Effect of laser shock peening without coating on fretting corrosion of copper contacts

Journal Pre-proofs Full Length Article Effect of laser shock peening without coating on fretting corrosion of copper contacts Changkyoo Park, Donghyuck Jung, Eun-Joon Chun, Sanghoon Ahn, Ho Jang, Yoon-Jun Kim PII: DOI: Reference:

S0169-4332(20)30673-5 https://doi.org/10.1016/j.apsusc.2020.145917 APSUSC 145917

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

27 December 2019 23 February 2020 25 February 2020

Please cite this article as: C. Park, D. Jung, E-J. Chun, S. Ahn, H. Jang, Y-J. Kim, Effect of laser shock peening without coating on fretting corrosion of copper contacts, Applied Surface Science (2020), doi: https://doi.org/ 10.1016/j.apsusc.2020.145917

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Effect of laser shock peening without coating on fretting corrosion of copper contacts

Changkyoo Parka,▽, Donghyuck Jung b,▽, Eun-Joon Chunc, Sanghoon Ahna, Ho Jangd, YoonJun Kimb,*

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Laser and Electron Beam Application Department, Korea Institute of Machinery and Materials, Daejeon, 34103, Republic of Korea

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Department of Materials Science and Engineering, Inha University, Incheon, 22212, Republic of Korea

Department of Nano Materials Science and Engineering, Kyungnam University, Changwon, 51767, Republic of Korea d

Department of Materials Science and Engineering, Korea University, Seoul, 02841, Republic of Korea

*Corresponding author: Yoon-Jun Kim, E-mail: [email protected] ▽Changkyoo

Park and Donghyuck Jung contributed equally to this work.

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Abstract The effects of laser shock peening without coating (LSPwC) on the degradation of copper electrical contact was investigated. A Nd:YAG laser with laser energy densities of 5.3 and 10.6 GW/cm2 was used for the LSPwC process. Surface hardness was enhanced from 55 HV to 110 and 120 HV for the laser shock-peened copper at 5.3 GW/cm2 and 10.6 GW/cm2, respectively. Moreover, near the copper surface, LSPwC introduced the max. compressive residual stress of 387.5 and 385.5 MPa for laser energy densities of 5.3 and 10.6 GW/cm2, respectively. Electron backscatter diffraction and transmission electron microscopy revealed that LSPwC introduced dislocation rearrangement, deformation twins, and grain refinement. The laser shock-peened copper exhibited superior wear resistance compared with the base metal. During the fretting test, the wear loss of the base metal was 1.61 × 10-3 mm3, and this decreased to 0.99 × 10-3 and 0.94 × 10-3 mm3 for the laser shock-peened copper at 5.3 and 10.6 GW/cm2, respectively. Thus, the laser shock-peened copper maintained a low electrical contact resistance during the fretting test, resulting in electrical contact failure delay from 2790 cycles for the base metal to 5011 and 5210 cycles for laser shock-peened copper at 5.3 and 10.6 GW/cm2, respectively.

Keywords: Laser shock peening without coating, copper, grain refinement, compressive residual stress, fretting corrosion

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1. Introduction Premature electrical failure occurs because of the friction-induced wear debris of materials that serve as metallic contacts in electric components such as switches and power connectors [1-3]. In particular, electrical devices used in transportation are subject to various conditions that can induce malfunctioning of electrical contacts. The wear at the electrical contacts of electrical components in automobiles occurs because of the small amplitude of reciprocal motion at these contacts produced by driving-induced vibrations [4]. This causes fretting wear, followed by the corrosion of the wear debris. The agglomerate of corroded wear debris generates an insulating layer at the electrical contacts, interrupting the electrical signal transmission. Eventually, the fretting corrosion at the contact junction causes an electrical failure [5-9]. Over the past two decades, electrical devices in automobiles have been increasingly developed to achieve a high degree of safety and comfort. Therefore, their robust design with high durability electrical junctions has been studied by developing appropriate materials [10]. Copper is widely used to manufacture electrical contacts because of its low cost and high electrical conductivity [11]. Noble metals including gold, silver, and silver alloys have been used as coating materials to improve the electrical conductivity and oxidation resistance of these contacts [12-14]; however, the high cost of these metals limits their use. Sn-coated Cu has also been utilized owing to the low friction caused by the relatively soft Sn layer at the electrical contact junctions. However, Sn often forms hard oxide particles under fretting motion, resulting in early electrical failure [9, 15-17]. Further, several studies have investigated the grain size effect on fretting wear [18-22]. Noh et al. prepared different grain sizes of copper by controlling the annealing time to improve wear resistance [22]. 3

