A modified dip-coating method to prepare BN coating on SiC fiber by introducing the sol-gel process

Boron nitride (BN) coatings were synthesized on silicon carbide (SiC) fibers by a modified dip-coating method using boric acid and urea as BN precursors and acrylamide as gel-casting monomer heated at 850 °C for 3 h under N2 atmosphere. For comparison, a conventional dip-coating method was also adopted to prepare BN coatings under the same conditions. The structure of BN coatings was characterized by X-ray diffraction, Fourier transform infrared spectroscopy and transmission electron microscopy. The morphology of the fibers after impregnation and heat treatment was observed by the electron micrographs. Fiber tensile strength was evaluated by single fiber tensile test. The effects of polyacrylamide on the formation of BN coatings were also investigated. The results show that the BN coatings, prepared by two cycles of impregnation in the modified dip-coating process, covered the SiC fibers well, and exhibited a compact turbostratic-amorphous hybrid structure with a thickness of 550 nm. The tensile strength of the coated SiC fibers increased by ~ 22% after the conventional dip-coating method was modified. The introduction of the sol-gel process enabled the BN precursors to form a film on the surface of SiC fibers with an even distribution in the impregnation process and to retain the evenness in the heat treatment process. Based on the two effects, the conventional dip-coating method was improved.