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

Abstract

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 NaCl 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 NaCl solutions below 1 wt%. However, significant degradation was observed at 5 wt% NaCl 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.

Effects of combustion and salt-fog exposure on fatigue behavior of two ceramic matrix composites and a superalloy

“…Thus, even a small amount of salt affects the fatigue resistance of N720/AS under the combustion fatigue. For the sake of comparison, it should be reiterated that a previous study found that there was no degradation in N720 fiber only from the exposure of salt solution below 1 wt% over the temperature range between 900 and 1150°C [22]. However, significant degradation was observed with 5 wt% salt exposure over this temperature range even in ambient environment [22].…”

“…However, significant degradation was observed with 5 wt% NaCl exposure over the temperature range between 900 and 1150°C, while no degradation was observed upon exposure to 1200°C. The relative contribution of fiber strength loss to composite degradation was estimated to be nearly 50 %, indicating that both fibers and matrix/interface degrade from the exposure to salt water [22].…”

“…The contribution of fiber strength loss on the degradation of the tensile behavior of Nextel TM 720/aluminosilicate composites after exposure to salt-fog (ASTM B117) was investigated in [22]. Single filament tension tests were performed on Nextel TM 720 fibers after immersion in NaCl solutions followed by high-temperature exposure in air.…”

“…Effects of exposure to the salt-fog on CMCs' performance have been also investigated [20][21][22][23][24]. The contribution of fiber strength loss on the degradation of the tensile behavior of Nextel TM 720/aluminosilicate composites after exposure to salt-fog (ASTM B117) was investigated in [22].…”