Densification and Mechanical Properties of B4C with Al2O3 as a Sintering Aid

The densification behavior and mechanical properties of B₄C hot-pressed at 2000°C for 1 h with additions of Al₂O₃ up to 10 vol% were investigated. Sinterability was greatly improved by the addition of a small amount of Al₂O₃. The improvement was attributed to the enhanced mobility of elements through the Al₂O₃ near the melting temperature or a reaction product formed at the grain boundaries. As a result of this improvement in the density, mechanical properties, such as hardness, elastic modulus, strength, and fracture toughness, increased remarkably. However, when the amount of Al₂O₃ exceeded 5 vol%, the level of improvement in the mechanical properties, except for fracture toughness, was reduced presumably because of the high thermal mismatch between B₄C and Al₂O₃.     

Hardness and Fracture Toughness of Pressureless-Sintered Boron Carbide (B4C)

This is a companion work to our previous study on the pressureless sintering of boron carbide (B₄C). The Vickers hardness and indentation fracture toughness of B₄C compacts were measured after various sintering heat treatments. Increases in hardness and decreases in indentation fracture toughness as the grain size decreased in sintered B₄C were attributed to the effects of more rapid strain hardening associated with dislocation pileups at grain boundaries.     

Combustion Synthesis of Si3N4–SiC Composite Powders

Fine Si₃N₄-SiC composite powders were synthesized in various SiC compositions to 46 vol% by nitriding combustion of silicon and carbon. The powders were composed of α-Si₃N₄, β-Si₃N₄, and β-SiC. The reaction analysis suggested that the SiC formation is assisted by the high reaction heat of Si nitridation. The sintered bodies consisted of uniformly dispersed grains of β-Si₃N₄, β-SiC, and a few Si₂N₂O.     

Synthesis of Dense TiB2-TiN Nanocrystalline Composites through Mechanical and Field Activation

The synthesis of dense nanometric composites of TiN-TiB₂ by mechanical and field activation was investigated. Powder mixtures of Ti, BN, and B were mechanically activated through ball milling. Some powders were milled to reduce crystallite size but to avoid initiating a reaction. In other cases powders were milled and allowed to partially react. All these were subsequently reacted in a spark plasma synthesis (SPS) apparatus. The products were composites with equimolar nitride and boride components with relative densities ranging from 90.1% to 97.2%. Crystallite size analyses using the XRD treatments of Williamson-Hall and Halder-Wagner gave crystallite sizes for the TiN and TiB₂ components in the range 38.5–62.5 and 31.2–58.8 nm, respectively. Vickers microhardness measurements (at 2 N force) on the dense samples gave values ranging from 14.8 to 21.8 GPa and fracture toughness determinations (at 20 N) resulted in values ranging from 3.32 to 6.50 MPa·m^1/2.     

Solubility and Diffusion of Si in B4C

Diffusion couples of B₄C/SiC single crystals were annealed at 1800° to 2100°C. Electron probe tnicroanalysis of the joined diffusion couple showed that the solubility limit and diffusion coefficient, D, for Si in B₄C are 0.27 to 0.36% wt% and D = 0.165 exp(—101200/RT), respectively.     

Al-B-C Phase Development and Effects on Mechanical Properties of B4C/Al-Derived Composites

B₄C/A1 offers a family of engineering materials in which a range of properties can be developed by postdensiflcation heat treatment. In applications where hardness and high modulus are required, heat treatment above 600°C provides a multiphase ceramic material containing only a small amount of residual metal. Heat treatment between 600° and 700°C produces mainly A1B₂; 700° and 900°C results in a mixture of A1B₂ and A1₄BC; 900° and 980°C produces primarily A1₄BC; and 1000° to 1050°C results in A1B₂₄C₄ with small amounts of A1₄C₃ if the heating does not exceed 5 h. Deleterious A1₄C₃ is avoided by processing below 1000°C. All of these phases tend to form large clusters of grains and result in lower strength regardless of which phase forms. Toughness is also reduced; the least determinal phase is A1B₂. The highest hardness (88 Rockwell A) and Young's modulus (310 GPa) are obtained in Al₄BC-rich samples. AlB₂-containing samples exhibit lower hardness and Young's modulus but higher fracture toughness. While the modulus, Poisson's ratio, and hardness of multiphase B₄C/A1 composites containing 5–10 vol% free metal are comparable to ceramics, the unique advantage of this family of materials is low density (>2.7 g/cm³) and higher than 7 MPa-m^1/2 fracture toughness.     

Processing and Mechanical Properties of Dense Si3N4-SiC-Whisker Composites without Sintering Aids

Si₃N₄ with 20 vol% SiC whisker was fabricated without sintering aids by hot isostatic pressing. Density higher than 99.5% was attained after sintering at 2000°C and 170 MPa for 1 h. Careful mixing procedures and the use of an appropriate amount of a dispersant was found to be effective in avoiding whisker segregation and inhomogeneity. Mechanical properties of the composite were investigated by measurements of flexural strength, microhardness, frature toughness, and Young's modulus as a function of temperature. At room temerature, Vickers microhardness and Young's modulus increased from the matrix value about 20% and 5%, respectively. Toughness was about 30% higher, without reduction in flexural strength, up to 1400Deg;C.     

