**5. Discussion**

The femtosecond laser micro−/nano-texturing process to die substrate materials is driven by the ablation process, where each constituent matter of dies is vaporized by high-intensity beam spot during the laser irradiation duration of sub-ps without thermal effect. As studied in [4, 5, 27, 28], this high-intensity ablation process is described by the mass density effect on the micro−/nano-texturing. When texturing the CNT (Carbon Nano- Tube) coating, the removal depth in laser-drilling reached 10s–100 μm by a series of shots. In the case of laser-texturing of the glassy carbon, this removal rate under the same laser irradiation condition decreased down to 5–10 μm. On the other hand, the sp3-rich DLC and the diamond coatings needed more irradiation shots for drilling the depth of coatings [29]. This dependency on the mass density of carbon base coatings reveals that the power and fluence of laser irradiation must be tuned to each substrate materials.

In addition to this high intensity power deposition effect to ablation process, the polarization effect on the texturing is taken into account. After [30–32], the surface structuring in sub-mm range is significantly affected by the twisted laser beam through the polarization. The straight microgroove textures are formed and aligned under a selected polarization condition, while the nano-islands are formed and aligned under another condition. This polarization effect on the nanotexturing is also affected by the substrate material. As seen in the nano-texturing onto the DLC-die in **Figures 10** and **12**, the straight sub-mm sized nanogrooves are formed in regular alignment onto the DLC-die in almost all the polarization conditions. As already discussed in [7, 23] and seen in **Figure 12**, the nanogroove orientation is controllable by this polarization. On the other hand, the nano-island pattern is formed on the nitrogen supersaturated AISI420 die surface instead of formation of regularly aligned nanogrooves, as seen in **Figure 13**. To be noticed, the polarization condition is varied to form a mixture of nano-islands and nano-grooves as shown in **Figure 20**.

The straight nano-grooves are formed in the controlled orientation together with the nano-islands under the tuned polarization conditions. This nano-island formation by the polarization control suggests that the metallic nano-particle formation [33] and the carbon nano-dot deposition [34] are also tunable by locally twisting the laser beam.

In the cold imprinting process, the micro−/nano-textured cavity in the DLC-die is filled by the metallic work through the elasto-plastic flow. In the conventional metal forming, the metallic work is compressed by the loading sequence to fill into mm−/sub-mm sized cavities of dies. The filling volume fraction is determined when *Femtosecond Laser Micro-/Nano-Texturing to Die Substrates for Fine Imprinting to Products DOI: http://dx.doi.org/10.5772/intechopen.105795*

### **Figure 20.**

*Polarization effect on the formation of nano-islands mixed with nano-grooves under the controlled polarization when femtosecond laser texturing the nitrogen supersaturated AISI420 surface.*

the applied stress is in equilibrium with the resistance flow stress of work materials. In the present micro-filling process into μm−/sub-μm sized grooves, the grain size of work has an influence on the polycrystalline plastic behavior. Let us describe the microscopic plastic flow of aluminum alloy work in filling the micro-textured and nano-textured grooves.

The filling process into two neighboring segments in the star-shaped emblem is considered in the following. Two segments on the DLC-die in **Figure 21** are imprinted onto the aluminum alloy plate in **Figure 21c**. As depicted in **Figure 21b**, the width of micro-groove terrace between two adjacent edges is 10 μm, smaller than the average grain size of 20 μm in the aluminum alloy work. Under the mechanical constraint by the grain boundaries, the work is thought to be plastically flown into the microterrace cavities of DLC-die by CNC-imprinting. Three-dimensional profilometer was utilized to describe this micro-filling process.

**Figure 22** compares the surface profiles on the DLC-die and the textured aluminum alloy plate. The aluminum alloy work flew into the micro-cavity with the die terrace width (Wdie) of 10.1 μm and the maximum terrace depth (Hdie) of 0.7 μm as depicted in **Figure 22a**. **Figure 22b** measures the aluminum work after cold imprinting and releasing from the DLC-die. The convex micro-bump with the work width (Wwork) of 9.2 μm and the work height (Hwork) of 0.3 μm was formed after indenting the aluminum work into the concave terrace of DLC-die and releasing the work. Since Hwork < Hdie, this die terrace cavity was not fully filled by the work in this cold imprinting process. Since Wwork < Wdie, the imprinted work surface after releasing from the die, shrunk by 0.9 μm from the elasto-plastically deformed geometry during loading. As studied in [35], the spring-back of work occurs after releasing the work from the die by the fraction of elastic strains. The shrinkage of 9% in the above reveals that structural recovery in elasticity takes place with the material spring-back after releasing the work from the die. The micro-edges in the DLC-die indent into the work and play as a wedge to fix the work. The work is backward extruded into the die terrace cavity during this indentation of micro-edges into the work. This local plastic micro-flow of mono-grained aluminum work drives this micro-filling into the die terrace.

### **Figure 21.**

*Comparison of the micro-textured grooves on the DLC-die and the textured aluminum alloy plate.*

### **Figure 22.**

*Comparison of the surface profile between the micro-grooved terrace in the DLC-die and the shaped bump in the aluminum alloy plate by imprinting the DLC-die.*

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

The side surfaces of extruded work peaks are partially in contact with the root of micro-edge and terrace surface in the DLC-die in **Figure 22**. The nanotextures on the DLC-die are thought to be imprinted into these side surfaces. As depicted in **Figure 15c** and **d**, the nano-textures were formed along the microgrooves.

