High‐Temperature Sintering Behavior and Microstructural Evolution of Boron Nitride Nanosheet Composite Si–Al–C–O Fibers

ABSTRACT To construct a high‐temperature stable phase in Si–Al–C–O ceramic fibers, hexagonal boron nitride nanosheets (h‐BNNS) were incorporated into the polyaluminocarbosilane (PACS) precursor to prepare boron nitride nanosheet composite Si–Al–C–O (Si–Al–C–O/BNNS) fibers. The polymer‐to‐ceramic process and high‐temperature sintering behavior of Si–Al–C–O/BNNS fibers were investigated. The results demonstrated that no reaction between PACS and h‐BNNS occurred in the polymer‐to‐ceramic process. In comparison to pure PACS, PACS/BNNS‐5% exhibited a high ceramic yield of 60.15% at 1600°C (29.48% for pure PACS). The as‐fabricated Si–Al–C–O/BNNS‐0.5% fibers had an improved tensile strength of up to 1022 MPa, representing a 63% increase compared to the original Si–Al–C–O fibers. After high‐temperature annealing at 1600°C, a novel graphitic‐like BN(C)/B ₄ C phase was initially formed by reacting B ₂ O ₃ (generated from the reaction of h‐BNNS with SiO and graphite) with graphitic carbon. At 1900°C, the Si–Al–C–O/BNNS fibers were mainly composed of β‐SiC grains, with trace amounts of h‐BNNS, graphitic‐like BN(C)/B ₄ C phase, and graphitic carbon. Furthermore, the introduction of h‐BNNS as a secondary sintering aid suppressed the abnormal growth of β‐SiC grains and promoted fiber densification. The stabilization role of boron nitride within the Si–Al–C–O fibers provides novel insights into the design and fabrication of high‐temperature‐resistant ceramic fibers.