Evaluation of the Temperature Range Suitable for the Synthesis of B4C–TiB2 and B4C–ZrB2 Powder Composite Materials

Inorganic Materials - Based on analysis of phase diagram data and thermodynamic modeling, we have evaluated the optimal temperature ranges of the processes underlying the preparation of...     

Low-temperature processing of B4C–Al composites via infiltration technique

An infiltration technique is commonly used for the fabrication of B₄C–Al composites. However, due to the low wettability of aluminum on B₄C, the process is currently performed at a relatively high temperature (1200°C) in industries. As a result, a large amount of reaction products are formed in the system, causing a significant loss of the initial B₄C and Al which are required for practical applications. In this study a sol–gel technique was employed to coat the B₄C powders with TiO₂. A subsequent heat treatment produced TiB₂ layers which enhance the wettability of Al. This process facilitated aluminum infiltration at or below 1000°C. In addition, this low-temperature process suppressed the formation of various reaction phases including deleterious Al₄C₃. It was also noted from the measurements of mechanical properties that the influences of undesirable reaction phases are considerably reduced.     

Diamond Crystallization in the System B4C–C

Superhard polycrystalline diamond material consisting of crystallites less than 20 m in size and containing less than 5 wt % B₄C is synthesized in the graphite–B₄C system at 2600–2800 K and 8–9 GPa. In the Raman spectrum of this material, the main band (1332 cm^–1) is shifted to lower frequencies by 40 cm^–1, typical of heavily boron-doped diamond films. Based on experimental data, a mechanism is proposed for the transformation of graphite into polycrystalline diamond at temperatures between the melting points of the B₄C–diamond and B₄C–graphite eutectics.     

Production and characterization of AA6061–B4C stir cast composite

This work focuses on the fabrication of aluminum (6061-T6) matrix composites (AMCs) reinforced with various weight percentage of B₄C particulates by modified stir casting route. The wettability of B₄C particles in the matrix has been improved by adding K₂TiF₆ flux into the melt. The microstructure and mechanical properties of the fabricated AMCs are analyzed. The optical microstructure and scanning electron microscope (SEM) images reveal the homogeneous dispersion of B₄C particles in the matrix. The reinforcement dispersion has also been identified with X-ray diffraction (XRD). The mechanical properties like hardness and tensile strength have improved with the increase in weight percentage of B₄C particulates in the aluminum matrix.     

Influence of the hard coated B4C particulates on wear resistance of Al–Cu alloys

In the present study, sliding wear tests were carried out on different sizes and volume fractions of coated B₄C particles reinforced 2024 aluminum alloy composites fabricated by a squeeze casting method. Microstructural examination showed that the B₄C distributions were generally homogeneous in the matrix while some particle clusterings were observed at relatively high particle containing composites. As compared to the 2024 Al matrix alloy, the hardness of the composites was found to be greater. It is observed that the wear resistance of the composites was significantly higher than that of the unreinforced aluminum alloy, and increased with increasing B₄C particles content and size. The hard B₄C particles act as a protrusion over the matrix, carries a major portion of the applied load and protect the abrasives from penetration into the specimen surface. Combination of rough and smooth regions is distinguished on the worn surface of the composites. The depth and number of grooves in composites decreased with increasing volume fraction of B₄C particles, and the worn surfaces of composites were relatively smooth.     

Machining behavior of AA6351–SiC–B4C hybrid composites fabricated by stir casting method

ABSTRACT

This study presents an effective approach to assess the machinability of 6351 aluminum alloy matrix, reinforced with 5 wt.% silicon carbide (SiC) and (0, 5, and 10 wt.%) boron carbide (B₄C) particles. The turning tests are carried out with a polycrystalline diamond (PCD) tool to identify the effect of the B₄C particles addition to the composite, with an objective to improve the material removal rate (MRR) and to reduce the surface roughness (Ra) and power consumption (P). The significant level of each factor, which contributes to affect the output response, is found through analysis of variance (ANOVA). The results show that the inclusion of B₄C particles in the hybrid composite significantly affects the machinability, with a contribution to the surface roughness by 7.87% and P by 6.36%. The increase in MRR affects the quality of the material, irrespective of the composites.     

Towards the controlled CVD growth of graphitic B–C–N atomic layer films: The key role of B–C delivery molecular precursor

Graphene-like, ternary system B–C–N atomic layer materials promise highly tunable electronic properties and a plethora of potential applications. However, thus far, experimental synthesis of the B–C–N atomic layers normally yields a microscopic phase-segregated structure consisting of pure C and BN domains. Further, growing the truly ternary B–C–N phase layers with homogenous atomic arrangements has proven to be very challenging. Here, in designing a bettercontrolled process for the chemical vapor deposition (CVD) growth of B–C–N atomic layer films with the minimized C and BN phase segregation, we selected trimethyl borane (TMB), a gaseous organoboron compound with pre-existing B–C bonds, as the molecular precursor to react with ammonia (NH₃) gas that serves as the nitrification agent. The use of this unique B–C delivery precursor allows for the successful synthesis of high-quality and large-area B–C–N atomic layer films. Moreover, the TMB/NH₃ reactant combination can offer a high level of tunability and control of the overall chemical composition of B–C–N atomic layers by regulating the relative partial pressure of two gaseous reactants. Electrical transport measurements show that a finite energy gap can be opened in the as-grown B–C–N atomic layers and its tunability is essentially dependent on the relative C to BN atomic compositions. On the basis of carefully controlled experiments, we show that the pre-existing B–C bonds in the TMB molecular precursor have played a crucial role in effectively reducing the C and BN phase segregation problem, thereby facilitating the formation of truly ternary B–C–N phase atomic layers.

