Growth Stress in SiO2 during Oxidation of SiC Fibers (Preprint)

Abstract

Abstract : A method to calculate the three principal growth stresses in SiO2 scales formed during SiC fiber oxidation has been developed. The method assumes that during oxidation the initial volume expansion at the SiC-SiO2 interface is three-dimensional and equal in all directions, and that subsequent SiO2 shear stress relaxation is described by the stress-dependent Eyring viscosity model. Large compressive stresses of 10 GPa in SiO2 adjacent to the SiC-SiO2 interface are relaxed to much lower levels at all temperatures. At 1200 deg.- 1300 deg. C viscous flow of amorphous SiO2 further relaxes stress to negligible levels. At 700 deg. - 900 deg. C, axial and hoop stress at the GPa level persist. Radial stresses only reach values greater than 100 MPa at 700 deg. - 900 deg. C for scales thicker than0.1 fiber radii. Radial expansion of the scale eventually causes hoop stress and later axial stress to become tensile in the outer scale. Differences in stress-states developed for crystallized and uncrystallized scales are considered. Some tentative calculations for crystalline SiO2 scales are compared with experimental evidence for stress in the crystalline SiO2 scales of Hi-Nicalon(trademark)M-S SiC fibers.