Micro-/Nano-Texturing of Aluminum by Precise Coining for Functional Surface Decoration

metals

Article Micro-/Nano-Texturing of Aluminum by Precise Coining for Functional Surface Decoration

Tatsuhiko Aizawa 1,* , Tomoaki Yoshino 2 and Tadahiko Inohara 3 1 Surface Engineering Design Laboratory, Shibaura Institute of Technology, Tokyo 144-0045, Japan 2 Komatsu-Seiki Kosakusho, Co., Ltd., Suwa 392-0012, Japan; [email protected] 3 LPS-Works, Co., Ltd., Tokyo 144-0033, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-3-6424-8615

Received: 15 June 2020; Accepted: 24 July 2020; Published: 3 August 2020

Abstract: The AISI316 austenitic stainless steel die was prepared and nitrided at 673 K for 14.4 ks. Through this low temperature plasma nitriding, the AISI316 die was homogeneously hardened up to 1400 HV within its surface layer of 50 µm. This nitrided AISI316 die was utilized to print the tailored micropattern with nanotextures onto its surface by the femtosecond laser processing. Each micropattern consisted of the tailored segments to have unidirectional nanotextures with diﬀerent orientations. Each segment was recognized by its intrinsic surface plasmonic brilliance to tailored nanotextures. The CNC (Computer Numerical Control) stamping system was used to coin these micropatterns with nanotextures onto the AA1060 aluminum plates with the thickness of 1 mm. SEM (Scanning Electron Microscopy) and optical microscopy were employed to characterize the original micro-/nano-textures on the AISI316 die as well as the coined nanotextured patterns on the AA1060 plate surfaces.

Keywords: functional decoration; femtosecond micro-/nano-texturing; nitrided punch; coining; pure aluminum

1. Introduction Various drawings, fonts, and patterns have been printed onto the metallic sheets, plates, and solids not only for commercial service but also for functional decoration [1]. These things are digitally designed and transformed into CAD (Computer Aided Design) data to directly drive and control the printing and machining tools. In addition to surface coloring by the grating technique [2], the micro-/nano-textures on the metallic plates and products are expected to significantly change the original surface properties and to improve the heat transfer process. The hydrophilic surface of AISI304 plates was modified by the femtosecond laser nanotexturing to be hydrophobic or super-hydrophobic [3–5]. The overall heat penetration capacity of an aluminum heat sink was improved by its microtextures [6,7]. The boiling heat transfer process was enhanced by micro-/nano-textures on the pure aluminum and copper devices [8–10]. In addition to these nano-texturing effects on the surface and engineering fields, various mini-/micro-/nano-textured metallic sheets and plates are necessary in mass for health-control and medical operations to deal with medicines, polymer parts, cells, viruses, etc. [11,12]. Mini-/micro-textures on the metallic plates provide a visible work-field to preserve the culturing platform; nanotextures yield a space to culture the targeting cells, viruses, and microorganisms [13]. A new printing system is necessary to duplicate the tailored micro- and nano-patterns onto the metallic sheets and plates and to fabricate the functionally decorated devices and parts. Femtosecond laser processing becomes a powerful tooling system to print the microtextures as well as the nanotextures onto any material substrates [14–16]. In particular, LIPSS (Laser Induced Periodic Surface Structuring) [17] forms various nanotextured morphologies on the work material

