Effect of the Initial-Flaw on Crack Propagation in Two-Step Cutting of Monocrystalline Sapphire

Though superior multifunctional performances of sapphire, heavy surface/subsurface damage during the process limit applications of the material. Many efforts have been made to improve the machinability of the material, by utilizing the ductile-regime cutting. Here, in addition to the efforts on control of material behavior, machining strategies consisting of aggressive rough cutting and fine finish cutting were utilized in the machining of sapphire to improve the process throughput. To achieve this goal, it is required to investigate the effects of existing flaws in materials because their shapes and size significantly influence crack propagation in subsequent cutting. Considering this, machining behavior of primary and subsequent cutting was analyzed in terms of cutting direction of each, and the effects of existing cracks were investigated with respect to the crystal orientation and stress at the ductile-brittle transition point. Experimental results showed that existing flaws distinctively influence the crack propagation in subsequent cutting; the ductile cutting regime varied up to about 2 times with the combination of different primary and subsequent cutting directions. Based on the experimental results, a two-step cutting was performed as a proof of concept to optimize the material removal rate.