Synthesis of boron nitride nanotubes by catalytic pyrolysis of organic-inorganic hybrid precursor

Large quantities of hexagonal boron nitride (h-BN) nanotubes (BNNTs) with high purity have been successfully synthesized under ammonia gas flow at 1200 °C via catalytic pyrolysis of organic-inorganic hybrid precursor which was pre-prepared through a wet chemistry method at 95 °C. Several characterizations, such as SEM, TEM, XRD, FTIR, EDS, XPS and SAED measurements, were used to confirm the morphology, composition and crystalline structure of the as-synthesized powders. It was observed that the obtained product was a kind of nanotubes (NTs) in hexagonal BN phase with a curved shape and smooth surface. The diameter of BNNTs was distributed in a range of 60–200 nm while its length was about tens of microns. The possible growth mechanism of BNNTs was also proposed in this paper.     

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.     

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.     

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.     

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.     

Reaction coupling preparation of high sintering activity boron carbide nano-powders

Large scale B₄C nano-powders were synthesized via a novel ball milling assisted reaction coupling self-propagating high temperature synthesis method using Mg, B₂O₃ and CH₂H₃Cl as the starting materials. The XRD, FTIR, Raman, EDX, FSEM, TEM, HRTEM and SAED were used to characterize the B₄C samples. The optimum endothermic rate was 35%, when the samples presented fine and uniform regular morphology with an average particle diameter of about 100 nm. In addition, the reaction coupling principle, possible chemical reaction mechanism and the effects of the endothermic reaction rate were also discussed. Moreover, the commercial B₄C (C-B₄C) and homemade B₄C (H-B₄C) ceramics were prepared by spark-plasma sintering method at 1700 °C under 30 Mpa. Compared with the C-B₄C ceramic, the values of relative density, vickers hardness and fracture toughness of the H-B₄C ceramic were increased by 2.1%, 9.2% and 20.1%, respectively, demonstrating high sintering activity of the homemade B₄C nano-powders.     

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.     

Synthesis and properties of ceramic-based nanocomposite layer of aluminum carbide embedded with oriented carbon nanotubes

In the present work, carbon nanotubes (CNTs) were embedded in aluminum carbide coating in desired vertical/horizontal direction in order to fabricate a nanocomposite layer with unidirectional enhanced mechanical properties. A novel method based on monopolar pulsed plasma electrolysis under magnetic field was used for this purpose. Nanostructure of the obtained nanocomposite layer was examined with high precision figure analysis of SEM, AFM and TEM nanostructures. The mechanical and tribological properties of these coatings were investigated with respect to the direction of the embedded CNTs. The coefficient of friction was lowered from 0.2 to less than 0.1 in a pin-on-disc test against steel with dramatic affected coating wear rate by a decrease to near 400% with respect to raw substrate. The lower friction is attributed to more extensive creation of amorphous carbon on the counter surface and also in the coating wear track. As a conclusion, this method is appropriate for fabrication of hard coating on the surface of low-melting-point metals and light alloys.     

Linear organodecaborane block copolymer as a single-source precursor for porous boron carbide ceramics

Boron carbide is one of the most widely used non-oxide ceramics as it possesses excellent physical and chemical properties. Much attention has been paid to prepare boron carbide ceramics via precursor derived method. In this work, poly(6-norbornenyldecaborane)-b-poly(6-cyclooctenyldecaborane) (PND-b-PCD) block copolymer was synthesized by the ruthenium-catalyzed ring-opening metathesis polymerization (ROMP) of 6-norbornenyldecaborane with 6-cyclooctenyldecaborane. The synthesized boron carbide preceramic polymer had good solubility and film-forming ability with a high ceramic yield of 75% at 850 °C. TGA, XRD and TG-IR-GC–MS were used to investigate the ceramization process of the precursor. Boron carbide ceramic foams were prepared by the precursor via replicating polyurethane foam template. The component, crystalline and morphology were investigated in detail. The ceramic foams showed a good high temperature performance and could keep their structure even up to 1800 °C.     

