## 3 Experimental Results
### Simultaneous Nanotexturing with Femtosecond Laser Trimming  
Figure 4 compares the geometry of the CVD-diamond coated WC (Co) punches before and after femtosecond laser trimming. The side surface of the diamond coating in the vicinity of the punch edge was trimmed down to 150 \(\upmu\)m from the punch head in the axial direction of the punch with a depth of 3.5 \(\upmu\)m. The diamond coating on the punch head was also trimmed to reduce the maximum toughness below 0.5 \(\upmu\)m.  
Figure 5 shows SEM and LM images of the diamond coated WC (Co) punch after trimming. As depicted in Figure 5a, the laser-machined traces are seen in the circumferential direction. The incident laser has the possibility to engage in optical interactions with the scattered beam via the rough diamond coating surfaces and to form nanostructured ripples through this interaction. Figure 5b proves that the original rough edge with a radius of curvature of 12.5 \(\upmu\)m was reduced down to an edge width less than 2 \(\upmu\)m via this laser trimming. Figure 5c also demonstrates that nanotextures with an LIPSS-period of 300 nm were formed in the axial direction from the vicinity of the punch edge in this laser trimming process by using second harmonics with a fluence of 0.6 J/cm\({}^{2}\). The agreement between this measured nanotexture width of 300 nm and the theoretical estimate of 250 nm assures that the measured nanotextures in Figure 5c are simultaneously induced by the present femtosecond laser trimming process. Let us consider the directivity of the unidirectional nanotextures in Figure 5c. As discussed in [13; 16; 17], the nanotextured ripples in the lateral direction are induced by femtosecond laser scanning in the longitudinal direction on a relatively smooth surface with a low roughness. In the present trimming process of the diamond coating on the cylindrical punch, the scattering laser was skewed by the local curvature on the trimmed diamond surface so that every nanotextured ripple was formed in the axial direction with a skewed angle. This nanotexturing process involving trimming the cylindrical diamond coating was mainly governed by the local curvature of the cylindrical punch, as well as the original roughness of the diamond coating. On the other hand, the laser trimming conditions, such as the trimmed depth of the cut (d), have less of an influence on this nanotexturing. In order  
Figure 4: Comparison of CVD (Chemical Vapor Deposition)-diamond coated WC (Co) punches before and after femtosecond laser trimming: (**a**) before trimming and (**b**) after trimming. to demonstrate the insensitivity of the trimmed depth to the induced nanotexturing process, three diamond punches were prepared and trimmed by d = 1.8, 2.4, and 3.6 \(\upmu\)m.  
Figure 6 compares the SEM images of the nanotextures on each trimmed surface using the three punches. Although the peak-to-valley ratio of the nanotextures increases with d, the unidirectional formation of the nanotextures with a skewed angle is common among the three punches. Similar nanotextures are simultaneously machined onto the diamond coating with trimming once the local curvature and roughness of the original diamond coating are predetermined as a tool geometry in addition to the femtosecond laser capacity. WLI is utilized as a nondestructive evaluation method for the diagnosis of nanotextures on the profiles of trimmed punch surfaces. This WLI is usually used to measure the geometric angulation of polished and buffed die surfaces with relatively small curvature radii; e.g., the local imperfections on the die surface profile are detected as a Moire pattern, from which the local surface condition is shown to involve geometric angulation with spatial frequencies [18]. The trimmed diamond-coated punch in Figure 5 was employed as a specimen for WLI to measure the spatial period of nanotextured ripples on the trimmed surface. Figure 7a depicts the two-dimensional surface angulation on the trimmed diamond coating.