Combined Effect of Salt Water and High‐Temperature Exposure on the Strength Retention of Nextel™720 Fibers and Nextel™720‐Aluminosilicate Composites

The relative contribution of fiber strength loss to reported degradation in the mechanical behavior of Nextel™720-aluminosilicate composites after exposure to salt fog (ASTM B117) was explored. Single filament tension tests were performed on Nextel™720 (3M, Inc., Minneapolis, MN) fibers after immersion in NaCI solutions followed by high-temperature exposure in air. The results were compared with the behavior of control specimens which received high-temperature exposure but were not immersed in NaCl solution. There was no degradation in fiber strengths for NaCI solutions below 1 wt%. However, significant degradation was observed at 5 wt% NaCI upon exposure to temperatures between 900° and 1150°C, while no degradation was observed upon an exposure to 1200°C. The relative contribution of fiber strength loss to composite degradation was estimated as nearly 50%, indicating that both fibers and matrix/ interface degrade from exposure to salt water. X-ray diffraction and transmission electron microscopy of the exposed fibers and composites were conducted to help rationalize the observations. Microstructure of degraded fibers showed presence of Na at grain boundaries near the surface, without any evidence of a crystalline phase, indicating weakening from segregation or formation of an amorphous phase. The degraded composites showed that matrix and fiber/matrix interfaces had Na rich regions/phases.