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Molecular Dynamics Simulations of Perfluoropolyether Lubricant Degradation in the Presence of Oxygen, Water, and Oxide Nanoparticles using a ReaxFF Reactive Force Field

Summary

This slug registers an alternate PDF (Lotfi_Lubricant_surfaces_JPC_C_2018.pdf) for the same J. Phys. Chem. C article as [[2018roghayyeh-lotfi-j-phys-chem-molecular-dynamics]], DOI 10.1021/acs.jpcc.7b09660, volume 122, pages 2684–2695 (2018). The work addresses perfluoropolyether (PFPE) D4OH lubricant degradation in hard disk drive-relevant environments using ReaxFF reactive MD. Simulations place nine D4OH strands with gas-phase O₂ and H₂O and with oxide nanoparticles (SiO₂, goethite FeO(OH), Fe₂O₃) at T = 1500 K to accelerate bond-making and bond-breaking. Nanoparticles appear in three surface treatments: (1) untreated fragments cut from crystals; (2) dry-air pretreatment; (3) wet-air pretreatment—intended to mimic realistic contamination and humidity. Motivation ties to heat-assisted magnetic recording (HAMR), where elevated temperatures and contaminants can accelerate lubricant loss at the head–disk interface. The introduction in the article reviews PFPE chemistries (Fomblin/Demnum families), places D4OH in the Demnum class with specified end groups, and frames degradation modes relevant to HDD reliability (thermal, catalytic, mechanical shear, and electron-mediated pathways) before presenting the 1500 K accelerated MD strategy.

Methods

1 — MD application (PFPE degradation). ReaxFF simulations of nine Demnum-class D4OH strands are run with O₂, H₂O, and oxide nanoparticles (SiO₂, goethite FeO(OH), Fe₂O₃) using ADF/LAMMPS-class ReaxFF integration as described in J. Phys. Chem. C (package name appears as ADF in the abstract/methods excerpt: “All MD simulations were performed with the ADF package”). Ensemble: NVT at 1500 K for the accelerated degradation chemistry. Timestep: 0.1 fs (selected timestep in the article). Thermostat: Berendsen with 250 fs damping constant as quoted. Nanoparticles: 16 Å diameter silica example size appears in the methods discussion; surfaces are prepared untreated, dry-air pretreated, or wet-air pretreated. Trajectory lengths: representative segments include 500 000 steps (~50 ps) and 1 000 000 steps (~100 ps) at 1500 K for specific degradation stages quoted in the paper. PBC: three-dimensional PBC for gas+NP+strand supercells. Barostat / pressure: N/A — NVT gas-phase cells without NPT barostat in the excerpted protocol. Electric fields / enhanced sampling: N/A — not used.

2 — Force-field training. N/A — applies a published C/F/O/H (PFPE + oxide) ReaxFF parametrization.

3 — Experiments / continuum. N/A — motivation references HDD tribology literature rather than new experiments here.

Duplicate PDF note: manifest alternate bytes at papers/Lotfi_Lubricant_surfaces_JPC_C_2018.pdf for the same article as 2018roghayyeh-lotfi-j-phys-chem-molecular-dynamics.

Findings

Outcomes / mechanisms: Water strongly accelerates D4OH degradation versus O₂-only environments at the 1500 K accelerated conditions; any nanoparticle presence accelerates chemistry versus gas-only controls. Untreated SiO₂ and goethite outperform their dry/wet-air passivated counterparts in degradation rate, whereas wet-air-treated Fe₂O₃ gives the strongest acceleration among the iron-oxide cases studied.

Comparisons: trends are discussed relative to industry HDD/HAMR humidity concerns and prior PFPE degradation literature.

Sensitivity: NP chemistry, pretreatment, and humidity are primary levers; 1500 K is a computational acceleration temperature.

Limitations: device-relevant temperatures and shear are not fully captured by the high-T gas-cell protocol.

Corpus honesty: numbers above come from papers/Lotfi_CF_OxideNP_JPC_C_2018.pdf; confirm wording against your VOR PDF.

Corpus honesty: same scientific content as sibling slug; cite whichever PDF your tree mounts.

Limitations

Duplicate PDF paths should be consolidated when safe for provenance. 1500 K is a computational acceleration temperature; quantitative rates are not direct device predictions without extrapolation. HAMR device temperatures and shear-driven chemistry are not fully captured by gas-phase NP + strand setups alone, so the study is best read as a compositional screen of humidity, O₂, and oxide effects on PFPE scission propensity.

Relevance to group

Adri C. T. van Duin co-authors; ReaxFF application to PFPE tribochemistry with oxide contaminants and humidity.

Citations and evidence anchors

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