A theoretical and experimental approach to the active-to-passive transition in the oxidation of silicon carbide - Experiments at high temperatures and low total pressures

摘要

Active-to-passive oxidation transition in chemical vapour deposited beta-SiC was investigated in the temperature range 1300 less than or equal to T less than or equal to 1700 degrees C under low total pressures (100 less than or equal to P-tot less than or equal to 800 Pa) and relatively high linear gas flow rates (10 less than or equal to Vgas less than or equal to 60 ms?(-1) by thermogravimetric analysis. For given T, P-tot and the oxygen partial pressure at the transition, P-02f(t) corresponds to the value where the mass-loss rate per unit area of the oxidized sample, R, is maximum. Logarithms of P-02(t) are linear functions of reciprocal temperature for given and P-tot, and V-gas. V-gas a significant influence on the position of the transition log(P-02(t))-T-1 line. P-02(t) is also slightly affected by an increase of from 100 Pa to 800 Pa. In passive oxidation at high P-tot temperatures (> 1500 degrees C), large bubbles form in the silica film which is then disrupted leading to a loss of material. In active oxidation, R significantly depends on V-gas: the kinetics is diffusion or mass transfer controlled under the conditions investigated in the present study. In both active and passive oxidation regimes, a mass loss of the test specimen is always observed; an explanation is proposed. [References: 17]