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Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide aqueous solution

Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide... Adv. Manuf. (2020) 8:265–278 https://doi.org/10.1007/s40436-020-00309-y Investigation of electropolishing characteristics of tungsten in eco- friendly sodium hydroxide aqueous solution 1 1,2 Wei Han Feng-Zhou Fang Received: 2 February 2020 / Revised: 17 March 2020 / Accepted: 4 May 2020 / Published online: 26 May 2020 The Author(s) 2020 Abstract In this study, an eco-friendly electrolyte for Keywords Electropolishing  NaOH solution  Surface electropolishing tungsten and the minimum material roughness  Tungsten  Etching removal depth on the electropolished tungsten surface are investigated using an electrochemical etching method. Using a concentrated acid electrolyte, the polarization 1 Introduction curve and current density transient are observed. For a NaOH electrolyte, the effects of interelectrode gap and Tungsten has the highest melting point of all available electrolyte concentration on electropolishing are investi- metals with a melting temperature at 3 422 C and is gated. The differences in electropolishing characteristics applied extensively in various fields, such as arc-welding are compared among different electrolyte types. Micro- electrodes [1, 2] and heat-resistant coatings [3]. Further- holes are etched on the electropolished tungsten surface to more, it is the most typically used material for preparing determine the minimum material removal depth on the scanning probe microscopy and scanning tunneling spec- tungsten surface. Experimental results indicate the color troscopy probes owing to its good physical and chemical effect due to a change in the thickness of the oxide film on properties [4, 5]. Moreover, tungsten has high stiffness and the tungsten surface after electropolishing with a concen- good electrical conductivity and is generally used as tool trated acid electrolyte. The surface roughness decreases electrodes in electrical discharge machining [6] and elec- with the interelectrode gap width owing to the increased trochemical machining (ECM) [7, 8]. current density when using the NaOH electrolyte. How- As a hard-brittle material, tungsten is difficult to ever, the electropolishing effect is less prominent with a machine using conventional machining methods, such as significantly smaller gap because the generated bubbles are cutting and grinding, owing to its low machinability unable to escape from the narrow working gap in time. A [9, 10]. It has been reported that the ultraprecision diamond material removal depth of less than 10 nm is achieved on cutting of tungsten is significantly affected by the adhesion the tungsten surface in an area of diameter 300 lm, using of tungsten to the tool, rapid tool wear, and brittle fracture the electrochemical etching method. [11]. Hence, tungsten is typically machined using non- conventional machining methods. Wire electrical discharge machining (WEDM) is an effective solution for machining hard materials such as zirconium, titanium, and tungsten carbide, which are difficult to machine using conventional & Feng-Zhou Fang fengzhou.fang@ucd.ie machining methods [12]; additionally, it is an alternative for tungsten machining [13, 14]. However, WEDM is a Centre of Micro/Nano manufacturing Technology (MNMT- thermal process in which the material is removed by Dublin), University College Dublin, Dublin 4, Ireland melting, and vaporization and the formation of a heat-af- State Key Laboratory of Precision Measuring Technology fected layer on the machined surface are inevitable [15]. In and Instruments, Centre of Micro/Nano Manufacturing addition, tool wear due to thermal processes occurs in Technology (MNMT), Tianjin University, Tianjin 300072, People’s Republic of China 123 266 W. Han, F.-Z. Fang WEDM, resulting in a deterioration in machining of electropolishing tungsten has been studied by Wang precision. et al. [21], and they discovered that electropolishing To polish tungsten, chemical-mechanical polishing tungsten in a NaOH aqueous solution with different applied (CMP) is often used, in which synergetic effects of potentials could be categorized into three stages: etching, chemical and mechanical interactions are involved to brightening, and pitting. However, it is noteworthy that achieve global planarization [16, 17]. Bielmann et al. [18] electropolishing tungsten in a NaOH aqueous solution reported that the tungsten removal rate increased with differs from the conventional electropolishing conducted in decreasing particle size and increasing solid loading. Lar- a concentrated acid electrolyte owing to the different sen-Basse and Liang [19] studied the contributions of physical and chemical characteristics of electrolytes used. abrasion in the CMP of tungsten and concluded that it was In a concentrated acid electrolyte, a thick viscous film layer a synergistic process of passive film removal by abrasives is formed on the workpiece surface because the dissolved and the reformation of a film by the action-passive reaction metal ions cannot diffuse into the viscous bulk electrolyte of a bare surface with a slurry. Although a chemical in time [26]. Previous studies have not focused on the reaction is involved in CMP, its fundamental is based on differences between electropolishing tungsten in a NaOH the traditional mechanical polishing process [20]. Slurry aqueous solution and the conventional electropolishing in a particles and polishing byproducts that are pressed onto the concentrated acid electrolyte [21, 27]. workpiece surface owing to mechanical forces are serious In this study, the electropolishing characteristics of defects. It is still challenging to directly apply a polished tungsten were investigated using different types of elec- workpiece by CMP because of the dirty surface; therefore, trolytes, i.e., the conventional concentrated acid electrolyte a post-CMP cleaning process is required. In addition, the and a NaOH aqueous solution. Subsequently, the minimum low material removal rate and significant slurry consump- value of the material removal depth on the tungsten surface tion render CMP a high-cost polishing method [21]. was determined based on the electropolished tungsten Moreover, electropolishing, also known as electro- surface using an electrochemical etching method. For the chemical polishing, anodic polishing, or electrolytic pol- study using the concentrated acid electrolyte, the polar- ishing, is a promising method for polishing tungsten ization curve and current density transient during elec- because the material is removed by electrochemical reac- tropolishing tungsten were characterized. For the study tions, which is a non-mechanical contact and damage-free using the NaOH aqueous solution, the effects of the processes without considering the hardness and brittleness interelectrode gap width and electrolyte concentration on of a workpiece [22–24]. The electropolishing method has the electropolishing of tungsten were investigated. Holstein been applied extensively in the surface treatment of metals et al. [28] reported the achievable minimum dimension of 100 lm of metallic tungsten adhesion elements in round with complex features, such as coronary stents and niobium superconducting radio frequency cavities. As CMP, which and square-edged variations using the ECM method. In this is widely used for polishing tungsten, is a high-cost fin- study, microholes of diameter 300 lm were etched on an ishing method owing to its requirement of a large amount electropolished tungsten surface to investigate the mini- of consumed slurry, the electropolishing of tungsten in mum material removal depth on the tungsten surface using NaOH electrolyte is an effective alternative as electropol- the ECM method. ishing is an easy and simple approach. High polishing efficiency can be achieved by increasing the current density and no post-treatments are required compared with CMP. 2 Experimental approach Schubert et al. [25] studied the anodic dissolution behavior of tungsten carbide in an alkaline electrolyte under elec- 2.1 Material and solution trochemical machining conditions and discovered that near the interface, an adherent, supersaturated, viscous film of Table 1 shows the electrolytes and electrodes used for polytungstates was formed, which was then continuously electropolishing tungsten. The concentrated acid elec- dissolved and reproduced. The anodic dissolution process trolyte was composed of a phosphoric acid aqueous Table 1 Electrolytes and electrodes used for electropolishing tungsten Electrolyte type Composition Workpiece electrode Tool electrode Concentrated acid H PO (81% v:v) : Glycerol = 3:1 Tungsten wire Copper sheet 3 4 electrolyte (u1 mm) (30 mm 9 10 mm 9 0.1 mm) NaOH electrolyte NaOH aqueous solution (0.27 mol/L, Tungsten wire Copper sheet 0.5 mol/L) (u1 mm) (30 mm 9 10 mm 9 0.1 mm) 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 267 solution (H PO , 81% in volume ratio) and glycerol in the 3 4 volume ratio of 3:1. The NaOH electrolyte was prepared by dissolving NaOH powder in distilled water to the concen- trations of 0.27 mol/L and 0.5 mol/L. The same workpiece and tool electrode were used for the two types of elec- trolytes. The workpiece electrode was tungsten wire with a diameter of 1 mm and was mounted in a resin of diameter 30 mm to expose only the end surface. Hence, the effective surface area was a circle of diameter 1 mm in the elec- tropolishing. The tool electrode was a copper sheet mea- suring 30 mm 9 10 mm 9 0.1 mm, and one side was insulated by tape to reduce the effects of stray currents. Table 2 shows the electrolyte and electrodes used for the nanoscale etching of tungsten. The electropolished tung- sten was used as a workpiece with the exposed end surface measuring 1 mm in diameter. Copper wire of diameter Fig. 1 Experimental setup used for electropolishing 300 lm was used as the tool electrode and was mounted in a resin of diameter 30 mm. The resin was visible owing to It is crucial to reduce the initial surface roughness of a the easy observation of the electrodes’ relative position. workpiece before electropolishing because the electropol- The mounted copper wire was ground and mechanically ishing method is limited in terms of polishing ability to polished