106 Experimental Study on the Ultra-Precision Ductile Machinability of Single-Crystal Germanium

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Single-crystal germanium is an important infrared optical material for dark-field imaging systems. In the present work, single point diamond turning experiments on single-crystal germanium (100), (110) and (111) planes were conducted in order to examine the ductile machinability. It was found that three kinds of surface textures and chip morphologies were generated during brittle-ductile transition. Ductile machinability exhibits strong crystallographic anisotropy, leading to micro-fractured surface regions distributing in a radial pattern from the workpiece center. Compared to wet cuts, dry cuts were beneficial for ductile machining on specific crystal orientations. However, the minimum critical undeformed chip thicknesses for obtaining homogeneous ductile surfaces on all orientations of the three crystal planes were the same, approximately 60 nm. Under this boundary condition, ductile cut surfaces with nanometric surface roughness can be produced with yielding flow type chips. The findings in this study provide criterions for determining process parameters for the fabrication of aspherical and diffractive infrared optics.
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