Microstructural engineering of boron nitride-coated SiC fibers via ultrafast joule heating for high-temperature creep resistance

Near-stoichiometric silicon carbide (SiC) fibers, such as Hi-Nicalon S (HNS), suffer from fine-grained microstructures and residual impurities that limit their high-temperature applications. Here, we report a combined strategy of in-situ boron nitride (h-BN) coating (initial BN thickness 155 nm) and ultrafast Joule heating to enhance the thermomechanical performance of domestic Cansas-3303 SiC fibers. The optimized fibers (treated at 2100 °C for 15 s) have average grain size coarsened from 14.5 nm to 46.9 nm—a threefold increase compared to the as-received state. Microstructural analysis reveals a radial grain size gradient (core: 36 nm → surface: 67 nm) and BN-induced β→α-SiC phase transformation, with XRD confirming the presence of 4H/6H polytypes. These optimized fibers achieve a tensile strength of 1.94 GPa (62.4 % retention), which is only ∼20 % lower than that of commercial Hi-Nicalon S (HNS) fiber (∼2.4 GPa). Additionally, they demonstrate superior creep resistance relative to HNS. The h-BN coating suppresses surface decomposition and defect formation, while Joule heating drives rapid grain coarsening and crystallinity improvement. These microstructural modifications collectively lead to the enhanced strength and creep resistance. This work provides a scalable processing route for high-performance SiC fibers suitable for extreme-temperature structural applications.