Influence of surface hydroxylation on the mechanical properties of iron nanopillars containing different thicknesses of pre-oxide layers

This study employs reactive molecular dynamics (ReaxFF-MD) simulations to examine the impact of surface hydroxylation on the mechanical performance of iron nanopillars with natively controlled oxide layers. Sequential oxidation and hydroxylation leading to core-shell nanostructures consisting of metallic Fe cores coated by amorphous Fe-oxyhydroxide shells with 5–20 % initial oxide fractions are produced. Hydroxylation is found to severely degrade mechanical performance compared to oxide-only systems in uniaxial compression tests. Hydroxylation softens Young's modulus by 10–20 % relative to oxide-only analogs; the softening arises due to partial substitution of strong load-carrying Fe–O bonds by weaker Fe–O–H bonds and increased interfacial amorphization, weakening the effective load-carrying network. Yield strength exhibits a monotonically decreasing correlation with the shell thickness due to flexible hydroxide-rich shells that promote premature dislocation nucleation. Size effects play an extremely dominant role in thinner nanopillars, where high surface-area-to-volume ratios enhance hydroxylation-induced mechanical softening. Hydroxylated shells have inferior load-carrying ability compared to pure oxide films, as indicated by radial distribution function analysis revealing disordered atomic coordination and reduced Fe–Fe metallic bonding density. More importantly, hydroxylation transforms mechanically protective oxide layers into structurally weak interfaces that facilitate plasticity initiation in spite of providing improved chemical passivation. These findings enlighten the intrinsic trade-offs between mechanical strength and chemical passivation in iron-based nanomaterials and offer critical insights into corrosion-resistant design principles and the development of high-performance functional nanocomposites.