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Energy absorption mechanisms of nanoscopic multilayer structures under ballistic impact loading

Summary

Large-scale molecular dynamics in LAMMPS studies ballistic impact of rigid nanoscale projectiles on ultrathin aluminum and polyurea films and their multilayers. The interatomic model combines EAM aluminum with PCFF plus the Interface Force Field (IFF) for polyurea nonbonded and cross-link behavior as defined in the article. MSST Hugoniot validation on NPT-equilibrated (≈300 K) cubic ~5 nm supercells precedes NEMD ballistic impact in NVE NEMD at about 1.5 km s⁻¹ on the named stacks (Al3, Pu3, Al3_Pu3_3) with polyurea on the strike or back face as the manuscript describes. The work emphasizes how layer order and tensile release-wave reverberation affect absorbed energy in these nanolaminate targets.

Methods

Interatomic models and Hugoniot validation (MSST, NPT preshock)

EAM aluminum and PCFF+IFF polyurea follow the Comput. Mater. Sci. 195:110504 reference set. Cubic ≈5 nm supercells undergo NPT equilibration at 300 K and are then shock-loaded with the multi-scale shock technique MSST along z with shock speed \(U_s\) above the bulk sound speed. Hugoniot \(U_s\)–\(U_p\) behavior is compared in the text to Wang–Li and Nellis aluminum and Mock polyurea shock data. N/A for a de novo ReaxFF/QM reparameterization study: these are classical FFs from the literature per the citations.

MD application (ballistic NEMD in LAMMPS)

  • Engine / code: LAMMPS molecular dynamics models rigid cylindrical projectiles (~7.7 nm diameter, ~8.1 nm height) impacting multilayer targets (Al3, Pu3, Al3_Pu3_3), varying which face carries polyurea on the striker side, default 1.5 km s⁻¹ unless a velocity sweep changes it. The target disk is ~43 nm across with a fixed outer rim and a ~40 nm deformable interior; in-plane PBC; full atom counts and all dimensions in pdf_path.
  • Ensemble, timestep, duration, thermostat, barostat, pressure, temperature: after MSST Hugoniot stages, NEMD impact uses NVE NEMD production (no Nose–Hoover or Berendsen thermostat during that NVE segment); the Hugoniot leg is NPT (300 K)-class equilibration before shock (see the NVT/NPT vs NVE split in pdf_path. Time step in fs and run ps / ns in the 10-step protocol: article and SI.
  • Shock, stress, barostat, pressure, temperature: virial stress and shock pressure (time-averaged in 2.5 fs windows) in the stated protocol; local temperature under the striker with center-of-mass subtractions as in the manuscript. GPa-scale stress (continuum data in the text) and 1 bar reference for NPT preshock cell; N/A for NPT Parrinello–Rahman barostat during the NVE NEMD impact leg.
  • Shear, electric field, umbrella, metadynamics, replica exchange: NEMD impact after MSST as the paper defines; electric field and replica/umbrella sampling N/A in the summarized protocol. Optional bi-crystal Al models in the publication probe grain-boundary effects.

Findings

Nanoscale simulations show a polyurea strike face is associated with lower damage relative to a back-face polyurea arrangement where shock reverberation weakens the polyurea in thinner films in the trends the authors report. Shock pressure and adiabatic heating increase as aluminum or polyurea films thin in the stated scans. Perforation tracks two resistance stages tied to tensile release after shock reflection at free surfaces; nanometer-scale striker diameter affects absorbed energy in the NEMD curves. MSST \(U_s\)–\(U_p\) relations for Al and polyurea rationalize wave reverberation arrival and timing next to NEMD deformation features; laboratory ballistic limits differ in the authored Limitations. Citable numerical data live in the VOR PDF in pdf_path—extract-thin web text here is a summary only.

Limitations

Reactive bond breaking is not fully enabled in the polymer model (PCFF_IFF); projectile rigidity neglects penetrator deformation. Nanoscale conditions differ from macroscopic ballistic tests emphasized in the introduction.

Relevance to group

Non-ReaxFF atomistic mechanics paper in the corpus; useful contrast to reactive MD workflows elsewhere. The MSST Hugoniot validation step is a concrete hook for comparing continuum shock relations against atomistic wave structure in nanoscale targets.

Citations and evidence anchors

  • theme-reactive-md-corpus (contrasts non-ReaxFF classical MD / MSST Hugoniot validation with reactive workflows elsewhere in the KB)