Effect of Thermal Exposures on the Strengths of Nextel™ 550 and 720 Filaments

The effects of thermal exposure on the strengths of Nextel™ 550 and 720 tows, bare and coated with carbon, were determined by room-temperature tensile testing of single filaments extracted from tows that had been exposed to different thermal environments (i.e., air or vacuum) at temperatures from 550° to 1400°C. The results help define the allowable composite processing conditions when using these tows. A 28% drop in the strength of Nextel 550 filaments occurred after a thermal exposure at 1100°C for 2 h in air. After an exposure of 1300°C/2 h/air, a strength degradation of ∼47% resulted. Filaments exposed above 1100°C under vacuum showed more severe strength degradation than filaments exposed in air. The observed strength degradation may stem from a combination of phase transformations of the alumina, the onset of mullite crystallization, and/or exaggerated mullite grain growth. Strength after heat treatment under vacuum at 1050° and 1150°C did not deteriorate as rapidly as after heat treatment under vacuum between 950° and 1050°C or between 1150° and 1250°C. This may be a result of the competition between healing of flaws by the amorphous silica and its evaporation (leading to an increase in its viscosity or loss) and/or densification of the filaments. Nextel 720 filaments exhibited about 9% strength loss after an exposure at 1100°C/2 h/air. The filaments maintained 75% of their strength after a 1300°C/2 h/air heat treatment. The observed strength degradation may stem from thermal grooving, grain growth, and/or annealing of the mullite subgrain boundaries. Thermal exposure of >10 h at 1300°C was required to produce measurable grain growth. Strength loss between 1200° and 1300°C (air heat treatment) was not as great as between 1100° and 1200°C or 1300° and 1400°C.