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The laser shock peening (LSP) process is a versatile and competitive surface modification technique, which induces the formation of compressive residual stress, surface hardening, and grain refinement. As a result, the LSP process helps improve fatigue strength [23-25], wear resistance [26-29], and corrosion resistance [30-32]. For the LSP process, high laser pulse energy (< 100 J) with nanosecond-range pulse duration is utilized [33]. The specimen is prepared with a thin coating layer (e.g., black paint or Al and Cu foils), which absorbs the energy from the laser beam. Transparent materials (e.g., glass or water) are used as confining mediums, covering the specimen. High-energy laser pulses are applied to a coating layer through the confining medium. The coating layer subsequently vaporizes and forms a plasma. With the continued application of laser pulses, the plasma drastically expands, while the confining medium restrains the plasma expansion. Thus, a high-pressure plasma is created between the specimens and the confining medium, inducing the propagation of a shock wave in the specimen [34]. If the shock wave is sufficiently large, exceeding the Hugoniot elastic limit (HEL) of the specimen, the evolution of microstructure and the modification of mechanical properties are introduced at the surface of specimen. However, the preparation of coating layer on specimen results in a slow LSP process, which is not economical for industrial applications [35]. Therefore, another LSP technique, called laser shock peening without coating (LSPwC), was developed in 1995 by Mukai et al. [36], which does not involve a coating layer. Relatively lower laser energy, shorter pulse duration, and higher overlap ratio are required compared with those of the conventional LSP processes make LSPwC possible to use no coating materials [37]. Several studies have investigated the influence of the LSP on copper [3, 38-41]. Fabbro et al. [38] reported the duration and pressure of the laser-generated plasma in confined 4

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geometry. Ye et al. [40] and Chen et al. [41] used finite element modeling to predict the plastic deformation behavior induced by LSP. Ye et al. [3] studied the influence of temperature of the LSP process on microstructure and mechanical properties of copper. However, to the best of our knowledge, only a few researchers have studied the influence of LSPwC process on the mechanical properties and microstructures of copper, and its effect on fretting corrosion. In this study, the effects of LSPwC process on the durability of the copper electrical contacts were investigated. The fretting test was conducted for the base metal and laser shock-peened copper to analyze the fretting cycles of electrical contact failure. The LSPwC-induced microstructure evolution was studied by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), and the surface hardness and residual stress was measured before and after the LSPwC. Moreover, the reason of electrical contact failure was examined by the surface morphology and chemical composition of the contact area.

2. Materials and methods 2.1. Specimen preparation and experimental setup for LSP A cold worked-oxygen free high conductivity (OFHC) copper plate (purity: 99.99%) was furnace-annealed in Ar atmosphere at 180 °C for 1 h, and subsequently cooled down to 25 °C by a furnace cooling [42]. The experimental setup for the laser shock peening without coating (LSPwC) process is described in Figure 1. A Q-switch Nd:YAG laser (Powerlite Furie, Continuum Inc.) with a wavelength of 1064 nm, pulse duration of 12 ns, and frequency of 10 Hz was used for the LSPwC process. The diameter of the laser beam was 2 mm with a uniform flat-top energy distribution. No ablative coating material was applied, and a water jet setup 5

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with distilled water was employed for the confining medium. Approximately 1–2 mm thickness of water flow was maintained to prevent the formation of water bubble and impurities concentration. The overlap ratio of the laser beam was 50% in both the x- and y-directions. The applied laser energies were 2 and 4 J, corresponding to the laser energy densities of 5.3 and 10.6 GW/cm2, respectively.

2.2. Microhardness and residual stress The hardness of the base metal and laser shock-peened copper was measured using a Vickers hardness tester (MMT-X7, Matsuzawa) with a load of 200 gf and a holding time of 10 s. For X-ray diffraction analysis, the copper samples were prepared in a size of 8 × 8 mm2 using a low-speed cutting wheel, then polished by SiC papers with a grit size from 200 to 3000. The removed thickness of copper samples was approximately 100 μm. The qualitative analysis of the base metal and laser shock-peened copper was conducted via X-ray diffraction (XRD; SmartLab, Rigaku) with Cu-Kα radiation (λ = 1.5406 Å). The tube voltage and current were set as 40 kV and 30 mA, respectively. Moreover, the transverse residual stress (σT) was measured at depth intervals of 200 μm by X-ray diffraction (XRD; D/MAX-2500/PC, Rigaku) with Cu-Kα radiation using a two-angle sin2Ψ technique. The residual stress was measured three times to improve reliability; further, Table 1 lists the parameters used in the residual stress measurement.