Densification and mechanical properties of boron carbide with micro-hole array by micro-powder injection molding

For the purpose of obtaining fully dense B₄C with micro hole array of high quality and precision, synthesis of B₄C powders were carried out by micro powder injection molding. Five kinds of additive systems were used and their influences on mechanical properties were investigated. The relative density, Vickers hardness, bending strength, and fracture toughness of the B₄C ceramics with BS10AY additive sintered at 2000 °C for 2 h could reach 97.56%, 3580.4 HV, 355.3 MPa and 5.87 MPa m^1/2, respectively. The temperature was generally 100–200 °C lower than reported temperatures which was attributed to the additives. The improvement in mechanical properties was attributed to grain refinement. A mixture of intragranular and transgranular fractures occurred due to the fine microstructures and the additive systems in the B₄C ceramics after sintering. Micro-hole array with the diameter of 450 μm and the length-diameter ratio of more than 8 were obtained. The inhomogeneous filling of feedstock from the substrate to the thin wall between two adjacent micro holes caused the inhomogeneous shrinkage of the substrate and micro holes.     

Microstructure,phase and electrical conductivity analyses of spark plasma sintered boron carbide machined with WEDM

Electrical conductivity is an essential property for machining of sintered boron carbide especially by wire electrical discharge machining (WEDM) process. Pure boron carbide was spark plasma sintered to full density at 2050 °C. Rietveld refinement on XRD analysis confirmed presence of B₁₃C₂ as the major phase in the powder as well as in the sintered samples.Electrical conductivity was found to be ~48 Ω⁻¹m⁻¹. The sintered specimens were successfully machined using WEDM technique. The microstructure of powder, machined and fractured surfaces of the sintered boron carbide were analyzed. At low power of WEDM with pulse current less than 140 A formation of molten, oxidized phases of boron carbide was observed as well as the development of surface cracks were minimum on the machined surface. Thus this work is aiming at achieving better product quality with sintered boron carbide specimens which are machined by WEDM.     

Preparation of SiC nanowires-filled cellular SiCO ceramics from polymeric precursor

SiC nanowires-filled cellular SiCO ceramics were prepared using polyurethane sponge as a porous template infiltrated with silicone resin by pyrolysis at 1400 °C under Ar atmosphere. The pyrolysis temperature was an important parameter affecting the formation of SiC nanowires. The as-prepared sample obtained at 1000 °C was composed of SiCO glasses and turbostratic carbon. The SiCO ceramic was further converted into SiO₂ crystals and amorphous carbon by pyrolysis at 1200 °C. With the increasing pyrolysis temperature, SiC nanocrystals embedded in the non-crystalline SiCO matrix were observed. Furthermore, the SiC nanowires were formed in the pores of the SiCO ceramic. The diameters of the SiC nanowires are in the range 80–150 nm and the lengths are up to several tens of micrometers. The growth mechanism of the nanowires was supported by the vapor-solid mechanism.     

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.     

Toughness control of boron carbide obtained by spark plasma sintering in nitrogen atmosphere

Boron carbide ceramic was prepared by reactive Spark Plasma Sintering under N₂-atmosphere and for different heating times and maximum pressure regimes. Split-Hopkinson Pressure Bar (SHPB), indentation, XRD and microscopy measurements were performed for samples characterization. It is shown that SHPB toughness control depending on SPS regime is possible and the main reason is introduction of nitrogen into B₄C ceramic. Complex relationships between processing conditions, sintering mechanism, material's specifics, static and dynamic mechanical properties are discussed. Improvement of dynamic toughness is through mechanisms resembling those working for static load conditions such as cracks deflection and pull out, but there are also significant differences.     

