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Operando tribochemical formation of onion-like carbon leads to macroscale superlubricity

Experiments on hydrogenated DLC sliding on MoS₂ with nanodiamond in dry N₂ are paired with ReaxFF MD and DFT to show sulfur-driven amorphization of nanodiamond into onion-like carbon that supports macroscale superlubricity.

Summary

Combined experiments and reactive molecular dynamics show tribochemical evolution of hydrogenated DLC (H-DLC) slid against 2D MoS₂ with nanodiamond in dry nitrogen: sulfur from dissociated MoS₂ drives amorphization of nanodiamond and formation of onion-like carbon (OLC) in the contact, enabling macroscale superlubricity through reduced real contact and incommensurate interface structure. The study is motivated by superlubricity regimes where third-body carbon chemistry and 2D sulfide decomposition co-determine interface composition, not by DLC or MoS₂ alone.

Methods

From the article PDF (pdf_path).

  • Experiments: Dry N2 tribology with H-DLC on MoS2 2D material plus nanodiamond third body; operando surface analysis (instrumentation in the article and Methods there).
  • Reactive MD (ReaxFF): C / Mo / S interactions use ReaxFF parameters from Mattsson et al. (as cited). S-doped nanodiamond models (~3 nm diameter, 7200 C atoms) with 1-15% S substitution; initial Maxwell velocities at 2000 K; canonical RMD in LAMMPS with 0.25 fs timestep and Nose-Hoover thermostat (2000 K segment 1 ns, then cool 2000 K to 300 K over 2 ns as stated). Supercells use three-dimensional periodic boundary conditions as in the published setup. Barostat: N/A — the quoted heating/cooling trajectory is NVT-style canonical RMD without a Parrinello–Rahman hydrostatic barostat; separate MoS₂ contact models that quote ~1 GPa stress are described in Supplementary figures and should be read there for any pressure coupling details.
  • DFT (VASP): PBE PAW calculations for substitutional defect energetics (520 eV cutoff, Gamma-centered 6x6x6 k-mesh, 64-atom diamond supercell) as described under Density functional theory in the article.

Correspondence to experiment: Dry N₂ tribometry with H-DLC/MoS₂/nanodiamond contacts is paired with operando surface probes in the article so macroscopic friction trends can be read alongside evolving carbon/sulfur chemistry inferred from simulation and spectroscopy.

Findings

  • Tribochemical reactions occur even under dry, additive-free conditions when H-DLC, MoS₂, and nanodiamond co-participate in the contact.
  • Simulations support that S transport from MoS₂ contributes to nanodiamond amorphization and subsequent OLC formation.
  • In situ OLC is argued to reduce contact area and promote incommensurate sliding against H-DLC, consistent with measured superlubricity.
  • Experimental operando characterization in the article ties macroscopic friction reduction to evolving surface carbon phases; the MD portion supplies an atomistic sulfur-mediated amorphization pathway consistent with those observations.

Limitations

  • Transferability to liquid-lubricated or different DLC chemistries requires separate validation.
  • Reactive MD at 2000 K explores sulfur-driven amorphization kinetics that are far from room-temperature tribology; quantitative timescales therefore require careful mapping back to experimentally relevant contact temperatures and shear rates discussed in the Nature Communications article.