Electrical Properties of Boron Nitride Matrix Composites: III, Observations near the Percolation Threshold in BN–B4C Composites

In this third paper of the series, we discuss the electrical resistivity of BN–B₄C composites with compositions ranging from 0% to 100% B₄C. After establishing the response of samples whose compositions lie far away from the percolation region, where effective medium models apply, we focus attention on samples with compositions at or near the percolation threshold (∼60% BN–40% B₄C). The large differences in electrical properties among samples with the same nominal composition can be explained by invoking a connectivity parameter. Since the difference in the electrical resistivity of BN and B₄C is about 9 orders of magnitude, the degree of connectivity of the two components at the percolation threshold determines the resultant composite resistivity. Connectivity in these composites was quantified by taking BN peak height ratios in X-ray diffraction patterns of all samples containing 60% BN–40% B₄C. The degree of preferred orientation of the BN platelets can be correlated with systematic increases in the electrical resistivity of the composites.     

High-Temperature Mechanical Properties of SiC–Mo5(Si,Al)3C Composites

SiC–Mo₅(Si,Al)₃C composites were fabricated by the melt infiltration process, and the infiltration characteristics were studied in detail. Fracture strength and toughness were measured up to 1600°C using a three-point bending test and indentation strength method, respectively. Both fracture strength and toughness significantly increased at 1400°C with respect to the values at room temperature. These increases were mainly attributed to plastic deformation of the infiltrated Mo₅(Si,Al)₃C phases at elevated temperatures, which acted as ductile toughening inclusions. Compressive creep tests were used to study the creep behavior of the composite in the range of 1550°–1650°C and 150–200 MPa. The stress exponent and activation energy were 1.3 and 277 kJ/mol, respectively. Preliminary oxidation tests showed that the composites exhibited good oxidation resistance at 1500°C because of the formation of a dense oxide scale.     

Pressure-Assisted Combustion Synthesis of Dense Layered Ti3AlC2 and its Mechanical Properties

A near-single-phase Ti₃AlC₂ ternary carbide was synthesized from 3Ti–1.1Al–1.8C powder blend, both by the wave propagation and thermal explosion (TE) modes of self-propagating high-temperature synthesis. The application of a moderate (28 MPa) pressure immediately after TE at 800°C (reactive forging) yielded a 95% dense material containing, in addition to Ti₃AlC₂, an appreciable amount of TiC_{1− x} . By adjusting the starting composition, a 99% dense material containing up to 90 vol.% Ti₃AlC₂ was obtained. The material had a fine-layered microstructure with Ti₃AlC₂ grain size not exceeding 10 μm. The samples were readily machinable and had a high compressive strength of ∼800 MPa up to 700°C.     

Addition of carbon fibers into B4C infiltrated with molten silicon

Boron carbide added with 0–20?wt% carbon fibers was subject to Si infiltration. Samples mainly consist of B₁₃C₂, β-SiC and unreacted Si. Some amount of SiB₆ and α-SiC was also detected, while formation of B₁₂(B,C,Si)₃ phase was suppressed due to short infiltration time. The carbon fibers react with Si and result in formation of a composite core-shell fiber with SiC-shell and C-core. Theoretical estimations suggest that these composite fibers have a strong influence on the enhancement of the bending strength. Although apparently in good agreement with experimental data showing an increase of bending strength up to 510?±?27?MPa in the sample with 10?wt% carbon fiber, the implications of phase changes with the carbon fiber amount has to be carefully considered. At higher amounts of carbon fibers, bending strength decreases.     

Mechanical Properties of Hot-Pressed B-B4C Materials

Elastic modulus and flexural strength were measured at room temperature for solid pieces hot-pressed at 1900 to 2600 K from mixtures of B₄C powder and ≤15 wt% B powder. Regression analysis showed that elastic modulus and flexural strength are not significantly affected by these boron additions. Elastic modulus is related to porosity and flexural strength to porosity and grain size. The fracture surface energy of boron carbide was evaluated. X-ray diffraction and chemical analyses showed that the specimens were composed entirely of boron carbide after hot-pressing. Lattice constants increased with the initial boron content.     

