Abstract
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.
Highlights
Nd-doped phosphate laser (PL) glasses are an ideal gain medium for high peak-power solid-state lasers because of their high optical energy-storage capacity
All the glass samples were stored in an electronic moisture-proof box, where the relative humidity was set below 10% and the temperature was maintained at room temperature
These results revealed that the wear of PL glass was much more severe in humid air than in dry air at the nanoscale
Summary
Nd-doped phosphate laser (PL) glasses are an ideal gain medium for high peak-power solid-state lasers because of their high optical energy-storage capacity. With increasing humidity, the COF increased but the wear rate decreased at the macroscale, because the wear mechanism changed from mechanical to tribo-chemical wear Despite all these studies, a quantitatively comparative analysis of the nanoand macro-tribological properties of glasses has not been performed. Due to the different contact modes at different scales (single-asperity contact for nanowear, multiasperity contact for macrowear), the contact stress can vary with respect to nanoand macro-tribological properties. This can further affect the damage mode of glass directly and the degree of water-related tribo-chemical wear. The results provide new insights into the tribological properties of glasses in relation to surface processing
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