Fabrication and high-temperature reliability of 2.5D Nextel 720 fiber-reinforced alumina composites

As the operating temperatures of next-generation aerospace engines continue to rise, all-oxide ceramic matrix composites have emerged as indispensable structural materials due to their intrinsic oxidation resistance. To overcome the catastrophic inter-laminar delamination and extreme stress sensitivity inherent to traditional 2D all-oxide laminates under severe thermo-mechanical loadings, 2.5D Nextel 720 fiber-reinforced alumina composites (2.5D N720/Al₂O₃) with a shallow cross-linked interlocking architecture were fabricated via a sol-gel process. The effects of sintering temperature ranging from 1000 °C to 1300 °C on the microstructural evolution, phase transformation, and mechanical properties of the composites were systematically investigated. Results indicated that the optimal sintering temperature was 1200 °C, yielding a maximum flexural strength of 214.49 ± 11.61 MPa, which was attributed to aggressive matrix densification and the completion of the α-Al₂O₃ phase transformation. The 2.5D architecture exhibited significant mechanical anisotropy; the flexural strength in the warp direction (214.49 ± 11.61 MPa) was approximately 2.55 times higher than that in the weft direction (84.06 ± 7.6 MPa), closely correlating with the fiber bundle density ratio. High-temperature tests revealed that the composites maintain excellent mechanical stability up to 1200 °C (252.68 ± 12.84 MPa), followed by a transition to “pseudo-plastic deformation” at 1300 °C and 1400 °C due to matrix softening and high-temperature creep. Tensile creep tests at 1200 °C and 1300 °C obtained stress exponents of 4.19 and 5.55, respectively. These values are notably lower than those reported for 2D laminates, suggesting the superior damage tolerance and global stress redistribution capabilities provided by the 2.5D interlocking architecture. This study provides a comprehensive theoretical foundation for the reliability and application of 2.5D all-oxide CMCs in extreme aerospace environments.