Effects of thermal oxidation on electromagnetic interference shielding properties of SiCf/SiC composites

SiCf/SiC composites containing PyC interphase were prepared by chemical vapor infiltration and their properties were investigated for electromagnetic interference (EMI) shielding applications in the frequency of 8.2–12.4 GHz. The room temperature direct current electrical conductivity (σdc) and EMI shielding effectiveness (SE) were tested after thermal oxidation for various time at 900 °C. It was found that the EMI was shielded mainly by absorption for both as-received and oxidized SiCf/SiC composites. Both σdc and SE of SiCf/SiC composites decrease after thermal oxidation at 900 °C. It was proposed that the free carbon, including PyC interphase and free carbon in matrix and fibers, mainly determines the EMI SE of composites. The decrease of σdc and SE of composites after oxidation are attributed to the consumption of free carbon. The SE of SiCf/SiC composites with a thickness of 2 mm remains above 15 dB after thermal oxidation for 4 h in air at 900 °C.