Microstructural evolution during the infiltration of boron carbide with molten silicon

The previously reported model that accounts for the formation of the core-rim structure in reaction-bonded boron carbide composites (RBBC) is expanded and validated by additional experimental observations and by a thermodynamic analysis of the ternary B–C–Si system. The microstructure of the RBBC composites consists of boron carbide particles with a core-rim structure, β-SiC and some residual silicon. The SiC carbide particles have a polygonal shape in composites fabricated in the presence of free carbon, in contrast to the plate-like morphology when the initial boron carbide is the sole source of carbon. In the course of the infiltration process, the original B₄C particles dissolve partly or fully in molten silicon, and a local equilibrium is established between boron carbide, molten silicon and SiC. Overall equilibrium in the system is achieved as a result of the precipitation of the ternary boron carbide phase B₁₂(B,C,Si)₃ at the surface of the original boron carbide particles and leads to the formation of the rim regions. This feature is well accounted for by the “stoichiometric saturation” approach, which takes into account the congruent dissolution of B₄C particles. The SiC phase, which precipitates form the silicon melt adopts the β-allotropic structure and grows preferably as single plate-like particles with an {1 1 1}_β habit plane. The morphology of the SiC particles is determined by the amount of carbon available for their formation.     

Pressureless sintering and properties of boron carbide composite materials

Boron carbide and Tantalum boride composites were prepared by pressureless sintering of B₄C with addition of TaC powder. The effect of TaC addition on the sinterability of boron carbide was studied. High densified ceramic with a relative density of 98.7% was obtained at sintering temperature of 2250°C. The composition and the microstructure of the dense composites are characterized by means of x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive x-ray spectroscopy (EDX). The studies show that the composites contain boron carbide, TaB₂, and carbon phases with a homogeneous structure. In addition, the correlation between the composition and the electrical conductivity was investigated. The electrical conductivity of the composite increased with increasing addition of TaC, and a change in conduction behavior from semiconducting to metallic was observed. High hardness value of 28.49 ± 1.33 GPa was obtained by the sample with 30 wt% TaC addition.     

Experimental observations of amorphization in multiple generations of boron carbide

Boron carbide is an excellent armor material due to its light weight and ultrahigh hardness. However, high-rate mechanical behavior can be degraded by stress-induced amorphization. In this paper, we review the progressive advances in the understanding of amorphization in three successive generations of boron carbide: stoichiometric (undoped), B-rich, and B/Si codoped boron carbides. For each generation of boron carbide, the crystal structure and microstructure are first discussed. Then, we outline the experimental observations of amorphization made by Raman spectroscopy and transmission electron microscopy. The susceptibility of amorphization in each generation of boron carbide will be compared and the fundamental mechanisms that explain the reduction in amorphization for B-rich and B/Si codoped boron carbides elucidated. Comments on future research directions to further broaden and deepen the understanding of stress-induced amorphization of boron carbide are also provided.     

制备高B6.5C相含量碳化硼粉末的工艺探讨

用电弧炉碳热还原法制备高B6．5C相含量碳化硼粉末，其化学成分可达到核工业级标准的要求。用X射线衍射法分析，其B6．5C相质量分数可达到85％以上。实验证明，采用提高冶炼温度和延长冶炼时间，在电弧炉中冶炼高B．5C相碳化硼粉是可行的。     

Densification and characterization of rapid carbothermal synthesized boron carbide

Submicrometer boron carbide powders were synthesized using rapid carbothermal reduction (RCR) method. Synthesized boron carbide powders had smaller particle size, lower free carbon, and high density of twins compared to commercial samples. Powders were sintered using spark plasma sintering at different temperatures and dwell times to compare sintering behavior. Synthesized boron carbide powders reached >99% TD at lower temperature and shorter dwell times compared to commercial powders. Improved microhardness observed in the densified RCR samples was likely caused by the combination of higher purity, better stoichiometry control, finer grain size, and a higher density of twin boundaries.     

