Machinability Study of Single-Crystal Sapphire in a Ball-End Milling Process

Sapphire has received increasing attention as an engineering material even though its machinability is still an issue. In this study, the influence of the milling process parameters on the surface quality of single-crystal sapphire is investigated. Due to the hardness and brittleness of the sapphire substrate, crack initiation becomes the main concern with respect to up- and down-milling, feed rate, tool inclination angle and tool wear. With an inclined ball-end mill cutter in feed direction various cutting tests were performed to find optimal parameters for machining microchannel. It was observed that the feed rate and depth of cut are dominant factors for the surface quality. The tool inclination angle shows a parabolic relation to the quality of the machined surface. After tool wear events, such as peeling-off of the tool coating, an increased surface roughness is observed.