**1. Introduction**

Due to the advantages of high melting point, great corrosion resistance, superior electrical conductivity, and high-temperature strength [1–3], tungsten is widely used in integrated circuits, nuclear material, the military industry, medical equipment, and other fields [4–6]. The surface quality of the material has a significant influence on the service performance and service life of the workpiece. For example, as the primary candidate for plasma facing materials (PFMs) in the diverter of the ITER and DEMO fusion reactors, tungsten needs to withstand the impact of high-energy particles. The surface quality will affect its radiation resistance to a certain extent, thus affecting the service life of the nuclear fusion reactor [7]. In semiconductor devices, tungsten is deposited on the device surface by chemical vapor deposition. The surface quality of tungsten film will affect the interconnect performance of devices, and therefore the surface of tungsten film needs to be polished [8]. Ultra-precision polishing is the final processing method to reduce the surface roughness of workpiece, remove the damaged layer, and obtain high surface accuracy and excellent surface quality [9–11]. However, as a typical hard and brittle material, there are great challenges in the ultra-precision polishing of tungsten due to the high hardness, high brittleness, and great wear resistance of material [12,13].

**Citation:** Xu, L.; Wang, L.; Chen, H.; Wang, X.; Chen, F.; Lyu, B.; Hang, W.; Zhao, W.; Yuan, J. Effects of pH Values and H2O<sup>2</sup> Concentrations on the Chemical Enhanced Shear Dilatancy Polishing of Tungsten. *Micromachines* **2022**, *13*, 762. https:// doi.org/10.3390/mi13050762

Academic Editors: Xiuqing Hao, Duanzhi Duan and Youqiang Xing

Received: 15 April 2022 Accepted: 10 May 2022 Published: 12 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In order to achieve high-efficiency and high-quality polishing, researchers have conducted extensive research on tungsten polishing methods and processes. Poddar et al. [14] used a mixed oxidant composed of H2O<sup>2</sup> and Fe(NO3)<sup>3</sup> for chemical–mechanical polishing of tungsten and found that the mixed oxidant would generate OH with stronger oxidizing ability, and its removal efficiency was higher than that of a single oxidant. Lim et al. [15] found that the polishing rate could be divided into two regions with increases in Fe(NO3)<sup>3</sup> concentration; when Fe(NO3)<sup>3</sup> concentration was less than 0.1 wt.%, the polishing rate of tungsten increased rapidly, while when the Fe(NO3)<sup>3</sup> concentration was higher than 0.1 wt.%, the polishing rate increased slowly. Han et al. [16] used an alkaline electrolyte containing 0.27 mol/L NaOH for electrochemical polishing of tungsten under the optimal electrode gap width, and surface roughness (*R*a) of the polished tungsten reached 7.5 nm. Based on polishing experiments and theoretical analysis, Wang et al. [17] proposed that electrochemical polishing of tungsten should be divided into three stages: corrosion stage, bright stage, and pitting stage. Under the optimal process, surface roughness *S*<sup>a</sup> was only 3.73 nm after polishing for 10 min. Chen et al. [18] proposed a high-efficiency electrochemical polishing method for tungsten surfaces combining forced convection and natural convection. After electrochemical polishing by forced convection for 3 min and natural convection for 3 min, surface roughness *R*<sup>a</sup> was reduced to 17.2 nm. Zhou et al. [19] developed a dynamic electrochemical polishing process using a bi-layer NaOH electrolyte to uniformly polish tungsten microfluidic molds. With the optimized parameters, surface roughness *S*<sup>a</sup> was reduced from 205.98 nm to 4.14 nm after 10 cycles of dynamic electrochemical polishing. Tungsten will be widely used in various fields in the future because of its excellent comprehensive mechanical properties. Therefore, it is particularly important to explore new ultra-precision polishing methods to improve the surface quality and service life of tungsten workpieces.

Shear Dilatancy Polishing (SDP) is a high-efficiency, high-quality, low-cost surface polishing method that has emerged in recent years. The principle is to use viscoelastic material with non-Newtonian fluid properties to prepare a specific shear pad in order to enhance the control of abrasive particles and improve the stress evenness and contact pressure on the workpiece surface based on the shear dilatancy and solidification effects under high-pressure and high-speed conditions to finally achieve high-efficiency and highquality polishing. The scratching effect of abrasives on the workpiece is the key to achieving material removal [20,21]. Compared with conventional polishing pads, the dilatancy pad can hold more abrasives. Doi et al. [22,23] used viscoelastic materials such as asphalt and potato starch to prepare a specific dilatancy pad, which could reduce the surface defects caused by stress concentrations in local areas during processing. Results showed that under low–medium speed/pressure, the material removal rate of the SiC wafer after the dilatancy pad processing was more than three times of that after metal tin plate processing, the surface scratches after the dilatancy pad processing were lower than 1% of the latter, and the depth of the subsurface damage layer was less than 10% of the latter.

