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Friction-induced subsurface densification of glass at contact stress far below indentation damage threshold

Summary

The paper shows that frictional shear can drive subsurface densification in borosilicate glass at a nominal Hertzian contact pressure near ~0.5 GPa in liquid water—far below pressures usually associated with indentation-induced densification. Experiments combine sub-Tg annealing volume recovery, nanoindentation, and pH 13 dissolution tests on wear tracks, complemented by ReaxFF molecular dynamics indicating that subsurface structural change is facilitated under shear even when purely normal loading would remain elastic. The mechanochemical framing connects to sliding contacts in precision optics and microfluidic devices where wear must be understood as chemistry-coupled shear rather than purely mechanical abrasion.

Methods

  • Experiments: Reciprocating sliding of borosilicate glass against a smooth stainless-steel ball in water, with characterization of wear-track topography and subsurface changes via sub-glass-transition annealing, nanoindentation, and alkaline dissolution measurements as reported in the article.
  • Simulation: molecular dynamics in LAMMPS-style code with a ReaxFF reactive force field on amorphous borosilicate-like slab supercells (thousands of atoms); PBC in-plane with NVT thermostat-controlled temperature (K); femtosecond timestep; nanoseconds-scale production duration; N/A — barostat if no NPT relax stage. Shear applies via boundary conditions; Hertzian ~0.5 GPa matches experiments qualitatively. N/A — electric field; N/A — metadynamics.

Correlative logic: annealing recovery is a fingerprint of densified SiO\(_x\)-like regions; MD asks whether coordination changes arise under shear when purely normal stress (without shear) would stay elastic at similar contact GPa pressure scale.

Findings

  • Under the stated low-load frictional conditions, subsurface cracking is suppressed relative to common brittle damage modes; wear proceeds via mechanochemical processes.
  • Sub-Tg annealing reveals volume recovery consistent with densified subsurface material in the wear track.
  • Nanoindentation and dissolution tests support altered subsurface properties along the track.
  • ReaxFF MD suggests subsurface structural change can occur readily when friction is included at low contact pressure, complementing the experimental observations.
  • Together, the tests support a shear-triggered densification pathway distinct from classic indentation yield at GPa contact pressures.

Compared to the high-pressure indentation literature, the work argues friction-biased subsurface restructuring can track annealing-recoverable densification at much lower Hertzian pressure when shear is present, agreement in trend between experiment and ReaxFF MD. Sensitivity to temperature in reciprocating tests and pH 13 dissolution ties chemical reactivity to the worn track. Limitations include kinetic access of kinetically trapped states; uncertain how far MD time and sliding rate map to the lab. Version-of-record text for exact conditions: VOR PDF, not a proof duplicate slug if present in the tree.

Limitations

Mapping laboratory contact mechanics to simulation boundary conditions involves modeling choices; long-time evolution of roughness and chemistry may require extrapolation beyond simulated windows. Third-body particles, lubricant additives, and pH swings outside the tested alkaline dissolution protocol may alter wear mechanisms relative to the borosilicate/steel/water case emphasized here. Load cycles and stroke length in reciprocating tests can change flash temperature fields compared to single-pass MD shear.

Relevance to group

Strong PSU collaboration link (He / Hahn / van Duin / Kim) connecting tribochemistry, glass mechanics, and ReaxFF.

Citations and evidence anchors

Reader notes (navigation)

Pair with tribochemistry/glass theme notes for mechanochemical densification vocabulary used across the oxide corpus.