Metals 2020, 10, 1044; doi:10.3390/met10081044 www.mdpi.com/journal/metals Metals 2020, 10, 1044x FOR PEER REVIEW 2 of 10 different LIPSS-ripple periods in the function of laser beam capacity such as fluence and pulse width surfacein addition with to dithefferent work LIPSS-ripplesurface condition. periods As survey in theed function in [18–20], of laser this beamLIPSS capacityprocess is such useful as to fluence make andnanotexturing pulse width onto in the addition punches to and the dies work and surface to tran condition.scribe the tool-surface As surveyed nanostructures in [18–20], this onto LIPSS the processmetal, polymer, is useful and to make oxide-glass nanotexturing products onto by the stam punchesping. Optical and dies property and to transcribe and structure the tool-surface control by nanostructuresfemtosecond laser onto nanotexturing the metal, polymer, has a anddirect oxide-glass influence productson the decoration by stamping. on the Optical metallic property products and structure[21,22]. control by femtosecond laser nanotexturing has a direct influence on the decoration on the metallicIn the products present [21 paper,,22]. the micro-/nano-textured AISI316 dies are fabricated by the femtosecond laser Inmachining the present to paper,coin the the textured micro-/nano-textured punch surface AISI316 to the diesaluminum are fabricated plate by by CNC the femtosecond (Computer laserNumerical machining Control)-stamping. to coin the textured The low punchtemperature surface plasma to the nitriding aluminum is employed plate by CNCto preharden (Computer the NumericalAISI316 substrate Control)-stamping. as a mother punch The low with temperature the tailoredplasma micro-/nano-textures. nitriding is employed In this design to preharden of original the AISI316micro-/nano-patterns, substrate as a motherevery micropattern punch with consists the tailored of geometric micro-/nano-textures. unit-segments, In which this design are represented of original micro-by an /assemblynano-patterns, of nanopatterns every micropattern with different consists orientations. of geometric The unit-segments, femtosecond which laser are machining represented is byemployed an assembly to print of these nanopatterns tailored micro-/nano-pattern with different orientations.s as micro-/nano-textures The femtosecond on laser the mother machining punch is employedsurface. First, to print 8 × 8 thesematrix tailored with 64 micro- unit segments/nano-patterns is designed as micro- by/ nano-texturesallocating the nanopattern on the mother with punch the surface.specified First,orientation 8 8 matrixto each with designated 64 unit segments segment. is This designed CAD by data allocating are transformed the nanopattern into CAM with × the(Computer specified Aided orientation Machining) to each data designated to drive the segment. laser control This in CAD the femtosecond data are transformed laser machining. into CAM Six (Computernanotextures Aided are formed Machining) onto the data specified to drive segment the laser to control build inup the the femtosecond matrix as an laser assembly machining. of 64 Sixnanotextured nanotextures segments. are formed Each onto segment the specified is color decor segmentated to in build blue upby the surface matrix asplasmon an assembly intrinsic of 64to nanotexturedeach nanotexture. segments. Second, Each the mother segment punch is color with decorated seven microtextures in blue by the is prepared surface plasmon for precise intrinsic coining to eachof these nanotexture. microtextures Second, with the mothernanotextures punch withonto seven the microtexturespure aluminum is prepared plates. forSEM precise and coiningoptical ofmicroscopy these microtextures is utilized withto evaluate nanotextures the micro- onto theand pure nano-textures aluminum plates.of mother SEM punch and optical as well microscopy as coined isspecimens. utilized to evaluate the micro- and nano-textures of mother punch as well as coined specimens.

2. Experimental Procedure 3 AISI316 die substrates with the size of 24 14 53 mm were surface-polished, nitrided, and AISI316 die substrates with the size of 24 × 14× × 5 mm× were surface-polished, nitrided, and laserlaser-machinedmachined to form to formthese thesemicropatterns micropatterns with withnanote nanotextures.xtures. This laser-processed This laser-processed punch punch was fixed was fixed into intoa cassette a cassette die to die coin to cointhese these micropatterns micropatterns with with nanotextures nanotextures onto onto pure pure aluminum aluminum plates. plates. As thethe firstfirst step, step, the the plasma plasma nitriding nitriding system system in Figure in Figure1a was 1a utilized was utilized to nitride to thenitride bu ff -polishedthe buff- AISI316polished die AISI316 substrate die substrate at 673 K forat 673 14.4 K ks for at 14.4 70 Pa ks underat 70 Pa the under nitrogen-hydrogen the nitrogen-hydrogen mixture gasmixture with gas the flowwith ratethe flow of 160 rate mL of/min 160 formL/min nitrogen for nitrogen and 30 mL and/min 30 mL/min for hydrogen, for hydrogen, respectively. respectively. Although Although no heat treatmentno heat treatment was utilized was utilized before nitriding,before nitriding, the hardness the hardness of nitrided of nitrided punch increasespunch increases up to 1400 up to HV 1400 in averageHV in average within within the nitrided the nitrided layer thicknesslayer thickness of 50 ofµm. 50 Owingμm. Owing to this to nitrogenthis nitrogen supersaturation, supersaturation, the averagethe average nitrogen nitrogen solute solute content content becomes becomes 4 mass 4 mass % homogeneously % homogeneously in the in nitridedthe nitrided layer. layer.

Figure 1. ThreeThree step experimental procedures procedures for the decorative coining process to duplicate the computer aidedaided design design (CAD) (CAD) data data of micropatternsof micropattern withs with nanotextures nanotextures onto theonto metallic the metallic plate. ( aplate.) Plasma (a) nitridingPlasma nitriding for fabrication for fabricat of theion die of substrate, the die substrate, (b) femtosecond (b) femtosecond laser texturing laser texturing onto the dieonto substrate, the die andsubstrate, (c) computer and (c) numericalcomputer numerical control (CNC)-stamping control (CNC)-stamp for preciseing for coining precise of coining micro-/nano-textures of micro-/nano- to metallictextures plates.to metallic plates.

A femtosecond femtosecond laser laser machining machining system system in inFigure Figure 1b1 isb utilized is utilized to form to form the tailored the tailored spatial spatial nano- nano-texturestextures onto ontothe nitrided the nitrided AISI316 AISI316 punch punch with with the the head head size size of of10 10 × 2020 mm mm2.2 .The The wave wave length length of × femtosecond laser was 515 nm, the pulse width was 200 fs, and the pulse repetition rate was 20 MHz.

