Preparation of reaction bonded silicon carbide (RBSC) using boron carbide as an alternative source of carbon

RBSC composites are fully dense materials fabricated by infiltration of compacted mixtures of silicon carbide and carbon by molten silicon. Free carbon is usually added in the form of an organic resin that undergoes subsequent pyrolysis. The environmentally unfriendly pyrolysis process and the presence of residual silicon are serious drawbacks of this process. The study describes an alternative approach that minimizes the residual silicon fraction by making use of a multimodal particle size distribution, in order to increase the green density of the preforms prior infiltration. The addition of boron carbide provides an alternative source of carbon, thereby eliminating the need for pyrolized organic compounds. The residual silicon fraction in the RBSC composites, prepared according to the novel processing route, is significantly reduced. Their mechanical properties, in particular the specific flexural strength is by 15% higher than the value reported for RBSC composites prepared by the conventional approach.     

Fabrication of carbon-coated boron carbide particle and its role in the reaction bonding of boron carbide by silicon infiltration

To tackle the dissolution problem of boron carbide particles in silicon infiltration process, carbon-coated boron carbide particles were fabricated for the preparation of the reaction-bonded boron carbide composites. The carbon coating can effectively protect the boron carbide from reacting with liquid Si and their dissolution, thus maintaining the irregular shape of boron carbide particles and preventing the growth of boron carbide particles and reaction formed SiC regions. Furthermore, the nano-SiC particles, originated from the reaction of the carbon coating and the infiltrated Si, uniformly coated on the surfaces of boron carbide particles, thus forming a ceramic skeleton of the nano-SiC particles-coated and -bonded boron carbide particles. The Vickers hardness, flexural strength and fracture toughness of the composites can be increased by 26 %, 45 %, and 37 % respectively, by using carbon-coated boron carbide particles as raw materials.     

Preparation of novel carborane-containing boron carbide precursor and its derived ceramic hollow microsphere

High melting point and hardness of boron carbide make it extremely difficult to be directly prepared as hollow microsphere. However, precursor derived method is an effective approach to prepare ceramic materials with complex shape. Therefore, in this work a novel boron carbide precursor, poly[1,7-bis(4-chlorophenyl)-m-carborane] (P4CB), was synthesized. The ceramic yield of the precursor P4CB reached as high as 90.25% at 900 °C in nitrogen. Oxidation of P4CB in air was barely observed below 500 °C, and a passive oxidation was exhibited beyond 700 °C. The P4CB/PAN slurry was prepared and coated on a polyoxymethylene (POM) ball substrate. After air crosslinking, substrate decomposition and heat-treatment at 1100 °C in Ar atmosphere, boron carbide hollow microsphere with diameter of approximate 1.34 mm and average shell thickness of 30 μm was finally obtained. The novel precursor could be also utilized to fabricate boron carbide ceramics with different shapes due to its high ceramic yield.     

Effect of powder purification by heat-treatment on the creep behaviour of dense boron carbide monoliths

High temperature compressive creep tests have been performed at 1650−1750 °C under applied stresses of 50−150 MPa on sintered boron carbide samples exhibiting high relative density and a mean grain size of 0.5 μm. The creep behaviour of two types of materials, sintered by spark plasma sintering from both raw and heat-treated powders, are characterized. For both materials, the identification of creep parameters (i.e. apparent activation energy and stress exponent values) coupled with TEM structural observations suggest a power law creep regime controlled by dislocation glide, which is limited by the presence of twins. However, the TP material exhibits lower stationary strain rates. This improved creep resistance seems to be directly correlated to the stoichiometry modification of the carbide induced by the powder pre-heat treatment, i.e. increase of structural carbon content and slight decrease of oxygen amount.     

