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Microstructure evolution, densification behavior and mechanical properties of nano-HfB2 sintered under high pressure
Abstract
A series of pure HfB2 ceramics have been prepared by sintering nano-grained powder using high-energy ball milling at 700–1600 °C and 5.5 GPa. The HfB2 ceramics are characterized via various techniques for their residual stress, grain size, density, microstructures and defects, hardness, fracture toughness, thermal stability, and oxidation resistance. All properties strongly depend on the treatment temperature, but the exact manner of dependence for each property varies. The results identified that the HfB2 ceramic sintered at a relatively low temperature of 1000 °C and 5.5 GPa – a bulk pure nano-grained composite for the first time – has the best overall performance. It has a relative density of 99.6%, a Vickers hardness of 26 GPa, a fracture toughness of 5.2 MPa m1/2, and excellent thermal stability and oxidation resistance at high temperatures. Additional strengthening and stabilizing effects are provided by microstructures and defects such as large-angle grain boundaries, stacking faults and twinning. Simultaneous high temperature and high pressure is an effective sintering route for HfB2 ceramics with grain-size ranging from nanometer to micron.
... As a result, HPHT technology has emerged as a highly effective approach for synthesizing high-density, fine-grained HfB 2 and TaB 2 ceramics. 17,18 Previous studies on the high-pressure behavior of HfB 2 and TaB 2 ceramics have primarily focused on their compression characteristics and equations of state, employing first-principles calculations and in situ high-pressure synchrotron x-ray diffraction (HP-XRD) techniques. For instance, Zhang et al. 19 investigated the structural, elastic, and electronic properties of TaB 2 under pressures up to 120 GPa using first-principles calculations. ...
... The unique microstructures induced by HPHT conditions appear to have a positive impact on the material properties, a fact that has been substantiated by previous studies. 17,18,28 The densities of the sintered HfB 2 and TaB 2 bulk sample, measured using the Archimedes immersion method on an analytical balance, were determined to be 11.106 and 12.353 g/cm 3 , respectively. These values correspond to 99.1% and 98.3% of their theoretical densities, demonstrating a high degree of densification achieved during the sintering process. ...
... 46 Under an applied load of 9.8 N, the asymptotic Vickers hardness values for polycrystalline HfB 2 and TaB 2 were determined to be 21.3 and 17.8 GPa, respectively. The Vickers hardness of our synthesized HfB 2 is lower than that of the nanocrystalline HfB 2 (26 GPa) reported by Liang et al., 17 but it is comparable to the hardness values they obtained for micro HfB 2 . This discrepancy can be attributed to the differences in grain size and microstructural characteristics, as nanocrystalline materials typically exhibit enhanced hardness due to the Hall-Petch effect. ...
Ultra‐high‐temperature ceramics (UHTCs), especially hafnium diboride (HfB2) and tantalum diboride (TaB2), have become indispensable for extreme‐environment applications due to their unparalleled combination of mechanical strength, exceptional thermal stability, and ultra‐high melting temperatures exceeding 3200°C. While these refractory ceramics hold great promise for advanced technologies, their fundamental behavior under high‐pressure conditions remains insufficiently understood, posing a critical barrier to their full potential utilization. We successfully fabricated fully dense, phase‐pure polycrystalline HfB2 and TaB2 ceramics via precisely controlled high‐pressure, high‐temperature processing (5 GPa, 1300°C). Comprehensive microstructural characterization verifies the deliberate incorporation of advantageous lattice defects and tailored crystallographic distortions, which are engineered to optimize material performance. The mechanical, elastic, and thermodynamic properties of these materials were systematically investigated under varying pressures using in situ high‐pressure ultrasonic interferometry. Third‐order finite strain equation analysis of the experimental data at ambient temperature reveals distinct elastic properties for these UHTCs: HfB2 exhibits superior stiffness with a bulk modulus B0 = 313 ± 2 GPa (pressure derivative ∂B/∂P = 3.8 ± 0.1) and shear modulus G0 = 247 ± 3 GPa (∂G/∂P = 2.7 ± 0.1), while TaB2 shows relatively compliant behavior with B0 = 232 ± 3 GPa (∂B/∂P = 4.3 ± 0.2) and G0 = 151 ± 2 GPa (∂G/∂P = 3.6 ± 0.1). Thermodynamic analysis at high‐pressure conditions yields the following thermal transport properties: For HfB2, we obtain a Debye temperature Θ = 740.6 K, Grüneisen parameter γ = 1.242, and acoustic thermal conductivity λacoustic = 391.2 W/mK. The corresponding values for TaB2 are Θ = 584.9 K, γ = 1.418, and λacoustic = 153.5 W/mK. Our results reveal pressure‐dependent behaviors in elastic moduli, hardness, fracture toughness, and thermal transport, emphasizing the role of microstructure and lattice dynamics. Both HfB2 and TaB2 exhibit nearly linear relationships between wave velocity, bulk modulus, shear modulus, and Debye temperature with pressure. Notably, hardness decreases with increasing pressure, while fracture toughness increases, showing an inverse relationship. The Grüneisen parameter rises with pressure, while acoustic thermal conductivity decreases, reflecting distinct pressure‐dependent thermal and mechanical behaviors. Our findings reveal previously unrecognized microstructure‐dependent mechanisms in UHTCs under extreme conditions, enabling transformative design strategies for future thermal protection systems and radiation‐tolerant structural components.
... High pressure and high temperature (HPHT) can improve the density of materials and optimize their microstructure, resulting in unexpected improvements in their overall physical and chemical properties. HPHT technology has been widely used in the sintering of superhard materials [8,12,13], refractory carbides (e.g., TaC, HfC, WC, HfB 2 , ReB 2 , WB 4 , etc.), and even transition metal carbide high-entropy ceramics [11,[14][15][16][17][18][19]. ...
... The microstructure of the consolidated β-Mo 2 C samples is strongly dependent on the temperature at high pressure, corresponding to physicochemical properties such as Vickers' hardness (H V ), fracture toughness (K IC ), and Young's modulus (E) also showing correlations. The H V values obtained at different loading forces for the consolidated samples at 15 GPa and a typical temperature of 1100 • C are shown in Fig. 5a, illustrating that the H V values decrease with increasing loading force, which hardness curve yields a geometrically asymptotic at a loading force of 29.4 N. As well known that moderate loading forces are usually required to generate sufficiently well-defined microcracks in the material to analyze its toughness [16,56]. Fig. 5b shows the indentation SEM picture at 29.4 N of the consolidated representative β-Mo 2 C sample, where the diamond-shaped Vickers indentation and extension lines are visualized. ...
High-strain-induced microstructural refinement and dislocations are critical for the strengthening of metallic materials; however, this is difficult to achieve in ceramic materials due to their unique bonding characteristics and electronic structure. Here, a series of β-Mo2C bulk ceramics are consolidated by the high pressure and high temperature (HPHT) strategy. Our results demonstrate that high strain-induced grain plastic deformation at high pressure produces a lamellar sub-grain structure with high-density dislocations, and that the dislocations at the lath-like grain boundaries eventually evolve into low-angle grain boundaries. The mechanical properties, superconducting behavior, and onset of oxidation of the specimens are investigated. It is found that superconductivity and hardness arise from the high density of states at the Fermi level, while the high intrinsic hardness is attributed to the strong hybridization between Mo-4d orbitals and C-2p orbitals. Furthermore, strain-induced defect structure (dislocations and low-angle grain boundaries) mainly mainly enhances the intrinsic structure of β-Mo2C.