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Metals 2020, 10, x FOR PEER REVIEW 3 of 10 femtosecond laser was 515 nm, the pulse width was 200 fs, and the pulse repetition rate was 20 MHz. TheThe maximummaximum powerpower inin averageaverage waswas 4040 W,W, andand thethe maximummaximum pulsepulse energyenergy waswas 5050 µμJ.J. Hence,Hence, thethe powerpower byby irradiationirradiation ofof aa singlesingle pulsepulse isis estimatedestimated toto bebe 0.250.25 GW.GW. TheThe ﬂuencefluence ofof laserlaser irradiationirradiation isis constantconstant byby 0.610.61 JJ/cm/cm22.. The beambeam spotspot isis 11 µμmm whenwhen usingusing thethe ﬁxedfixed lens.lens. TheThe maximummaximum scanningscanning speedspeed isis 30003000 mm/s/s forfor thethe areaarea ofof 3030 × 3030 mm.mm. The beam overlap ratio isis 98%.98%. × TheThe CNCCNC stampingstamping systemsystem (ZEN90;(ZEN90; Hoden-Seimitsu,Hoden-Seimitsu, Co.,Co., Ltd.,Ltd., Kanagawa,Kanagawa, Tokyo)Tokyo) inin FigureFigure1 1cc waswas usedused toto makemake embossingembossing ofof thethe laser-processedlaser-processed punchpunch ontoonto thethe purepure aluminumaluminum platesplates withwith thethe sizesize ofof 2020 × 10 × 11 mm mm33.. The The stroke stroke was was controlled controlled to to move move down down the the punch punch from the originaloriginal positionposition × × downdown toto thethe startingstarting positionposition wherewhere thethe punchpunch waswas contactedcontacted toto work.work. TheThe upsettingupsetting waswas performedperformed untiluntil thethe totaltotal strokestroke becamebecame 150150µ μmm byby thethe appliedapplied loadload ofof 33 kN. kN.

3.3. ExperimentalExperimental ResultsResults

3.1.3.1. FormationFormation ofof UnidirectionalUnidirectional NanotexturesNanotextures ontoonto StainlessStainless SteelsSteels WithWith useuse ofof thethe galvanometer,galvanometer, thethe laserlaser beambeam pathpath waswas controlledcontrolled toto formform thethe unidirectionalunidirectional nanotexturesnanotextures ontoonto thethe AISI304AISI304 stainlessstainless steelsteel sheetssheets onon thethe X-YX-Y stagestage byby thethe present femtosecondfemtosecond laserlaser processing.processing. The laser beam was scannedscanned in thethe speciﬁedspecified angleangle ((θθ)) byby thisthis galvanometer;galvanometer; thisthis angleangle waswas incrementallyincrementally rotatedrotated clockwise.clockwise. InIn thethe casecase whenwhen thethe laserlaser ﬂuencefluence waswas 0.60.6 JJ/cm/cm22 andand thethe wavewave lengthlength waswas 512512 nm,nm, thethe LIPSSLIPSS rippleripple periodperiod waswas estimatedestimated toto bebe 300300 nm.nm. FigureFigure2 2 depicts depicts the the six six nanotexturesnanotextures formedformed ontoonto thethe AISI304AISI304 sheetsheet withwith aa clockwiseclockwise rotation ofof nanotexture orientationorientation fromfrom 00°◦ forfor FigureFigure2 2aa to to 180 180°◦ forfor FigureFigure2 2ff with with an an incremental incremental angle angle of of 30 30°.◦.

FigureFigure 2.2. Six nanotextures formedformed ontoonto thethe stainlessstainless steelsteel platesplates byby thethe opticaloptical pathpath control.control. Directivity ofof thethe nanotextures nanotextures is is controlled controlled to to rotate rotate from from (a– f().a– Inf). theIn following,the following, each each nanotextured nanotextured segment segment with thewith specified the specified orientation orientation was identified was identified by each by number; each number; e.g., a nanotexturede.g., a nanotextured segment segment in Figure in2 aFigure was identified2a was identified by No. 1.by No. 1.

TheThe nanotexturesnanotextures inin FigureFigure2 2a–fa–f are are aligned aligned to to thethe speciﬁedspecified orientationorientation inin parallelparallel toto eacheach other.other. AmongAmong FigureFigure2 a–f,2a–f, the the nanotexture nanotexture width width remains remains to to be be constant constant by by 300 300 nm. nm. This This directional directional controllabilitycontrollability ofof nanotexturesnanotextures revealsreveals thatthat aa two-dimensionaltwo-dimensional segmentsegment isis printedprinted asas aa unidirectionalunidirectional assemblyassembly ofof nanotexturenanotexture withwith thethe speciﬁedspecified orientationorientation andand thenthen everyevery two-dimensional microtexturemicrotexture isis designeddesigned andand constructedconstructed byby aa patchworkpatchwork ofof segmentssegments toto havehave distributeddistributed nanotextures.nanotextures. InIn orderorder toto demonstratedemonstrate thisthis possibilitypossibility toto formform thethe patchworkedpatchworked microtexturemicrotexture byby nanotexturednanotextured segments, a two-dimensional matrix with 8 rows 8 columns is designed to be built up by assembling segments, a two-dimensional matrix with 8 rows× × 8 columns is designed to be built up by assembling the 8 × 8 (= 64) segments with six different nanotextures. Figure 3a represents how to distribute the unidirectional nanotextures for each segment; e.g., the nanotexture in Figure 2c is allocated to (1,1)- segment in the matrix by notation of No.3 while the nanotexture in Figure 2b, to (8,8)-segment by

