Microstructure and mechanical properties of B4C based ceramics with Fe3Al as sintering aid by spark plasma sintering

B₄C based ceramics were fabricated with different Fe₃Al contents as sintering aids by spark plasma sintering at relatively low temperature (1700 °C) in vacuum by applying 50 MPa pressure and held at 1700 °C for 5 min. The effect of Fe₃Al additions (from 0 to 9 wt%) on the microstructure and mechanical properties of B₄C has been studied. The composition and microstructure of as-prepared samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA) equipped with WDS (wavelength dispersive spectrometry) and EDS. The mixtures of B₄C and Fe₃Al underwent a major reaction in which the metal borides and B₄C were encountered as major crystallographic phases. The sample with 7 wt% of Fe₃Al as a sintering aid was found to have 32.46 GPa Vickers hardness, 483.40 MPa flexural strength, and 4.1 MPa m^1/2 fracture toughness which is higher than that of pure B₄C.     

Processing,microstructure, and mechanical properties of zirconium diboride-boron carbide ceramics

The processing, microstructure, and mechanical properties of zirconium diboride-boron carbide (ZrB₂-B₄C) ceramics were characterized. Ceramics containing nominally 5, 10, 20, 30, and 40 vol% B₄C were hot-pressed to full density at 1900 °C. The ZrB₂ grain size decreased from 4 to 2 µm and B₄C inclusion size increased from 3 to 5 µm for B₄C additions of 5 and 40 vol% B₄C, respectively. Elastic modulus decreased from 525 to 515 GPa and Vickers hardness increased from 15 to 21 GPa as the B₄C content increased from 5 to 40 vol%, respectively, following trends predicted using linear rules of mixtures. Flexure strength and fracture toughness both increased with increasing B₄C content. Fracture toughness increased from 4.1 MPa m^½ at 5 vol% B₄C to 5.3 MPa m^½ at 40 vol% B₄C additions. Flexure strength was 450 MPa with a 5 vol% B₄C addition, increasing to 590 MPa for a 40 vol% addition. The critical flaw size was calculated to be ~30 µm for all compositions, and analysis of the fracture surfaces indicated that strength was controlled by edge flaws generated by machining induced sub-surface damage. Increasing amounts of B₄C added to ZrB₂ led to increasing hardness due to the higher hardness of B₄C compared to ZrB₂ and increased crack deflection. Additions of B₄C also lead to increases in fracture toughness due to increased crack deflection and intergranular fracture.     

Microstructures and mechanical properties of B4C–SiC intergranular/intragranular nanocomposite ceramics fabricated from B4C,Si, and graphite powders

B₄C–SiC intergranular/intragranular nanocomposites with high hardness and high toughness were fabricated through mechanochemical processing with B₄C, Si, and graphite powders and subsequent hot pressing without any sintering aid. The milled powders are composed of stacking-disordered SiC and nanocrystalline B₄C. Most nano/micron-sized SiC particles are homogeneously dispersed in B₄C matrix, and some nano-sized SiC and B₄C particles are embedded into B₄C grains to form an intergranular/intragranular structure. The disordered characteristic of the milled powders is the essential factor for the formation of the intragranular structure, sudden densification within the narrow temperature range (1700–1900 °C), and the preparation of dense samples under a relatively low temperature (1900 °C). The relative density, Vickers hardness, and fracture toughness of the samples sintered at 1950 °C are 98.6%, 34.3 GPa, and 6.0 MPa m^1/2, respectively. The intergranular/intragranular structure plays an important role in improving fracture toughness and hardness of the composites.     

Microstructure and mechanical properties of hot-pressed B4C/TiC/Mo ceramic composites

Boron carbide (B₄C)/TiC/Mo ceramic composites with different content of TiC were produced by hot pressing. The effect of TiC content on the microstructure and mechanical properties of the composites has been studied. Results showed that chemical reaction took place for this system during hot pressing sintering, and resulted in a B₄C/TiB₂/Mo composite with high density and improved mechanical properties compared to monolithic B₄C ceramic. Densification rates of the B₄C/TiC/Mo composites were found to be affected by additions of TiC. Increasing TiC content led to increase in the densification rates of the composites. The sintering temperature was lowered from 2150 °C for monolithic B₄C to 1950 °C for the B₄C/TiC/Mo composites. The fracture toughness, flexural strength, and hardness of the composites increased with increasing TiC content up to 10 wt.%. The maximum values of fracture toughness, flexural strength, and hardness are 4.3 MPa m^1/2, 695 MPa, and 25.0 GPa, respectively.     