in sequence to obtain a smoother surface. The improve the surface roughness [24]. The final electropol- electropolishing of copper was performed in H PO aque- 3 4 ished surface is better with a lower initial surface rough- ous solution with a concentration of 81%, and the tool ness [29]. Therefore, the workpiece electrodes used in electrode was the same as that shown in Table 1. electropolishing were ground and mechanically polished The surface morphology was characterized using a prior to electropolishing to obtain a better electropolishing digital microscope (VHX-5000), and the surface topogra- effect. They were uniformly ground with 600, 1 200, and 2 phy was measured by a noncontact optical profile 500 grit abrasive sandpaper sequentially and then (NPFLEX). Contact mode atomic force microscopy (MFP- mechanically polished using a colloidal silica polishing 3D) was used for the analysis of the image topography. suspension of micro sizes 3 lm and \ 1 lm. Figure 2 shows the ground and mechanically polished tungsten 2.2 Experimental method surfaces. Because tungsten is a significantly hard material, scratches that form due to grinding could not be removed Figure 1 shows the experimental setup used for elec- completely by the subsequent mechanical polishing, as tropolishing in this study. A typical electrochemical cell shown in Fig. 2b. It was observed that the number of with three electrodes was used, and the reference electrode scratches in the vicinity of the edge was less than that at the was a Ag/AgCl reference element surrounded by an elec- center because the refresh of colloidal silica polishing trolyte of 4 mol/L KCl aqueous solution saturated with suspension was more efficient during mechanical AgCl. A potentiostat/galvanostat (CS310) was used to polishing. supply the applied potential between the electrodes. A Table 3 shows the experimental conditions used to study magnetic stir bar was used to stir the electrolyte at a stirring the effects of the interelectrode gap width and electrolyte speed of 667 r/min. The setup show in Fig. 1 was also used concentration on the electropolishing of tungsten in NaOH for the nanoscale etching of tungsten surface with a copper tool electrode mounted in resin. Table 2 Electrolyte and electrodes used for the nanoscale etching of tungsten Electrolyte Workpiece electrode Tool electrode NaOH aqueous Electropolished tungsten Copper wire (u solution (0.5 mol/L) wire (u1 mm) 300 lm) Fig. 2 a Ground and b mechanical polished tungsten surface 123 268 W. Han, F.-Z. Fang Table 3 Experimental conditions used to study the influences of interelectrode gap width and electrolyte concentration on the elec- tropolishing effect of tungsten in NaOH electrolyte Parameter Value Applied potential/V 8 Electrolyte (NaOH aq.) concentration/ 0.27, 0.5 -1 (molL ) Electropolishing duration/s 200, 300, 400, 500 Interelectrode gap widths/mm 0.15, 0.5, 1.0, 1.5 electrolyte. The interelectrode gap width was varied to 0.15, 0.5, 1.0, and 1.5 mm, and the concentrations of the Fig. 3 Copper tool surface after mechanical polishing NaOH aqueous solution were 0.27 mol/L and 0.5 mol/L. To determine the optimum applied potential, the potential polishing. The scratches shown in Fig. 3 might have gen- was reduced step by step from the maximum value of 10 V erated during grinding, and the mechanical polishing failed supplied by the potentiostat/galvanostat while monitoring to remove some significantly deep scratches caused by for a significant oscillation in the current density; a final large abrasives during grinding. Furthermore, this might be potential of 8 V was decided. caused by contaminative particles, such as abrasives from Table 4 shows the experimental conditions used for the grinding, during the mechanical polishing. Because copper nanoscale etching of the tungsten surface. The electrolyte has a low hardness, scratches can be formed easily even was a NaOH aqueous solution of concentration 0.5 mol/L. when only a few contaminative particles exist in the pol- The interelectrode gap width was 0.3 mm, and the stirring ishing suspension. The mechanically polished surface speed of the magnetic stir bar was 667 r/min. The etching shown in Fig. 3 was treated by electropolishing to obtain a durations were 35 s and 75 s. The surface quality of the significantly smooth surface. tool electrode is critical in nanoscale etching because the surface shape of the tool electrode can be converted into the machined surface [30]. The copper tool electrode, 3 Results and discussion which was used as a tool electrode for the nanoscale etching of the electropolished tungsten, was ground and 3.1 Electropolishing tungsten in a concentrated acid mechanically polished sequentially based on the methods electrolyte for preparing tungsten. Subsequently, electropolishing was performed to reduce the grain boundaries on the copper Figure 4 shows the polarization curve that was obtained by surface by optimizing the process parameters. sweeping the applied potential from 3 V to 0 V (vs Ag/ Figure 3 shows the copper tool surface after grinding AgCl ) at a scan rate of - 20 mV/s, and the current sat and mechanical polishing. Grain boundaries can be density transient measured in the concentrated acid elec- observed clearly on the surface. Moreover, scratches trolyte. It has been reported that the potential must be appeared after the mechanical polishing. They were typi- swept from the positive values in the cathodic direction to cally generated on the copper surface by the grinding obtain reproducible measurements [31]. Four typical process and most of them disappeared after the mechanical potential regions, i.e., etching, passivation, limiting current density plateau, and gas evolution were observed on the polarization curve, as shown in Fig. 4a. The current density Table 4 Experimental conditions used for the nanoscale etching of oscillated with a large amplitude in the gas evolution tungsten region because of the evolution of oxygen gas from the Parameter Value tungsten surface. Because the best electropolishing effect is Applied potential/V 5 typically obtained in the limiting current density plateau -1 Electrolyte (NaOH aq.) concentration/ (molL ) 0.5 region [24], 2 V was applied as the potential for elec- Inter-electrode gap width /mm 0.3 tropolishing. Figure 4b shows the typical current density -1 Stir speed/(rmin ) 667 transient in electropolishing. The decrease in the current Etching duration/s 35, 75 density at the initial stage was due to the generation of a thick oxide film layer on the workpiece surface. 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 269 H PO solution and glycerol in the volume ratio of 3:1. 3 4 Because the thick oxide film caused a high interelectrode gap resistance, the current density was significantly low, as shown in Fig. 4, resulting in a low material removal rate and the electropolishing effect. The overall electrode reactions can be expressed by the following chemical reactions when electropolishing tungsten using the con- centrated acid electrolyte [32, 33] Cathode 2H þ 2e ! H ðÞ g ð1Þ Anode WðÞ s þ 3H O ¼ WO ðÞ s þ 6H þ 6e ð2Þ 2 3 WO ðÞ s þ H O ! WO  H OsðÞ ¼ H WO ðÞ aq ð3Þ 3 2 3 2 2 4 The dissolution of the oxide film layer by reaction (3)isan extremely slow process, thereby resulting in the low material removal rate in electropolishing. It has been dis- covered that WO can be dissolved using H when the solution pH is lower than 1, based on the following reaction [33, 34] þ þ Cathode WO ðÞ s þ H ! WO OH ðÞ aq ð4Þ 3 2 However, the pH of the 81% H PO aqueous solution was 3 4 between 1.0–1.6, resulting in the dissolution of the oxide film layer in a H O-assisted process, as shown in reaction (3), with an extremely low dissolution rate; this is verified by the electropolished results shown in Fig. 5. Meanwhile, tungsten can be electropolished in a concentrated acid electrolyte of pH lower than 1 owing to the dissolution of the oxide film layer by reaction (4), which necessitates Fig. 4 a Polarization curve measured with the scan rate of - 20 mV/ more investigation in the future. s and potential range of 3–0 V and b current density transient with the Moreover, Fig. 5 shows that the surface color of tung- applied potential of 2 V and electropolishing duration of 30 min sten changed after electropolishing though the electropol- ishing effect was not evident. Evans et al. [35] studied the composition and thickness of the colored oxide films on a stainless steel surface after they were immersed in a solu- tion containing chromic and sulfuric acids at 70 C. It was reported that the colors on the steel surface area were produced by interference between the light reflected from the metal/film interface and that reflected from the film/air interface. The film thickness is changed as the reaction proceeds in the solution, thus producing changes in color. The electropolishing process failed to improve the tungsten Fig. 5 Images of tungsten surface a before and b after surface significantly; however, the thickness of oxide film electropolishing was changed, resulting in the different colors shown in Subsequently, it stabilized with increasing electropolishing Fig. 5b. The change of the oxide film layer could also be duration because of the balance achieved between the verified by the initial decrease in the current density tran- generation and dissolution of the oxide film layer. sient shown in Fig. 4b. Figure 5 shows the images of the tungsten surface before and after electropolishing. The surface finish 3.2 Electropolishing tungsten in NaOH aqueous showed almost no improvement; therefore, it was con- solution cluded that a slight electropolishing effect occurred in tungsten electropolishing when the conventional concen- Figure 6 shows the current density transitions with differ- trated acid electrolyte composed of 81% concentration ent interelectrode gap widths. At the large gap width of 123 270 W. Han, F.-Z. Fang Cathode 6H O þ 6e ! 