Self-healing mechanisms of a SiC fiber reinforced multi-layered ceramic matrix composite in high pressure steam environments

Non-oxide ceramic matrix composites are potential candidates to replace the current nickel-based alloys for a variety of high temperature applications in the aerospace field. The durability of a SiC(f)/PyC(i)/[Si,C,B](m) composite with a sequenced self-sealing matrix and Hi-Nicalon fibers was investigated at 600 °C for exposure durations up to 600 h. The specimens are aged in a variety of slow-flowing air/steam gas mixtures and total pressures, ranging from atmospheric pressure with a 10–50% water vapor content to 1 MPa with 10–20% water vapor content. The degradation of the composite was determined from the measurement of residual strength and strain to failure on post-exposure specimens and correlated with microstructural observations, weight changes and characterizations of the generated oxides. All of the post-exposure characterizations demonstrate the ability of the sequenced [Si,C,B] matrix to protect the PyC interphase from environmental attacks. Two different oxidation modes of the matrix, depending on the total pressure are discussed in terms of the reactivity of the boron-containing layers, and their relative positions in the sequenced matrix. In high pressure environments, a strong localized dissolving of a small amount of SiC fibers in the boria-containing oxide is evidenced at 600 °C.