Bubble Nucleation During Oxidation of SiC

• Published in: Journal of the American Ceramic Society 2015-08, Vol.98 (8), p.2579-2586
• Main Authors: Raj, Rishi, Terauds, Kalvis
• Format: Article
• Language: English
• Subjects: Bubbles; Carbon monoxide; Experiments; Gas pressure; Grain boundaries; Nucleation; Oxidation; Partial pressure; Precipitates; Silicon carbide; Silicon dioxide; Substrates; Superalloys; Viscosity; Viscous flow
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/jace.13613

Oxidation of SiC at 1400°C–1700°C produces carbon monoxide (CO), which can form bubbles at the interface between the substrate and the silica (SiO2) overgrowth. The bubbles add complexity to the mechanistic understanding of the oxidation process. We analyze the kinetics of bubble formation, where the driving force for their nucleation depends on the partial pressure of CO while their growth is determined by the viscosity of silica. The mechanism bears analogy to oxygen migrating from the atmosphere into superalloys, and reacting with carbide precipitates at the grain boundaries to nucleate bubbles of carbon monoxide, or hydrogen migrating into carbon‐steel to form methane. The unique feature of the present case is the ability of silica to release the gas pressure within the bubbles by viscous flow, and then, to heal the crater that is left behind. As the bubbles grow out from the interface and escape into the atmosphere, they can expose bare surface to the atmosphere rendering silicon carbide vulnerable to active oxidation. The relative significance of the mechanisms of oxidation of SiC is presented in the form of an oxidation map, where atmospheric pO2 and oxidation temperature are the experimental variables. We discover that experiments often lie in the regime of bubble formation.