Effect of the gas layer evolution on electrolytic plasma polishing of stainless steel

Abstract

In recent years, electrolytic plasma polishing technology has attracted wide attention due to its advantages of shape adaptability, high efficiency, better precision, environmental friendliness, and non-contact polishing. However, the lack of research on the evolution mechanism of the gas layer at the anode interface restricts the improvement of the material removal mechanism and the regulation of the polishing effect. Firstly, the thermodynamic conditions of gas layer formation were analyzed based on the Clapeyron-Clausius equation, and the key parameters affecting the gas layer were identified. Secondly, the laws of voltage and electrolyte temperature on the dynamic evolution of the gas layer and its polishing effect were revealed. Additionally, the influence of the gas layer on the voltage-current characteristics was also investigated by analyzing the experimental phenomena. The results indicate that the optimal polishing effect is achieved at a voltage level of 300 V resulting in a decrease in Ra from 0.451 μm to 0.076 μm. Similarly, superior polishing results are obtained when the electrolyte temperature is 80 °C, with a decrease in R_a from 0.451 μm to 0.075 μm. This study provides theoretical guidance for the further development and application of electrolytic plasma polishing technology.

Introduction

Electrolytic plasma polishing (EPP) is an advanced technology for polishing metals and alloys. It is based on the phenomenon of plasma-assisted anode dissolution, using an inexpensive neutral inorganic salt solution as the electrolyte. This technology does not use corrosive and toxic reagents, does not produce harmful substances during the polishing process^1–3. The cited article highlights the advantages of using this method of processing, which include high efficiency, excellent surface quality and the ability to process a wide range of materials without the introduction of macroscopic stresses or micro-cracks^4–7. Especially in the processing of parts with complex shapes, it has incomparable advantages over other processing methods such as mechanical polishing, chemical polishing, and electrochemical polishing^8–11.

During the polishing process, the electrolyte on the anode surface locally boils and generates a large number of bubbles due to ohmic heating, and a macroscopic vapor-gaseous envelope (abbreviation: gas layer) is formed as the bubbles continue to be generated and move upward as well as merge between the bubbles^12–14. Wang Ji et al. studied the mechanism of the gas layer in the electrolytic plasma polishing process of 304 stainless steel on the basis of gas discharge theory, and the results showed that the microscopic raised position on the anode surface formed localized gas explosion and melted and etched away the surface metal under high electric field¹⁵. Zhou et al. simulated and modeled the evolution of the gas layer during electrolytic plasma polishing of nickel-based high-temperature alloys, and calculated the heat flow density at the gas-liquid interface at different voltage and electrolyte temperature. The effects of voltage and electrolyte temperature on surface quality and polishing effect were investigated experimentally. The results showed that a discontinuous and fluctuating gas layer is favorable for the removal of surface materials from the workpiece, and when the voltage exceeds 400 V, the heat flow at the gas-liquid interface exceeds the critical heat flow and the bubbles evolve into gas films, which is unfavorable for electrolytic plasma polishing of nickel-based high temperature alloys¹⁶.

Electrolytic plasma polishing is characterised by the physical bombardment effect of instantaneous high temperature on surface materials of the workpiece caused by gas layer discharge. However, this mechanism alone cannot explain why different types of metals or alloys require different electrolytes for polishing^11,17,18. Meanwhile, there have been few studies on the influencing factors and thermodynamic conditions during the formation process of the gas layer in the electrolytic plasma polishing process. Additionally, the connection between the gas layer evolution process and electrolytic plasma polishing is relatively ambiguous, which leads to a deviation in the understanding of the polishing mechanism. This study establishes a thermodynamic model for the formation of gas layers during the process of electrolytic plasma polishing of stainless steel. The model is used to analyse the influence of gas layer evolution on voltage-current characteristics, study the impact of process parameters on the dynamic evolution of the gas layer, surface roughness R_a, and material removal rate. The study also aims to reveal the mechanism of gas layer evolution during the polishing process.

