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 (10BxC, x>4) is currently used in high-technology industries—fast-breeders, lightweight armors and high-temperature thermoelectric conversion.

The contents of this review are: (1) introduction; (2) preparations—industrial preparative routes, powders, sintering (additives, pressureless, hot pressing, HIP); laboratory methods of synthesis (CVD, PVD, plasma, crystal growth); (3) analytical characterization; (4) phase diagram—a peritectic, nearly pure boron, and a wide phase homogeneity range (B4C-B10·5C); (5) rhombohedral crystal structure—a comprehensive model of the whole solid solution is proposed; (6) chemical properties; (7) physical properties—density, mechanical (strength, hardness, toughness) and thermo-electrical properties; (8) main industrial applications; (9) conclusion.

Boron carbide ― a comprehensive review

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.

Boron Carbide: Structure, Properties, and Stability under Stress

Boron carbide is a strategic material, finding applications in nuclear industry, armour for personnel and vehicle safety, rocket propellant, etc. Its high hardness makes it suitable for grinding and cutting tools, ceramic bearing, wire drawing dies, etc. Boron carbide is commercially produced either by carbothermic reduction of boric acid in electric furnaces or by magnesiothermy in presence of carbon. Since many specialty applications of boron carbide require dense bodies, its densification is of great importance. Hot pressing and hot isostatic pressing are the main processes employed for densification. In the recent past, various researchers have made attempts to improve the existing methods and also invent new processes for synthesis and consolidation of boron carbide. All the techniques on synthesis and consolidation of boron carbide are discussed in detail and critically reviewed.

Synthesis and consolidation of boron carbide: a review

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.

Pressureless Sintering of Boron Carbide

This paper reports how the sintering behavior of boron carbide was investigated with particular attention given to microstructure development at various stages in the sintering process. Hot-pressing and pressureless sintering techniques were employed and the effects of heating rate, firing atmosphere, and composition were used to characterize the sintering behavior. Pressureless sintering at temperatures up to 2300{degrees} C produces only limited densification. Microstructural coarsening is responsible for this since it leads to conditions where densification is slow. Hot-pressing and carbon additions suppressed coarsening and permitted densification to {gt} 95% of theoretical density.

Microstructural Coarsening During Sintering of Boron Carbide

It was found that boron carbide can be sintered pressureless to ultrafine-grained bodies with densities of more than 95% of the theoretical density of B 4 C, if small additions of carbon are made to boron carbide powders of submicron particle size and if the compacts are heated in inert atmospheres to temperatures near 2150 °C. In particular the carbon acts as an inhibitor of surface-to-surface matter transport so that densification via grain boundary and/or lattice diffusion is enhanced at lower sintering temperatures. The mechanical properties of this new sintered material are summarized and compared with those of the commercial hot-pressed product.

K.A. Schwetz

Papers

The influence of carbon on the microstructure and mechanical properties of sintered boron carbide