Oxidation embrittlement of SiC fibers at intermediate temperatures

Abstract

SiC-based fibers are key constituents for the reinforcement of SiC/SiC thermo-structural composites. Their tensile strength is however known to decrease if oxidized because of stresses associated to the oxide scale growth, which in turns is affected by the fiber type (chemical composition, microstructure). This phenomenon is here reported for several types of fibers (five in total, Nicalon® or Tyranno®) between 450 and 950 °C in ambient air. The highest strength loss rate was noticed on Tyranno® Grade S, also known to be more sensitive to oxidation. The initial strength was nevertheless recovered when the oxide scale was removed. This report uses a simplistic empirical model to fit the embrittlement phenomenon, relating the tensile strength to x−1/2. The strength distribution and the projection to larger scale thicknesses are discussed. Assuming the same conditions (environment, scale crystallization rate), the oxidation kinetic could be used to estimate the embrittlement phenomenon.