Thermodynamic model of gas layer formation

The essence of gas layer formation during electrolytic plasma polishing process is that the electrolyte near the anode is instantly vaporized under the action of ohmic heat and a large number of small bubbles are generated and attached to the anode surface¹⁹. As the bubbles continue to grow and merge with adjacent bubbles before leaving the surface of the workpiece, the anode is enveloped by a macroscopic vapor-gas envelope, which main components are water vapor, oxygen produced by the electrolysis reaction at the anode, and plasma discharge products²⁰. As a result of the separation speed of the bubbles from the anode being lower than the generation speed, the anode is enveloped and forms a macroscopic vapor-gas envelope. Therefore, bubble generation is the basis for the formation of the gas layer. According to the Clapeyron-Clausius equation, the gas in the bubble is assumed to be an ideal gas, and the degree of superheat required to generate the bubble is²¹:

1

Where: σ_l is the surface tension of the electrolyte, mN/m²; R_g is the gas constant; T_s is the thermodynamic temperature of the anode surface, K; p_l is the saturated vapor pressure of the electrolyte, Pa; ρ_l is the electrolyte density, g/cm³; ρ_g is the water vapor density, g/cm³; i_l−g is the parameter of physical property of the gas-liquid interface. r_c is the critical radius of the bubble from the anode surface, mm. In the process of electrolytic plasma polishing, the electrolyte is in a nonisothermal flow boiling state with heat transfer, and the bubble growth process is the continuous evaporation process of the electrolyte interface in the bubble, accompanied by continuous heat transfer to the liquid phase. In this case, the electrolyte around the anode is in a superheated state, and the vapor layer is also in a superheated state. With the condition of ideal gas content:

2

Where: p_g is the pressure of vapour; v_g is the volume of the bubble; T_g is the temperature of vapour; R_g is the molar gas constant of vapour. Since ρ_l >>ρ_g, ρ_l/(ρ_l-ρ_g) ≈ 1, the Eq. (1) can be simplified as:

3

As the anode continues to be heated by Ohmic heating, and under the condition of neglecting the heat taken away by solution evaporation, the temperature of the electrolyte around the anode gradually rises, and the overall liquid phase enters a metastable equilibrium state. With increasing superheat ΔT_s, the critical bubble radius r_c decreases. When the action of thermal fluctuations and phase inhomogeneity coexist, bubble clusters are formed, known as bubble nuclei, leading to boiling on the surface of the workpiece. The formation of bubble nuclei requires energy consumption. For the formation of a bubble nucleus with a radius r in the liquid phase, the energy consumed is mainly used to overcome the free energy and the interfacial surface energy²².

4

Where: g_g represents specific free enthalpy of activated molecular groups of gases; g_l represents specific free enthalpy of activated molecular groups of liquids. According to Eq. (3), the critical bubble radius can be obtained as follows:

5

The critical bubble radius for the formation of macroscopic gas layer on the anode surface during electrolytic plasma polishing is r_c, assuming that the number of water vapor molecular clusters forming a single bubble with radius r_c is n(r_c), according to the probability relation and Boltzmann energy distribution law, the rate of generating critical bubble nuclei v_bg can be expressed according to molecular motion theory as follows²³:

6

Where: represents collision frequency; κ stands for Boltzmann constant; h_p stands for Planck’s constant; E_c represents the energy required to produce critical bubbles; τ represents time. The differential of Eq. (4) can be obtained as follows:

7

Suppose that the energy consumption corresponding to the maximum value of r_c according to the bubble size can be obtained from :

8

Combined Eqs. (5) and (8) obtain:

9

Substituting Eqs. (8) and (9) into Eq. (4) yields:

10

By substituting Eq. (10) into Eq. (6), the rate of generating critical bubble nuclei is as follows:

11

The logarithm of Eq. (11) yields:

12

The r_c can be obtained as

13

Substituting Eq. (13) into (5) yields the following.

14

It can be concluded that the surface tension of the solution has an effect on the degree of superheat generated by the bubbles, but the value of the electrolyte concentration near the anode during the polishing process is in a relatively stable range, so that the electrolyte concentration has a low degree of influence on the surface tension of the solution. In general, bubbles can be formed at the macrolevel when the bubble rate v_bg reaches 10⁹-10¹³/ (m³·s)²⁴. In the electrolytic plasma polishing process of stainless steel, atmospheric pressure p = 0.1 MPa, thermodynamic temperature T ≈ 373 K near the anode, surface tension σ_l > 58.9 mN /m² of ammonium sulfate electrolyte, collision frequency range 10⁹-10¹³/s, ΔT_s ≈ 220 K can be calculated according to Eq. (14). However, in the actual situation, as shown in Fig. 1, the surface of stainless steel is uneven, and bubbles are not uniformly generated on its surface. In the state of electrification, the charge density at the microconvex position is relatively concentrated, and the ohmic heat generated in the transient state is much higher than that in the surrounding depression, and the temperature rises faster, making it easier to reach the superheat required for bubble nucleation. Therefore, from a thermodynamic point of view, bubbles are always generated from the bulge and continue to grow and merge with the bubbles generated by the surrounding bulge until they leave the anode surface.

