Detailed study of interphase degradation in SiC/BN/SiC ceramic matrix composites after elevated temperature tensile testing

Ceramic matrix composites of silicon carbide fibres in a silicon carbide matrix with boron nitride interphase are promising candidates for replacing superalloys in the hottest part of aerospace engines, reducing the need for cooling and increasing the fuel efficiency. This needs a thorough understanding of how these materials degrade under high levels of stress combined with high temperatures in an oxidative environment. This work presents a detailed investigation of the degradation in the interphase and surrounding interfaces. Advanced electron microscopy and electron energy loss spectroscopy are used to extract information on the degradation process. It was found that silica and boria form along with a migration of silica into the interphase. At 1000 °C the degradation along the surface leads to early fracture at the surface and eventually complete fracture of the composite, lower temperature allows for the oxidation to reach the centre of the sample before complete failure. • Investigation of SiC/BN/SiC composites after tensile testing to failure at elevated temperature performed in high detail, including chemical information with nanoscale spatial resolution. • Silica formation and silica migration identified at all tested temperatures (400, 600 and 1000 °C), with the migration being fast enough at 1000 °C to prevent the formation of silica directly adjacent to the fibre. • Significant amounts of turbostratic BN interphase found in all samples also at failure. • Failure at 1000 °C happens through early fracture along the edges, likely with oxide formation welding adjacent fibres acting as crack initiation sites, followed by transfer of load and complete failure before the oxidation can reach the centre portions of the sample. • Failure at 400 °C happens through continued accumulation of cracks and oxidation throughout the sample.