Kinetics of Passive Oxidation of Hi‐Nicalon‐S SiC Fibers in Wet Air: Relationships between SiO 2 Scale Thickness, Crystallization, and Fiber Strength

Abstract

Abstract : The strengths of Hi-NicalonTM-S SiC fibers were measured after oxidation in wet air between 700 deg and 1300 deg C. The oxidation and scale crystallization kinetics were also measured. Thicknesses of amorphous and crystalline scale were measured by TEM. Oxidation initially produces an amorphous scale that starts to crystallize to cristobalite and tridymite in 100 hours at 1000 deg C or in one hour at 1300 deg C. Crystallization kinetics for oxidation in wet air were slightly slower than those for dry air. The activation energy of 249 kJ/mol for parabolic oxidation to uncrystallized SiO2 scale in wet air was indistinguishable from that for dry air oxidation, but the pre-exponential factor was 2x higher. SiC fiber strength changes with oxidation in dry and wet air were very similar. The fiber strength increased by approximately 10% for SiO2 scale thickness up to 100 nm, and decreased for thicker scales. No significant strength degradation was observed for amorphous scales. All fibers with significantly degraded strength had crystallized or partially crystallized scales.