Coarsening of boron carbide grains during the infiltration of porous boron carbide preforms by molten silicon

This work investigates the coarsening of boron carbide grains during the infiltration of porous boron carbide preforms by molten silicon with respect to fabrication of reaction-bonded boron carbide ceramics. Experimental results reveal that the shape of boron carbide grains evolve from the irregular shape to faceted shape due to dissolution-precipitation during infiltration. For infiltration temperatures below 1750 °C, the boron carbide grains are irregular and exhibit an unimodal size distribution, which can be ascribed to the normal grain growth. The growth of the irregular grains follow a cubic law of diffusion control. In contrast, for infiltration temperatures above 1750 °C, the boron carbide grains become faceted and exhibit a bimodal size distribution, indicative of the typical abnormal grain growth. The abnormal growth of faceted grains is proposed to be controlled by coalescence-enhanced two-dimensional nucleation.     

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.     

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。     

Optimization of the decarbonizing ratio of boron carbide powders by reverse flotation using response surface methodology

Free carbon is the main impurity in boron carbide and has many side effects on the quality of boron carbide. In this study, reverse flotation was used for the first time to remove free carbon in boron carbide. The response surface methodology was utilized to optimize the reverse flotation factors, and the samples were analyzed by X-ray diffraction, scanning electron microscope, laser particle size analyzer, and chemical analysis. The study results reveal that the main factors affecting the decarbonization ratio were slurry concentration, collector dosage, foaming agent dosage and pH value. Furthermore, the results also show that reverse flotation could be applied effectively to the removal of free carbon in boron carbide. Slurry concentration of 25.14%, collector dosage of 567.9 g/t, foaming agent dosage of 199.32 g/t and pH value of 8.4 were found to be the best conditions. Under the optimal conditions, the decarbonization ratio is 84.23%. Mass ratio of free carbon in boron carbide reduced from 2.98 to 0.47.     

碳化硼微粉的低温合成

以聚乙烯醇和硼酸为原料,首先合成聚乙烯醇硼酸酯前驱物凝胶,然后将前驱物热解及碳热还原制备碳化硼粉末。考察了聚乙烯醇与硼酸的物质的量比,前驱物热解温度,碳热还原温度以及还原时间等因素对碳化硼合成的影响。采用IR、化学分析、XRD、离心粒度分析、SEM等方法对中间物及产物进行了表征,确定了中间物及产物的组成、物相、粒度分布及形貌。研究结果表明：前驱物合成的适宜原料配比是n(聚乙烯醇)∶n(硼酸)=4∶1;前驱物在600 ℃下热解2 h,在1 300 ℃下碳热还原2 h,得到粒径为10 μm左右的碳化硼微粉。     

The effect of microwave heating on the microstructure and the mechanical properties of reaction-bonded boron carbide

Reaction-bonded boron carbide composites were fabricated by both microwave (under Ar/10% H₂) and conventional heating (under vacuum or Ar/10% H₂). Silicon carbide (SiC) formation occurred in all cases and was slightly favored in the case of microwave heating under Ar/H₂. The resulting microstructures were influenced by the heating process and atmosphere; the SiC existed in the form of needles with conventional heating under vacuum. SiC small polygonal grains were present after microwave heating under Ar/H₂. Both the atmosphere and the electromagnetic field influence the SiC morphology. Despite this difference, the hardness and toughness of composites obtained by both heating techniques were similar.     

Boron Carbide Reinforced Alumina Composites

The mechanical properties of alumina have been successfully improved by adding isolated boron carbide particles of two different shapes. A K _Ic of 7.26 ± 0.20 MPa · m^1/2 for alumina—boron carbide whiskerlike composites and of 5.27 ± 0.12 MPa · m^1/2 alumina—boron carbide shardlike particle composites has been achieved. The fracture toughness of these composites is dependent on the volume fraction of the boron carbide particles as well as their size and shape. The flexural strength is also appreciably enhanced to a constant value with from 5 to 20 vol% boron carbide additions. The whiskerlike particles increase the flexural strength by 25% and the shardlike particles produce a 47% improvement.     

Strengthening boron carbide by doping Si into grain boundaries

Grain boundaries, ubiquitous in real materials, play an important role in the mechanical properties of ceramics. Using boron carbide as a typical superhard but brittle material under hypervelocity impact, we report atomistic reactive molecular dynamics simulations using the ReaxFF reactive force field fitted to quantum mechanics to examine grain-boundary engineering strategies aimed at improving the mechanical properties. In particular, we examine the dynamical mechanical response of two grain-boundary models with or without doped Si as a function of finite shear deformation. Our simulations show that doping Si into the grain boundary significantly increases the shear strength and stress threshold for amorphization and failure for both grain-boundary structures. These results provide validation of our suggestions that Si doping provides a promising approach to mitigate amorphous band formation and failure in superhard boron carbide.     

