Crystallization kinetics for SiO2 formed during SiC fiber oxidation in steam

Abstract

AbstractThe 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.