Spark plasma sintering and pressureless sintering of SiC using aluminum borocarbide additives

Aluminum borocarbide powders (Al₃BC₃ and Al₈B₄C₇) were synthesized, and the ternary powders were used as a sintering additive of SiC. The densification of SiC was nearly completed at 1670 °C using spark plasma sintering (SPS) and pressureless sintering was possible at 1950 °C. The sintering behavior of SiC using the new additive systems was nearly identical with that using the conventional Al–B–C system, but grain growth was suppressed when adding the borocarbides. In addition, oxidation of the fine additive powders did not intensively occur in air, which has been a problem in the case of the Al–B–C system for industrial application. The hardness, Young's modulus and fracture toughness of a sintered SiC specimen were 21.6 GPa, 439 GPa and 4.6 MPa m^1/2, respectively. The ternary borocarbide powders are efficient sintering additives of SiC.     

Microstructure and properties of B4C-SiC composites prepared by polycarbosilane-coating/B4C powder route

B₄C-SiC composites with fine grains were fabricated with hot-pressing pyrolyzed mixtures of polycarbosilane-coated B₄C powder without or with the addition of Si at 1950 °C for 1 h under the pressure of 30 MPa. SiC derived from PCS promoted the densification of B₄C effectively and enhanced the fracture toughness of the composites. The sinterability and mechanical properties of the composites could be further improved by the addition of Si due to the formation of liquid Si and the elimination of free carbon during sintering. The relative density, Vickers hardness and fracture toughness of the composites prepared with PCS and 8 wt% Si reached 99.1%, 33.5 GPa, and 5.57 MPa m^1/2, respectively. A number of layered structures and dislocations were observed in the B₄C-SiC composites. The complicated microstructure and crack bridging by homogeneously dispersed SiC grains as well as crack deflection by SiC nanoparticles may be responsible for the improvement in toughness.     

Densification and mechanical properties of pulsed electric current sintered B4C with in situ synthesized Al3BC obtained by the molten-salt method

The in situ generated ternary compound Al₃BC shows good dispersion on the surfaces of B₄C particles (10 ± 0.6 μm), obtained using the molten-salt method rather than the ball-milling method, which can form a conductive network in the pulse electric current sintering process to improve the sinterability of B₄C. The sintering behaviour, microstructure and mechanical properties of compacts were investigated at different sintering temperatures and Al₃BC contents. The study found that the relative density, elastic modulus, Vickers hardness, and fracture toughness of samples reached as high as 100%, 495 GPa, 37.0 GPa, and 6.32 MPa m^1/2 at 1700 °C, respectively, when the content of Al₃BC was 18 wt.%. The crack propagation mode can provide an explanation for the higher toughness than that of pure B₄C (3.19 MPa m^0.5). Additionally, the good dispersion of Al₃BC in B₄C was found to change the fracture mode from the single transgranular mode to a mixture of intergranular and transgranular modes. The compositions of the compacts were almost pure B₄C.     

Effect of Al addition on pressureless sintering of B4C

The effect of Al addition on pressureless sintering of B₄C ceramic was analyzed in the present work. Different amounts of Al, from 0 to 5 wt.% were added to the base material and pressureless sintering was conducted at 2050 and 2150 °C under argon atmosphere. Microstructure, crystal phases and density evolution were studied and correlated to Al additions and firing temperature. Density and grain size of sintered samples, increased significantly with Al load while less evidence is the effect of sintering temperature; 94% dense material was obtained by adding 4 wt.% Al regardless of the maximum firing temperature.     