Metals 2020, 10, 1044 4 of 10 the 8 8 (= 64) segments with six diﬀerent nanotextures. Figure3a represents how to distribute × Metalsthe unidirectional 2020, 10, x FOR PEER nanotextures REVIEW for each segment; e.g., the nanotexture in Figure2c is allocated4 of to10 (1,1)-segment in the matrix by notation of No.3 while the nanotexture in Figure2b, to (8,8)-segment by notationnotation of of No.2. No.2. Figure Figure 3b3b shows the optical microscopy image on the patchworked microtexture. EachEach surface surface plasmonic plasmonic color color correspon correspondsds to each segment in the matrix.

Figure 3. Construction of two-dimensional microtextures as the 8 8 matrix of 64 segments with Figure 3. Construction of two-dimensional microtextures as the 8 ×× 8 matrix of 64 segments with different nanotextures allocated to each one. (a) Allocation of nanotextures from “1” in Figure2a to different nanotextures allocated to each one. (a) Allocation of nanotextures from “1” in Figure 2a to “6” in Figure2f to each segment with (i,j) for 1 < i, j < 8 of matrix, and (b) surface plasmonic brilliant “6” in Figure 2f to each segment with (i,j) for 1 < i, j < 8 of matrix, and (b) surface plasmonic brilliant decoration of stainless steel plate surface on the matrix. decoration of stainless steel plate surface on the matrix. If this collaring were induced by the grating techniques, the tone in collaring could be insensitive If this collaring were induced by the grating techniques, the tone in collaring could be insensitive to the change of nanotexture orientations. As depicted in Figure3b, the segments allocated with the to the change of nanotexture orientations. As depicted in Figure 3b, the segments allocated with the same nanotextures in Figure3a have a di fferent tone in blue; e.g., the segment at (8,2) colors light blue same nanotextures in Figure 3a have a different tone in blue; e.g., the segment at (8,2) colors light but the segment at (8,8) colors darker blue even when the same nanotexture with Figure2b is allocated blue but the segment at (8,8) colors darker blue even when the same nanotexture with Figure 2b is to two segments in Figure3a. This sensitiveness of tone in the blue range under normal lighting allocated to two segments in Figure 3a. This sensitiveness of tone in the blue range under normal explains that this surface color decoration is induced by plasmonic brilliance on the nanotexture with lighting explains that this surface color decoration is induced by plasmonic brilliance on the the period of 300 nm. nanotexture with the period of 300 nm. 3.2. Fabrication of AISI316 Micro-/Nano-Textured Punch 3.2. Fabrication of AISI316 Micro-/Nano-Textured Punch The nitrided AISI316 substrate was micro-/nanotextured by the femtosecond laser processing to prepareThe nitrided for a punchAISI316 for substrate precise embossing. was micro-/nano- Figure textur4 showsed by seven the patternsfemtosecond (M1–M7) laser processing which are tomicro-textured prepare for a onto punch the surfacefor precise of nitrided embossing. AISI316 Figure punch. 4 shows They consistseven patterns of three circular(M1–M7) patterns which (M1,are micro-texturedM2, M3), three onto polygonal the surface patterns of nitrided (M4, M5, AISI3 M6),16 and punch. a star-shaped They consist one (M7).of three As circular stated inpatterns [20,21], (M1,each micro-texturedM2, M3), threepattern polygonal is colored patterns in dark(M4, blue M5, by M6), surface and plasmonica star-shaped brilliance one (M7). in optics. As stated Since thein [20,21],LIPSS-ripple each micro-textured period is around pattern 300 nm is colored in these in nanotextures, dark blue by thissurface blue plasmonic color range brilliance is selected in optics. in the Sinceoptical the di LIPSS-rippleffraction to nanotextures. period is around 300 nm in these nanotextures, this blue color range is selected in theThis optical diff ractivediffraction color to is nanotextures. different in each segment of seven micropatterns in Figure4, as suggested by coloring in the matrix in Figure3. This reveals that allocation of nanotextured ripples with their tailored orientation to each segment controls the color tone of micropatterns for surface decoration. The M6 micropattern is employed to explain this controllability. Figure5 depicts the optical microscopic image on the polygonal micropattern, M6. This microtexture consists of five repetitive sections; each section is further divided into six segments from #1 to #6 in Figure5 with different nanotextures. As stated in 3.1, each segment has different surface plasmonic brilliances with dependence on the orientation of nanotextures. Figure6 lists the SEM images on the nanotextures allocated to six segments for five sections in the microtexture of M6 on the die. The orientation of nanotexture is controlled to rotate clockwise (a)–(f) in correspondence from segment #1 to #6 in Figure5. Irrespective of

Figure 4. Seven microtexture patterns directly printed onto the nitrided AISI316 die surface.

