HIGH-TEMPERATURE OXIDATION OF CVD BETA-SIC .1. EXPERIMENTAL-STUDY

The oxidation kinetics of beta-SiC prepared by chemical vapor depositi on (CVD) in dry oxygen (3 x 10(-2)-80 atm.) was investigated over the temperature range 1200 to 1550-degrees-C. In situ X-ray diffraction wa s used to measure the silica layer thickness at P(O2) less-than-or-equ al-to 1 atm. At higher pressures, profilometry, interferometry and sca nning electron microscopy were employed. The silica growth was parabol ic after an initial linear step. The parabolic oxidation rate constant s (K(p)) varied with the oxygen partial pressure indicating that oxyge n diffusion controlled the oxidation process. Two different apparent a ctivation energies were found. at low temperatures (T less-than-or-equ al-to 1350-degrees-C) and high pressures (P(O2) > 1 atm) close to 1 eV /at, a value which corresponds to the molecular oxygen permeation thro ugh amorphous SiO2: at high temperatures (T greater-than-or-equal-to 1 350-degrees-C) and low pressures (P(O2) less-than-or-equal-to 1 atm) t he partial crystallization of the silica layer must be taken into acco unt to interpret correctly the value of the apparent activation energy (2-3 eV/at).