Wear resistant boron carbide compacts produced by pressureless sintering

Boron carbide compacts were produced by pressureless sintering at 2200 °C/2 h and 2250 °C/2 h in Ar atmosphere, using a starting powder with a particle size smaller than 3 µm. Effects of carbon addition (3.5 wt%) and methanol washing of the starting powder were investigated on the densification, Vickers hardness, and micro-abrasive wear resistance of the samples. The removal of oxide phases by methanol washing allowed the production, with no sintering additive, of highly densified (93.6% of theoretical density), hard (25.4 GPa), and highly wear resistant (wear coefficient =2.9×10^–14 m³/N.m) boron carbide compacts sintered at 2250 °C. This optimized combination of properties was a consequence of a reduced grain growth without the deleterious effects associated to the carbon addition. Methanol washing of the starting powder is a simple and general approach to produce, without additives, high quality, wear resistant boron carbide compacts by pressureless sintering.     

Low-temperature synthetic route for boron carbide

Boron carbide is one of the hardest materials with diamond-like mechanical properties, and is already used for a variety of applications including armor plating, blasting nozzles, and mechanical seal faces, as well as for grinding and cutting tools. It is produced on an industrial scale by classical carbothermal reduction of boric oxides at high temperatures, but the formation of pure boron carbide in processed forms, such as films and fibers, is difficult. As an alternative to high-temperature powder techniques, there is recently great interest in the development of polymer precursors to produce ceramic materials. The aim of the present work is to develop a cost effective and low-temperature manufacturing process to synthesize boron carbide from cheap and easily available raw materials. The initial objective of our research is the design and synthesis of a new type of boron–carbon polymer, which would serve as precursor for boron carbide. The polymeric precursor is synthesized by the reaction of boric acid and polyvinyl alcohol that after pyrolysis at 400 °C and 800 °C gives boron carbide. The polymeric precursor and its pyrolyzed products are characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). X-ray diffraction shows that boron carbide (B₄C) obtained from this method has an orthorhombic crystal structure. It is a unique low-temperature (∼400 °C) synthetic route for boron carbide.     

Thermal expansion of nanocrystalline boron carbide

The lattice thermal expansion behaviour of nanocrystalline and microcrystalline boron carbides has been measured by high temperature X-ray diffraction technique in the temperature range 298–1773 K. The lattice parameters of both were found to increase with increase in temperature. The average thermal expansion coefficients in this temperature range for the nanocrystalline and the microcrystalline boron carbides were found to be 7.76 × 10^?6 and 7.06 × 10^?6 K^?1, respectively. The difference observed in the thermal expansion behaviour is probably due to increase in the surface energy of the lattice in the nanocrystalline material.     

Microstructure and thermophysical properties of B4C/graphite composites containing substitutional boron

B₄C/graphite composites (BGC) containing substitutional boron were fabricated by pressureless sintering of powder mixtures of petroleum coke, coal tar pitch and B₄C. After sintering at 900 °C and graphitizing at 2200 °C, the microstructure of BGC was characterized by SEM, TEM, XRD, Raman spectroscopy and optical microscopy. XPS measurements revealed the formation of BC₃, and the matrix carbon contained around 6 wt.% substitutional boron. The thermal conductivity of the BGC at room temperature is 52.7 W/m K and the flexural strength is up to 35.1 MPa. The bulk density and electrical resistivity are 1.72 g/cm³ and 13.4 μΩ m, respectively. The correlation between microstructure and properties was investigated. The results showed that the microstructure improvement of the BGC has obvious effect on the thermal conductivity, flexural strength, and electrical resistivity.     

Processing factors influencing the free carbon contents in boron carbide powder by rapid carbothermal reduction

High quality boron carbide powder without free carbon is desired for many applications. In this study, the factors that influence free carbon content in boron carbide powders synthesized by rapid carbothermal reduction reaction were evaluated. The dominant factors affecting free carbon contents in boron carbide powder were reaction temperature, precursor homogeneity, the particle size of reactants, and excess boron reactant amount. The reaction temperature at 1850 °C was sufficient to synthesize boron carbide with low free carbon content. Depending on process conditions, precursor homogeneity was also affected by the calcination temperature and time. Smaller particle size of reactants contributed to less carbon content and more uniformity in synthesized boron carbide. Excess boric acid effectively compensated for B₂O₃ volatilization. In the optimal sample, using 80 mol% excess nano boric acid and calcined at 500 °C, the free carbon in the synthesized boron carbide was negligible (0.048 wt.%).     