3H ðÞ g þ 6OH ð5Þ 2 2 Anode WðÞ s þ 6OH ! WO ðÞ s þ 3H O þ 6e ð6Þ 3 2 WO ðÞ s þ 2OH ! WO þ H O ð7Þ 3 2 The oxide film layer can be dissolved rapidly on the tungsten surface by reaction (7) in the NaOH aqueous solution; subsequently, an obvious electropolishing effect can be obtained. An air-formed oxide film layer is gener- ated on the tungsten surface before the electropolishing; therefore, reactions (3) and (7) should occur first when electropolishing tungsten in a concentrated acid electrolyte and NaOH aqueous solution, respectively. However, the oxide film layer can be easily dissolved by reaction (7) and Fig. 6 Current density transitions with different interelectrode gap the dissolution rate is extremely low owing to reaction (3). widths Therefore, the electropolishing of tungsten indicated a higher current density in the NaOH aqueous solution, as 1.5 mm, the current density decreased owing to the high shown in Figs. 4 and 6. gap resistance. It was discovered that the current density Figure 7 shows the electropolished tungsten surfaces decreased significantly at the gap width of 0.15 mm. This with different interelectrode gap widths. Figure 8 shows occurred because the gas produced by electrochemical the topographies of the electropolished tungsten surfaces reactions could not dissipate from the narrow working gap with the interelectrode gap widths of 0.15 mm and 1.0 mm. efficiently, resulting in a higher gap resistance and lower With the interelectrode gap width of 0.15 mm, some bulges current density. Moreover, four spikes in the current den- were observed, as shown in Figs. 7a and 8a. The gas sity transition were observed at 82, 140, 161, and 187 s at bubbles attached to these positions and protected the sur- the gap width of 0.15 mm. It was speculated that as the face against dissolution during electropolishing process. total number of generated gas bubbles increased with the Figure 9 shows the schematic diagram of the protection electropolishing duration, some small gas bubbles merged effect. In addition, Fig. 7d shows that the electropolished to form a bigger bubble and then escaped from the narrow surface was rougher at the interelectrode gap width was working gap under the action of electrolyte stirring, 1.5 mm because of the lower current density—a higher resulting in some fresh electrolytes flowing into the narrow current density generates a better surface finish in ECM. working gap. Hence, the current density increased quickly owing to the decreased working gap resistance. Subse- quently, the current density decreased slowly after the spike, as shown in Fig. 6, because new gas bubbles were generated and gathered in the narrow working gap again. Shimasaki and Kunieda [36] observed the gap phenomenon of ECM using transparent electrodes made of SiC single crystal as the cathode and discovered that the working gap was filled with gas bubbles within several milliseconds, which significantly affected the gap resistance and current density. Hence, for the electropolishing of tungsten, the interelectrode gap width should be optimized to avoid the effects of gas bubbles in a narrow working gap. Moreover, it was discovered that the current density transitions in the NaOH aqueous solution differed from those in the concentrated acid electrolyte shown in Fig. 4b. Firstly, the current density was much higher when the NaOH aqueous solution was used. This might be due to the higher applied potential or the dissolution of the oxide film layer in the NaOH aqueous solution. When electropolish- ing tungsten with a NaOH aqueous solution, the overall Fig. 7 Electropolished tungsten surfaces with different interelectrode electrode reactions can be expressed as follows [21, 37] gap widths of a 0.15 mm, b 0.5 mm, c 1.0 mm and d 1.5 mm 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 271 Fig. 8 Topographies of electropolished tungsten surfaces with different interelectrode gap widths electropolishing duration of 200 s. The R was 26.2 nm (as shown in Fig. 6b), which was much rougher than the R of 9.6 nm; it was obtained when the electrolyte concentration of 0.5 mol/L was used for the same duration of 200 s. The current density decreased with the electrolyte concentration owing to the reduced electrical conductivity, resulting in a lower material removal rate. Figure 12 shows the current density transitions with the NaOH concentrations of 0.27 mol/L and 0.5 mol/L, which shows that the current density with the concentration of 0.27 mol/L is half of that with the concentration of 0.5 mol/L. Therefore, the tung- sten surface might not be surface treated sufficiently with Fig. 9 Protection effect of gas bubbles on the tungsten surface in the concentration of 0.27 mol/L and duration of 200 s. electropolishing Figure 13 shows the electropolishing results with increased durations using the electrolyte concentration of 0.27 mol/ L. As shown, the tungsten surfaces were similar for the durations of 400 s and 500 s; this implied that the tungsten surface was sufficiently electropolished after 400 s with the electrolyte concentration of 0.27 mol/L. Figure 14 shows the three-dimensional (3D) plot of the electropolished tungsten surface based on a duration of 400 s and the NaOH concentration of 0.27 mol/L. A low R of 7.9 nm was obtained. Figure 15 shows the R obtained using dif- ferent NaOH concentrations. The minimum R decreased from 9.6 nm to 7.5 nm when the NaOH concentration was decreased from 0.5 mol/L to 0.27 mol/L. With the con- centrated acid electrolyte, the effect of electrolyte con- centration on the electropolishing effect is likely attributed Fig. 10 Surface roughness R obtained with different interelectrode to the concentration of the limitation species, which are gap widths responsible for the mass transportation limitation in the viscous film layer [24, 26]. However, a thick viscous film With the optimized interelectrode gap width of 1.0 mm, the layer cannot be formed on the tungsten surface with the surface roughness R decreased to 9.6 nm, as shown in NaOH aqueous solution because the dissolved ions can Fig. 8b. Figure 10 shows the R obtained using different diffuse away from the tungsten surface in time with a low interelectrode gap widths, and the optimized interelectrode viscosity of the NaOH electrolyte. Therefore, the effect of gap of 1.0 mm generated the lowest surface roughness R . NaOH concentration on electropolishing cannot be Figure 11 shows the electropolished tungsten surface explained by the mass transport limitation theory. with the NaOH concentration of 0.27 mol/L and the 123 272 W. Han, F.-Z. Fang Fig. 11 Electropolished tungsten surface with the NaOH concentration of 0.27 mol/L and duration of 200 s accuracy. Furthermore, the lower electrolyte concentration generated comparatively less sludge in the narrow working gap by not only providing lower amounts of precipitates, but also by minimizing the machining allowance. Mean- while, Krauss et al. [38] investigated the electrochemical behavior of tungsten in electrolytes with different pH val- ues. It was discovered that a pH value higher than 12 was not required for the electrochemical dissolution of tung- sten, and reactions other than tungsten dissolution did not occur in the high pH range (approximately 10). In this study, the pH values of the NaOH aqueous solution were higher than 13 with the concentrations of 0.25 mol/L and 0.5 mol/L. Therefore, it was assumed that the slight Fig. 12 Current density transitions with the NaOH concentrations of decrease in the minimum R with decreasing electrolyte 0.27 mol/L and 0.5 mol/L concentration, as shown in Fig. 15, could be limited by the high pH values of the electrolyte. It is assumed that localization effects, which localize the In the electropolishing of tungsten with different types electrochemical dissolution process in a small gap width, of electrolytes, tungsten could not be polished in the con- are greater when the electrolyte concentration is lower centrated acid electrolyte owing to the oxide film on the [15], enabling a more prominent electropolishing effect to surface, and a clear electropolishing effect was obtained in be realized with a lower NaOH concentration. The anodic the NaOH electrolyte. The oxide film on the tungsten dissolution process favors the nearest position between the surface can be dissolved by reaction (7), resulting in anode and cathode electrodes owing to the higher current material removal in the NaOH electrolyte. As for the density, and a lower electrolyte concentration reduces the electropolishing of tungsten in the NaOH electrolyte, the effective interelectrode gap width for anodic dissolution, current density increased with decreasing interelectrode resulting in better localization effects and machining Fig. 13 Electropolished tungsten surfaces with different electropolishing durations and the NaOH electrolyte concentration of 0.27 mol/L 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 273 of the high viscosity and a thick viscous film layer formed on the workpiece surface, a larger interelectrode gap width was required for the bubbles to escape from the working gap with the concentrated acid electrolyte. 3.3 Preparation of copper tool electrode for nanoscale etching Figure 16a shows the polarization curve of the copper tool electrode in the H PO electrolyte. It was obtained by 3 4 sweeping the applied potential from 2.5 V to 0 V (vs Ag/ AgCl ) at a scan rate of - 20 mV/s. A limiting current sat density plateau region is shown clearly in the potential range from 0.35 V to 1.6 V, and the best electropolishing Fig. 14 3D Plot of electropolished tungsten surface with a duration effect can be obtained in this region [24]. Figure 16b shows of 400 s and NaOH concentration of 0.27 mol/L the current density transients measured with different applied potentials along the limiting current density plateau region. The current density shows a clear passivation phenomenon at the beginning of the electropolishing, in which the current density decreases quickly with the potential. Subsequently, it shifts to a constant value with Fig. 15 Surface roughness R obtained with different NaOH concentrations gap width because of the decreased gap resistance. How- ever, the current density decreased when the interelectrode gap width was extremely narrow because the generated bubbles could not escape from the working gap in time, resulting in increased gap resistance. These electropolish- ing characterizations exhibit similar features with those obtained from the electropolishing process with the con- ventional concentrated acid electrolyte. The electropolish- ing effect was investigated based on different interelectrode gap widths of 3, 5 and 7 mm when elec- tropolishing 316L stainless steel with an electrolyte com- posed of phosphoric acid, sulfuric acid, glycerin, and deionized water [37]. The optimal electrode gap width was 5 mm; however, a smooth surface was achievable with a gap width of 7 mm. The surface roughness was worse when the gap width was 3 mm because a small gap obstructed the bubbles from escaping from the working gap. However, it was observed that the interelectrode gap Fig. 16 a Polarization curve measured with the scan rate of - 20 mV/s and potential range of 0–2.5 V and b current density width was much larger when the concentrated acid elec- transient with different applied potentials and electropolishing trolyte rather than the NaOH electrolyte was used. Because duration of 300 s 123 274 W. Han, F.