Thermodynamic modeling of amorphous Si–C–N ceramics derived from polymer precursors

A thermodynamic model has been developed to rationalize the enhanced persistence of amorphous Si–C–N polymer derived ceramics (PDC). Typically, the complex morphology of such materials is separated into regions of amorphous carbon and amorphous domains of a composition SiCxNy on the tie line joining the phases SiC and Si3N4 in the Si–C–N phase diagram. These amorphous domains have been modeled as a solution phase of five stoichiometric constituents corresponding to short-range ordered SiCiN4−i atomic configurations identified in the PDC structure. Parameters of the model Gibbs energy of the am-SICN phase have been estimated on the basis of the available thermodynamic assessments of the phases SiC and Si3N4 and additional assumptions concerning energies of Si–N and Si–C atomic bonds in the phase constituents. The Gibbs energy derived for the am-SICN phase has been compared, at temperatures of 1300 K ⩽ T ⩽ 2000 K, with a Gibbs energy of the equivalent state composed exclusively of SiC and Si3N4 phases. This comparison proves an enhanced resistance to crystallization for the am-SICN phase of certain SiCxNy compositions, being in agreement with experimental evidence.