The ultimate sharpness of single-crystal diamond cutting tools—Part I: Theoretical analyses and predictions

In this paper, the brittle–ductile transition lapping mechanism is presented for mechanical lapping of single-crystal diamond cutting tools. The calculated critical depths of cut for controlling brittle–ductile transition in different orientations and on different planes are used to direct tool lapping and deduce the dynamic micro-mechanical strengths (such as tensile, shearing and compressive strengths) of diamond cutting tools, while in the tool fabrication. The investigation illustrates that the dynamic micro-mechanical strengths have great anisotropy, and the strengths in the ‘soft’ direction are all less than those in the ‘hard’ direction on any crystal plane. The lapping of designed cutting tools, oriented (1 1 0)–(1 0 0) and (1 0 0)–(1 0 0) as tool rake and flank faces respectively, is carried out in ductile mode based on the optimal lapping parameters as selected. Both atomic force microscope (AFM) and scanning electron microscope (SEM) measurements results show that the lapping quality of diamond cutting tools is much improved with the proposed lapping approach. The surface roughness on the tool rake face is 0.8 nm (Ra) and lapped cutting edge radius is 35–50 nm as achieved when (1 1 0)–(1 0 0) orientations are selected as rake and flank faces; and the rake face surface roughness is 0.7 nm (Ra) and lapped cutting edge radius is 30–40 nm as the rake and flank faces oriented with (1 0 0)–(1 0 0) combination. Theoretical analyses of the dynamic impact effects on the cutting edge radius are undertaken to predict their ultimate cutting edge radii. Theoretical calculations indicate that the extreme cutting edge radius can be sharpened down to 1–6 nm of the (1 1 0)–(1 0 0) oriented tools, and 2–5 nm for the (1 0 0)–(1 0 0) oriented cutting tools.