Nanosecond Laser Etching of Aluminum-Plated Composite Materials Applied to Frequency Selective Surfaces

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Article Nanosecond Laser Etching of Aluminum-Plated Composite Materials Applied to Frequency Selective Surfaces

Jian Cheng 1, Shufeng Jing 1, Deyuan Lou 1, Qibiao Yang 1, Qing Tao 1, Zhong Zheng 1, Lie Chen 1, Xuefeng Yang 2 and Dun Liu 1,* 1 School of Mechanical Engineering, Hubei University of Technology, Wuhan 430068, China; [email protected] (J.C.); [email protected] (S.J.); [email protected] (D.L.); [email protected] (Q.Y.); [email protected] (Q.T.); [email protected] (Z.Z.); [email protected] (L.C.) 2 School of Mechanical Engineering, Jinan University, Jinan 250022, China; [email protected] * Correspondence: [email protected]; Tel.: +86-136-3866-1098; Fax: +86-27-5975-0416

Received: 4 May 2020; Accepted: 16 June 2020; Published: 22 June 2020

Abstract: High-quality frequency selective surfaces (FSSs) are important for electromagnetic signal absorption/filtration. Usually, they are made from wave-transparent composite materials covered with a thin metal layer. Current machining methods show some disadvantages when performing fabrication on the structure. Based on its flexibility and uncontactable processing characteristics, nanosecond laser etching of aluminum-plated composite materials applied to FSSs was investigated. To observe the influence of the laser light incident angle, etching of a series of square areas with different incident angles was performed. Thereafter, an image processing method, named the image gray variance (IGV), was employed to perform etching quality evaluation analysis. The observed microscopic pictures of experimental samples were consistent with those of the IGV evaluation. The potential reasons that might affect the etching quality were analyzed. Following all the efforts above, an incident angle range of 15 was recommended, and the best etching result was obtained ± ◦ at the incident angle of 10◦. To observe the influence of the laser pulse overlap and focal spot size on the etched area border uniformity and on the potential damage to the base materials, a theoretical equation was given, and then its prediction of area border edge burrs fluctuation was compared with the experiments. Furthermore, SEM pictures of etched samples were examined. Based on the study, a processing window of the laser pulse overlap and focal spot size was recommended. To conclude, optimal etching results of the FSS materials could be guaranteed by using the right laser operating parameters with the nanosecond laser.

Keywords: nanosecond laser; composite material; incident angle; pulse overlap; frequency selective surface

1. Introduction Frequency selective surfaces (FSSs) are widely used in aeronautical, astronautical and warship facilities due to their excellent electromagnetic signal absorption/ﬁltration performance [1–5]. For instance, Shang demonstrated FSS validation on radar cross-section reduction [6], while Li showed that FSSs could be used as meta-surfaces for cloaking and stealth purposes [7]. Recently, research on possible application in the THz spectrum has been reported, indicating its potential remote-sensing function [8]. Additionally, Zhang’s study enhanced this approach by designing a multi-layer FSS and controlling the chemical potential of a graphene layer [9]. Furthermore, they could also be applied to forthcoming 5G communication to improve signal transmission performance [10]. Usually, FSSs are made from wave-transparent composite materials covered with a thin metal

Materials 2020, 13, 2808; doi:10.3390/ma13122808 www.mdpi.com/journal/materials Materials 2020, 13, 2808 2 of 11 layer. To achieve the electromagnetic signal absorption/filtration function, the metal layer should be machined into high-accuracy periodic patches without damaging the based wave-transparent materials. Wave-transparent materials include glass fiber composites, silica fiber composites, foam, polyimide (PI), etc. [4,11–13]. Additionally, gold, copper and aluminum are usually selected as candidates for the top thin metal layer [8,14,15]. Many different types of machining methods, ranging from computer numerical controlled (CNC) milling to chemical etching, and even continuous lasers and pulsed lasers, have been reported to perform FSS parts fabrication [16–18]. A 5-axis robot CNC milling machine has been employed by Zhu et al. to successfully process a 38-µm copper layer on FSSs [16]. Kim adopted an e-beam evaporator to form the copper-layer pattern on a glass/epoxy prepreg and thus fabricated a new kind of FSS [19]. FSSs could also be printed on dielectric layers using a resistive ink made of a suspension of carbon nanotubes [20]. An Nd:YAG laser was used by Zeng et al. to etch a ceramic substrate [21], and ultrafast femtosecond laser micromachining of two concentric hexagon-shaped metal slots on a Teflon substrate has also been investigated [8]. Femtosecond laser etching is a promising technique; however, its complicated optical delivery system and high cost will block its quick and wide application in FSS fabrication at least for the next 1–2 years. With its high flexibility of processing, high energy density and strong adaptability of materials, nanosecond fiber laser usage and study have become more and more popular. For instance, Hua used a 100-watt nanosecond laser to do aluminum-oxide-layer cleaning [22]. Zhang et al. simulated the temperature distribution of metal thin films on a PI substrate under nanosecond laser irradiation, and their results were instructive and helpful to understand the actual laser etching process [23]. A method to fabricate and characterize terahertz frequency selective surface filters from a low-cost silver adhesive tape with a 20-nanosecond laser has been reported in [24]. However, nanosecond laser processing showed some thermal effects and accuracy shortages due to its pulse duration and improper processing procedures. Taking the disadvantages above into account, the main aim of this work is to study the limitations of three key operating parameters that may determine nanosecond laser etching quality and accuracy, i.e., to clarify the influence of the laser incident angle, focal spot size and pulse overlap. The samples used were silica reinforced resin matrix composites with a 20-µm-thick aluminum film on top, provided by an aviation company in Jinan, China. An IPG nanosecond laser with a 100-ns pulse duration was adopted to perform the etching test. Thereafter, the influence of the laser light incident angle on etching quality and the influence of the pulse overlap and focal spot size on etching accuracy and uniformity were studied experimentally and theoretically. Since FSS materials have similar structures, the possible findings in this study could hopefully be applied for different FSS material compositions.

