Boron carbide is characterized by a unique combination of properties that make it a material of choice for a wide range of engineering applications. Boron carbide is used in refractory applications ...due to its high melting point and thermal stability; it is used as abrasive powders and coatings due to its extreme abrasion resistance; it excels in ballistic performance due to its high hardness and low density; and it is commonly used in nuclear applications as neutron radiation absorbent. In addition, boron carbide is a high temperature semiconductor that can potentially be used for novel electronic applications. This paper provides a comprehensive review of the recent advances in understanding of structural and chemical variations in boron carbide and their influence on electronic, optical, vibrational, mechanical, and ballistic properties. Structural instability of boron carbide under high stresses associated with external loading and the nature of the resulting disordered phase are also discussed.
High dense (>99% density) SiC ceramics were produced with addition of C and B4C by spark plasma sintering method at 1950 °C under 50 MPa applied pressure for 5 min. To remove the oxygen from the SiC, ...it was essential to add C. Two different mixture method were used, dry mixing (specktromill) and wet mixing (ball milling). The effect of different levels of carbon additive and mixture method on density, microstructure, elastic modulus, polytype of SiC, Vickers hardness, and fracture toughness were examined. Precisely, 1.5 wt.% C addition was sufficient to remove oxide layer from SiC and improve the properties of dense SiC ceramics. The highest hardness and elastic modulus values were 27.96 and 450 GPa, respectively. Results showed that the 4H polytype caused large elongated grains, while the 6H polytype caused small coaxial grains. It has been observed that it was important to remove oxygen to achieve high density and improve properties of SiC. Other key factor was to include sufficient amount of carbon to remove oxide layer. The results showed that excess carbon prevented to achieve high density with high elastic modulus and hardness.
Commercially available boron carbide ceramics typically have heterogeneous microstructures that contain distributions of processing‐induced inclusions. The inclusions that are rich in carbon (i.e., ...carbonaceous) govern the underlying mechanisms of brittle fracture through wing crack formation, and thus dictate the mechanical response of the ceramic. In this study, we investigate the dynamic failure of five boron carbide ceramic materials with different inclusion populations. All of the materials were prepared by hot‐pressing; four of these boron carbides contained different sizes and concentrations of carbonaceous inclusions, while one contained no carbonaceous inclusions. The heterogeneity distributions were characterized in some detail for statistical analysis using scanning electron microscopy and quantitative image analysis. A modified compression Kolsky bar setup with in situ ultra‐high‐speed microscopic imaging (10 million frames per second) was then used to study the influence of the inclusion distributions on the dynamic failure processes in these materials, at nominal high strain rates of 102–103 s−1. The in situ ultra‐high‐speed microscopy highlighted the link between micro and macroscale failure processes and demonstrated that the carbonaceous inclusions are indeed the preferential sites for nucleation of wing cracks, as previously hypothesized based on post‐mortem observations. The relative orientation of an inclusion with respect to the compression axis was shown to affect the likelihood that it would participate in crack nucleation. All of the ceramics were also found to have orientation‐dependent peak compressive stress, regardless of the presence of carbonaceous inclusions, suggesting that grain orientation distributions are also important.
Boron carbide is a notable ceramic, with its high hardness and low density. However, it suffers a sudden loss in strength under high shear stress. Doping boron carbide with Si/B is widely used to ...increase its resistance to amorphization. High purity boron and silicon hexaboride precursor powders are used for doping boron carbide, but these materials have high costs and supply chain constraints. This research investigated the effect of substituting lower purity B or using pure Si powder instead of SiB6 on materials’ properties such as elastic and mechanical properties, microstructure, and amorphization resistances. It was observed that using lower‐purity boron or pure Si powder instead of SiB6 did not significantly affect critical properties, such as fracture toughness, hardness, or amorphization resistance. However, Young's modulus values decreased as B purity decreased and as Si was used instead of SiB6. These findings demonstrate that substituting precursor materials in Si/B co‐doped B4C is possible with little change in the material's properties. This facilitates the use of easier‐to‐access, cheaper production routes to be used for silicon‐doped boron carbide products.
Non‐uniform morphology and existence of free carbon are two main problems for commercial boron carbide powders. This work proposes a method for eliminating free carbon and changing the morphology of ...commercial powders using Rapid Carbothermal Reduction (RCR) process. Free carbon is eliminated from commercial boron carbide powders and morphology is evolved to less angular shapes with limited particle size growth. Commercial and modified powders were densified by Spark Plasma Sintering at 1900°C with 0, 5, and 20 minutes dwell. Despite the particle size growth, modified boron carbide powders reached >99% TD with shorter dwell times compared with commercial starting powders. Improved microhardness observed with dense modified samples as a result of enhanced morphology and increased twinning.
