*4.1. Rheological Analysis of Shear Dilatancy Material*

Rheological testing of shear dilatancy material was performed by a rotational rheometer (MCR302, Anton Paar, Graz, Austria) at a constant testing temperature of 25 ◦C. During the test, the distance between the plate clamp and rotor (both 25 mm in diameter) was 1 mm. The strain was constant at 0.1%, and frequency sweep tests were performed from 0.1 to 10 Hz. The measurement for each sample was repeated three times in order to quantify the measurement error.

Figure 4a,b respectively show the trends of *G'* and *tanδ* of samples with different abrasive concentrations as the frequency increased. The *G'* represents the ability of material to store elastic deformation energy, which is used to characterize the elasticity of material. The *tanδ* represents the viscoelastic properties of material. When the *tanδ* is smaller, the elasticity of material is greater. The frequency (*tanδ* = 1) is the critical frequency of the solid–liquid phase transition, beyond which the material transforms from a liquid-like state to a solid-like state. It can be clearly found that with increasing frequency, the *G'* of sample increased, while the *tanδ* decreased. The material experienced an obvious shear hardening effect, which met the requirements of shear dilatancy polishing. As shown in Figure 4a,b, the abrasive concentrations significantly affected the rheological properties of the shear dilatancy material. With the increase of abrasive concentrations, the *G'* of the sample increased, and the *tanδ* decreased. The elasticity of the material was improved, and its

phase transition frequency decreased. In other words, the material transition to a "flexible fixed abrasive tool" requires a lower polishing speed, which makes the material more prone to transition from liquid-like to solid-like. When the abrasive concentration was 30 wt.%, the *G'* increased significantly, which was much higher than that of 20 wt.%. Combined with the rheological test results and polishing requirements, VM-30 wt.% Dimond was selected for the preparation of the dilatancy pad. and its phase transition frequency decreased. In other words, the material transition to a "flexible fixed abrasive tool" requires a lower polishing speed, which makes the material more prone to transition from liquid‐like to solid‐like. When the abrasive concentration was 30 wt.%, the *G'* increased significantly, which was much higher than that of 20 wt.%. Combined with the rheological test results and polishing requirements, VM‐30 wt.% Dimond was selected for the preparation of the dilatancy pad.

Figure 4a,b, the abrasive concentrations significantly affected the rheological properties of the shear dilatancy material. With the increase of abrasive concentrations, the *G'* of the sample increased, and the *tanδ* decreased. The elasticity of the material was improved,

*Micromachines* **2022**, *13*, x FOR PEER REVIEW 6 of 17

**Figure 4.** Rheological curves of the shear dilatancy material: (**a**) storage modulus curves; (**b**) loss factor curves. **Figure 4.** Rheological curves of the shear dilatancy material: (**a**) storage modulus curves; (**b**) loss factor curves.

### *4.2. Effect of pH Values on the Polishing Performance of Tungsten 4.2. Effect of pH Values on the Polishing Performance of Tungsten*

pH value is an important component of chemical polishing slurry, which determines the basic polishing environment of C‐SDP and directly affects the polishing quality [25]. The effects of pH values on tungsten C‐SDP are shown in Figure 5. As shown in Figure 5a, with the increase of pH, the material removal rate of tungsten showed a continuous increasing trend, and it increased from 6.69 μm/h to 13.67 μm/h. In neutral and alkaline environments, the oxide formed on tungsten surfaces is unstable and dissolves in solution at a very low rate to form tungstate ions (WO4−) [26,27], which can remove the oxide on the tungsten surface to a certain extent. On the other hand, since the oxide is softer and easier to remove than tungsten, the corresponding mechanical removal effect is also more pronounced. Because the dissolution rate of this oxide is low, the mechanical action of abrasives will remove most of the generated oxide, thereby exposing a new tungsten sur‐ face to continue the chemical reaction. In the neutral and alkaline conditions of the pol‐ ishing slurry, the removal rate increases with the increase of pH, which is also related to pH value is an important component of chemical polishing slurry, which determines the basic polishing environment of C-SDP and directly affects the polishing quality [25]. The effects of pH values on tungsten C-SDP are shown in Figure 5. As shown in Figure 5a, with the increase of pH, the material removal rate of tungsten showed a continuous increasing trend, and it increased from 6.69 µm/h to 13.67 µm/h. In neutral and alkaline environments, the oxide formed on tungsten surfaces is unstable and dissolves in solution at a very low rate to form tungstate ions (WO<sup>4</sup> −) [26,27], which can remove the oxide on the tungsten surface to a certain extent. On the other hand, since the oxide is softer and easier to remove than tungsten, the corresponding mechanical removal effect is also more pronounced. Because the dissolution rate of this oxide is low, the mechanical action of abrasives will remove most of the generated oxide, thereby exposing a new tungsten surface to continue the chemical reaction. In the neutral and alkaline conditions of the polishing slurry, the removal rate increases with the increase of pH, which is also related to the dissolution rate. The material removal rate includes the mechanical removal of abrasives and the dissolution of oxide.

the dissolution rate. The material removal rate includes the mechanical removal of abra‐ sives and the dissolution of oxide. When pH = 7, the material removal was mainly achieved by the mechanical action of diamond abrasives. Because the chemical action was very small at this time, the corrosion effect on the tungsten surface was extremely weak, resulting in a minimum material removal rate. When pH > 7, it was easy for tungsten to react with the alkaline slurry. Micro-convex peaks on the tungsten surface could be oxidized into relatively soft oxides. Under the mechanical grinding of diamond abrasives and the dissolution of alkaline solution, the generated oxides could be easily removed, and thus the material removal rate was improved.

