Reaction Bonded Boron Carbide Research Articles

Gel‐casting process‐derived 3D‐interconnected porous carbon/B4C preform for reaction‐bonded boron carbide composites

AbstractA novel kind of 3D interconnected porous carbon‐bonded B4C preform with flexural strength of about 20MPa is fabricated by gel‐casting method. As the control, the B4C/C preforms via molding‐produced method are fabricated. The B4C/C preforms own the same carbon content and almost the same porosity with different molding method. Compared with the conventional compression molding‐produced B4C/C preform, the 3D‐interconnected porous carbon‐bonded boron carbide preforms afford three‐fold advantages in the fabrication of reaction‐bonded boron carbide (RBBC) composite via the liquid silicon infiltration (LSI) process, including induced formation of continuous SiC‐bonded boron carbide skeleton structure, a reduction in the size of residual silicon, and prevented the coarsening of ceramic grains as well. As a result, the RBBC composite fabricated by gel‐casting appears as a more refined structure resulting in enhanced mechanical properties: the Vickers hardness, flexural strength, and fracture toughness of I‐GC composites fabricated from 3D‐interconnected porous carbon/B4C preforms are 19.4 ± 1.3 GPa, 389 ± 17 MPa, and 4.37 ± 0.1 MPa·m1/2, respectively, which are statistically higher than those of RBBC fabricated by the conventional compression molding‐produced B4C/C preform.

Structural evolution in reaction-bonded silicon carbide and boron carbide composites (RBSBC)

Reaction-bonded silicon carbide and boron carbide composites (RBSBC) are promising for laser mirror, armor and other applications. However, the structural evolution in RBSBC has not been fully understood. From microscopic characterization and thermodynamic analysis, we propose the mechanisms which can successfully explain observed microstructures, e.g., core-rim structure in SiC grains and plate-like SiC inside B4C grains, respectively. In addition, we find that the pre-existing SiC particles may hinder the formation of polygonal SiC in Si and lead to more B4C dissolution during the fabrication of RBSBC due to its lower B4C percentage than that in corresponding reaction-bonded boron carbide composites (RBBC). We believe that the results would shed light on potential fabrication optimization of RBSBC.

Microstructure and mechanical properties of reaction bonded B4C-SiC composites: The effect of polycarbosilane addition

Reaction bonded B4C-SiC composites were prepared by infiltrating silicon melt into porous B4C-SiC green preforms at 1500°C in vacuum. The porous green preform was obtained from a mixture of polycarbosilane (PCS) and particle size graded B4C after pre-sintering at 1600°C. For the first time, PCS was used to adjust the phase composition and microstructure of the reaction bonded boron carbide composites. It is indicated that the addition of PCS and its content has a significant influence on the microstructure as well as the mechanical properties of the subsequent reaction bonded B4C-SiC composites. For the B4C-SiC composite with 5wt% PCS added, a flexural strength of 319±12MPa, and an elastic modulus of 402±18GPa can be achieved, which is 23% and 15% higher than those of the composite without PCS addition, respectively. While, with the higher content of PCS addition, the mechanical properties of the composites are decreased drastically due to the large amount of residual Si agglomeration in the composites. The reaction mechanisms as well as their microstructure evolution processes correlated with the mechanical properties of the reaction bonded B4C-SiC composites are further discussed in our work.

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 (16wt% carbon) exhibit a strength of 34±1MPa. 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 (16wt% carbon) are 24GPa, 452MPa and 4.32MPam1/2, respectively.

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.

Raman spectroscopic characterization of the core-rim structure in reaction bonded boron carbide ceramics

Raman spectroscopy was used to characterize the microstructure of reaction bonded boron carbide ceramics. Compositional and structural gradation in the silicon-doped boron carbide phase (rim), which develops around the parent boron carbide region (core) due to the reaction between silicon and boron carbide, was evaluated using changes in Raman peak position and intensity. Peak shifting and intensity variation from the core to the rim region was attributed to changes in the boron carbide crystal structure based on experimental Raman observations and ab initio calculations reported in literature. The results were consistent with compositional analysis determined by energy dispersive spectroscopy. The Raman analysis revealed the substitution of silicon atoms first into the linear 3-atom chain, and then into icosahedral units of the boron carbide structure. Thus, micro-Raman spectroscopy provided a non-destructive means of identifying the preferential positions of Si atoms in the boron carbide lattice.

Study of Reaction-Bonded Boron Carbide Properties

Results are given for a study by scanning electron microscopy of the effect of original powder composition on physicomechanical properties of reaction-bonded boron carbide and during new material structure formation in the B–Si–C system.

