Oxidation behavior and thermal stability of Si-C-O fibers (Nicalon) in carbon dioxide

Abstract

The oxidation behavior and thermal stability of Si-C-O fibers (Nicalon) in CO₂ gas were examined at 1273 to 1773 K through mass change determination, XRD analysis, resistivity measurement, SEM observation and tensile test. Large mass gains and SiO₂ film formation show that Nicalon fibers are passively oxidized in CO₂, as well as Si-C fibers (Hi-Nicalon). The grain growth of β-SiC, the drop of resistivity and the crystallization of SiO₂ film were observed above 1573 K. On being oxidized at high temperature, the thermal decomposition of SiC _X O _Y and the formation of imperfections in SiO₂ film caused the degradation of the room-temperature tensile strength of both the as-oxidized fibers and unoxidized cores. It became evident that the oxygen partial pressure for the active-to-passive oxidation transition of Nicalon fibers is between 1.00 × 10² and 1.78 × 10² Pa at 1773 K.