2. Experimental Procedure

2.1. Materials Commercially available aluminum-plated silica-reinforced resin matrix composite sheet (see Figure1), which is a kind of raw material for FSS parts, was given by an aviation industry company in Shandong (Jinan, China). The sheet was then cut into 100 100 mm pieces. The aluminum × layer was about 20 µm thick, and the silica reinforced resin matrix composite was about 4.2 mm thick. Nanosecond laser etching tests were then executed on the cut pieces. MaterialsMaterials2020 2020,,13 13,, 2808 x FOR PEER REVIEW 33 ofof 1112

Figure 1. A cross-section schematic of the studied composite sheet (a), a top view of the sample (b) and theFigure raw 1. sample A cross-section sheet (c). The schematic thicknesses of the for studied the Al layercomposite and the sheet composite (a), a top materials view of were the 20sampleµm and (b) 4.2and mm, the respectively.raw sample sheet (c). The thicknesses for the Al layer and the composite materials were 20 2.2. Experimentalμm and 4.2 mm, respectively.

2.2. ExperimentalLaser etching experiments were performed by using a nanosecond fiber laser (IPG YLP-100, IPG Laser GmbH, Burbach, Germany). Figure2 is the schematic diagram of the laser etching system. Laser etching experiments were performed by using a nanosecond fiber laser (IPG YLP-100, IPG Briefly, it consisted of a laser source, an optical delivering system, a scanning galvanometer and an Laser GmbH, Burbach, Germany). Figure 2 is the schematic diagram of the laser etching system. x–y–z three-dimensional stage. The laser source was an IPG nanosecond laser with a wavelength of Briefly, it consisted of a laser source, an optical delivering system, a scanning galvanometer and an 1064 nm, a pulse width of 100 ns (Full Width at Half Maximum), a maximum output power of 100 watts x–y–z three-dimensional stage. The laser source was an IPG nanosecond laser with a wavelength of and an M2 1.3. To observe the influence of the laser light incident angle on etching quality, samples 1064 nm, a≤ pulse width of 100 ns (Full Width at Half Maximum), a maximum output power of 100 on the stage were tilted from 0 to 35 to etch 1 1 mm squared areas, assuring the maximum height watts and an M2 ≤ 1.3. To observe◦ the◦ influence× of the laser light incident angle on etching quality, difference depth of focus. The aluminum-layer ablation threshold was tested to be 3.15 J/cm2 by samples on≤ the stage were tilted from 0° to 35° to etch 1 × 1 mm squared areas, assuring the maximum using a single-pulse drilling method, which has been mentioned in [25]. The single-pulse drilling study height difference ≤ depth of focus. The aluminum-layer ablation threshold was tested to be 3.15 J/cm2 on Al with a nanosecond laser was done by Zhang et al. recently. Their study showed that melting by using a single-pulse drilling method, which has been mentioned in [25]. The single-pulse drilling happened, and the heat-affected zone (HAZ) increased with the pulse energy density in a logarithmic study on Al with a nanosecond laser was done by Zhang et al. recently. Their study showed that pattern when doing laser irradiation [26]. In reference to that paper, the laser fluence adopted was only melting happened, and the heat-affected zone (HAZ) increased with the pulse energy density in a several times that of the threshold energy, so as to hinder the HAZ. All parameters used during the logarithmic pattern when doing laser irradiation [26]. In reference to that paper, the laser fluence laser etching are shown in Table1, with the number of overscans being 1. To observe the influence adopted was only several times that of the threshold energy, so as to hinder the HAZ. All parameters of the focal spot size and pulse overlap on the etching accuracy and etched area border uniformity, used during the laser etching are shown in Table 1, with the number of overscans being 1. To observe two different focal spot sizes of 32 and 50 µm were tested and compared, consequently. The laser the influence of the focal spot size and pulse overlap on the etching accuracy and etched area border parameters used during the laser etching are shown in Table2. After laser processing, the surface uniformity, two different focal spot sizes of 32 and 50 μm were tested and compared, consequently. profiles of the as-prepared and etched composite materials were observed via a Nikon 3100 optical The laser parameters used during the laser etching are shown in Table 2. After laser processing, the microscope (Nikon Instruments Inc, Tokyo, Japan) and a Zeiss Gemini 300 SEM system (Carl Zeiss surface profiles of the as-prepared and etched composite materials were observed via a Nikon 3100 Microscopy GmbH, Jena, Germany). optical microscope (Nikon Instruments Inc, Tokyo, Japan) and a Zeiss Gemini 300 SEM system (Carl Zeiss MicroscopyTable 1.GmLaserbH, parametersJena, Germany). for the study of the laser light incident angle’s influence.