Synthesis,structure, and properties of polymer-derived,metal-reinforced boron carbide cermet composites

We report on a novel polymer-derived synthesis approach that yields boron carbide monoliths and metal-reinforced B₄C cermets. This single-step approach relies on a preceramic powder blend of boron and an acetylenic resin with a high char yield. At low temperatures below 1500°C and without applied pressures, preceramic precursor reaction bond together and form nanocrystalline refractory B₄C matrices. Resulting near net shape boron, carbide monoliths exhibit small crystal grain sizes, maintain chemical purity, and exhibit morphological homogeneity. We reinforce the refractory carbide with five different metals and demonstrate the influence of each on the density, hardness, oxidation stability, and electronic conductivity of resulting cermet composites. We assess the optimal synthesis and reinforcement strategies that tailor these nanostructured materials for inexpensive and high-performing engineering applications.     

Improvement of toughness of reaction bonded silicon carbide composites reinforced by surface-modified SiC whiskers

To improve the reliability, especially the toughness, of the reaction bonded silicon carbide (RBSC) ceramics, silicon carbide whiskers coated with pyrolytic carbon layer (PyC-SiC_w) by chemical vapor deposition (CVD) were introduced into the RBSC ceramics to fabricate the SiC_w/RBSC composites in this study. The microstructures and properties of the PyC-SiC_w/RBSC composites under different mass fraction of nano carbon black and PyC-SiC_w were investigated methodically. As a result, a bending strength of 550 MPa was achieved for the composites with 25 wt% nano carbon black, and the residual silicon decreased to 11.01 vol% from 26.58 vol% compared with the composite of 15 vol% nano carbon black. The fracture toughness of the composites reinforced with 10 wt% PyC-SiC_w, reached a high value of 5.28 MPa m^1/2, which increased by 39% compared to the RBSC composites with 10 wt% SiC_w. The residual Si in the composites deceased below to 7 vol%, resulting from the combined actively reaction of nano carbon black and PyC with more Si. SEM and TEM results illustrated that the SiC_w were protected by PyC coating. A thin SiC layer formed of outer surface of whiskers can provide a suitable whisker-matrix interface, which is in favor of crack deflection, SiC_w bridging and pullout to improve the bending strength and toughness of the SiC_w/RBSC composites.     

Optical characterization of boron carbide powders synthesized with varying B-to-C ratios

Boron carbide powders were synthesized from elemental powders and studied using X-ray diffraction (XRD) and UV–visible diffuse reflectance, Raman, and diffuse reflectance IR spectroscopies. Following reaction at 1400°C for 6 h, synthesized powders exhibited possible faulting as suggested by XRD patterns. B₃C, B_4.3C, and B₅C powders contained graphitic carbon whereas the boron carbides with higher B/C ratios contained no residual carbon, suggesting that the carbon rich phase boundary is likely temperature dependent. Analysis by Raman and IR spectroscopy suggested that Raman spectra are influenced by excitation frequency due to resonance. We suggest that measurement of boron carbides with resonant Raman lifts the selection rules to allow measurement of Raman silent modes that are present in the IR spectra. Optical reflectance of the boron carbide powders revealed that the B/C ratio governed the indirect and direct optical band gaps of the faulted powders. B₃C and B_4.3C powders were light gray in spite of the presence of the carbon, whereas B₅C, B_6.5C, B₁₀C, and B₁₂C were gray, green, brown, and dark brown, respectively. Increasing carbon content increased the optical indirect band gap from 1.3 eV for B₁₂C to 3.2 eV for B₃C, causing the observed color changes.     