In this study, Chemical enhanced Shear Dilatancy Polishing (C-SDP) as a novel ultraprecision polishing method was proposed to obtain high surface quality tungsten. The effects of pH values and H2O<sup>2</sup> concentrations of polishing slurry on material removal rate (*MRR*) and surface roughness (*R*a) in the tungsten C-SDP process were studied. In addition, the corrosion behaviors of tungsten in solutions with different pH values and H2O<sup>2</sup> concentrations were analyzed by electrochemical experiments, and the valence states of elements on the tungsten surface were analyzed by XPS.

### **2. Principle of Chemical Enhanced Shear Dilatancy Polishing**

Figure 1a,b are schematic illustrations of the SDP and C-SDP principles, respectively. In the SDP processing, the abrasives can be trapped in the viscoelastic material to avoid the height difference caused by different abrasive particle sizes and improve the uniformity of force on the workpiece surface. C-SDP is a polishing method with the synergistic effect of mechanical action and chemical action. It selectively removes workpiece surface

roughness peaks based on chemical etching of oxidants and efficient mechanical removal of the dilatancy pad, resulting in higher efficiency and greater surface quality of workpieces than can be achieve using SDP. piece surface roughness peaks based on chemical etching of oxidants and efficient me‐ chanical removal of the dilatancy pad, resulting in higher efficiency and greater surface quality of workpieces than can be achieve using SDP. quality of workpieces than can be achieve using SDP.

synergistic effect of mechanical action and chemical action. It selectively removes work‐

synergistic effect of mechanical action and chemical action. It selectively removes work‐ piece surface roughness peaks based on chemical etching of oxidants and efficient me‐ chanical removal of the dilatancy pad, resulting in higher efficiency and greater surface

Chemical enhanced Shear Dilatancy Polishing (C‐SDP).

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*Micromachines* **2022**, *13*, x FOR PEER REVIEW 3 of 17

**Figure 1.** Schematic illustrations of the polishing principle: (**a**) Shear Dilatancy Polishing (SDP); (**b**) Chemical enhanced Shear Dilatancy Polishing (C‐SDP). **Figure 1.** Schematic illustrations of the polishing principle: (**a**) Shear Dilatancy Polishing (SDP); (**b**) Chemical enhanced Shear Dilatancy Polishing (C-SDP). During C‐SDP processing, the chemical polishing slurry contacts the tungsten sur‐ face so that a chemical reaction occurs, forming a passivation film with a lower hardness

During C‐SDP processing, the chemical polishing slurry contacts the tungsten sur‐ face so that a chemical reaction occurs, forming a passivation film with a lower hardness than tungsten. Moreover, relative movement between the dilatancy pad and tungsten workpiece will happen. Under shear force and pressure, viscoelastic material with non‐ Newtonian fluid properties in the contact area instantly exhibits shear dilatancy and so‐ lidification effects, forming a "flexible fixed abrasive tool (FFAT)" in the processing area, which can realize micro‐cutting removal of the reaction layer. C‐SDP is a synergistic pro‐ cess in which the abrasives remove the passivation film, and the bare surface reacts ac‐ tively–passively with the polishing slurry to reform the passivation film [24]. In the pro‐ cess of polishing, chemical corrosion of the polishing fluid and mechanical grinding of the abrasives are coupled to achieve materialremoval at the atomic level and efficientremoval of the tungsten surface. During C-SDP processing, the chemical polishing slurry contacts the tungsten surface so that a chemical reaction occurs, forming a passivation film with a lower hardness than tungsten. Moreover, relative movement between the dilatancy pad and tungsten workpiece will happen. Under shear force and pressure, viscoelastic material with non-Newtonian fluid properties in the contact area instantly exhibits shear dilatancy and solidification effects, forming a "flexible fixed abrasive tool (FFAT)" in the processing area, which can realize micro-cutting removal of the reaction layer. C-SDP is a synergistic process in which the abrasives remove the passivation film, and the bare surface reacts actively– passively with the polishing slurry to reform the passivation film [24]. In the process of polishing, chemical corrosion of the polishing fluid and mechanical grinding of the abrasives are coupled to achieve material removal at the atomic level and efficient removal of the tungsten surface. than tungsten. Moreover, relative movement between the dilatancy pad and tungsten workpiece will happen. Under shear force and pressure, viscoelastic material with non‐ Newtonian fluid properties in the contact area instantly exhibits shear dilatancy and so‐ lidification effects, forming a "flexible fixed abrasive tool (FFAT)" in the processing area, which can realize micro‐cutting removal of the reaction layer. C‐SDP is a synergistic pro‐ cess in which the abrasives remove the passivation film, and the bare surface reacts ac‐ tively–passively with the polishing slurry to reform the passivation film [24]. In the pro‐ cess of polishing, chemical corrosion of the polishing fluid and mechanical grinding of the abrasives are coupled to achieve materialremoval at the atomic level and efficientremoval of the tungsten surface. **3. Experiments**