2.3. Microstructure For electron backscatter diffraction (EBSD; Quanta 200 FEG, FEI) analysis, the 6

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copper specimens with a dimension of 10 × 10 mm2 were prepared using a low-speed cutting wheel. The specimens were polished by SiC papers with a grit size from 200 to 4000, and then fine-polished with 3, 1, and 0.04 μm colloidal silica suspensions. The EBSD microstructure analysis was conducted with a 5.5 μm beam step size, and OIM Analysis™ v8 was used to determine the average grain size. The chemical composition and surface morphology of the contact area were investigated using energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM; SU5000, Hitachi). To analyze dislocation structures and grain refinement, transmission electron microscopy (TEM; JEM-2200FS, JEOL) observation was conducted at 200 kV. The TEM samples were prepared at the top surface of the LSPwC area (e.g. parallel cut to LSPwC) by focused ion beam (FIB; Helios NanoLab DualBeam 650, FEI) using Ga+ ions. After several steps of milling, the TEM samples were located on FIB lifeout copper grid (Omniprobe), and then multiple steps of thinning and cleaning were conducted.

2.4. Fretting test The fretting test was conducted using a reciprocating friction instrument (RFW160, NeoPlus). The average surface roughness (Ra) of the polished copper specimens used for the fretting test was less than 0.4 μm. The surface roughness was based on area measurement and conducted by laser scanning microscopy (VK-8710, Keyence) with a wavelength of 658 nm and depth resolution of 0.1 μm. The dimension of the copper specimen was 40 (W) × 40 (D) × 10 (H) mm3, and a A194-2H steel ball with a diameter of 9.5 mm was used as the counter material (Figure 2). Different normal loads of 10, 15, and 20 N, frequency of 4 Hz, and displacement amplitude of 300 μm were applied to the copper specimen for 10,000 cycles of 7

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the fretting test. A constant direct current of 1 A was supplied to both the copper specimen and A194-2H steel ball during the fretting test. Simultaneously, the resistance contact (Rc) was measured using a four-point probe setup, as shown in Figure 2. The fretting tests were conducted at 25 °C at a relative humidity of 40–60%. After the fretting test, the total wear loss was analyzed via laser scanning microscopy (VK-8710, Keyence). All fretting tests were repeated three times, and the obtained wear losses were averaged.

3. Results and discussion 3.1. Microstructures Figure 3 shows the X-ray diffraction profiles of the base metal and laser shock-peened copper surfaces. No phase transformation was detected after the LSPwC process was conducted, as shown in Figure 3 (a). However, the marginal peak shifting toward higher 2θ values and broadening was observed in the XRD profile after the LSPwC process. Figure 3 (b) shows the magnified XRD peak of the (1 1 1) plane where peak shifting and broadening are identified for the laser shock-peened copper. Figure 3 (c) shows the XRD peaks width at full width half maximum (FWHM) for different diffraction planes. The laser shock-peened copper exhibits relatively larger peak width in comparison to that of the base metal. This peak shifting and broadening can be attributed to the presence of compressive residual stress and surface hardening [27, 43-45] in the microstructure after LSP. Ye et al. [3] conducted LSP on copper at room and cryogenic temperatures, and both results also exhibited the XRD peak broadening due to the high strain-rate deformation. Figure 4 shows EBSD inverse pole figure (IPF) maps of the cross-sectional 8

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microstructure of the base metal and laser shock-peened copper at laser energy densities of 5.3 and 10.6 GW/cm2. Grain refinement and twin structures were induced by the LSPwC process at both 5.3 and 10.6 GW/cm2. That is, relatively smaller grain size with the number of twinning was detected for laser shock-peened copper compared with the base metal. Owing to high stack fault energy (78 mJ/m2 [46]), the high strain rate is necessary to generate the deformation twinning in copper. Ye et al. [3] reported that the deformation twinning in copper was not formed by room temperature LSP. However, LSP conducted at cryogenic temperature generated the deformation twinning by inhibiting dislocation slip. In this study, the deformation twinning in copper was formed by room temperature LSP without using an ablative coating material. Direct ablation of copper may develop relatively higher confined plasma pressure in comparison to that of the LSP process with coating materials, resulting in large shock wave propagation and high strain-rate in copper. The average grain size was found to be 159.3 μm for the base metal. Conversely, for laser shock-peened copper at 5.3 GW/cm2, the average grain sizes were 102.1 and 96.5 μm for region 1 (from the surface to 500 μm depth) and region 2 (from 500 to 1000 μm depth), respectively. These values increased to 130.3 μm for region 3 (from 1000 to 1500 μm depth). The LSPwC-induced grain refinement was detected up to a depth of approximately 1000 μm. Moreover, for laser shock-peened copper at 10.6 GW/cm2, the measured average grain sizes for regions 1, 2, and 3 were measured as 107, 111.9, and 107.9 μm, respectively. The grain size increased to 133.9 μm for region 4 (from 1500 to 2000 μm depth) and 159.2 μm for region 5 (from 2000 to 2500 μm depth). The LSPwC-induced grain refinement was observed up to a depth of approximately 1500 μm. Figure 5, 6, and 7 show the TEM analysis of the dislocation arrangements and grain 9

