Oxidation Kinetics and Its Impact on the Strength of Carbon Short Fiber Reinforced C/SiC Ceramics

Because of the excellent fracture toughness and oxidation resistance, carbon short fiber reinforced ceramics have a sound potential for a variety of high-temperature applications. For the composite's reliable use in long-term applications, however, the oxidation effects on the mechanical strength are of crucial interest and have hitherto not been investigated in detail yet. In this study, the weight change of carbon short fiber reinforced C/SiC composites with carbon fiber lengths of 9mm (long cut fiber composite LCFC) and 2mm (short milled fiber composite SMFC), respectively, are evaluated in the temperature range from 500 to 1200 degrees C for two different fiber orientations. Both types of composites were additionally tested in bending mode after 25 and 50min of exposure to air at 700 degrees C depending on fiber orientation. After 3h of exposure at 1000 degrees C, a total weight loss of up to 37% for LCFC and 47% for SMFC could be determined. The bending test carried out after 50min exposure at 700 degrees C showed a dramatic decrease down to 50% of the initial flexural strength for LCFC samples with fibers orientated in parallel to the beam axis. The smallest strength decrease of around 13% was found for SMFC samples with fibers orientated perpendicularly to the beam axis. Microstructural investigations suggest strongly that oxidation along the fibers is the most dominating weakening mechanism and is therefore directly affected by the fiber orientation and length.