Crystallization kinetics for SiO₂ formed during SiC fiber oxidation in steam

Abstract

The crystallization kinetics for SiO2 formed by oxidation of Hi‐Nicalon™‐S SiC fiber between 800 and 1600°C in Si(OH)4(g) saturated steam were determined. Glass SiO2 scale always formed first. Glass scale eventually crystallized to cristobalite, and during further oxidation the scale formed directly as cristobalite. Growth stress relaxed by viscous flow in SiO2 that formed as glass. Cristobalite formed by crystallization of this glass was relatively undeformed. In SiO2 that formed directly as cristobalite, growth stress relaxed by intense plastic deformation accompanied by dynamic recrystallization. There were therefore two layers in cristobalite scale: a heavily deformed inner layer and an undeformed outer layer. These layers were distinguished by TEM. SiO2 crystallization times were determined from the thicknesses of undeformed cristobalite and the SiC oxidation kinetics for glass scale formation. SiO2 crystallization kinetics were determined from the crystallization time distributions at different SiC oxidation temperatures in steam. For all temperatures the crystallization time growth exponent (n) was 1. There was a large decrease in crystallization rate between 1000 and 1100°C. Between 800 and 1000°C the activation energy (Q) for crystallization was 65 kJ/mol, between 1100 and 1500°C it was 110 kJ/mol, and at 1600°C it was ~500 kJ/mol. Analysis methods and results are discussed.

Static fatigue of Hi‐Nicalon™‐S fiber at elevated temperature in air, steam, and silicic acid‐saturated steam

“…In some areas these scales were crystalline and highly variable in thickness, with some areas locally over a micron thick (Figure C,D). These types of crystalline scales with erratic thickness were previously observed, and are associated with scale dewetting and high growth rates attributed to long range vapor phase transport of silica as Si(OH) 4 (g) from bare fiber surface to the growing crystalline SiO 2 areas. The observations of active oxidation motivated modification of the test facility to permit testing of fiber tow at elevated temperature in steam saturated with Si(OH) 4 (g) throughout the test section.…”

Oxidation of 3D‐printed SiC in air and steam environments

“…During high‐temperature steam oxidation of SiC, initial silica formation is amorphous 26,29,32 . As oxidation proceeds, the outer portion of the scale crystallizes and subsequent SiO 2 forms as cristobalite 26 .…”

“…The growth stress will depend on when crystallization occurred and also the extent of crystallization. At 1400°–1500°C in steam saturated with Si(OH) 4 , the scale is anticipated to be ~80% cristobalite within the first 100 min of exposure, and for 1500°C, fully crystalline by ~200 min 26 . It is likely that tensile hoop stress cracking does not occur until the scale is primarily crystalline, as viscous glass flow may be sufficient to relax growth stresses in a predominantly amorphous scale but it is also unclear what magnitudes of tensile stresses a fully crystalline scale can withstand prior to cracking and how creep may relax these stresses.…”

Evaluating steam oxidation kinetics of environmental barrier coatings

“…However, quantification of this reduced oxidation is hindered by specimen volatilization; a bare silica former would exhibit paralinear kinetics, 23 while the silica thermally grown oxide (TGO) forming at a coating-substrate interface, protected against volatilization, may exhibit parabolic growth. 24 To effect useful comparison, SiC 25,26 or fused silica 9,10,27,28 reaction tubes can be used to mitigate volatilization in steam by generating a Si(OH) x overpressure. Then, scales formed on an EBC can be directly compared to uncoated substrates, and a measure of reduced oxidation, or effectiveness, can be determined.…”