Accelerated oxidation and microstructure evolution of SiC in the presence of NaF

Abstract Oxidation behaviors of SiC in the presence of some impurities are very important for the performance of SiC used in high temperature environments such as aerospace vehicles and nuclear reactors. In this study, the effects of NaF on the oxidation behaviors of SiC powder with grain sizes from 0.3~2 μm were investigated. NaF with contents of 0.2~10 wt.% was added to the 6H-SiC powder (~ 1 μm) and the mixtures were oxidized at 500~800 °C for 30 min in air atmosphere. Compared with pure SiC, enhanced oxidation was observed with the NaF addition as low as 0.2 wt.%. When the content of NaF was increased to 2.0%, the weight gain ratio of SiC reached 23.0% at the oxidation temperature of 700 °C, which was only 0.6% for pure SiC powder. In the TG analysis, the onset oxidation temperature for the SiC-2.0%NaF sample was significantly decreased to 570 °C. The weight gain was increased with the decrease of SiC grain size. The intensive oxidation was confirmed by SEM, TEM, XRD and XPS characterizations. The formation of free carbon during the oxidation process was demonstrated by XRD and Raman spectroscopy. New polytype of SiC of 15R and defects were formed in the 6H-SiC matrix after oxidation. The results showed that NaF could greatly accelerate the oxidation of SiC and played a very important role in the microstructure evolution of SiC. Possible mechanism for the effects of NaF on the oxidation behaviors of SiC was discussed.