... Concerning the synthesis approaches, there are several methods to get individual ZrB 2 or HfB 2 , such as synthesis from elements [14,[21][22][23], oxide reduction [24][25][26], molten salt method [15]. These methods can potentially be used to produce the Zr₁₋ₓHfₓB₂ solid solutions and some of them have already been tested in experiment, namely synthesis from elements [14] and the molten salt method by reaction of ZrO 2 , HfO 2 and B in a NaCl/KC melt at 1100 • C [15]. ...
... The FWHM is influenced by grain size 098101-2 and stress. [33,34] There is mechanical bonding between grains at low temperatures. Following decompression, a significant amount of the local strain generated at high pressure is released. ...
Silicon carbide (SiC) is a high-performance structural ceramic material with excellent comprehensive properties unmatched by metals and other structural materials. In this paper, the raw SiC powder with an average grain size of 5 μm was sintered by an isothermal compression process at 5.0 GPa and 1500 °C, and the maximum hardness of the sintered samples is 31.3 GPa. Subsequently, scanning electron microscopy was used to observe the microscopic morphology of the recovered SiC samples treated in the temperature and extended pressure range of 0 - 1500 °C and 0 - 16.0 GPa, respectively. The defects and plastic deformation in SiC grains were further analyzed by transmission electron microscopy. Further, high pressure in situ synchrotron radiation x-ray diffraction was used to study the intergranular stress distribution and yield strength under non-hydrostatic compression. This study provides a new viewpoint for the sintering of pure phase micron sized SiC particles.
... It can be seen that the grain size does affect the elastic constants of CoCrNi MEA to some extent since its neighbor grains own large-angle grain boundaries, and this prediction is consistent with reported observations [32][33][34]. More specifically, Fig. 3(a) ...
In this paper, the molecular dynamics method was used to analyze mechanical properties and microscopic deformation mechanisms of CoCrNi medium-entropy alloy with different average grain sizes at various temperatures. Its elastic modulus and Poisson’s ratio were first calculated by the constant pressure molecular dynamics method. It is found that the elastic modulus increases with the average grain size increasing and is reduced at elevated temperatures. However, its Poisson's ratio decreases with the average grain size increasing and is not sensitive to temperatures. Simulations of simple tension were carried out and the results show that: (1) when the average grain size exceeds 15.2 nm, its yield stress and maximum flow stress decreased with the average grain size raising (Hall-Petch relationship), in this situation it is speculated that the dislocation slips and deformation twins within the grains dominate the plastic deformation; (2) when the grain size is smaller than 15.2 nm, the two stress parameters instead increase with the average grain size increasing (Inverse Hall-Petch relationship), such a plastic deformation mechanism is understood mainly due to grain boundaries migrations and grain rotations. In the end, as temperature effects on microscopic deformation mechanisms are concerned, it is found that more dislocations tend to be plugged near grain boundaries which have lower mobility at lower temperatures. Accordingly, the two stress parameters increase as the temperature decreases.
... It is worth mentioning that the reports on dense boride based ceramics with grain size < 100 nm are really rare in the literature. Unlike the processing under routine condition developed in this work, all of their consolidations require extreme high pressure (> 5 GPa) [40,41]. On the one hand, as the reaction and densification rates are comparable in this system, superfine TiB 2 based nanoparticles could be continuously generated; On the other hand, the required sintering temperature in T55 (1700°C) is also lower than most of the studies on TiB 2 based ceramics in the literature [23]. ...
In this work, dense TiB2-B4C based ceramics with different amounts of hBN additions were spark plasma sintered at 1700−1850 °C through in-situ reactions involving with TiCxN1-x (x = 0.5 and 0.7) and boron. The molar Gibbs’ free energy of TiCN is lower than that of TiC and TiN, thereby the boronizing of TiCN is more favorable in multiple steps which suppressed the combustion process in the TiCN-B system. Taking advantage of this, the reaction rate in the TiCN-B system was dragged to accompany with that of densification, resulting that nanometric TiB2 grains were gradually produced and microstructures composed of B4C-hBN reinforcement and TiB2 –hBN(C) matrix were finally built. Although hBN additions reduce the Young’s modulus, TiB2-B4C composites with 17.4 vol% hBN additions densified at 1700 °C still exhibit excellent mechanical properties. The present study suggests solid solution powders are a class of promising reactants for achieving nanoceramics with better performance.
... The temperature of the sample chamber was obtained directly using W-Re thermocouples. The cell pressure was calibrated using the pressure-induced phase change of known substances such as Bi, Ti, Ba, ZnTe, and ZnS [22]. In our experiments, the samples were gradually compressed to 13 GPa for the synthesis experiment at room temperature. ...
Cubic boron nitride (cBN), a typical superhard material, has garnered significant interest for both fundamental scientific research and technical applications. Here, nanostructured cBN with high-density nanotwins and stacking faults was synthesized by rapid quenching using hexagonal boron nitride (hBN) as the starting material at a moderate temperature (2200 K) and low pressure (13 GPa). This synthetic nanostructured cBN exhibits high Vickers hardness (78 GPa), high fracture toughness (11.8 MPa·m1/2), and extremely high oxidization temperature (1565 K). We propose that the refined grain structure and excellent mechanical properties of the cBN synthesized in this study is attributed mainly to dislocation slips and (111) mechanical nanotwins that dominate this phase transformation.
... The HfB 2 -15 wt% TiSi 2 composites reportedly exhibited good combination of properties. For the first time, Liang et al. [264] developed nano-structured bulk HfB 2 composites at very low temperature of 1000°C and high pressure of 5.5 GPa with high density (99.6%), hardness (26 GPa) and fracture toughness (5.2 MPa·m 1/ 2 ). As mentioned earlier in Section 4.5, Demiskyi et al. [195] used both FSPS and CSPS processes to densify HfB 2 . ...
Ceramic materials having melting points higher than 3000 °C and suitable for structural applications at above 2000 °C are commonly known as Ultra-High Temperature Ceramics (UHTCs). Several transition metal di-borides, possessing the desired combinations of thermo-mechanical, physical and chemical properties, form an important sub-class of the UHTCs. Over the last couple of decades, there has been a growing interest for UHTCs in general, and for the transition metal di-borides in particular, due to the increasing demands in hypersonic aerospace vehicles, atmospheric re-entry vehicles and energy applications. However, problems pertaining to sintering, moderate fracture toughness and experimental challenges associated with reliably measuring the elevated temperature properties, as well as the properties that determine the performances at the actual service conditions, have limited their widespread applications. This paper comprehensively reviews the various routes/techniques, including the advanced ones, as adopted for the synthesis and densification of the di-borides. The effects of sinter-additives and reinforcements on the densification, microstructure and various properties, including elevated temperature properties have been discussed in critical terms. Due attention has been paid towards understanding the challenges associated with the experimental measurements of the high temperature properties under extreme environmental conditions and the very recently developed techniques for the same. Some of the existing and futuristic applications of transition metal di-borides have also been discussed. Finally, the review concludes with an outlook towards some of the outstanding issues.