The Role of Infiltration Temperature in the Reaction Bonding of Boron Carbide by Silicon Infiltration

The present paper is concerned on the effect of infiltration temperature on the components, microstructure, and mechanical properties of reaction‐bonded boron carbide (RBBC) ceramics. RBBC ceramics were fabricated by reactive infiltration of molten silicon (Si) into porous preforms containing boron carbide (B4C) and free carbon. It has been found that infiltration temperatures have significant influence on the infiltration reactions involved and therefore the evolution of different phases formed in the RBBC ceramics. An increase in grain size of boron carbide particles through the coalescence of neighboring grains was observed at certain infiltration temperatures. The morphology of silicon carbide (SiC) phases developed from discontinuous and cloud‐like SiC to continuous and integrated SiC zones with the increase of infiltration temperatures. With increasing temperatures up to 1600°C, the hardness, flexural strength, and fracture toughness all increased. When the temperatures exceeded 1600°C, while the hardness and flexural strength decreased, the fracture toughness continued to increase.

Si/SiC-Based Layer Deposited on Boron Carbide Particles via CVD

The application of B4C as structural materials has been restricted largely because of its poor sinter-ability. Compared with pressure-less and hot pressing methods, sintering the B4C via the reaction-bonded boron carbide (RBBC) can to great extent circumvent such problem, which can even be conducted at low temperature, when Si/Si-based alloy was used as binding phase. However molten Si/Si-based alloy infiltrated into performs of bare B4C powders can strongly react with and significantly consume B4C particles based on molten infiltration reaction method. The present study aims to encapsulate B4C particles with a protective layer to block off its contact with molten Si/ Si-based alloy via chemical vapor deposition (CVD) method in CH3SiCl3(MTS)-H2-Ar system at low temperature (900-1100 °C) under atmospheric pressure. The phase composition and surface morphology of encapsulated B4C particles were studied using X-ray diffraction (XRD) , scanning electron microscopy (SEM) plus energy dispersive spectrometer (EDS), respectively. It was found that the deposition temperatures have a significant effect on microstructure and composition of deposited coating. The studies on surface morphology revealed that spine-like crystals, nodular growth, island structure and whiskers can be deposited onto the surface of boron carbide particles. When deposited at 900-1000 °C,the coatings is Si + SiC co-deposition, while pure SiC coatings form only at as high deposition temperature at 1050 °Cand 1100 °C.

Reaction bonded boron carbide: recent developments

Reaction bonded boron carbide composites are among the hardest ceramics with low specific gravity values, a combination that makes them eminently suitable for light armour applications. The present review covers the main issues involved in the reaction bonding process based on the infiltration of boron carbide preforms with molten silicon. The importance of achieving high green density values before infiltration is emphasised. Separate sections deal with the morphology of the composite material, the thermodynamics of its microstructure formation, the static mechanical properties and the dynamic mechanical properties at strain rates up to 105 s−1. All along the review, emphasis is placed on describing the effect of free carbon presence in the initial preform on the morphology and properties of the final composite.

Carbon fiber/reaction-bonded carbide matrix for composite materials – Manufacture and characterization

Abstract The processing of self-healing ceramic matrix composites by a short time and low cost process was studied. This process is based on the deposition of fiber dual interphases by chemical vapor infiltration and on the densification of the matrix by reactive melt infiltration of silicon. To prevent fibers (ex-PAN carbon fibers) from oxidation in service, a self-healing matrix made of reaction bonded silicon carbide and reaction bonded boron carbide was used. Boron carbide is introduced inside the fiber preform from ceramic suspension whereas silicon carbide is formed by the reaction of liquid silicon with a porous carbon xerogel in the preform. The ceramic matrix composites obtained are near net shape, have a bending stress at failure at room temperature around 300 MPa and have shown their ability to self-healing in oxidizing conditions.

Effect of particle size on the mechanical properties of reaction bonded boron carbide ceramics

In the present communication, effect of boron carbide particle size on the mechanical properties such as hardness, fracture toughness and flexural strength of reaction bonded boron carbide (RBBC) ceramics were investigated. RBBC composites were produced by the reactive infiltration of molten silicon into porous preform containing boron carbide and free carbon. Boron carbide powders with mean particle size of 18.65μm, 33.70μm and 63.35μm were chosen for the RBBC composites. The experimental results show that hardness increases from 1261.70±64.74kg/mm2 to 1674.90±100.00kg/mm2 and fracture toughness drops from 5.76±0.26MPam1/2 to 3.4±0.37MPam1/2. However, flexural strength decreases from 403.41±5.70MPa to 256.15±25.05MPa with the increase in particle size. Indentation induced cracks in RBBC are mainly median type and number of cracks increase with the increase of starting particle size.

Compressive property of liquid silicon (infiltrated) boron carbide

Compressive deformation of reaction bonded boron carbide has been carried out over a strain rate range 10−4 to 10−2 s−1. The maximum compressive strength of reaction bonded boron carbide at strain rate of 10−4 s−1 is 370 MPa and 470 MPa at strain rate of 10−2 s−1. It is not possible to carryout quasi-static compression test at very higher strain rates (≥ 102 s−1). However, the data generated at low strain rates can be extrapolated to higher strain rates and this data correlate well with the experimental compressive strength values generated by dynamic compression tests.

