## 1 Introduction  
Motor cores have grown to become one of the most essential parts of electric vehicles and passenger cars. Since pierced electrical steel sheets and magnetic amorphous films are stacked in the motor core, each constituent sheet and film must be precisely punched out to minimize geometric irregularities, such as burrs and fractured surfaces, as well as the affected zones in these work materials to achieve a significant reduction of iron loss [1]. Tungsten carbide cobalt (WC (Co)) punches and dies with a normal WC-grain size are selected as a standard punch and core die during die-manufacturing and stamping of the iron core because of their greater hardness than tool steel [2]. They often suffer from severe chipping and failure even in the early stages of the piercing process, as stated in [3]. Since normal WC-grains can easily drop from the punch edge during piercing, the punch edge curvature is always designed to be more than 10 \(\upmu\)m, which is enough to be free from chipping. Hence, the burr height and the affected zones increase inevitably by increasing the number of shots in piercing, as noted in [4, 5]. Three methods were developed to accomplish fine piercing without failure [5]. Refinement of WC-grains was the first approach used to promote edge strength in piercing. As proposed in [6], ion milling is available to sharpen the punch edge down to 1 to 2 \(\upmu\)m and to remove the mechanically affected layers by grinding and polishing. The second method involves using low temperature plasma nitriding to harden and control the microstructure of SKD11/AISI316 punches and dies to significantly improve dry piercing performance [7; 8]. Due to their higher hardness than 1400 HV, as well as their finer grain size than 0.1 \(\upmu\)m, no chipping and failure occur at the punch edge, even after continuous dry piercing over 1000 repetitions. In addition, the nearly-zero clearance between the punch and core-die increases the burnished area ratio by up to 85% and reduces the burr height down to 10% of the work sheet thickness. A diamond protective coating of the normally grained WC (Co) punch is the third method used to avoid chipping, even after long-term usage [9]. Due to the dull edge and rough punch surfaces of this diamond-coated WC (Co) punch, the fractured surface is left in part in the pierced hole of the stainless steel workpiece. This third method requires some surface treatments to shape the diamond coating. Among these treatments, the laser trimming process is effective not only to reduce surface roughness but also to adjust the punch surface configuration. The excimer laser treatment was first utilized for the trimming of a diamond-coated WC (Co) punch [9]. The diamond-coated punch head geometry was adjusted accurately within tolerance ranges from +0.1 to +0.6 \(\upmu\)m without a significant change in the Raman spectrum of diamond coating by trimming. The femtosecond laser trimming method also improved the dimensional accuracy of the fine piercing punches [10; 11]. The edge curvature radius of the diamond coated WC (Co) punch was reduced from 12.5 \(\upmu\)m down to 2.5 \(\upmu\)m. AISI316L stainless steel sheets were successfully pierced by this trimmed punch to narrow the affected zone in the cross-section of the pierced sheets. When using this femtosecond laser irradiation, a nanotexture was formed on the irradiated surface by LIPSS (Laser Induced Periodic Surface Structuring) [12]. Since this first discovery on LIPSS, many studies have reported its theoretical and experimental factors, as well as its application to the modification of surfaces and optical properties and its tribological performance [13]. This nanotexturing has the possibility to work simultaneously with the laser trimming of diamond-coated punches. As illustrated in Figure 1a, directionally nanotextured ripples are superposed on the trimmed diamond coatings by the simultaneous optical interference between the incident light and the scattered beams due to the trimmed surface roughness.