... www.springer.com/journal/40145 HfB 2 ceramics, as a member of TMB 2 ceramics, have recently attracted extensive attentions because they exhibit potential applications in high-temperature electrodes and thermal protection systems for hypersonic aerospace vehicles [17][18][19][20]. In this work, we successfully synthesized the nanocrystalline HfB 2 powders by the molten salt synthesis technique at a relatively low temperature of 1373 K using the inexpensive HfO 2 and B powders as reaction precursors within the inexpensive KCl/NaCl molten salts. ...
Nanocrystalline HfB2 powders were successfully synthesized by molten salt synthesis technique at 1373 K using B and HfO2 as precursors within KCl/NaCl molten salts. The results showed that the as-synthesized powders exhibited an irregular polyhedral morphology with the average particle size of 155 nm and possessed a single-crystalline structure. From a fundamental aspect, we demonstrated the molten-salt assisted formation mechanism that the molten salts could accelerate the diffusion rate of the reactants and improve the chemical reaction rate of the reactants in the system to induce the synthesis of the high-purity nanocrystalline powders. Thermogravimetric analysis showed that the oxidation of the as-synthesized HfB2 powders at 773–1073 K in air was the weight gain process and the corresponding oxidation behavior followed parabolic kinetics governed by the diffusion of oxygen in the oxide layer.
... These values are consistent with the Vickers hardness of TiB 2 ($33 GPa), B 6 O ($45 GPa), and HfB 2 ($25 GPa). [44][45][46][47][48] The accurate determination of pressure plays an important role in high-pressure experiments. 22,23,49,50 In Runs I and III, a 5 lm ruby sphere and a small piece of Pt flake with a diameter of $5 lm foil are used as pressure gauges to obtain the internally consistent pressure scale. ...
Hafnium diboride (HfB2) is one of the most promising hard-brittle ceramic materials with unique physical properties. Here, we have synthesized the well-crystallized HfB2 by a high-pressure solid-state reaction and employ in situ high-pressure synchrotron radiation angle-dispersive X-ray diffraction to investigate the size-effect of HfB2. An abnormal physical behavior of HfB2 under high pressure is observed. The microsized HfB2 shows slight anisotropy along a and c axes; however, the nanosized HfB2 reveals a different compression behavior with pressure-induced shell structural transition from a disordered symmetric amorphous shell state to an asymmetric amorphous shell state. In addition, the results indicate that the pressure calibrations are sensitive to the grain size. The present findings offer insights into the physical behaviors of different sized HfB2, which may also provide valuable information for other transition metal borides under high pressure.
Following the diverse structural characteristics and primary usage, diamond products include nano‐polycrystalline diamond (NPD), micron‐polycrystalline diamond (MPD), diamond film, porous diamond, and diamond wire drawing die. Among them, porous diamond possesses a distinctive combination of flexible surface functionality and a remarkably high surface area‐to‐volume ratio (SA/V) compared to traditional bulk materials, which contributes to cross‐cutting applications in catalysis, adsorption, and electrochemistry while retaining the superior traits of diamond, particularly its exceptional chemical inertia. To avoid etching or microwave plasma chemical vapor deposition (MPCVD) techniques, this study proposes a high‐temperature and high‐pressure method based on a soluble skeleton (HPHT‐ss) as an efficient and inexpensive approach for synthesizing millimeter‐level porous diamonds. Interestingly, porous diamond synthesized by HPHT‐ss exhibits multiscale pores distributed as macropores (average 75 µm) and mesopores (average 19 nm), which gives it a unique feature compared with other methods. Pertinent temperature‐pressure conditions, HPHT‐ss synthesis, and the formation mechanism of porous diamonds are also thoroughly discussed.
Context:
The mechanical properties and deformation mechanisms of CoCrNi medium-entropy alloy are studied through molecular dynamics simulations. The effects of temperature and average grain size on the elastic modulus, Poisson's ratio, yield stress, and maximum flow stress are investigated.
Methods:
The constant pressure molecular dynamics method is used to calculate the elastic modulus and Poisson's ratio of the alloy at different temperatures and average grain sizes. Simple tension simulations are conducted to determine the yield stress and maximum flow stress as a function of average grain size. The study also analyzes the dislocation behavior near grain boundaries at different temperatures using molecular dynamics simulations. The Hall-Petch and inverse Hall-Petch relationships are employed to describe the grain size-dependent deformation behavior of the alloy.
HfC coatings with different microstructure were successfully fabricated by chemical vapor deposition. The mechanical properties and ablation resistance of HfC coatings were evaluated, and the coefficient of thermal expansion (CTE) and ablation behavior of the as-prepared coatings were investigated in detail. Results showed that HfC coating with the dense coarse columnar structure exhibits good mechanical properties, and the equiaxed structure has a low CTE. The fine columnar structure coating has better ablation resistance compared with other microstructures, and the mass ablation rate and linear rate are only 2.02 mg/s and 1.25 μm/s after ablation for 90 s, respectively.
Tantalum diboride (TaB2) is an important transition metal (TM) diboride that has attracted considerable scientific and industrial interest. Herein, we fabricated TaB2 ceramic samples by combining high-pressure sintering with high-energy ball milling (HEBM) and investigated their microstructure and mechanical properties under high pressure at different sintering temperatures; these results were also compared with those of the bulk TaB2 ceramic. Thus, we evaluated the Vickers hardness, fracture toughness, Young's modulus, and relative density of our TaB2 ceramic at 1200 °C, under 5.5 GPa, to be 26 GPa, 3.8 MPa m1/2, 402 GPa, and 95.11%, respectively. Furthermore, we observed that the Vickers hardness, thermal stability, and fracture toughness are critically controlled with grain size and microstructures of the samples. These observations provide not only useful insights into the physical properties of TaB2 under high pressure and temperatures, but also avenues for the synthesis and design of novel functional ceramics.
Hafnium carbide (HfC) is a potential candidate of ultrahigh-temperature ceramics (UHTCs) and has attracted significantly widespread interest in recent years. Here, we have synthesized high-purity HfC samples with NaCl-type structure by using a high-pressure solid-solid reaction. The structural stability, equation of state, plastic deformation, yield strength, and bonding properties under high pressure are investigated by a series of in situ high-pressure synchrotron-radiation angle-dispersive X-ray diffraction experiments combined with first-principles calculations. The yield strength of HfC (∼18 GPa) is obtained from analyzing the plastic deformation behavior under high pressure. In addition, we have successfully prepared bulk HfC ceramics with high density using a high-pressure and high-temperature method. The synthesized sample possesses a desirable Vickers hardness of 24.2 GPa and an excellent fracture toughness of 5.0 MPa·m1/2. The present results offer insights into the achievable application of HfC ceramics under extreme conditions and provide a powerful guide for the further design and synthesis of other high-performance UHTCs.
Ultra-high temperature ceramics, UHTCs, are a group of materials with high technological interest because their use in extreme environments. However, their characterization at high temperatures represents the main obstacle for their fast development. Obstacles are found from a experimental point of view, where only few laboratories around the world have the resources to test these materials under extreme conditions, and also from a theoretical point of view, where actual methods are extremely expensive. Here, a new theoretical high-throughput framework for the prediction of the thermoelastic properties of materials is introduced. This approach can be systematically applied to any kind of crystalline material, drastically reducing the computational cost of previous methodologies. Elastic constants for UHTCs have been calculated at a wide range of temperatures with excellent agreement with experimentally reported values. Moreover, other mechanical properties such a bulk modulus, shear modulus or Poisson ration have been also explored. Other frameworks with similar computational cost have been used only for predicting isotropic or averaged properties, however this new approach opens the door to the calculation of anisotropic properties at a very low computational cost.
