Thermal effect on brittle-ductile transition in CaF <inf>2</inf> single crystals

Abstract

Single crystal calcium fluoride (CaF 2 ) is widely used to manufacture optical lenses due to its excellent optical properties. Conventionally CaF 2 is fabricated using grinding and lapping for aspherical and free form optics. However, these fabrication processes are time-consuming and difficult to control; hence single crystal diamond turning is often used to fabricate optical components with the nanoscale surface finish. The diamond turning process is conducted at the optimal uncut chip thickness to ensure ductile mode machining of the brittle CaF 2 single crystals. In general, the ductile mode machining of CaF 2 is performed at an extremely low uncut chip thickness (<100 nm) which adversely increases the machining time and cost. This work proposes a novel method to potentially increase the critical uncut chip thickness by modifying the brittle-ductile transition characteristics through elevating the temperature of CaF 2 crystals. Three crystallographic planes, (111), (110), and (100), have been studied in microhardness tests under different heating conditions. A significant decrease in hardness has been observed for all the crystallographic orientations as the temperature increases. Further observations on the indentation morphology indicate that crack formation can be suppressed at elevated temperatures, which implies a thermally enhanced plasticity of CaF 2 single crystals.