Molecular dynamics simulation study of nanoscale passive oxide growth on Ni-Al alloy surfaces at low temperatures

Abstract

Oxidation kinetics of Ni-Al ͑100͒ alloy surface is investigated at low temperatures ͑300-600 K͒ and at different gas pressures using molecular dynamics ͑MD͒ simulations with dynamic charge transfer between atoms. Monte Carlo simulations employing the bond order simulation model are used to generate the surface segregated minimum energy initial alloy configurations for use in the MD simulations. In the simulated temperature-pressure-composition regime for Ni-Al alloys, we find that the oxide growth curves follow a logarithmic law beyond an initial transient regime. The oxidation rates for Ni-Al alloys were found to decrease with increasing Ni composition. Structure and dynamical correlations in the metal/oxide/gas environments are used to gain insights into the evolution and morphology of the growing oxide film. Oxidation of Ni-Al alloys is characterized by the absence of Ni-O bond formation. Oxide films formed on the various simulated metal surfaces are amorphous in nature and have a limiting thickness ranging from ϳ1.7 nm for pure Al to 1.1 nm for 15% Ni-Al surfaces. Oxide scale analysis indicates significant charge transfer as well as variation in the morphology and structure of the oxide film formed on pure Al and 5% Ni-Al alloy. For oxide scales thicker than 1 nm, the oxide structure in case of pure Al exhibits a mixed tetrahedral ͑AlO 4 ϳ 37%͒ and octahedral ͑AlO 6 ϳ 19%͒ environment, whereas the oxide scale on Ni-Al alloy surface is almost entirely composed of tetrahedral environment ͑AlO 4 ϳ 60%͒ with very little AlO 6 ͑Ͻ1%͒. The oxide growth kinetic curves are fitted to Arrhenius-type plots to get an estimate of the activation energy barriers for metal oxidation. The activation energy barrier for oxidation on pure Al was found to be 0.3 eV lower than that on 5% Ni-Al surface. Atomistic observations as well as calculated dynamical correlation functions indicate a layer by layer growth on pure Al, whereas a transition from an initial island growth mode ͑Ͻ75 ps͒ to a layer by layer mode ͑Ͼ100 ps͒ occurs in case of 5% Ni-Al alloy. The oxide growth on both pure Al and Ni-Al alloy surfaces occurs by inward anion and outward cation diffusions. The cation diffusion in both the cases is similar, whereas the anion diffusion in case of 5% Ni-Al is 25% lower than pure Al, thereby resulting in reduced self-limiting thickness of oxide scale on the alloy surface. The simulation findings agree well with previously reported experimental observations of oxidation on Ni-Al alloy surface.

Evolutionary Optimization of a Charge Transfer Ionic Potential Model for Ta/Ta-Oxide Heterointerfaces

“…= 6.25!MV/cm in our calculations here. This is similar to values used by Sankaranarayan et al's prior MD studies on oxidation of Zr and Al [25,26,27]. Using an estimated charge (q) of 4.65 ± 0.18!…”

“…The optimized parameters are used to investigate the initial stages of oxidation of metallic Ta by employing a similar approach used by Sankaranarayanan et al [25,26,27]. The LAMMPS MD package [40] is used to perform all the simulations in this work.…”

“…To model such a scenario, one requires a potential model such as charge transfer ionic potential (CTIP) that seamlessly allows for switching between an environment dominated by ionic interactions in the oxide and metallic interactions in the metal region. This formalism has been successfully used in the past to investigate oxidation of several metal and alloy surfaces and nanoparticles [25,26,27,28]. To determine the force-field parameters for complex functional forms like CTIP, we require a systematic approach that can enable efficient sampling of most of the available parameter space.…”

LIPSS on thin metallic films: New insights from multiplicity of laser-excited electromagnetic modes and efficiency of metal oxidation

“…In the hot lines of the absorption maxima, oxidation is much more efficient compared to the lines of the absorption minima due to a non-linear nature of the oxidation process [38]. The metal cations diffuse through the growing oxide layer to the sample surface while the oxygen anions diffuse in the opposite direction [35,36], thus enhancing oxidation ( Fig. 5(b)).…”

<title>Formation of microstructure on liquid metal surface under nanosecond laser ablation</title>

“…Here we discuss the microstructure growth through the surface oxidation, assuming that the proposed scenario is also valid for other reactive atmospheres which can create a rigid skeleton on the liquid metal surface. The oxidation process proceeds through the diffusion of metal cations through the growing oxide layer to the sample surface while the oxygen anions diffuse in the opposite direction 40,41 . This process is characterized by the Arrenius type behavior, implying that the higher the temperature, the faster oxidation proceeds.…”