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.     

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.     

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.     

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.     

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.     

Properties of ZrB2–SiC Ceramics by Pressureless Sintering

Pressureless sintering was used to densify ZrB₂–SiC ultra-high temperature ceramics. The physical, mechanical, thermal, electrical, and high temperature properties were investigated. This comprehensive set of properties was measured for ZrB₂ containing 20 vol% SiC in which B₄C and C were used as the sintering aids. The three-point flexural strength was 361±44 MPa and the elastic modulus was 374±25 GPa. The Vickers hardness and fracture toughness were 14.7±0.2 GPa and 4.0±0.5 MPa·m^1/2 respectively. Scanning electron microscopy studies of the microstructure of ZrB₂–SiC showed that SiC particles were distributed homogenously in the ZrB₂ matrix with little residual porosity.     

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.     

B6O: A Correlation Between Mechanical Properties and Microstructure Evolution upon Al2O3 Addition During Hot Pressing

B₆O powders were hot pressed with and without Al₂O₃ as a sintering additive at temperatures up to 1900°C and a pressure of 50 MPa. The microstructure of a doped and undoped sample was studied by transmission electron microscopy techniques. This paper aims at studying the correlation between micro/nanostructure evolution and the resulting mechanical properties; i.e., hardness and fracture toughness. The addition of alumina yields the formation of a secondary aluminum borate phase in addition to promoting grain growth strongly. While the addition of Al₂O₃ slightly decreased the hardness of the B₆O polycrystals, the corresponding fracture toughness was strongly improved, as compared with the undoped material.     

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.     

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.     

B6O–c-BN Composites Prepared by High-Pressure Sintering

High-pressure sintering behavior in the B₆O– c -BN system was investigated using in-laboratory-synthesized B₆O and commercially available c -BN powders (with an average grain size of 0.5, 3, or 6 μm). No reaction occurred between the two components under the high-pressure (4–6 GPa) and high-temperature (1500°–1800°C) conditions that have been investigated. Well-dispersed, sintered B₆O– x ( c -BN) composites (where x = 0–60 vol%) of almost-full density were prepared by sintering at a pressure of 6 GPa and temperature of 1800°C for 20 min. The maximum Vickers microhardness (46 GPa) of these composites was attained by adding 40 vol% c -BN with an average grain size of 0.5 μm. The fracture toughness of these composites increased as the c -BN content increased; the maximum fracture toughness (1.5–1.8 MPa^.m^1/2) was observed for x = 40–60 vol%. Crack deflection along the B₆O– c -BN grain boundary contributed to increasing the fracture toughness.     

Synthesis, Microstructure, and Mechanical Properties of Al3BC3

In situ synthesis of bulk Al₃BC₃ was achieved via a reactive hot-pressing method using Al, B₄C, and graphite powders at 1800°C for 2 h. The reaction path for synthesizing Al₃BC₃ was investigated. It was found that Al₃BC₃ formed via the reaction of C, B₄C, and Al₄C₃ above 1180°C. Dense Al₃BC₃ was prepared with a little B₄C and graphite remained. Microstructure observations revealed the plate-like morphology of Al₃BC₃ grains. Moreover, the mechanical properties of Al₃BC₃ were characterized (Vickers hardness of 11.1 GPa, bending strength of 185 MPa, fracture toughness of 2.3 MPa·m^1/2, and Young's modulus of 163 GPa). Young's modulus decreased slowly with increasing temperature, and at 1600°C remained 79% of that at ambient temperature. These results show that Al₃BC₃ is a promising lightweight high temperature structural material.     

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.     

Fracture Toughness of TiB2 and B4C Using the Single-Edge Precracked Beam, Indentation Strength, Chevron Notched Beam, and Indentation Strength Methods

The mode I fracture toughness ( K _Ic) of boron carbide (B₄C) and titanium diboride (TiB₂) was determined using four competing techniques. The indentation strength (IS), chevron notched beam (CNB), and indentation fracture (IF) methods are common techniques that were compared to the recently standardized single-edge precrack beam (SEPB) method. The SEPB method was more difficult to apply, but it represents the most rigorous method for K _Ic determination, because it uses few assumptions and requires a direct measurement of crack length. The IS method was an expeditious and economical alternative when low indentation loads were used. CNB K _Ic values were virtually rate-independent when displacement rates less than or equal to 0.5 mm/min were used. The IF method was the least satisfactory technique, because of high variability in K _c values and because of the low differentiation between the two materials studied.     

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.     

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.     

Elevated-Temperature Toughness and Hardness of a Hot-Pressed Al2O3—WC—Co Composite

The fracture toughness and hardness of an Al₂O₃80WC10Co composite were investigated in air at elevated temperatures. The primary phases in the composite were WC, α-Al₂O₃, and Co₃W₃C, but small amounts of Co and C (graphite) appeared at elevated temperatures, related to decomposition of the Co₃W₃C phase. The fracture toughness of the composite was constant with increasing temperature up to 330°C and then increased in the range 400° to 550°C. A transition of brittle to ductile behavior occurred at about 700°C. The enhancement of fracture toughness at elevated temperature is attributed to the decomposition of Co₃W₃C to Co and C, and enhanced crack deflection and bridging. Decreases in hardness with increasing temperature are attributed to the softening of WC matrix and decomposition of Co₃W₃C.     

Low-Temperature Pressureless Sintering of alpha-SiC with Al4C3-B4C-C Additions

Various combinations of Al₄C₃, B₄C, and carbon were used as sintering aids for pressureless sintering of alpha-SiC (94% 6 H + 6% 15 R ) powders. Densification behavior, polytypic transformation, microstructural development, and mechanical properties were studied. Appropriate amounts of Al₄C₃-B₄C-C additions could facilitate alpha-SiC powders being pressureless sintered to high densities (>95% TD) at temperatures greaterthan equal to1850°C. Even using an alpha-SiC as a starting powder, many elongated grains developed in the microstructures during sintering at low temperature that might contribute to toughening. Anisotropic grain growth was found to be associated with the faulted structures and the 6 H → 4 H phase transformation.     

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.     

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.