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refinement for the base metal and laser shock-peened copper surfaces at laser energy densities of 5.3 and 10.6 GW/cm2. Randomly arranged low density of dislocation and dislocation line (DL) were observed for the base metal, as shown in Figure 5. In contrast, the laser shockpeened copper at 5.3 GW/cm2 (Figure 6) showed complex and random dislocation structures via the LSPwC-induced severe plastic deformation. In Figure 6 (a), the dislocation structures in the form of DLs, dislocation tangles (DTs), dense dislocation walls (DDWs), and dislocation cells were observed, and these structures can be generated by the high strain-induced dislocation rearrangement and accumulation. In Figure 6 (b), 60–200 nm sized of ultra-fine grains and 20–50 nm size of nano-grains were detected next to the sub-grains, which substantiates the grain refinement at the laser shock-peened surface. Figure 6 (c) also shows the high density of dislocation structures and nano-grain formation. Similar dislocation structures and microstructures were obtained for the laser shock-peened copper at 10.6 GW/cm2, as shown in Figure 7. The dense dislocation structures including DTs, DDWs, and dislocation cells, as well as 60–200 nm size of ultra-fine grains and 20–50 nm size of nanograins were detected in the microstructures. The EBSD and TEM analyses reveal that the LSPwC process significantly refines the microstructure of copper. For the aluminum alloy, the grain refinement mechanism induced by the laser shock peening process was previously studied by Lu et al. [47] and Trdan et al. [48]. On the contrary, Mishra et al. [49] and Li et al. [50] proposed the grain refinement mechanisms of copper caused by equal channel angular pressing (ECAP) and dynamic plastic deformation (DPD), respectively. The grain refinement mechanism did not differ based on the difference in material. Therefore, the grain refinement mechanism proposed by Lu et al. and Trdan et al. can be adopted to the LSPwC process of copper; their theory is summarized as follows. The LSPwC 10

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process causes severe plastic deformation and increased strain-rate in the materials, resulting in the formation of high density of dislocation structures including dislocation tangles, dense dislocation walls, and dislocation cells via dislocation movement and accumulation. Further LSPwC treatment facilitates the formation of sub-grains in the original coarse grains with the generation of new grain boundaries by DTs and DDWs. Eventually, the significantly high strains induced by the repetitive LSPwC treatment produce 60–200 nm size of ultra-fine grains and 20–50 nm size of nano-grains via division of the sub-grains. Therefore, the total energy in the materials is minimized due to dislocation annihilation.

3.2. Microhardness and residual stress Figure 8 shows the microhardness as a function of the distance from the surface before and after the LSPwC process is conducted. The microhardness was 55 HV at the base metal surface and increased to 110 and 120 HV after LSPwC at laser energy densities of 5.3 and 10.6 GW/cm2, respectively. For the laser shock-peened copper at 5.3 GW/cm2, a comparable microhardness with the surface microhardness was detected to a depth of 800 μm, and relatively lower values were measured at 900 and 1000 μm. Eventually, equivalent microhardness with the base metal was measured at a depth of 1100 μm. The decrease in hardness was attributed to the reduction of shock wave propagation due to confined plasma along the depth. A similar trend was also observed for the laser shock-peened cooper at 10.6 GW/cm2, and increased microhardness was detected up to a depth of 1400 μm. A higher laser energy density caused a greater depth of the hardening effect than a lower laser energy density. The development of nanostructures and deformation twins was attributable to an increase in hardness. Ye et al. [3] also reported that the hardness of copper can be increased by laser shock 11

