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.

SiC fiber strength after low pO2 oxidation

“…SiC must endure high‐temperature exposure to high partial pressures of oxygen, water, and other combustion products for long times in these turbine engine applications. Hypersonic aerospace and spacecraft structures are another area with extensive basic and applied SiC–SiC CMC development . Hypersonic and spacecraft applications may expose structural material to low oxygen partial pressures (pO 2 ) and very high temperatures, but exposure times are generally much shorter than those experienced by turbine engine materials.…”

“…SiC fiber strength is a critical parameter that defines many SiC–SiC CMC mechanical properties . Passive and active oxidation changes SiC fiber strength in various ways, depending on oxidation temperature, pO 2 , pH 2 O, flow rate, and whether the SiO 2 scale is crystalline or glass (R. S. Hay, P. Mogilevsky, submitted) . Severe strength degradation is common after passive oxidation with formation of a crystalline scale .…”

“…Hypersonic aerospace and spacecraft structures are another area with extensive basic and applied SiC-SiC CMC development. [3][4][5][6][7][8][9][10] Hypersonic and spacecraft applications may expose structural material to low oxygen partial pressures (pO 2 ) and very high temperatures, but exposure times are generally much shorter than those experienced by turbine engine materials. SiC fibers are also be exposed to low pO 2 at high temperatures in nuclear power applications, 11,12 and during CMC processing.…”

“…13 Passive and active oxidation changes SiC fiber strength in various ways, depending on oxidation temperature, pO 2 , pH 2 O, flow rate, and whether the SiO 2 scale is crystalline or glass (R. S. Hay, P. Mogilevsky, submitted). 9,[14][15][16][17][18][19][20][21][22] Severe strength degradation is common after passive oxidation with formation of a crystalline scale. 14,23 Severe degradation is also observed after active oxidation, [18][19][20]22,[23][24][25][26] which is a concern for SiC-based CMCs used in hypersonic or nuclear applications.…”

The Influence of Spherical Nano-SiO2 Content on the Thermal Protection Performance of Thermal Insulation Ablation Resistant Coated Fabrics

“…Therefore, these types of fabrics offer excellent thermal protection in a high temperature fire environment, but there has been more focus on their ablation resistance and other aspects of thermal stability performance. At present, there are very few mechanisms and experimental studies when combining ablation resistance with other demands of fire protection [4,5].…”