Influence of B4C Additive on Sintering and Crystallization of Fused Silica Ceramics

The green bodies were prepared by adding B₄C （0.5%,1.5%,2.5%and 3.5%in mass） or B₂O₃ （6%and 9%in mass） infused silica powders,respectively and then were fired at 1 150℃for 4 h,1 200℃for 4 h,and 1 350℃for 0.5 h,respectively.The bulk density,apparent porosity,cold modulus of rupture and thermal expansion coefficient of the fired specimens were determined,and phase compositions and microstructure were researched.The results show that:（1 ） B₄C promotes the sintering of fused silica ceramics,but many closed pores will form easily with B₄ C increase; （2） fused silica ceramics with 3.5 mass%B₄C have 1.045×10^-6 V^-1 of the lowest average thermal expansion coefficient between 40℃and 730℃;（3 ） B₄C additive is more effective to restrain crystallization of fused silica than B₂O₃ with the same content of boron.     

Preparation of High-Purity Vitreous B2S3

Preparation schemes for high-purity vitreous B₂S₃ ( v -B₂S₃) with a S:B ratio of 1.478 ± 0.032 are described. Preparation schemes reported in the literature are shown by very sensitive FT-IR measurements to yield v -B₂S₃ with significant oxygen contamination. Through extreme care taken in carbonizing silica reaction tubes and through the use of very high purity starting materials, oxygen contamination of less than 1% has been achieved. The IR spectrum for very pure v -B₂S₃ contains only two absorption bands.     

Wetting of Binary Aluminum Alloys in Contact with Be, B4C, and Graphite

Sessile drop contact angles between liquid aluminum alloys and solid beryllium, boron carbide, and graphite were measured to 840°C under vacuum and in helium. Little wetting occurred between most of the combinations, but at 20% magnesium the contact angle on beryllium decreased to 68°. Low contact angles were noted for the binary aluminum alloys on graphite coated with titanium.     

Synthesis of Al4SiC4

The conditions necessary for synthesizing Al₄SiC₄ from mixtures of aluminum, silicon, and carbon and kaolin, aluminum, and carbon, as starting materials, were examined in the present study. The standard Gibbs energy of formation for the thermodynamic reaction SiC( s ) + Al₄C₃( s ) = Al₄SiC₄( s ) changed from positive to negative at 1106°C. SiC and Al₄C₃ formed as intermediate products when the mixture of aluminum, silicon, and carbon was heated in argon gas, and Al₄SiC₄ then formed by reaction of the SiC and Al₄C₃ at >1200°C. Al₄C₃, SiO₂, Al₂O₃, SiC, and Al₄O₄C formed as intermediate products when the mixture of kaolin, aluminum, and carbon was heated under vacuum, and Al₄SiC₄ formed from a reaction of those intermediate products at >1600°C.     

Microstructural Evolution of Titanium Carbide–Chromium Carbide (TiC–Cr3C2) Composites Produced via Combustion Synthesis

A microstructural analysis of compounds produced by combustion synthesis coupled with hot pressing, for reactions between titanium, chromium, and carbon, was conducted. The reactions were aimed to produce composites of Cr₃C₂ and TiC at three different volume fractions of each carbide (25/75, 50/50, and 75/25). Large amounts of chromium and carbon were found to be in solution in the B1 rock-salt structure of TiC. The materials with 25 and 50 vol% of Cr₃C₂ consisted of 100% (Ti,Cr)C_ss solid solution, while the composition with 75 vol% Cr₃C₂ was formed by Cr₃C₂+ (Ti,Cr)C_ss. Some precipitation of Cr₃C₂ was achieved by annealing, but a minimum of 20 wt% was always in solution. The 50 vol% Cr₃C₂–50 vol% TiC composition was the most affected by the heat treatments. Discontinuous and general precipitation were observed, depending on the annealing conditions. A TTT-type diagram was plotted for this material.     

Crack Propagation in Irradiated B4C Induced by Swelling and Thermal Gradients

Boron carbide pellets, used as the neutron absorber materials in fast breeder reactors, were observed to crack in two distinctly different ways. Radial thermal gradients are produced as a result of internal heat generation. In larger-diameter pellets, these thermal expansion gradients are large and it was observed that pellet cracking occurred early in operation. It was also observed that swelling gradients were established at high burnup levels. These pellets were also cracked but in a different manner.     

Formation of MgO-B4C Composite via aThermite-Based Combustion Reaction

The combustion synthesis of MgO-B₄C composites was investigated by coupling a highly exothermic Mg-B₂O₃ thermite reaction with a weakly exothermic B₄C formation reaction. Unlike the case of using Al as the reducing agent, the interaction between Mg and B₂O₃ depends on the surrounding inert gas pressure due to the high vapor pressure of Mg. The interaction changes from one involving predominantly gaseous Mg and liquid B₂O₃ to one involving liquid Mg and liquid B₂O₃ as the pressure increases. At low inert gas pressure, the initiation temperature is found to be just below the melting point of Mg (650°C). As the inert gas pressure increases, the vaporization loss of reactants is reduced, and this in turn increases the combustion temperature, which promotes greater grain growth of the product phases, MgO and B₄C. The particle size of B₄C increased from about 0.2 to 5 μm as the pressure changed from 1 to 30 atm.