Metals 2020, 10, x FOR PEER REVIEW 4 of 10

notation of No.2. Figure 3b shows the optical microscopy image on the patchworked microtexture. Each surface plasmonic color corresponds to each segment in the matrix.

Figure 3. Construction of two-dimensional microtextures as the 8 × 8 matrix of 64 segments with different nanotextures allocated to each one. (a) Allocation of nanotextures from “1” in Figure 2a to “6” in Figure 2f to each segment with (i,j) for 1 < i, j < 8 of matrix, and (b) surface plasmonic brilliant decoration of stainless steel plate surface on the matrix.

If this collaring were induced by the grating techniques, the tone in collaring could be insensitive to the change of nanotexture orientations. As depicted in Figure 3b, the segments allocated with the same nanotextures in Figure 3a have a different tone in blue; e.g., the segment at (8,2) colors light blue but the segment at (8,8) colors darker blue even when the same nanotexture with Figure 2b is allocated to two segments in Figure 3a. This sensitiveness of tone in the blue range under normal lighting explains that this surface color decoration is induced by plasmonic brilliance on the nanotexture with the period of 300 nm.

3.2. Fabrication of AISI316 Micro-/Nano-Textured Punch The nitrided AISI316 substrate was micro-/nanotextured by the femtosecond laser processing Metalsto prepare2020, 10, 1044for a punch for precise embossing. Figure 4 shows seven patterns (M1–M7) which are5 of 10 micro-textured onto the surface of nitrided AISI316 punch. They consist of three circular patterns (M1, M2, M3), three polygonal patterns (M4, M5, M6), and a star-shaped one (M7). As stated in thisMetals[20,21], rotational 2020 each, 10, control, xmicro-textured FOR PEER the REVIEW spatial pattern period is colored of six nanotextures in dark blue isby constant surface plasmonic by 300 nm brilliance for all segments in optics.5 of 10 and sectionsSince inthe this LIPSS-ripple microtexture period of M6. is around Figures 3005 and nm6 indemonstrate these nanotextures, that the microtexturesthis blue color of range M6 isis shapedselected as a regularlyin theThis optical tailoreddiffractive diffraction assembly color to nanotextures. ofis nanotexturesdifferent in each with theirsegment controlled of seven orientations. micropatterns in Figure 4, as suggested by coloring in the matrix in Figure 3. This reveals that allocation of nanotextured ripples with theirMetals 2020tailored, 10, x FOR orientation PEER REVIEW to each segment controls the color tone of micropatterns for5 of surface 10 decoration.This The diffractive M6 micropattern color is differentis employed in each to explainsegment this of controllability.seven micropatterns in Figure 4, as Figuresuggested 5 depictsby coloring the in optical the matrix microscopic in Figure 3. imagThis revealse on thethat polygonalallocation of micropattern,nanotextured ripples M6. This microtexturewith their consists tailored of orientation five repetitive to each sections; segment each cont sectionrols the iscolor further tone dividedof micropatterns into six segmentsfor surface from #1 to #6decoration. in Figure The 5 withM6 micropattern different nanotextures. is employed to As explain stated this in controllability. 3.1, each segment has different surface plasmonicFigure brilliances 5 depicts with the dependence optical microscopic on the orie imagntatione on theof nanotextures.polygonal micropattern, Figure 6 listsM6. theThis SEM imagesmicrotexture on the nanotextures consists of five allocated repetitive to sections; six segments each section for five is further sections divided in the into microtexture six segments fromof M6 on the die.#1 Theto #6 orientation in Figure 5 ofwith nanotexture different nanotextures. is controlled As to stated rotate in clockwise 3.1, each se (a)–(f)gment inhas correspondence different surface from plasmonic brilliances with dependence on the orientation of nanotextures. Figure 6 lists the SEM segment #1 to #6 in Figure 5. Irrespective of this rotational control, the spatial period of six images on the nanotextures allocated to six segments for five sections in the microtexture of M6 on nanotextures is constant by 300 nm for all segments and sections in this microtexture of M6. Figures the die. The orientation of nanotexture is controlled to rotate clockwise (a)–(f) in correspondence from 5 andsegment 6 demonstrate #1 to #6 thatin Figure the microtextures 5. Irrespective of of M6this is rotational shaped ascontrol, a regularly the spatial tailored period assembly of six of nanotexturesnanotexturesFigureFigure with 4. 4.Seven Sevenis their constant microtexture microtexture controlled by 300 nm patternsorientations. patterns for all directlydirectlysegments printedprinte and dsections onto onto the the in nitridedthis nitrided microtexture AISI316 AISI316 die of die M6.surface. surface. Figures 5 and 6 demonstrate that the microtextures of M6 is shaped as a regularly tailored assembly of nanotextures with their controlled orientations.

