Effect of Machining on Mechanical Properties of Borosilicate Glasses

Although optical properties are essential for materials for optical purposes, their mechanical properties must also be considered as they directly govern their practical applicability as well as their machining. The machining of optical glasses is mainly carried out by mechanical removal of the material using free or bonded abrasives. The relationship between the local mechanical properties of three types of borosilicate glasses used to produce optical elements (mirrors) and the type of surface machining was investigated. The samples were machined using two approaches, namely 7-axis CNC surface machining and conventional manual machining. Advanced analysis of nano/micro-mechanical properties was performed on glass samples using quasi-static nanoindentation and microindentation and dynamic impact test complemented by the analysis of the acoustic emission signals generated during the mechanical tests.

The results show a difference in mechanical properties between industrial and optical borosilicate glasses. The type of machining has a strong influence on studied mechanical properties and surface quality. Glasses manufactured by CNC have more pronounced acoustic emission signal and crack more than glasses manufactured by hand. The different behavior of hand- and CNC-machined surfaces can be linked to different subsurface damage caused by different contact conditions between the workpiece and the instrument/abrasive. As a result, a different density of surface and especially (near)-subsurface flaws are present in the glass. Simax is slightly softer than BG33 glass, which together with the less pronounced AE may indicate a lower amount of subsurface cracks/damage.