Silicon nitride phase diagram

Rogi, P. and J.C. Schuster Phase Diagrams of Ternary Boron Nitride and Silicon Nitride Systems, ASM International, Materials Park, OH. 1992. 

The thermodynamics of the above-elucidated SiC/C and SijN Si composites are determined by the decomposition of silicon carbide and silicon nitride, respectively, into their elements. The chemistry of ternary Si-C-N composites is more complex. If producing Si-C-N ceramics for applications at elevated temperature, reactions between carbon and silicon nitride have to be considered. Figure 18.2, which exhibits a ternary phase diagram valid up to 1484°C (1 bar N2) displays the situation. The only stable crystalline phases under these conditions are silicon carbide and silicon nitride. Ceramics with compositions in the three-phase field SiC/Si3N4/N are unknown (this is a consequence of the thermal instability of C-N bonds). Although composites within the three-phase field SiC/Si3N4/Si are thermodynamically stable even above 1500°C, such materials are rare. The reasons are difficulties in the synthesis of the required precursors and silicon melting above 1414°C. The latter aspect is of relevance, since liquid silicon dramatically worsens the mechanical properties of the derived ceramics. 

Use of Phase Diagrams in the Study of Silicon Nitride Ceramics... 

This chapter is not intended to be a literature survey, since an excellent review paper on phase diagrams of silicon nitride-metal oxide systems has been written by Sorrell [5], This chapter, is, however, intended to illustrate the use of phase diagrams to study silicon nitride ceramics. 

Silicon nitride ceramics are one of the most promising materials for structural applications. The properties of the silicon nitride ceramics depend on the phases present and their microstructures. This paper describes how to approach microstructural design of these materials by using the phase equilibrium diagrams of silicon nitride-metal oxide systems. 

Fio. 38. The Mg-Nd-Si-O phase diagram showing compositional lines corresponding to mole ratios of 4 and 1 used as additive compositions to silicon nitride containing Si02. After Hampshire et al. [34], Reproduced with permission of the American Ceramic Society, Westerville. 

Experimental work on the phase diagram of this system is meager. Some information on selected phase equilibria can be derived indirectly from work on the development of commercial materials in the Si-B-N system. Gugel et al. (1972) [223] suggested a tentative phase equilibrium diagram. Equilibria between silicon Sorides and silicon nitride were assumed. According to this result, silicon does not show an equilibrium with BN. Kato et al. [227]... 

Rog] Rogl, R, The System B-N-Fe m Phase Diagrams of Ternary Boron Nitride and Silicon Nitride Systems, , Rogl, P. Schuster, J.C., (Eds.), ASM, Materials Park, OH 1992, 33-36 (Crys. Structure, Thermodyn., Phase Diagram, Phase Relations, Experimental, Review, 9)... 

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