#### **3. Experiments 3. Experiments** *3.1. Preparation of Dilatancy Pad and Polishing Slurry*

#### *3.1. Preparation of Dilatancy Pad and Polishing Slurry 3.1. Preparation of Dilatancy Pad and Polishing Slurry* Viscoelastic materials, ingredients, and abrasives with a certain mass ratio were

Viscoelastic materials, ingredients, and abrasives with a certain mass ratio were evenly mixed by mechanical agitation at 80 °C. After that, the mixture was cooled to room temperature to obtain shear dilatancy material suitable for SDP. In our research, the vis‐ coelastic material was a polymer material with non‐Newtonian fluid properties, and fumed silica was used as the ingredient to improve the mechanical properties of viscoe‐ lastic material. This viscoelastic material is prone to shear hardening under the action of external forces, which can enhance the holding force of abrasives. The dilatancy pad was obtained by filling the shear dilatancy material in a special polyurethane polishing pad, as shown in Figure 2a. The structure of the polyurethane dilatancy pad is shown in Figure 2b. The polyurethane pad filled with the shear dilatancy material is attached to the rigid layer to achieve a certain rigid support effect. The magnetic layer at the bottom makes the dilatancy pad magnetically adsorbed on the surface of the polishing base plate for easy Viscoelastic materials, ingredients, and abrasives with a certain mass ratio were evenly mixed by mechanical agitation at 80 ◦C. After that, the mixture was cooled to room temperature to obtain shear dilatancy material suitable for SDP. In our research, the viscoelastic material was a polymer material with non-Newtonian fluid properties, and fumed silica was used as the ingredient to improve the mechanical properties of viscoelastic material. This viscoelastic material is prone to shear hardening under the action of external forces, which can enhance the holding force of abrasives. The dilatancy pad was obtained by filling the shear dilatancy material in a special polyurethane polishing pad, as shown in Figure 2a. The structure of the polyurethane dilatancy pad is shown in Figure 2b. The polyurethane pad filled with the shear dilatancy material is attached to the rigid layer to achieve a certain rigid support effect. The magnetic layer at the bottom makes the dilatancy pad magnetically adsorbed on the surface of the polishing base plate for easy replacement. evenly mixed by mechanical agitation at 80 °C. After that, the mixture was cooled to room temperature to obtain shear dilatancy material suitable for SDP. In our research, the vis‐ coelastic material was a polymer material with non‐Newtonian fluid properties, and fumed silica was used as the ingredient to improve the mechanical properties of viscoe‐ lastic material. This viscoelastic material is prone to shear hardening under the action of external forces, which can enhance the holding force of abrasives. The dilatancy pad was obtained by filling the shear dilatancy material in a special polyurethane polishing pad, as shown in Figure 2a. The structure of the polyurethane dilatancy pad is shown in Figure 2b. The polyurethane pad filled with the shear dilatancy material is attached to the rigid layer to achieve a certain rigid support effect. The magnetic layer at the bottom makes the dilatancy pad magnetically adsorbed on the surface of the polishing base plate for easy replacement.

**Figure 2. Figure 2.**Polyurethane dilatancy pad: ( Polyurethane dilatancy pad: (**aa** ) physical diagram; ( ) physical diagram; (**b b** ) structure diagram. ) structure diagram.

**Figure 2.** Polyurethane dilatancy pad: (**a**) physical diagram; (**b**) structure diagram. The polishing base slurry was prepared by deionized water, the dispersant, and the active agent. Diamond micro-powders with a particle size of 0.5 µm were used as the abrasives. H2O<sup>2</sup> with a concentration of 30 vol.% was used as the oxidant. The pH value of the polishing slurry was adjusted by NaOH. The slurry was continuously stirred for 30 min to ensure that all components were well mixed for the polishing experiments. *3.2. Experimental Process and Conditions*

min to ensure that all components were well mixed for the polishing experiments.

