The Si-B-C System

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. 

The role of boron for favorable sintering of SiC is not completely solved. However, a minor solution of boron in SiC seems to play an important role for enhancing grain boundary diffusion [216]. 

The existence of pseudobinary sections was tracked by most of the groups dealing with Si-B-C. Most of the authors assumed a composition B4+aC with 5 = 0. The phase equilibria of four different isopleths have been m2iinly investigated experimentally in this system B4C-SiC, B-SiC, B = const. = 80 at.% and B4C-Si. An overview of experimental investigations of the Si-B-C phase diagram is shown in Table 16 [4, 217-224], 

Calculated isothermal sections at 2500 K and 1400 K are shown in Figs. 10a and b, respectively. 

The calculated section B4.6C-SiC in comparison with experimental data is shown in Fig. 11. 

---

A calculated isothermal section for the Si-B-C system at T = 1400 K was presented by Goujard et al. (1994) [75, 76]. The data were derived from JANAF [140] and assessments of the phases SiBi4, SiBg, SiB3 and the solid solution phase boron carbide. The isothermal section calculated using the data of Kasper (1996) [33] (Fig. 10b) deviates significantly. Experimental investigations at these temperatures suffer from slow reaction kinetics and decisions... 

Fig. 11. Calculated B4.6C-SiC section in the Si-B-C system. The horizontal line corresponds...

Schulz S (2002) Synthesis, Structure and Reactivity of Group 13/15 Compounds Containing the Heavier Elements of Group 15, Sb and Bi 103 117-166 Seifert HJ, Aldinger F (2002) Phase Equilibria in the Si-B-C-N System. 101 1-58 Stahlke D, see Mahalakshmi L (2002) 103 85-116... 

This paper gives an overview of experimental results and thermodynamic calculations in the Si-B-C-N system and its binary and ternary subsystems. 

The compositions of solid phases appearing in the Si-B-C-N system are indicated in the concentration tetrahedron as shown in Fig. 1. Some of them appear in different modifications. The basic crystallographic data of stable phases are given in Table 1. 

The CALPHAD method of computer coupling of phase diagrams and thermodynamics [24] was used to explore the phase equilibria in the Si-B-C-N system. Analytical descriptions of the Gibbs free energies for all stable phases and gaseous species of the system were established in the literature and by the... 

The ternary system Si-B-C was optimized in the scope of our work on the Si-B-C-N system [33, 34]. The other three ternary systems (Si-B-N, Si-C-N, B-C-N) could be calculated comprehensively by thermodynamic extrapolations since all solid phases have negligible ranges of solubility. Thermodynamic data of the ternary phases pSiCJti4 and Si2CN4, are not known yet, however they are reported not to be stable at the conditions treated here (P = 1 bar, T > 1300 K) [21]. 

Table 2. Gas species in the Si-B-C-N system. Thermodynamic data were used from [32]...

---