Structure and stability of Fe₃C-cementite surfaces from first principles

We report results of gradient-corrected pseudopotential-based density functional theory calculations on bulk Fe₃C in the cementite structure and its (0 0 1), (1 1 0), (0 1 1), (1 0 0), (1 0 1), (0 1 0), and (1 1 1) surfaces. Bulk properties are in reasonable agreement with available experimental data. The cementite local density of states shows predominantly metallic character, along with some polar covalent bonding contributions (charge transfer from iron to carbon) for both bulk and surfaces. We predict cementite surface energies in the range of 2.0-2.5 J/m², most of which are lower than all pure Fe surface energies. In particular, we predict the Fe₃C (0 0 1) surface to be the most stable and the Fe₃C (1 0 0) surface to be the least stable. We show that greater stability is associated with localized Fe-C bonding at the surface, smoother surfaces created, e.g., by large C atom relaxation into the bulk, and more uniform coordination at the surface. The relatively greater stability of Fe₃C surfaces is suggested to provide the driving force for cementite to form at the surfaces of bcc iron. Implications for the carburization erosion mechanism for steel, such as cracking and melting, are discussed.