RELATIONSHIPS BETWEEN FIBER STRENGTH .PASSIVE OXIDATION AND SCALE CRYSTALLIZATION KINETICS OF HI-NICALON tm-S SIC FIBERS

摘要

The strengths of Hi-Nicalon™-S SiC fibers were measured after oxidation in dry air between 700° and 1400°C. The oxidation and scale crystallization kinetics were also measured Scale thickness was measured by TEM so that amorphous scale could be clearly distinguished from crystalline scale Oxidation initially produces an amorphous scale that has significant carbon. These scales begin to crystallize to cristobalite and tridymite in 100 hours at 1000°C or in one hour at 1300°C Crystallization nucleates at the scale surface with rapid growth parallel to the surface relative to through thickness. The activation energy for parabolic oxidation for uncrystallized SiO_2 scale was 248 kJ/mol. The fiber strength increased by approximately 10% for SiO_2 scale thickness up to ～100 nm and decreased for thicker scales. The onset of strength degradation was stronglv correlated with crystallization of the scale. Possible mechanisms for strength increases and decreases with scale thickness are discussed.