Fig. 1.

Schematic diagram of gas layer formation on anode during electrolytic plasma polishing process.

Materials and methods

The basic electrolytic plasma polishing model is shown in Fig. 2, which is mainly composed of a power supply system, electrolytic cell, electrode, and electrolyte temperature control system. The test material in this study was 316 L stainless steel produced by TISCO. The chemical composition is as follows: (wt%) C-0.03, Cr-18.5, Ni-12.7, Mo-2.10, Mn-2.00, P-0.035, S-0.02, Si-0.80, and balance Fe. The sample was cut into smaller pieces of dimensions 20 × 15 × 3 mm following the wire-electrode cutting method. Both sides of the sample were milled to remove the oxide layer on the surface and obtain a consistent initial surface. After pretreatment, the mean arithmetic roughness Ra of 316 L stainless steel surface is approximately 0.45 μm. Table 1 shows the experimental programme, the electrolyte was 3% (wt%) ammonium sulfate aqueous solution, the polishing voltage was set at 100 V, 300 V, 400 V, and 500 V respectively, and the electrolyte temperature was set to 70 ℃, 80 ℃, and 90 ℃ respectively, and the processing time was set at 10 min. All measurements were repeated three times for each group. The dynamic evolution process of the vapor-gas envelope layer was observed with the high-speed camera mode of the Canon M6 camera during the polishing process. The average temperature of the gas layer and the electrolyte interface during the polishing process was measured with the Fluke Ti401 PRO infrared thermal imager. Furthermore, by applying a linear increase voltage ranging from 0 to 500 V with a change rate of 1 V/s to the anode workpiece, the loop current and resistance change with voltage data are measured.

Fig. 2.

The model of electrolytic plasma polishing.

Table 1.

The experimental scheme.

Number	Voltage/V	(NH4)2SO4/wt%	Temperature/℃	Time/min
NO.1	100	3	80	10
NO.2	300	3	80	10
NO.3	400	3	80	10
NO.4	500	3	80	10
NO.5	300	3	70	10
NO.6	300	3	80	10
NO.7	300	3	90	10

As a commonly used surface quality evaluation index in the machining industry, surface roughness is a quantitative analysis of the irregular characteristics of machined surfaces with continuous peak and valley values and different heights, depths and intervals. In this study, the improvement of the surface line roughness value of polished samples was taken as the evaluation index of electrolytic plasma polishing, referring to the product geometry technical specification GB/T 3505 − 2009 (ISO 4287: 1997) terms, definitions, and technical specifications of surface structure, taking the contour arithmetic mean roughness Ra in surface roughness as the measurement index. The measuring equipment is the MarSurf M400 high precision surface roughness measuring instrument from the German Marsurf Company. The resolution is 0.8 nm and the contact measuring force is 0.75 mN. During measurement, all samples were measured at the uniform location 5 times and the average value was taken as the test result. The average material removal rate MRR (unit: µm/min) of electrolytic plasma polished stainless steel was calculated using the “weight loss method” as follows:

15

Where, Δm is the mass change of the sample before and after polishing, the unit is g, which is measured by a high precision electronic balance; ρ is the sample density of 316 L stainless steel, 7.98 g/cm³; S is the total effective polished surface area of the sample, the unit is cm²; t is the polishing time, the unit is min.

Results and discussion

The influence of voltage on gas layer evolution

The dynamic evolution process of the steam-gas envelope layer of 316 L stainless steel polished by electrolytic plasma under different voltages is shown in Fig. 3. Figure 3 (a) shows the dynamic evolution process of the gas layer when the voltage is 100 V. With the ohmic thermal effect and the anodic electrolytic reaction, the electrolyte on the surface of the stainless steel workpiece evaporates rapidly in a short time, and the strong volume expansion causes the explosion of vapor gas. There are a lot of visible bubbles around the stainless steel anode, and no stable steam-gas envelope layer is formed.