Improving specific stiffness of silicon carbide ceramics by adding boron carbide

A strategy for improving the specific stiffness of silicon carbide (SiC) ceramics by adding B₄C was developed. The addition of B₄C is effective because (1) the mass density of B₄C is lower than that of SiC, (2) its Young’s modulus is higher than that of SiC, and (3) B₄C is an effective additive for sintering SiC ceramics. Specifically, the specific stiffness of SiC ceramics increased from ~142 × 10⁶ m²?s^?2 to ~153 × 10⁶ m²?s^?2 when the B₄C content was increased from 0.7 wt% to 25 wt%. The strength of the SiC ceramics was maximal with the incorporation of 10 wt% B₄C (755 MPa), and the thermal conductivity decreased linearly from ~183 to ~81 W?m^?1?K^?1 when the B₄C content was increased from 0.7 to 30 wt%. The flexural strength and thermal conductivity of the developed SiC ceramic containing 25 wt% B₄C were ~690 MPa and ~95 W?m^?1?K^?1, respectively.     

Synthesis and characterization of submicron silicon carbide powders with silicon and phenolic resin

A novel three-step process is used to fabricate submicron silicon carbide powders in this paper. The commercially available silicon powders and phenolic resin are used as raw materials. In the first step, precursor powders are produced by coating each silicon powder with phenolic resin shell. Then, precursor powders are converted into carbonized powders by decomposing the phenolic resin shell. The submicron silicon carbide powders are formed in the reaction of silicon with carbon during the third step of thermal treatment. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and thermogravimetric (TG) analyses are employed to characterize the microstructure, phase composition and free carbon content. It is found that the sintered powders consist of β-SiC with less than 0.2 wt.% of free carbon. The particle size of the obtained silicon carbide powders varies from 0.1 to 0.4 μm and the mean particle size is 0.2 μm. The silicon carbide formation mechanism of this method is based on the liquid-solid reaction between liquid silicon and carbon derived from phenolic resin. The heat generated during the reaction leads to great thermal stress in silicon carbide shell, which plays an important role in its fragmenting into submicron powders.     

前驱体转化法制备碳化硼粉体的研究进展

碳化硼材料具有优良的物理和化学性能,被广泛应用于各个领域。目前,传统的碳热还原方法生产碳化硼粉体存在温度高、产率低、环境污染重等问题。相比之下,前驱体转化法具有能耗低、工艺简便、原料易得、产品尺寸小等特点,在制备碳化硼粉体方面有明显的优势。详细介绍了前驱体转化法合成碳化硼粉体的制备过程,综述了使用不同碳源经前驱体转化法合成碳化硼粉体的最新研究进展,并展望了前驱体转化法合成碳化硼的研究方向。     

Effect of initial compositions on boron carbide synthesis and corresponding growth mechanism

ABSTRACT

The yield of boron carbide (B₄C) synthesised by carbothermal reduction (CTR) is low (40–50%) owing to the inappropriate initial composition. In this paper, thermodynamic equilibrium simulations for effects of initial compositions on B₄C synthesis were performed, while initial compositions on the quality and yield of B₄C synthesised by CTR from B₂O₃-C mixture at atmospheric pressure were systemically investigated. The reaction mechanisms for B₄C synthesis process were thermodynamically analysed. Moreover, growth mechanisms for B₄C particles were speculated based on the thermodynamic and experimental results. The results show that the simulations agree well with experimental results, and optimal initial composition is 1.906 (B₂O₃/C weight ratio). Under optimal condition, B₄C powder prepared at 1973?K is high-quality with well crystallinity, purity of 93.58% and yield of 90.05%. Additionally, the prepared B₄C particles show two types of morphologies, aggregate of small equiaxed particles and needle-like particle, which are formed through different reaction mechanisms, respectively.     

Drucker-Prager-Cap modelling of boron carbide powder for coupled electrical-thermal-mechanical finite element simulation of spark plasma sintering

The coupled electrical-thermal-mechanical finite element method in the continuum scale has been widely used to investigate the spark plasma sintering process. An accurate constitutive model of powder material is pivotal for precise continuum finite element simulation. In this study, the Drucker-Prager-Cap model, which is highly accurate in describing the densification behaviour of powder material, was adopted to numerically analyse the spark plasma sintering process of boron carbide powder. First, the parameters of the model were defined to be dependent on temperature and density for higher accuracy; they were determined by minimising the discrepancy between the simulated and experimental results. Based on a spark plasma sintering experiment with a cylindrical sample, the parameters of the Drucker-Prager-Cap model were identified at 1500 °C, 1600 °C, 1700 °C, 1800 °C, and 1900 °C. A coupled electrical-thermal-mechanical finite element simulation with the model was performed for spark plasma sintering of boron carbide powder at 1750 °C and 1850 °C. The temperature, stress, and relative density were investigated numerically. By comparing the model results with the temperature and relative density measured in the experiment, the continuum finite element method with the Drucker-Prager-Cap model was validated.     