To obtain high density boron carbide-silicon carbide composites, the spark plasma sintering method was used. 50% B
4
C–1.5% C–48.5% SiC mixture compositions were sintered at four different ...temperatures (1800, 1850, 1900, 1950°C) under 50 MPa pressure and four different applied pressures (20, 30, 40, and 50 MPa) at a constant temperature of 1950°C. The boron carbide-silicon carbide composites reached full density (>99% th. density) at 1950°C and under 50 MPa pressure. Samples were characterized using SEM, XRD, and ultrasound analysis. Density, Vickers hardness, Berkovich hardness and fracture toughness were also evaluated. Ultrasound analysis showed that increasing the sintering temperature and applied pressure increased the elastic modulus, shear, and bulk modulus of the composites. The samples densified at 1950°C under 50 MPa pressure, had 409 GPa elastic modulus, 176 GPa shear modulus, and 203 GPa bulk modulus. With increasing sintering temperature and pressure, the hardness and fracture toughness of the composites also increased. Vickers hardness values dramatically increased from 17.55 GPa (1800°C) to 30.78 GPa (1950°C) with increasing sintering temperature. The highest Berkovich hardness value was obtained as 37.37 GPa in the sample sintered at 1950°C under 50 MPa. The highest calculated fracture toughness values were 2.64 MPa m
1/2
(1950°C under 50 MPa).
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•Fabricated Si/B co-doped and its composite with TiB2 through reaction hot-pressing.•Reported the mechanical properties of Si/B co-doped boron carbide and its composite.•TiB2 ...reinforcement yielded an improvement to the overall mechanical properties.
Boron carbide experiences glass-like brittle behavior when subjected to high shear stresses due to stress-induced amorphization. Strategies to mitigate amorphization have primarily focused on Si/B co-doping. However, the overall mechanical properties of Si/B co-doped boron carbide are unknown. Here, we delivered a comprehensive report on the mechanical properties of Si/B co-doped boron carbide; and proposed an effective strategy to improve the mechanical properties through TiB2 reinforcement. Vickers hardness of 25.0 GPa, indentation toughness of 2.1 MPa·m0.5, Young’s modulus of 430 GPa, and flexural strength of 338 MPa were measured for Si/B co-doped boron carbide. The reinforcement with TiB2 increased the overall mechanical properties to 26.6 GPa for hardness, 2.3 MPa·m0.5 for indentation toughness, and 410 MPa for flexural strength. The improvements were attributed to the combined effects of grain size refinement and crack propagation alteration due to TiB2 addition.
Agglomerates are a large cluster of primary particles, which may result in the flaws of the final ceramics. Stereolithography suspensions could pose challenges to identify their coarse agglomerates ...without modifications to their properties; the suspensions are mostly high turbid and viscous in some cases. Any modifications, such as dilution, can alter the actual degree of agglomeration. In this study, a means to characterize the degree of dispersion was developed using the automated fineness of grind gauge. The fineness of grind gauge was visualized to determine the nature of the trapped agglomerates and their distribution. The resulted agglomerate size distribution was treated mathematically as a power law distribution in a selected region. The power law function was found to describe the degree of dispersion of suspensions, which was associated with the coarse agglomerate distribution behavior.
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•Identified the degree of dispersion of stereolithography suspensions.•Defined the power law coefficients of the agglomerate size distribution.•Provided a correlation between coarse agglomerate and power law exponent.
Superstrength through Nanotwinning An, Qi; Xie, Kelvin Y; Sim, Gi-dong ...
Nano letters,
12/2016, Letnik:
16, Številka:
12
Journal Article
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Odprti dostop
The theoretical strength of a material is the minimum stress to deform or fracture the perfect single crystal material that has no defects. This theoretical strength is considered as an upper bound ...on the attainable strength for a real crystal. In contradiction to this expectation, we use quantum mechanics (QM) simulations to show that for the boron carbide (B4C) hard ceramic, this theoretical shear strength can be exceeded by 11% by imposing nanoscale twins. We also predict from QM that the indentation strength of nanotwinned B4C is 12% higher than that of the perfect crystal. Further, we validate this effect experimentally, showing that nanotwinned samples are harder by 2.3% than the twin-free counterpart of B4C. The origin of this strengthening mechanism is suppression of twin boundary (TB) slip within the nanotwins due to the directional nature of covalent bonds at the TB.
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