Thermodynamic analysis of isothermal crystallization of amorphous Si-C-N ceramics derived from polymer precursors

Thermodynamic modelling was used to explain structural transformations during crystallisation of amorphous Si-C-N polymer derived ceramics. Nanocrystalline SiC and nanocrystalline Si3N4 identified in ceramic microstructure were regarded as metastable NASIC and NASIN phases of the Si-C-N system. Gibbs energies G(NASIC) and G(NASIN) were derived considering an excess energy coupled with interfaces of nanocrystallites. The metastable phase equilibria including the nanocrystalline phases and the amorphous am-SICN phase were analysed. Formation of NASIC and NASIN phases by eutectoid transformation of am-SICN phase and continuous growth of nanocrystallites were ascertained as a possible method of crystallisation. The resistance of Si3N4 against disintegration by reaction with carbon is explained as a thermodynamic effect of nanocrystalline structure. 26 refs.