Stabilization of highly-loaded boron carbide aqueous suspensions

Injection molding of boron carbide (B₄C) slurries affords the production of complex-shaped personal armor. To injection mold, however, requires preparation of a well dispersed, flowable suspension with >45 vol% B₄C loadings to reduce porosity that must be removed during sintering. In the present study, the preparation of highly-loaded B₄C suspensions is investigated using zeta potential and rheological measurements, varying dispersant type, molecular weight, and amount. Of those dispersants investigated, polyethylenimine (PEI) with a molecular weight of 25,000 g/mol was found to produce suspensions with up to 56 vol% B₄C and the requisite rheological properties suitable for injection molding. A PEI concentration of 1.83 mg/m² was established as the appropriate to produce highly-loaded B₄C suspensions. The effect of a prior B₄C powder treatment (ethanol washed or attrition milled) on rheological properties of the suspensions was also investigated. The PEI was completely burned out in argon, nitrogen, and air at 450 °C.     

The effect of carbon nanotubes introduction on the mechanical properties of reaction bonded boron carbide ceramics

The influence of carbon nanotubes (CNTs) on the mechanical properties and structure formation during reactive sintering of B₄C materials with Si addition was studied. Upon infiltrating the B₄C structure with molten silicon, a non-porous composite was formed with a density of 2.45-2.55 g/cm³ and a hardness of 22-27 GPa. The formation of highly dispersed B-C-Si phases was observed in the interphase of adjacent B₄C particles due to the incorporation of Si into B₄C structures. These phases increase the bonding strength between B₄C particles. In spite of the fact that the addition of 1-5 wt% Multi-Walled Carbon Nanotubes decreases the green density of the compacts, the flexural strength of the infiltrated material significantly increased. The improvement of the strength of ceramics modified with MWCNTs was interpreted in terms of the formation of thin flattened SiC crystals at the interfaces between B₄C and B-C-Si particles, which strengthen the interfaces between ceramic particles.     

Preparation of fully dense boron carbide ceramics by Fused Filament Fabrication (FFF)

Boron carbide is the third hardest material known, with a high melting point (2450 °C) and poor sintering ability. Therefore, boron carbide is a challenging material for shaping by conventional processing routes and can still be considered as unsuitable for commercial production of ceramics parts by additive manufacturing technologies. This work reports the first successful preparation of boron carbide ceramics fabricated by fused filament fabrication from a newly developed composite filament containing 65 wt% of micron-sized boron carbide powder dispersed in a thermoplastic binder. A commercial FFF desktop printer with a 0.40 mm nozzle was used for manufacturing of complex-shaped green bodies. Almost fully dense boron carbide ceramics with printed parts sized up to 4 centimeters and relative density higher than 96% after sintering were prepared. The DTA/TG analysis of composite filament and heat microscopy technique were used to set the debinding temperature program with critical temperature at 140 °C, due to the thermal decomposition of the binder. Microstructure SEM images after sintering showed excellent material homogeneity, while micro-CT images showed very well retained experimental shapes of collimator-like printed grids. The x-ray diffraction proved the presence of boron carbide phase with the free carbon phase at the level of about 1 wt% without significant influence on the measured hardness value of 29.88 ± 1.27 GPa.     

