Sensitivity of SiC Grain Boundaries to Oxidation

Abstract

Molecular dynamics simulations of dry oxidation of bicrystals with incoherent and coherent grain boundaries (GBs) in 3C–SiC are performed at 2000 K and the results are compared to oxidation of single-crystal SiC. Oxidation near incoherent GBs is found faster than that in single crystals and in coherent GBs, whereas oxidation of coherent GBs is comparable to that of single crystals. The accelerated oxidation near incoherent GBs is attributed to strain and the presence of under-coordinated Si within the GB region, both of which reduce the positive charge on silicon atoms, making them more reactive with oxygen. Although atoms with similar properties are found in dislocation cores of coherent GBs, dislocation cores are isolated from each other by crystalline regions, which in turn control the rate of oxidation.

Effects of point defects on oxidation of 3C–SiC

“…By exposing an unirradiated SiC to the supercritical water, Tan et al have found that incoherent GBs are more significantly corroded than the coherent ones [11]. These experimental results are consistent with our recent molecular dynamics (MD) simulations [12]. Specifically, we have previously shown that incoherent tilt GBs oxidize faster than the bulk SiC whereas coherent GBs do not.…”

“…were randomly and uniformly inserted into a 40 Å vacuum region above the SiC surface; the corresponding gas pressure at 2000 K is ~552.5 MPa. Our simulated oxygen partial pressure is significantly higher than pressures used in dry thermal oxidation experiments, which is in the order of tens of kPa [12]. This high pressure used in our simulations is necessary to observe reactions on the time scales of MD simulations.…”

“…We have tested that the choice of the number of atoms (different from three) does not change the qualitative trends reported in this paper. The thickness defined with this method represents the average oxide thickness, as discussed in Ref [12]. In the bicrystal, we report the average oxide thickness within a 10 Å thick region centered at the boundary.…”

“…2. We can see that all samples (with and without defects) show a logarithmic dependence of the oxide thickness on time during the very early stages of oxidation (~100 ps) [12]. During this period oxygen molecules are first adsorbed and dissociated onto the SiC surface, and then the dissociated oxygen atoms react with surface Si atoms to form a silica oxide layer.…”

“…In addition, recent experimental and theoretical investigations have found that grain boundaries (GBs) in SiC are more vulnerable to oxidation [11,12]. By exposing an unirradiated SiC to the supercritical water, Tan et al have found that incoherent GBs are more significantly corroded than the coherent ones [11].…”