Fabrication of Near-Stoichiometric Crystalline SiC Fibers From Polyborocarbosilane and Polyaluminoborocarbosilane

Silicon carbide (SiC) fibers are indispensable reinforcements for ceramic matrix composites (CMCs) in extreme environments. However, achieving near-stoichiometric SiC compositions with low oxygen content and a dense microstructure still remains a challenge. This study introduces a novel fabrication route for producing near-stoichiometric crystalline SiC fibers. This approach utilizes polyborocarbosilane (PBCS) and polyaluminoborocarbosilane (PABCS) precursors, synthesized from liquid polysilacarbosilane (PSCS), o -carborane, and aluminum acetylacetonate (Al(acac) 3 ) via a high-temperature/pressure method. Our findings show that incorporating o -carborane markedly accelerates polymerization kinetics under mild conditions (360°C–380°C). Boron (B) is integrated into the polycarbosilane (PCS) molecular structure through the formation of B─Si and B─C bonds, resulting from reactions between Si-H, Si-CH 3 , and B-H groups. Continuous precursor fibers are successfully melt-spun by tailoring precursor properties. These fibers can be oxidatively cured under milder conditions (100°C, 1–2 h) due to an elevated concentration of Si radicals. Subsequent pyrolysis and sintering of both PBCS- and PABCS-derived cured fibers yield near-stoichiometric crystalline SiC fibers with low oxygen content and a dense structure. We observed that the formation of B─C/B─Si bonds promotes densification of the SiC fibers. Notably, PABCS-derived SiC fibers exhibit superior densification compared to PBCS-derived fibers, attributed to the synergistic sintering effects of boron and aluminum, potentially mediated by the formation of boron, aluminum, and their respective eutectic liquid phases.