Abstract: SiC_f/SiC composites exhibit advantages such as high temperature resistance, oxidation resistance, and high strength, making them a "star" candidate material in the field of aerospace thermal protection. Under operational conditions, these materials are subjected to prolonged multiple coupled fields such as heat, water, and oxygen, exhibiting complex failure mechanisms and damage evolution patterns. This study investigated the integrated oxidation mechanism of the matrix/interface/fiber in Mini-SiC_f/BN/SiC composites under cyclic oxidation at 1100 ℃ in a water-oxygen coupled environment, by using multi-scale macro/micro characterization techniques. The results showed that during the initial oxidation stage, an amorphous SiO₂ glass layer with relatively smooth morphology formed on the material surface. However, with the increase in crystallinity, localized spallation occurred in the oxide layer, causing the surface roughness (S_a) to initially decrease before subsequently increasing. XRM results showed that numerous micro-defects were generated within the material after cyclic oxidation, and the number of defects increased by orders of magnitude (about 107 fold). The majority of these micro-defects were mainly distributed on the matrix surface, and the oxidation products played a certain filling role in these defects. The tensile strength showed no significant variation before ((328.47±32.84) MPa) and after ((343.27±35.71) MPa) cyclic oxidation, indicating the continued effectiveness of the synergistic toughening mechanism of "strong matrix-weak interface". These observations indicate that an integrated oxidation protection mechanism involving the matrix, interface, and fiber is established. This mechanism is predicated on the filling of defects by SiO₂ and borosilicate glass generated by the interface layer and adjacent matrix and fibers in the direction parallel to the fiber axis, and the dynamic "outer porous sacrificial layer-middle dense SiO₂-inner SiC matrix" three-dimensional protective barrier of the matrix in the direction perpendicular to the fiber axis. This dual-protection system substantially alleviates material degradation under cyclic thermal water oxidative conditions.

Key words: Mini-SiC_f/BN/SiC, cyclic oxidation, high temperature water oxygen environment, multi-scale representation, integrated oxidation mechanism