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peening. Room temperature and cryogenic temperature laser shock peening achieved a surface hardness of 99.7 and 124.4 HV, respectively. A relatively higher surface hardness was achieved using cryogenic temperature laser shock peening due to the existence of deformation twinning. Figure 9 shows the transverse residual stress as a function of the distance from the surface for the base metal and laser shock-peened copper at laser energy densities of 5.3 and 10.6 GW/cm2. The base metal showed marginal tensile stress, while compressive residual stress was detected for both laser shock-peened coppers. For the laser shock-peened copper at 5.3 GW/cm2, the compressive residual stress was maintained to a depth of 1000 μm, and was converted to marginal tensile residual stress at 1200 μm. For the laser shock-peened copper at 10.6 GW/cm2, the compressive residual stress was measured to a depth of 1400 μm and transitioned to a low tensile residual stress at 1600 μm. For both laser shock-peened coppers, a slightly lower compressive residual stress was observed at the surface, which may have originated from the local surface heating or melting due to an absence of ablative coating material. Moreover, a marginal tensile stress was generated at 1200 μm depth for the LSP at 5.3 GW/cm2 and at 1600 μm for the LSP at 10.6 GW/cm2 to balance the formed-compressive residual stress [51]. The microhardness and residual stress results are in agreement with the EBSD-based microstructure analysis in terms of the effected-depth of the LSPwC process.

3.3. Fretting test Fretting tests were performed for 10,000 cycles at normal loads of 10, 15, and 20 N for the base metal and laser shock-peened copper. Simultaneously, the four-point probe setup measured the contact resistance (Rc) between the copper specimen and counter material. Figure 12

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10 shows the average coefficient of friction (COF) as a function of the normal load where the base metal showed the highest average COF at each normal load. In contrast, comparable and relatively lower average COF values were obtained for the laser shock-peened copper at 5.3 and 10.6 GW/cm2 compared to those of the base metal. Higher average COF values were observed with increasing normal loads for both the base metal and laser shock-peened copper. Figure 11 shows the COF values and electrical contact resistance as a function of fretting cycles at a normal load of 10 N. In this study, the critical number of fretting cycles was defined as the value where the contact resistance exceeded 0.1 Ω for more than 10 subsequent cycles [22, 52]; this is indicated with an arrow. For the base metal in Figure 11 (a), the average COF was 0.36, and the critical number of fretting cycles was 2790 cycles. For the laser shockpeened copper, the average COF values were 0.33 and 0.32 for laser energy densities of 5.3 GW/cm2 (Figure 9 (b)) and 10.6 GW/cm2 (Figure 9 (c)), respectively. Moreover, the critical numbers of fretting cycles were 5011 and 5210 for the LSP at 5.3 and 10.6 GW/cm2, respectively. For the base metal, as shown in Figure 11 (a), the COF values drastically increase during the initial stages of the fretting test (up to approximately 520 fretting cycles), and the slope of the COF values subsequently decrease with a more gradual increase in the COF values (from approximately 520 to 2370 fretting cycles). Then, relatively stable and uniform COF values were detected from approximately 2370 to 10,000 fretting cycles. In contrast, the contact resistance showed relatively low and uniform values until approximately 2000 fretting cycles, and then an abrupt increase with large variation was observed. The COF and contact resistance values for both laser shock-peened coppers showed similar trends as those of the base metal. However, for laser shock-peened copper, the period of abrupt increase in contact resistance was significantly delayed compared with that of the base metal. 13

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The contact area surface morphology was examined to investigate the increased contact resistance during the fretting tests. Figure 12 shows SEM images of the contact area for the base metal after 100, 1000, 2000, and 5000 fretting cycles at a normal load of 10 N. The locations of the high-magnification SEM images are marked with dotted white boxes in the low magnification SEM images. At 100 fretting cycles, a relatively small and smooth contact area was detected with minimal damages, as shown in Figure 12 (a). At 1000 fretting cycles (Figure 12 (b) and (c)), delamination and abrasion grooves were observed at the original top surface. Moreover, the accumulation of wear debris was detected at the worn surface, which was only limited at the center of contact area. At 2000 fretting cycles, the wear accumulation was spread over the contact area, and the original copper top surface was only maintained at the edge of the contact area (Figure 12 (d)). The formation of a wear debris layer was observed in the magnified-image (Figure 12 (d)), and this may have developed due to a combination of wear debris accumulation and normal load application during the fretting test. At 5000 fretting cycles, the wear debris accumulation and formation of wear debris layer were observed over the entire contact area, as shown in Figure 12 (f). In the high-magnification SEM image (Figure 12 (g)), the formation of a wear debris layer was detected, and EDX analysis was conducted to investigate its chemical composition. Oxygen was identified from the chemical composition analysis, substantiating that the wear debris layer underwent oxidation. Small amounts of Fe and Cr were detected due to the wear of the counter material (A194-2H) during the fretting test. Noh et al. [22] also reported the formation of the oxidized-wear debris layer during the fretting tests of copper, and X-ray photoelectron spectroscope (XPS) analysis revealed the main chemical composition of wear debris layer was Cu2O. The COF and electrical contact resistance values (Figure 11 (a)) appear to be correlated 14