In the hot CNC-imprinting of glass materials, they deform visco-elastically in the temperature range from the glass transition temperature to the softening one. Hence, the holding duration in stamping has much influence on the micro-filling of glasses into the DLC-die cavities.

The hot mold-stamping tests were performed to investigate the effect of holding duration on the filling process of glass work into concave textures. The inline measured time history of temperature is controlled as indicated in **Figure 23**. Owing to the IH heating, the heating transient to the specified holding temperature of 440°C had no over- and under-shooting steps; the measured temperature monotonously increased to TH. The holding duration was directly controlled in this sequence. The cooling process was also controlled to be free from the thermal cracking on the contact surface between the die and the glass work.

The surface profiles of textured glass works were measured to calculate the average peak height (H) of glass works. **Figure 24** depicts the variation of H with increasing the holding duration (τH). H monotonously converged to the die cavity depth (Hdie)

### **Figure 23.**

*A typically controlled temperature history for hot CNC-imprinting the textures onto the L-PHL2 preforms.*

**Figure 24.** *Variation of the average peak height of textured L-PHL2 preform surface with increasing the holding duration.*

with τH. This monotonic convergence of H to Hdie reveals that the filling process of glass materials into the textured cavity in nitrided die is governed by the visco-elastic, time-dependent deformation of glass works.

When tailoring the fundamental periodic micro-texture in **Figure 11**, the monotonic filling process is sustained in the present hot imprinting procedure to fabricate the textured glass preform with the periodic textures in **Figure 18**. When the tailored textures are synthesized and formed from some periodic structures in **Figure 4b**, this filling process must be affected by the inhomogeneous deformation of glasses into the textured cavities in the nitrided die.

Let us investigate this topological effect of micro-textures on the nitrided die to the hot imprinting behavior. As depicted in **Figure 25a**, two periodic surface structures were synthesized and machined onto the nitrided die. Compared to the fundamental surface structure in **Figure 11**, the peak height and valley depth of lasertextured surface profile distribute on the die surface.

When hot mold-stamping the glass preform onto this die, a local filling process of glass material into each cavity in the die, advances in a different manner at each position. **Figure 25b** depicts the surface profile of imprinted glass preform. Four peaks of this surface profile (P1, P2, P3, P4) were formed by micro-filling of glass materials into four cavities (C1, C2, C3, C4) on the DLC-die in **Figure 25a**. If the filling process advances homogeneously in a similar manner to the imprinting process in **Figure 18**, the width and height of four peaks must corresponding to the width and depth of four cavities. As noticed in **Figure 25**, the geometric correspondence between micro-peaks in **Figure 25a** and micro-cavities in **Figure 25b** is not well-defined by the difference in micro-viscous flow of glass materials. Due to this inhomogeneous filling process in local, the maximum peak height of glass preform was reduced from 95% in **Figure 18** down to 81% in **Figure 25**. This reveals that the loading sequence during τH in **Figure 23** must be tailored to incrementally drive the local micro-viscous flow of glasses.

Finally, let us consider the application of femtosecond laser micro−/nanotexturing process with a direct imprinting process. As surveyed in [15], almost all

### **Figure 25.**

*Comparison of micro-textured surface on the nitrogen supersaturated die by the plasma nitriding with the textured glass work surface by hot imprinting. a) Synthesized surface periodic structure on the nitrided die, and b) imprinted surface profile of glass preform.*

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

the tool surfaces can be DLC-coated with a significant thickness of more than 10 μm. Using this laser texturing technique, almost all the DLC coated tools are available as a mother die with the tailored micro−/nano-textures. Through the cold, warm, and hot imprinting processes, every metallic, polymer and ceramic product surfaces are decorated by the color-grating and surface plasmonic brilliance. In particular, the imprinting with the use of textured DLC-roll is effective to make large-area imprinting of textures onto metallic and polymer product surfaces. A surface decoration by the surface textures with a high aspect ratio is expected in cold imprinting instead of the polymer-based imprinting procedure [36].

The nitrogen supersaturated tool steel and stainless-steel dies are suitable for fine laser- and mechanical machining for micro−/nano-texturing. As stated in [37–39], PCD (PolyCrystalline Diamond) – chipped tools were available in fine texturing without the tearing of machined work-material surfaces and without the significant wear of PCD. This preciseness in dimension with robustness in texturing comes from the chemical stability of nitrogen supersaturated layer. Even in the femtosecond laser texturing, this chemical stability has an influence on the local ablation process by increasing the nitrogen solute content.

The hot imprinting of die textures is effective to change the original surface of glass preforms to be hydrophobic or super-hydrophobic during the mold-stamping of optical lens. In particular, a miniature lens in the endoscope and a micro-lens array in the detector are often covered by the surfactants such as the blood and body solution drops and the raindrops. This hydrophobicity works to prevent these lens surfaces from swelling with surfactants on them. In case of the meniscus lenses, its transparency is controllable by optimizing the microtexture depth.