Fig. 3.

The dynamic evolution process of the gas layer at various voltages.

When the voltage is 300 V, a stable gas layer is formed on the surface of the stainless steel anode as shown in Fig. 3 (b). In this state, a large number of bubbles will be formed at the microconvex position on the anode surface. As the bubbles continue to grow and leave the anode-electrolyte interface, the bubbles will combine and form a macroscopic vapor gas film. The anode is wrapped in vapor gas film, due to the high charge density and ionization breakdown effect, the anode edge produces a weak micro-arc discharge phenomenon, forms a plasma channel, and enters a stable electrolytic plasma polishing state.

When the voltage is increased to 400 V and 500 V, a stable steam-gas envelope appears on the surface of the stainless steel anode. When the voltage is 400 V, as shown in Fig. 3 (c), the thickness of the gas layer on the anode surface increases, a clear phenomenon of microarc discharge can be seen, and the luminous region spreads from the edge to the center. When the voltage continues to increase to 500 V, as shown in Fig. 3 (d), the phenomenon of microarc discharge on the anode surface becomes more intense, the luminous intensity further increases, the color becomes brighter, and the gas layer thickness also increases. This happens because when the voltage increases, the transient temperature on the anode surface increases, and the saturation temperature of the electrolyte remains unchanged, thus the superheat generated by bubbles at the interface increases. According to Eq. (5), an increase in energy transfer from the anode to the electrolyte results in a reduction in the critical bubble radius and the generation of more bubbling cores per unit area. This phenomenon is accompanied by a sharp increase in bubble size, which subsequently merges, leading to an expansion in the thickness of the gas layer.

The level of voltage in the polishing process directly determines the total input energy of the entire system, and the output energy also increases with increasing power supply voltage. Generation and maintenance of the gas layer are mainly formed by the boiling and vaporization of the electrolyte under the action of ohmic heat. The initial electrolyte temperature of 316 L stainless steel is 90 ± 2℃ at different voltages. The average temperature distribution between the vapor gas envelope layer and the electrolyte during the electrolytic plasma polishing process is shown in Fig. 4. It can be seen from the figure that the temperature at the interface between the anode stainless steel and the gas layer is always the highest during the entire polishing process.

Fig. 4.

Temperature distribution of gas-anode interface at various voltages.

When the voltage is 100 V, no stable vapor layer is formed around the anode, and a large number of steam bubbles are formed in the high temperature area, and the temperature is close to the boiling temperature of the electrolyte, as shown in Fig. 4 (a). This may be due to the electrolytic reaction of ammonium sulfate solution occurring at the anode at this time, and there is no obvious temperature spreading area. When the voltage is 300 V, a stable gas layer gradually forms around the anode, as shown in Fig. 4(b). It can be seen that the temperature of the gas layer is higher than the temperature of the surrounding electrolyte, and the electrolyte around the anode forms a convective diffusion zone, and when the gas layer leaves the liquid surface, the temperature is lower than the boiling temperature of the electrolyte. At a voltage of 400–500 V, the temperature of the gas layer upon exiting the liquid surface is comparable to or exceeds the boiling temperature of the electrolyte, as illustrated in Fig. 4 (c) and (d). This could be due to the input energy of the power supply is significantly greater than the energy required to maintain the stability of the gas layer. In this instance, the mean heat flux at the gas-liquid interface surpasses the critical heat flux of the electrolyte.

The influence of the electrolyte temperature on gas layer evolution

Figure 5 illustrates the process of dynamic evolution of the gas layer in electrolytic plasma polishing of 316 L stainless steel at different electrolyte temperatures when the voltage is 300 V. It can be observed that the stability of the gas layer is not directly influenced by the electrolyte temperature. The electrolyte surrounding the anode workpiece reaches a stable convective heat transfer state upon entering the polishing mode. As the temperature of the electrolyte increases, the superheat of bubble formation at the gas-liquid interface decreases. According to Eq. (5), the critical bubble radius increases, leading to an increase in the thickness of the gas layer. Therefore, the temperature of the electrolyte has an impact on the thickness of the gas layer.

Fig. 5.