Effects of pressure on densification behaviour,microstructures and mechanical properties of boron carbide ceramics fabricated by hot pressing

Effects of pressure, from ordinary (30 MPa) to high pressure (110 MPa), on densification behaviour, microstructures and mechanical properties of boron carbide ceramics sintered by hot pressing are investigated. With increasing pressure, the relative density sharply increases within 30–75 MPa, slowly increases within 75–100 MPa and finally stagnates. For samples within 75–100 MPa, densification begins at approximately 1000 °C, and the dominant densification process ends before the soaking stage. High relative densities of 98.49% and 99.76% are achieved. For samples within 30–50 MPa, densification begins at approximately 1500 °C, and the soaking stage (initial 20 min) is still important for the dominant densification process. The final relative densities are only 87.90% and 92.32%. The above-mentioned differences are derived from contributions of pressure, and the dominant densification mechanism under high pressure is plastic deformation. The average grain size of the samples slightly increases with increasing soaking time. The grain size under higher pressure is larger than that under lower pressure at corresponding periods because grains grow easily with reduced pores. Vickers hardness and fracture toughness increase as grain size decreases in fully dense samples. However, when the samples do not achieve full density, relative density becomes more influential than grain size in hardness and toughness. A soaking time of 30 min is enough for samples under 100 MPa. Prolonging the soaking time has deleterious effects on mechanical properties. The relative density, grain size, hardness and fracture toughness of the samples under 100 MPa for 30 min are 99.73%, 1.96 µm, 37.85 GPa and 3.94 MPa m^1/2, respectively.     

Mechanical properties of submicronic and nanometric boron carbides obtained by Spark Plasma Sintering: Influence of B/C ratio and oxygen content

Boron carbide samples exhibiting nanometric and submicronic microstructure were sintered by Spark Plasma Sintering to investigate the effect of grain size on mechanical properties. The mechanical properties of sintered monoliths were characterized at the grain and macroscopic scales. Although nanostructured material exhibits finer grains than the submicronic material (i.e. mean diameter of 82 vs. 474?nm), its apparent rigidity and hardness are found to be reduced by 6.8% and 8.4% respectively. This contradiction with the Hall-Petch law is linked to the chemical compositions of both materials, which show significant difference in terms of B/C ratio and higher structural oxygen content especially for nanostructured material.     

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.     

Facile synthesis of boron carbide elongated nanostructures via a simple in situ thermal evaporation process

Boron carbide elongated nanostructures such as nanowires, nanobelts and nanosheets have been synthesized via a low-cost and simple in situ thermal evaporation process using commercially available B₄C powders as the main precursor. Heat treatments were done in the temperature range of 1400-1600 °C in the presence of Co nanoparticles (and NiCl₂ in some experiments) as the catalyst material. The growth mechanism of the nanostructures was proposed to be a cooperative growth procedure including surface diffusion, vapor-liquid-solid (VLS) and solid-liquid-solid (SLS) growth mechanisms. The final product, containing some of the initial B₄C particles and as-synthesized elongated nanostructures may be potentially applicable as an excellent reinforcing phase in composite materials. Moreover, nanostructures with right angle junctions were obtained from the sidewalls of the graphite boats, which may be operative in MEMS and NEMS devices. The samples have been characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and photoluminescence spectroscopy.     

Corrosive wear behavior of chromium carbide coatings deposited by air plasma spraying

The corrosive wear behavior of chromium carbide coatings deposited by air plasma spraying was studied, through wet pin-on-disk wear experiments. During the wear tests, the samples were immersed in corrosive environments consisting of watery hydrochloric acid with the acid concentrations of 5, 10 and 15 vol%. The wear tests were performed at both room temperature and 80 °C. The results showed that the wet environment significantly increased the wear rate. In addition, the increase of the acid concentration and temperature considerably deteriorated the wear resistance of the coated samples. It was also realized that, compared to the dry condition, the wear mechanism changed from abrasive to adhesive in the wet environment where a tribochemical wear was observed.