Ductile behaviour in single-point diamond-turning of single-crystal silicon

Single-crystal silicon is usually considered to be extremely brittle and exhibit little ductility in machining. In work reported in the literature during indentation tests, cracks appeared around the impression at loads as small as 5 g and the crystallographic direction was found to have a strong influence on the crack initiation and propagation; however, spiral and flake-like swarf were observed in the single-point diamond-turning of single-crystal silicon. This means that, under some carefully chosen machining conditions, it is possible for single-crystal silicon to be deformed in a ductile manner. The implication of this “ductile behaviour” is to enable single-crystal silicon to be diamond-turned/ground in ductile-mode. In the present research, diamond tools with different tool geometries were employed to diamond-turned single-crystal silicon. Cutting speeds ranged from near 0 to 150 m/min. The results showed that, given the same machining conditions, where rapid tool wear and poor surface finish were obtained when round-nosed tools were used, surfaces with Ra better than 5 nm and no obvious tool wear were achieved by using a ruling tool. This means that the tool geometry has profound effects on the material removal mechanisms involved and on the tool wear rate and that it is possible to reduce tool wear and improve surface finish by carefully modifying the tool shape.