High strength continuous SiC fibers are synthesized from polycarbosilane. The fiber which consists of ultrafine grain crystallites of β-SiC has high tensile strength at high temperatures in air. But, the behavior of the fiber after exposure for long time at high temperatures, has not been investigated yet. Therefore, the changes of strength and structure of the SiC fiber after exposure were studied by measuring tensile strength, analyzing X-ray-diffraction, scanning electron microscopical observation, and chemical element analysis. The SiC fibers were exposed at 800 to 1400°C, for 0.5 to 1000hours, in air or argon gas atmosphere. The strength and structure of the fibers were measured at room temperature. The results obtained are summarized as follows: (1) The SiC fibers maintained the strength higher than 2GPa after exposure at 800°C for 1000 hours, at 1000°C for 100 hours and at 1200°C for one hour in either atmosphere. (2) The strength decreased at temperatures above 1200°C. (3) At 1400°C in argon gas, the decrease of strength results from the crystal growth and the escape of CO gas formed by the interaction of C and O contained in the fibers. (4) At 1400°C in air, the decrease of strength results from the cracks of the SiO2 skin formed around the fiber.
