The characterization of the thin oxide film formed over Fe₃C at 300°C

Abstract

The oxide formed over polished iron-carbon alloys at 300°C and in a 100 Torr dry oxygen atmosphere was primarily due to the oxidation of the ferrite phase, but a thin protective oxide film (∼ 150 Å) was formed on the carbide phase. The protective nature was attributed to the existence of a kinetic barrier of CO and CO₂ at the carbide-oxide interface. The initial oxide film formed on the carbide phase comprised many small, randomly oriented crystallites; approximately 70 Å in size, of γ-Fe₂O₃. This film then underwent a grain growth process via a strain-induced grain boundary migration accompanied by a phase transformation from γ-Fe₂O₃ to α-Fe₂O₃. This transformation produced a tensile stress in the oxide film which was later relieved by the generation of cracks along the newly grown grain boundaries. These cracks exposed fresh carbide surface, released the gas barrier at the carbide-oxide interface, and provided rapid diffusional paths for further oxidation. As a result, the oxide film (α-Fe₂O₃) was reduced to Fe₃O₄ and raised ridges appeared at the crack sites. Once the cracks were healed, the film again became protective.