-Z. Fang increasing electropolishing duration, which is a typical current density transient in the electropolishing process [39]. Because the mass transportation process is limited in the limiting current density plateau region, the current densities are almost the same for the magnitudes of stable current density with different applied potentials. Figure 17 shows the electropolished copper surfaces with different applied potentials corresponding to the current density transients shown in Fig. 16b. The best surface finish was obtained with the high applied potential of 1.5 V, and only a few grain boundaries remained on the electropolished surface. Figure 18 shows the surface counter and 3D plot of the electropolished copper surface with the applied potential of 1.5 V. The R was reduced to 18.1 nm although a few grain boundaries remained. Because the left grain boundaries did not indicate any effect in the subsequent experiments when it was directly used as a tool electrode for nanoscale etching, this elec- tropolished copper tool was used in this study. 3.4 Nanoscale etching of electropolished tungsten Figure 19 shows the current density transients with dif- ferent etching durations. The current density oscillated significantly because of the effect of gas bubbles generated Fig. 18 Surface counter and 3D plot of the electropolished copper surface with the applied potential of 1.5 V Fig. 19 Current density transients with different etching durations in the narrow working gap on the gap resistance. Figure 20 shows the results of etching for 75 s. An etched circular area was observed, and the material removal depth was 9.4 nm. Figure 21 shows the results of etching for 35 s. It was difficult to identify the etching area from the contour Fig. 17 Electropolished copper surfaces with different applied image; consequently, the material removal depth could not potentials 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 275 Fig. 20 Etching results with a duration of 75 s Fig. 21 Etching results with a duration of 35 s be measured effectively based on the profile, as shown in electrode surface can be utilized to localize the electro- Fig. 21b. chemical dissolution in a small working gap [40]. Therefore, the tungsten etched for 35 s was measured using atomic force microscope (AFM); the results are shown in Fig. 22. Some black lines appeared owing to the 4 Conclusions effect of noise. The arc ‘‘mn’’ was the etched edge and the lines ‘‘AB,’’ ‘‘CD,’’ and ‘‘EF’’ crossed the machined edge The electropolishing of tungsten was studied using differ- at different positions. A material removal depth of less than ent types of electrolytes, and the minimum material 10 nm was achieved. Nonetheless, the surface finish of the removal depth on the tungsten surface was investigated electropolished tungsten should be further improved in the using the electrochemical etching method. A smooth sur- future because, as shown in Fig. 22, the etched edge was face could not be obtained when the conventional con- not clear in the three abovementioned lines because of the centrated acid electrolyte was used; on the contrary, a rough surface. Therefore, the R of 7.5 nm used in this sufficient electropolishing effect was achieved when a study might have affected the results when the material NaOH electrolyte was used. The following conclusions removal depth was further decreased. Moreover, Fig. 23 were obtained. shows the amplified image at the machined edge, which shows that the machined edge is not a steep step. This (i) The electropolished tungsten in a concentrated occurred because the etching did not have sufficient acid electrolyte indicated a slight electropolishing localization ability to limit the electrochemical dissolution effect owing to the thick oxide film on the to a sufficiently small area. It is assumed that a pulse or tungsten surface. Furthermore, the surface color ultrashort pulse voltage etching can yield a smaller etching removal depth because an electrical double layer on the 123 276 W. Han, F.-Z. Fang Fig. 22 Etching results with a duration of 35 s measured by AFM (‘‘mn’’ is the machined edge, ‘‘AB,’’ ‘‘CD,’’ and ‘‘EF’’ are the three lines crossing the machined edge ‘‘mn’’ at different positions) Fig. 23 Amplified image at the etching edge with a duration of 35 s of tungsten changed after electropolishing because not escape from the working gap in time, resulting of a change in the oxide film thickness. in a high gap resistance. At a large gap width, the (ii) When electropolishing tungsten in a NaOH aque- surface finish was deteriorated by the low current ous solution, an optimum interelectrode gap width density owing to the large gap resistance. to obtain the best electropolishing effect existed. (iii) The R decreased from 9.6 nm to 7.5 nm when the At an extremely small gap width, the current NaOH concentration was increased from 0.5 mol/ density decreased because the gas bubbles could L to 0.27 mol/L. Localization effects were more 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 277 8. Wang XL, Han LH, Geng YQ et al (2019) The simulation and prominent at a lower electrolyte concentration, research of etching function based on scanning electrochemical enabling a more pronounced electropolishing microscopy. Nanomanuf Metrol 2:160–167 effect to be realized than when a higher electrolyte 9. Fang FZ, Zhang N, Guo D et al (2019) Towards atomic and concentration was used. Furthermore, the lower close-to-atomic scale manufacturing. Int J Extrem Manuf 1:1–33 10. Fang FZ, Xu F (2018) Recent advances in micro/nano-cutting: electrolyte concentrations generated compara- effect of tool edge and material properties. Nanomanuf Metrol tively less sludge in the narrow working gap. 1:4–31 (iv) A material removal depth of less than 10 nm was 11. Suzuki N, Haritani M, Yang J et al (2007) Elliptical vibration achieved when the etching area measured 300 lm cutting of tungsten alloy molds for optical glass parts. CIRP Ann Manuf Technol 56:127–130 in diameter on the electropolished tungsten sur- 12. Sarkar S, Sekh M, Mitra S et al (2008) Modeling and optimiza- face. In addition, the machined edge was not a tion of wire electrical discharge machining of c-TiAl in trim steep step because the ECM did not possess cutting operation. J Mater Process Technol 17:525–536 sufficient localization ability to limit the material 13. Chen HC, Lin JC, Yang YK et al (2010) Optimization of wire electrical discharge machining for pure tungsten using a neural dissolution process to a sufficiently small area. network integrated simulated annealing approach. Expert Syst Appl 37:7147–7153 Acknowledgements The authors would like to thank the support 14. Yang RT, Tzeng CJ, Yang YK et al (2012) Optimization of wire received from the Science Foundation Ireland (SFI) (Grant No. electrical discharge machining process parameters for cutting 15/RP/B3208) and the National Natural Science Foundation of China tungsten. Int J Adv Manuf Technol 60:135–147 (NSFC) (Grant No. 61635008). This project has also received funding 15. Masuzawa T (2000) State of the art of micromachining. CIRP from the Enterprise Ireland and the European Union’s Horizon 2020 Ann Manuf Technol 49:473–488 Research and Innovation Programme under the Marie Skłodowska– 16. Reinhardt KA, Kern W (2018) Handbook of silicon wafer Curie Grant agreement (Grant No 713654). cleaning technology, 3rd edn. William Andrew, Park Ridge 17. Fang FZ, Zhang XD, Gao W et al (2017) Nanomanufacturing— Open Access This article is licensed under a Creative Commons perspective and applications. CIRP Ann Manuf Technol Attribution 4.0 International License, which permits use, sharing, 66:683–705 adaptation, distribution and reproduction in any medium or format, as 18. Bielmann M, Mahajan U, Singh RK (1999) Effect of particle size long as you give appropriate credit to the original author(s) and the during tungsten chemical mechanical polishing. Mater Res Soc source, provide a link to the Creative Commons licence, and indicate Symp Proc 2:401–403 if changes were made. The images or other third party material in this 19. 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J Electrochem Soc 149:B224–B233 has been doing both fundamen- 34. Di PA, Di QF, Sunseri C (1980) Anodic oxide films on tungsten- tal studies and application I. The influence of anodizing parameters on charging curves and development in the areas of film composition. Corros Sci 20:1067–1078 optical freeform design and 35. Evans TE, Hart AC, Skedgell AN (1973) The nature of the film manufacturing, bio-medical on coloured stainless steel. Trans IMF 51:108–112 manufacturing, ACSM, ultra- 36. Shimasaki T, Kunieda M (2016) Study on influences of bubbles precision machining and mea- on ECM gap phenomena using transparent electrode. CIRP Ann surement benefiting a variety of Manuf Technol 65:225–228 industries in medical devices, 37. Zhang R, Ivey DG (1996) Preparation of sharp polycrystalline bio-implants, optics and mold sectors. tungsten tips for scanning tunneling microscopy imaging. J Vac Sci Technol B Microelectron Nanom Struct 14:1–10 38. Krauss W, Holstein N, Konys J (2007) Strategies in electro- chemical machining of tungsten for divertor application. Fusion Eng Des 82:1799–1805 39. Park JJ, Il PS, Lee SB (2004) Growth kinetics of passivating oxide film of Inconel alloy 600 in 0.1 M Na SO solution at 2 4 25–300 C using the abrading electrode technique and ac impe- dance spectroscopy. Electrochim Acta 49:281–292 40. Schuster R, Kirchner V, Allongue P et al (2000) Electrochemical micromachining. Science 289:98–111 Wei Han is a Marie Sklo- dowska-Curie Career-FIT Research Fellow at the Centre of Micro/ Nano Manufacturing Technology (MNMT-Dublin) at University College Dublin. He received his Ph.D. from The University of Tokyo, Japan in 2016. His research interests focus on the development of eco-friendly electrolyte for the electropolishing of biomedical devices and the micro-electro- chemical machining of easily passivated materials using an ultra-short pulse current. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Advances in Manufacturing Springer Journals

Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide aqueous solution

Advances in Manufacturing , Volume 8 (3) – Sep 26, 2020

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Adv. Manuf. (2020) 8:265–278 https://doi.org/10.1007/s40436-020-00309-y Investigation of electropolishing characteristics of tungsten in eco- friendly sodium hydroxide aqueous solution 1 1,2 Wei Han Feng-Zhou Fang Received: 2 February 2020 / Revised: 17 March 2020 / Accepted: 4 May 2020 / Published online: 26 May 2020 The Author(s) 2020 Abstract In this study, an eco-friendly electrolyte for Keywords Electropolishing  NaOH solution  Surface electropolishing tungsten and the minimum material roughness  Tungsten  Etching removal depth on the electropolished tungsten surface are investigated using an electrochemical etching method. Using a concentrated acid electrolyte, the polarization 1 Introduction curve and current density transient are observed. For a NaOH electrolyte, the effects of interelectrode gap and Tungsten has the highest melting point of all available electrolyte concentration on electropolishing are investi- metals with a melting temperature at 3 422 C and is gated. The differences in electropolishing characteristics applied extensively in various fields, such as arc-welding are compared among different electrolyte types. Micro- electrodes [1, 2] and heat-resistant coatings [3]. Further- holes are etched on the electropolished tungsten surface to more, it is the most typically used material for preparing determine the minimum material removal depth on the scanning probe microscopy and scanning tunneling spec- tungsten surface. Experimental results indicate the color troscopy probes owing to its good physical and chemical effect due to a change in the thickness of the oxide film on properties [4, 5]. Moreover, tungsten has high stiffness and the tungsten surface after electropolishing with a concen- good electrical conductivity and is generally used as tool trated acid electrolyte. The surface roughness decreases electrodes in electrical discharge machining [6] and elec- with the interelectrode gap width owing to the increased trochemical machining (ECM) [7, 8]. current density when using the NaOH electrolyte. How- As a hard-brittle material, tungsten is difficult to ever, the electropolishing effect is less prominent with a machine using conventional machining methods, such as significantly smaller gap because the generated bubbles are cutting and grinding, owing to its low machinability unable to escape from the narrow working gap in time. A [9, 10]. It has been reported that the ultraprecision diamond material removal depth of less than 10 nm is achieved on cutting of tungsten is significantly affected by the adhesion the tungsten surface in an area of diameter 300 lm, using of tungsten to the tool, rapid tool wear, and brittle fracture the electrochemical etching method. [11]. Hence, tungsten is typically machined using non- conventional machining methods. Wire electrical discharge machining (WEDM) is an effective solution for machining hard materials such as zirconium, titanium, and tungsten carbide, which are difficult to machine using conventional & Feng-Zhou Fang fengzhou.fang@ucd.ie machining methods [12]; additionally, it is an alternative for tungsten machining [13, 14]. However, WEDM is a Centre of Micro/Nano manufacturing Technology (MNMT- thermal process in which the material is removed by Dublin), University College Dublin, Dublin 4, Ireland melting, and vaporization and the formation of a heat-af- State Key Laboratory of Precision Measuring Technology fected layer on the machined surface are inevitable [15]. In and Instruments, Centre of Micro/Nano Manufacturing addition, tool wear due to thermal processes occurs in Technology (MNMT), Tianjin University, Tianjin 300072, People’s Republic of China 123 266 W. Han, F.-Z. Fang WEDM, resulting in a deterioration in machining of electropolishing tungsten has been studied by Wang precision. et al. [21], and they discovered that electropolishing To polish tungsten, chemical-mechanical polishing tungsten in a NaOH aqueous solution with different applied (CMP) is often used, in which synergetic effects of potentials could be categorized into three stages: etching, chemical and mechanical interactions are involved to brightening, and pitting. However, it is noteworthy that achieve global planarization [16, 17]. Bielmann et al. [18] electropolishing tungsten in a NaOH aqueous solution reported that the tungsten removal rate increased with differs from the conventional electropolishing conducted in decreasing particle size and increasing solid loading. Lar- a concentrated acid electrolyte owing to the different sen-Basse and Liang [19] studied the contributions of physical and chemical characteristics of electrolytes used. abrasion in the CMP of tungsten and concluded that it was In a concentrated acid electrolyte, a thick viscous film layer a synergistic process of passive film removal by abrasives is formed on the workpiece surface because the dissolved and the reformation of a film by the action-passive reaction metal ions cannot diffuse into the viscous bulk electrolyte of a bare surface with a slurry. Although a chemical in time [26]. Previous studies have not focused on the reaction is involved in CMP, its fundamental is based on differences between electropolishing tungsten in a NaOH the traditional mechanical polishing process [20]. Slurry aqueous solution and the conventional electropolishing in a particles and polishing byproducts that are pressed onto the concentrated acid electrolyte [21, 27]. workpiece surface owing to mechanical forces are serious In this study, the electropolishing characteristics of defects. It is still challenging to directly apply a polished tungsten were investigated using different types of elec- workpiece by CMP because of the dirty surface; therefore, trolytes, i.e., the conventional concentrated acid electrolyte a post-CMP cleaning process is required. In addition, the and a NaOH aqueous solution. Subsequently, the minimum low material removal rate and significant slurry consump- value of the material removal depth on the tungsten surface tion render CMP a high-cost polishing method [21]. was determined based on the electropolished tungsten Moreover, electropolishing, also known as electro- surface using an electrochemical etching method. For the chemical polishing, anodic polishing, or electrolytic pol- study using the concentrated acid electrolyte, the polar- ishing, is a promising method for polishing tungsten ization curve and current density transient during elec- because the material is removed by electrochemical reac- tropolishing tungsten were characterized. For the study tions, which is a non-mechanical contact and damage-free using the NaOH aqueous solution, the effects of the processes without considering the hardness and brittleness interelectrode gap width and electrolyte concentration on of a workpiece [22–24]. The electropolishing method has the electropolishing of tungsten were investigated. Holstein been applied extensively in the surface treatment of metals et al. [28] reported the achievable minimum dimension of 100 lm of metallic tungsten adhesion elements in round with complex features, such as coronary stents and niobium superconducting radio frequency cavities. As CMP, which and square-edged variations using the ECM method. In this is widely used for polishing tungsten, is a high-cost fin- study, microholes of diameter 300 lm were etched on an ishing method owing to its requirement of a large amount electropolished tungsten surface to investigate the mini- of consumed slurry, the electropolishing of tungsten in mum material removal depth on the tungsten surface using NaOH electrolyte is an effective alternative as electropol- the ECM method. ishing is an easy and simple approach. High polishing efficiency can be achieved by increasing the current density and no post-treatments are required compared with CMP. 2 Experimental approach Schubert et al. [25] studied the anodic dissolution behavior of tungsten carbide in an alkaline electrolyte under elec- 2.1 Material and solution trochemical machining conditions and discovered that near the interface, an adherent, supersaturated, viscous film of Table 1 shows the electrolytes and electrodes used for polytungstates was formed, which was then continuously electropolishing tungsten. The concentrated acid elec- dissolved and reproduced. The anodic dissolution process trolyte was composed of a phosphoric acid aqueous Table 1 Electrolytes and electrodes used for electropolishing tungsten Electrolyte type Composition Workpiece electrode Tool electrode Concentrated acid H PO (81% v:v) : Glycerol = 3:1 Tungsten wire Copper sheet 3 4 electrolyte (u1 mm) (30 mm 9 10 mm 9 0.1 mm) NaOH electrolyte NaOH aqueous solution (0.27 mol/L, Tungsten wire Copper sheet 0.5 mol/L) (u1 mm) (30 mm 9 10 mm 9 0.1 mm) 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 267 solution (H PO , 81% in volume ratio) and glycerol in the 3 4 volume ratio of 3:1. The NaOH electrolyte was prepared by dissolving NaOH powder in distilled water to the concen- trations of 0.27 mol/L and 0.5 mol/L. The same workpiece and tool electrode were used for the two types of elec- trolytes. The workpiece electrode was tungsten wire with a diameter of 1 mm and was mounted in a resin of diameter 30 mm to expose only the end surface. Hence, the effective surface area was a circle of diameter 1 mm in the elec- tropolishing. The tool electrode was a copper sheet mea- suring 30 mm 9 10 mm 9 0.1 mm, and one side was insulated by tape to reduce the effects of stray currents. Table 2 shows the electrolyte and electrodes used for the nanoscale etching of tungsten. The electropolished tung- sten was used as a workpiece with the exposed end surface measuring 1 mm in diameter. Copper wire of diameter Fig. 1 