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 18.65 μm, 33.70 μm and 63.35 μm were chosen for the RBBC composites. The experimental results show that hardness increases from 1261.70±64.74 kg/mm² to 1674.90±100.00 kg/mm² and fracture toughness drops from 5.76±0.26 MPa m^1/2 to 3.4±0.37 MPa m^1/2. However, flexural strength decreases from 403.41±5.70 MPa to 256.15±25.05 MPa 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.     

Bulk boron carbide nanostructured ceramics by reactive spark plasma sintering

Bulk boron carbide (B₄C) ceramics was fabricated from a boron and carbon mixture by use of one-step reactive spark plasma sintering (RSPS). It was also demonstrated that preliminary high-energy ball milling (HEBM) of the B+C powder mixture leads to the formation of B/C composite particles with enhanced reactivity. Using these reactive composites in RSPS permits tuning of synthesized B₄C ceramic microstructure. Optimization of HEBM + RSPS conditions allows rapid (less than 30 min of SPS) fabrication of B₄C ceramics with porosity less than 2%, hardness of ~35 GPa and fracture toughness of ~ 4.5 MPa m ^1/2     

Microstructure and deposition mechanism of CVD amorphous boron carbide coatings deposited on SiC substrates at low temperature

Amorphous boron carbide (α-B₄C) coatings were prepared on SiC substrates by chemical vapor deposition (CVD) from CH₄/BCl₃/H₂/Ar mixtures at low temperature (900–1050 °C) and reduced pressure (10 kPa). The deposited coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that two kinds of α-B₄C coatings were deposited with different microstructures and phase compositions, and the effect of deposition temperature was significant. When deposited at 1000 °C and 1050 °C, the coatings exhibited a nodular morphology and had a relatively low content of boron. The free carbon was distributed in them inhomogeneously; in contrast, when deposited at 900 °C and 950 °C, the coatings presented a comparatively flat morphology and had a uniform internal structure and high boron content. They did not contain free carbon. At the last of this paper, the pertinent mechanisms resulting in differences in microstructure and phase composition were discussed.     

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.     

Pressureless sintering of titanium carbide doped with boron or boron carbide

Titanium carbide ceramics with different contents of boron or B₄C were pressureless sintered at temperatures from 2100 °C to 2300 °C. Due to the removal of oxide impurities, the onset temperature for TiC grain growth was lowered to 2100 °C and near fully dense (>98%) TiC ceramics were obtained at 2200 °C. TiB₂ platelets and graphite flakes were formed during sintering process. They retard TiC grains from fast growth and reduced the entrapped pores in TiC grains. Therefore, TiC doped with boron or B₄C could achieve higher relative density (>99.5%) than pure TiC (96.67%) at 2300 °C. Mechanical properties including Vickers’ hardness, fracture toughness and flexural strength were investigated. Highest fracture toughness (4.79 ± 0.50 MPa m^1/2) and flexural strength (552.6 ± 23.1 MPa) have been obtained when TiC mixed with B₄C by the mass ratio of 100:5.11. The main toughening mechanisms include crack deflection and pull-out of TiB₂ platelets.     

Pyrolysis behavior of poly[(n-propylamino/methylamino)borazine]

A processable poly[(n-propylamino/methylamino)borazine] (PPAB) has been pyrolyzed in Ar to study its thermal decomposition behavior. The structural evolution and chemical composition change during pyrolysis were characterized by chemical analysis, thermal gravimetry–mass spectrometry (TG–MS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results indicated that the polymer-to-ceramic transition of PPAB involved two steps. Below 400 °C, the gas species were mainly methane and methylamine, while from 400 to 900 °C those were methane and n-propylamine. The PPAB displayed a ceramic yield of 52 wt% at 1000 °C and the pyrolyzed product was amorphous boron nitride (BN) with a small quantity of carbon impurity, in presence as CC and CN bonds. Moreover, for the pyrolyzed product, further heat treatment resulted in the occurrence of a transformation from amorphous to turbostratic.     

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.     