HfB2-based ultrafine composites with 25 vol% SiC and various volume fractions of HfC (5–20 vol%) have been successfully fabricated using spark plasma sintering. The composites were sintered in both 1800 °C and 1850 °C for different soaking times of 5 and 15 min and flexural strength, hardness and fracture toughness of specimens were measured. Sintering mechanism and mechanical properties of composites and HfC impact on them were investigated. The highest relative density (99.97 ± 0.02%) and the lowest apparent porosity (0.07 ± 0.03) were obtained for the specimen with 15 vol% of HfC. This composite also showed the maximum flexural strength (592 ± 17 MPa) and fracture toughness (4.98 ± 0.12 MPam1/2), even more than the composite with 20 vol% of HfC which had the highest Vickers hardness (22.7 ± 0.9 GPa).
Conventional metals become harder with decreasing grain sizes, following the classical Hall-Petch relationship. However, this relationship fails and softening occurs at some grain sizes in the nanometer regime for some alloys. In this study, we discovered that plastic deformation mechanism of extremely fine nanograined metals and their hardness are adjustable through tailoring grain boundary (GB) stability. The electrodeposited nanograined nickel-molybdenum (Ni–Mo) samples become softened for grain sizes below 10 nanometers because of GB-mediated processes. With GB stabilization through relaxation and Mo segregation, ultrahigh hardness is achieved in the nanograined samples with a plastic deformation mechanism dominated by generation of extended partial dislocations. Grain boundary stability provides an alternative dimension, in addition to grain size, for producing novel nanograined metals with extraordinary properties.
Transparent ceramics are important for scientific and industrial applications because of the superior optical and mechanical properties. It has been suggested that optical transparency and mechanical strength are substantially enhanced if transparent ceramics with nano-crystals are available. However, synthesis of the highly transparent nano-crystalline ceramics has been difficult using conventional sintering techniques at relatively low pressures. Here we show direct conversion from bulk glass starting material in mutianvil high-pressure apparatus leads to pore-free nano-polycrystalline silicate garnet at pressures above ∼10 GPa in a limited temperature range around 1,400 °C. The synthesized nano-polycrystalline garnet is optically as transparent as the single crystal for almost the entire visible light range and harder than the single crystal by ∼30%. The ultrahigh-pressure conversion technique should provide novel functional ceramics having various crystal structures, including those of high-pressure phases, as well as ideal specimens for some mineral physics applications.
Ultra-high-temperature composite materials HfB2-SiC containing 45 vol % SiC were prepared by spark plasma sintering. The behavior of a sample of this composition under exposure to a subsonic jet of dissociated air of a high-frequency induction plasmatron was studied; the total time was more than 30 min. Under certain test conditions, some regions of the sample were found to experience a rapid increase in temperature up to 2700°C. So, most of the surface area of the sample experienced exposure to temperatures up to 2500–2700°C for more than 15–18 min, while the rest of the surface had a temperature of 1700–1800°C during almost the entire duration of the experiment. The joint use of optical microscopy, scanning electron microscopy (with EDX analysis), X-ray powder diffraction, and X-ray computed microtomography enabled us to study the microstructure and composition of a structurally complex oxidized layer.
Good neutron absorber properties of hafnium diboride allow it to be a valuable candidate for control-rod application in pressurized water reactors. In order to understand irradiation effects in this ceramic, neutron irradiated samples are observed by SEM and TEM and thermal diffusivity measurements are carried out by the Flash-Laser method. A decrease of the diffusivity is observed and is partially related to the appearance of porosity at grain boundaries. This porosity does not seem to be helium bubbles, neither lithium precipitates. Defect structure is observed by TEM.
Nanostructured bulk NiAl materials were prepared at high pressure and temperature (0–5.0 GPa and 600–1500 °C, respectively). The sintered samples were characterized by x-ray diffraction, scanning electron microscope, density, and indentation hardness measurements. The results show that NiAl nanoparticles may have a compressed surface shell, which may be the reason why NiAl nanomaterials were difficult to densify sintering using conventional methods and why high-pressure sintering was an effective approach. We also observed that B2-structured NiAl could undergo a temperature-dependent phase transition and could be transformed into Al0.9Ni4.22 below 1000 °C for the first time. It is interesting to note that Vickers hardness decreased as grain size decreased below ∼30 nm, indicating that the inverse Hall-Petch effect may be observed in nano-polycrystalline NiAl (n-NiAl) samples. Moreover, a tentative interpretation was developed for high-pressure nanosintering, based on the shell-core model of nanoparticles.
We calculate the structural and electronic properties of OsB2 using density functional theory with or without taking into account the spin-orbit (SO) interaction. Our results show that the bulk modulus with and without SO interactions are 364 and 365 GPa, respectively, both are in good agreement with experiment (365–395 GPa). The evidence of covalent bonding of Os-B, which plays an important role to form a hard material, is indicated both in charge density, atoms in molecules analysis, and density of states analysis. The good metallicity and hardness of OsB2 might suggest its potential application as hard conductors.
Superplastic structural ceramics (Y-TZP, Al2O3, Si3N4, and their composites) that can withstand biaxial stretching to large strains have been developed recently. Microstructural design of these ceramics first requires an ultrafine grain size that is stable against coarsening during sintering and deformation. A low sintering temperature is a necessary, but not a sufficient, condition for achieving the required microstructure. In many cases, the selection of an appropriate phase, such as tetragonal phase in zirconia or α phase in silicon nitride, which is resistant to grain growth, is crucial. The use of sintering aids and grain-growth inhibitors, particularly those that segregate to the grain boundaries, can be beneficial. Second-phase particles are especially effective in suppressing static and dynamic grain growth. Another major concern is to maintain an adequate grain-boundary cohesive strength, relative to the flow stress, to mitigate cavitation or grain-boundary cracking during large strain deformation. Existing evidence suggests that a lower grain-boundary energy is instrumental in achieving this objective. The selection of an appropriate phase and the tailoring of the grain boundary or liquid-phase composition can sometimes drastically alter the cavitation resistance. Related observations on forming methods, forming characteristics, and sheet formability are also reviewed. The basic deformation characteristics are similar to diffusional creep and are dominated by grain-boundary diffusion. However, deformation characteristics are frequently altered by interface reactions, secondphase hardening/softening, and dynamic grain-growth-induced strain hardening. Ductility and formability, on the other hand, are controlled by the flow stress and flaw distribution, not by deformation instability as in superplastic metals. Analytical models and empirical correlations are presented to describe various constitutive relations pertaining to superplastic ceramics.
First-principles calculations were performed on the superhard material, W B 4 (Vicker hardness exceeding 46 GPa ), to reveal the origin of its high hardness. Our simulated lattice parameters, bulk modulus, and hardness are in excellent agreement with the experimental data. A three-dimensional B network with a peculiar B 2 dimer along the z -axis and a xy planar honeycomb B sublattice is uncovered to be mainly responsible for the high hardness. We further predicted that five other transition metal B compounds ( TM B 4 , TM = Re , Mo, Ta, Os, and Tc) within the W B 4 structure are potential superhard materials.