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with the contact area surface morphology (Figure 12). The sharp increase in COF values in the initial stages of the fretting test (up to approximately 520 cycles) was attributed to the oxide layer removal and surface leveling. The gradual increase in COF values (from approximately 520 to 2370 cycles) was likely caused by the increase in the contact area, interfacial shear of material, and wear debris formation by abrasion wear mechanism at the contact area [22]. The delamination caused by interfacial shear and wear debris formation and accumulation are observed from the contact area at 1000 and 2000 fretting cycles as shown in Figure 12 (c) and (e). Moreover, the steady-state period of COF (from approximately 2370 to 10,000 cycles) values was achieved when the produced wear debris stacked into big pieces, and finally formed the wear debris layer at the contact area [53]. As shown in Figure 12 (f) and (e), the development of wear debris layer is detected for the contact area at 5000 fretting cycles. This result agrees with the theory proposed by Hurricks et al. [54], who proposed three steps (e.g. the adhesion and metal transfer, the formation of oxidation of wear debris, and steady state wear condition) in the fretting wear process. On the contrary, the contact resistance maintained relatively low and stable values approximately until 2000 fretting cycles. Then, the abrupt increase with a large fluctuation of contact resistance was observed. The oxidized-wear debris layer plays a key role in electrical conduction at the electric contact. It acts as an electrically insulating layer, resulting in an increase in the contact resistance, and finally induces an electrical contact failure. At the early stage of the fretting test, a low contract resistance was obtained because the oxidized-wear debris layer was not fully developed at the contact area, as shown in Figure 12 (b) and (d). When the formation of oxidized-wear debris layer was spread over the contact area, electrical conduction was hindered, thereby increasing contact resistance and electrical failure [22, 55, 56]. Moreover, the large variation of contact resistance in this 15

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period may have been induced by repetitive formation and collapse of the wear debris layer during the fretting test [57]. Figures 13 shows SEM images of the contact area for the laser shock-peened copper at 5.3 GW/cm2 at 2000, 4000, and 6000 fretting cycles at a normal load of 10 N. Figure 13 (a) shows the surface morphology of the contact area for 2000 fretting cycles, and a relatively smaller size of wear scar and damage by reciprocal motion were detected for the contact area than those of the base metal. The delamination and abrasion grooves with the remainder of the original top surface were detected at the contact area. At 4000 fretting cycles (Figure 13 (b)), the debris accumulation and wear debris layer were observed at the contact area, while the original top surface still remained so as to preserve metallic contacts. Thus, it may help to maintain a low contact resistance as shown in Figure 11 (b). Figure 13 (c) shows the SEM image and EDX analysis at the contact area for 6000 fretting cycles. The wear debris accumulation and formation of wear debris layer were detected over the entire contact area, which induced an increase in contact resistance as shown in Figure 11 (b). However, a relatively smaller amount of oxygen was detected by the EDX analysis in comparison to that of the base metal at 5000 fretting cycles. This result indicates that the laser shock-peened copper underwent a slower oxidation process than that of the base metal due to the late formation of wear debris particles and layer. The increased microhardness and formation of compressive residual stress of the laser shock-peened copper resulted in reduced wear debris in comparison to that of the base metal, thereby hindering the formation of the electrically insulating oxidized-wear debris layer at the contact area [22, 44, 58, 59]. Therefore, the laser shock-peened copper maintained a low level of electrical contact resistance and indicated a long and stable lifetimes as electrical contacts. 16

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Figure 14 presents the critical number of fretting cycles as a function of applied normal load for the base metal and laser shock-peened copper. The base metal showed the lowest critical number of fretting cycles at each normal load, and comparable critical numbers were obtained for the laser shock-peened copper at 5.3 and 10.6 GW/cm2. The critical number of fretting cycles decreased with increasing normal load for both the base metal and laser shockpeened copper. Figure 15 shows the total wear loss after 10,000 fretting cycles for the base metal and laser shock-peened copper. The base metal showed the highest wear loss at each normal load, whereas the laser shock-peened copper at 5.3 and 10.6 GW/cm2 demonstrated relatively lower wear losses than that of the base metal. For the base metal, the total wear loss was 1.61 × 10-3 mm3 at a normal load of 10 N, which decreased to 0.99 × 10-3 mm3 at 5.3 GW/cm2 and 0.94 × 10-3 mm3 at 10.6 GW/cm2. The earlier contact failure of the base metal compared to the laser shock-peened copper was likely induced by the larger wear loss. The produced wear debris during the fretting test eventually forms an electrically insulating oxide layer at the contact area, causing the electrical contact to fail. The earlier contact failure with increasing normal load for both the base metal and laser shock-peened copper can also be attributed to the larger wear loss. Figure 16 shows the depth profiles of the wear scars developed after 10,000 fretting cycles at a normal load of 10 N. The greatest depth and largest length of the wear scar was observed in the base metal, which showed the largest wear loss. In contrast, comparable wear scar depths and lengths were detected for the laser shock-peened copper at 5.3 and 10.6 GW/cm2. It showed relatively smaller wear scars and lesser wear loss than the base metal. Figure 17 shows SEM images of the developed wear scars on the base metal (Figure 17 (a)) and laser shock-peened copper at 5.3 GW/cm2 (Figure 17 (b)) and 10.6 GW/cm2 (Figure 17 17

