Oxidation behavior of Si-C-O-(Ti) fibers from 450° to 1140°C: Comparing the kinetic of oxide scale growth to the slow crack growth

Abstract

The oxidation behavior of Tyranno® Grade-S SiC-tows was investigated under ambient air in the 450–1140 °C range. Below 650 °C, a linear oxidation rate was identified for oxide scale thicknesses up to 1.2 µm, attributed to the hydroxyl (Si-OH) groups degrading the silica structure as shown by 1H and 1H–29Si CP/MAS NMR. Above this temperature, the classical parabolic oxidation rate (diffusion controlled) was identified with a dehydroxylated silica scale. These results bring new insights to interpret the slow crack growth (SCG) transition observed at 650 °C. Above 650 °C the activation energies (Ea) for oxidation and SCG are close to 50 kJ mol−1 indicating the diffusion might be the rate limiting factor; whereas for lower temperatures EaSCG < Eaoxidation as a consequence of the stress-enhanced chemical reactivity. The oxide thicknesses at failure time (predicted with power or exponential laws) were assessed and highlight some incoherencies in the lifetime projection to lower stresses.