to analyze the defect structure and grain surface characteristics of the reaction products. The results indicate that the reaction of B with ZrO2 and C exothermically alters the nucleation environment of ZrB2 grains. The helical dislocations generated act as a source of crystal growth steps, promoting the growth of smooth interfaces and leading to a transformation of the ZrB2 grain growth mechanism. Quantitative calculations of the dislocation density of ZrB2 powders, using the convolutional multiple whole profile fitting (CMWP-fit), suggest that the inclusion of boron as an additive can modulate the defect concentration of the products. ZrB2 powders with varied dislocation densities were achieved through meticulously controlled synthesis methods, followed by subsequent densification via hot pressing sintering. The results indicated that the relative density of ZrB2 ceramics increased from 83.9 % to 96.7 % as the dislocation density of the powder increased from 2.7 × 1013 m−2 to 7.6 × 1014 m−2 with the rise in boron content from 0 wt% to 3 wt%. In addition, the defects create extra sintering-driving forces that induce a shift in the densification mechanism from grain boundary diffusion to dislocation slip during hot-pressing sintering. This shift results in an increase in the apparent activation energy from 105 kJ/mol to 210 kJ/mol.

Dual plate-like grains of (Ti, Zr)B2 and W2B5 toughened solid solution composites fabricated via in-situ reaction spark plasma sintering of (Ti, W)C and ZrB2 powders

Novel dual plate-like grains of (Ti, Zr)B2 and W2B5 toughened solid solution composites were fabricated by the in-situ reaction spark plasma sintering at 1800 °C using (Ti, W)C and ZrB2 powders. The composites were predominated and composed of the (Zr, Ti, W)C solid solutions and plate-like (Ti, Zr)B2. In the (Ti, W)C-ZrB2 reaction system, W promotes the anisotropic growth of (Ti, Zr)B2 grains, while Ti makes the formation reactions of WxBy thermodynamically feasible. When the addition of ZrB2 exceeds 40 mol%, W begins to be boronized into WxBy which is mainly composed of plate-like W2B5. The (Ti, W)C-50 mol% ZrB2 composite with dual plate-like grains of (Ti, Zr)B2 and W2B5 exhibited the best comprehensive mechanical properties of hardness of 26.5 GPa, flexural strength of 620 MPa, and fracture toughness of 6.6 MPa·m1/2, which provide a new approach for the enhanced toughness without the introducing of metal impurities.

A TEM study of nanostructures and interfaces in the hot-press sintered ZrB2–SiC–Si3N4 composites

A fully dense ZrB2–30 vol% SiC composite containing 5 wt% Si3N4 and 4 wt% phenolic resin (1.6 wt% carbon) was sintered using the hot-pressing route under the external pressure of 10 MPa at 1900 ºC for 2 h. The microstructural evolution and interfacial phenomena were scrutinized using advanced microscopy facilities such as high-resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM). The FESEM images showed the ZrB2 and SiC grains without any evidence of Si3N4. The formation of the hexagonal BN (hBN) phase was proven in the sintered composite. The hBN nanosheets had a graphite-like morphology with an average thickness of 20 nm. This phase has a perpendicular orientation to the pressure direction and prevents abnormal ZrB2 grain growth. Two types of ZrB2/SiC interfaces were detected, which exhibited an amorphous phase along with the grain boundary and a clean/smooth interface, resulting from the Si3N4 addition. HRTEM and inverse fast Fourier transform (IFFT) observations disclosed that the d-spacing value in the ZrB2 grain (0.335 nm) is higher than those reported in the literature. Furthermore, it was found that the exerted pressure during the sintering distorted atomic planes. The presence of numerous dislocations within the ZrB2 grains confirmed dislocation creep as the main densification mechanism.

Reactive spark plasma sintering of an electrically conductive B4C-SiC-ZrB2 composite with enhanced mechanical properties

Ternary B4C-SiC-ZrB2 ceramics with eutectic compositions were prepared in situ by reactive spark plasma sintering at 1600–1800 °C. The two-step sintering, including an in situ reaction 2ZrSi2 + 5B4C+ 3 C= 4B4C+ 4SiC+ 2ZrB2 at 1600 °C and sintering at 1800 °C, is superior to combined one-step sintering at 1800 °C. The as-sintered 40B4C-40SiC-20ZrB2 composites (sample 16R18) had grain sizes of microns with a density over 97%. The Vickers hardness, bending strength and fracture toughness of the ternary ceramic reached 28.6 GPa, 631 MPa and 5.50 MPa m1/2 respectively. Crack deflection and branching were ascribed to improved fracture toughness. The elongated plate-like ZrB2 grains not only had toughening effects but also helped to lower the percolation threshold of the electrical conductivity. The 16R18 sample possessed an electrical conductivity of 5.38 × 104 S/m, owing to the tiny conductive ZrB2 grains with aspect ratios of 3–4, which is beneficial for the electro-discharge machining of the B4C-SiC based composites.

