Boron densification

Hot reactive pressing is possible . Densification is easier when sintering takes place simultaneously with synthesis, starting from boron and carbon powders . 

Using computer-interactive (APL programming language) methods " , it is possible to determine the models of densification kinetics of boron carbides. The simplest is ... 

S.C. Zhang, GE. Hilmas, et al.. Pressureless Densification of Zirconium Diboride with Boron Carbide Additions, J. zlm. Ceram. Soc., 89 5, 1544-1550(2006). 

Synthesis from the Elements Prochazka [39] synthesized submicron pSiC powder with a surface area of 7 m g using direct reaction of high-purity silicon powder with carbon-black at temperatures between 1500 and 1650°C, but saw little densification with boron and carbon additions, suggesting that free sihcon is detrimental to densification. 

Figure 35. Densification of boron carbide with 5-7 mass-% C (after [197]).

As introduced above, the reaction of boron carbide with metal carbides can be used to fabricate metal borides or metal boride/boron carbide composites in a controlled way during densification if boron carbide or free boron is used in excess, or if carbon is bonded by another additive. Although the incompatibility of B4C and metal carbides is well known, many attempts have been undertaken to produce composites or coatings thereof but failed as soon as equilibrium conditions were approached. Physical or chemical vapor deposition of B4C on hard metal substrates, or WC coatings on boron carbides are typical problems (e.g., [252]). In both cases, interlayers of graphite form and hence result in an unsatisfactory adhesion of the deposited coating to the substrate. 

According to Greskovich and Rosolovski [372] and Prochazka [373], the specific surface area during the initial state of sintering is mainly consumed by a coarsening of the particles and pores, which in turn reduces the driving force for densification (local chemical potential). Densification comes to an end before pore closure is obtained. This so-called terminated density has been observed for pure boron carbide, SiC and silicon nitride, when no sintering aids are present. 

Figure 4.13 Densification of boron carbide with addition of carbon. After Ref [395].

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... 

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. 

The mechanical properties of self-bonded boron carbides produced by different densification techniques are summarized in Table 11.2. 

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