Electrical resistivity monitoring of a SiC/[Si-B-C] composite under oxidizing environments

The introduction of Ceramic Matrix Composites parts in civil aeronautics requires a thorough understanding of their evolution under the oxidizing environments present within the engines. In this respect, a SiCf/PyC/[Si-B-C]m material has been tested in fatigue at 450 °C and 100 MPa (which is a typical stress during take-off) under two types of environmental conditions: ambient air, and moist air with an imposed water pressure of 10 kPa. Static fatigue and cyclic fatigue at a frequency of 1 Hz were both performed with these two conditions. As expected, the additional presence of moisture contributes to increase the degradation of the mechanical properties of the material, leading to shorter lifetimes and higher increases in electrical resistivity. It is shown that the pyrocarbon interphases are the main electrical conductors in this material: the electrical resistance can therefore be an accurate indicator of the damage state of these interphases, which are sensitive to the oxidizing environment. The global resistance increase presents two distinct phases of evolution in the four tests performed, with a transition around 35-40% of the time to failure. A model is proposed to account for this global resistance change, which proves to be in good agreement with experimental results. Moreover, the evolution of the electrical resistivity during the interposed unload-reload cycles can give key information about the state of the fiber/matrix interfaces which are critical for mechanical properties. Finally, electrical resistance monitoring seems to provide information on the damage state of the material complementary to acoustic emission results, allowing an unprecedented assessment of the evolution of the interphases state during ageing under oxidizing environments.

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