In-situ synthesis of hierarchically high porosity ZrB2 ceramics from carbon aerogel template with excellent performance in thermal insulation and light absorption

This work introduces an innovative methodology combining the in-situ boron/carbon thermal reduction (BCTR) reaction and replication carbon aerogel template to synthesize hierarchically high porosity ZrB2 ceramics. The meticulously designed ceramic liquid-phase precursor ensures higher solubility of solutes while reducing the temperature requisite for boron/carbon thermal reduction. An additional carbon source is incorporated in the precursor to ensure the uniformity of resulting ZrB2 grains, concurrently maintaining ZrB2 phase purity. The ultimate prepared ZrB2 porous ceramics present a uniform microstructure, with grain sizes at the hundred-nanometer scale, and exhibit a hierarchical pore structure. The ZrB2 porous ceramics exhibited high porosity (85.4–92.9%), low density (0.264–0.482 g/cm3), reasonable compressive strength (0.26–0.51 MPa), low thermal conductivity (0.103–0.212 W/m·K), and efficient light absorption in the range of 400–2000 nm (>94%). The fabricated ZrB2 porous ceramics hold significant promise for applications in high-temperature insulation and light absorption within elevated-temperature environments.

Effect of Zr2Al4C5 Content on the Mechanical Properties and Oxidation Behavior of ZrB2-SiC-Zr2Al4C5 Ceramics.

ZrB2-SiC-Zr2Al4C5 multi-phase ceramics with uniform structure and high density were successfully prepared through the introduction of in situ synthesized Zr2Al4C5 into ZrB2-SiC ceramic via SPS at 1800 °C. A systematic analysis and discussion of the experimental results and proposed mechanisms were carried out to demonstrate the composition-dependent sintering properties, mechanical properties and oxidation behavior. The results showed that the in situ synthesized Zr2Al4C5 could be evenly distributed in the ZrB2-SiC ceramic matrix and inhibited the growth of ZrB2 grains, which played a positive role in the sintering densification of the composite ceramics. With increasing Zr2Al4C5 content, the Vickers hardness and Young's modulus of composite ceramics gradually decreased. The fracture toughness showed a trend that first increased and then decreased, and was increased by about 30% compared with ZrB2-SiC ceramics. The major phases resulting from the oxidation of samples were ZrO2, ZrSiO4, aluminosilicate and SiO2 glass. With increasing Zr2Al4C5 content, the oxidative weight showed a trend that first increased then decreased; the composite ceramic with 30 vol.% Zr2Al4C5 showed the smallest oxidative weight gain. We believe that the presence of Zr2Al4C5 results in the formation of Al2O3 during the oxidation process, subsequently resulting in a lowering of the viscosity of the glassy silica scale, which in turn intensifies the oxidation of the composite ceramics. This would also increase oxygen permeation through the scale, adversely affecting the oxidation resistance of the composites with high Zr2Al4C5 content.

Synthesis of zirconium boride (ZrB2) rod crystals through salt-assisted boro/carbothermal reduction

High-aspect-ratio ZrB2 rod crystals were synthesized via a molten-salt-mediated boro/carbothermal reduction using ZrOCl2· 8H2O/B4C/C as the raw material and NaCl as the flux material. This synthesis may improve the poor fracture toughness of monolithic ZrB2 ceramics. X-ray diffraction and scanning electron microscopy confirmed that the heat-treatment temperature of 1550 °C and the addition of 50 wt% NaCl were the most effective in the preparation of ZrB2 grains with a high aspect ratio. Thermogravimetric and differential scanning calorimetry was used to study the influence of NaCl addition, which increased the liquid-phase content. Finally, the growth of ZrB2 grains along the c-axis into rod-like morphology at elevated temperatures was investigated by transmission electron microscopy. This work revealed that the addition of NaCl has a positive effect on the reducing of synthesis temperature and increasing of products particle size. The ZrB2 rod crystals synthesized have diameters and aspect ratios of approximately 1 μm and 30, respectively.

