Mapping ductile-to-fragile transition and the effect of tool nose radius in diamond turning of single-crystal silicon

Although it has long been known that tools with more negative rake angles allow the ductile regime when machining monocrystalline silicon, little has been discussed about the tool-material interaction. The microgeometric contact of the tool tip at this interface plays an essential role in the material remotion (ductile or brittle). In this paper, the tool rake angle was varied in order to change the value of the undeformed chip thickness once the tool cutting radius, formed in front of the tool rake face, changes when the tool rake angle becomes more negative. Based on the statistical design of the experiment applied to cutting tests, a map is built to relate the values of transition pressure in different crystallographic directions. This map assisted in determining the machining conditions with a ductile response into a broader spectrum based on the variation of the tool rake angle. The results obtained allowed to answer questions under which machining conditions and tool geometry account for better surface finishes, lower machining forces, and lower residual stresses. The response surfaces, from statistical design, provided answers capable of establishing under which cutting radii yielded more ductile material removal and avoided a brittle response related to the anisotropic behavior of the material. Finally, the brittle-to-ductile transition mapping determined a more suitable machining condition to diamond turn Fresnel lenses in single crystal silicon.