# Structural Evolution and Mechanical Modulation of Cf/SiC Interfaces During PIP Ceramization: A ReaxFF Molecular Dynamics Study

## TL;DR

This study uses simulations to understand how the structure and strength of carbon fiber/silicon carbide interfaces change during a manufacturing process called PIP, offering insights for better material design.

## Contribution

The paper introduces a ReaxFF MD approach to reveal how interface orientation and pyrolysis affect mechanical properties of Cf/SiC composites.

## Key findings

- High-orientation-angle substrates promote covalent bonding and ductile failure modes in Cf/SiC interfaces.
- The OA = 55° interface achieves a peak shear strength of 15 GPa with a balance of strength and ductility.
- Brittle failure occurs in the OA = 85° interface despite high peak stress due to crack propagation into the SiC matrix.

## Abstract

The precursor infiltration and pyrolysis (PIP) route is widely adopted to fabricate carbon fiber-reinforced silicon carbide (Cf/SiC) composites; however, the atomic-scale restructuring of the pyrolytic carbon/silicon carbide (PyC/SiC) interface during ceramization—and its impact on mechanical integrity—remains elusive. Here, reactive molecular dynamics (ReaxFF MD) simulations elucidate the coupled thermochemical–mechanical evolution of polycarbosilane (PCS) precursors on PyC substrates with orientation angles (OAs) of 0°, 25°, 55°, and 85°. Dynamic pyrolysis triggers a pivotal transition from sp2 to sp3 hybridization at the interface. High-OA substrates (55° and 85°) present a dense population of reactive edge sites, fostering extensive cross-interfacial covalent bonding. Subsequent shear loading reveals that these pyrolysis-induced chemical bridges govern failure modes, shifting from interlayer sliding dominated by weak non-bonded interactions (0°) to ductile fracture featuring uniform plasticity and crack deflection. The OA = 55° interface attains a theoretical peak shear strength of 15 GPa and exhibits the most favorable combination of high strength and ductile failure under tensile loading, owing to an optimal balance between reactive site availability and interlayer steric openness. In contrast, the OA = 85° interface, despite comparable peak stress, fails via brittle crack penetration into the SiC matrix. By correlating atomistic structure with macroscopic performance, this study provides a bottom-up framework for engineering Cf/SiC composites via interfacial texturing and optimized pyrolysis protocols.