Ductile-brittle transition at large negative tool rake angles

Effects of tool rake angle on ductile-brittle transition in diamond turning of single crystal silicon are investigated. Straight-nosed diamond tools are used and rake angles are varied from -80° to 0°. Chips and machined surfaces are observed using scanning electron microscope and atomic force microscope, and micro cutting forces are measured with a piezoelectric dynamometer. It is found that critical depth reaches the maximum at medium negative rake angle around -40° as the result of appropriate hydrostatic pressure. The ductile-brittle transition behavior becomes remarkably different as rake angle is varied. The cutting forces decrease when the cutting mode transits from ductile to brittle at small negative rake angles, whereas at large negative rake angles they keep increasing. Large negative rake angle leads to significant downward material flow and subsurface plastic deformation. Stagnation region is formed in front of the cutting edge, which destroys the transcription accuracy in ductile regime. In brittle regime, chip formation behind tool flank face is also observed, which indicates that lateral crack occurs along the elastic-plastic boundary after the tool pass due to the elastic response of large subsurface deformation.