The effects of oxidation in air and corrosion in high‐temperature, high‐pressure water on the mechanical properties of three commercially available amorphous Si‐Ti‐C‐O (Tyranno) fibers with different oxygen contents (12%–18%) and diameters (8–11 μm) were investigated. The fibers were exposed to isothermal treatments at elevated temperatures and subsequently tested at room temperature. Structural changes in the fibers after oxidation and corrosion were also studied in order to understand better the degradation mechanisms of the fibers. Oxidation resulted in the formation of vitreous silica films and decreases of strength and Young's modulus of the fibers. Hydrothermal corrosion under 100 MPa water pressure started above 300°C and resulted in the formation of a carbon layer on the surface of the fibers. Dissolution of silica in water during the treatment was observed. Corrosion at temperatures above 400°C led to the formation of relatively thick carbon films which delaminated easily. It caused a decrease of strength and Young's modulus of the fibers. The hydrothermal method can be used for producing carbon coatings with thickness up to 2 μm on the surface of silicon carbide fibers. The degrading of the mechanical properties after oxidation and corrosion was controlled by the thickness of the oxide or carbon layer. Based on this fact, it is possible to predict changes in the mechanical properties from the oxidation data.
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