Spot Size Pulse Width Repetition Rate Scanning Speed Peak Power Peak Fluence (µm) (ns) (kHz) (mm/s) (Mw) (J/cm2) 32 100 100 1920 16.6 20.64

Table 2. Laser parameters for the study of the focal spot size and the pulse overlap’s inﬂuence.

Spot Size Pulse Width Repetition Rate Scanning Speed Peak Power Peak Fluence (µm) (ns) (kHz) (mm/s) (Mw) (J/cm2) 32 100 100 960–2560 16.6 20.64 50 100 100 1000–4000 39.8 20.27

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Figure 2. A schematic of the experimental setup. With a PC controller, the output laser beam and the action of the scanning galvanometer could be well synchronized. By raising one side of the samples andFigure then 2. measuring A schematic with of athe level experimental meter, the incidentsetup. With angle a PC could controller, be tuned. the output laser beam and the action of the scanning galvanometer could be well synchronized. By raising one side of the samples 3. Resultsand then and measuring Discussion with a level meter, the incident angle could be tuned.

3.1. The InﬂuenceTable of Laser1. Laser Incident parameters Angle for on the Etching study Qualityof the laser and light Accuracy incident angle’s influence.

SpotIn the Size case Pulse of flat-surface Width Repetition laser etching, Rate it isScanning not very Speed difficult Peak to find Power fixed laserPeak processingFluence parameters(μm) for high-accuracy(ns) results.(kHz) However, FSSs are(mm/s) not regular flat surfaces.(Mw) Indeed,(J/cm most2) FSSs are three-dimensional32 100 freeform surfaces 100 that hugely increase 1920 the difficulty of 16.6 uniform etching.20.64 With this condition considered, tolerant windows for main laser processing parameters are required. One key factor thatTable may 2. Laser seriously parameters affect for the the etching study of resultsthe focal isspot the size laser and lightthe pulse incident overlap’s angle. influence. Therefore, experimental results and some discussions of the effect of the laser light incident angle were provided.Spot Size In thisPulse case, Width the maximumRepetition height Rate diScanningfference could Speed be Peak calculated Power to bePeak 0.57 Fluence mm via 2 1 mm (μsin35m) = 0.57(ns) mm . The depth(kHz) of focus (DOF) for(mm/s) the optical system(Mw) was 1.4 mm,(J/cm according) to × ◦ ± Equation32 (1) [27]. 100 100 960–2560 16.6 20.64 50 100 100DOF = 2.56 f 1000–40002M2λ 39.8 20.27 (1) ± 2 2 where3. Resultsf is and the ratioDiscussion of focal length to the expanded laser beam diameter, M is the beam quality value and λ is the laser wavelength. Therefore, the laser fluence could be regarded as uniform in vertical direction3.1. The Influence in this experiment. of Laser Incident Angle on Etching Quality and Accuracy Figure3 shows the etching evolution diagram with an increased laser incident angle under In the case of flat-surface laser etching, it is not very difficult to find fixed laser processing previously mentioned parameters in Table1. It can be seen that laser etching uniformity improved parameters for high-accuracy results. However, FSSs are not regular flat surfaces. Indeed, most FSSs a little from 0 to 5 and 10◦, and then reversed gradually to 20◦. With the incident angle further are three-dimensional freeform surfaces that hugely increase the difficulty of uniform etching. With increasing, non-etching areas appeared from the left sides and quickly grew up to the right sides. this condition considered, tolerant windows for main laser processing parameters are required. One Finally, the etching quality deteriorated to an unacceptable level. key factor that may seriously affect the etching results is the laser light incident angle. Therefore, In order to further characterize the surface etching quality, an image processing method, named experimental results and some discussions of the effect of the laser light incident angle were provided. the image gray variance (IGV) [28], was adopted for the evaluation. With IGV, selected sample surfaces In this case, the maximum height difference could be calculated to be 0.57 mm via 1 mm × sin35° = were captured by using a microscope under the same light angle and intensity. Image pixel gray levels 0.57 mm. The depth of focus (DOF) for the optical system was ±1.4 mm, according to Equation (1) were collected, and then gray variances were calculated with Equation (2). Usually, the smaller the IGV [27]. value, the better the uniformity of the image, which means a better laser etching quality was achieved in this case. DOF = ± 2.56f (1) M N 1 X Xh i2 where f is the ratio of focal lengthS =to the expandedf ( i,laserj) fbeam diameter, M2 is the beam quality(2) MN − value and λ is the laser wavelength. Therefore,i=1 j =the1 laser fluence could be regarded as uniform in vertical direction in this experiment. Figure 3 shows the etching evolution diagram with an increased laser incident angle under previously mentioned parameters in Table 1. It can be seen that laser etching uniformity improved a