Strong HfB2 powder green bodies with compressive strengths exceeding 20MPa have been produced by incorporating 1 wt% of glassy carbon derived from a pyrolyzed phenolic resin binder. Starting from the green body after debinding in argon, non-textured HfB2 ceramics (diameter ~20 mm and height ~4 mm) with a relative density of 95.1% were achieved using flash spark plasma sintering (FSPS) with a discharge time of 20 s, peak power of 19 kW and applied pressure of ~16 MPa. The lack of a need for a pre-sintering step resulted in energy savings compared to the densification of HfB2 ceramics by FSPS.
We report an approach to strengthen micro-grained polycrystalline diamond (MPD) compact through work hardening under high pressure and high temperature, in which both hardness and fracture toughness are simultaneously boosted. Micro-sized diamond powders are treated without any additives under a high pressure of 14 GPa and temperatures ranging from 1000 °C to 2000 °C. It was found that the high pressure and high temperature environments could constrain the brittle feature and cause a severe plastic deformation of starting diamond grains to form a mutual bonded diamond network. The relative density is increased with temperature to nearly fully dense at 1600 °C. The Vickers hardness of the well-prepared MPD bulks at 14 GPa and 1900 °C reaches the top limit of the single crystal diamond of 120 GPa, and the near-metallic fracture toughness of the sample is as high as 18.7 MPa m1/2.
Using B4C and C additives, a HfB2-SiC composite with an enhanced strength up to 1600 °C was prepared using high-energy ball milling followed by hot pressing. The composite microstructure comprised equiaxed large HfB2 and fine SiC grains and an intergranular amorphous phase. The mechanical behaviour of the composite was evaluated up to 1600 °C via a four-point bending test. At or below 1500 °C, only a linear stress-strain response was observed. At 1600 °C, however, the initial linear response was followed by nonlinear deformation behaviour. The flexural strength was constant between room temperature and 1400 °C; subsequently, the flexural strength significantly increased with increasing temperature up to 1600 °C, with strengths in the range of 650-750 MPa.
A mechanically induced self-sustaining reaction (MSR) was used to synthesize hafnium diboride nanoparticles. Along this route, magnesium was selected as a robust reducing agent for co-reduction of boron and hafnium oxides in a combustive manner. Combustion occurred after a short milling period of 12 min. The hafnium diboride nanoparticles had a polygonal faceted morphology and were 50–250 nm in diameter. The assessment of the processing mechanism revealed that the initial combustive reduction of B2O3 to elemental B by Mg was the major step for progressing the overall reaction. After that, HfO2 can be reduced to elemental Hf, followed by the synthesis of HfB2 phase.
B4C-HfB2 composites were prepared by arc-melting using B4C and HfB2 as raw materials. The eutectic composition of B4C-HfB2 system was 70B(4)C-30HfB(2) (mol%) with a lamellar eutectic microstructure. HfB2 about 1 mu m in thickness was dispersed in B4C matrix uniformly of the eutectic composite, much smaller than raw powders. At the eutectic composition, the B4C-HfB2 composites showed the maximum Vickers hardness (31.2 GPa) and fracture toughness (5.3 MPa m(1/2)) at room temperature, and maximum thermal expansion coefficient (7.1 x 10(-6) K-1) from 293 to 1273 K. The electrical and thermal conductivity of B4C-HfB2 composites increased with increasing HfB2 content. The electrical conductivity of B4C-HfB2 eutectic composites decreased from 8.94 x 10(4) to 7.43 x 10(4) Sm-1 with increasing temperature from 298 to 800 K, showing a metallic electrical behavior. The thermal conductivity of B4C-HfB2 eutectic composite was 16-18 WK-1 m(-1) from 298 to 973 K.
The behavior of Hafnium di-boride (HfB2) under neutron irradiation has been simulated in a wide range of energy from 0.025 eV up to 14 MeV. The simulation and the analysis have been carried out using Geant4 and its related database. From the radiation shielding perspective, it was observed that, under thermal neutron irradiation, HfB2 scatters neutrons with a marginally higher energy than the incident neutrons and also produces prompt gamma rays up to 11 MeV. These results would indicate that, for high-energy neutron 14 MeV, not only is HfB2 unacceptable as a reasonable neutron absorber but also produces 20 MeV prompt gamma rays.
The correlation between structural stability and mechanical properties of Hf-B compounds is systemically investigated by first-principle calculations. The convex hull indicates that the HfB2 is more stability than other Hf-B compounds at ground state. The calculated bulk modulus, shear modulus, Young’s modulus and theoretical hardness of HfB2 are bigger than other Hf-B compounds, which are consistent with the variation of Poisson’s ratio and B/G ratio. Moreover, the calculated Vickers hardness of HfB2 is 40.7 GPa and the HfB2 has strong stiffness deformation resistant along the a-direction compared with the c-direction. The structural stability and mechanical properties of Hf-B compounds are related not only to boron concentration in a system but also to the localized hybridization and bond characteristic. We conclude that the large elastic modulus and high hardness of Hf-B compounds are mainly attributed to the network B-B covalent bonds. In particular, the bond orientation plays an important role in mechanical properties.
Densification behavior and thermal properties of hot‐pressed HfB2–Bx C particulate composites were studied. Boron carbide powders were synthesized throughout the homogeneity range (B4.3C–B˜10C) and beyond the carbon‐saturated (B3C) and boron‐saturated (B12C) limits for use as sintering additives in HfB2. Changes in the densification behavior of HfB2 were observed with changes in the B/C ratio of the boron carbide sintering additive. The most effective sintering additives were either the carbon or boron saturated boron carbides which produced composites with the highest relative densities. The improved densification behavior was caused by a combination of removal of surface oxides due to chemical reactivity and changes in the B‐ or Hf‐vacancy concentration in the HfB2 matrix phase. Thermal conductivities of the ceramics were only affected slightly by boron carbide stoichiometry. Thermal conductivities of HfB2 with 10 mol% additions of boron carbides ranged from 114 to 131 W·(m·K)−1 at room temperature and from 87 to 93 W·(m·K)−1 at 1000°C. Measureable changes in composite thermal conductivity with composition were attributed to slight differences in hot‐pressed densities that resulted from different boron carbide B/C stoichiometries.
In order to reduce production cost and effectively improve the oxidation protection ability of the ultra-high temperature ceramics (UHTCs) ZrB2–SiC coating for the SiC-coated carbon/carbon (C/C) composites, a ZrB2–SiC outer coating was prepared by in-situ reaction method on SiC-coated C/C. Instead of costly ZrB2 powder, inexpensive B2O3, ZrO2, Si and C powders were used as raw materials to prepare the outer ZrB2–SiC coating with well-developed microstructures. The ZrB2 and SiC phases in the outer coating were synchronously obtained during the preparation of the coating. The isothermal oxidation behavior of the coated C/C composites at 1773 K was investigated. The double-layer ZrB2–SiC/SiC coating could prevent C/C composites from oxidation at 1773 K for 550 h. During oxidation, a kind of ‘embedding structure’ containing ZrO2 and ZrSiO4 will be formed on the surface of the compound silicate glass layer, which is responsible for the excellent oxidation resistance of the ZrB2–SiC outer coating.