Applying the Taguchi to Optimization the densification, and flexural strength of ZrB2–SiC–ZrC-CNFs

The purpose of this research is to optimize the densification, microstructure and flexural strength of ZrB2-25 vol% SiC composites reinforced by carbon nano fibers (CNFs) and zirconium carbide (ZrC). Spark Plasma Sintering (SPS) was applied for composites consolidation. Taguchi orthogonal array L9 as the design of statistical experiments (DOE) was evaluated to optimize the additives, as well as SPS parameters (temperature and time). Three levels were defined for each factor. The relative density (R.D%) and flexural strength of the samples were calculated based on Archimedes' law and the three-point bending test. Moreover, analysis of variance (ANOVA) was utilized to determine the importance of effective parameters and to optimize the SPS variables and compounds (the amount of CNFs and ZrC). The microscopic structure of surfaces was examined through field emission scanning electron microscopy equipped with energy dispersive spectroscopy (FE-SEM, EDS). The results showed that the best output of relative density and flexural strength was equal to 99.8% and 610 MPa, respectively, with an average ZrB2 grain size of 9.6 μm in a composite containing 15 vol% CNFs and 10 vol% ZrC and SPSed at 1875 °C for 7 min.

Melt/solid interaction and melt super-saturation effect on the microstructure asset of ultra-refractory composites prepared by reactive melting infiltration

The reactions occurring between a Zr2Cu melting phase and possible compounds intended for use in ultra-refractory ceramic matrix composite, i.e. SiC, TiB2 and ZrB2 are first explored through sessile drop tests, confirming a promising infiltration capability of the melt. Then, a multiphasic ultra-high temperature ceramic matrix composite was prepared by reactive melt infiltration (RMI), using a TiB2-coated carbon fiber preform infiltrated with Zr2Cu at 1500 °C which resulted in the precipitation of ZrB2 and ZrC sub-micrometric faceted grains and a general damage of both fiber and fiber coating, which were penetrated by the melt. Then, to decrease the RMI process temperature down to 1200 °C and limit the fiber damage degree, a second composite was prepared by introducing an additional ZrB2-B infiltration step. Although processing at 300 °C lower, notable alteration of the fiber occurred upon reaction with the Zr2Cu melt and developed a ZrC layer around them, but still allowing pull-out. In addition, the local B-rich environment promoted the development of a dual-phase microstructure within the matrix composed of new sub-micrometric grains and of larger ZrB2 grains, partially dissolved in the melt and re-precipitated acquiring a nano-sized lamellar eutectic structure. The combination of factors on multiple scale length, from the micro- of the fiber to the nano-of the lamella, seems to provide synergistic reinforcing phenomena and thus could be a valuable approach to develop materials for extreme environments.

Effect of SiC on ablation mechanism and morphological evolution of in situ synthesized ZrB2–SiC composites

In the current study, the synthesis of ZrB2–SiC composites was carried out by in-situ reduction of ZrO2, B4C, Si, and graphite (C) followed by hot pressing at 1500 °C. The thermodynamic calculation revealed the feasibility of the reactions. The amount of SiC increased from 12 wt% to 30 wt% as the B4C/Si and C/Si molar ratio decreased from 1.0 to 0.4 and 4.0 to 2.2, respectively. In situ formed SiC particles are homogeneously distributed on ZrB2 grains, which inhibits grains growth and enhances densification. The HR-SEM and EDS analysis revealed the microstructure and elemental analysis of the composite before and after the ablation test. The SiC particle provides better dimensional and thermal stability during ablation at 2600 °C due to the formation of SiO2 and ZrO2/SiO2 layers on the surface of oxidized composites. Also, the SiO2-dense layer protects the base composite due to the low diffusion coefficient of oxygen and the development of the SiC-depleted layer.

Thermal conductivity of hot-pressed ZrB2-ZrC ceramics

The thermal conductivity of ZrB2 with 0–10 vol.% ZrC was studied from 25 to 2000 °C. The ZrB2-ZrC ceramics were prepared by attrition milling the starting powders and adding 0.5 wt.% C as a sintering aid. The ceramics were hot-pressed to full density at 1900 °C for 30 min in a flowing Ar atmosphere. The ceramics contained graphitic carbon as a secondary phase, with ZrB2 and ZrC grain sizes of approx. 3.5 and 1.5 μm, respectively. At room temperature, thermal conductivity was 75 W/m·K for monolithic ZrB2, increasing to 79 W/m·K for 2.5 vol.% ZrC, and decreasing to 72 W/m·K for 10 vol.% ZrC. At 2000 °C, thermal conductivity was 72 W/m·K for ZrB2, decreasing to 69 W/m·K for 10 vol.% ZrC. Electrical resistivity behavior was similar to thermal conductivity, decreasing with the addition of ZrC up to 2.5 vol.% (9.9 µΩ·cm at 25 °C), then increasing with further ZrC additions. The increase in thermal conductivity with ZrC addition up to 2.5 vol.% was the result of preferential segregation of W from the ZrB2 phase to the ZrC phase. The reduction in thermal conductivity with greater ZrC additions was due to the lower thermal conductivity of ZrC.

