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Tribochemistry of phosphoric acid sheared between quartz surfaces: A reactive molecular dynamics study

Evidence and attribution

Authority of statements

Prose below summarizes the publication identified by doi, title, and pdf_path. For friction coefficients, temperature ranges, and reaction details, use the peer-reviewed PDF.

Summary

ReaxFF simulations are reported for tribochemistry in a silica / phosphoric acid sliding contact. The coefficient of friction is described as strongly correlated with the number of interfacial hydrogen bonds, so that a weaker H-bond network aligns with lower friction. Two temperature regimes are discussed: at moderate temperatures (300–600 K), friction decreases mainly through thermal activation of motion and weakening of the H-bond network without new tribochemical chemistry; at higher temperatures (800–1400 K), tribochemical reactions produce polymerized / clustered phosphoric species and water near the interface, enabling a very low friction state (reported friction coefficient ~0.02 in the abstract).

Methods

  • Force field: ReaxFF with Si/O/H/P parameters merged from bulk silica and phosphoric acid training sets and refined against DFT for interfacial interactions (as described in the Methods section).
  • Model: Hydroxyl-terminated α-quartz (101̄0) slabs (~29.46 Å × 21.608 Å × 8.504 Å each); 44 orthophosphoric acid molecules fill the gap; the interface is pre-relaxed at 300 K to remove bad contacts before sliding.
  • MD protocol (LAMMPS): Velocity-Verlet integration with Δt = 0.25 fs; normal pressure 600 MPa on the upper slab (−z); the top rigid layer slides at 100 m/s (1 Å/ps) along +x while the bottom rigid layer stays frozen; Langevin thermostats (damping 100 fs) act on the two layers adjacent to the rigid plates; PBC in plane.
  • Temperature schedule: Eight runs ramp from 300 K to target temperatures 300, 400, 500, 600, 800, 1000, 1200, and 1400 K to span thermally activated friction versus tribochemical regimes.
  • Ensemble / pressure coupling: NVT-like constant-temperature control via Langevin on the thermostated layers together with imposed normal stress (600 MPa, −z) on the stack; this is not a textbook isotropic NPT barostat on the entire cell—hydrostatic NPT is N/A — not the stated protocol.

Findings

Outcomes & mechanisms. The cumulative friction coefficient μ correlates positively with the number of interfacial hydrogen bonds; weaker H-bond networks align with lower μ. For 300 K ≤ T ≤ 600 K, no tribochemical reaction is reported; friction falls mainly because molecular motion accelerates and the H-bond network weakens thermally. For 800 K ≤ T ≤ 1400 K, tribochemistry polymerizes/clusterizes phosphoric species and generates interfacial water, enabling an ultralow-friction branch with μ ≈ 0.02 in the authors’ high-temperature trajectories.

Comparisons. The paper emphasizes correlation of μ with interfacial hydrogen-bond counts rather than a single Arrhenius picture across both regimes.

Sensitivity & design levers. Friction and chemistry shift strongly with temperature across the 300–1400 K schedule and with whether tribochemical products appear.

Limitations & outlook. Idealized flat quartz–acid contact and high-T chemistry; experimental superlubricity systems include roughness and additives not fully represented (## Limitations).

Corpus honesty. Production times in ps/ns per stage are not duplicated here; read pdf_path for exact run lengths beyond this summary.

Limitations

  • Idealized quartz–acid interface and high-temperature conditions for chemistry; experimental superlubricity systems include broader chemistry and roughness not fully represented.

Relevance to group

Adri C. T. van Duin (Penn State) coauthors; exemplifies ReaxFF for oxide–acid tribology and interfacial H-bond control.

Citations and evidence anchors

  • DOI: https://doi.org/10.1021/jp406360u (papers/Yue_Ma_Yeon_JPCC_tribochemistry_2013.pdf).