Sintering of Boron Carbide

Another reason for the poor sinterability is the extraordinarily high vapor pressure of boron oxides and suboxides. Since boron carbide powders are generally coated by a B2O3 layer [172] which quickly reacts to form boric acid, H3BO3, in humid atmosphere, vapor phase reactions are active at higher temperatures, in particular above 1500 C, providing a fast transport of boron compounds. Redox reactions such as 

The only technically important sintering additive for boron carbide is carbon, as discovered almost simultaneously by Schwetz and Vogt [189], Henney and Jones [190], and Suzuki et al. [191], which today allows the routine production of dense parts. An amount ranging from 1 to 6 mass-% is sufficient to obtain almost theoretical density. Schwetz and Grellner [192] added phenolic resin (corresponding to 1-3 mass-% C) to a submicron B4C powder and obtained 98% density at 2150°C. The resins are pyrolized at temperatures up to 1000°C and they leave a 

A typical microstructure of B4C doped with 5 mass-% free carbon and sintered at 2150°C is shown in Fig. 36. In general the microstructure is very uniform and fine grained, but some grains have already entered the regime of discontinuous growth. This process is more clearly reflected in Fig. 37 While the grain size increases rather slowly below 2150°C, a rapid rise of the mean intercept length is observed beyond that temperature. 

Recently, boron carbide was successfully densified with TiC by pressureless sintering [207]. Since TiC reacts with B4C by the formation of TiB2 and free carbon. 

Injection molding of boron carbide with 2-5 mass.-% carbon black was developed by Schwetz et al. [210]. Like in conventional processes known for oxide and nitride ceramics, the spray-dried powder blend was mixed with 18 mass-% organic binder and molded at 120°C and 45 MPa. Dewaxing was accomplished by heating in an atmosphere-controlled furnace at 100 mbar. The binder components decomposed thermally by cracking and evaporated within four days and temperatures up to 

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The addition of free graphite yields fine-grained compounds near the theoretical density (94-100 %) . Carbon is better added by the in-situ pyrolysis of a phenolfor-maldehyde resin (i 9 wt Pressure-less sintering of boron-carbide is now... 

The Si-B-C system was mainly investigated with view to the understanding of the sintering mechanisms of SiC with boron in combination with carbon and the sintering of boron carbide with silicon [4]. The silicon solubility of about 2.5 at.% in B4C at 2323 K and the comparatively low temperatures of liquid phase formation in the ternary system enhance the sintering of boron carbide. 

Figure 11.3 Results of pressureless sintering of boron carbide of different stoichiometric compositions (green density 60%, 1 h soaking time at final temperature, 1 bar argon atmosphere). The numbers in italics refer to percentage theoretical density.

Adrian Goldstein, Ygal Geffen, Ayala Goldenberg. Boron Carbide-Zirconium Boride In Situ Composites by the Reactive Pressureless Sintering of Boron Carbide-Zirconia Mixtures. Journal of the American Ceramic Society 2001 84 642. 

The hardness of boron carbide (carbon hexaboride) is not well defined because it is made as sintered compacts which have variable densities, compositions, and defect densities. It is very hard (up to 4400kg/mm2), and of relatively low density, so it has been used extensively as body-armor (McColm,... 

Hafnium-like boron is known to be a neutron absorber or neutron moderator element, and, therefore, composites of boron carbide, B4C, and hafnium diboride, HfB2, can be considered as nuclear materials. These boron compounds after sintering and °B/"B isotopic ratio adapting are found to be heterogeneous polyphone cermets useful for nuclear applications (Beauvy et al. 1999). Boron acid obtained from the °B enriched boron trifluoride also was used in nuclear reactors (Shalamberidze et al. 2005). Amorphous boron powders enriched both in °B and "B, boron carbide, and zirconium diboride (ZrB2) powders and pallets labeled with °B isotope And applications in nuclear engineering too. The °B enriched Fe-B and Ni-B alloys are useful for the production of casks for spent nuclear fuel transfer and storage. 

Boron carbide is among the hardest materials yielding only to diamond and boron nitride. It is also one of the most corrosion-resistant compounds at room or moderate temperatures. When considering the corrosion resistance of boron carbide materials, it is important to remember that they are rarely stoichiometric, with the carbon content varying from 9.88 to 23.4% [96] Many of them contain free carbon or sintering aids. Thus their behavior depends on the chemical composition. 

Thermodynamic calculations by Dorner [78], Lukas [93] and Lim and Lukas [52], however, clearly demonstrated the existence of a binary phase equilibrium of boron carbide and a Si- and B-containing melt above 1560°C. The theoretical results were confirmed by hot pressing, liquid phase sintering and infiltration experiments by Lange and Holleck [75], Telle [83], Telle and Petzow [94], and Telle [54], which also yielded more details on the extension of the homogeneity field of boron carbide towards the Si-rich corner of the system B-C-Si. 

Figure 36. Microstructure of boron carbide pressurelessly sintered with carbon black (after [197]).

The physical and chemical properties of boron carbide have been reviewed by Lipp [159], Thevenot and Bouchacourt [256], Thevenot [164,165], and Schwetz (1999) [223]. Special problems while presenting the physical properties arise from the large homogeneity range of boron carbide. Furthermore, its poor sinterability requests additives that are usually unspecified and results in residual porosity and various grain sizes which are often also not considered in the publications. Most variation and discrepancies in the properties reported come from the undefined composition of the materials studied. 

The densification of boron carbide using silicon or aluminum is difficult to obtain due to the high vapor pressures of liquid silicon or aluminum at reasonable sintering temperatures, and which may hinder densification due to the formation of degassing channels and entrapment of residual gas in the closed pores. Telle and Petzow [404, 405] have demonstrated the existence of a binary equilibrium between a Bi2(B,C,Si)3 solid solution and boron-rich liquid silicon above 1560 ° C, which might be beneficial... 

Different processing routes have been applied to synthesize this class of composite material. Besides the classical method of powder mixing using submicron SiC, B4C and carbon or carbon-containing precursors, followed by forming and subsequent heat treatment ]313], the infiltration of boron carbide preforms with organic precursors such as PCS, followed by heat treatment [400, 401] or the pressure-assisted reaction sintering of silicon borides with elemental carbon [423], have been used. 

Campbell, J., Klusewitz, M., LaSalvia, J, et at (2008) Novel processing of boron carbide plasma synthesized nano powders and pressureless sintering forming of complex shapes. Proceedings 26th Army Science Conference, December 2008, Orlando, Florida,... 

Prochazka S (1975) The role of boron and carbon in the sintering of silicon carbide. In Popper P (ed.) Special ceramics, 6th edn. British Ceramic Research Association, Stoke-on-Trent, p 171... 

Various results to furtiier develop these materials as TE materials have been obtained such as the densification of these materials through usage of sintering additives, doping with transition metals,and modification of the [B]/[C] composition, which previously had led to significant improvement in the TE properties of boron carbide. 

Synthesis temperature affects the particle size of the boride powder which is extremely important for densification. Powders obtained at lower synthesis temperature have finer size and better sinterability. Particle size of boron carbide also affects the reaction temperature and finally the product quality. As boron carbide grain size increases, the oxide rednction processes and diffusional processes slow... 

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