Synthesis of coatings on SiC fibers and their effects on microstructure and mechanical properties of SiCf/SiBCN composites

In this study, the amorphous C, ZrB₂, and BN single-layer coatings as well as C/BN, C/ZrB₂, ZrB₂/BN, and C/ZrB₂/BN composite coatings were prepared on SiC fibers (SiC_f) by an in situ synthesis and solution impregnation–pyrolysis method. Subsequently, SiC_f/SiBCN composites were fabricated by hot-pressing sintering at 1900℃/60 MPa/30 min to explore the influence of different coatings on the microstructure and mechanical performance of resulting composites. After the preparation of single-layer-coated SiC_f, the SiC_f(BN) or SiC_f(ZrB₂) tended to be overlapped with each other, whereas the dispersion of amorphous C–coated SiC_f was satisfying. Besides, some uneven areas and attached particles have appeared on fiber surfaces of the SiC_f(BN) or SiC_f(ZrB₂), whereas smooth and dense surfaces of amorphous C–coated SiC_f were observed. Because the uniformity of ZrB₂ coatings can be partially damaged by the subsequent coating process of BN, the composite coatings of ZrB₂/BN and C/ZrB₂/BN were thereby not suitable for strengthening SiBCN matrix. The SiC_f/SiBCN composites with C/ZrB₂ coatings have desirable comprehensive mechanical properties. Nevertheless, the conventional toughening mechanisms such as fiber pull-out and bridging, and crack deflection are not available for these composites because the serious crystallization of SiC_f leading to great strength loss, resulting in catastrophic brittle fracture.