Production of titanium-containing metal-ceramic composites based on boron carbide in the nanocrystalline state

ABSTRACT

The results of the study of the production technology, phase composition, structure and physico-mechanical properties of metal-ceramic materials based on boron carbide and their components are presented. Boron carbide was obtained by direct synthesis from chemical elements using amorphous boron and carbon black. By mechanical dispersion, solid reagents were converted into an ultrafine state. Using a chemical method, nanoscale (70–80?nm) boron carbide was synthesised from suspension solutions of amorphous boron and liquid hydrocarbons. Boron carbide-based metal-ceramic composite powder B₄C–(Co–Ni–Ti) was obtained by mechanical dispersion of the constituent components. Based on results of studying of the temperature-dependence of wetting angle of boron carbide with Co–Ni–Ti metallic alloy, the compacting modes of metal-ceramic composite powders by plasma-spark sintering and hot pressing have been developed. The influence of the component content of the binder metal (alloy) on some physico-mechanical properties (linear expansion coefficient, hardness, and bending strength) of hardmetal-ceramic materials based on boron carbide was studied. It was found that the optimum content of the metal component in the composite is ～ 25?wt-%. In the temperature range 300–600°C, the materials obtained are characterised by stable dimensional factors, since in this temperature range the thermal conductivity coefficient does not depend much on temperature. At room temperature, their bending strength is about 1?GPa. A new method of chemical synthesis of nanocrystalline ceramic compositions of boron carbide and titanium diboride using suspension solutions for the preparation of powders and their spark plasma sintering was also developed to obtain a compacted material of composition B₄C+30?wt-%TiB₂, which has a high hardness of 95 HRA (with maximum microhardness 45.6?GPa) and sufficient strength (with a bending strength of 834?MPa).     

Gel-cast hierarchical porous B4C/C preform and its role in fabricating reaction bonded boron carbide composites

The resorcinol-formaldehyde (RF) gel-casting system is employed for the first time to fabricate a hierarchical porous B₄C/C preform, which was subsequently used for the fabrication of reaction bonded boron carbide (RBBC) composites via a liquid silicon infiltration process. The effect of the carbon content and carbon structures of this perform on the microstructures and mechanical properties of B₄C/C preform and the resultant RBBC composites is reported. The B₄C/C preform (16 wt% carbon) exhibit a strength of 34±1 MPa. The obtained RBBC composites shown uniform microstructure is consisted of SiC particles bonded boron carbide scaffold and an interpenetrating residual silicon phase. The Vickers hardness, flexural strength and fracture toughness of the RBBC composites (16 wt% carbon) are 24 GPa, 452 MPa and 4.32 MPa m^1/2, respectively.     

Preparation of submicron boron carbide powder through gas-solid reaction method

Boron carbide submicron powder was synthesized with boron oxide and graphene as starting materials by gas-solid reaction method using two different apparatuses. The effects of calcination temperature and holding time, apparatus type and B₂O₃/C ratio of the starting materials on the phase composition and morphology of the synthesized powders were evaluated. A newly formed residual carbon morphology distinct from original graphene were present in samples synthesized at a higher B₂O₃/C ratio or temperature. The synthesis temperature of ∼1500 °C was found to be more suitable to obtain boron carbide powder without the existence of residual carbon. The new type of apparatus enabled the synthesis of boron carbide phase at a relatively lower temperature, due to its more efficient use of B₂O₃ vapor.     

Intrinsic hardness of boron carbide: Influence of polymorphism and stoichiometry

Boron carbide comprises of polymorphs that differ in crystallographic arrangement and stoichiometry. Consequently, specimens extracted from the same batch can exhibit variability in mechanical properties depending on the constituent mixture of polymorphs. In this work, density functional theory simulations and estimates from three models (bond resistance model, bond strength model, and electronegativity model) are utilized to (i) investigate the influence of polymorphism and stoichiometry on the intrinsic hardness of boron carbide, (ii) reveal the sensitivity of the estimates to the model used, and (iii) test their conformance to experimental data. The study finds intrinsic hardness of boron carbide to be primarily a function of stoichiometry, with polymorphism having a lower influence. Furthermore, hardness estimates are shown to exhibit substantial sensitivity to the model used, differing by as much as 9 GPa for the same polymorph. Thus, the search for new superhard materials should be guided by more than just one model. Our analysis finds bond resistance model to offer the best conformance to experimental data, indicating that bond length is a much stronger influencer of intrinsic hardness in covalent crystals than coordination numbers and electronegativities of bonding atoms.