Investigation of nanoscratch anisotropy of C-plane sapphire wafer using friction force microscope

Abstract

In this study, nanoscratch experiments using side-forward multiple scans by friction force microscope with triangular pyramidal monocrystalline diamond tip were conducted on a (0001) plane sapphire wafer substrate to clarify the ductile-regime machining mechanism including its anisotropy. Various characteristics, such as scratch force, depth and specific energy for each scratch direction on C-plane of sapphire, were manifested by a friction force microscope, and the specific scratch energy showed a trend of three-fold symmetry. Subsequently, cutting chips which adhere to the diamond tip exhibited plastic deformations features were testified by a scanning election microscope. Based on slip/twinning systems of sapphire, Schmid factor was calculated for each scratch direction using the resultant force, and the calculated Schmid factors on the rhombohedral planes became the highest. Following the results, the resolved shear stresses along the twinning directions on the rhombohedral planes were calculated using a simple chip formation model, and they were several times larger than the critical resolved shear stress for rhombohedral twinning. As a result, it was found that when the scratch direction is set so that one of the three faces of the diamond tip is close to parallel to one of the rhombohedral planes, the rhombohedral plane acts as the chip shear plane and the specific scratch/cutting energy can be minimized. The cross-sectional transmission electron microscope images of scratched sub-surface also distinctly displayed the existence of subsurface damages consisting of twins and stacking faults relating rhombohedral planes.