Processing and characterization of B 4C–SiC–Si–TiB 2 composites

B 4C–SiC–Si–TiB 2 composites were synthesized by a two step process. TiB 2 particles in the size range 2–5 μm were generated in situ in the first step and were distributed in the residual silicon present in the reaction bonded boron carbide, in the second step. The composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe micro analysis (EPMA) and micro-hardness testing. The density and average hardness of siliconized boron carbide samples with and without TiB 2 particle reinforcement were found to be 2.67 g/cm 3 and 25 GPa and 2.54 g/cm 3 and 21 GPa, respectively.

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 B 4C 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 B 12(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 B 4C 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.

The effect of aluminum on the microstructure and phase composition of boron carbide infiltrated with silicon

Reaction-bonded boron carbide is prepared by pressureless infiltration of boron carbide preforms with molten silicon in a graphite furnace under vacuum. The presence of Al 2O 3 parts in the heated zone, even though not in contact with the boron carbide preform, causes aluminum to appear in the liquid silicon. The formation of aluminum sub-oxide (Al 2O) stands behind the transport of aluminum into the composite. The presence of aluminum in the boron carbide–silicon system accelerates the transformation of the initial boron carbide particles into B x (C,Si,Al) y and Al 1.36B 24C 4, newly formed carbide phases . It also leads during cooling to the formation of some Si–Al solid solution particles. The effect of Al on the microstructural evolution is well accounted for by the calculated isothermal section of the quaternary Al–B–C–Si phase diagram, according to which the solubility of boron in liquid silicon increases with increasing aluminum content. This feature is a key factor in the evolution of the microstructure of the infiltrated composites.

Rim region growth and its composition in reaction bonded boron carbide composites with core-rim structure

Aluminum was detected in reaction-bonded boron carbide that had been prepared by pressureless infiltration of boron carbide preforms with molten silicon in a graphite furnace under vacuum. The presence of Al2O3 in the heated zone, even though not in contact with the boron carbide preform, stands behind the presence of aluminium in the rim region that interconnects the initial boron carbide particles. The composition of the rim corresponds to the Bx(C,Si,Al)y quaternary carbide phase. The reaction of alumina with graphite and the formation of a gaseous aluminum suboxide (Al2O) accounts for the transfer of aluminum in the melt and, subsequently in the rim regions. The presence of Al increases the solubility of boron in liquid silicon, but with increasing aluminum content the activity of boron decreases. These features dominate the structural evolution of the rim-core in the presence of aluminum in the melt.

The morphology of ceramic phases in B xC –SiC –Si infiltrated composites

The present communication is concerned with the effect of the carbon source on the morphology of reaction bonded boron carbide (B 4C). Molten silicon reacts strongly and rapidly with free carbon to form large, faceted, regular polygon-shaped SiC particles, usually embedded in residual silicon pools. In the absence of free carbon, the formation of SiC relies on carbon that originates from within the boron carbide particles. Examination of the reaction bonded boron carbide revealed a core–rim microstructure consisting of boron carbide particles surrounded by secondary boron carbide containing some dissolved silicon. This microstructure is generated as the outcome of a dissolution–precipitation process. In the course of the infiltration process molten Si dissolves some boron carbide until its saturation with B and C. Subsequently, precipitation of secondary boron carbide enriched with boron and silicon takes place. In parallel, elongated, strongly twinned, faceted SiC particles are generated by rapid growth along preferred crystallographic directions. This sequence of events is supported by X-ray diffraction and microcompositional analysis and well accounted for by the thermodynamic analysis of the ternary B–C–Si system.

Pressureless sintering and evaluation of silicon carbide-boron carbide composites: K.Nihara et al. (Osaka University, Ibaraki, Japan.) J. Jpn Soc. Powder Powder Metall., vol 44, no 12, 1997, 1078–1082. (In Japanese.)

The present communication is concerned with the effect of the carbon source on the morphology of reaction bonded boron carbide (B4C). Molten silicon reacts strongly and rapidly with free carbon to form large, faceted, regular polygon-shaped SiC particles, usually embedded in residual silicon pools. In the absence of free carbon, the formation of SiC relies on carbon that originates from within the boron carbide particles. Examination of the reaction bonded boron carbide revealed a core–rim microstructure consisting of boron carbide particles surrounded by secondary boron carbide containing some dissolved silicon. This microstructure is generated as the outcome of a dissolution–precipitation process. In the course of the infiltration process molten Si dissolves some boron carbide until its saturation with B and C. Subsequently, precipitation of secondary boron carbide enriched with boron and silicon takes place. In parallel, elongated, strongly twinned, faceted SiC particles are generated by rapid growth along preferred crystallographic directions. This sequence of events is supported by X-ray diffraction and microcompositional analysis and well accounted for by the thermodynamic analysis of the ternary B–C–Si system.