We have demonstrated the ability to perform a ductile material removal operation, via single-point diamond turning, on single-crystal silicon carbide (6H). To our knowledge, this is the first reported work on the ductile machining of single-crystal silicon carbide (SiC). SiC experiences a ductile-to-brittle transition similar to other nominally brittle materials such as silicon, germanium, and silicon nitride. It is believed that the ductility of SiC during machining is due to the formation of a high-pressure phase at the cutting edge, which encompasses the chip formation zone and its associated material volume. This high-pressure phase transformation mechanism is similar to that found with other semiconductors and ceramics, leading to a plastic response rather than brittle fracture at small size scales.

In situ x-ray diffraction study of the hexagonal 6H SiC under pressure and shear in rotational diamond anvil cell is performed that reveals phase transformation to the new high-density amorphous (hda) phase SiC. In contrast to known low-density amorphous SiC, hda-SiC is promoted by pressure and unstable under pressure release. The critical combination of pressure ∼30 GPa and rotation of an anvil of 2160° that causes disordering is determined. In situ x-ray diffraction study of the hexagonal 6H SiC under pressure and shear in rotational diamond anvil cell is performed that reveals phase transformation to the new high-density amorphous (hda) phase SiC. In contrast to known low-density amorphous SiC, hda-SiC is promoted by pressure and unstable under pressure release. The critical combination of pressure ∼30 GPa and rotation of an anvil of 2160 Keywords• that causes disordering is determined.

A germanium surface and the chips produced from a singlepoint diamond turning process operated in the "ductile regime" have been analyzed by transmission electron microscopy and parallel electron-energy-loss spectroscopy. Lack of fracture damage on the finished surface and continuous chip formation are indicative of a ductile removal process. Periodic thickness variations perpendicular to the machining direction also are observed on these chips and are identified as ductile shear lamellae. The chips consist of an amorphous, elemental germanium matrix containing varying amounts of microcrystalline germanium fragments. The relative orientation of machining marks and crystallographic fragment texture are used to position individual chips with respect to the initial angular cutting zone on the wafer. Chips with high fragment content correlate directly to cutting zones subject to the highest resolved tensile stress on cleavage planes. These findings are explained in the context of a high-pressure metallization (brittle-to-ductile) transformation with ductility limited by the onset of classical brittle fracture.

Nanoindentation tests were performed on ultraprecision diamond-turned silicon wafers and the results were compared with those of pristine silicon wafers. Remarkable differences were found between the two kinds of test results in terms of load-displacement characteristics and indent topologies. The machining-induced amorphous layer was found to have significantly higher microplasticity and lower hardness than pristine silicon. When machining silicon in the ductile mode, we are in essence always machining amorphous silicon left behind by the preceding tool pass; thus, it is the amorphous phase that dominates the machining performance. This work indicated the feasibility of detecting the presence and the mechanical properties of the machining-induced amorphous layers by nanoindentation.

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