Anomalous high-temperature oxidation behavior of SiC coatings on C_f/SiC Composites: Degradation mechanisms and microstructural evolution from 1200 °C to 1400 °C

SiC coatings on C_f/SiC composites undergo oxidation in high-temperature environments, reacting with O₂ to form SiO₂ layers. The low diffusion rate of O₂ through SiO₂ underscores its role in enhancing oxidation resistance. Pre-oxidation of SiC coated C_f/SiC composites (C_f/SiC–SiC) facilitates the formation of a protective SiO₂ layer. This process significantly enhances the long-term oxidation resistance of the resulting C_f/SiC–SiO₂ composites, with the degree of improvement being critically dependent on pre-oxidation temperature, SiO₂ formation kinetics, oxygen diffusion rates, and other thermodynamic factors. In this investigation, C_f/SiC–SiC composites were subjected to isothermal oxidation at 1200 °C, 1300 °C, and 1400 °C for durations of 24 h, 48 h, 72 h, and 96 h. Results showed that at 1300 °C, the composite exhibited the lowest weight loss rate and optimum oxidation resistance. Specifically, the weight loss rate exhibited a continuous decrease from 1200 °C to 1300 °C, reaching its minimum value at 1300 °C. However, weight loss rate rapidly increased at 1400 °C, leading to a diminished protective performance. This phenomenon can be attributed variations into the density, uniformity, and fluidity of the oxide layer at different temperatures. The microscopic mechanisms underlying the differences were discussed in terms of atomic stress, gas diffusion, and phase transformation. A comprehensive analysis and validation of the findings were conducted using reactive molecular dynamics (MD) simulations and high-temperature oxidation experiments. Additionally, Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), and Fourier Transform Infrared (FTIR) Spectroscopy, were utilized to analyze the microstructural evolution and chemical composition of the oxidized composites.