Strength boron carbides

In general, the purified boron carbide is ultimately obtained as a granular soHd that subsequendy may be molded or bonded into usehil shapes. To achieve high density and strength, it is hot pressed at 1800—2400°C in graphite molds. 

Figure 20 Particle strength a of various solids as a function of particle size d. a, Glass beads b, boron carbide c, crystaline boron d, cement clinker e, marble f, cane sugar g, quartz h, limestone i, coal. Source From Ref. 22.

The carbides and nitrides are well known for their hardness and strength, and this section will briefly compare a number of these properties with those of the pure metals. Concentration will be placed here on the first row compounds, since these constitute a complete series, and Mo and W, since these are the most commonly studied metals. As will be shown, the physical and mechanical properties of carbides and nitrides resemble those of ceramics not those of metals. Comparisons will be made with boron carbide (B4C), silicon carbide (SiC), aluminium nitride (AIN), silicon nitride (Si3N4), aluminium oxide (A1203), and diamond, as representative ceramic materials. 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

Ceramic fibers of the nonoxide variety such as silioon carbide, silicon oxycarbide such as Nicalon, silicon nitride, boron carbide, etc. have become very important because of their attractive combination of high stiffiiess, high strength and low density. We give brief description of some important nonoxide fibers. 

Boron carbide is a relatively inexpensive hard material, which is used for its mechanical properties of strength and extreme hardness in armor-plates for body protection, in sandblast nozzles, and as an abrasive for grinding and cutoff wheels. In nuclear plants, boron carbide is used as the neutron absorbing material of the control rods. 

At temperatures above 2000°C, the structural stability and strength of the boronated graphite materials degrade. Boron carbide melts at 2140-2450°C and reacts with the graphite matrix. 

Boron carbide is similar in hardness to diamond, and boron nitride is similar in structure and mechanical properties to graphite, but, unlike graphite, boron nitride does not conduct electricity. -> A1 has widespread use in construction and aerospace industries. Because it is a soft metal, its strength is improved by alloy formation with Cu and Si. 

The mechanical strength of sintered boron carbide is 15-20 % lower than that of hot-pressed samples this may be owing to intercrystalline defects in the former case, and transcrystalline ones in the latter case. 

B4C-SiC The presence of particulate SiC limits grain growth in sintered B4C-SiC compacts, thus leading to fine-grained materials with improved strength compared to the single-phase boron carbide materials [313, 423]. 

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