Oxidation Behavior of SiC_f/SiC Minicomposites with Multilayered (BN/SiC)_n Interfacial Coatings under Humid Environment

SiC_f/SiC minicomposites with BN, double-layered BN/SiC, and multi-layered (BN/SiC)₂ interfacial coatings were fabricated via the chemical vapor infiltration and precursor infiltration pyrolysis methods. The oxidation behavior and mechanical properties of the minicomposites after high-temperature oxidation in dry air and under a humid environment were studied and compared with those of the uncoated minicomposites. The weight of the coated minicomposites increased linearly in dry air over a period of 20 h, while that of the coated minicomposites decreased linearly after a short weight gain in a water/oxygen atmosphere, and the minicomposites with BN/SiC coating lost the most weight. The oxidation of BN to B₂O₃ competes with the volatilization of H_xB_yO_z, and the latter becomes dominant after oxidation for 1 h. The introduction of the SiC layers caused the dry air or water/oxygen mixture to diffuse into the minicomposites along the direction parallel rather than perpendicular to the fiber axis, eventually resulting in the degradation in the BN coatings in the inner composites and a deterioration in the mechanical properties. After 20 h of oxidation in a water/oxygen atmosphere, the fracture strength of the minicomposites with the BN/SiC and (BN/SiC)₂ coatings decreased from 582.3 to 231.7 MPa and from 558.2 to 271.6 MPa, respectively.