**3. Femtosecond laser texturing**

The femtosecond laser machining system was utilized to make a surface decoration of DLC coating die and to control the surface properties of nitrogen supersaturated die by the laser micro−/nano-texturing.

**Die preparation for surface decoration and surface property control.** Thick DLC coating was deposited onto the SKD11 substrate with the size of 100 mm × 100 mm × 5 t mm by using the MF (Medium Frequency) – PECVD (Plasma Enhanced Chemical Vapor Deposition) system (KOBELCO, Japan). This DLC-deposited substrate was further cut and finished to a die shape with a head size of 10 mm × 20 mm. **Figure 9a** depicts the DLC-coated SKD11 die. After [23], this amorphous carbon layer has a homogeneous nanostructure with the constant hardness of 22 GPa even by varying the layer thickness and the PECVD processing conditions. The average surface roughness was much less than 0.1 μm. This DLC coating was utilized for laser surface decoration and direct imprinting in cold and warm.

The largest drawback of this DLC coating is low thermal resistance at elevated temperature. As pointed out in [24], the amorphous carbon becomes chemically unstable when the holding temperature is higher than 623 K or 350°C. Hence, another substrate material must be selected for hot imprinting of textures into the glass work materials.

The nitrogen supersaturated AISI420 substrate was utilized as a mold for hot imprinting process. As-machined AISI420, mold with a diameter of 12 mm was

### **Figure 9.**

*Two types of dies are to be laser-textured for surface decoration and surface property control. a) DLC coated SKD11 die with the amorphous carbon layer thickness of 20 μm, and b) nitrogen supersaturated AISI420 die with the nitrided layer thickness of 50 μm.*

prepared and nitrided at 673 K for 14.4 ks (or 4 h) to increase the surface hardness from the matrix hardness of 260 HV to 1100 HV and to be supersaturated in the 50 μm nitrided layer by higher nitrogen content of 7 mass%. **Figure 9b** depicts the nitrogen supersaturated AISI420 mold. No surface roughing was observed on the top surface of mold; no disturbance in dimension was also detected after nitriding. Without additional grinding and polishing, as nitrided AISI420 mold was employed for laser texturing.

**Femtosecond laser micro−/nano-texturing.** These dies and molds were used for femtosecond laser micro−/nano-texturing. A star-shaped emblem was textured onto the DLC die to describe the femtosecond laser processing. **Figure 10** depicts this textured emblem, which is represented by eight polygonal segments including the nano-grooves with their tailored orientation. Each segment in this emblem is distinguished by its own color-grating on the micro-textured surface with a pitch of 10 mm. The nanotextured zone was colored by its surface-plasmonic brilliance. In this laser texturing operation, the whole DLC-die surface was once ground down to the depth of 7 μm except for a square area with the size of 6 mm × 6 mm. Each constituent segment was laser-cut and shaped onto this square region to build up an emblem on the DLC die.

The nitrogen supersaturated AISI420 mold was laser-textured to have tailored micro−/nano-grooves with the pitch of 20 μm and the height of 4 μm. **Figure 11** depicts the laser-textured mold surface profile. The average pitch and height of

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

### **Figure 11.**

*Surface profiles of the nitrogen supersaturated AISI420 die after laser-texturing along the x-axis with the tailored pitch of 20 μm and the height of 4 μm. a) Bird-view of laser-textured surface, and b) its cross-sectional view.*

those micro-grooves were measured to be 19.8 mm and 3.8 mm, respectively. This dimensional accuracy in laser texturing reflects on the surface property. AISI420 mold before texturing was hydrophilic with the contact angle (θ) of 70° while the micro-grooved AISI420 mold became hydrophobic with θ = 140°. After [20, 22, 25, 26], the hydrophobic surface with high contact angle is thought to have high repellency with low falling angle (ϕ). The pure droplet falls along this laser micro-grooved mold surface by ϕ = 25°. This reveals that the textured mold surface in **Figure 11** has sufficiently low surface energy with low wettability.

**Characterization.** Let us investigate the microstructures of these laser-textured DLC-die and nitrogen supersaturated die surfaces. **Figure 12** depicts the SEM of micro−/nano-textures formed onto the DLC-die with varying magnifications from **Figure 12a–d**. A-zone in **Figure 12a** turns to be **Figure 12b, B**-zone in **Figure 12b**

#### **Figure 12.**

*SEM image on the micro−/nano-textures formed on the DLC coating die with varying the magnitude from left to right.*

**Figure 13.**

*SEM image on the micro−/nano-textures formed on the nitrogen supersaturated AISI420 mold surface. a) SEM image in low magnification, and b) SEM image in high magnification.*

turns to be **Figure 12c**, and **C**-zone in **Figure 12c** turns to be **Figure 12d**. **Figure 12a** and **b** show that each segment consists of a regular alignment of micro-grooves with a pitch of 1-mm. Each micro-groove is formed by two micro-edges and concave terrace. As depicted in **Figure 12c** and **d**, these micro-edges and terraces are nano-textured to have an alignment of ripples with the LIPSS-period of 300 nm. This fine multi-dimensional surface structure characterizes the micro−/nano-textured DLC-die surface.

**Figure 13** depicts the SEM image of the textures formed on the nitrided AISI420 mold surface with different magnifications. Micro-edges were regularly formed with a pitch of 20 μm. The nano-grooves were also formed in the longitudinal direction on the terrace between two adjacent micro-edges. This regular alignment of micro- and nano-grooves is expected to be responsible for high contact angle and high repellency in the above.

Comparing the microstructures in **Figures 12** and **13**, the amorphous carbon layer is physically laser-textured by controlling the ablation process within the beam spot to form the tailored surface structure even local. When laser-texturing the nitrogen supersaturated AISI420 die, a μm- to sub-μm sized island is formed on the terrace surfaces of microgrooves and the nanotextures are induced across these islands. This difference in induced microstructure by leaser-texturing comes from the ablation process of local heterogeneous microstructure and the chemical reaction after laser-irradiation, to be discussed later.