In Situ ZrB2 Formation in B4C Ceramics and Its Strengthening Mechanism on Mechanical Properties.

In order to reduce the sintering temperature and improve the mechanical properties of B4C ceramics, ZrB2 was formed in situ using the SPS sintering method with ZrO2 and B4C as raw materials. Thermodynamic calculations revealed that CO pressure affected the formation of ZrB2 at temperatures from 814 °C to 1100 °C. The experimental results showed that the ZrB2 grain size was <5 µm and that the grains were uniformly distributed within the B4C ceramics. With an increase in ZrO2 content, the Vickers hardness and flexural strength of the B4C ceramics first increased and then decreased, while the fracture toughness continuously increased. When the content of ZrO2 was 15 wt%, the Vickers hardness, fracture toughness and flexural strength of B4C ceramics were 35.5 ± 0.63 GPa, 3.6 ± 0.24 MPa·m1/2 and 403 ± 10 MPa, respectively. These results suggest that ZrB2 inhibits B4C grain growth, eliminates crack tip stress, and provides fine grain to strengthen and toughen B4C ceramics.

High heterogeneous compatibility of HfB2-SiC ceramic and Zr-4 alloy with in-situ assembled interface

HfB2-based ceramics, such as HfB2-SiC, are promising materials of neutron control rods. Under nuclear conditions, the interfacial compatibility between the HfB2-SiC control rod and the guide tube (made of Zr-4 alloy) is critical in the operation. The present study demonstrated the high compatibility of the two heterogeneous materials even at 1400 °C. The formation of an in-situ assembled triple diffusion layer with stable phases and dense structures largely improved the interface compatibility. In addition, the as-produced phases with unique microstructures were organized at the interface. ZrSi formed typical columnar crystals with strong [001] fiber texture, which extended in the direction favorable to the interface. ZrB2 grains grew as needle shapes and entered the Zr-4 alloy substrate. These unique morphologies clearly revealed an interface with strong stability and high compatibility. The results provide the basis for the application of HfB2-based ceramics in nuclear infrastructures.

Effect of LaB6 addition on compressive creep behavior with simultaneous oxide scale growth of spark plasma sintered ZrB2‐SiC composites

AbstractA study has been carried out to examine the effect of LaB6 addition on the compressive creep behavior of ZrB2‐SiC composites at 1300–1400°C under stresses between 47 and 78 MPa in laboratory air. The ZrB2‐20 vol% SiC composites containing LaB6 (10% in ZSBCL‐10 and 14% in ZSBCL‐14) besides 5.6% B4C and 4.8% C as additives were prepared by spark plasma sintering at 1600°C. Due to cleaner interfaces and superior oxidation resistance, the ZSBCL‐14 composite has exhibited a lower steady‐state creep rate at 1300°C than the ZSBCL‐10. The obtained stress exponent (n ∼ 2 ± 0.1) along with cracking at ZrB2 grain boundaries and ZrB2‐SiC interfaces are considered evidence of grain boundary sliding during creep of the ZSBCL‐10 composite. However, the values of n ∼ 1 and apparent activation energy ∼700 kJ/mol obtained for the ZSBCL‐14 composite at 1300–1400°C suggest that ZrB2 grain boundary diffusion is the rate‐limiting mechanism of creep. The thickness of the damaged outer layer containing cracks scales with temperature and applied stress, indicating their role in facilitating the ingress of oxygen causing oxide scale growth. Decreasing oxidation‐induced defect density with depth to a limit of ∼280 μm, indicates the predominance of creep‐based deformation and damage at the inner core of samples.

