Investigation on the Degradation Mechanisms and Mechanical Property Evolution of 3D-SiO2-Fiber-Reinforced Polyimide Composites After Thermal Aging

Understanding the thermal aging behavior of SiO2f/PI composites is crucial for ensuring their reliability and longevity in high-temperature aerospace applications. In this study, the thermal aging behavior of three-dimensional quartz-fiber-reinforced polyimide (3D-SiO2f/PI) composites was investigated. The composites were fabricated through a modified prepreg compression molding (PCM) method, including a vacuum-assisted impregnation process followed by thermal compression molding. Long-term thermal aging was carried out to evaluate the high-temperature aging resistance. The flexural properties were tested with a three-point bending test method. The micromechanical characterization was performed though nanoindentation. Interfacial properties were evaluated through single-fiber push-in tests, and the interfacial shear strength (IFSS) was calculated. The thermal aging experiment results revealed that the composites maintained excellent long-term thermal stability. The PI matrix showed no detectable oxidation or thermal degradation of after 100 h aging at 350 °C, yet no significant porosity or cracks were observed for the composite. However, further prolonged thermal exposure led to significant mechanical property degradation. The flexural strength retention rates dropped to below 15% after 300 h at 350 °C or 100 h at 400 °C aging. On the other hand, 350 °C isothermal aging for 300 h led to a progressive decrease in IFSS from 128.6 MPa to near-zero. The degradation mechanism was primarily attributed to two factors: increased porosity due to thermal decomposition of the PI matrix and significant deterioration of fiber–matrix interfacial bonding strength. These findings provide critical insights into the durability prediction and failure mechanisms of 3D-SiO2f/PI composites in high-temperature usage environments.