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.     

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.     

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 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.     

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.     

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.     

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.     

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.     

Oxidation Resistances of Ternary Mixtures of the Carbides of Titanium, Silicon, and Boron

Dense compacts of mixtures of Tic, Sic, and B,C were prepared by sintering in an argon atmosphere and by hot pressing. Titanium carbide and B₄C reacted during heating to form TiB₂. The oxidation resistances of these ternary compacts were determined by the weight gained at 2000°F. in air. Compacts containing 10% B₄C, 70 to 50% Tic, and the remainder Sic had superior oxidation resistances. The oxide coating on these compositions consisted of rutile, silica, and an amorphous third phase. A compact composed of 73.5% Tic, 17% silicon, and 9.5% boron was hot-pressed. It had zero porosity and better oxidation resistance than any other composition tested.     

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.     

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.     

Hot Pressing of Tantalum Carbide With and Without Sintering Additives

Densification of tantalum carbide (TaC) was studied by hot pressing at temperatures ranging from 1900° to 2400°C with and without sintering additives. Without sintering additives, the relative density increased from 75% at 1900°C to 96% at 2400°C. A microstructural examination showed no observable grain growth up to 2300°C. Densification was enhanced with carbon (C) and/or B₄C additions. TaC with a 0.78 wt% C addition achieved a relative density of 97% at 2300°C. Additions of 0.36 wt% B₄C or 0.43 wt% B₄C and 0.13 wt% C increased the relative density to 98% at 2200°C, accompanied by rapid grain growth at 2100°C and higher temperatures.     

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.     

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.     

Boron Carbide–Zirconium Boride In Situ Composites by the Reactive Pressureless Sintering of Boron Carbide–Zirconia Mixtures

The heating of B₄C–YTZP (where YTZP denotes yttria-stabilized zirconia polycrystals) mixtures, under an argon atmosphere, generates B₄C–ZrB₂ composites, because of a low-temperature (<1500°C) carbide–oxide reaction. Composites derived from mixtures that include ≥15% YTZP are better sintered than monolithic B₄C that has been fired under the same conditions. Firing to ∼2160°C (1 h dwell) generates specimens with a bulk density of ≥91% of the theoretical density (TD) for cases where the initial mixture includes ≥15% YTZP. Mixtures that include 30% YTZP allow a fired density of ≥97.5% TD to be attained. The behavior of the B₄C–YTZP system is similar to that of the B₄C–TiO₂ system. Dense B₄C–ZrB₂ composites attain a hardness (Vickers) of 30–33 GPa.     

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.     

Effects of Additives on the Pressure-Assisted Densification and Properties of Silicon Carbide

The densification of silicon carbide (SiC) was studied using a variety of additives (Al, AlN, Al₂O₃, B₄C, C, Si₃N₄, and Y₂O₃). The onset of densification of SiC with small amounts of additives occurred at temperatures between 1500° and 1900°C with 28 MPa applied pressure. Al, B₄C, and C promoted densification, while N (added as AlN or Si₃N₄) retarded sintering. A 96.75 wt% SiC–2 wt% Al–1 wt% C–0.25 wt% B₄C starting composition yielded the same percent of theoretical density (in the range of 70%–90% theoretical density) 400°C lower than a 95 wt% SiC–5 wt% AlN material. Yttria additions promoted intergranular fracture, which increased the single-edged precracked beam fracture toughness. The appropriate selection and amount of additives allowed for the tailoring of grain size and intergranular fracture, thus controlling the mechanical properties. While oxygen was present in all materials containing aluminum, the incorporation of additional oxygen as alumina resulted in reduced sintering activity compared with Al metal. Corrosion resistance decreased in both HF and NaOH solutions at 80°C for materials containing a grain boundary phase.     

The Study on Carbon Nanofiber (CNF)-Dispersed B4C Composites

Carbon nanofiber (CNF)-dispersed B₄C composites have been synthesized and consolidated directly from mixtures of elemental raw powders by pulsed electric current pressure sintering (1800°C/10 min/30 MPa). A 15 vol% CNF/B₄C composite with ∼99% of dense homogeneous microstructures (∼0.40 μm grains) revealed excellent mechanical properties at room temperature and high temperatures: a high bending strength (σ_b) of ∼710 MPa, a Vickers hardness ( H _v) of ∼36 GPa, a fracture toughness ( K _{I C} ) of ∼7.9 MPa m^1/2, and high-temperature σ_b of 590 MPa at 1600°C in N₂. Interfaces between the CNF and the B₄C matrix were investigated using high-resolution transmission electron microscopy, EDS, and electron energy-loss spectroscopy.     

Reactive Hot Pressing of ZrB2–SiC–ZrC Composites at 1600°C

A ZrB₂–SiC–ZrC ceramic was produced by reactive hot pressing using Zr, Si, and B₄C as raw materials. The kinetics of the reaction process was studied. The reduction of powders by ball milling is of crucial importance for the sintering. The self-propagating high-temperature synthesis reaction between the raw powders can be ignited by controlling the sintering conditions, which leads to a sintering temperature as low as 1600°C, the lowest sintering temperature reported thus far. The relative density is 97.3%, with an open porosity of 0.6%, and the mechanical properties are comparable to the composites that sintered at higher temperatures. The depletion of oxygen impurities during the sintering was discussed.     

Effects of B2O3 Addition on the Dielectric and Ferroelectric Properties of Ba0.7Sr0.3TiO3 Ceramics

The effects of B₂O₃ addition on the sintering behavior and the dielectric and ferroelectric properties of Ba_0.7Sr_0.3TiO₃ (BST) ceramics were investigated. The dielectric and ferroelectric properties of a BST sample with 0.5 wt% B₂O₃ sintered at <1150°C were as good as those of undoped BST sintered at 1350°C, and the dielectric loss was better. When >1.0 wt% B₂O₃ was added to BST, the overdoped B₂O₃ did not form a liquid phase or volatilize; it remained in the samples and formed a secondary phase that lowered the sintering behavior and the dielectric and ferroelectric properties of the BST.