Multi‐Stage Decomposition Mechanism of Polycarbosilane During Pyrolysis by a ReaxFF Molecular Dynamics Study

ABSTRACT Polycarbosilane (PCS) is a critical precursor for silicon carbide (SiC)‐based ceramics, although its atomistic pyrolysis mechanism remains insufficiently understood. In this study, large‐scale reactive molecular dynamics simulations using the ReaxFF force field were conducted to elucidate the product evolution and decomposition pathways of PCS under high‐temperature pyrolytic conditions. A crosslinked PCS model (93.75% crosslinking, density = 1.11 g/cm 3 ) was simulated under 2000–4000 K to quantitatively characterize molecular species evolution, bond dissociation sequences, and radical formation kinetics. The simulations revealed a multi‐stage decomposition mechanism involving initial Si─Si bond cleavage (≈ 2200 K), subsequent Si─C and C─C bond dissociation (2500–3200 K), and final stabilization into small thermodynamic species. Quantitatively, the formation of ─CH 3 and ─H radicals reached 340 and 200 molecules at 4000 K, corresponding to a CH 3 /H evolution ratio of 1.7, in close agreement with theoretical predictions (≈ 1.4). Product yield analysis showed that gaseous species such as H 2 and CH 4 dominate above 3500 K, while Si‐containing fragments (e.g., SiCH x ) decrease sharply, indicating a complete breakdown of the Si‐based framework. These results establish a direct correlation between bond dissociation energy hierarchy (Si─Si < Si─C < C─C) and temperature‐dependent product selectivity. This study provides the first quantitative atomistic validation of PCS pyrolysis stages and offers a mechanistic framework for optimizing preceramic polymer design and enhancing SiC ceramic yield.