SEM and AFM images. (a) SEM image of DLC-coated triangular pyramid tip, together with the schematic of the scratch direction. (b) The AFM images of the typical grooves scratched at 10 μN of tip force, 1 μm/s of speed. (c) The cross-section profiles of the grooves at the position as indicated by the line. (d) Schematic of oblique cutting, inclination angle θ defined as the angle between the directions of scratching and cutting face.