High Temperature Passive Oxidation Mechanism of CVD SiC

Abstract

The passive oxidation mechanism of CVD SiC was discussed from experimental results with high-temperature thermogravimetry and thermodynamic analyses. The bubble formation temperature around 1900 K could be too low for an oxygen inward diffusion limited process but conform to a CO outward diffusion limited process. The parabolic rate constant (kp) had weak oxygen partial pressure (PO2) dependence, kp ∝ PO2 n where n = 0.09 to 0.12. These n values may be consistent with the CO outward diffusion limited process. The activation energy of kp obtained in the present study, 210 kJ/mol, could suggest a different mechanism from the well-approved oxygen molecule permeation limited process at lower temperatures below 1600 K. Amorphous phase was significantly contained in SiO2 scales formed in an N2-O2 atmosphere. No effect of the amorphous formation on kp was identified.

High‐temperature oxidation behavior of the SiC layer of TRISO particles in low‐pressure oxygen

“…The presence of similar oxide morphologies on the particles depicted in Figures 1 and 2 agrees with the kinetic analysis. The activation energy of 188 kJ/mol is similar to historically reported values for oxidation controlled by the diffusion of oxygen in amorphous and crystalline SiO 2 27–29 . At the same time, the activation energy calculated here is somewhat higher than those reported by Cao et al.…”

“…The activation energy of 188 kJ/mol is similar to historically reported values for oxidation controlled by the diffusion of oxygen in amorphous and crystalline SiO 2 . [27][28][29] At the same time, the activation energy calculated here is somewhat higher than those reported by Cao et al (146 ± 5 kJ/mol) 21 and Liu et al (145 kJ/mol) 20 following experiments on oxidation of TRISO particles in air for up to 50 h, yielding oxides up to approximately 2 µm thick. Discrepancies in activation energy between this study and past studies may be related to oxidation time or gas flow, as the studies by Cao et al and Liu et al were conducted in stagnant air instead of flowing.…”

“…Discrepancies in activation energy between this study and past studies may be related to oxidation time or gas flow, as the studies by Cao et al and Liu et al were conducted in stagnant air instead of flowing. Prior studies [27][28][29] also reported a transition in oxidation mechanism and its corresponding increase in activation energy between 1300 and 1500 • C, which was associated with a transition from gaseous O 2 permeation in SiO 2 (80-120 kJ/mol) to O 2− ion diffusion (200-400 kJ/mol). No clear change in the mechanism was observed between 1200 and 1600 • C in this work based on oxide morphology or kinetic analysis, although the calculated value of 188 kJ/mol is somewhat lower than the anticipated value for oxidation controlled by the diffusion of O 2− ions in amorphous SiO 2 and the data below the expected transition point is limited.…”

“…Goto et al. reported reduced crystallization behavior when oxidizing CVD SiC in N 2 –O 2 atmospheres compared to Ar–O 2 29 . This may explain why crystallization was suppressed overall, but not why it occurred in the SiO 2 nodules.…”

Deposition Rate, Texture, and Mechanical Properties of SiC Coatings Produced by Chemical Vapor Deposition at Different Temperatures

“…To our knowledge, although much research has been focused on improving the oxidation properties of SiC coatings, not much systematic work has been carried out so far to improve their mechanical properties. For applications, there is a need to determine the mechanical properties to improve the reliability and lifetime performance of these coatings.…”