The growth of carbon coating layers on iron particles and the effect of alloying the iron with silicon

Research highlights: → Dissolution of Si in Fe controls the generation of Fe3C. → Si helps in the decomposition of Fe-C. → And then, the growth of C coating layers was facilitated. → The coatings become thick and homogeneous by Si addition. → The C coating layers result from the precipitation of C from Fe. - Abstract: Magnetic fine particles of metallic Fe coated with graphitic carbon layers were synthesized by annealing Fe2O3 particles with carbon powders at 1673 K in nitrogen atmosphere. For comparison, SiC was added to Fe2O3. X-ray diffraction measurement showed that the lattice constants of Fe changed depending on the Si contents. Moessbauer spectroscopy confirmed that Fe-Si alloys were formed by the Si addition and that the iron carbide disappeared. Electron microscope images revealed that the thickness of carbon coating layers increased from 24 nm to 36 nm as a result of the Si addition. Soaking tests showed that the corrosion resistance of the carbon-coated Fe particles was improved by the addition of Si. The results suggest that Si caused C to leave the Fe cores and move to the surface to form a carbon coating.