| Summary: | Boron nitride (BN) ceramic fibers containing amounts of silicon nitride (Si<sub>3</sub>N<sub>4</sub>) were prepared using hybrid precursors of poly(tri(methylamino)borazine) (PBN) and polycarbosilane (PCS) via melt-spinning, curing, decarburization under NH<sub>3</sub> to 1000 °C and pyrolysis up to 1600 °C under N<sub>2</sub>. The effect of Si<sub>3</sub>N<sub>4</sub> contents on the microstructure of the BN/Si<sub>3</sub>N<sub>4</sub> composite ceramics was investigated. Series of the BN/Si<sub>3</sub>N<sub>4</sub> composite fibers containing various amounts of Si<sub>3</sub>N<sub>4</sub> from 5 wt% to 25 wt% were fabricated. It was found that the crystallization of Si<sub>3</sub>N<sub>4</sub> could be totally restrained when its content was below 25 wt% in the BN/Si<sub>3</sub>N<sub>4</sub> composite ceramics at 1600 °C, and the amorphous BN/Si<sub>3</sub>N<sub>4</sub> composite ceramic could be obtained with a certain ratio. The mean tensile strength and Young’s modulus of the composite fibers correlated positively with the Si<sub>3</sub>N<sub>4</sub> mass content, while an obvious BN (shell)/Si<sub>3</sub>N<sub>4</sub> (core) was formed only when the Si<sub>3</sub>N<sub>4</sub> content reached 25 wt%.
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