**2. Micro**−**/nano-texturing procedure**

Various micro−/nano-textures are required for surface decoration and surface property control to improve the product quality and function. First, a texture design is proposed for surface profiling to be installed by the femtosecond laser texturing. Two types of die substrate materials are utilized for this laser texturing; e.g., thick DLC coated SKD11 die and nitrogen supersaturated AISI316 die. The former die is selected for surface decoration by laser texturing. The latter die is used for surface property control. These dies are utilized to transcribe the die textures into the work materials by the CNC-stamping system. SEM and three-dimensional surface profilometer are employed for characterization of the surface profiles of dies and works.

**Texture Design for Surface Decoration and Surface Property Control.** In the laser texturing for surface decoration, the symbols, the fonts, the image, the pictures, the patterns, and the figures are represented by the simple geometric model to reduce the efforts to prepare an amount of CAM (Computer Aided Machining) data for laser machining. As shown in **Figure 4a**, each unit geometry is modeled by a polygonal segment, which consists of the lines and dots for laser microtexturing. Nanotextures are induced onto the edges and terraces of micro-textured zones by the LIPSS (Laser-Induced Periodic Surface Structuring)-effect [19]. **Figure 4b** depicts a typical one-dimensional surface profile; some periodic profiles are tailored and synthesized to this profile in order that a Fractal dimension along the laser-scanned direction is optimally controlled for well-defined surface property [20]. In this surface property control, the nanotextures by the LIPSS-effect are formed onto the micro-textured edges and terraces to preserve the self-similar surface with the same Fractal dimension as specified by micro-texturing.

### **Figure 4.**

*Texture design for surface decoration and surface property control. a) Polygonal model for laser path control to print the micro-textures, and b) synthesizing the periodic structures into a surface profile.*

*Femtosecond Laser Micro-/Nano-Texturing to Die Substrates for Fine Imprinting to Products DOI: http://dx.doi.org/10.5772/intechopen.105795*

**Figure 5.**

*Simultaneous nanotexturing by LIPSS with microtexturing. a) Schematic view on LIPSS, and b) formation of nanotextures with the LIPSS-period.*

As illustrated in **Figure 5**, these LIPSS-ripples are induced by optical interaction between the incident laser beam and the scattered beam by the surface roughness. After [21], this LIPSS-period is affected by the wavelength and fluence in the laser irradiation. Both higher and lower frequency nanotextures against the original wavelength are formed onto the irradiated surface. In addition, the orientation of nanotextures is also controllable by using optical polarization or by twisting the laser beam. In general, this laser nanotexturing is rather insensitive to the work material selection; to be discussed later, the ablation steps by laser irradiation might be influenced by the microstructure of materials.

**Femtosecond Laser Texturing System.** A femtosecond laser system (FEM-1; LPS-Works, Co., Ltd., Tokyo, Japan) was used to print the tailored spatial textures directly onto the DLC coating surface. The wavelength (λ) of the laser was 515 nm, with a pulse width of 200 fs and a pulse repetition rate of 400 kHz. The maximum average power was 40 W, and the maximum pulse energy was 50 μJ. The working area was 300 mm × 300 mm. In practical operation, a working plate with the size of 280 mm × 150 mm was placed on the work table as depicted in **Figure 6**. The irradiation power of a single pulse is estimated to be 0.25 GW. This high-power irradiation in the 200-fs interval drives a well-defined ablation into the targeting materials. The femtosecond laser machining process was controlled by the CAM (Computer Aided Manufacturing) data. In this experiment, each microtexture is represented by the assembly of line segments. Nanotexture is cut into each micro-texture by the LIPSS effects. In this LIPSS, each nano-groove is formed by the nonlinear optical interaction between the controlled incidental laser beam and the traveling beam on the surface. Depending on the laser irradiation parameters and the surface condition, the nanogroove depth (dL) is uniquely determined; in this case, dL ~ 400 nm. On the other hand, the LIPSS-period (Λ) or the nano-groove width is also determined by the laser processing conditions. In this case, Λ ~ 300 nm.

**Die substrate material selection.** Different from conventional metal forming, a die substrate material has an amorphous carbon film or a nano-size grain-structured surface layer. Otherwise, the grain boundaries are easy to be imprinted together with the micro−/nano-textures when using the polycrystalline metals, alloys, ceramics, and thermets with the normal grain sizes as used in the normal die and mold. In the following experiments, both the DLC-coated SKD11 die and the nitrogen supersaturated AISI316 mold are utilized for femtosecond laser micro−/nano-texturing. In this material selection, how to control the pulsed laser ablation becomes a key to efficiently subtract the amorphous carbon and nitrogen supersaturated Fr−Cr (N). As stated in [5], DLC

**Figure 6.**

*Femtosecond laser micro−/nano-texturing system. a) A schematic view of laser processing in operation, and b) an overview of the system.*

coating is efficiently machined by low-power application without the deposition of carbon particles onto the DLC die. Laser power and fluency must be optimized for laser texturing of nitrogen solute bearing tool steels and stainless steel molds.

**CNC-Imprinting.** Two types of CNC-stamping systems were utilized to transcribe the original micro−/nano-textures on the dies into the work materials. The cold and warm CNC-stamping system (ZEN90, Hoden-Seimitsu, Co., Ltd.; Kanagawa, Japan) was used for imprinting the textured die surface into the metallic and polymer works with relatively low melting temperature as shown in **Figure 7**. The textured die was placed into the upper die set. Both the upper and lower cassette die-sets were respectively fixed to the upper and lower bolsters of this system, respectively. In the cold and warm imprinting process, the upper bolster was incrementally lowered to imprint the mother textures on the die onto the work surface after the starting position in contact with die surface of the work. The stroke velocity was constant by 0.05 mm/s; various loading schedules can be programmed in this CNC-imprinting system. This cold upsetting process was performed until the total stroke of 150 μm by the applied load of 3 kN [7].

In the hot imprinting system, the IH (Induction Heating)-unit was used for prompt and accurate thermal transient control in **Figure 8**. Both the upper and lower

### **Figure 7.**

*Cold CNC-stamping system for imprinting the mother textured die onto the metallic and polymer sheets at room temperature. a) A schematic view of cold imprinting process, and b) an overview of cold imprinting system.*

*Femtosecond Laser Micro-/Nano-Texturing to Die Substrates for Fine Imprinting to Products DOI: http://dx.doi.org/10.5772/intechopen.105795*

**Figure 8.**

*Hot CNC-stamping system for imprinting the mother textured mold onto the glasses above the glass transition temperature. a) A schematic view of hot imprinting process, and b) an overview of hot imprinting system.*

molds were located on the inside of IH-coil for uniform heating. As stated in [22], the heating and cooling steps were PID (Proportional-Integral-Differentiation) controlled to narrow the temperature deviation of molds within ±1 K in the inline temperature measurement by the embedded thermocouples into the upper mold. Both the loading sequence and temperature history were controlled by the personal computer. How to control the temperature history, is discussed later.

**Characterization.** SEM (Scanning Electron Microscopy; JOEL, Tokyo, Japan) was utilized for surface analysis on the textured die and work surfaces. Three-dimensional profilometer (NT91001, Bruker AEX Co., Ltd.; Tokyo, Japan) and laser microscopy (Olympus Co., Ltd., Tokyo, Japan) were also used to describe the depth profiles of micro-textures.
