Degradation mechanism of near stoichiometric SiC fibers after air and Ar–H₂O–O₂corrosion at 1000–1500°C

Abstract

The near stoichiometric SiC fiber has been reported to play significant roles in the application of aeroengine field. An in‐depth understanding on the degradation mechanism of the fiber during its corrosion in air and under a simulated aeroengine environment (PH2O:PO2:PAr = 14:8:78 kPa) will shine a light on the performance evaluations of the near stoichiometric SiC fiber‐based materials as well as the development of their potential applications. In this study, X‐ray diffraction, scanning electron microscope, and FIB‐TEM were utilized to analyze the mechanical properties and microstructure of the fiber. After oxidation in dry air and Ar–H2O–O2 for 1 h, respectively, the fiber strength retention rate has been found to decrease with the increased oxidation temperature. The raise in oxidation temperature also led to the increase of the thickness and the crystallization rate of the oxide scale. The most different oxidation behaviors of SiC being treated under the simulated environment than in air are the lower oxidation activation energy and the higher crystallization activation energy for cristobalite. Water vapor can promote the oxidation reaction and inhibit the crystallization of cristobalite in the oxide scale. Few significant differences have been observed otherwise in the oxidation process and oxidation chromatography crystallization mechanism of fibers being treated under different conditions. The increase of oxide layer thickness and the formation of cristobalite impair the structural integrity and compactness of the oxide scale and thus lead to the deterioration of the mechanical properties of SiC fibers. Therefore, it is proposed that oxidation resistance of SiC fiber can be improved by insulating the reaction between the oxidizing agents and the SiC fiber or by increasing the crystallization temperature of cristobalite in the oxidation process and reducing the crystallization rate.

Oxidation under stress of SiC‐based fibers at intermediate temperature and ambient air

“…It is, however, not expected for temperatures below 1000°C (below the creep initiation 49 ), even for the first generation SiCO fibers 27,28 . The strength loss is hence rather explained by the scale growth and its crystallization generating longitudinal and hoop stresses 20,44,50 …”

“…27,28 The strength loss is hence rather explained by the scale growth and its crystallization generating longitudinal and hoop stresses. 20,44,50 This embrittlement is empirically related to the inverse square root of the scale thickness 21 (Equation ( 6)), where the constant C can be approximated by K IC /(Y π 1/2 ). As mentioned in the introduction, several authors associated this expression to the fracture mechanics law Equation ( 1) with x = a.…”