To protect the carbon/carbon (C/C) composites from oxidation, an outer ultra-high-temperature ceramics (UHTCs) HfB2-SiC coating was prepared on SiC-coated C/C composites by in situ reaction method. The outer HfB2-SiC coating consists of HfB2 and SiC, which are synchronously obtained. During the heat treatment process, the formed fluid silicon melt is responsible for the preparation of the outer HfB2-SiC coating. The HfB2-SiC/SiC coating could protect the C/C from oxidation for 265 h with only 0.41 × 10−2 g/cm2 weight loss at 1773 K in air. During the oxidation process, SiO2 glass and HfO2 are generated. SiO2 glass has a self-sealing ability, which can cover the defects in the coating, thus blocking the penetration of oxygen and providing an effective protection for the C/C substrate. In addition, SiO2 glass can react with the formed HfO2, thus forming the HfSiO4 phase. Owing to the “pinning effect” of HfSiO4 phase, crack deflecting and crack termination are occurred, which will prevent the spread of cracks and effectively improve the oxidation resistance of the coating.
The present work describes a simple process to synthesise HfB2 powder with sub-micron sized particles. Hafnium chloride and boric acid were used as the elemental sources whilst several carbon sources including sucrose, graphite, carbon black, carbon nanotubes and liquid and powder phenolic resin were used. The carbon sources were characterised using thermogravimetric analysis and transmission electron microscope. The mechanism by which the structure of the carbon source used, affects the size and morphology of the resultant HfB2 powder was studied; the HfB2 powders were characterised using X-ray diffraction and scanning and transmission electron microscopy. The powder synthesised using powder phenolic resin had a surface area of 21 m2 g−1 and a particle size distribution between 30 and 150 nm. This was sintered using SPS to a relative density of 94% of theoretical density (TD) at 2100 °C and 50 MPa pressure without the help of any sintering aids.
Flexural strengths at room temperature, at 1400 °C in air and at room temperature after 1 h oxidation at 1400 °C were determined for ZrB2- and HfB2-based ultra-high temperature ceramics (UHTCs). Defects caused by electrical discharge machining (EDM) lowered measured strengths significantly and were used to calculate fracture toughness via a fracture mechanics approach. ZrB2 with 20 vol.% SiC had room temperature strength of 700 ± 90 MPa, fracture toughness of 6.4 ± 0.6 MPa, Vickers hardness at 9.8 N load of 21.1 ± 0.6 GPa, 1400 °C strength of 400 ± 30 MPa and room temperature strength after 1 h oxidation at 1400 °C of 678 ± 15 MPa with an oxide layer thickness of 45 ± 5 μm. HfB2 with 20 vol.% SiC showed room temperature strength of 620 ± 50 MPa, fracture toughness of 5.0 ± 0.4 MPa, Vickers hardness at 9.8 N load of 27.0 ± 0.6 GPa, 1400 °C strength of 590 ± 150 MPa and room temperature strength after 1 h oxidation at 1400 °C of 660 ± 25 MPa with an oxide layer thickness of 12 ± 1 μm. 2 wt.% La2O3 addition to UHTCs slightly reduced mechanical performance while increasing tolerance to property degradation after oxidation and effectively aided internal stress relaxation during spark plasma sintering (SPS) cooling, as quantified by X-ray diffraction (XRD). Slow crack growth was suggested as the failure mechanism at high temperatures as a consequence of sharp cracks formation during oxidation.
Despite success of diamond anvil cells in all fields of high-pressure research, there have been continuous efforts to search for new anvil materials that are complementary to diamond but not limited by its cost, availability, and crystal size. In this regard, hexagonal silicon carbide, moissanite, has been found to be an ideal material (Xu and Mao, 2000). It is believed that moissanite anvil cell will open a new window for studies that require large sample volumes as well as stable and accurate temperature measurements. This study reports the yield strength measurements of moissanite at high temperature, which is one of the fundamental properties that define the ultimate performance of this material at high temperature conditions. We use the principles outlined by Weidner et al. (1994) to obtain information of stress and strength in the powder samples from x-ray diffraction signals. Two experiments have been performed at pressures up to 18.3 GPa and temperatures up to 1200 ° C using a DIA-type cubic anvil apparatus and a newly developed "T-Cup" high pressure system. At room temperature, the moissanite crystal behaves elastically with increasing pressure up to 13.7 GPa. At higher pressures applied, the sample is yielded, and the yield strength of moissanite is determined to be 13.6 GPa. Upon heating at 18.3 GPa, significant stress relaxation is observed at temperatures above 400 ° C, and the yield strength of moissanite decreases rapidly from 12.8 GPa at 400 ° C to 4.2 GPa at 1200 ° C. Such behavior will place severe limitations on the use of moissanite as anvil material when external heating is desired for high pressure and temperature experiments. References: Xu and Mao (2000), Science 290, 783-786. Weidner et al. (1994), Geophy. Res. Lett. 21, 753-756.
The synthesis and consolidation in one processing step of single phase tantalum diboride by Spark Plasma Sintering (SPS) is proposed. TaB2 formation from Ta and B elemental powders takes place at about 800°C through a solid-state combustion reaction while product densification requires higher temperatures. Consolidation is significantly improved (~96% density) when increasing the applied pressure from 20 to 60MPa immediately after the synthesis reaction occurs. Simplicity, short duration, milder temperatures and pressure conditions, in comparison with other routes utilized so far for obtaining bulk TaB2 are, along with the good mechanical properties of the obtained product, the main benefits of the adopted approach.
The sintering kinetics of α-Al2O3 powder are reviewed in this paper. The initial sintering of α-Al2O3 micropowder and α-Al2O3 nanopowder is all controlled by grain boundary diffusion. The sintering kinetics dominate up to a relative density of 0.77, where the coarsening kinetics dominate during further densification. Herring's scaling law can be used to predict the approximate sintering temperature of α-Al2O3 powder and demonstrates that if the particle size can be reduced to <20 nm, sintering below 1000°C may be possible.
HIP experiments carried out on pure alumina show that grain growth is enhanced both during and after the densification process. These results are compared with those published in the literature on several superplastic ceramics and metals, and a general phenomenological model for grain growth enhancement is proposed. This model is used to simulate grain size gradients generated during the HIP densification of a complex part. TEM examination of the samples shows that point defects and dislocation loops are responsible for grain growth enhancement.RésuméDes expériences de CIC menées sur l'alumine pure montrent que la croissance granulaire est accélérée durant et aprés la densification. Ces résultats sont comparés avec ceux publiés dans la littérature sur des céramiques et des métaux superplastiques. Nous proposons un mo déle général phénoménologique pour représenter la croissance accélérée. Ce modéle est ensuite utilisé pour simuler l'apparition d'un gradient de taille de grain durant la densification par CIC dans une piéce complexe. L'examen des échantillons au MET indique que les défauts ponctuels et les boucles de dislocation sont responsables de la croissance accélérée.ZusammenfassungReines Aluminiumoxid wird heiβ-isostatisch gepreβt; es zeigt sich, daβ das Kornwachstum sowohl während als auch nach der Verdichtung verstärkt abläuft. Diese Beobachtungen werden mit den in der Literatur für verschiedene superplastische Keramiken und Metalle veröffentlichten Ergebnissen verglichen. Es wird ein allgemeines phänomenologisches Modell für das verstärkte Kornwachstum vorgeschlagen. Mit diesem Modell werden die Gradienten, die in der Korngröβe während des Verdichtungsprozesses eines komplizierten Teiles entstehen, simuliert. Untersuchungen im Durchstrahlungselektronenmikroskop zeigen, daβ Punktfehler und versetzungsringe für das verstärkte Kornwachstum verantwortlich sind.
