Oxidation Transitions for SiC Part I. Active‐to‐Passive Transitions

Abstract

Oxidation of SiC can occur in a passive mode where a protective film is generated or in an active mode where a volatile suboxide is generated and can lead to rapid material consumption. The transition between these two modes of oxidation is a critical issue. Evidence indicates that this transition occurs via a different mechanism for the active‐to‐passive transition as compared with that of the passive‐to‐active transition. In Part I of this article, the former (active‐to‐passive mode) is explored. Three different types of SiC are examined: Si‐rich SiC, stoichiometric SiC, and C‐rich SiC. Evidence suggests that the SiO2/SiC equilibrium requirements as well as formation of SiO(g) at the SiC surface and subsequent oxidation to SiO2(s) are critical issues in the active‐to‐passive transition.

Microstructure evolution of SiC-ZrB2-ZrC coating on C/C composites at 1773 K under different oxygen partial pressures

“…5(c)). The similar appearance has also been observed by some other researchers [26,27]. The formation of the SiO 2 rods might be ascribed to the generation of SiO and its subsequent oxidation on the glass film.…”

“…It is believed that the formation of SiO 2 rods is an intermediate step of forming protective film. As the density of rods becomes higher, a dense and protective SiO 2 scale forms [26]. The present work suggests that compared with the pure SiC coating, the introduction of ZrB 2 and ZrC is beneficial to forming a protective SiO 2 -rich glass film at lower oxygen partial pressure and shorter oxidation time, which might result in the better oxidation resistance.…”

Formation mechanism of Si3N4 in reaction-bonded Si3N4-SiC composites

“…The phase stability diagram of the system at 1723 K is shown in SiO 2 (s), SiO(g) and O 2 (g) are at a balanced state [16,17]. The change of p(O 2 ) and p(SiO) in different sections of blocks is shown in Fig.…”

Oxidation behavior of ZrB2/SiC laminates: Effect of composition on microstructure and mechanical strength

“…The conditions of temperature and oxygen partial pressure that cause the passive to active transition at the SiO 2 -SiC interface have been reported in the literature. 41,[47][48][49][50][51] When oxidation occurs in air or under oxygen/inert mixture at 1600 • C the passive to active transition is believed to happen if the oxygen partial pressure becomes lower than 10 −1 atm. [47][48][49] In these particular environmental conditions several reactions of SiC active oxidation can occur:…”