Materials 2020, 13, x FOR PEER REVIEW 5 of 12 little from 0 to 5 and 10°, and then reversed gradually to 20°. With the incident angle further increasing, non-etching areas appeared from the left sides and quickly grew up to the right sides. Finally, the etching quality deteriorated to an unacceptable level.

Materials 2020, 13, x FOR PEER REVIEW 5 of 12 little from 0 to 5 and 10°, and then reversed gradually to 20°. With the incident angle further increasing, non-etching areas appeared from the left sides and quickly grew up to the right sides. Materials 2020, 13, 2808 5 of 11 Finally, the etching quality deteriorated to an unacceptable level.

Figure 3. Etching evolution pictures captured with an optical microscope for the increased laser incident angle from 0 to 35°. The etched area was a series of 1 × 1 mm squares.

In order to further characterize the surface etching quality, an image processing method, named the image gray variance (IGV) [28], was adopted for the evaluation. With IGV, selected sample surfaces were captured by using a microscope under the same light angle and intensity. Image pixel gray levels were collected, and then gray variances were calculated with Equation (2). Usually, the smaller the IGV value, the better the uniformity of the image, which means a better laser etching qualityFigure was 3.3. achieved EtchingEtching evolution evolutionin this case. pictures pictures captured captured with with an optical an optical microscope microscope for the for increased the increased laser incident laser angle from 0 to 35◦. The etched area was a series of 1 1 mm squares. incident angle from 0 to 35°. The etched= area∑∑ was a series×, of 1 − × 1 mm squares. (2) In the equation, S is the IGV value of the captured area, f(i,j) is the gray level of a pixel and f is InIn orderthe equation, to further S ischaracterize the IGV value the surfaceof the captured etching quality,area, f(i,j) an is image the gray processing level of method,a pixel and named is the averaged gray level of the area, and M and N are the sampling numbers in x and y directions, thethe imageaveraged gray gray variance level of (IGV) the area, [28], and was M adopted and N arefor thethe sampling evaluation. numbers With IGV,in x andselected y directions, sample respectively. The analysis with the IGV method is given in Figure4. Generally, the trend line of IGV surfacesrespectively. were The captured analysis by withusing the a microscope IGV method under is given the insame Figure light 4. angle Generall andy, intensity. the trend Image line of pixel IGV coincides with the observed results in Figure3 very well. A minimum IGV value of 62.81 was achieved graycoincides levels with were the collected, observed and results then gray in Figure variances 3 very were well. calculated A minimum with Equation IGV value (2). ofUsually, 62.81 wasthe at the laser light incident angle of 10◦; comparably, a maximum IGV value of 1631.1 was achieved at smallerachieved the at IGVthe laservalue, light the incidentbetter the angle uniformity of 10°; comparably,of the image, a whichmaximum means IGV a valuebetter oflaser 1631.1 etching was the laser light incident angle of 35◦. There were some slightly diﬀerences between the angles of 0 to qualityachieved was at achievedthe laser inlight this incident case. angle of 35°. There were some slightly differences between the 15◦. After 15◦, the slopes of the line graph rose very fast, which meant unstable etching conditions. angles of 0 to 15°. After 15°, the slopes of the line graph rose very fast, which meant unstable etching Hence, for a 1 1 mm area, a 30◦ (from= 15∑∑ to +15◦) processing, − window for laser light incident angle(2) conditions. Hence,× for a 1 × 1 mm area,− a 30° (from −15 to +15°) processing window for laser light could be clearly inferred. incidentIn the angle equation, could Sbe is clearly the IGV inferred. value of the captured area, f(i,j) is the gray level of a pixel and is the averaged gray level of the area, and M and N are the sampling numbers in x and y directions, respectively. The analysis with the IGV method is given in Figure 4. Generally, the trend line of IGV coincides with the observed results in Figure 3 very well. A minimum IGV value of 62.81 was achieved at the laser light incident angle of 10°; comparably, a maximum IGV value of 1631.1 was achieved at the laser light incident angle of 35°. There were some slightly differences between the angles of 0 to 15°. After 15°, the slopes of the line graph rose very fast, which meant unstable etching conditions. Hence, for a 1 × 1 mm area, a 30° (from −15 to +15°) processing window for laser light incident angle could be clearly inferred.

