In situ tensile behavior of Hi-Nicalon silicon carbide fibers exposed to high-temperature argon plasma

Direct mechanical characterization of silicon carbide fibers in extreme hypersonic aerothermal environment is critical to the development of next-generation inflatable thermal protection systems that could enable delivery of large payloads to planetary surfaces. In this article, we report direct measurements of tensile properties in Hi-Nicalon silicon carbide fibers exposed to high-temperature argon plasma exceeding 1100 degrees C using an in situ mechanical testing system integrated into a 30-kW inductively coupled plasma torch chamber simulating the hypersonic atmospheric entry conditions. As a comparison, ex situ tensile tests were performed on virgin Hi-Nicalon silicon carbide fibers in both ambient air and vacuum conditions. In situ thermal and optical imaging was used to obtain a real-time resolution of the thermo-mechanical events occurring on the fibers during the high-temperature argon plasma exposure. It is found that the high-temperature tensile strength of Hi-Nicalon fiber tows exposed to argon plasma is 0.74 +/- 0.19 GPa, which denotes a 59% reduction from the virgin fiber strength in ambient air (1.81 +/- 0.19 GPa). Fractographic characterization by scanning electron microscopy shows that the substantial degradation of tensile strength in Hi-Nicalon fibers results from a reduction of fiber cross-section due to active surface attack from high-temperature argon plasma.