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Effects of Manufacturing Processes on Microstructure and Properties of Al/A356–B4C Composites

Liquid and semi-solid stir casting processes were applied to fabricate B₄C particles-reinforced aluminum–matrix composites. The effects of manufacturing processes on particle distribution, particle/matrix interface, and mechanical properties of the prepared composites were studied. The results show that particle distribution can be significantly improved by using K₂TiF₆–flux and Ti powders in the liquid stir casting process, whereas in the semi-solid stir casting process it could be improved by decreasing the temperature of the slurry. With additions of Ti, the decomposition of B₄C was prevented, and the interfacial bonding strength was significantly improved due to the fact that a TiB₂ layer formed at the particle/matrix interface. Compared to the matrix, the hardness and tensile strength of the Al–B₄C composite fabricated by the liquid stir casting process were increased by 89.6% and 128.8%, respectively; those of the A356–B₄C composite fabricated by the semi-solid stir casting process had no significant improvement due to the weak particle/matrix interface and the presence of particle porosity clusters.     

Initial nucleation of amorphous Si–B–C–N ceramics derived from polymer-precursors

Nucleation behavior of amorphous Si–B–C–N ceramics derived from boron-modified polyvinylsilazane procusors was systematically investigated by transmission electron microscopy(TEM) combined with spatially-resolved electron energy-loss spectroscopy(EELS) analysis. The ceramics were pyrolyzed at1000?C followed by further annealing in N2, and SiC nano-crystallites start to emerge at 1200?C and dominate at 1500?C. Observed by high-angle annular dark-field imaging, bright and dark clusters were revealed as universal nano-structured features in ceramic matrices before and after nucleation, and the growth of cluster size saturated before reaching 5 nm at 1400?C. EELS analysis demonstrated the gradual development of bonding structures successively into SiC, graphetic BNCxand Si_3N_4 phases, as well as a constant presence of unexpected oxygen in the matrices. Furthermore, EELS profiling revealed the bright SiC clusters and less bright Si_3N_4-like clusters at 1200–1400?C. Since the amorphous matrix has already phase separated into SiCN and carbon clusters, another phase separation of SiCN into SiC and Si_3N_4-like clusters might occur by annealing to accompany their nucleation and growth, albeit one crystallized and another remained in amorphous structure. Hinderance of the cluster growth and further crystallization was owing to the formation of BNCxlayers that developed between SiC and Si_3N_4-like clusters as well as from the excessive oxygen to form the stable SiO_2.     

Special features of the diamond crystallization in the Mg–Zn–B–C system

The diamond crystallization in the Mg–Zn–B–C system occurring in the diamond thermal stability region have been considered. The phase transformations, which take place during the preparation of the alloy–solvent for carbon and its structure, the diamond crystallization and properties of the resultant diamond crystals have been studied. The formation of the acceptor centers and inclusions in diamond crystals caused by the addition of boron into the growth system have been considered. It has been found that the use of the diamond powder produced in this system for abrasive machining surfaces of sapphire parts makes it possible to increase the machining efficiency and quality as compared with that of the powder produced in the Ni–Mn–C system.     

Magnetic susceptibility of electroless Ni84P16 and Ni84P16–B4C

X-ray amorphous and crystallised NiP alloys possess structural inhomogeneities with nano-scale sizes, linked to the existence of a distribution of excess free volumes. The way the observed magnetic inhomogeneities are related to structural ones is not yet fully understood. In particular the dispersion of ceramic particles in a NiP-matrix composite adds further, uninvestigated structural peculiarities. In this work we focus our attention on the possibility of antiferromagnetic exchange between nickel atoms in the relevant materials. The magnetic susceptibility was investigated by the Faraday method in the temperature range 290–770 K with magnetic fields B=0.4–1.2 Tl. Amorphous pure-matrix and composite samples show one and two spin-glass transitions, respectively. Crystallised pure-matrix material shows field-independent susceptibility and ferromagnetic behaviour. Crystallised composites show field-dependent susceptibility, but are superparamagnetic.     

Pyrolytic-grown B–C–N and BN nanotubes

We report the growth of pyrolytic boron–carbon–nitrogen (B–C–N) nanotubes on iron (Fe) and nickel (Ni) catalysts. It was discovered that different catalysts had effect on the elemental compositions of B–C–N nanotubes, which may allow one to tune the transport properties of B–C–N nanotubes in a wide range. A new synthetic route was also developed to generate H₃N:BH₃ as the precursor and yield boron nitride (BN) nanotubes by pyrolysis. The typical growth scenario of multi-wall BN tubes will be discussed.     

