Femtosecond Laser Micro-/Nano-Texturing to Die Substrates for Fine Imprinting to Products

Abstract

A femtosecond laser micro−/nano-texturing was proposed to fabricate the coated and surface treated dies with the tailored textures for surface decoration and surface property control of metal, polymer and glass products. The polygonal model for microtextures with nanotextures by the LIPSS-effect was utilized to fabricate a DLC-coated SKD11 die with a star-shaped emblem. This die was set up into the cassette die set for directly imprinting this emblem into aluminum alloy and PET sheets. The periodic surface structure was synthesized as a surface geometry model to build up the super-hydrophobic surface on the nitrogen supersaturated AISI316 die. This die was also set up into a hot stamping system to directly imprint the hydrophobic surface onto the phosphorous glass products. Through the femtosecond laser micro−/nano-texturing and CNC-imprinting, the metal, polymer and glass product surfaces were optically decorated to have color grating and plasmonic brilliance and functionally controlled to be hydrophobic.

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 into the die surface to transcribe this negative pattern to the product surface [4, 5, 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.

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.

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 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.

Figure 2.

Various products with directly imprinted micro−/nano-textures. a) Fresnel-patterned flat lens, b) heat-transferring aluminum device with regular micro-cavity alignment, and c) dispensing stainless steel nozzle with micro−/nano-textures around its 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 micro-texturing. 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, 17, 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.

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.

In the present study, a thick DLC coating is utilized as a die to imprint the laser-textured 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 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.

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.

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.

Figure 5.

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

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 nano-groove depth (d_L) is uniquely determined; in this case, d_L ~ 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.

Figure 6.

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

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 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].

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.

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 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.

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.

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.

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.

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.

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 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.

Figure 10.

DLC coating die with a star-shaped emblem, which was laser micro−/nano-textured.

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 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.

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.

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 12bturns 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 12.

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

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.

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.

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.

4. CNC-imprinting of textures to metals, polymers, and glasses

Both the DLC-coating die and the nitrogen supersaturated mold were utilized for CNC-imprinting the mother micro−/nano-textures on the dies into various work materials. The DLC-die was utilized in the cold and warm CNC-imprinting processes. The nitrogen supersaturated AISI420 mold was used in the hot CNC-imprinting process.

CNC-imprinting of textures to aluminum alloy. The CNC stamping system in Figure 7 was utilized to imprint the star-shaped texture on the DLC-coating die onto the AA1060 aluminum alloy plate with the thickness of 1 mm and the as-rolled surface roughness. Figure 14a shows a star-shaped replica that is imprinted onto the aluminum plate by coining the micro-textured DLC punch. The original star-shaped emblem in Figure 10 corresponds to the imprinted replica in Figure 14a in the mirror-inverse reflection. In parallel with the geometric imprinting from the microtextures on the DLC-die to the plate, the color-grating and surface plasmonic brilliance are also duplicated on this aluminum plate. This reveals that the intrinsic color grating and surface plasmonic brilliance to micro−/nano-textures on the DLC-die can be reproduced on any metallic work product surfaces by the means of accurate imprinting.

Figure 14.

Micro−/nano-textured AA1060 aluminum alloy plate by CNC-imprinting in cold. a) CNC-imprinting of the star-shaped emblem including the micro−/nano-textures onto the as rolled aluminum alloy plate with the thickness of 1 mm, and b) optical microscopy image of the star-shaped emblem.

Surface decoration observed in Figure 14 in a similar manner to Figure 10, reveals that the mother micro−/nano-textures are accurately imprinted onto the aluminum alloy plate and that the microstructure reproduced on the product surface is responsible for the similar surface decoration to the textured DLC-die. Let us investigate the microstructure of replica textures imprinted onto the plate. Figure 15 depicts the SEM image on the micro−/nano-textures imprinted onto the AA1060 aluminum alloy plate with varying magnification from a) to d). A-zone in Figure 15a turns to be Figure 15b, B-zone in Figure 15b turns to be Figure 15c, and C-zone in Figure 15cturns to be Figure 15d. In contrast to the mother micro−/nano-textures in Figure 12, the micro-textures in Figure 15a and b are just in the mirror-image reversal to the mother textures on the DLC-die in Figure 12a and b. Simply comparing Figure 15c and d to Figure 12c and d, the nanotextures are laterally aligned in Figure 15c and d against the longitudinal alignment of mother nanotextures in Figure 12c and d. This difference comes from the plastic flow of polycrystalline aluminum alloy with an average grain size of 20 μm into the DLC-die micro-cavities, to be discussed later.

Figure 15.

SEM image on the micro−/nano-textures imprinted on the AA1060 aluminum alloy plate with varying the magnification from a) to d).

CNC-imprinting of textures to PET. A PET (Poly-Ethylene Terephthalate) film was employed as a polymer work material for warm CNC-imprinting of mother textures on the DLC-die. In a similar manner to imprinting onto the aluminum alloy sheet, the DLC-die was indented into the PET film with a thickness of 0.2 mm. In this warm imprinting process, the thermocouple was embedded into the lower die. This die temperature was varied and optimized to search for the appropriate holding temperature (T_H) and holding time (T_i). In the following experiment, T_H = 290°, and T_i = 90 s.

Figure 16 depicts the center part of the star-shaped replica imprinted onto the PET film. Micro-edges of textures in the DLC-die were indented into PET film to form the micro-grooves. This proves that originally tailored textures on the DLC-die are accurately reproduced onto the polymer products by this warm imprinting process.

Figure 16.