Figure 4. The image gray variance values changing with the incremental incident angle. Figure 4. The image gray variance values changing with the incremental incident angle. When changing the incident angle, at least four issues may be changed: laser light polarization, laser fluence, reflectivity and the hatch space between the lines. As a result, the etching quality and accuracy are affected more or less. In this study:

(1) The polarization of the outcome laser beam was circular; thus, it should be irrelevant to the etching direction and result. (2) When the laser light was normal to the surface (i.e., 0◦ of the incident angle), the projected area showed a circle shape. With the incident angle increasing, the projected area became elliptical, thus reducingFigure 4. The the image real laser gray fluencevariance tovalues the samplechanging surface, with theas incremental shown in incident Figure 5angle.. In the figure, dw is the raw focused diameter, θ is the incident angle and dx and dy are the projected oval axis diameters. The reduction of laser fluence may have caused less etching depth. However, Materials 2020, 13, 2808 6 of 11

this variation was in the range of depth of focus, and the laser fluence was about six times that of the threshold fluence. For this situation, the fluence change due to the incident angle changing could be a minor factor. (3) The reflectivity of the surface may have changed, which could also have had some effect on the etching [29,30]. For example, Chang et al. argued that the reflectivity was kept consistent and had not much difference on the ablation phenomenon when the incident angle was below 20◦ [31]. Materials 2020, 13, x FOR PEER REVIEW 6 of 12 Their conclusion fitted this study very well. With the incident angle going up further, it seemed thereWhen was changing an obvious the jumpincident of the angle, reflectivity, at least thusfour causingissues may uneven be changed: etching results.laser light A very polarization, similar laserphenomenon fluence, reflectivity was reported and the by Liaohatch et space al. when betw studyingeen the lines. laser weldingAs a result, [30]. the All etching the experimental quality and accuracyresults are suggested affected theremore shouldor less. beIn quitethis study: little reflectivity change when the incident angle breaks through a threshold value, and this change caused a worse processing quality. (4)(1) When The polarization doing area etching, of the outcome line-by-line laser hatching beam was was circular; usually adopted.thus, it should With thebe irrelevant incident angle to the varying,etching the direction hatch space and result. was also affected (see Figure6a for details). To simplify the situation, only (2) the When hatching the laser distance light in was the normaly direction to the was surface elongated. (i.e., 0° After of the the incident projection angle), transition, the projected distortion area happened:showed a thecircle square shape. area With changed the incident to a rectangle. angle increasing, Consequently, the projected the etching area dimension became elliptical, in the y thusdirection reducing could the have real been lasera fluenceffected fromto theL sample+ 2r to surface, (L + 2r) as/cos shownθ (see in Figure Figure6b,c). 5. In In the Figure figure,7, dw theis calculatedthe raw focused and measured diameter, dimension θ is the incident changes inangle the yanddirection dx and aredy are plotted, the projected correspondingly. oval axis Itdiameters. can be seen The that reduction the measured of laser experimental fluence may dimensions have caused are smallerless etching than depth. the calculated However, ones. this Thevariation smaller was the incidentin the range angle of was,depth the of less focu thes, and distortion the laser was. fluence The best was coincidence about six times was foundthat of at the threshold fluence. For this situation, the fluence change due to the incident angle changing could the incident angle of 10◦. be a minor factor.

Materials 2020, 13, x FOR PEER REVIEW 7 of 12

Figure 5. The projected spot size changing with the incident angle. θ is the angle between the laser Figure 5. The projected spot size changing with the incident angle. θ is the angle between the laser light and the normal vector of the sample surface. light and the normal vector of the sample surface.

(3) The reflectivity of the surface may have changed, which could also have had some effect on the etching [29,30]. For example, Chang et al. argued that the reflectivity was kept consistent and had not much difference on the ablation phenomenon when the incident angle was below 20° [31]. Their conclusion fitted this study very well. With the incident angle going up further, it seemed there was an obvious jump of the reflectivity, thus causing uneven etching results. A very similar phenomenon was reported by Liao et al. when studying laser welding [30]. All the experimental results suggested there should be quite little reflectivity change when the incident angle breaks through a threshold value, and this change caused a worse processing quality. (4) When doing area etching, line-by-line hatching was usually adopted. With the incident angle varying, the hatch space was also affected (see Figure 6a for details). To simplify the situation, only the hatching distance in the y direction was elongated. After the projection transition, Figuredistortion 6. A schematic happened: of the the projected square etching area changed area changing to a with rectangle. the incident Consequently, angle (a), the the ideal etching Figureetchingdimension 6. area A schematic (b )in and the the y of realdirection the elongated projected could etching etching have area areabeen (c ).changing affected Due to distortion with from the L inincident+ the2r toy direction, (angleL + 2r ()/cosa), the the etchingθ ideal(see Figure etchingaccuracy6b,c). area In was Figure (b reduced.) and 7, the the real calculated elongated andetching measured area (c). dimensionDue to distortion changes in the in y thedirection, y direction the etching are plotted, accuracycorrespondingly. was reduced. It can be seen that the measured experimental dimensions are smaller than the calculated ones. The smaller the incident angle was, the less the distortion was. The best coincidence was found at the incident angle of 10°.

