Degradation mechanisms of a self-healing SiC_(f)/BN_(i)/[SiC-B₄C]_(m) composite at high temperature under different oxidizing atmospheres

Chemical vapor-infiltrated self-healing SiC_(f)/BN_(i)/[SiC-B₄C]_(m) composite specimens were exposed at 1300 °C for 300 h at atmospheric pressure under two different oxidizing atmospheres (i.e., wet (12%H₂O:8%O₂:80%Ar) and dry (0.01%O₂:99.99%Ar)) representative of rich and poor oxidizing conditions, respectively. Mechanical testing, microstructural observations, and element analyses were performed on the treated specimens. The flexural strength retentions of the specimens were 47.9 and 39.4% under wet and dry oxygen conditions, respectively. The SiC and B₄C matrices were severely oxidized under wet oxygen conditions, whereas the BN interphase remained intact. The BN interphase and the B₄C layered phase were both partially oxidized under dry oxygen conditions. Thus, the SiC_(f)/BN_(i)/[SiC-B₄C]_(m) composites exhibited improved oxidation resistance under wet oxygen atmospheres as compared to dry oxygen conditions as a result of the formation of borosilicate glasses. In addition, two different degradation mechanisms for the composites during the oxidation process were discussed.