Nanocrystalline hafnium diboride (HfB2) has been prepared by the reaction of HfCl4 with NaBH4 at 600°C in an autoclave. The X-ray powder diffraction (XRD) pattern can be indexed as hexagonal HfB2 with the lattice constants of a=3.146 and c=3.456Å. The transmission electron microscopy (TEM) image shows a particle morphology with an average size of 25nm. The selected area electron diffraction (SAED) pattern confirms the presence of hexagonal HfB2. The oxidation behavior of HfB2 is studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA).
First-principle calculations are performed to investigate the structural, elastic, and electronic properties of ReB2 and WB2. The calculated equilibrium structural parameters of ReB2 are consistent with the available experimental data. The calculations indicate that WB2 in the P63∕mmc space group is more energetically stable under the ambient condition than in the P6∕mmm. Based on the calculated bulk modulus, shear modulus of polycrystalline aggregate, ReB2 and WB2 can be regarded as potential candidates of ultra-incompressible and hard materials. Furthermore, the elastic anisotropy is discussed by investigating the elastic stiffness constants. Density of states and electron density analysis unravel the covalent bonding between the transition metal atoms and the boron atoms as the driving force of the high bulk modulus and high shear modulus as well as small Poisson's ratio.
The strength of tungsten was determined under static high pressures to 69 GPa using x-ray diffraction techniques in a diamond anvil cell. Analysis of x-ray diffraction peak broadening and measurement of peak shifts associated with lattice strains are two different methods for strength determination of materials under large nonhydrostatic compressions. Here these methods are directly compared under uniaxial compression in a diamond anvil cell. Our results demonstrate the consistency of the two approaches, and show that the yield strength of tungsten increases with compression, reaching a value of 5.3 GPa at the highest pressure. The obtained yield strength of tungsten is also compared with previous experimental data involving shock wave and static compression measurements, and with theoretical predictions. The high-pressure strength of tungsten is comparable to that of other dense metals such as Re and Mo, and ratio of yield strength to shear modulus is about 0.02 for all these materials between 20 and 70 GPa. The static strength of tungsten is much greater than values observed for W under shock loading but is very similar to values observed under quasi-isentropic loading.
The recent interest in the properties of ceramics with grain sizes less than 100 nm has created a need for processing routes with which to manufacture such ceramics. This article briefly reviews the variety of techniques currently used to manufacture ultrafine starting powders, compact the powders, and sinter them into bulk nanocrystalline ceramics. The unique challenges associated with processing such fine structures are discussed together with each technique. Major obstacles have included the difficulty (now largely overcome) of producing sufficient quantities of ultrafine powders; the strong tendency of nanocrystalline powders to agglomerate; the difficulty of manufacturing homogeneous, stress free compacts from ultrafine powders; and the ever present obstacle of unwanted grain growth during sintering. Despite numerous technical difficulties, researchers have managed to develop techniques for processing ultrafine powders into nanocrystalline ceramics which are both fully dense (or near fully dense) and still retain a grain size less than 100 nm. Pressureless sintering, sinter-forging, hot pressing, and hot isostatic pressing, when carried out under the correct conditions, have all been shown to be capable of producing nanocrystalline ceramics. Microwave sintering, rapid rate sintering, plasma activated sintering, and shock compaction techniques have produced near-nanocrystalline ceramics. At the root of these accomplishments is an improved understanding of, and exploitation of, the microstructural events which occur during traditional powder processing procedures.
A pressureless sintering process, using a small amount of boron carbide (≤2 wt%) as sintering aid, was developed for the densification of hafnium diboride. Hafnium diboride ceramics with high relative density were obtained when the sintering temperature changed from 2100 °C to 2350 °C. However, the sintering mechanism was varied from solid state sintering (SSS, below 2300 °C) to liquid phase sintering (LPS, above 2300 °C). Boron carbide addition improved densification by removing the oxide impurities during solid state sintering and by forming a liquid phase which was well wetting hafnium diboride grains during liquid phase sintering process. The different roles of B4C on the microstructure development and mechanical properties of the sintered ceramics were investigated.
The fabrication temperature was the principal variable in a kinetic study of the densification of hafnium diboride in high-pressure hot-pressing. Densification studies for conventional hot-pressing were reviewed and correlated with the high-pressure hot-pressing results. The consolidation of HfB2 in the open pore region during high-pressure hot-pressing is attributed to particle rearrangement caused by grain boundary sliding and fragmentation. The final stage of densification (relative density >90%) was analyzed in terms of the Nabarro-Herring vacancy creep model. An activation energy of 22,900 cal/mole was obtained for the rate-controlling step in the creep process.
HfB2–SiC-based ultra-high-temperature ceramics with aluminum nitride (AlN) as a sintering aid were hot pressed at 1850°C. The sinterability and mechanical properties were investigated and compared with the composite without a sintering aid. It was shown that the addition of AlN greatly improved the powder sinterability and enabled the production of a nearly full-dense composite. The mechanical properties, especially the flexural strength, were enhanced remarkably through the improvement in the sinterability and microstructure. The oxidation resistance of a composite doped with 10 vol% AlN was evaluated by a plasma arc heater and the ablation mechanism was discussed.
A study was conducted to investigate boron-rich compounds of the systems Ru-B,Os-B, and W-B. The compounds OsB, Os2B32, OsB, RuB, RuB2, WB2, and WB4 were synthesized by arc melting and subsequent annealing. ReB2 and the ternary (Os 0.5W0.5)B2 were synthesized to study the influence of the isoelectronic substitution of Re by a mixture of W and Os. The structural stability and compressibility was analyzed through the use of X-ray powder diffraction up to pressures of 34 GPa at ambient temperature using ETH-type diamond anvil cell (DAC) and synchrotron radiation. The compressibility curves of OsB and Os2B3 also were found to be comparable to that of diamond, which indicates ultra-incompressible materials.
A growth equation for individual grains in single-phase materials is suggested. It is used to calculate a rate equation for normal grain growth and the size distribution in the material. It predicts a maximum size of twice the average size. The theory is modified to take into account the effect of second-phase particles. In an alternative treatment the array of grains is described in terms of a kind of defects introduced into a perfect array. The defects move through the array during grain growth. The rate of grain growth is calculated from the number of defects and their mobility. The defect concentration is predicted by comparing the two treatments. The defect-model predicts two grain size limits due to second-phase particles. Normal grain growth takes place below the lower limit. Abnormal grain growth can take place between the two limits if the material contains at least one very large grain. No grain growth can take place above the higher limit. Several possible mechanisms for the development of abnormal grain growth are examined. An explanation is offered for the observation that most of the well-known cases occur as the second-phase is dissolving.
Dense transition metal borides have recently been identified as superhard materials that offer the possibility of ambient pressure synthesis compared to the conventional high pressure, high temperature approach. This feature article begins with a discussion of the relevant physical properties for this class of compounds, followed by a summary of the synthesis and properties of several transition metal borides. A strong emphasis is placed on correlating mechanical properties with electronic and atomic structure of these materials in an effort to better predict new superhard compounds. It concludes with a perspective of future research directions, highlighting some recent results and presenting several new ideas that remain to be tested.
