Quantitative analysis of the tribological properties of phosphate glass at the nano- and macro-scales

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.