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Atomistic understanding of surface wear process of sodium silicate glass in dry versus humid environments

Summary

Glass wear in humid environments is often attributed to stress corrosion and hydrolysis, but atomistic mechanisms coupling shear, water, and network modifiers such as Na⁺ remain under-resolved. Hahn, Liu, Kim, and van Duin simulate sodium silicate versus silica counterfaces with ReaxFF molecular dynamics, contrasting dry and humid sliding where interfacial water is treated explicitly. The study focuses on how Si–O–Si bridging bonds form across the interface under shear and how sodium participates in hydrolytic pathways that generate hydroxyl-rich surfaces.

Methods

The authors prepare amorphous Na–Si–O and SiO₂ slab-like supercells (thousands of atoms), then impose sliding with PBC in the shear plane as in J. Am. Ceram. Soc. Molecular dynamics in LAMMPS with ReaxFF; NVT or equivalent thermostat-controlled temperature (stated in K in the paper); timestep in femtoseconds; duration of nanoseconds-scale production relative to the shear protocol. N/A for NPT barostat / GPa hydrostatic pressure in typical shear stages—confirm in PDF. Humid cases add H₂O at the interface; N/A — electric field; N/A — metadynamics. Analysis tracks bond formation, subsurface mixing, and hydroxylation under load.

Findings

Under dry shear, simulations show aggressive wear accompanied by interfacial Si–O–Si bridges that couple the two bodies and transmit stress into the subsurface, promoting plastic rearrangement and material transfer. Adding interfacial water suppresses formation of these bridging motifs, consistent with lower wear in humid conditions within the same modeling framework. Leachable Na⁺ facilitates water-driven reactions that produce silanol-rich layers; the manuscript discusses how this modifier-assisted hydrolysis alters the mechanochemical balance compared with pure silica contacts. The Introduction highlights a broader experimental puzzle: some sodium-bearing silicate glasses show improved wear resistance as relative humidity rises—contrary to naïve stress-corrosion expectations—whereas several non-leachable glasses instead show monotonic wear increases with humidity; the atomistic study is positioned to rationalize such contrasts through ion leaching and interfacial chemistry rather than humidity alone.

Post-processing in the article separates wear fragments by connectivity to each slab so third-body formation can be quantified as a function of RH and composition.

Limitations

ReaxFF remains empirical: transferability to other glass compositions, exact load/velocity mapping to experiment, and long-time wear volumes require the caveats stated in the paper.

For retrieval, pair this entry with bulk hydrated silica mechanics (2019mei-acta-materia-effects-water) when questions ask how water speciation differs between volume and sliding interface environments.

Relevance to group

Core tribochemistry / glass wear reference from the van Duin collaboration—pairs naturally with other Si–O and mechanochemical entries in the corpus.

Reader notes (navigation)

Cross-link hydrated silica mechanical entries such as [[2019mei-acta-materia-effects-water]] when questions separate bulk water speciation from interfacial wear chemistry. Load and sliding speed ranges in the publication define where ReaxFF remains tractable; extrapolate to MPa contacts only with explicit multiscale justification.

Citations and evidence anchors