Brittle-ductile transition and nano-surface generation in diamond turning of single-crystal germanium

Single-crystal germanium (Ge) has been widely used in infrared optics and ultra-precision components, however, the appearance of alternating brittle and ductile fan-shaped patterns on machined surfaces seriously affects the machined surface quality. The present research focused on the brittle-ductile transition and its effect on the nanosurface generation in single-point diamond turning (SPDT) of (100), (110) and (111) crystal planes of Ge. Face cutting and plunge cutting experiments were performed, the surface anisotropy and brittle-ductile transition regime during cutting were analyzed, and the evolution of crystalline structure was discussed with molecular dynamics (MD) simulation. Theoretical and experimental results show: (1) The anisotropy of the machined surface is closely related to the atomic arrangement order and lattice structure. The anisotropy of the (111) crystal plane is stable while the (110) is unstable. (2) The critical length and depth of both the elastic-ductile and brittle-ductile transition are anisotropic; the depth and length of brittle regions are shallower and shorter than those of ductile regions and proportional to the tool edge radius. (3) The critical depth and thrust force of the elastic-ductile transition obtained in MD simulation can be employed to predict the changing trend of the brittleductile transition. (4) The thickness of amorphous layer produced on the machined surface is proportional to the ratio of cut depth to tool edge radius. The [110] crystal orientation of the (111) crystal plane shows the best cutting performance. This paper provides deep insight into the brittle-ductile transition and nano-surface generation in SPDT of Ge.