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(c)) at a normal load of 10 N after 10,000 fretting cycles. For both the base metal and laser shock-peened copper, a large amount of wear debris accumulated, forming an electrically insulating oxide layer over the wear scar, which induced electrical contact failure. Slightly larger wear scars were detected for the base metal compared to that of the LSP copper. The length and width of the wear scar on the base metal were 1215 and 762 μm, respectively. In contrast, a wear scar with a length of 1114 μm and a width of 701 μm was detected at 5.3 GW/cm2, and a wear scar with a length of 1091 μm and a width of 703 μm was observed at 10.6 GW/cm2.

4. Conclusion This study investigated the influence of LSPwC on the mechanical properties and microstructure evolution of high purity copper. The following conclusions were drawn: 1. Microstructures: No phase transformation was observed in the X-ray diffraction profile, while marginal peak shifting and broadening was detected after LSPwC. The EBSD analysis revealed that the microstructure was significantly refined with the formation of deformation twins after LSPwC. The measured average grain size at the sub-surface of the base metal was 159.3 μm; for it decreased to 102.1 and 107 μm for the laser shock-peened copper at 5.3 and 10.6 GW/cm2, respectively. Moreover, the effected-depth after LSPwC treatment was found to be approximately 1000 μm at 5.3 GW/cm2 and 1500 μm at 10.6 GW/cm2. Dense dislocation structures in the form of dislocation tangles, dense dislocation walls, and dislocation cells were detected in the laser shock-peened copper by the TEM analysis. Moreover, 60–200 nm size ultra-fine grains and 20–50 nm size nano-grains were observed owing to grain refinement. 18

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2. Surface hardness and residual stress: The laser shock-peened copper showed increased microhardness from 55 to 110 HV at a laser energy density of 5.3 GW/cm2 and to 120 HV at a laser energy density of 10.6 GW/cm2. The base metal showed a marginal tensile stress, while compressive residual stress was observed in the laser shock-peened copper at 5.3 GW/cm2 (max. compressive residual stress: 387.5 MPa) and 10.6 GW/cm2 (max. compressive residual stress: 379 MPa). The surface hardness and residual stress results showed that the effecteddepth was 1000 and 1400 μm after LSPwC at 5.3 and 10.6 GW/cm2, respectively. These results agreed well with those of the EBSD analysis. 3. Wear behavior: Owing to the microstructural evolution, mechanical properties improvement, and compressive residual stress formation by LSPwC, the laser shock-peened copper showed superior wear resistance in comparison to the base metal. For the base metal, the average coefficient of friction and wear loss were 0.36 and 1.61 × 10-3 mm3, respectively, at a normal load of 10 N. These values decreased to 0.33 and 0.99 × 10-3 mm3 for the laser shock-peened copper at 5.3 GW/cm2 and 0.32 and 0.94 × 10-3 mm3 for the laser shock-peened copper at 10.6 GW/cm2. A relatively smaller wear loss of the laser shock-peened copper delayed the formation of an oxidized-wear debris layer at the contact area, which acted as an electrical insulator. Therefore, the laser shock-peened copper maintained a low electrical contact resistance and retarded electrical contact failure during the fretting test. The critical number of fretting cycles was 5011 and 5210 cycles for the laser shock-peened copper at 5.3 and 10.6 GW/cm2, respectively, while that for the base metal was 2790.

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Acknowledgements This research was supported the National Research Council of Science and Technology, Republic of Korea [Project number: NK217C, 2019].

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23

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Table 1. Experimental parameters in the measurement of residual stress.

X-ray target

Cu-Kα1

Wavelength

1.5406 Å

Voltage

40 kV

Current

200 mA

Diffractive plane

(1 1 1)

Diffractive angle

43.32°

2θ range

42 – 45°

2θ width

0.02°

PSI range

0 – 30°

Radiation area

1.2 mm2

24

25

Fig. 1. Experimental setup for laser shock peening without coating.

