Structure, bonding, and adhesion at the ZrC(100)/Fe(110) interface from first principles*1

Abstract

We report a pseudopotential-based density functional theory (DFT) investigation of the interface between ZrC and bcc Fe. ZrC is a potential candidate for a protective coating for ferritic steels because of its favorable physical, chemical, and mechanical properties. Here we predict the atomic structure, bonding, and ideal work of adhesion ( W ad ideal ) of the interface formed between the most stable surfaces of ZrC and bcc Fe, namely, the ZrC(1 0 0)/Fe(1 1 0) interface. Bulk properties of ZrC and bcc Fe, calculated for calibration of the DFT approximations involved, are in good agreement with experiment. Further, all the low-index surfaces of ZrC and bcc Fe are observed to retain near ideal bulk termination, as observed experimentally. Stabilities of both ZrC and bcc Fe surfaces follow their respective packing density sequence, i.e., (1 1 0) < (1 1 1) < (1 0 0) for ZrC and (1 1 1) < (1 0 0) < (1 1 0) for bcc Fe. Based on surface energy values, we estimate that the critical stress required for crack propagation in bcc Fe to be at least 30% larger than in ZrC. The ZrC(1 0 0)/Fe(1 1 0) interface is well lattice-matched (≈1.7% strain) producing a fairly smooth interface with little structural relaxation. Bonding at the interface consists of covalent C p–Fe d mixing and some metallic Zr d–Fe d interactions. As the coating grows, decreases in inter-metallic bonding, coupled with increased intraceramic interactions, weaken the interfacial bonding. While a monolayer of ZrC is tightly bound to an Fe substrate ( W ideal =3.05 J/m 2 ), the adhesion decreases upon film thickening, with the asymptotic value of W ad ideal reached for 4 ML of ZrC (∼2.3 J/m 2 ). This interface strength is sufficiently high that it may survive as a coating for steels in extreme environments, though it is much smaller than the minimum energy required for fracture of either ZrC or Fe.