The polishing base slurry was prepared by deionized water, the dispersant, and the active agent. Diamond micro‐powders with a particle size of 0.5 μm were used as the abrasives. H2O2 with a concentration of 30 vol.% was used as the oxidant. The pH value of the polishing slurry was adjusted by NaOH. The slurry was continuously stirred for 30

### *3.2. Experimental Process and Conditions* Tungsten workpieces used for the polishing experiments were obtained by a rolling

Tungsten workpieces used for the polishing experiments were obtained by a rolling process, which gave them a high density. The characteristics of tungsten are shown in Table 1. In this study, the plane workpiece was taken as the research target. Tungsten samples were 10 mm in diameter and 0.3 mm in thickness. C-SDP polishing experiments were carried out on the experimental device, as shown in Figure 3. During the polishing process, tungsten samples fixed on the fixture rotated along the normal direction with a certain pressure to ensure a uniform polishing of the workpiece surface. process, which gave them a high density. The characteristics of tungsten are shown in Table 1. In this study, the plane workpiece was taken as the research target. Tungsten samples were 10 mm in diameter and 0.3 mm in thickness. C‐SDP polishing experiments were carried out on the experimental device, as shown in Figure 3. During the polishing process, tungsten samples fixed on the fixture rotated along the normal direction with a certain pressure to ensure a uniform polishing of the workpiece surface.


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**Figure 3.** The experimental device of C‐SDP. **Figure 3.** The experimental device of C-SDP.

The experimental conditions are shown in Table 2. In order to achieve better mechan‐ ical removal effects of the dilatancy pads, the C‐SDP polishing experiments were carried out under the condition of high‐pressure and high‐speed. Tungsten has a high dissolution rate under alkaline conditions, which can increase the material removal rate. Therefore, the effects of polishing slurries at pH 7, 8, 9, 10, 11, and 12 on the polishing effects of tungsten were studied. Moreover, the effects of H2O2 concentrations (0–2.0 vol.%) on the The experimental conditions are shown in Table 2. In order to achieve better mechanical removal effects of the dilatancy pads, the C-SDP polishing experiments were carried out under the condition of high-pressure and high-speed. Tungsten has a high dissolution rate under alkaline conditions, which can increase the material removal rate. Therefore, the effects of polishing slurries at pH 7, 8, 9, 10, 11, and 12 on the polishing effects of tungsten were studied. Moreover, the effects of H2O<sup>2</sup> concentrations (0–2.0 vol.%) on the polishing performance of tungsten were also investigated.

polishing performance of tungsten were also investigated.


**Table 2.** Experimental conditions.

### *3.3. Measurement and Testing*

The pH values of the polishing slurries were measured by a glass electrode pH meter (PB-10, Sartorius, Germany, resolution: 0.01). The *MRR* of tungsten was measured by a precision balance (ME36S, Sartorius, Germany, resolution: 0.001 mg). The formula for calculating *MRR* is as follows:

$$MRR = \frac{\Delta m}{\rho ts} \tag{1}$$

∆*m* (g) is the quality difference of the tungsten sample before and after polishing, *ρ* (g/cm<sup>3</sup> ) is the density of the tungsten sample, *s* (cm<sup>2</sup> ) is the area of the tungsten sample, *t* (h) is the polishing time, and the unit of *MRR* is µm/h. Each polishing experiment was repeated three times, and the mean value was calculated.

After polishing, the surface roughness (*R*a) of tungsten was measured by a 3D profile White Light Interferometer (Super View W1, Chotest, Shenzhen, China), and the sampling range of the White Light Interferometer was 0.5 × 0.5 mm. The surface morphology of the workpiece was observed by a large-field-depth digital microscope (VHX-7000, Keyence, Osaka, Japan). The dynamic potential polarization curves of tungsten in abrasive-free solutions with different pH values and H2O<sup>2</sup> concentrations were tested by an electrochemical system with a three-electrode cell (CHI760E, CH Instruments, Shanghai, China). The chemical reactions between the components of the polishing slurry and tungsten were analyzed by X-ray photoelectron spectroscopy (ESCALAB 250Xi, Thermo Fisher, Waltham, MA, USA).

### **4. Results and Discussion**
