Chapter 31. Effect of Temperature on Tungsten Core SiC Monofilament

Summary

The thermal stability of a tungsten core SiC monofilament fiber, SIGMA™, is studied with special emphasis on the determination of the origin of fiber strength increase under specific heat-treatment conditions. Tensile strength of the SiC fiber is evaluated by room-temperature tensile test for as-received and thermally exposed fiber. Fibers subjected to thermal exposure for 1 h in air and in argon at temperatures >800°C experienced up to a 41% loss of strength. Exposure at 400° and 600°C in air, however, resulted in a 33% and 25% increase in tensile strength, respectively. Additionally, exposure at 600°C in argon resulted in a 47% increase in strength. Heat treatment at 400°-600°C appears to optimize the strength potential of this fiber.

The presence of an SiO₂ film is confirmed through scanning Auger spectroscopy. It is suggested thal he presence of the silica layer formed under the heat treatment conditions of 1 h aposure in air or argon at temperatures between 400° and 600°C functions as a surface-healingfilm by blunting surface flaws. The result is enhanced fiber strength.