Solid-State Diffusion Bonding of Carbon–Carbon Composites with Borides and Carbides

Solid-state diffusion bonding of carbon–carbon (C─C) composites by using boride and carbide interlayers has been investigated. The interlayer materials used in this study were single-phase borides (TiB₂ or ZrB₂), eutectic mixtures of borides and carbides (ZrB₂+ ZrC or TiB₂+ B₄C), and mixtures of TiB₂+ SiC + B₄C produced in situ by chemical reactions between B₄C, Ti, and Si or between TiC, Si, and B. The double-notch shear strengths of the joints produced by solid-state reaction sintering of B₄C + Ti + Si interlayers were much higher than those of joints produced with other interlayers. The maximum strength was achieved for C─C specimens bonded at 2000°C with a 2:1:1 mole ratio of Ti, Si, and B₄C powders. The reaction products identified in the interlayers, after joining, were TiB₂, SiC, and TiC. The joint shear strength increased with the test temperature, from 8.99 MPa at room temperature to an average value of 14.51 MPa at 2000°C.     

Sintering and Mechanical Properties of Mullite-Reinforced Boron Carbide Matrix Composite

In order to improve the mechanical properties of boron carbide (B₄C) ceramic, a mullite-reinforced B₄C matrix ceramic with complete densification was fabricated via hot pressing for the first time. The dense sintering mechanism of mullite-reinforced B₄C ceramic was discussed through the phase and element analysis. A new dense sintering mechanism was found in which the diffusion of Si in mullite through the B₄C matrix enhances the sintering of mullite-reinforced B₄C ceramics effectively. The mechanical properties and microstructure of the composite ceramics were investigated in contrast with monolithic B₄C and one kind of commercial B₄C ceramic. The flexural strength and fractural toughness of B₄C with 3 vol% mullite addition reached 560 MPa and 3.33 MPa·m^1/2, which is 154% and 96% higher than that of monolithic B₄C, respectively.     

Pressureless Sintering of Boron Carbide

B₄C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B₄C–B₄C contact between particles permitted via volatilization of B₂O₃ particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B₄C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B₄C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B₂O₃ to form B₄C and CO gas, thereby extracting the B₂O₃ coatings, permitting sintering to start at ∼1350°C.     

Effect of Iron and Boron Carbide on the Densification and Mechanical Properties of Titanium Diboride Ceramics

The effect of Fe and B₄C on the sintering behavior and mechanical properties of TiB₂ ceramics have been studied. Sintering was performed in an Ar atmosphere at 2000° using attrition-milled TiB₂ powder (mean particle size = 0.8 μm). When a small amount of Fe (0.5 wt%) was added, abnormal grain growth occurred and the sintered density was low. In the case of B₄C added along with 0.5 wt% Fe, however, abnormal grain growth was remarkably suppressed, and the sintered density was increased up to 95% of theoretical. But with excess Fe addition (5 wt%), B₄C grains did not act as a grain growth inhibitor, and B₄C grains were frequently trapped in large TiB₂ grains. The best mechanical properties were obtained for the TiB₂–10 wt% B₄C–0.5 wt% Fe ceramics, which exhibited a three-point bending strength of 400 MPa and a fracture toughness of 5.5 MPa · m^1/2.     

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.     

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.     

Phase and Property Studies of Boron Carbide–Boron Nitride Composites

Boron carbide–boron nitride particulate composites were fabricated by vacuum hot-pressing. Near-theoretical densities of B₄C were obtained, but percent theoretical densities decreased with increasing amounts of BN. The grain size of B₄C and BN was not affected by composition, but the amount of twinning in B₄C decreased with increasing BN content. No third phase was found at the B₄C–BN interface by analytical STEM analysis. Lattice parameter measurements indicated slight solubility of B₄C in BN, but no solubility of BN in B₄C for samples hot-pressed at 2250°C. Room-temperature flexural strength measurements revealed a sharply decreasing strength with increasing BN content up to 40% BN, and then relatively constant values with greater amounts of BN.     