Optical microscopy image on the micro−/nano-textures imprinted onto the PET film by compression of the DLC-die.

CNC-imprinting of textures to phosphorous glasses. A phosphorous glass type L-PHL2 has been widely utilized as a work for the fabrication of functional optical lenses and DOE. It is characterized by a low glass transition temperature of 381°C and low softening point of 440°C. The flow stress of this glass significantly decreases between these two temperature points; the hot mold stamping process makes use of this temperature range to shape the glass preform to an optical element and lens.

In this hot imprinting process, a cylindrical L-PHL2 preform with a diameter of 5 mm was used as a work and mold-stamped at 440°C for 90 s. Figure 17 compares the L-PHL2 preform before and after hot imprinting process. The micro−/nano-textures were shaped onto the preform surface in Figure 17b. Three-dimensional surface profilomer was also utilized to measure this surface profile.

Figure 17.

A phosphorous glass type L-PHL2 preform before and after hot imprinting with the use of nitrided and laser-textured AISI420 mold.

Figure 18 shows the measured surface profile of L-PHL2 glass after CNC-imprinting. With comparison to the mother surface profile of DLC-die in Figure 11, both profiles are corresponding to each other with fairly good geometric compatibility in its peak-to-valley. This compatibility is dependent on the filling process of glass material work into the cavities of nitrided AISI420 mold. To be discussed in later, the imprinting process control has much influence on this filling process.

Figure 18.

The preform surface profile shaped by hot imprinting the laser-textured mold. a) Bird view of preform surface profile, and b) preform surface profile scanned in the x-axis.

The wettability test was performed to investigate the surface property change before and after micro-texturing onto the L-PHL2 glasses by CNC-imprinting. As shown in Figure 19a, the swelling on the textured surface proves that the original hydrophilic glass surface changes to be hydrophobic. As measured in Figure 19b, the contact angle changed from 60° before imprinting to 114° after imprinting. The surface property of glasses can be controlled from the original hydrophilicity to the hydrophobicity by imprinting the tailored micro−/nano-textures.

Figure 19.

Wettability on the textured L-PHL2 glasses by CNC-imprinting. a) A pure water droplet on the textured L-PHL2 glass surface, and b) hydrophobic surface with a contact angle of 114°.

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, 31, 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.

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 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 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 micro-terrace cavities of DLC-die by CNC-imprinting. Three-dimensional profilometer was utilized to describe this micro-filling process.

Figure 21.

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

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 (W_die) of 10.1 μm and the maximum terrace depth (H_die) 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 (W_work) of 9.2 μm and the work height (H_work) of 0.3 μm was formed after indenting the aluminum work into the concave terrace of DLC-die and releasing the work. Since H_work < H_die, this die terrace cavity was not fully filled by the work in this cold imprinting process. Since W_work < W_die, 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 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.

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 T_H. 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.

Figure 23.

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

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 (τ_H). H monotonously converged to the die cavity depth (H_die) with τ_H. This monotonic convergence of H to H_die 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.

Figure 24.

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

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 laser-textured surface profile distribute on the die surface.

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.

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 τ_H 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−/nano-texturing process with a direct imprinting process. As surveyed in [15], almost all 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, 38, 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.

6. Conclusion

Two-step laser-texturing base procedure is proposed to make surface decoration and surface property control on the metal, plastic, and glass products. The femtosecond laser micro−/nano-texturing works as the first step to form the tailored textures onto the hard-coating die and the surface-treated mold. Using the polygonal geometric models, various emblems, symbols, images, fonts, and pictures are textured onto the die surface. In particular, the textured DLC-die has sufficient hardness and chemical stability for long-term usage in cold and warm conditions. The nitrogen supersaturated tools by the low-temperature plasma nitriding plays as a reliable die to be micro−/nano-textured by using the femtosecond laser processing. Different from the homogeneous ablation process of DLC-die, the polarization in laser texturing must be controlled to form the nanotextures onto the micro-textured surfaces. This nitrided die has sufficient hardness and chemical stability in fine machining and in hot imprinting.

The CNC-imprinting is utilized as the second step to transcribe the mother textures on the die and mold them to the product surfaces. The cold imprinting of textures onto metallic product works well to reproduce the tailored micro−/nano-textured on the DLC-die. In this mechanical imprinting, the grain-size effect on the micro-plastic flow of metallic work has an influence on the accuracy of reproducibility in nanotexturing. The warm imprinting onto polymer product is also available to reproduce the micro−/nano-textures.

Hot imprinting procedure is needed to transcribe the laser-textured die surface into the glass preform. The hydrophobic die surface profile is imprinted onto the glass preform; the hydrophilic glass surface changes to be hydrophobic. This hot imprinting procedure is improved to increase the dimensional accuracy in transcription of micro−/nano-textures and to duplicate the tailored die surface profile with synthesized periodic structure.

The laser-textured DLC-die can be widely utilized to make large-area imprinting even onto the curved product surface and to form the textured product surface with a high aspect ratio. The nitrided stainless steel die is utilized to form the mother textured die surface by using the laser-processing and mechanical finishing processes. Various applications are acceptable to the present approach for micro−/nano-texturing of industrial and medical products.

Acknowledgments

The authors would like to express their gratitude to Okabe T. (LPS-Works Co., Ltd.), Yoshino T. (Komatsu-Seiki Kosakusho, Co., Ltd.), Hasegawa T., Miyagawa T. (Graduate School of Engineering, Shibaura Institute of Technology), and Kurozumi S. (Nano-Film Coat) for their help in experiments.

Conflict of interest

The authors declare no conflict of interest.