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.