Experimental setup used for electropolishing 300 lm was used as the tool electrode and was mounted in a resin of diameter 30 mm. The resin was visible owing to It is crucial to reduce the initial surface roughness of a the easy observation of the electrodes’ relative position. workpiece before electropolishing because the electropol- The mounted copper wire was ground and mechanically ishing method is limited in terms of polishing ability to polished in sequence to obtain a smoother surface. The improve the surface roughness [24]. The final electropol- electropolishing of copper was performed in H PO aque- 3 4 ished surface is better with a lower initial surface rough- ous solution with a concentration of 81%, and the tool ness [29]. Therefore, the workpiece electrodes used in electrode was the same as that shown in Table 1. electropolishing were ground and mechanically polished The surface morphology was characterized using a prior to electropolishing to obtain a better electropolishing digital microscope (VHX-5000), and the surface topogra- effect. They were uniformly ground with 600, 1 200, and 2 phy was measured by a noncontact optical profile 500 grit abrasive sandpaper sequentially and then (NPFLEX). Contact mode atomic force microscopy (MFP- mechanically polished using a colloidal silica polishing 3D) was used for the analysis of the image topography. suspension of micro sizes 3 lm and \ 1 lm. Figure 2 shows the ground and mechanically polished tungsten 2.2 Experimental method surfaces. Because tungsten is a significantly hard material, scratches that form due to grinding could not be removed Figure 1 shows the experimental setup used for elec- completely by the subsequent mechanical polishing, as tropolishing in this study. A typical electrochemical cell shown in Fig. 2b. It was observed that the number of with three electrodes was used, and the reference electrode scratches in the vicinity of the edge was less than that at the was a Ag/AgCl reference element surrounded by an elec- center because the refresh of colloidal silica polishing trolyte of 4 mol/L KCl aqueous solution saturated with suspension was more efficient during mechanical AgCl. A potentiostat/galvanostat (CS310) was used to polishing. supply the applied potential between the electrodes. A Table 3 shows the experimental conditions used to study magnetic stir bar was used to stir the electrolyte at a stirring the effects of the interelectrode gap width and electrolyte speed of 667 r/min. The setup show in Fig. 1 was also used concentration on the electropolishing of tungsten in NaOH for the nanoscale etching of tungsten surface with a copper tool electrode mounted in resin. Table 2 Electrolyte and electrodes used for the nanoscale etching of tungsten Electrolyte Workpiece electrode Tool electrode NaOH aqueous Electropolished tungsten Copper wire (u solution (0.5 mol/L) wire (u1 mm) 300 lm) Fig. 2 a Ground and b mechanical polished tungsten surface 123 268 W. Han, F.-Z. Fang Table 3 Experimental conditions used to study the influences of interelectrode gap width and electrolyte concentration on the elec- tropolishing effect of tungsten in NaOH electrolyte Parameter Value Applied potential/V 8 Electrolyte (NaOH aq.) concentration/ 0.27, 0.5 -1 (molL ) Electropolishing duration/s 200, 300, 400, 500 Interelectrode gap widths/mm 0.15, 0.5, 1.0, 1.5 electrolyte. The interelectrode gap width was varied to 0.15, 0.5, 1.0, and 1.5 mm, and the concentrations of the Fig. 3 Copper tool surface after mechanical polishing NaOH aqueous solution were 0.27 mol/L and 0.5 mol/L. To determine the optimum applied potential, the potential polishing. The scratches shown in Fig. 3 might have gen- was reduced step by step from the maximum value of 10 V erated during grinding, and the mechanical polishing failed supplied by the potentiostat/galvanostat while monitoring to remove some significantly deep scratches caused by for a significant oscillation in the current density; a final large abrasives during grinding. Furthermore, this might be potential of 8 V was decided. caused by contaminative particles, such as abrasives from Table 4 shows the experimental conditions used for the grinding, during the mechanical polishing. Because copper nanoscale etching of the tungsten surface. The electrolyte has a low hardness, scratches can be formed easily even was a NaOH aqueous solution of concentration 0.5 mol/L. when only a few contaminative particles exist in the pol- The interelectrode gap width was 0.3 mm, and the stirring ishing suspension. The mechanically polished surface speed of the magnetic stir bar was 667 r/min. The etching shown in Fig. 3 was treated by electropolishing to obtain a durations were 35 s and 75 s. The surface quality of the significantly smooth surface. tool electrode is critical in nanoscale etching because the surface shape of the tool electrode can be converted into the machined surface [30]. The copper tool electrode, 3 Results and discussion which was used as a tool electrode for the nanoscale etching of the electropolished tungsten, was ground and 3.1 Electropolishing tungsten in a concentrated acid mechanically polished sequentially based on the methods electrolyte for preparing tungsten. Subsequently, electropolishing was performed to reduce the grain boundaries on the copper Figure 4 shows the polarization curve that was obtained by surface by optimizing the process parameters. sweeping the applied potential from 3 V to 0 V (vs Ag/ Figure 3 shows the copper tool surface after grinding AgCl ) at a scan rate of - 20 mV/s, and the current sat and mechanical polishing. Grain boundaries can be density transient measured in the concentrated acid elec- observed clearly on the surface. Moreover, scratches trolyte. It has been reported that the potential must be appeared after the mechanical polishing. They were typi- swept from the positive values in the cathodic direction to cally generated on the copper surface by the grinding obtain reproducible measurements [31]. Four typical process and most of them disappeared after the mechanical potential regions, i.e., etching, passivation, limiting current density plateau, and gas evolution were observed on the polarization curve, as shown in Fig. 4a. The current density Table 4 Experimental conditions used for the nanoscale etching of oscillated with a large amplitude in the gas evolution tungsten region because of the evolution of oxygen gas from the Parameter Value tungsten surface. Because the best electropolishing effect is Applied potential/V 5 typically obtained in the limiting current density plateau -1 Electrolyte (NaOH aq.) concentration/ (molL ) 0.5 region [24], 2 V was applied as the potential for elec- Inter-electrode gap width /mm 0.3 tropolishing. Figure 4b shows the typical current density -1 Stir speed/(rmin ) 667 transient in electropolishing. The decrease in the current Etching duration/s 35, 75 density at the initial stage was due to the generation of a thick oxide film layer on the workpiece surface. 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 269 H PO solution and glycerol in the volume ratio of 3:1. 3 4 Because the thick oxide film caused a high interelectrode gap resistance, the current density was significantly low, as shown in Fig. 4, resulting in a low material removal rate and the electropolishing effect. The overall electrode reactions can be expressed by the following chemical reactions when electropolishing tungsten using the con- centrated acid electrolyte [32, 33] Cathode 2H þ 2e ! H ðÞ g ð1Þ Anode WðÞ s þ 3H O ¼ WO ðÞ s þ 6H þ 6e ð2Þ 2 3 WO ðÞ s þ H O ! WO  H OsðÞ ¼ H WO ðÞ aq ð3Þ 3 2 3 2 2 4 The dissolution of the oxide film layer by reaction (3)isan extremely slow process, thereby resulting in the low material removal rate in electropolishing. It has been dis- covered that WO can be dissolved using H when the solution pH is lower than 1, based on the following reaction [33, 34] þ þ Cathode WO ðÞ s þ H ! WO OH ðÞ aq ð4Þ 3 2 However, the pH of the 81% H PO aqueous solution was 3 4 between 1.0–1.6, resulting in the dissolution of the oxide film layer in a H O-assisted process, as shown in reaction (3), with an extremely low dissolution rate; this is verified by the electropolished results shown in Fig. 5. Meanwhile, tungsten can be electropolished in a concentrated acid electrolyte of pH lower than 1 owing to the dissolution of the oxide film layer by reaction (4), which necessitates Fig. 4 a Polarization curve measured with the scan rate of - 20 mV/ more investigation in the future. s and potential range of 3–0 V and b current density transient with the Moreover, Fig. 5 shows that the surface color of tung- applied potential of 2 V and electropolishing duration of 30 min sten changed after electropolishing though the electropol- ishing effect was not evident. Evans et al. [35] studied the composition and thickness of the colored oxide films on a stainless steel surface after they were immersed in a solu- tion containing chromic and sulfuric acids at 70 C. It was reported that the colors on the steel surface area were produced by interference between the light reflected from the metal/film interface and that reflected from the film/air interface. The film thickness is changed as the reaction proceeds in the solution, thus producing changes in color. The electropolishing process failed to improve the tungsten Fig. 5 Images of tungsten surface a before and b after surface significantly; however, the thickness of oxide film electropolishing was changed, resulting in the different colors shown in Subsequently, it stabilized with increasing electropolishing Fig. 5b. The change of the oxide film layer could also be duration because of the balance achieved between the verified by the initial decrease in the current density tran- generation and dissolution of the oxide film layer. sient shown in Fig. 4b. Figure 5 shows the images of the tungsten surface before and after electropolishing. The surface finish 3.2 Electropolishing tungsten in NaOH aqueous showed almost no improvement; therefore, it was con- solution cluded that a slight electropolishing effect occurred in tungsten electropolishing when the conventional concen- Figure 6 shows the current density transitions with differ- trated acid electrolyte composed of 81% concentration ent interelectrode gap widths. At the large gap width of 123 270 W. Han, F.