Hafnium diboride (HfB2)- and hafnium carbide (HfC)-based materials containing MoSi2 as sintering aid in the volumetric range 1%–9% were densified by spark plasma sintering at temperatures between 1750° and 1950°C. Fully dense samples were obtained with an initial MoSi2 content of 3 and 9 vol% at 1750°–1800°C. When the doping level was reduced, it was necessary to raise the sintering temperature in order to obtain samples with densities higher than 97%. Undoped powders had to be sintered at 2100°–2200°C. For doped materials, fine microstructures were obtained when the thermal treatment was lower than 1850°C. Silicon carbide formation was observed in both carbide- and boride-based materials. Nanoindentation hardness values were in the range of 25–28 GPa and were independent of the starting composition. The nanoindentation Young's modulus and the fracture toughness of the HfB2-based materials were higher than those of the HfC-based materials. The flexural strength of the HfB2-based material with 9 vol% of MoSi2 was higher at 1500°C than at room temperature.
The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this first part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elastic/plastic indentation fracture mechanics. It is thereby asserted that the key to the radial crack response lies in the residual component of the contact field. This residual term has important implications concerning the crack evolution, including the possibility of post indentation slow growth under environment-sensitive conditions. Fractographic observations of cracks in selected “reference” materials are used to determine the magnitude of this effect and to investigate other potential complications associated with departures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the Indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely simple in procedure and economic in its use of material.
This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.
Photomicrographs of pore and grain boundary structures in sintered powder compacts are presented to provide the basis for qualitative description of the important phases of the course of densification. From this guide, appropriate grain shapes and pore shapes and locations are selected for the formulation of diffusion sintering models. The principle models presented are for bulk diffusion transport with the grain boundaries as vacancy sinks when the pore phase is continuous and coincident with three grain edges, and also when the pore phase is discontinuous and located at four‐grain corners. These models predict that the rate of density change is constant when the diffusion coefficient and grain size are constant. The need for simultaneous isothermal densification and grain growth data is indicated. The explicit change in densification rate with discontinuous grain growth is predicted in terms of pore spacing and grain size.
It is shown that when certain plausible assumptions are fulfilled simple scaling laws govern the times required to produce, by sintering at a given temperature, geometrically similar changes in two or more systems of solid particles which are identical geometrically except for a difference of scale. It is suggested that experimental studies of the effect of such a change of scale may prove valuable in identifying the predominant mechanism responsible for sintering under any particular set of conditions, and may also help to decide certain fundamental questions in fields such as creep and crystal growth.
Models for initial‐, intermediate‐, and final‐stage densification under pressure have been developed, which explicitly include both the surface energy and applied pressure as driving forces. For the initial stage, the time dependences and size effects given by the integrated equations are identical to those reported earlier for surface energy (alone) as the driving force. The only modification is that the surface energy (γ) is expanded into (γ+P a R/π), where P a is the applied pressure and R is the particle radius. For the intermediate stage of the process, the Nabarro‐Herring and Coble creep models may be adapted to give approximate (∼4×) densification rates for lattice and boundary diffusion models, respectively. In these cases the complex driving force is written as: (P a /D+γk), where D is the relative density, and k is the pore surface curvature. At the final stage of the process those models are invalid; an alternate model is developed based on diffusive transport between concentric spherical shells which will give a better assessment of the time dependence of densification high density (≫95%); the driving force is (P a /D+γk) in this case also. Because of the fact that the pore size is some unknown function of density, the rate equations cannot be integrated without further information. It is shown that of the various relations which have been assumed in development of models for hot pressing, for the effective stress in relationship to the applied stress and the porosity, (P a /D) is the only form which satifies the criteria demanded by self‐consistency in generation of steady‐state diffusion models.
Basic mechanical properties of single crystal gallium nitride are measured. A Vickers (diamond) indentation method was used to determine the hardness and fracture toughness under an applied load of 2N. The average hardness was measured as 12±2 GPa and the average fracture toughness was measured as 0.79±0.10 MPa√m. These values are consistent with the properties of brittle ceramic materials and about twice the values for GaAs. A methodology for examining fracture problems in GaN is discussed. © 1996 American Institute of Physics.
We have measured the pressure dependence of the solid phase epitaxial growth rate of self‐implanted Si (100) by using the in situ time‐resolved interferometric technique in a high‐temperature and high‐pressure diamond anvil cell. With fluid argon as the pressure transmission medium, a clean and perfectly hydrostatic pressure environment is achieved around the sample. The external heating geometry employed provides a uniform temperature across the sample. At temperatures in the range of 530–550 °C and pressures up to 3.2 GPa (32 kbar), the growth rate is enhanced by up to a factor of 5 over that at 1 atmosphere pressure. The results are characterized by a negative activation volume of approximately -3.3 cm3/mole (-28% of the atomic volume). These results show a significantly weaker pressure dependence than does the previous work of Nygren et al. [Appl. Phys. Lett. 47, 232 (1985)], who found an activation volume of -8.7 cm3/mole. The implication of this measurement for the nature of the defects responsible for crystal growth is discussed.
The structural, elastic, and electronic properties of Tc B 2 and recent synthesized superhard Re B 2 were studied by the first-principles calculations. Both Re B 2 and Tc B 2 are found to be elastically stable with the hexagonal and orthorhombic structures. For the two materials, the hexagonal structure is more stable than the orthorhombic one. Moreover, hexagonal Re B 2 and Tc B 2 have stronger directional bonding between ions than Os B 2 , Ir N 2 , Os N 2 , and Pt N 2 . The author’s calculations show that both Re B 2 and Tc B 2 are potentially superhard materials on the basis of their large bulk moduli and also their large shear to bulk modulus ratios. The band structure shows that experimentally synthesized hexagonal Re B 2 is metallic. A pseudogap appears around the Fermi level of the total density of states of hexagonal Re B 2 , which may contribute to its stability.
The use of silicon nitride as a sintering aid (5 vol.%) greatly improves the powder sinterability of zirconium diboride, in comparison to additive free ZrB2. Nearly full dense monolithic material is obtained by hot pressing at 1700 °C. The microstructure consists of fine regular ZrB2 grains and of various secondary grain boundary phases (e.g. BN, t-ZrO2, BN-rich glassy phase), mainly located at triple points. The addition of 20 vol.% of silicon carbide as a reinforcing particulate phase to the ZrB2+5vol.%Si3N4 powder mixture slows down the densification rate of ZrB2, therefore a higher hot pressing temperature (i.e. 1870 °C) is necessary to achieve nearly full density. Further addition of oxide additives (1vol.%Al2O3+0.5vol.%Y2O3) to the ZrB2–20vol.%SiC–5vol.%Si3N4 system enables the production of near fully dense composites at lower hot pressing temperature (1760 °C). The presence of SiC particles in both the ZrB2-based composites effectively improves strength, hardness and toughness, compared to monolithic zirconium diboride. Some mechanical properties are very interesting: flexural strength up to 700 and 600 MPa are measured at room temperature and 1000 °C, respectively. The properties are discussed in terms of the microstructural features.
A pressureless sintering process with two pre-sintering heat treatments was developed for the densification of HfB2. The use of two isothermal holdings, at 1650 and 2000 °C, prior to the final sintering temperature (2200 °C) proved to be more effective at removing oxygen impurities and rearranging the initial powder compact microstructure. The mechanical properties of HfB2 with 2 wt.%B4C, including hardness (19.5 GPa), E modulus (529 GPa) and strength (469 MPa), were comparable to hot-pressed samples or higher than the values using other sintering aids.