FigureFigure 5.Figure 5.Allocation Allocation 5. Allocation of nanotextures nanotexturesof nanotextures with withwith different different diﬀerent orieorie orientationsntations into into six intosix segments segments six segments (#1–#6) (#1–#6) in (#1–#6) the in M6the inM6 the M6micropattern. micropattern.micropattern.

Figure 6.FigureNanotextures 6. Nanotextures of six of segmentssix segments in in M6 M6 with with a clockwise rotation rotation of their of their orientations orientations (a–f) in (a–f) in correspondencecorrespondence of #1 toof #1 #6 to in #6 Figure in Figure5 by 5 by the the femtosecond femtosecond laser texturing. texturing.

Figure 6. Nanotextures of six segments in M6 with a clockwise rotation of their orientations (a–f) in correspondence of #1 to #6 in Figure 5 by the femtosecond laser texturing.

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EachMetals 2020 of, the 10, x otherFOR PEER six REVIEW micropatterns in Figure5 is also composed of geometrically designed6 of 10 assembly of sections, each section of which consists of segments with designated nanotextures as seen in FiguresEach5 and of6. the To beother noticed six micropatterns in Figure6, eachin Figure nanotexture 5 is also is composed continuous of acrossgeometrically the grain designed and zone boundariesassembly of of nitrided sections, AISI316 each section punch. of which This consists proves of that segments femtosecond with designa laserted processing nanotextures works as seen as a tool in Figures 5 and 6. To be noticed in Figure 6, each nanotexture is continuous across the grain and to print the original nanotextures onto the ﬁne-grained and hardened AISI316 punch. This punch is zone boundaries of nitrided AISI316 punch. This proves that femtosecond laser processing works as used for precise coining onto the metallic products. a tool to print the original nanotextures onto the fine-grained and hardened AISI316 punch. This punch is used for precise coining onto the metallic products. 3.3. Coining of Micro-/Nano-Textures onto AA1060 Aluminum Plates A3.3. single-mode Coining of Micro-/Nano-Textu loading sequenceres wasonto AA1060 used to Aluminum compress Plates the upper punch with the constant velocity. First, theA punch single-mode was lowered loading fromsequence the originalwas used positionto compress down the toupper the startingpunch with position, the constant where the punchvelocity. touched First, just the on punch the work was lowered material from surface. the origin Then,al position the stroke down was to the moved starting down position, to 150 whereµm from the startingthe punch position touched by just the on constant the work velocitymaterial surface. of 0.02 Then, mm/ s.the After stroke holding was moved this down stroke to for150 10μm s, the punchfrom was the moved starting up position by the by constant the constant velocity velocity of 0.1of 0.02 mm mm/s./s. Figure After 7holding depicts this the stroke embossed for 10 s, AA1060 the aluminiumpunch was plate moved after single-shotup by the constant loading velocity in the above.of 0.1 mm/s. This strokeFigure of7 depicts 150 µm the is embossed larger than AA1060 the depths aluminium plate after single-shot loading in the above. This stroke of 150 μm is larger than the depths of microtextures and nanotextures in Figure7. This is because the elastic deformation of AA1060 of microtextures and nanotextures in Figure 7. This is because the elastic deformation of AA1060 aluminum surface roughness is included in this total stroke together with the elastic response in the aluminum surface roughness is included in this total stroke together with the elastic response in the supportingsupporting jig of jig aluminum of aluminum work. work. In addition,In addition, the the depths depths of of coined coined texturestextures on the aluminum aluminum plate plate are not constantare not constant in the whole in the whole area during area during coining coining because because of uneven of uneven distribution distribution of of coining coining pressure and elasticand deformation elastic deformation of punch of and punch die. and In fact,die. theIn fact ratio, the of ratio depth ofof depth coined of texturescoined textures on the on aluminum the platealuminum to the height plate of to textures the height on of the textures punch on wasthe punch limited was by limited 80%. Theby 80%. incremental The incremental loading loading sequence mightsequence be well might employed be well to employed increase thisto increase ratio. this ratio.

Figure 7. Pure aluminum plate with coined seven microtextures, each of which has nanotextured Figure 7. Pure aluminum plate with coined seven microtextures, each of which has nanotextured segments. Each microtexture M1–M7 in the above just corresponds to the original one from M1 to M7 segments. Each microtexture M1–M7 in the above just corresponds to the original one from M1 to M7 in Figure4. in Figure 4.

