Boron Carbide A Comprehensive Review

Boron carbide, which has a high melting point, outstanding hardness, good mechanical properties, low specific weight, great resistance to chemical agents and high neutron absorption cross-section (l°BxC, x > 4) is currently used in high-technology industries–fast-breeders, lightweight armors and high-temperature thermoelectric conversion.

In the group of the most important non-metallic hard materials (alumina, silicon carbide, silicon nitride, diamond or cubic boron nitride), boron carbide occupies a specific place. A boron carbide compound was discovered in
1858, t then Joly in 1883 and Moissan in 1894 prepared and identified the compounds B3C and B6C, respectively. The ‘stoichiometric formula’ B4C was only assigned in 1934, 2 Then many diverse formulae were proposed by Russian authors, which have not been confirmed; in fact, there is a wide phase homogeneity range B,.0C-BI0.5C . After 1950, numerous studies were carried out, especially concerning structures and properties. Due
to the limited size of this review, only some of the most important or recent references and the more appropriate reviews will be cited.

Hardness and wear resistance

Hardness Boron carbide is among the hardest materials (diamond, BNc). Hardness measurements are difficult; the preparation of samples and conditions of measure are uncertain or unknown, therefore values are scattered, and difficult to be compared. 17,75,78,13 ~ The influence of carbon content is controversial; for instance Allen 13z found an increase of hardness with C content, whereas Lipp and Schwetz did not find any influence.~31 Therefore the conditions of Knoop microhardness (HK) determination on planar electron-gun melted samples were studied. ~ v,v 5,78.133 Mechanical polishing induces a surface strain, therefore HK decreases when the time of electrolytic etching is increasing; HK reaches a plateau after 10 s. HK decreases when the load P increases. The variation of log P versus logL (L = length of the print), allows the determination of n in Meyer’s law P = aL”; after mechanical or electrolytic polishing, this variation is represented by a broken line with two slopes: i.e. (i) n = 1.01 and 1.75 after mechanical polishing and (ii) n = 1.20 and 1.80 after electrolytic etching. The Knoop microhardness increases linearly with the C content in the phase homogeneity range. For instance, after mechanical polishing, HK2oog= 2910 + 90 kg mm- 2 (29″ 1 GPa) for 10.6 at. C %, and reaches 3770 + 80 kg/mm 2 for 20 at.C % (B4C, with the minimum volume cell (Section 5) and the maximum density (Section 7.1)). After electrolytic etching HK drops by 25%: HK2o0~=2840+ 60 kg/mm 2 for B4C. HK2o o = 25.5 ___ 2.4 GPa for pressureless sintered samples and 29.0 + 1-5 for hot-pressed samples, x°’~ Values depend on microstructures, i.e. on processing and densification parameters. Vickers hardness (HV) similarly increases with the C content in CVD samples. 134,135 Extreme hardness values were obtained by microwave plasma deposition (Section 2.3.2). There is an evolution of HV with the deposition temperature of CVD; a minimum was observed for 1373 K, which corresponds to the microstructure with a maximum grain size.X 34 Hardness decreases in the presence of free graphite in electron-beam melted, 75 sintered 11 or CVD 135 samples or AI-Si-C phase in hot-pressed boron carbide? 36 Rebound measurements of the hardness of boron carbide indicate no decrease with temperature up to 1300°C. However, static indentation measurements show a continuous decrease of the hardness with temperature137 (Fig. 16).



Because of its high hardness and strength, boron carbide is inferior in abrasive resistance only to diamond; expressed in arbitrary units, the abrasive resistance of diamond is the top of the scale with 0.613, then boron carbide with 0.4-0.422, and silicon carbide 0-314. 8 The abrasion mechanism of B4C was studied at 20-1400°C; plastic deformation was significant at temperatures higher than 800°C in air and vacuum; the depth of the deformation layer after reciprocal abrasion of two similar objects made of B4C increased with the increase of the material grain size, and was 10-15 pm at 800-1000°C. 138 The friction coefficient and wear resistance of hotpressed B4C cylinders were studied under vacuum in the temperature range 20-1400°C; hollow cylinder face-to-face friction was carried out, under a load of 1 MPa and an average slip velocity of 0.01 m/s; the friction coefficient decreases continuously during heating from room temperature up to 1400°C, whereas the wear rate gradually increased up to 400°C, then decreases and becomes very low at 800-1000°C, and finally increases. 139’14° B203, BO(OH) and BO3H3 were formed on the friction interface in air. 1 a s,x 39 The erosion resistance by SiC particles impact is better for B4C than for zirconia, alumina or silicon carbide-based systems


Almost all the mechanical properties measured on hot-pressed (or sintered) boron carbide samples, with close to theoretical densities, differ and depend on specific impurity contents (especially A1, Si and C used as dopants) and distribution, porosity, clusters of diffusion pores, grain size, etc. Therefore measurements are hardly comparable.~ 36 Strength depends on the hot-pressing temperature and the stoichiometry of the carbide; small boron additions eliminate free graphite, thus improve strength; 16 strength would increase with the carbon content of the homogeneous carbide phase. 144 Notice that three point bending tests give higher values than four point. 1°5’136 Hot-pressed 0rV= 300-500 MPa), or post-HIP treated 26 samples have higher characteristics than sintered ones (av= 150-350 MPa) 10- 12’14’26’39″t05’136’142- 144 (Table 3; 39 sintering conditions are not clearly indicated, but pressureless sintering conditions are likely to be 1900-2150°C, under reduced atmosphere (0″1 mbar), hold time 30min (cf. Ref. 26), and hotpressing conditions: 2100-2200°C, 20-40MPa, under vacuum, for 15M5min (cf. Section Strength is highly variable as Weibull modulus is low (5); for all etching depths, the Weibull modulus and strength remain constant, thus the flaws are inherent to the materials. ~43

Our products include Boron carbide ball

B4C Grinding bead

Boron carbide ball Pformance units,   Composition B4C Density,g/cm3, 2.50-2.65 Modulus of elasticity,Gpa 510 KNOOP hardness 3300 Flexural strength,Mpa 400-650 Compressive strength,Mpa 4100 Fracture toughness,Mpa m1/2 4.5-7.0 Thermal expansion coefficient.1/0k 4.5*106 Thermal conductivity/m 0k 29 Maximum use temperature in air,℃ 1500


Boron carbide flak

Boron carbide flak Performance units,   Composition B4C Density,g/cm3, 2.50-2.65 Modulus of elasticity,Gpa 510 KNOOP hardness 3300 Flexural strength,Mpa 400-650 Compressive strength,Mpa 4100 Fracture toughness,Mpa m1/2 4.5-7.0 Thermal expansion coefficient.1/0k 4.5*106 Thermal conductivity/m 0k 29 Maximum use temperature in air,℃ 1500



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