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.     

Sintering of Boron Carbide Heat-Treated with Hydrogen

Hydrogen gas (H₂) was used to extract B₂O₃ coatings from boron carbide (B₄C) particles, permitting a lower temperature onset of sintering and restricting coarsening via solution and precipitation of B₄C in B₂O₃ liquid. Remnant H₂ had to be removed from the furnace before specimens were heated through temperature ranges in which evaporation-condensation coarsening competed with sintering (2010°–2140°C), because the presence of H₂ increased the B₄C vapor pressure. Heat treatment of B₄C compacts in a 50:50 H₂-He mixture at 1350°C, followed by a purge of the H₂ gas and then rapid heating to 2230°C, resulted in a percentage of theoretical density of 94.7%. This is higher than the value of 92.8%, which was the highest achieved without the use of H₂.     

Conical Boron Nitride Nanorods Synthesized Via the Ball-Milling and Annealing Method

A boron nitride (BN) nanostructure, conical BN nanorod, has been synthesized in a large quantity on Si substrates for the first time via the ball-milling and annealing method. Nitridation of milled boron carbide (B₄C) powders was performed in nitrogen gas at 1300°C on the surface of the substrates to form the BN nanorods. The highly crystallized nanorods consist of conical BN basal layers stacked along the nanorod axis. Ball milling of the B₄C powders can significantly enhance the nitridation of the powders and thus facilitate the formation of nanorods during the annealing process.     

In Situ TiC/TiB2 Particulate Locally Reinforced Steel Matrix Composites Fabricated Via the SHS Reaction of Ni–Ti–B4C System

The composites synthesized with three kinds of B₄C particles mainly consist of TiC, TiB₂, and the alloy austenite containing Ni element. Ceramic particulate sizes in the composites synthesized with ∼3.5 and ∼45 μm B₄C particles are larger than that synthesized with ∼140 μm B₄C particle. No pores are found between the reinforcing region and matrix in the composites synthesized with ∼3.5 and ∼45 μm B₄C particles, while some large pores exist in the composites synthesized with ∼140 μm B₄C particle. With the decrease of B₄C particle size, the pores in the composites become fewer and the hardness and wear resistance of the composites increase.     

Reactive Hot Pressing of ZrB2–SiC Composites

A ZrB₂–SiC composite was prepared from a mixture of zirconium, silicon, and B₄C via reactive hot pressing. The three-point bending strength was 506 ± 43 MPa, and the fracture toughness was 4.0 MPa·m^1/2. The microstructure of the composite was observed via scanning electron microscopy; the in-situ -formed ZrB₂ and SiC were found in agglomerates with a size that was in the particle-size ranges of the zirconium and silicon starting powders, respectively. A model of the microstructure formation mechanism of the composite was proposed, to explain the features of the phase distributions. It is considered that, in the reactive hot-pressing process, the B and C atoms in B₄C will diffuse into the Zr and Si sites and form ZrB₂ and SiC in situ , respectively. Because the diffusion of Zr and Si atoms is slow, the microstructure (phase distributions) of the obtained composite shows the features of the zirconium and silicon starting powders.     

The Effect of Stoichiometry on Mechanical Properties of Boron Carbide

The mechanical properties of chemically vapor-deposited boron carbides (B₄C) with varied B/C ratios were investigated as a function of composition. The maximum hardness, H, and fracture toughness, K_1c, were observed at an almost stoichiometric composition. For nonstoichiometric B₄C (B/C>4), H and K_1c decreased with increasing B content, suggesting that excess B diminishes the bond strength in the B₄C structure. The decrease in H and K_1C at B /C <4 was attributed to free C in the microstructure.     

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.     

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.     