Figure 7. A comparison of etching distortion between the theory and the experiment.

3.2. The Influence of Pulse Overlap and Focal Spot Size on Etching Uniformity and Base Materials Generally speaking, the electromagnetic absorption/filtration sensitivity of the FSSs relies largely on the etched area border uniformity rather than on the shape and dimensional accuracy. Pulse overlap and focal spot size are directly related to the etching area uniformity, which could be expressed by the burrs along the border edge. Figure 8 indicates the burrs variation with the pulse overlap changing. In the figure, EC presents the burrs dimension and could be calculated with the following equation:

2 EC=r（（）） 1- 1- 1-ε (3) where r is the radius of the spot size and ε is the percentage of pulse overlap, respectively. When OA = 0, two pulses totally overlapped and ε = 100%; when OA = r, two pulses just departed and ε = 0.

Materials 2020, 13, x FOR PEER REVIEW 7 of 12

Figure 6. A schematic of the projected etching area changing with the incident angle (a), the ideal etching area (b) and the real elongated etching area (c). Due to distortion in the y direction, the etching accuracy was reduced.

Materials 2020, 13, 2808 7 of 11

Figure 7. A comparison of etching distortion between the theory and the experiment. Figure 7. A comparison of etching distortion between the theory and the experiment. 3.2. The Influence of Pulse Overlap and Focal Spot Size on Etching Uniformity and Base Materials 3.2. TheGenerally Influence speaking, of Pulse Overlap the electromagnetic and Focal Spot absorption Size on Etching/filtration Uniformity sensitivity and ofBase the Materials FSSs relies largely on theGenerally etched area speaking, border uniformity the electromagnetic rather than onabsorption/filtration the shape and dimensional sensitivity accuracy. of the Pulse FSSs overlap relies largelyand focal on spot the sizeetched are directlyarea border related uniformity to the etching rather area than uniformity, on the shape which and could dimensional be expressed accuracy. by the Pulseburrs alongoverlap the and border focal edge. spot size Figure are8 directlyindicates related the burrs to the variation etching with area theuniformity, pulse overlap which changing. could be expressedIn the figure, by ECthepresents burrs along the burrsthe border dimension edge. andFigu couldre 8 indicates be calculated the burrs with thevariation following with equation: the pulse overlap changing. In the figure, EC presents the qburrs dimension and could be calculated with the following equation: EC= r(1 1 (1 ε)2) (3) − − − 2 Materialswhere r 2020is the, 13, radius x FOR PEER of the REVIEW spot size and EC=rε is the（（）） percentage 1- 1- 1-ε of pulse overlap, respectively. When OA8 of(3)= 120 , two pulses totally overlapped and ε = 100%; when OA = r, two pulses just departed and ε = 0. where r is the radius of the spot size and ε is the percentage of pulse overlap, respectively. When OA = 0, two pulses totally overlapped and ε = 100%; when OA = r, two pulses just departed and ε = 0.

Figure 8. A schematic diagram showing the burrs dimension changing with the pulse overlap. As can Figurebe seen, 8. theA schematicEC increases diagram with showing the prolonging the burrs of OA dimension. changing with the pulse overlap. As can be seen, the EC increases with the prolonging of OA. In Figure9, the burrs fluctuation with pulse overlap for di fferent spot sizes is shown. It is clear that the smaller the spot size and the more the pulse overlaps, the better the border uniformity. Thus far, In Figure 9, the burrs fluctuation with pulse overlap for different spot sizes is shown. It is clear a 15-µm border edge accuracy is an acceptable accuracy standard for FSS fabrication. It means the that the smaller the spot size and the more the pulse overlaps, the better the border uniformity. Thus laser parameters below the red dashed line are favorable in Figure9, theoretically. In order to further far, a 15-μm border edge accuracy is an acceptable accuracy standard for FSS fabrication. It means investigate the influence of the pulse overlap and focal spot size on the etching area uniformity, the laser parameters below the red dashed line are favorable in Figure 9, theoretically. In order to FSS pattern etching under spot sizes of 32 and 50 µm was carried out with the laser parameters in further investigate the influence of the pulse overlap and focal spot size on the etching area Table2, and the results are given in Figures 10 and 11. For the purposes of comparison, the laser fluences uniformity, FSS pattern etching under spot sizes of 32 and 50 μm was carried out with the laser used in the experiments were almost the same as 20.64 J/cm2 for the 32-µm laser spot and 20.27 J/cm2 parameters in Table 2, and the results are given in Figures 10 and 11. For the purposes of comparison, for the 50-µm laser spot. In the figures, the thermal effect is more and more obvious when the pulse the laser fluences used in the experiments were almost the same as 20.64 J/cm2 for the 32-μm laser overlap increased. The dark area indicates that some carbonized damage occurred. Additionally, spot and 20.27 J/cm2 for the 50-μm laser spot. In the figures, the thermal effect is more and more the border burrs effects showed that the smaller spot size favored stable etching uniformity and obvious when the pulse overlap increased. The dark area indicates that some carbonized damage stability. Optimal etching results usually occurred with pulse overlap at about 40%, which has been occurred. Additionally, the border burrs effects showed that the smaller spot± size favored stable explained in [32]. Combining the theoretical and experimental results, it could be inferred that the etching uniformity and stability. Optimal etching results usually occurred with pulse overlap at about ±40%, which has been explained in [32]. Combining the theoretical and experimental results, it could be inferred that the actual processing windows are smaller than those calculated, when the thermal effect and etching uniformity are considered, which are very important for FSS machining.

