Fundamental Understanding of Carbon Evolution in Polycarbosilane Derived SiC Ceramics: A ReaxFF Molecular Dynamics Study

Abstract

The in situ formed free carbon (C_free ) phase in polycarbosilane (PCS)-derived SiC ceramics plays a dual role, providing essential functionalities while compromising structural stability. However, a predictive design of these materials is currently hindered by an incomplete understanding of the atomic-scale mechanisms governing C_free evolution during pyrolysis. In this work, this knowledge gap is bridged using large-scale reactive molecular dynamics simulations, enabled by a newly optimized ReaxFF force field. A multi-tiered control mechanism is revealed. The initial C/Si ratio is demonstrated to dictate the macroscopic phase separation pathway, driving a definitive transition from nucleation-and-growth to spinodal decomposition at high carbon contents. Within this framework, the atomic-scale nucleation pathway is shown to be governed by the chemical form of the carbon source; aliphatic side groups yield amorphous clusters via radical polymerization, whereas pre-aromatic molecules template the formation of ordered domains through direct aggregation. Furthermore, a novel, self-perpetuating 'poisoning' mechanism is uncovered, by which methane actively inhibits graphitization through the continuous creation of stable sp³ defects on graphitic surfaces. These mechanistic insights replace the traditional 'black box' view of PDC pyrolysis with a predictive, science-based framework, providing validated design principles for rationally engineering the C_free nanostructure to achieve tailored properties in advanced SiC ceramics.