Pyrolytic-grown B–C–N and BN nanotubes

We report the growth of pyrolytic boron–carbon–nitrogen (B–C–N) nanotubes on iron (Fe) and nickel (Ni) catalysts. It was discovered that different catalysts had effect on the elemental compositions of B–C–N nanotubes, which may allow one to tune the transport properties of B–C–N nanotubes in a wide range. A new synthetic route was also developed to generate H₃N:BH₃ as the precursor and yield boron nitride (BN) nanotubes by pyrolysis. The typical growth scenario of multi-wall BN tubes will be discussed.     

Superplasticity and production of mechanically milled Ti–6Al–4V–0.5B

Abstract

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material with a fine grain size is required to allow either, an increase in the forming rate or a reduction in the deformation temperature. This study details the manufacture of Ti–6Al–4V–0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which ～2.5 vol.-%TiB was formed by an in situ reaction between the titanium and boron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 nm–4 µm after consolidation. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 310%was achieved when testing at 900°C at a strain rate of 6×10^-2 s^-1 compared with 220% for conventional Ti–6Al–4V sheet. However, extensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved.     

Laser cladding composite coatings by Ni–Cr–Ti–B4C with different process parameters

To investigate the effect of laser process parameters on microstructure and properties of composite coating, the composite coatings were manufactured by laser cladding Ni–Cr–Ti–B₄C mixed powder on Q235 mild steel with different process parameters. The coatings are bonded with the substrate by remarkable metallurgical binding without cracks and pores. The composite coatings are consisted of in situ synthesized solid solution Ni–Cr–Fe, intermetallic compound (IMC) Ni₃Ti, Cr₂Ti, and ceramic reinforcements TiB₂, TiC. Results of scanning electron microscopy (SEM) revealed that the ceramic reinforcements became coarser with higher specific energy (E_s). There were independent ceramics TiB₂, TiC, eutectic ceramic TiB₂–TiC in coatings, and eutectic alloy–ceramic was detected. Compared with the substrate, the microhardness of coatings was increased significantly, and the maximum microhardness of coatings was approximately five times as high as the substrate. The wear resistance of coatings was improved dramatically than the substrate. Compared to the coatings with lower E_s, higher E_s led to lower microhardness and worse wear resistance ascribing to more Fe diffused into the coating from the substrate.     

Synthesis of B–C–N nanotubes by means of gas arc discharge with a rotating anode

B–C–N nanotubes were synthesized by plasma rotating electrode process (PREP) and examined by scanning electron microscope (SEM) and X-ray diffraction (XRD). Compared with results at 0 rpm of the anode, the number of B–C–N nanotubes was increased by rotating the anode. In addition, peak intensity ratio of h-(BN)_0.26C_0.74 to graphite was increased with rotation of the anode and/or the increase of metal concentration. That indicates the yield of B–C–N nanotubes was increased. These results could be explained by an increase of plasma temperature and a swirl in the plasma by rotation of the anode that promote the mixing to aid chemical reactions between evaporated species and atomic nitrogen.     

Spectral and Thermodynamic Analysis of Wear Products in a B4C–ShKh15-Steel Friction Couple

We investigate the phase composition of wear products on the sliding surface of boron carbide over steel by means of thermodynamic calculations as well as scanning electron microscopy and Auger electron spectroscopy. We show that graphitization on the friction surface is attributable to the decomposition of carbon monoxide.     

Thermal stability of magnetron sputtered Si–B–C–N materials at temperatures up to 1700 °C

Thermal stability of deposited Si–B–C–N materials (film fragments or powders without a substrate) in inert gases (He and Ar) up to 1700 °C was investigated using differential scanning calorimetry, high-resolution thermogravimetry and X-ray diffraction measurements. Amorphous Si–B–C–N films were fabricated by dc magnetron co-sputtering of a single B₄C–Si target in two nitrogen–argon gas mixtures (50% N₂ + 50% Ar or 25% N₂ + 75% Ar). It was found that the deposited Si–B–C–N materials can be more stable at high temperatures in the inert atmosphere than the usually used substrates (e.g. SiC or BN). The materials with the compositions (in at.%) Si_32–33B₁₀C₂N_50–51, for which N/(Si + B + C) = 1.1–1.2, retained their amorphous structure up to 1600 °C without any structural transformations and detectable mass changes.     

Synthesis and characterization of a new liquid polymer precursor for Si–B–C–N ceramics

A new liquid polyborosilazane precursor for Si–B–C–N ceramic was synthesized by co-condensation reaction of boron trichloride, organodichlorosilanes, and hexamethyldisilazane. The structure and properties of polyborosilazane were studied by means of Fourier transform-infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), rheology, and thermogravimetric analysis (TGA). The conversion of polymer to ceramic and the high-temperature behavior of the new polymer-derived ceramic were investigated by TG–MS, FT-IR, X-ray diffraction (XRD) and high-temperature TGA (HTGA). The ceramics showed good oxidative resistance and thermal stability with weight gain of 1.8 wt% at 1350 °C under air atmosphere and weight loss of 2.6% at 1900 °C under Ar atmosphere.