Figure 9. The relationship of the etching border burrs variation with the pulse overlap for different spot sizes. Under the current condition, an accuracy tolerance of 15 μm is required, which is shown with a red dashed line.

Materials 2020, 13, x FOR PEER REVIEW 8 of 12

Figure 8. A schematic diagram showing the burrs dimension changing with the pulse overlap. As can be seen, the EC increases with the prolonging of OA.

In Figure 9, the burrs fluctuation with pulse overlap for different spot sizes is shown. It is clear that the smaller the spot size and the more the pulse overlaps, the better the border uniformity. Thus far, a 15-μm border edge accuracy is an acceptable accuracy standard for FSS fabrication. It means the laser parameters below the red dashed line are favorable in Figure 9, theoretically. In order to further investigate the influence of the pulse overlap and focal spot size on the etching area uniformity, FSS pattern etching under spot sizes of 32 and 50 μm was carried out with the laser parameters in Table 2, and the results are given in Figures 10 and 11. For the purposes of comparison, the laser fluences used in the experiments were almost the same as 20.64 J/cm2 for the 32-μm laser spot and 20.27 J/cm2 for the 50-μm laser spot. In the figures, the thermal effect is more and more obvious when the pulse overlap increased. The dark area indicates that some carbonized damage Materialsoccurred.2020 ,Additionally,13, 2808 the border burrs effects showed that the smaller spot size favored 8stable of 11 etching uniformity and stability. Optimal etching results usually occurred with pulse overlap at about ±40%, which has been explained in [32]. Combining the theoretical and experimental results, it actual processing windows are smaller than those calculated, when the thermal eﬀect and etching could be inferred that the actual processing windows are smaller than those calculated, when the uniformity are considered, which are very important for FSS machining. thermal effect and etching uniformity are considered, which are very important for FSS machining.

Figure 9. The relationship of the etching border burrs variation with the pulse overlap for different Figure 9. The relationship of the etching border burrs variation with the pulse overlap for different spot sizes. Under the current condition, an accuracy tolerance of 15 µm is required, which is shown spot sizes. Under the current condition, an accuracy tolerance of 15 μm is required, which is shown with a red dashed line. with a red dashed line. In order to further characterize the possible damage to base materials after laser etching, SEM pictures were captured and analyzed. In Figure 12a, the plain surface of the Al layer before etching is given. It looks very smooth, indicating good original flatness. According to observations of Figures 10 and 11, samples etched with 30% pulse overlap and 40% line pitch showed good processing appearance and were then selected for SEM characterization. In Figure 12b,c, SEM images of base materials after laser etching were exposed, corresponding to the focal spot sizes of 32 and 50 µm, respectively. From the two figures, complete silica fibers still existed there because of the high melting point, revealing no obvious damage to the composite materials. Some resin residue on the fiber surface of the latter may indicate the etching consistence is inversely proportional to the focal spot size. On one hand, the smaller the focal spot size is, the better the etching consistence is; on the other hand, the smaller the focal spot size is, the lower the etching efficiency is. Therefore, a balance between the twoMaterials sides 2020 should, 13, x FOR be considered.PEER REVIEW 9 of 12

Figure 10. The etching results with diﬀerent pulse overlaps by using a 32-µm focal spot size. The areas Figure 10. The etching results with different pulse overlaps by using a 32-μm focal spot size. The areas looped with red lines showed better etching results. looped with red lines showed better etching results.

Figure 11. The etching results with different pulse overlaps by using a 50-μm focal spot size. The areas looped with red lines showed better etching results.

Materials 2020, 13, x FOR PEER REVIEW 9 of 12

MaterialsFigure2020 ,10.13, The 2808 etching results with different pulse overlaps by using a 32-μm focal spot size. The areas9 of 11 looped with red lines showed better etching results.

µ MaterialsFigure 2020 11., 13 ,The Thex FOR etching etching PEER resultsREVIEW with different diﬀerent pulse overlaps by using a 50-μm focal spot size. The areas 10 of 12 looped withwith redred lineslines showedshowed betterbetter etchingetching results.results.

