Surface Of B4C Particles Research Articles

Influence of thinner flaked Al powder with larger diameter on the distribution of B4C particles and tensile properties of B4C/Al composite

Abstract The thinner flaked Al powders are used to improve the strength of the B4C/Al composite. The effect of the flaked Al powder on distribution of B4C particles is investigated. The tensile properties of the B4C/Al composites with the flaked Al powders are also studied. The results show that during ball milling process, spherical Al powders are flaked with the thinner thicknesses and larger diameters. The thinner flaked Al powders are not easy to be deformed and are bonded together as the laminated structure after thermal compression. B4C particles are evenly dispersed and embedded in Al laminated structure. With increasing the volume fraction of B4C particles, the deformation of the thinner flaked Al powders become difficult and the thinner flaked Al powders coat on the surface of B4C particles, resulting in the disappearance of the laminated structure of the thinner flaked Al powder.

Effect of Ti element on the interfacial reactions and micro-structures of the Al-B4C composites fabricated by the stir-casting method

The effect of titanium (Ti) on the interfacial reactions and micro-structures of the Al-B4C composites were studied using theoretical calculations based on the first-principles approach of density functional theory in combination with experimental analysis. The experimental results showed that the addition of Ti caused a more uniform distribution of B4C particles and the disappearance of interfacial micro-cracks. Three interfacial reaction layers were formed on the surface of B4C particles, the inner layer was composed of Al3BC, the middle layer constituted fine-grained TiB2, and the outer layer consisted of coarse-grained TiB2. The thermodynamic analysis showed that the most favorable reaction paths in the Al-B4C and Al-Ti-B4C systems were 9Al + 2B4C → 2Al3BC + 3AlB2 and 6Al + 3Ti + 2B4C → 2Al3BC + 3TiB2, respectively. Finally, the formation mechanism of the interface reaction layers in Al–B4C and Al–Ti–B4C systems were discussed. The results indicated the inter-diffusion of interfacial atoms in the melt, atomic diffusion rate, atomic concentration, atomic size and lattice mismatch were the factors influencing the formation and growth of the B4C-Al3BC-TiB2 interface reaction layer.

Studies on Copper-Coated Boron Carbide Particle-Reinforced Copper-Matrix/Graphite Self-Lubricating Composite Materials

In this study, copper-coated (Cu-coated) boron carbide (B4C) particles were successfully prepared by electroless Cu plating onto B4C particle surfaces. These Cu-coated B4C particles were combined with Cu-coated graphite particles and Cu powder, which were then used to prepare a new type of Cu-matrix self-lubricating composite by powder metallurgy. Uncoated B4C particles were used to prepare corresponding composites and all composites characterized by phase composition, microstructure, density, porosity, hardness, compressive yield strength, and friction wear. The results showed that Cu-coated B4C particles improved wettability between the particles and matrix and possessed increased interface-bonding strength with the matrix. With the same B4C content, Cu-matrix/graphite composites reinforced by Cu-coated B4C had lower porosity and wear resistance, as well as higher hardness and compressive strength, than composites reinforced by uncoated B4C. Considering the wear and friction coefficients, composites containing 0.5 wt % Cu-coated B4C exhibited better friction and wear resistance properties.

Synthesis and properties of conductive B4C ceramic composites with TiB2 grain network

AbstractHigh electrical resistance and low fracture toughness of B4C ceramics are 2 of the primary challenges for further machining of B4C ceramics. This report illustrates that these 2 challenges can be overcome simultaneously using core‐shell B4C‐TiB2&TiC powder composites, which were prepared by molten‐salt method using B4C (10 ± 0.6 μm) and Ti powders as raw materials without co‐ball milling. Finally, the near completely dense (98%) B4C‐TiB2 interlayer ceramic composites were successfully fabricated by subsequent pulsed electric current sintering (PECS). The uniform conductive coating on the surface of B4C particles improved the mass transport by electro‐migration in PECS and thus enhanced the sinterability of the composites at a comparatively low temperature of 1700°C. The mechanical, electrical and thermal properties of the ceramic composites were investigated. The interconnected conductive TiB2 phase at the grain boundary of B4C significantly improved the properties of B4C‐TiB2 ceramic composites: in the case of B4C‐29.8 vol% TiB2 composite, the fracture toughness of 4.38 MPa·m1/2, the electrical conductivity of 4.06 × 105 S/m, and a high thermal conductivity of 33 W/mK were achieved.