Mechanical properties and deformation mechanisms of B4C–TiB2 eutectic composites

Samples of B₄C–TiB₂ eutectic are laser processed to produce composites with varying microstructural scales. The eutectic materials exhibit both load dependent and load independent hardness regimes with a transition occurring between 4 and 5 N indentation load. The load-independent hardness of eutectics with a microstructural scale smaller than 1 μm is about 31 GPa, and the indentation fracture toughness (5–10 N indenter load) of the eutectics is 2.47–4.76 MPa m^1/2. Indentation-induced cracks are deflected by TiB₂ lamellae, and indentation-induced spallation is reduced in the B₄C–TiB₂ eutectic compared to monolithic B₄C. Indentation-induced amorphization in monolithic B₄C and the B₄C phase of the eutectic is detected using Raman spectroscopy. Sub-surface damage is observed using TEM, including microcracking and amorphization damage in B₄C and B₄C–TiB₂ eutectics. Dislocations are observed in the TiB₂ phase of eutectics with an interlamellar spacing of 1.9 μm.     

Fabrication of toughened B4C composites with high electrical conductivity using MAX phase as a novel sintering aid

MAX phase Ti₃AlC₂ was chosen as a novel sintering aid to prepare electrically conductive B₄C composites with high strength and toughness. Dense B₄C composites can be obtained at a hot-pressing temperature as low as 1850 °C with 15 vol% Ti₃AlC₂. The enhanced sinterability was mainly ascribed to the in situ reactions between B₄C and Ti₃AlC₂ as well as the liquid phase decomposed from Ti₃AlC₂. Both the Vickers hardness and fracture toughness increase with increasing Ti₃AlC₂ amount, and high hardness and toughness values of 28.5 GPa and 7.02 MPa m^−1/2 respectively were achieved for B₄C composites sintered with 20 vol% Ti₃AlC₂ at 1900 °C. Crack deflection by homogenously distributed TiB₂ particles was identified as the main toughening mechanism. Besides, B₄C composites sintered with Ti₃AlC₂ show significantly improved electrical conductivity due to the percolation of highly conductive TiB₂ phase, which could enhance the machinability of B₄C composites largely by allowing electrical discharge machining.     

Densification and properties of superhard B6O materials with cobalt additions

The search for suitable additives for boron suboxide (B₆O) materials which could improve densification, reduce sintering temperature and tailor the microstructure has been productive. B₆O materials doped with 0–5 vol% cobalt addition were sintered at temperatures up to 1850 °C and pressure of 50 MPa for 20 min. Relationships between the formed phases, microstructures and mechanical properties of the sintered materials were investigated as a function of sintering conditions and added cobalt content. The hardness of the sintered B₆O materials increases with sintering temperature, while the fracture toughness increases with increasing cobalt content and reduces with increasing sintering temperature.     

SiC whisker reinforced multi-carbides composites prepared from B4C and pyrolyzed rice husks via reactive infiltration

SiC whisker reinfored carbide-based composites were fabricated by a reactive infiltration method by using Si as the infiltrate. Rice husks (RHs) were pyrolyzed to SiC whiskers, particles and amorphous carbon, and were then mixed with different contents of B₄C as well as Mo powders. The mixtures were molded to porous preforms for the infiltration. The SiC whiskers and particles in the preform remained in the composite. Molten Si reacted with the amorphous carbon, B₄C as well as Mo in the preform during the infiltration, forming newly SiC, B₁₂(C,Si,B)₃ as well as MoSi₂. The upper values of elastic modulus, hardness and fracture toughness of the composites are 297.8 GPa, 16.8 ± 0.8 GPa, and 3.8 ± 0.2 MPa m^1/2, respectively. The influence of the phase composition of the composites on the mechanical properties and the fracture mechanism are discussed.     

Mechanical properties of reactively processed W/Ta2C-based composites

The mechanical properties of reactively processed W/Ta₂C cermet composites were studied. Dense W/Ta₂C cermets with a nominal composition of 40.4 vol% W(Ta)ss, 48.9 vol% Ta₂C and 10.6 vol% Ta₂WO₈ were fabricated using a pressureless reactive processing method. Four-point bend strength, fracture toughness, elastic modulus, and microhardness were measured at room temperature. The average flexure strength was 584 MPa which is lower than for pure W; however the strength of pure Ta₂C is unknown. The fracture toughness was 8.3 MPa m^1/2 which fits a rule of mixtures between literature values for the fracture toughness of the W and Ta₂C phases. The elastic modulus was 476 GPa, and the microhardness was 13.4 GPa. Both Young's modulus and hardness were higher than values reported for other W-based cermets.     

