Although enhanced structural relaxation is generally believed to be an important contributor to the work hardening of metallic glasses subjected to a triaxial stress state, direct observation of the relaxation process in response to work hardening has not been achieved in metallic glasses. Here, we show that nanoscratching an atomically flat bulk metallic glass surface, a small atomic force microscopy tip with a radius of ~10 nm, results in a high hydrostatic stress within the stressed volume. This enables densifying plastic flow via enhanced structural relaxation and leads to work hardening behavior, as evidenced by an obvious decrease in friction force signals within scratched regions. Further examination of the atomic structure beneath the scratched surface via high-resolution transmission electron microscopy reveals a relaxed structural configuration, which is indicated by dispersed clusters of medium-range order scale in the case of line scratching and nucleated nanocrystals in the case of cyclic scratching. This study provides compelling evidence for stress-driven structural relaxation, greatly deepening the understanding of the work-hardening mechanism in metallic glasses.

Tribology behaviors of energetic crystals play critical roles in the friction-induced hotspot in high-energy explosive, however, the binder and energetic crystals are not distinguished properly in previous investigations. In this study, for the first time, the nanoscale friction of β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β-HMX) crystal is studied with nanoscratch tests under the ramping load mode. The results show that the nanoscale friction and wear of β-HMX crystal, as a typical energetic material, is highly depended on the applied load. The friction coefficient of β-HMX crystal is initially high when no discernible wear is observed, and then it decreases to a stable value which varies from ~0.2 to ~0.7, depending on the applied load, scratch direction, and crystal planes. The β-HMX (011) surfaces show weakly friction and wear anisotropy behavior; in contrast, the β-HMX (110) surfaces show strongly friction and wear anisotropy behavior where the friction coefficient, critical load for the elastic–plastic deformation transition and plastic–cracking deformation transition, and deformation index at higher normal load are highly depended on the scratch directions. Further analyses indicate the slip system and direction of β-HMX surfaces play key roles in determining the nanoscale friction and wear of β-HMX surfaces. The obtained results can provide deeper insight into the friction and wear of energetic crystal materials.

Processing (grinding, polishing) of phosphate laser (PL) glass involves material removal at two vastly different (spatial) scales. In this study, the nano- and macro-tribological properties of PL glass are investigated by rubbing the glass against a SiO₂ counter-surface in both dry and humid conditions. The results indicate that the friction of the PL glass/SiO₂ pair has opposing trends at the nano- and macro-scales. At the nanoscale, the friction coefficient (COF) in humid air is much higher than in dry air, which is attributed to the capillary effect of the absorbed water-film at the interface. At the macroscale, on the other hand, the COF in humid air is lower than in dry air, because the water-related mechanochemical wear makes the worn surface less susceptible to cracking. Material removal for PL glass is better facilitated by humid air than by dry air at both scales, because the stress-enhanced hydrolysis accelerates the material-removal process in glass. Moreover, the material-removal is more sensitive to contact pressure at the macroscale, because stronger mechanical-interaction occurs during material removal at the macroscale with the multi asperity contact mode. At the macroscale, the material removal is more sensitive to contact pressure in humid air compared to dry air. Because almost all mechanical energy is used to remove material in humid air, and most of the mechanical energy is used to produce cracks in PL glass in dry air. The results of this study can help optimize the multi-scale surface processing of optical glasses.