Fracture Behavior of Woven Silicon Carbide Fibers Exposed to High‐Temperature Nitrogen and Oxygen Plasmas

Abstract

High‐temperature aero‐thermal heating in a 30 kW inductively coupled plasma torch was used to replicate the effects of harsh oxidizing environments during hypersonic atmospheric entry on fracture behavior and microstructure of two‐dimensional woven SiC fibers. Hi‐Nicalon SiC woven cloths were exposed to surface temperatures over 1400°C with different high‐enthalpy dissociated oxygen and nitrogen plasma flows, and were subsequently deformed in pure tension at room temperature. Changes in fiber microstructure and surface chemistry after thermal exposure were examined by scanning electron microscopy. Pure nitrogen plasmas resulted in a 50% decrease of strength in woven SiC fibers with minimal effects on the fiber structure, except for highly localized surface pitting caused by partial decomposition of silicon oxycarbonitride phase at high temperature. In contrast, exposure to dissociated oxygen and air plasmas led to severe strength reduction and embrittlement over significantly short time scales, corresponding to degradation rates up to 200 times higher than those reported with static heating at equivalent temperatures. The origin of accelerated embrittlement at microscopic scale was found related to complex gas‐surface interactions and high‐temperature oxidizing processes involving the formation of SiO2 bubbles and microcracks on the surface. These findings are important for the development of outer fabric materials for new flexible thermal protection systems in space applications.