25

26

Fig. 2. Experimental setup for the fretting test. The four-point probe method was used to measure the electrical contact resistance.

26

27

(a)

27

28

(b)

28

29

(c)

Fig. 3. (a) XRD diffraction pattern of the base metal and laser shock-peened copper at laser energy densities of 5.3 and 10.6 GW/cm2, (b) magnified (1 1 1) plane XRD peak, and (c) XRD peak broadening at different planes for the base metal and laser shock-peened copper.

29

30

Fig. 4. EBSD patterns of the base metal and laser shock-peened copper at laser energy densities of 5.3 and 10.6 GW/cm2.

30

31

Fig. 5. TEM image of the base metal at a low dislocation density.

31

32

(a)

(b)

32

33

(c)

Fig. 6. TEM images of the laser shock-peened copper at laser energy densities of 5.3 GW/cm2; (a) dislocation structures with dislocation cells, dislocation tangles, and dense dislocation walls, (b) formation of ultra-fine and nano-grains next to sub-grains, (c) nano-grain formation.

33

34

(a)

(b)

34

35

(c)

Fig. 7. TEM images of the laser shock-peened copper at laser energy densities of 10.6 GW/cm2; (a) dislocation lines, dislocation tangles, and dense dislocation walls with ultra-fine grains, (b) formation of dislocation cells and sub-grains, (c) ultra-fine and nano-grains formation.

35

36

Fig. 8. Vickers hardness of the base metal and laser shock-peened copper as a function of the depth from the surface.

36

37

Fig. 9. Transverse residual stress of the laser shock-peened copper as a function of the depth from the surface.

37

38

Fig. 10. Average coefficient of friction of the base metal and laser shock-peened copper as a function of the normal load during the fretting test.

38

39

(a)

39

40

(b)

40

41

(c)

Fig. 11. Coefficient of friction and electrical contact resistance for the (a) base metal and laser shock-peened copper at laser energy densities of (b) 5.3 GW/cm2 and (c) 10.6 GW/cm2 over 10,000 fretting cycles. The normal load was set as 10 N.

41

42

(a)

42

43

(b)

43

44

(c)

44

45

(d)

45

46

(e)

46

47

(f)

47

48

(g)

Fig. 12. Contact area of the base metal; (a) surface morphology for 100 fretting cycles, (b) surface morphology for 1000 fretting cycles, (c) high magnification SEM image of white dotted box in (b), (d) surface morphology for 2000 fretting cycles, (e) high magnification SEM image of white dotted box in (d), (e) surface morphology for 5000 fretting cycles, and (f) high magnification SEM image, EDX spectrum, and chemical composition of the white dotted box in (e).

48

49

(a)

49

50

(b)

50

51

(c)

Fig. 13. Surface morphology of the contact area of the laser shock-peened copper at 5.3 GW/cm2 for (a) 2000 fretting cycles, (b) 4000 fretting cycles, and (c) 6000 fretting cycles with EDX spectrum and chemical composition at the location of the white dotted box.

51

52

Fig. 14. Critical number of fretting cycles as a function of the applied normal load for the base metal and laser shock-peened copper.

52

53

Fig. 15. Wear loss of the base metal and laser shock-peened copper at applied normal loads of 10, 15, and 20 N during 10,000 fretting cycles.

53

54

Fig. 16. Depth profiles of the wear scars on the base metal and laser shock-peened copper after 1000 fretting cycles at a normal load of 10 N.

54

55

(a)

55

56

(b)

56

57

(c)

Fig. 17. Surface morphologies of the developed wear scars after 10,000 fretting cycles at a normal load of 10 N for the (a) base metal and laser shock-peened copper at (b) 5.3 GW/cm2 and (c) 10.6 GW/cm2.

57

Declaration of interests █The authors declare that they have no known competing financial interests or personal relationships

that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

N/A

Highlights █ Performing the laser shock peening without coating process on high purity copper █ Increment of micro-hardness and formation of compressive residual stress █ Microstructures modification; Dislocation re-arrangement and grain refinement █ Showing superior wear resistance during the fretting test █ Maintaining a low contact resistance, resulting in electrical contact failure delay

Credit Author Statement 1. Changkyoo Park: Laser shock peening test, materials characterization, and writing and reviewing 2. Donghyuck Jung: Materials characterization and writing and reviewing 3. Eun-Joon Chun: Laser shock peening test 4. Sanghoon Ahn: Reviewing and editing 5. Ho Jang: Reviewing and editing 6. Yoon-Jun Kim: Supervision