Strain-modulated early stage oxidation of Fe films
Summary
ReaxFF Fe/O/H molecular dynamics in LAMMPS simulates early oxidation of biaxially prestrained Fe(100), Fe(110), and Fe(111) films exposed to high O₂ pressure in a vacuum gap. Nosé–Hoover NPT equilibration, strain application, and 500 ps production at 1 fs with QEq each step bracket the oxidation kinetics reported below.
Summary¶
Early-stage metal oxidation couples oxygen flux, surface orientation, and mechanical strain in thin films. Wu et al. study bcc iron slabs with ReaxFF Fe/O/H parameters in LAMMPS, applying biaxial prestrain to Fe(100), Fe(110), and Fe(111) films before exposing surfaces to molecular oxygen at elevated gas-phase pressure relative to ambient. Charge equilibration each timestep captures polar bond evolution during oxidation. The study targets strain-engineered oxidation differences relevant to films grown on mismatched substrates where residual stress is unavoidable.
Methods¶
Engine / code: LAMMPS with ReaxFF Fe/O/H (as in J. Appl. Phys. 125, 245305). System: bcc Fe slabs; simulation cells are approximately 30 × 30 × 30 Å body-centered-cubic iron slabs with periodic boundaries in-plane and a vacuum gap holding on the order of 21–23 O₂ molecules corresponding to roughly 10 MPa equivalent ideal-gas pressure in the article’s estimate. Preparation equilibrates iron at 600 K for 30 ps using Nosé–Hoover NPT dynamics, then ramps biaxial prestrain to tensile limits near 8%, 7%, and 8% or compressive limits near −3%, −2.6%, and −1.5% for (100), (110), and (111) surfaces respectively, matching the values quoted in Section II. Production oxidation runs 500 ps with a 1 fs timestep and QEq every step, with an O₂ replenishment heuristic tied to oxygen charges near the surface. Oxide phase identification and diffusivity calculations follow post-processing definitions in the article. Full reproducibility requires the exact ReaxFF file, strain tensors, thermostat damping parameters, and boundary conditions from J. Appl. Phys. 125, 245305, DOI 10.1063/1.5094966.
Electric field: N/A — not used. Replica / metadynamics / umbrella: N/A — not reported. NPT during production oxidation: the excerpt above uses NVT-like production with O₂ gas—confirm against the PDF whether NPT applies to the slab+gas cell during oxidation; if strictly NVT/NVE, then NPT barostat is N/A for that stage per the article.
Findings¶
Oxygen occupies tetrahedral sites during oxidation with body-centered-cubic to face-centered-cubic transformations, especially on (100). Prestrain changes the relative oxidation rates among facets and steers ordered versus disordered oxide evolution. Fe/O diffusivities at 1:1 stoichiometry are very low in the modeled regime, suggesting FeO can slow further oxygen ingress. Together, the results link elastic loading to early oxidation microstructure. The facet-dependent ordering suggests that texture engineering could steer protective versus porous oxide morphologies even before long-term corrosion enters the picture. Corrosion modelers should import both the strain tensor and the facet distribution from experimental texture data when attempting quantitative match, because the paper shows strong orientation sensitivity even under identical oxygen pressure. The high O₂ pressure used to accelerate oxidation should be interpreted as a computational convenience analogous to elevated temperature in other rare-event studies: it reveals pathways that may also appear at ambient pressure over longer times, but relative rates may shift. Finally, QEq every timestep is computationally expensive; reproduction budgets should account for that cost when scaling to larger slabs.
Limitations¶
High O₂ pressure accelerates reactions beyond ambient corrosion timescales; thin-film strain states are idealized.
Relevance to group¶
ReaxFF iron oxidation benchmark with mechanical loading coupling.
Citations and evidence anchors¶
DOI: 10.1063/1.5094966