SevenSeven microtextures microtextures from from M1 M1 to to M7 M7 are are duplicatedduplicated on on the the aluminum aluminum plate plate surface surface to have to have their their surfacesurface plasmonic plasmonic brilliance. brilliance. Seven Seven microtextures microtextures fr fromom M1 M1 to to M7 M7 on on the the coined coined aluminum aluminum plate plate in in FigureFigure7 are 7 just are correspondingjust corresponding in mirrorin mirror image image to to sevenseven micropatterns from from M1 M1 to M7 to M7 on the on AISI316 the AISI316 punchpunch in Figure in Figure4. A 4. micropattern A micropattern of of M6 M6 is is employed employed as a representative representative to todescribe describe the thedimensional dimensional accuracyaccuracy in this in coiningthis coining process process by comparing by comparing Figures Figures5 and 58 and. As 8. stated, As stated, this M6this micropattern M6 micropattern consists of ﬁveconsists patch-works of five patch-works of six segments. of six segments. The segments The segments from #1 from to #6 #1 in to Figure#6 in Figure5 are duplicated5 are duplicated to #1 to #6 into Figure #1 to 8#6. Throughin Figure 8. coining, Through the coining, anti-clock the anti-clock wise numbering wise numbering of six segmentsof six segments in Figure in Figure5 turns 5 to be clockwiseturns to be in Figureclockwise8. Thisin Figure agreement 8. This betweenagreement the between original the M6 original micropattern M6 micropattern and the and coined the M6

Metals 2020, 10, 1044 7 of 10

Metals 2020, 10, x FOR PEER REVIEW 7 of 10 microtextureMetals 2020, implies 10, x FOR PEER that REVIEW each nanotexture in each segment is also accurately transcribed7 fromof 10 the coined M6 microtexture implies that each nanotexture in each segment is also accurately transcribed AISI316 punch to the aluminum work surface. coinedfrom the M6 AISI316 microtexture punch impliesto the aluminum that each nanotexturwork surface.e in each segment is also accurately transcribed from the AISI316 punch to the aluminum work surface.

FigureFigure 8. Optical 8. Optical microscopic microscopic image image on on the the microtexture microtexture of of M6, M6, coined coined onto onto the the surface surface AA1060 AA1060 plate. plate. The coined segments are aligned in the clockwise numbering in the mirror-image reversal to the The coinedFigure segments8. Optical microscopic are aligned image in the on clockwise the microtexture numbering of M6, in thecoined mirror-image onto the surface reversal AA1060 to the plate. original original anti-clockwise alignment in Figure 5. anti-clockwiseThe coined alignment segments inare Figure aligned5. in the clockwise numbering in the mirror-image reversal to the original anti-clockwise alignment in Figure 5. FigureFigure9 compares 9 compares the the nanopattern nanopattern with with LIPSS-ripples LIPSS-ripples for segment segment #1 #1 in in micropattern micropattern M6 M6on the on the AISI316AISI316 punchFigure punch to 9 compares the to nanotexturethe nanotexture the nanopattern for for segment segment with LIPSS-rippl #1 #1 inin microtexturees for segment M6 M6 of of the #1 the incoined coinedmicropattern AA1060 AA1060 aluminumM6 on aluminum the plate. The average nanotexture period remains to be the same as the original LIPS-period of 300 nm. plate.AISI316 The average punch to nanotexture the nanotexture period for segment remains #1 to in be microtexture the same asM6 the of the original coined LIPS-period AA1060 aluminum of 300 nm. The original orientation angle (θ) in Figure 9a is −30° while θ = +30° in Figure 9b. This mirror-image The originalplate. The orientation average nanotexture angle (θ) inperiod Figure remains9a is to30 be thewhile sameθ =as +the30 originalin Figure LIPS-period9b. This of mirror-image 300 nm. reverse relationship proves that the original nano− pattern◦ on the punch◦ is accurately transcribed to reverseThe relationship original orientation proves angle that the (θ) originalin Figure nanopattern 9a is −30° while on θ the = +30° punch in Figure is accurately 9b. This transcribedmirror-image to the reversethe aluminum relationship plate. proves that the original nanopattern on the punch is accurately transcribed to aluminum plate. the aluminum plate.

Figure 9. Comparison of the nanotextures of segment #1 in the microtexture of M6 between the punch and the coined AA1060 specimen. (a) Nanopattern of #1 on M6 of the AISI316 punch, and (b) FigureFigure 9. Comparison 9. Comparison of of thethe nanotextures of ofsegment segment #1 in#1 the in microtexture the microtexture of M6 between of M6 the between punch the nanotexture of #1 on the M6 of coined AA1060. punchand and the the coined coined AA1060 AA1060 specimen. specimen. (a) Nanopattern (a) Nanopattern of #1 on of #1M6 onof M6the ofAISI316 the AISI316 punch, and punch, (b) and (b) nanotexturenanotexture ofof #1#1 on the M6 M6 of of coined coined AA1060. AA1060.

