Experimental study and molecular dynamics simulation of oxidation mechanisms of direct-sintered silicon carbide

Abstract

Oxidation tests of direct-sintered SiC were conducted in tube furnace in static air at 1200 ℃, 1300 ℃ and 1400 ℃ for 1 h, 5 h, 12 h and in Thermogravimetric Analyzer (TGA) at the same temperature for 24 h to obtain continuous mass change curves. Grazing incidence X-ray diffraction (GIXRD), field emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS) were used to characterize the oxidation products and to reveal the underlying mechanisms. And the ReaxFF Reactive Molecular Dynamics (ReaxFF MD) simulation was conducted in open-source LAMMPS code to study the oxidation behaviors of 6H-SiC. The results of Oxidation tests show that the oxidation of direct-sintered SiC obeys parabolic law, indicating the oxidation process is controlled by the diffusion of O2. Besides, it takes on a 3-stages oxidation kinetics. Morphology of oxide layer initially shows a transition from amorphous SiO2 to spherulitic features accompanied by a decreasing oxidation rate. After a long-time oxidation, spherulitic features are transformed to fine grain structure along with an increasing oxidation rate. The transition of SiO2 structure and the variation in oxidation rate are probably associated with the specific diffusion mode of O2 in the oxide layer. In combination with ReaxFF MD simulation, the oxidation mechanism of 6H-SiC is obtained. It reveals that O2 diffusion inwards controls the oxidation reaction of 6H-SiC along with the formation of C element, followed by oxidation into CO and CO2 and escapes in the form of bubbles.

Keywords

• silicon carbide
• passive oxidation
• oxidation mechanism
• oxygen diffusion
• molecule dynamics