Electrical Properties of Boron Nitride Matrix Composites: I, Analysis of McLachlan Equation and Modeling of the Conductivity of Boron Nitride–Boron Carbide and Boron Nitride–Silicon Carbide Composites

The McLachlan equation, which incorporates both effective medium models and percolation, was used to predict the volume fraction–conductivity relationships of insulator–conductor composites, and results were compared with experimental data. Two composite systems were investigated (BN–B₄C and BN–SiC). Both systems are anisotropic, because of the orientation of BN platelets perpendicular to the hot-pressing direction. For BN–B₄C composites, with increasing B₄C content, the ac and dc conductivities are relatively constant to ∼40% B₄C (the critical volume fraction). At this composition, the conductivity suddenly increases to a value closer to that of B₄C and then resumes a gradual increase. Little difference is seen for measurements made perpendicular or parallel to the hot-pressing direction, i.e., perpendicular or parallel to the BN platelets. Similar results are found for the BN–SiC composites, except that the critical volume fraction is ∼20% SiC in this case. The experimental curves are in good agreement with those predicted by the McLachlan equation. The parameters s and t of the McLachlan equation relate to the morphology of the phases present in the microstructure. The critical volume fraction relates to the connectivity of the phases in the composites.     

Synthesis of Nanocrystalline Boron Carbide via a Solvothermal Reduction of CCl4 in the Presence of Amorphous Boron Powder

Nanocrystalline boron carbide (B₄C) was synthesized via a solvothermal reduction of carbon tetrachloride using metallic lithium as reductant in the presence of amorphous boron powder at 600°C in an autoclave. The X-ray diffraction pattern of the product powder was indexed to the hexagonal B₄C phase, with lattice constants a =5.606 and c =12.089 Å. The sample was also characterized by Raman spectrum, X-ray photoelectron spectroscopy and inductively coupled plasma atomic emission spectrometry. Transmission electron microscopy revealed that the B₄C nanocrystallites were slightly agglomerated, with a particle size of approximately 15–40 nm in diameter.     

Phase Equilibria in the System Boron Carbide-Silicon Carbide

The system boron carbide-silicon carbide is quasi binary and contains a eutectic of composition 70 ± 2 wt% B₄C and 30 ± 2 wt% Sic at a temperature of 2300°± 20° C. The solid solubility of both B₄C in Sic and Sic in B₄C is less than 2 wt% at this temperature, based on metallographic and X-ray analyses. The Sic corresponds to the hexagonal (a) allotrope when the compositions contain more than 50 wt% Sic. The cubic (p) form of Sic is stabilized in compositions which contain less than 50 wt% Sic.     

Synthesis of Calcium Hexaboride Powder via the Reaction of Calcium Carbonate with Boron Carbide and Carbon

The synthesis of calcium hexaboride (CaB₆) powder via the reaction of calcium carbonate (CaCO₃) with boron carbide (B₄C) and carbon has been investigated systematically in the present study. The influences of heating temperature and holding time on the reaction products have been studied using X-ray diffractometry, and the morphologies of CaB₆ obtained at various temperatures and holding times have been investigated via scanning electron microscopy. The interaction in the CaCO₃–B₄C–carbon system by which CaB₆ is formed is a solid-phase process that passes through the transition phases Ca₃B₂O₆ and CaB₂C₂. The optimal conditions for CaB₆ synthesis are a holding time of 2.5 h at a temperature of 1673 K, under vacuum (a pressure of 10⁻² Pa). CaB₆ powder has the same morphology as B₄C, and the properties and the shape of CaB₆ powders can be improved by choosing good-quality raw materials.     

Pressureless Densification of Zirconium Diboride with Boron Carbide Additions

Zirconium diboride (ZrB₂) was densified (>98% relative density) at temperatures as low as 1850°C by pressureless sintering. Sintering was activated by removing oxide impurities (B₂O₃ and ZrO₂) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (∼150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B₄C were evaluated as additives to facilitate the removal of ZrO₂. Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB₂ (surface area ∼1 m²/g) or attrition-milled ZrB₂ (surface area ∼7.5 m²/g) with WC and/or B₄C as a sintering aid. ZrB₂ containing only WC could be sintered to ∼95% relative density in 4 h at 2050°C under vacuum. In contrast, the addition of B₄C allowed for sintering to >98% relative density in 1 h at 1850°C under vacuum.