**Figure 5.** Effect of pH values on the removal rate and surface roughness: (**a**) material remove rate; (**b**) surface roughness. **Figure 5.** Effect of pH values on the removal rate and surface roughness: (**a**) material remove rate; (**b**) surface roughness.

When pH = 7, the material removal was mainly achieved by the mechanical action of diamond abrasives. Because the chemical action was very small at this time, the corrosion effect on the tungsten surface was extremely weak, resulting in a minimum material re‐ moval rate. When pH > 7, it was easy for tungsten to react with the alkaline slurry. Micro‐ convex peaks on the tungsten surface could be oxidized into relatively soft oxides. Under the mechanical grinding of diamond abrasives and the dissolution of alkaline solution, the generated oxides could be easily removed, and thus the material removal rate was improved. However, the final tungsten surface quality is determined by the rate of oxide disso‐ However, the final tungsten surface quality is determined by the rate of oxide dissolution, the rate of oxide production, and the mechanical action of abrasives. How to control and balance the relationship among the three factors is a key issue that needs to be considered during the polishing process. With the increase of pH values, the surface roughness of the polished tungsten firstly decreased and then increased, as shown in Figure 5b. The surface roughness *R*<sup>a</sup> was the lowest at pH = 9. As the pH increased from 7 to 9, *R*<sup>a</sup> decreased from 3.16 nm to 2.35 nm. However, as the pH sequentially increased to 12, *R*<sup>a</sup> subsequently increased to 8.25 nm.

lution, the rate of oxide production, and the mechanical action of abrasives. How to con‐ trol and balance the relationship among the three factors is a key issue that needs to be considered during the polishing process. With the increase of pH values, the surface roughness of the polished tungsten firstly decreased and then increased, as shown in Fig‐ ure 5b. The surface roughness *R*<sup>a</sup> was the lowest at pH = 9. As the pH increased from 7 to 9, *R*<sup>a</sup> decreased from 3.16 nm to 2.35 nm. However, as the pH sequentially increased to 12, *R*<sup>a</sup> subsequently increased to 8.25 nm. Figure 6 shows the surface morphologies of tungsten after polishing with different pH values. The tungsten surface after polishing was relatively smooth, as seen in Figure 6a–c. However, the tungsten surface became gradually uneven, as seen in Figure 6d–f, Figure 6 shows the surface morphologies of tungsten after polishing with different pH values. The tungsten surface after polishing was relatively smooth, as seen in Figure 6a–c. However, the tungsten surface became gradually uneven, as seen in Figure 6d–f, and the corrosion degree of tungsten gradually deepened. It can be inferred that when pH > 9, OH– in the slurry was very corrosive for tungsten, which likely caused uneven corrosion of micro-convex peaks on the tungsten surface, resulting in poor surface quality after polishing. Figure 7 shows the surface morphologies of tungsten under different pH values. Under strong alkaline conditions (pH = 11 and pH = 12), the tungsten surface was significantly corroded, as shown in Figure 7b,c. *Micromachines* **2022**, *13*, x FOR PEER REVIEW 8 of 17

**Figure 6.** Surface morphologies of polished tungsten under different pH values: (**a**) pH = 7; (**b**) pH = 8; (**c**) pH = 9; (**d**) pH = 10; (**e**) pH = 11; (**f**) pH = 12. **Figure 6.** Surface morphologies of polished tungsten under different pH values: (**a**) pH = 7; (**b**) pH = 8; (**c**) pH = 9; (**d**) pH = 10; (**e**) pH = 11; (**f**) pH = 12.

**Figure 7.** Surface morphologies of polished tungsten under different pH values: (**a**) pH = 9; (**b**) pH

During the polishing process, a passivation film is formed on the tungsten surface due to oxidizing agents. Because of the low hardness of oxide and the weak interface that exists between tungsten and oxide, the passive film is easily removed by the mechanical action of abrasives [28]. The concentrations of H2O2 can significantly affect the generation rate of the passivation film on the tungsten surface, which in turn affects the polishing performance of C‐SDP [29]. The effects of H2O2 concentrations on material removal rate and surface roughness of tungsten are shown in Figure 8. In the experiments, the pH val‐ ues of polishing slurries with different H2O2 concentrations were all 9. As shown in Figure 8a, with the increase of H2O2 concentration, the *MRR* of tungsten increased continuously, from 9.71 μm/h to 34.95 μm/h. After the concentration of H2O2 increased, the chemical corrosion effect of polishing slurry on the tungsten surface material was enhanced, which could generate faster or thicker soft passivation films. As shown in Figure 8b, the surface roughness of tungsten decreased firstly and then increased, and its surface roughness was the lowest when H2O2 concentration was 1 vol.%. As the H2O2 concentration increased from 0 to 1 vol.%, *R*<sup>a</sup> decreased from 2.35 nm to 1.87 nm. *R*<sup>a</sup> began to increase, reaching 4.14 nm when the H2O2 concentration was 2 vol.%, since the H2O2 concentration continued

*4.3. Effect of H2O2 Concentrations on the Polishing Performance of Tungsten*

= 11; (**c**) pH = 12.

to increase.

= 8; (**c**) pH = 9; (**d**) pH = 10; (**e**) pH = 11; (**f**) pH = 12.

**Figure 7.** Surface morphologies of polished tungsten under different pH values: (**a**) pH = 9; (**b**) pH = 11; (**c**) pH = 12. **Figure 7.** Surface morphologies of polished tungsten under different pH values: (**a**) pH = 9; (**b**) pH = 11; (**c**) pH = 12.

**Figure 6.** Surface morphologies of polished tungsten under different pH values: (**a**) pH = 7; (**b**) pH