Pressureless sintering of boron carbide with Cr3C2 as sintering additive

In this study, chromium carbide (Cr₃C₂) was selected as the sintering additive for the densification of boron carbide (B₄C). Cr₃C₂ can react with B₄C and form graphite and CrB₂ in situ, which is considered to be effective for the sintering of B₄C composites. The sintering behavior, microstructure development and mechanical properties of B₄C composites were studied. The density of B₄C composite increased with the increase of Cr₃C₂ content and sintering temperature. The formation of liquid phase could effectively improve the densification of B₄C composites. The abnormal grains began to appear at 2080 °C. The bending strength could reach 440 MPa for the 25 wt% and 30 wt% Cr₃C₂ samples after sintering at 2070 °C.     

Exotic grain growth law in twinned boron carbide under electric fields

Grain growth is a ubiquitous phenomenon in all materials, and it affects both structural and functional properties. Despite its intrinsic importance, a full comprehension of grain growth from a fundamental point of view—i.e., from the nanoscale to the macroscale—is still a pending issue. In practical terms, our knowledge relies on the classical kinetic laws reported sixty years ago.This paper reports the violation of such classical laws in boron carbide ceramics consolidated by spark plasma sintering. The conjunction of high temperature gradients with large compressive stress when a pulse electric current passes through the ceramic powders gives rise to an intense twinning–detwinning formation. These forming steps at the grain boundaries change the grain mobility drastically. Therefore, a new ‘exotic’ law for grain-growth kinetics is found and validated at different temperatures and dwell times.     

Fabrication and characterization of hot-pressed B-rich boron carbide

Boron carbide has a wide solubility range owing to the substitution of B and C atoms in the crystal. In this study, boron carbides with different stoichiometric ratios were prepared using a hot-pressing sintering method, and the influences of the B/C atomic ratio on the microstructures and properties were explored in detail. X-ray diffraction analysis showed that excessive B atoms caused lattice expansion. Raman spectroscopy analysis showed disordered substitution of B atoms in the chains and icosahedra. Analysis of the densification process and microstructure evolution revealed that the addition of B promoted densification, and more stacking faults and twins occurred in B-rich boron carbide, and result in the densification mechanism gradually changes from atomic diffusion mechanism driven by thermal energy to plastic deformation mechanism dominated by the proliferation of dislocation and substructures. The introduction of chemical composition changes by dissolving excessive B into boron carbide further affected the microstructure and consequently the mechanical properties. The Vickers hardness, modulus, and sound velocity all decreased with the increase in B content. Moreover, the fracture toughness improved with increased B content. The flexural strength of the samples was optimised at the B/C stoichiometric ratio of 6.1.     

Silicon in intericosahedra chains of boron carbide

Interaction between boron carbide and silicon was studied at 1400–1700 ℃ in a vacuum. The results at 1400 ℃ suggest that silicon substitutes a single atom of carbon per formula unit B₁₂C₃, producing a phase with increased lattice parameters. Temperature increase improves the degree of transformation, but at 1700 ℃, the product starts to lose silicon. Analysis of DFT formation energies of configurations with B₁₂C₃ and B₁₂C₂Si stoichiometries shows that B₁₂C₂Si may have two ground states with practically equal stability. The first is B₁₁Si(C-B-C) with silicon in icosahedral polar position, and the second is B₁₂(C^Si^C), where the silicon is in the middle of an angular chain. The latter is suggested to be more favorable during the synthesis, while the former is very close to equilibrium with its precursor B₁₁C(C-B-C), and its formation may be inhibited. Comparison of XRD patterns for the synthesis products and modeled structures seems to confirm this suggestion.     

