Passive oxidation kinetics for glass and cristobalite formation on Hi‐Nicalon™‐S SiC fibers in steam

Abstract

Parabolic rate constants for SiO2 glass (BG) and cristobalite (BC) scale formation during passive oxidation of SiC in steam were determined. Cristobalite scale that originally formed as glass and as scale that formed afterward, directly as cristobalite, was distinguished by TEM. A method to determine BG and BC from many thickness measurements of the 2 different scale layers was developed. The method was applied to Hi‐Nicalon™‐S SiC fiber oxidation in Si(OH)4 saturated steam between 500 and 1600°C. At 1500°C and lower temperatures, glass scale formed more rapidly than cristobalite scale. BG and BC had activation energies of 80 ± 5 kJ/mol and 95 ± 5 kJ/mol, respectively. At 1600°C, cristobalite scales formed much faster than glass scales. Many scales spheroidized after oxidation at temperatures beneath 1000°C, and continuous scales that did form had wide variation in thickness. This made kinetics analysis after low temperature steam oxidation problematic.

Crystallization kinetics for SiO2 formed during SiC fiber oxidation in steam

“…However, in scale that formed directly as cristobalite, growth stress relaxed by intense plastic deformation of cristobalite, accompanied by dynamic recrystallization. This formed small columnar grains, with a cristobalite SAD ring pattern . Outer cristobalite scale thicknesses ( x g ) that originally formed as glass were distinguished from the younger, heavily plastically deformed inner cristobalite scale thicknesses ( x c ) that formed directly in the crystalline state.…”

“…This is a concern for long‐term CMC environmental durability. Current applications for SiC–SiC CMCs are turbine shrouds for the Boeing 737 Max and the Airbus A320 . These SiC–SiC CMCs use the Hi‐Nicalon™‐S SiC fiber, which has properties that are described in detail elsewhere …”

“…During SiC oxidation SiO 2 crystallization starts between 700 and 1400°C . The wide range in temperature is due to the extreme sensitivity of silica crystallization to impurities, and to H 2 O and other gases in the oxidizing environment .…”

“…Many SiC high temperature structural applications involve steam exposure. The passive oxidation kinetics for Hi‐Nicalon™‐S SiC fibers in Si(OH) 4 saturated steam were recently determined for both glass and cristobalite scale formation . Outer parts of cristobalite scales originally formed as glass, and inner parts directly as cristobalite .…”

“…In glass SiO 2 , the growth stress associated with the 220% volume expansion during SiC oxidation relaxed by glass flow . If the glass eventually crystallized to cristobalite, that cristobalite was relatively undeformed, and had a near single‐crystal selected area electron diffraction (SAD) pattern . However, in scale that formed directly as cristobalite, growth stress relaxed by intense plastic deformation of cristobalite, accompanied by dynamic recrystallization.…”