Microstructure,mechanical and thermal properties of in situ toughened boron carbide-based ceramic composites co-doped with tungsten carbide and pyrolytic carbon

Boron carbide (B₄C)-based ceramics were pressureless sintered to a relative density of 96.1% at 2150 °C, with the co-incorporation of tungsten carbide and pyrolytic carbon. The as-batched boron carbide power was 7.89 m² g^?1 in surface area. A level of fracture toughness as high as 5.80 ± 0.12 MPa m^1/2 was achieved in the BW-6C composite. Sintering aids of carbon and tungsten boride were formed by an in situ reaction. The toughness improvement was attributed to the presence of thermal residual stress as well as the W₂B₅ platelets. The thermal conductivity and thermal expansivity of the BW-6C composite as a function of temperature are also reported in this work. Our current study demonstrated that the B₄C–W₂B₅ composites could be potential candidate materials for structural applications.     

Fabrication and microstructure of boron-doped isotropic pyrolytic carbon

Boron-doped isotropic pyrolytic carbon (pyrocarbon) was prepared by CVD using CH₄ + BCl₃ + H₂ as gaseous precursors. Microstructure of the deposited pyrocarbon on a graphite substrate was investigated using X-ray diffraction, X-ray photoelectron spectroscopy, polarized light microscopy, and scanning and transmission electron microscopy. It was found that the boron-containing pyrocarbon prepared exhibits a multi-scale structure, different from the pure isotropic pyrocarbon deposited under nearly the same experimental conditions. The multi-scale structure consists of carbon agglomerates of fine wrinkled graphitic sheets which contain substitutional boron and some micrometer-sized boron carbide particles uniformly distributed in the carbon agglomerates. Influence and mechanism of boron co-deposition during CVD process are also discussed.     

Effect of spark plasma sintering parameters on microstructure and room-temperature hardness and toughness of fine-grained boron carbide (B4C)

Spark plasma sintering (SPS) parameters are reported to have a remarkable effect on microstructure and fracture toughness of boron carbide. Fully dense fine-grained boron carbide samples have been sintered by SPS at 1700 °C for 3 min as optimal conditions. Both temperature and the current density applied during sintering seem to be important parameters for fully dense output and can induce significant changes on the final microstructure. The initial grain size of the powder is a crucial factor to perform fine-grained fully dense specimens. The improvement on room-temperature hardness and toughness is discussed.     

Carbothermal reduction synthesis of TiB2 ultrafine powders

Submicrometric TiB₂ powders were synthesized by carbothermal reduction process using titanium dioxide, boron carbide and carbon black as the starting materials. The influence of different amount of boron carbide (22.0–26.8 wt%), calcination temperature (1400–1900 °C) and holding time (15–90 min) on the composition and microstructure of the product was investigated. The resultant powders were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Results showed that hexagonal impurity-free TiB₂ crystalline powders with the grain size below 1.0 μm could be successfully prepared at 1600 °C for 30 min in Ar atmosphere when the amount of boron carbide was 25.3 wt%. The increase in temperature contributed to reaction completion and grain growth, but the abnormal grain growth and oversintering took place above 1800 °C.     

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.     

Synthesis of nanocrystalline boron carbide by sucrose precursor method-optimization of process conditions

Nanocrystalline boron carbide powder was synthesized by a precursor method using B₂O₃ as the source of boron and sucrose as the source of carbon. Precursor was prepared at different temperatures ranging from 300 to 800 °C. The optimum temperature for the precursor preparation was found to be 600 °C. All the precursors were heat treated at different temperatures from 1000 to 1600 °C for different duration of heating, ranging from 5 to 240 min under vacuum. The products thus obtained after heat treatment were characterized using X-ray diffraction. The boron carbide obtained was nanocrystalline and the average X-ray crystallite size was found to be ~ 33 nm. Boron, total carbon and free carbon contents also were determined. The free carbon content was found to be less than 3% for samples heated at 1600 °C for 10 min. Effect of heat treatment temperature on the morphology of the synthesized product was studied using scanning electron microscope.     

Microstructural characterization of spark plasma sintered boron carbide ceramics

Fully dense boron carbide specimens were fabricated by the spark plasma sintering (SPS) technology in the absence of any sintering additives. Densification starts at 1500 °C and the highest densification rate is reached at about 1900 °C. The microstructure of the ceramic sintered at 2200 °C, with heating rates in the 50–400 °C/min range, displays abnormal grain growth, while for a 600 °C/min heating rate a homogeneous distribution of finely equiaxed grains with 4.05 ± 1.62 μm average size was obtained. TEM analysis revealed the presence of W-based amorphous and of crystalline boron-rich B₅₀N₂ secondary phases at triple-junctions. No grain-boundary films were detected by HRTEM. The formation of a transient liquid alumino-silicate phase stands apparently behind the early stage of densification.     

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.