Processing and mechanical properties of hot-pressed zirconium diboride – zirconium carbide ceramics

ZrB2 was mixed with 0.5 wt% carbon and up to 10 vol% ZrC and densified by hot-pressing at 2000 °C. All compositions were > 99.8% dense following hot-pressing. The dense ceramics contained 1–1.5 vol% less ZrC than the nominal ZrC addition and had between 0.5 and 1 vol% residual carbon. Grain sizes for the ZrB2 phase decreased from 10.1 µm for 2.5 vol% ZrC to 4.2 µm for 10 vol% ZrC, while the ZrC cluster size increased from 1.3 µm to 2.2 µm over the same composition range. Elastic modulus was ~505 GPa and toughness was ~2.6 MPa·m½ for all compositions. Vickers hardness increased from 14.1 to 15.3 GPa as ZrC increased from 2.5 to 10 vol%. Flexure strength increased from 395 MPa for 2.5 vol% ZrC to 615 MPa for 10 vol% ZrC. Griffith-type analysis suggests ZrB2 grain pullout from machining as the strength limiting flaw for all compositions.

Hot-pressed ZrB2–SiC composite ceramics: Effect of various Ta-containing additives on the microstructure and mechanical properties

The ZrB2–SiC composites have been commercially used at ultrahigh temperatures, but it often failed due to their poor toughness. In order to solve this problem, four types of Ta-containing additives (Ta, TaC, TaB2 and TaSi2) were used as the “third phase” to regulate the microstructure, so as to enhance the mechanical properties of hot-pressed ZrB2–20SiC-based ceramics (in vol. %). The incorporation of the additives generated a core–shell structure, which comprised of a ZrB2 core and a (Zr, Ta)B2 solid solution shell. The additives helped refine the ZrB2 grains in addition to the metallic Ta and release the internal stress field generated by the thermal misfit. The interfacial structure was modified by the formation of the coherent core/shell interface and the semi-coherent interface of adjacent ZrB2 grains and the semi-coherent ZrB2/(Zr, Ta)C interface in the TaC-additive composite. The addition of TaB2 or TaC hardened the ZrB2–20SiC ceramics, whereas the addition of Ta or TaSi2 reduced the hardness. The fracture toughness was enhanced by the formation of the Ta-containing phases. These phases reduced the stress intensity factor of the crack tip, which was proportional to the intrinsic residual stress. However, the crack-propagation mechanism would be changed by the incorporation of various Ta-containing additives. The decrease in the crack deflection, which was induced by the stronger interfacial bonding force and the significant consumption of SiC, resulted in relatively low toughness in the Ta- and TaC-included samples. The weaker interfacial bonding force in the TaB2- and TaSi2-included samples caused an increase in deflection and generated branching, which enhanced the toughness of the TaSi2-included composites to ∼4.72 MPa⸱m1/2.

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.

Solid‐state formation mechanisms of core–shell microstructures in (Zr,Ta)B2 ceramics

AbstractTransition metal diborides with core–shell microstructures have demonstrated excellent mechanical properties at elevated temperatures. Previous studies concluded that core–shell microstructures were formed by liquid‐assisted mass transport mechanisms, but in this study, we propose a solid‐state formation mechanism for core‐shell microstructures in (Zr,Ta)B2 ceramics produced by reaction hot pressing and in ZrB2‐TaB2 diffusion couples. Diffusion couple experiments demonstrated that core–shell microstructures developed as a result of Ta diffusion along ZrB2 grain boundaries, which occurred concurrently with lattice diffusion of Ta into ZrB2. These findings suggest that with optimization of batching and processing parameters, core–shell diboride materials may be formed through solid‐state processes rather than liquid‐assisted processes, which could assist in raising the upper temperature limits of use for these materials.

Heating rate effects on the thermal and mechanical properties of ZrB2

AbstractZirconium diboride ceramics were densified by hot pressing and spark plasma sintering with heating rates varying from 5 to 300℃/min. Slower heating rates produced larger grains due to the longer times spent at temperatures between 1500 and 1900℃, which is the temperature range in which ZrB2 grains coarsen. Heating rates above 50℃/min resulted in rapid densification, but this led to the retention of up to 3.3 vol.% of ZrO2 particles in the ceramics. After densification, changes to the microstructure were evaluated to interpret the effects of heating rate on thermal and mechanical properties. The flexure strength of ceramics processed by hot pressing up to 80℃/min was proportional to the inverse square root of the maximum grain size based on the Griffith criteria. Conversely, densification by spark plasma sintering, which had heating rates of up to 300℃/min, resulted in microcracks that decreased the elastic modulus from >500 GPa for pristine specimens to <485 GPa for microcracked materials. The use of heating rates >20℃/min also reduced the thermal conductivity due to the presence of retained ZrO2, but improved the strength by reducing the maximum grain size.