In order to further characterize the possible damage to base materials after laser etching, SEM pictures were captured and analyzed. In Figure 12a, the plain surface of the Al layer before etching is given. It looks very smooth, indicating good original flatness. According to observations of Figures 10 and 11, samples etched with 30% pulse overlap and 40% line pitch showed good processing appearance and were then selected for SEM characterization. In Figure 12b,c, SEM images of base materials after laser etching were exposed, corresponding to the focal spot sizes of 32 and 50 μm, respectively. From the two figures, complete silica fibers still existed there because of the high melting point, revealing no obvious damage to the composite materials. Some resin residue on the fiber Figure 12. SEM images of the studied samples: (a) image of the unprocessed Al plate layer showing a surfaceFigure of the 12. SEMlatter images may indicate of the studied the etching samples: consis (a) imagetence of isthe inversely unprocessed proportional Al plate layer to showingthe focal a spot relatively flat surface, (b) sample processed with a 32-µm focal spot size and (c) sample processed with size. relativelyOn one hand, flat surface, the smaller (b) sample the fo processedcal spot size with is, a the32-μ betterm focal the spot etching size and consistence (c) sample is; processed on the other a 50-µm focal spot size, showing silica fibers undamaged. hand,with the a smaller50-μm focal the spot focal size, spot showing size is,silica the fibers lowe undamaged.r the etching efficiency is. Therefore, a balance 4.between Conclusions the two sides should be considered. 4. Conclusions Nanosecond laser etching of aluminum-plated composite materials applied to FSSs was demonstrated Nanosecond in thislaser paper. etching The of influencealuminum-plated of the laser composite incident materials angle, pulse applied overlap to FSSs and focalwas spotdemonstrated size were examinedin this paper. and The discussed. influence The of results the laser led toincident the following angle, pulse conclusions: overlap and focal spot size were examined and discussed. The results led to the following conclusions: The laser incident angle plays a pivotal role to guarantee FSS etching quality and accuracy. It is •  foundThe laser that incident an incident angle angleplays rangea pivotal of role15 tois gu suitablearantee for FSS 20- etchingµm aluminum-layer quality and accuracy. composite It is ± ◦ materialfound that etching an incident within theangle depth range of focus.of ±15° is suitable for 20-μm aluminum-layer composite material etching within the depth of focus. As the laser incident angle changes, the variation of laser light reflectivity and hatch space between • As the laser incident angle changes, the variation of laser light reflectivity and hatch space lines causes the etching quality instability. between lines causes the etching quality instability. Laser pulse overlap and focal spot size affects not only etching border accuracy and uniformity but • Laser pulse overlap and focal spot size affects not only etching border accuracy and uniformity also the material removal consistence. Combining the theoretical and experimental results, it is but also the material removal consistence. Combining the theoretical and experimental results, inferred that the actual processing windows are smaller than of those calculated, when thermal it is inferred that the actual processing windows are smaller than of those calculated, when effect and etching uniformity are considered. thermal effect and etching uniformity are considered. In order to ensure etching area uniformity and no damage to base materials, a pulse overlap of • In order to ensure etching area uniformity and no damage to base materials, a pulse overlap of 30~50% and a relatively small focal spot size are recommended from the experimental results. 30~50% and a relatively small focal spot size are recommended from the experimental results. By reasonably selecting a processing window, optimal etching results on FSSs with the nanosecond • By reasonably selecting a processing window, optimal etching results on FSSs with the fibernanosecond laser could fiber be laser obtained. could be The obtained. method The and method experimental and experime techniquesntal techniques in this study in this could study be generalizedcould be generalized to different to FSSdifferent material FSS composition material composition processing. processing.

Funding: This research is financially supported by the Natural Science Founding of Hubei (Grant Number: 2019CFB509 and 2019CFB163), Aero-science Research Funds (Grant Number: 201818X5002), Talent Recruitment Project for Hubei University of Technology (Grant Number: BSQD2019001) and the Key Research and Development Program of Shandong Province, China (Grant Number: 2018CXGC0809).

Acknowledgments: The authors appreciate the Aeronautical Science Key Lab for High Performance Electromagnetic Windows of China for providing technical supports in this paper.

Conflicts of Interest: The authors declare that there are no conflicts of interest regarding the publication of this paper.

References

Materials 2020, 13, 2808 10 of 11

Author Contributions: Conceptualization, D.L. (Dun Liu); formal analysis, D.L. (Deyuan Lou), Q.Y. and X.Y.; investigation, S.J., Q.T., Z.Z. and L.C.; methodology, J.C. All authors have read and agreed to the published version of the manuscript. Funding: This research is financially supported by the Natural Science Founding of Hubei (Grant Number: 2019CFB509 and 2019CFB163), Aero-science Research Funds (Grant Number: 201818X5002), Talent Recruitment Project for Hubei University of Technology (Grant Number: BSQD2019001) and the Key Research and Development Program of Shandong Province, China (Grant Number: 2018CXGC0809). Acknowledgments: The authors appreciate the Aeronautical Science Key Lab for High Performance Electromagnetic Windows of China for providing technical supports in this paper. Conflicts of Interest: The authors declare that there are no conflicts of interest regarding the publication of this paper.

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