High temperature oxidation and its effect on the mechanical behavior of SiC_f/BN/SiBCN composites

The toughness of SiC fiber reinforced ceramic matrix composites (CMCs) can be significantly improved by the adoption of a pyrolytic carbon (PyC) interphase but this cannot be exposed to oxidizing environments at high temperatures. Here we considered a BN interphase as an alternative and investigated the flexural properties of SiC_f/BN/SiBCN CMCs before and after high temperature exposure (1050 °C, 1200 °C and 1350 °C) in air. The flexural and interfacial shear strengths were determined by three-point bending and single fiber push-out tests, respectively. The flexural strength of the composites decreased from 325 ± 23 MPa to 165 ± 15 MPa, while the interfacial shear strength increased from 32 ± 9 MPa to 88 ± 18 MPa after oxidation at 1350 ^°C for 30 h. Digital image correlation (DIC) was used to visualize crack initiation and growth during the bending tests. This revealed that cracks formed on the tensile side of the specimens before being deflected and eventually passed through the fiber tow. The oxidation of the SiC_f/BN/SiBCN composite at high-temperature resulted in the formation of borosilicate glass along the interphase area, and a porous structure was observed after oxidation at high temperatures due to its evaporation. The interfacial shear strength between the fiber and matrix increased after oxidation at high temperatures, which resulted in brittle failure of the composite.