Porous TiC–TiB2–NiAl composites and effect of NiAl contents on pore structure and microstructure

Porous TiC–TiB2–NiAl composites were successfully synthesised by self-propagating high temperature synthesis (SHS) with Ti–B4C–Ni–Al mixture as reactant. The effects of Ni+Al contents and particle size of B4C powders on pore structure and microstructure of composites were investigated. The results show that the porous composites are composed of open and closed pores with desired TiC, TiB2 and NiAl phases. The closed pores are mainly from the decomposition of B4C particles, while TiB2 and TiC particles develop at the surface of B4C particles, which can provide a shell surrounding the remnant B4C particles and finally form a shell core structure. With the increase of Ni+Al contents, enough melted NiAl is formed during the SHS process. The NiAl liquid can cause the TiB2–TiC shell to collapse, then lead to the distribution of TiB2 and TiC particles in the NiAl matrix, and even fill in the space between the shells and cores, which result in the reducing or even disappearing of closed pores, and the increasing of open porosity.

Electroless Copper Coating on Boron Carbide Particles by Using Copper Activation Method

The present work is focused on electroless coating of copper nanolayer onto boron carbide (B4C) particle surfaces by using copper activation method. The B4C particles used are approximately 18.25μm in average size. B4C particle surfaces were washed by acetone and activated through copper activation method. In the electroless coating bath, copper sulfate, EDTA-2Na and seignette salt, hydrazine hydrate were used as the copper catalytic centers source, complexing agent and reducing agent respectively. The structure and morphology of the coating layers were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD and SEM observations show that B4C particle surfaces were successfully coated by a homogeneous and continuous copper layer.

Electroless nickel–phosphorus coating on boron carbide particles

The present work aimed at studying electroless coating of nickel–phosphorus (Ni–P) onto boron carbide (B4C) particles with average particle sizes of 93 and 32μm. Prior to the coating process, B4C particle surfaces were sensitized using SnCl2 and HCI solution and activated in PdCl2 and HCl solutions. In the electroless coating bath, nickel sulphate (NiSO4·6H2O), sodium hypophosphite (NaH2PO2·H2O) and sodium citrate (Na3C6H5O7.5, 5H2O) were used as Ni2+ source, reducing agent and complexing agent, respectively. The morphology and the chemical composition of the coating layers were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Characterization results showed that B4C particle surfaces were successfully coated by a homogeneous and continuous Ni–P layer.

Effect of titanium on microstructure and fluidity of Al–B4C composites

The effect of Ti on the interfacial reactions, microstructural characteristics, and the related fluidity of Al–12%B4C composites has been investigated. Without Ti addition, B4C decomposed heavily during holding, and a large quantity of reaction-induced compounds, Al3BC and AlB2, was generated. When Ti was added, a TiB2 layer was built surrounding B4C particle surfaces, which acted as a diffusion barrier to separate B4C from liquid aluminum. Thus, the decomposition of B4C slowed down remarkably. The fluidity of the composite without Ti was the shortest of all composites and deteriorated quickly during the holding time. The fluidity of the composite melt was improved significantly with increased Ti levels. The optimum Ti level for the best fluidity results lied between 1.0 and 1.5%. The solid particle volume and the particle agglomeration are the two main factors influencing the fluidity.

Nickel-Boron Nanolayer-Coated Boron Carbide Pressureless Sintering

Sintering of pure B4C and Ni2B nanolayer-coated B4C was studied from 1300° to 1600°C, with the holding time at the peak temperatures being 2 or 10 h. Compacts were made by uniaxial die compaction and combustion-driven compaction. Pure B4C sample shows less sintering at all conditions. Ni2B-coated B4C sample shows more extensive densification, neck formation, and grain shape accommodation. The combustion driven compaction process accelerates sintering by offering higher green density to start with. The Ni2B species on the B4C particle surfaces melts into a nickel–boron-containing liquid phase during heating, remains as liquid during sintering, and then transforms into Ni4B3 and NiB during cooling. High-resolution composition analysis shows that there is no nickel diffusion into bulk B4C during the sintering process. However, there is boron diffusion into the Ni2B coating layer. Carbon diffusion cannot be directly measured but is believed to be a simultaneous process as boron diffusion. A multievent sintering process has been proposed to explain the observations.

Compaction of different boron carbide powders using uniaxial die compaction and combustion driven compaction

Compaction of pure B4C and Ni2B nanolayer coated B4C was studied using uniaxial die compaction and combustion driven compaction techniques. Effects of different compaction techniques and the Ni2B nanolayer around B4C particle surfaces on green B4C sample characteristics are the focus of this study. The combustion driven compaction process yields much higher green density and strength than the uniaxial die compaction process. For the samples obtained from the same compaction technique, the Ni2B nanolayer on individual B4C particle surfaces improves the green density and strength of the B4C powder compacts. For the combustion driven compaction process, optical images show micro-cracks on the surface of pure B4C compact while crack-free surface is observed for Ni2B nanolayer coated B4C sample. Scanning electron microscopy analysis shows the same trend as the green density and strength measurements. Combustion driven compaction diagram for hard and brittle materials such as B4C is discussed.

