Influence of dip-coated boron nitride interphase on mechanical and dielectric properties of SiCf/SiC composites

The utilization of urea and boric acid solutions as precursors via dip-coating method for synthesis of BN coatings on KD-I SiC fibers was demonstrated. The effects of precursor compositions of BN on coatings microstructure, coatings morphology, mechanical and dielectric properties of SiCf/SiC composites were investigated. XRD, FT-IR and Raman spectra results show that the crystalline degree of t-BN decreases with urea and boric acid mole ratios increase from 3:1 to 12:1. The surface of as prepared BN coatings is smooth and its thickness is uniform when the ratio is higher than 6:1. From mechanical properties measurements, the higher mole ratio helps to improve the flexural strength and fracture ductility of the composites. The flexural strength and failure displacement of SiCf/SiC composites with 0.2 μm thickness BN interphase reach 278 MPa and 0.29 mm, which are both much higher than those of the composites without interphase, whose ultimate values are only 191 MPa and 0.13 mm, respectively. The flexural strength retention of the composites with BN interphase is higher than that of the composites with PyC interphase below 800 °C and there is no significant difference at higher temperatures. The BN coatings have little influence on the complex permittivity of SiC fabrics.