Amorphous alumina thin films (TFs) have attracted great attention in the last decade due to their unconventional properties that are distinct from other oxide glasses. The ion migration mechanism was systematically investigated for surface amorphous alumina TFs on aluminum in comparison with bulk amorphous/crystalline alumina by using reactive molecular dynamics simulations combined with free-energy calculations. The results reveal that the ion migration mechanism within the surface alumina TFs on Al is different from that of bulk alumina due to the incorporation of Al at the interface. We show that incorporating Al into surface alumina TF remarkably decreases the ion migration barriers to around 1 eV by increasing the ion site volume along the migration paths. The calculated ion diffusivity in surface alumina above the Al melting point significantly increases to ${10}^{-8}\sim {10}^{-6}\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$, lowering the temperature threshold for massive ion migration by $\sim 1000$ K. We finally discussed the temperature and structural anisotropy dependence for activating the ion migration in the surface oxide TF. The results reveal the atomistic origin of the enhanced ion migration within surface alumina TF and provide insights into thermal oxidation mechanism of metal/alloy-based energetic materials.

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• Diffusion
• Electronic structure of atoms & molecules
• First-principles calculations
• Phase transitions
• Plasticity
• Thermal properties