Sintering,mechanical, electrical and oxidation properties of ceramic intermetallic TiC–Ti3Al composites obtained from nano-TiC particles

The paper discusses the development of a new material system for interconnect application in Solid Oxide Fuel Cells (SOFC) based on TiC–Ti₃Al. Nano-sized TiC powders utilized in this research were synthesized using carbon coated TiO₂ precursors from a patented process. The pressureless sintering of TiC–Ti₃Al in a vacuum was applied at temperatures between 1100 °C and 1500 °C and content of Ti₃Al was varied in the range of 10–40 wt%. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for phase evaluation and sintering behavior. Relative density increased markedly with increasing sintering temperature because of grain growth and formation of the Ti₃AlC₂ secondary phase. Dense products (>95% TD) were prepared from nanosized TiC powders with 10 and 20 wt% Ti₃Al, but with about 8 to 10% porosity for 30 and 40 wt% Ti₃Al. The mechanical properties were determined from Vickers hardness and fracture toughness calculations. Vickers hardness decreased and fracture toughness increased with increasing Ti₃Al content. The electrical conductivity and oxidation behavior of TiC–Ti₃Al composites were investigated to evaluate the feasibility for SOFC interconnect application. The electrical conductivity measurements in the air at 800 °C for 100 h were made using the Kelvin 4-wire method.     

Microstructure and mechanical properties of Ti (C,N) based cermets reinforced with different ceramic particles processed by spark plasma sintering

The addition effect of different ceramic particles such as TiB₂, TiN and nano-Si₃N₄ on the microstructure and mechanical properties of TiCN-WC-Co-Cr₃C₂ based cermets, which are prepared by spark plasma sintering, was studied. Microstructural characterization of the cermets was done by scanning electron microscope. X-ray diffraction was performed to study the crystal structures. Mechanical properties such as hardness and fracture toughness were measured for the different developed cermets. The hardness and fracture toughness of the TiCN-WC-Co-Cr₃C₂ cermets without TiN, TiB₂, and nano-Si₃N₄ were 8.4 GPa and 3.4 MPa m^1/2, respectively. It was found that 5 wt% TiB₂ addition alone improved the corresponding hardness and fracture toughness to 19.2 GPa and 6.9 MPa m^1/2, respectively. The addition of 5 wt% TiN, improved the hardness and fracture toughness to 16.7 GPa and 6.9 MPa m^1/2, respectively. With the combination of 5 wt% TiN and 5 wt% TiB₂, the hardness and fracture toughness were improved to 15.5 GPa and 6.6 MPa m^1/2, respectively. But, the addition of 5 wt% Si₃N₄ showed a balanced improvement in both hardness (17.6 GPa) and toughness (6.9 MPa m^1/2). Fracture toughness did not change much for all the above cermets with different ceramic inclusions.     

Gel-cast hierarchical porous B4C/C preform and its role in fabricating reaction bonded boron carbide composites

The resorcinol-formaldehyde (RF) gel-casting system is employed for the first time to fabricate a hierarchical porous B₄C/C preform, which was subsequently used for the fabrication of reaction bonded boron carbide (RBBC) composites via a liquid silicon infiltration process. The effect of the carbon content and carbon structures of this perform on the microstructures and mechanical properties of B₄C/C preform and the resultant RBBC composites is reported. The B₄C/C preform (16 wt% carbon) exhibit a strength of 34±1 MPa. The obtained RBBC composites shown uniform microstructure is consisted of SiC particles bonded boron carbide scaffold and an interpenetrating residual silicon phase. The Vickers hardness, flexural strength and fracture toughness of the RBBC composites (16 wt% carbon) are 24 GPa, 452 MPa and 4.32 MPa m^1/2, respectively.     

