Active oxidation of silicon carbide

The oxidation resistance of SiC obviously depends on the high temperature stability of the Si02 layer. The oxidation reaction of SiC may be expressed as Eq. (14.6)  

The Si02 formation (rightward) reaction takes place at a certain Po, which can be calculated from thermodynamic data, e.g. 10 Pa at 1600 K. 

However, the protective Si02 layer (passive oxidation) may not form under such conditions because Si02 (s) is not stable and decomposes into SiO (g) and CO (g), i.e., the decomposition to SiO (g) and CO (g) is more significant than the formation of Si02 (s) even at Po. 10 Pa at 1600 K. Therefore, SiC will be severely degraded by O2 gas, and thus this oxidation mode is called active oxidation .  

If Psio at the Si (s) surface is higher than Si02 (s) will be stable at the Si (s) surface. The Fq is obtained from the Si(s)-SiO(g)-02(g) equilibrium. However, Po. at the Si (s) is significantly different from that in a bulk gas stream due to a gas boundary layer above the Si (s) surface. Therefore, Wagner proposed a model to estimate the Fq by assuming the steady state 

12 Models for the active-to-passive transition (a) Wagner model, 

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