Morphology and composition of nickel–boron nanolayer coating on boron carbide particles

This work is focused on electroless coating of Ni–B nanolayer on B4C particle surfaces. The B4C particles used are approximately 2 μm in average size. Effects of activation agent PdCl2, complexing agent C2H8N2, and reducing agent NaBH4 addition rate are studied. The solids loading of B4C is 0.625 g/L and the concentration of Ni2+ ions is 0.004 mol/L in the electroless coating solution. Scanning electron microscopy (SEM) shows that when B4C:Pd2+ molar ratio is 1:0.005, a Ni–B nanolayer with the smallest Ni–B nodule size covers the B4C particle surfaces. Complexing agent C2H8N2 decreases Ni2+ ion release rate. Ni:C2H8N2 ratio of 1:6 is the preferred complexing agent amount for achieving a continuous Ni–B nanolayer. The Ni–B nanolayer formation is also strongly dependent on the rate that Ni2+ ions are reduced. Slow Ni2+ reduction leads to increased Ni content in the Ni–B nanolayer. When the above three factors are combined at the optimal values for the electroless coating process, well-defined Ni–B nanolayer is obtained. SEM cross section analysis shows the Ni–B nanolayer completely covers the B4C particles with less than 55 nm thickness.

Property and Microstructure of Machinable B<sub>4</sub>C/BN Nanocomposites

The B4C/BN nanocomposites were fabricated by hot-pressing sintering of the B4C/BN nanocomposite powders at 1850oC for 1h under the pressure of 30MPa. The composite powders with the microstructure of B4C particles coated with nano-sized BN particles were prepared by the chemical reaction of H3BO3 and CO(NH2)2 on the surface of B4C particles at high temperature. The microstructure investigation of the nanocomposites sintered samples showed that the nano-sized h-BN particles were homogenously distributed in the B4C matrix. With the increasing content of h-BN, the density of the B4C/BN nanocomposites decreased gradually; the fracture strength and fracture toughness of the B4C/BN nanocomposites decreased gradually, the strength and toughness of the B4C/BN nanocomposites with the h-BN content of 10wt% and 20wt% achieved high values. The Vickers hardness of the B4C/BN nanocomposites decreased remarkably with the increasing content of h-BN, while the machinability of the B4C/BN nanocomposites was significantly improved. The B4C/BN nanocomposites with the h-BN content more than 20wt% exhibited excellent machinability.

Particle distribution and interfacial reactions of Al–7%Si–10%B4C die casting composite

Aluminum–boron carbide particle reinforced composite is an advanced material which can be used in applications such as neutron-shielding components, aircraft, and aerospace structures. In the microstructural characterization of an Al–7%Si–10%B4C die casting, attention is particularly focused on particle distribution and interface reaction products between B4C particles and the aluminum matrix. The quantitative analysis results show that, in a cross-section of the cast part, more particles concentrate in the center and fewer particles are present in the wall regions. Moreover, some particle segregation bands have been observed. The mechanisms of the particle migration are proposed to describe the phenomenon. However, the average particle fraction in any cross-section of the cast part is almost the same. A barrier layer consisting of several sublayers was detected on the surface of B4C particles. Using electron diffraction in selected areas, it is found that these sublayers are composed of Al3BC crystals, TiB2 crystals, Si crystals, and coarse stick-shaped TiB2 particles. In addition, it is observed that Si plays an important role in the formation of a dense barrier layer. The barrier layer can limit B4C decomposition and improve B4C stability in the aluminum melt.

Reaction and Its Suppression of B4C with Mullite.

The reactivity of B4C with mullite was investigated under Ar and reductive atmospheres, respectively. The results indicated that B4C could coexist stably with mullite under Ar atmosphere below 1350°C, above which some reaction would occur to form Al2O3 and Al3SiB48 phases. Under a reductive atmosphere, whereas, they exhibited a relatively higher co-existing temperature of 1500°C, which is deemed to be dependent on the partial pressure of CO2 gas in heating atmosphere. Coating technique was employed in this study to suppress the reaction between B4C and mullite by producing a TiO2 thin coating layer on the surface of B4C particles. TiO2 will react with B4C at high temperature to produce TiB2 and B2O3. The latter phase, existing on the grain boundary of B4C particles by a liquid state at high temperature, will, together with TiB2, serve as a barrier to inhibit the reaction between B4C and mullite. A2mass% TiO2 on B4C powders can effectively suppress the occurrence of this reaction.