Crystallization kinetics for SiO 2 formed during SiC fiber oxidation in steam

The crystallization kinetics for SiO 2 formed by oxidation of Hi‐Nicalon ™ ‐S SiC fiber between 800 and 1600°C in Si(OH) 4 (g) saturated steam were determined. Glass SiO 2 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 SiO 2 that formed as glass. Cristobalite formed by crystallization of this glass was relatively undeformed. In SiO 2 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. SiO 2 crystallization times were determined from the thicknesses of undeformed cristobalite and the SiC oxidation kinetics for glass scale formation. SiO 2 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.