**1. Introduction**

Surface decoration is an essential method for copy-proof of originally designed products. The holograph and color-grating techniques are utilized in a newly designed 10,000-yen paper billets [1], where a few holographic symbols are imprinted onto this billet to be free from forgery together with the accurately printed portrait of the late Mr. E. Shibusawa, a famous founder of enterprises in Japan. The color-grating method with surface plasmonic design is also utilized to decorate the polymer surface [2] and to modify the original surface properties [3]. In the conventional approach, the designed micro−/nano-textures are printed onto a plastic foil, which is further pasted onto the product surface, as depicted in **Figure 1a**. This approach is easy to be done but it has always a risk of foil delamination from the product surface in daily usage. **Figure 1b** depicts the two-step procedure where the designed textures are cut

**Figure 1.**

*Two approaches to form the micro−/nano-textures onto the product surface. a) Indirect formation of textures by pasting the textured foils onto the products, and b) direct imprinting of textures onto the products.*

into the die surface to transcribe this negative pattern to the product surface [4–6]. The femtosecond laser micro−/nano-texturing has been highlighted as a flexible tool to form the tailored surface and interface profiles onto any material substrates [4, 5]. Hard coating layers such as DLC (Diamond-Like Carbon) and diamond, were micro−/nano-textured by using the femtosecond machining [6, 7]. In particular, as stated in [8, 9], those hard-coated and surface-treated substrates were suitable as a special tool for directly imprinting the shaped micro−/nano-textures onto the work materials.

In this direct imprinting, by using the laser-treated dies, various textures in the order from mm down to sub-μm were transcribed onto the product surfaces. The accurately aligned micro-textures for diffraction optical elements were first shaped onto a die surface by femtosecond laser machining and then imprinted onto the plastic and glass products to fabricate an optical lens with DOE. **Figure 2a** and **b** illustrate the Fresnel-patterned flat lens cross-section and the top view of heat-transferring

### **Figure 2.**

*Various products with directly imprinted micro−/nano-textures. a) Fresnel-patterned flat lens, b) heattransferring aluminum device with regular micro-cavity alignment, and c) dispensing stainless steel nozzle with micro−/nano-textures around its outlet.*

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

aluminum device with regular micro-cavity array, respectively [10, 11]. Due to the fine micro-cavity alignment with the unit size of 3.5 μm × 3.5 μm, the heat flux in the boiling curve was increased five times higher than the non-textured aluminum plate. **Figure 2c** depicts the laser-treated stainless steel nozzle to dispense sub-nL to pL droplets for inkjet printing and line-drawing [12]. Due to the micro−/nano-textured around the nozzle outlet, the diameter of the dispensed droplet was preserved to be nearly equal to the inner diameter of the nozzle outlet.

In the direct imprinting in **Figure 1**, various kids of die material are selected to each application. Consider the flow stress of product materials in practice. Most of metallic products have a yield stress, the above which they begin to deform elasto-plastically and to shape themselves under the constraint of die surfaces [13]. The oxide glasses are fragile below their glass transition temperature; they are able to be elasto-viscously formed into the tailored optical element shape under the constraint of mold surfaces at the elevated temperature [14]. The die and mold materials are optimally designed to have sufficient hardness against the high flow stress of works, to have high erosion and corrosion toughness in contact to work materials in cold and hot, and to have high chemical stability for high laser-machinability. There are two die and mold design approaches for directly imprinting the tailored micro−/nano-textures into the die and mold substrates by using the femtosecond laser machining [15].

**Figure 3a** depicts the hard coating die material with significant film thickness. As discussed in [15], CVD (Chemical Vapor Deposition) coated DLC, diamond and β-SiC films have sufficient thickness to be working as a die substrate for laser microtexturing. On the other hand, the plasma nitriding and carburizing at low temperature are available to make nitrogen and carbon supersaturation into the Fe-Cr alloys and stainless steels, as depicted in **Figure 3b** [9, 16–18]. These supersaturation process provides the nitrogen- and carbon-alloyed layers without nitride and carbide precipitates; those secondary phases often play as an origin of fatigue cracking and deteriorate the original corrosion toughness of chromium-base die substrates. Both coated and surface-treated layers are expected to be working as a die and mold substrate for accurate micro−/nano-texturing with well-defined abrasion behavior.

In the present study, a thick DLC coating is utilized as a die to imprint the lasertextured surface into the metal and polymer sheets by using the CNC (Computer Numerical Control) cold stamping system. The optical properties of textured DLC-die are transcribed onto these sheet surfaces together with geometric and topological coining of die surface profile. This color-grating and plasmonic brilliance of imprinted micro−/nano-textures onto sheets decorate their surface. A thick nitrogen-supersaturated layer is also used as a mold to imprint the laser-textured

**Figure 3.**

*Two die and mold designs for directly imprinting the tailored micro−/nano-textures into the die and mold substrates by using the femtosecond laser machining.*

surface to the phosphorous glass works by using the CNC hot stamping system. The super-hydrophobic die surface is imprinted to the hydrophobic work surface with high contact angle. The glass work surface is controlled from hydrophilic to hydrophobic state by this CNC-imprinting. Owing to the appropriate die and mold material selection, these imprinting processes are free from galling or adhesion wear of work material debris particles. Various applications are discussed to make full use of this surface decoration and surface property control by the femtosecond laser micro−/ nano-texturing.