Direct ink writing of boron carbide monoliths

Direct ink writing – an extrusion-based additive manufacturing process – followed by pressureless sintering was investigated to produce boron carbide monoliths. The effects of ceramic powder loading and Pluronic binder concentration on the rheology of boron carbide pastes were studied and linked to both processing behaviour and final outcome in terms of sintered density and hardness. The effects of printing parameters, in particular orifice diameter and printing speed, were also investigated. Reducing the size of the extrusion nozzle from 584 μm to 406 μm led to significantly better shape retention, lower surface roughness, as well as higher density and hardness. A 203 μm printing orifice was also trialled but was unsuccessful due to faster drying kinetics that occurred with smaller ceramic struts resulting in rapid warping and nozzle clogging. Carbon-black – 8 wt% relative to B₄C – acted as an effective sintering aid to increase both density and hardness. After optimisation of feedstock and printing parameters, few-layer samples (3–5 layers) had a density as high as ∼ 97 % TD and a hardness of ∼ 30 GPa. On the other hand, 18-layer specimens had a sintered density of ∼ 87 % TD, despite a fully dense microstructure, due to the formation of a 3D array of inter-strut pores. Nevertheless, several issues that arose during manufacturing and post-processing were detrimental to the density and structural integrity of printed specimens; these issues were identified, discussed, and suggestions for future work are provided.     

Effects of reactants proportions on features of in-situ magnesiothermic self-propagating high temperature synthesized boron carbide powder

The effects of reactant proportions were investigated on features (phase composition, micromorphology and crystal development) of B₄C (boron carbide) powder synthesized by in-situ magnesiothermic SHS (self-propagating high temperature synthesis) method, which was based on the perspective of thermodynamic design. The results showed that the reactant proportions were the fundamental reasons affecting the phase composition of the products during the reaction process. Mg addition could decrease the Mg₃B₂O₆/MgO mass ratio of the SHS products. Moreover, C addition would increase the C/B mass ratio of the leached products. When the C molar ratio was 0.2, free boron appeared in the leached products, and the composition of boron carbide was B₁₃C₂. When the C molar ratio ≥0.6, the composition of boron carbide changed into B₄C, while free carbon began to appear. The lattice parameters a and c of boron carbide crystal dropped with the increase of the C/B mass ratio from 0.1 to 0.3. Meanwhile, carbon atoms gradually entered the boron carbide unit cell. When the C/B mass ratio exceeds 0.3, the lattice parameters tended to be constants of a = 5.608 Å, c = 12.039 Å, while free carbon began to appear in the leached products.     

Exciton and piezoelectricity in insulating 2-dimensional boron carbide

Using first-principles density functional theory, we predict a hexagonal structure of boron carbide with two shells, which consists of the sp² hybridized boron and carbon in (001) plane and the pz–pz (σ) bonding carbon along [001] direction. The calculated results show that the structure is thermodynamically stable and possesses lower formation energy than other candidates. In addition, the quasiparticle calculations within the GW approximation reveal that the boron carbide, which is a two dimensional insulator, exhibits the indirect band gap of 2.4 eV and large exciton bonding energy of 1.35 eV. In optical absorption spectra, a bright Frenkel class bound exciton has been discovered at about 2.98 eV, which is desirable for light emitting applications. Besides, the piezoelectric coefficient (e₂₂) of −2.38×10⁻¹⁰ Cm⁻¹ is predicted for monolayer boron carbide, which indicates that the monolayer boron carbide is a potential candidate for piezoelectric applications in the nanoelectromechanical systems.     

Optimization of wire electrical discharge machining parameters for cutting electrically conductive boron carbide

In this work, Pure boron carbide (B₄C) was consolidated using spark plasma sintering (SPS) at 2050 °C with a dwell of 10 min under 50 MPa uniaxial pressure in Argon atmosphere. The sintered specimen was >99% dense and offered characteristic Vickers hardness and fracture toughness of 31.4 GPa and 4.21 MPa-m^0.5, respectively, at 4.9 N indentation load. The specimen showed satisfactory wire electrical discharge machining (WEDM) performance because of its good electrical conductivity. The design of experiment (DOE) was arranged by L32 orthogonal array (OA) between the machining input parameters namely pulse on-time, pulse off-time, pulse peak current, dielectric fluid pressure and servo feed rate and the output responses like machining speed and surface roughness (R_a). Regression models were employed to establish the numerical correlation between the machining parameters and output responses. Experimental observations were utilized to formulate the first-order regression models to predict responses of WEDM. The optimized input parameters were 27 μs pulse on time, 48 μs pulse off time, 180 A pulse peak current, 7 kg/cm² water pressure and 2200 mm/min servo feed rate for the WEDM performance to produce an optimum machining speed and R_a.