A high strength alumina-silicon carbide-boron carbide triplex ceramic

A ceramic particulate composite composed of oxide, and carbide ceramics was found to have high strength, hardness, and fracture toughness values. A composition consisting of Al₂O₃ with 15 vol% SiC and 15 vol% B₄C additions was produced by hot-pressing at 1650 °C for 30 min, with full density reached after ~5 min at temperature. Both WB and WB₂ were observed, with the W source presumably an impurity from WC milling media, and Al₁₈B₄O₃₃ was also detected following densification. Strength was ~880 MPa, which is greater than values reported for comparable composites of Al₂O₃ containing 30 vol% SiC or B₄C. Vickers hardness was ~21 GPa, and fracture toughness was ~4.5 MPa m^½, comparable to values reported for the binary mixtures. The calculated critical flaw size of the material was similar to the size of the SiC/B₄C clusters and microcracking at grain boundaries. The latter resulting from thermal expansion mismatch between the Al₂O₃ matrix and SiC/B₄C reinforcing phases.     

Microstructure,hardness and toughness of boron carbide thin films deposited by pulse dc magnetron sputtering

Boron carbide thin films were deposited on (100) silicon substrates at ambient temperature via. pulse dc magnetron sputtering. Various frequency and duty cycles were applied to the hot-pressed B₄C target in order to understand their influence on the structure and mechanical properties of the B₄C films. X-ray Energy dispersive spectrum, Raman spectroscopy and Transmission electronic microscopy were used to characterize the composition and microstructure of the films. Nanoindenter was employed to measure the hardness and modulus. The film toughness was evaluated by a microindentation method. The results show that both pulse frequency and duty cycle significantly affect the B/C atomic ratio and then hardness and modulus in the boron carbide films. However, the amorphous structure of the films was maintained when the frequency and duty cycle changed. The maximum hardness of 29 GPa and modulus of 247 GPa combined with relative high toughness (3.3 MPa m^1/2) were achieved under 50 kHz frequency and 30% duty cycle. In addition, there was no evidence to prove that the graphite phase existed in the B₄C films although exceeded C concentration was detected.     

Densification behaviour and mechanical properties of B4C–SiC intergranular/intragranular nanocomposites fabricated through spark plasma sintering assisted by mechanochemistry

High-performance B₄C–SiC nanocomposites with intergranular/intragranular structure were fabricated through spark plasma sintering assisted by mechanochemistry with B₄C, Si and graphite powders as raw materials. Given their unique densification behaviour, two sudden shrinkages in the densification curve were observed at two very narrow temperature ranges (1000–1040 °C and 1600–1700 °C). The first sudden shrinkage was attributed to the volume change in SiC resulting from disorder–order transformation of the SiC crystal structure. The other sudden shrinkage was attributed to the accelerated densification rate resulting from the disorder–order transformation of the crystal structure. The high sintering activity of the synthesised powders could be utilised sufficiently because of the high heating rate, so dense B₄C–SiC nanocomposites were obtained at 1700 °C. In addition, the combination of high heating rate and the disordered feature of the synthesised powders prompted the formation of intergranular/intragranular structure (some SiC particles were homogeneously dispersed amongst B₄C grains and some nanosized B₄C and SiC particles were embedded into B₄C grains), which could effectively improve the fracture toughness of the composites. The relative density, Vickers hardness and fracture toughness of the samples sintered at 1800 °C reached 99.2±0.4%, 35.8±0.9 GPa and 6.8±0.2 MPa m^1/2, respectively. Spark plasma sintering assisted by mechanochemistry is a superior and reasonable route for preparing B₄C–SiC composites.     

The effect of La2O3 on the microstructure and room temperature mechanical properties of t-ZrO2

Structural and mechanical properties of La₂O₃ added (up to 5 wt%) t-ZrO₂ compacts were examined with the aim of optimizing hardness and fracture toughness for room temperature applications. Structural examinations were performed using an X-ray diffractometer and a scanning electron microscope. Mechanical properties of the compacts were determined as modulus of elasticity, hardness and fracture toughness by conducting Vickers indentation tests under test loads of 0.5, 1 and 10 kg. Addition of 0.5 wt% La₂O₃ deteriorated the room temperature stability of t-ZrO₂ by forming m-ZrO₂ and coarse polygonal grains in the matrix. Higher concentrations (2 and 5 wt% La₂O₃) caused precipitation of La₂Zr₂O₇ at the grain boundaries, which also accompanied by a reduction in hardness. Fracture toughness increased with increasing La₂O₃ content of the compact. Finally, an optimum combination between hardness and fracture toughness was obtained for the 0.5 wt% La₂O₃ containing compact.