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

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/cm³) 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 ─CH₃ and ─H radicals reached 340 and 200 molecules at 4000 K, corresponding to a CH₃/H evolution ratio of 1.7, in close agreement with theoretical predictions (≈ 1.4). Product yield analysis showed that gaseous species such as H₂ and CH₄ 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.