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Surface integrity in ultraprecision machined single crystal CaF₂

Hadidi, Mohammad
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Abstract

This research investigates the effects of machining parameters of tool rake angle and stepover on the surface quality, mechanical properties, and optical performance of single crystal CaF₂. Six CaF₂ specimens were machined on the (111) surface using single point diamond turning with variable tool rake angles (0° and -20°) and stepovers (0.5 µm, 1 µm, 3 µm). Atomic force microscopy showed an increase in surface roughness and fracture pits by using a -20° rake angle tool and machining with stepovers of 1 and 3 µm. Nanoindentation revealed an increase in hardness for the machined samples compared to pristine CaF₂, attributed to machining-induced defects. The extent of indentation size effect increased with using -20° rake angle and larger stepover, indicating greater subsurface damage. Stress-strain curves obtained from nanoindentation exhibited higher yield strengths for the machined crystals, comparable to the measured hardness. Optical characterization was performed using UV-Vis spectrophotometry and scatterometry. The results demonstrated that machining with a negative rake angle and increasing the stepover led to increased scattering and lower transmission, correlated to the surface roughness, and subsurface damage.

Date
2023-12
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