-Z. Fang Cathode 6H O þ 6e ! 3H ðÞ g þ 6OH ð5Þ 2 2 Anode WðÞ s þ 6OH ! WO ðÞ s þ 3H O þ 6e ð6Þ 3 2 WO ðÞ s þ 2OH ! WO þ H O ð7Þ 3 2 The oxide film layer can be dissolved rapidly on the tungsten surface by reaction (7) in the NaOH aqueous solution; subsequently, an obvious electropolishing effect can be obtained. An air-formed oxide film layer is gener- ated on the tungsten surface before the electropolishing; therefore, reactions (3) and (7) should occur first when electropolishing tungsten in a concentrated acid electrolyte and NaOH aqueous solution, respectively. However, the oxide film layer can be easily dissolved by reaction (7) and Fig. 6 Current density transitions with different interelectrode gap the dissolution rate is extremely low owing to reaction (3). widths Therefore, the electropolishing of tungsten indicated a higher current density in the NaOH aqueous solution, as 1.5 mm, the current density decreased owing to the high shown in Figs. 4 and 6. gap resistance. It was discovered that the current density Figure 7 shows the electropolished tungsten surfaces decreased significantly at the gap width of 0.15 mm. This with different interelectrode gap widths. Figure 8 shows occurred because the gas produced by electrochemical the topographies of the electropolished tungsten surfaces reactions could not dissipate from the narrow working gap with the interelectrode gap widths of 0.15 mm and 1.0 mm. efficiently, resulting in a higher gap resistance and lower With the interelectrode gap width of 0.15 mm, some bulges current density. Moreover, four spikes in the current den- were observed, as shown in Figs. 7a and 8a. The gas sity transition were observed at 82, 140, 161, and 187 s at bubbles attached to these positions and protected the sur- the gap width of 0.15 mm. It was speculated that as the face against dissolution during electropolishing process. total number of generated gas bubbles increased with the Figure 9 shows the schematic diagram of the protection electropolishing duration, some small gas bubbles merged effect. In addition, Fig. 7d shows that the electropolished to form a bigger bubble and then escaped from the narrow surface was rougher at the interelectrode gap width was working gap under the action of electrolyte stirring, 1.5 mm because of the lower current density—a higher resulting in some fresh electrolytes flowing into the narrow current density generates a better surface finish in ECM. working gap. Hence, the current density increased quickly owing to the decreased working gap resistance. Subse- quently, the current density decreased slowly after the spike, as shown in Fig. 6, because new gas bubbles were generated and gathered in the narrow working gap again. Shimasaki and Kunieda [36] observed the gap phenomenon of ECM using transparent electrodes made of SiC single crystal as the cathode and discovered that the working gap was filled with gas bubbles within several milliseconds, which significantly affected the gap resistance and current density. Hence, for the electropolishing of tungsten, the interelectrode gap width should be optimized to avoid the effects of gas bubbles in a narrow working gap. Moreover, it was discovered that the current density transitions in the NaOH aqueous solution differed from those in the concentrated acid electrolyte shown in Fig. 4b. Firstly, the current density was much higher when the NaOH aqueous solution was used. This might be due to the higher applied potential or the dissolution of the oxide film layer in the NaOH aqueous solution. When electropolish- ing tungsten with a NaOH aqueous solution, the overall Fig. 7 Electropolished tungsten surfaces with different interelectrode electrode reactions can be expressed as follows [21, 37] gap widths of a 0.15 mm, b 0.5 mm, c 1.0 mm and d 1.5 mm 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 271 Fig. 8 Topographies of electropolished tungsten surfaces with different interelectrode gap widths electropolishing duration of 200 s. The R was 26.2 nm (as shown in Fig. 6b), which was much rougher than the R of 9.6 nm; it was obtained when the electrolyte concentration of 0.5 mol/L was used for the same duration of 200 s. The current density decreased with the electrolyte concentration owing to the reduced electrical conductivity, resulting in a lower material removal rate. Figure 12 shows the current density transitions with the NaOH concentrations of 0.27 mol/L and 0.5 mol/L, which shows that the current density with the concentration of 0.27 mol/L is half of that with the concentration of 0.5 mol/L. Therefore, the tung- sten surface might not be surface treated sufficiently with Fig. 9 Protection effect of gas bubbles on the tungsten surface in the concentration of 0.27 mol/L and duration of 200 s. electropolishing Figure 13 shows the electropolishing results with increased durations using the electrolyte concentration of 0.27 mol/ L. As shown, the tungsten surfaces were similar for the durations of 400 s and 500 s; this implied that the tungsten surface was sufficiently electropolished after 400 s with the electrolyte concentration of 0.27 mol/L. Figure 14 shows the three-dimensional (3D) plot of the electropolished tungsten surface based on a duration of 400 s and the NaOH concentration of 0.27 mol/L. A low R of 7.9 nm was obtained. Figure 15 shows the R obtained using dif- ferent NaOH concentrations. The minimum R decreased from 9.6 nm to 7.5 nm when the NaOH concentration was decreased from 0.5 mol/L to 0.27 mol/L. With the con- centrated acid electrolyte, the effect of electrolyte con- centration on the electropolishing effect is likely attributed Fig. 10 Surface roughness R obtained with different interelectrode to the concentration of the limitation species, which are gap widths responsible for the mass transportation limitation in the viscous film layer [24, 26]. However, a thick viscous film With the optimized interelectrode gap width of 1.0 mm, the layer cannot be formed on the tungsten surface with the surface roughness R decreased to 9.6 nm, as shown in NaOH aqueous solution because the dissolved ions can Fig. 8b. Figure 10 shows the R obtained using different diffuse away from the tungsten surface in time with a low interelectrode gap widths, and the optimized interelectrode viscosity of the NaOH electrolyte. Therefore, the effect of gap of 1.0 mm generated the lowest surface roughness R . NaOH concentration on electropolishing cannot be Figure 11 shows the electropolished tungsten surface explained by the mass transport limitation theory. with the NaOH concentration of 0.27 mol/L and the 123 272 W. Han, F.-Z. Fang Fig. 11 Electropolished tungsten surface with the NaOH concentration of 0.27 mol/L and duration of 200 s accuracy. Furthermore, the lower electrolyte concentration generated comparatively less sludge in the narrow working gap by not only providing lower amounts of precipitates, but also by minimizing the machining allowance. Mean- while, Krauss et al. [38] investigated the electrochemical behavior of tungsten in electrolytes with different pH val- ues. It was discovered that a pH value higher than 12 was not required for the electrochemical dissolution of tung- sten, and reactions other than tungsten dissolution did not occur in the high pH range (approximately 10). In this study, the pH values of the NaOH aqueous solution were higher than 13 with the concentrations of 0.25 mol/L and 0.5 mol/L. Therefore, it was assumed that the slight Fig. 12 Current density transitions with the NaOH concentrations of decrease in the minimum R with decreasing electrolyte 0.27 mol/L and 0.5 mol/L concentration, as shown in Fig. 15, could be limited by the high pH values of the electrolyte. It is assumed that localization effects, which localize the In the electropolishing of tungsten with different types electrochemical dissolution process in a small gap width, of electrolytes, tungsten could not be polished in the con- are greater when the electrolyte concentration is lower centrated acid electrolyte owing to the oxide film on the [15], enabling a more prominent electropolishing effect to surface, and a clear electropolishing effect was obtained in be realized with a lower NaOH concentration. The anodic the NaOH electrolyte. The oxide film on the tungsten dissolution process favors the nearest position between the surface can be dissolved by reaction (7), resulting in anode and cathode electrodes owing to the higher current material removal in the NaOH electrolyte. As for the density, and a lower electrolyte concentration reduces the electropolishing of tungsten in the NaOH electrolyte, the effective interelectrode gap width for anodic dissolution, current density increased with decreasing interelectrode resulting in better localization effects and machining Fig. 13 Electropolished tungsten surfaces with different electropolishing durations and the NaOH electrolyte concentration of 0.27 mol/L 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 273 of the high viscosity and a thick viscous film layer formed on the workpiece surface, a larger interelectrode gap width was required for the bubbles to escape from the working gap with the concentrated acid electrolyte. 3.3 Preparation of copper tool electrode for nanoscale etching Figure 16a shows the polarization curve of the copper tool electrode in the H PO electrolyte. It was obtained by 3 4 sweeping the applied potential from 2.5 V to 0 V (vs Ag/ AgCl ) at a scan rate of - 20 mV/s. A limiting current sat density plateau region is shown clearly in the potential range from 0.35 V to 1.6 V, and the best electropolishing Fig. 14 3D Plot of electropolished tungsten surface with a duration effect can be obtained in this region [24]. Figure 16b shows of 400 s and NaOH concentration of 0.27 mol/L the current density transients measured with different applied potentials along the limiting current density plateau region. The current density shows a clear passivation phenomenon at the beginning of the electropolishing, in which the current density decreases quickly with the potential. Subsequently, it shifts to a constant value with Fig. 15 Surface roughness R obtained with different NaOH concentrations gap width because of the decreased gap resistance. How- ever, the current density decreased when the interelectrode gap width was extremely narrow because the generated bubbles could not escape from the working gap in time, resulting in increased gap resistance. These electropolish- ing characterizations exhibit similar features with those obtained from the electropolishing process with the con- ventional concentrated acid electrolyte. The electropolish- ing effect was investigated based on different interelectrode gap widths of 3, 5 and 7 mm when elec- tropolishing 316L stainless steel with an electrolyte com- posed of phosphoric acid, sulfuric acid, glycerin, and deionized water [37]. The optimal electrode gap width was 5 mm; however, a smooth surface was achievable with a gap width of 7 mm. The surface roughness was worse when the gap width was 3 mm because a small gap obstructed the bubbles from escaping from the working gap. However, it was observed that the interelectrode gap Fig. 16 a Polarization curve measured with the scan rate of - 20 mV/s and potential range of 0–2.5 V and b current density width was much larger when the concentrated acid elec- transient with different applied potentials and electropolishing trolyte rather than the NaOH electrolyte was used. Because duration of 300 s 123 274 W. Han, F.