Metals 2020, 10, 1044 8 of 10

4. Discussion Micro-/nano-textures on the nitrided AISI316 die were coined onto the pure aluminum plate surfaces by the CNC stamping system with the stroke control. The dimensional accuracy in this coining process can be evaluated by two approaches. The high resolution SEM analysis works as the first method to prove that nanotextures with the LIPSS-period of 300 nm are precisely coined to the aluminum surface. As shown in Figure9, the nanogroove texture with the orientation of 30 − ◦ in segment #1 in the M6 micropattern on the punch is transcribed into the AA1060 aluminum plate surface with the same LIPPS period of 300 nm and the orientation of +30◦. This mirror nanotexture reversal proves that each segment with an alignment of nanotextures and each microtexture with patchwork of segments can be also accurately transcribed into the aluminum plate surface. The surface plasmonic brilliance intrinsic to each nanotexture works as the second method to demonstrate that each segment and each microtexture can be coined with accuracy. Let us compare the original surface plasmonic brilliance to the micropattern M6 in Figure5 with that in the coined microtexture M6 in Figure8. As stated next, the color tone in each nanotexture is sensitive to positioning of the die and plate as well as lighting for observation. However, the variation of color tone from #1 to #6 in Figure5 is transcribed to that from #1 to #6 in Figure8. The fundamental optics in this optical decoration is determined by surface plasmonic characteristics on the nanotextures with the specified LIPSS period (Λ)[22]. In the present laser nano-texturing, Λ = 300 nm; the wave length of the induced surface plasmon at these nanotextures ranges around 300 nm. That is, the color tone ranges from light blue to dark blue. As seen in Figure3b, Figure4, Figure5, Figure7, and Figure8, every plasmonic brilliance on the die and AA1060 aluminum surfaces is characterized by the light to dark blue tone. As stated in Figure3, this color tone is sensitive to the orientation of unidirectional nanotextures. When the specimen with coined micro-/nano-textures does not deform by the external stimulus, its surface disturbance on the surface is detected by the change in this surface plasmonic color tone. Let us consider two types of stimulation to change the micro-/nano-textured specimen surface. When the liquid swells on a part of surface, the surface plasmon disappears on this local spot. As discussed in [18,19], every metallic material surface with micro-/nano-textures become hydrophobic and super-hydrophobic. Then, the surface plasmon disappears only on the hydrophobic spot while it remains the same as before on the other surface of textured surface. This reveals that the liquid deposition in a small volume can be detected by the change in the surface plasmonic pattern. Bacteria, virus or cell has a nature to deposit and swell on the material surface with specified nano-textures [23]. These solid deposits are detected by the selective disappearance of surface plasmon at the nanotextured segment with the designated LIPSS-period. The present laser-nanotexturing tool and coining process is also utilized to fabricate a metallic specimen by coining the tailored microtextures with aligned nanotextures even with the controlled LIPSS-period. Where the bacteria, the viruses, and the cells deposit on the coined metallic specimen is precisely detected by the local disappearance of surface plasmon. The current attack by COVID19 teaches us that every man-made part and tool must be anti-bacterial not to make infection to a human being only by touching these parts and tools. Several papers insisted that nano-textured surface decoration by formation of a nanostructured oxide layer was effective to make the titanium part anti-bacterial and to control the bio-activeness of titanium surface by nanotexturing [24–26]. Their approach stood on the naturally grown oxide nanostructure; the present surface decoration has a potential to vary the spatial frequency, distribution, and orientation of nanotextures and to control the anti-bacterial surface condition of any material products.

5. Conclusions The AISI316 substrate was plasma nitrided at 673 K for 14.4 ks as a mother die to be micro-/nano-textured by using the femtosecond laser processing. Seven micropatterns were machined onto this die; each microstructure was geometrically composed of several patchworks. Every patchwork Metals 2020, 10, 1044 9 of 10 also consisted of several nano-textured segments. This multi-dimensional design of micro-/nano-textures enables constructing various microstructures with the tailored nanotextures onto the mother die surface. In particular, the nanotexture period as well as its regular alignment and designable orientation are a key factor to make full use of the femtosecond laser surface decoration of dies. The preliminary plasma nitriding is useful to reduce the grain size of die material and to enhance its hardness and strength enough to be used in fine stamping for coining these micro-/nano-textures onto various metallic products. CNC-stamping is useful to transcribe the original micropatterns and nanopatterns on the die to every metallic work material. The original patchwork of nanotextured segments in each micropattern on the punch surface is preserved and coined onto the work surface. Every patchwork as well as each segment is reproduced in the mirror-image reversal. As suggested in [15], this coining of micro-/nano-textures can be performed simultaneously with near-net shaping the metallic sheet and plate into parts and tools. Surface plasmonic brilliance is a key factor in the present surface decoration. Since the nanotexture period in the present study is around 300 nm, its color tone ranges from light to dark blue. Each segment in every micropattern and microtexture is identiﬁed to have a diﬀerent color tone by this surface plasmon. Every microtexture on the coined work products is composed of a nanotextured segment assembly; it has its intrinsic surface plasmon color tone. Any deposits and adheres on this product surface can be detected by local disappearance of this color tone. No change in this optical characteristic on the product surface proves that its surface is antibacterial in practice by the present surface decoration.

Author Contributions: Conceptualization, T.A. and T.I.; methodology, T.A.; formal analysis, T.A.; investigation, T.A. and T.Y.; data curation, T.I.; writing—original draft preparation, T.A.; writing—review and editing, T.A.; visualization, T.A. and T.Y.; funding acquisition, T.I. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to express their gratitude to Yasuyuki Okabe (LPS-Works, Co., Ltd.) and Shu-Ichi Kurozumi (Nano-Film Coat, LLC.) for their help in the experiments. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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