-Z. Fang increasing electropolishing duration, which is a typical current density transient in the electropolishing process [39]. Because the mass transportation process is limited in the limiting current density plateau region, the current densities are almost the same for the magnitudes of stable current density with different applied potentials. Figure 17 shows the electropolished copper surfaces with different applied potentials corresponding to the current density transients shown in Fig. 16b. The best surface finish was obtained with the high applied potential of 1.5 V, and only a few grain boundaries remained on the electropolished surface. Figure 18 shows the surface counter and 3D plot of the electropolished copper surface with the applied potential of 1.5 V. The R was reduced to 18.1 nm although a few grain boundaries remained. Because the left grain boundaries did not indicate any effect in the subsequent experiments when it was directly used as a tool electrode for nanoscale etching, this elec- tropolished copper tool was used in this study. 3.4 Nanoscale etching of electropolished tungsten Figure 19 shows the current density transients with dif- ferent etching durations. The current density oscillated significantly because of the effect of gas bubbles generated Fig. 18 Surface counter and 3D plot of the electropolished copper surface with the applied potential of 1.5 V Fig. 19 Current density transients with different etching durations in the narrow working gap on the gap resistance. Figure 20 shows the results of etching for 75 s. An etched circular area was observed, and the material removal depth was 9.4 nm. Figure 21 shows the results of etching for 35 s. It was difficult to identify the etching area from the contour Fig. 17 Electropolished copper surfaces with different applied image; consequently, the material removal depth could not potentials 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 275 Fig. 20 Etching results with a duration of 75 s Fig. 21 Etching results with a duration of 35 s be measured effectively based on the profile, as shown in electrode surface can be utilized to localize the electro- Fig. 21b. chemical dissolution in a small working gap [40]. Therefore, the tungsten etched for 35 s was measured using atomic force microscope (AFM); the results are shown in Fig. 22. Some black lines appeared owing to the 4 Conclusions effect of noise. The arc ‘‘mn’’ was the etched edge and the lines ‘‘AB,’’ ‘‘CD,’’ and ‘‘EF’’ crossed the machined edge The electropolishing of tungsten was studied using differ- at different positions. A material removal depth of less than ent types of electrolytes, and the minimum material 10 nm was achieved. Nonetheless, the surface finish of the removal depth on the tungsten surface was investigated electropolished tungsten should be further improved in the using the electrochemical etching method. A smooth sur- future because, as shown in Fig. 22, the etched edge was face could not be obtained when the conventional con- not clear in the three abovementioned lines because of the centrated acid electrolyte was used; on the contrary, a rough surface. Therefore, the R of 7.5 nm used in this sufficient electropolishing effect was achieved when a study might have affected the results when the material NaOH electrolyte was used. The following conclusions removal depth was further decreased. Moreover, Fig. 23 were obtained. shows the amplified image at the machined edge, which shows that the machined edge is not a steep step. This (i) The electropolished tungsten in a concentrated occurred because the etching did not have sufficient acid electrolyte indicated a slight electropolishing localization ability to limit the electrochemical dissolution effect owing to the thick oxide film on the to a sufficiently small area. It is assumed that a pulse or tungsten surface. Furthermore, the surface color ultrashort pulse voltage etching can yield a smaller etching removal depth because an electrical double layer on the 123 276 W. Han, F.-Z. Fang Fig. 22 Etching results with a duration of 35 s measured by AFM (‘‘mn’’ is the machined edge, ‘‘AB,’’ ‘‘CD,’’ and ‘‘EF’’ are the three lines crossing the machined edge ‘‘mn’’ at different positions) Fig. 23 Amplified image at the etching edge with a duration of 35 s of tungsten changed after electropolishing because not escape from the working gap in time, resulting of a change in the oxide film thickness. in a high gap resistance. At a large gap width, the (ii) When electropolishing tungsten in a NaOH aque- surface finish was deteriorated by the low current ous solution, an optimum interelectrode gap width density owing to the large gap resistance. to obtain the best electropolishing effect existed. (iii) The R decreased from 9.6 nm to 7.5 nm when the At an extremely small gap width, the current NaOH concentration was increased from 0.5 mol/ density decreased because the gas bubbles could L to 0.27 mol/L. Localization effects were more 123 Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide… 277 8. Wang XL, Han LH, Geng YQ et al (2019) The simulation and prominent at a lower electrolyte concentration, research of etching function based on scanning electrochemical enabling a more pronounced electropolishing microscopy. Nanomanuf Metrol 2:160–167 effect to be realized than when a higher electrolyte 9. Fang FZ, Zhang N, Guo D et al (2019) Towards atomic and concentration was used. Furthermore, the lower close-to-atomic scale manufacturing. Int J Extrem Manuf 1:1–33 10. Fang FZ, Xu F (2018) Recent advances in micro/nano-cutting: electrolyte concentrations generated compara- effect of tool edge and material properties. Nanomanuf Metrol tively less sludge in the narrow working gap. 1:4–31 (iv) A material removal depth of less than 10 nm was 11. Suzuki N, Haritani M, Yang J et al (2007) Elliptical vibration achieved when the etching area measured 300 lm cutting of tungsten alloy molds for optical glass parts. CIRP Ann Manuf Technol 56:127–130 in diameter on the electropolished tungsten sur- 12. Sarkar S, Sekh M, Mitra S et al (2008) Modeling and optimiza- face. In addition, the machined edge was not a tion of wire electrical discharge machining of c-TiAl in trim steep step because the ECM did not possess cutting operation. J Mater Process Technol 17:525–536 sufficient localization ability to limit the material 13. Chen HC, Lin JC, Yang YK et al (2010) Optimization of wire electrical discharge machining for pure tungsten using a neural dissolution process to a sufficiently small area. network integrated simulated annealing approach. Expert Syst Appl 37:7147–7153 Acknowledgements The authors would like to thank the support 14. Yang RT, Tzeng CJ, Yang YK et al (2012) Optimization of wire received from the Science Foundation Ireland (SFI) (Grant No. electrical discharge machining process parameters for cutting 15/RP/B3208) and the National Natural Science Foundation of China tungsten. Int J Adv Manuf Technol 60:135–147 (NSFC) (Grant No. 61635008). This project has also received funding 15. Masuzawa T (2000) State of the art of micromachining. CIRP from the Enterprise Ireland and the European Union’s Horizon 2020 Ann Manuf Technol 49:473–488 Research and Innovation Programme under the Marie Skłodowska– 16. Reinhardt KA, Kern W (2018) Handbook of silicon wafer Curie Grant agreement (Grant No 713654). cleaning technology, 3rd edn. William Andrew, Park Ridge 17. Fang FZ, Zhang XD, Gao W et al (2017) Nanomanufacturing— Open Access This article is licensed under a Creative Commons perspective and applications. CIRP Ann Manuf Technol Attribution 4.0 International License, which permits use, sharing, 66:683–705 adaptation, distribution and reproduction in any medium or format, as 18. Bielmann M, Mahajan U, Singh RK (1999) Effect of particle size long as you give appropriate credit to the original author(s) and the during tungsten chemical mechanical polishing. Mater Res Soc source, provide a link to the Creative Commons licence, and indicate Symp Proc 2:401–403 if changes were made. The images or other third party material in this 19. 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Krauss W, Holstein N, Konys J (2007) Strategies in electro- chemical machining of tungsten for divertor application. Fusion Eng Des 82:1799–1805 39. Park JJ, Il PS, Lee SB (2004) Growth kinetics of passivating oxide film of Inconel alloy 600 in 0.1 M Na SO solution at 2 4 25–300 C using the abrading electrode technique and ac impe- dance spectroscopy. Electrochim Acta 49:281–292 40. Schuster R, Kirchner V, Allongue P et al (2000) Electrochemical micromachining. Science 289:98–111 Wei Han is a Marie Sklo- dowska-Curie Career-FIT Research Fellow at the Centre of Micro/ Nano Manufacturing Technology (MNMT-Dublin) at University College Dublin. He received his Ph.D. from The University of Tokyo, Japan in 2016. His research interests focus on the development of eco-friendly electrolyte for the electropolishing of biomedical devices and the micro-electro- chemical machining of easily passivated materials using an ultra-short pulse current.

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Published: Sep 26, 2020

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