The dynamic evolution process of the gas layer at various electrolyte temperatures.

The average temperature distribution of the steam-gas envelope layer and electrolyte of 316 L stainless steel at different electrolyte temperatures is shown in Fig. 6. It can be seen that the gas layer temperature is always higher than the electrolyte temperature during the polishing process. With the increase in the electrolyte temperature, the temperature of gas layer around the anode increases gradually, and an obvious temperature gradient area can be seen, which will affect the diffusion rate of polishing products near the anode. The electrolyte temperature is stable during the polishing process, and the influence of the atmosphere is smaller. The increase in the temperature of the electrolyte will reduce the power input of the power supply to the polishing system. As the electrolyte gradually approaches the boiling temperature, the average heat flux of the gas-liquid interface gradually decreases and is lower than the critical heat flux. During the polishing process, the electrolyte always maintains the local bubble nucleation boiling mode near the surface of the workpiece.

Fig. 6.

Temperature distribution of the gas-anode interface at various electrolyte temperatures.

The influence of gas layer on the voltage-current characteristic

The characteristic curve of loop current and resistance of 316L stainless steel in ammonium sulfate solution with voltage change is shown in Fig. 7, which is measured by applying a linear increase voltage with a change rate of 1 V/s from 0 to 500 V. The voltage-current curve can be divided into four regions according to the change characteristics of loop current and resistance when the voltage increases linearly: (I) The voltage between 0–40 V is Faraday's law region, (II) the voltage between 40–180 V is a negative differential resistance region, (III) the voltage between 180–380 V is a stable polishing region, and (IV) the voltage above 380 V is an unstable polishing region or microarc discharge region. In the region (I) stage, the loop current and voltage are linear, and the loop resistance is mainly the resistance value of the electrolyte and the wire between the electrodes, which belongs to the typical Faraday electrolytic mode. When the voltage is increased to region (II), due to the large number of bubbles produced by the anode with the action of an electroanalytic oxygen reaction and ohmic heat boiling, and the formation of a gas film with a large resistance around the anode, the resistance begins to increase and shows obvious negative differential characteristics. In the stage of region (III), a dynamic and stable VGE is formed around the stainless steel anode, and the loop current is relatively stable. Compared to regions (I) and (II), the resistance value increases rapidly under the action of the gas layer. Therefore, the region (III) stage can be considered as the stable voltage range of electrolytic plasma polishing for 316L stainless steel. In region (IV), the loop current increases gradually with the increase of voltage, and the unstable gas layer increases the contact frequency between the anode workpiece and the electrolyte, resulting in a decrease in resistance. Moreover, a strong microarc discharge occurs at high voltage, and the gas layer thickness reaches the maximum value.

Fig. 7.

The characteristic curve of current and resistance changes with voltage.

The influence of voltage on surface roughness and material removal rate

When the electrolyte concentration, electrolyte temperature, and processing time are clear, the arithmetic average roughness Ra and material removal rate measured at different voltages are shown in Fig. 8 (a) and (b). It can be seen that the mean arithmetic roughness Ra decreases first and then increases with the increase of voltage, and the lowest roughness decreases from the initial 0.451 μm to 0.076 μm at 300 V. The material removal rate gradually decreased with the increase of voltage. When the voltage is 100 V, a stable gas layer has not been formed, and the metal ions on the surface of the stainless steel anode undergo an electrochemical oxidation reaction to form a precipitate and adhere to the surface. At this time, the passivation layer on the anode surface dissolved rapidly with the action of current, and the anode showed a violent oxygen evolution reaction and electrochemical etching processing. Although the material removal rate was high, the roughness of the workpiece surface did not decrease. When the voltage is 300 V, a stable gas layer is gradually formed, and the plasma electrochemical oxidation reaction at the interface occurs when the electrolyte is in contact with the workpiece. The generation of the gas layer causes a strong voltage drop between the anode and the electrolyte, and plasma discharge occurs. Plasma discharge is often accompanied by an instant high temperature and high pressure, and the interface plasma electrochemical dissolution reaction occurs. Thus, the surface of stainless steel is polished. When the voltage exceeds 400 V, due to the existence of local ultra-high voltage, the plasma discharge intensity in the gas layer further increases, and microarc discharge is formed in some areas, most of the energy is consumed in the microarc discharge between the anode and the electrolyte, and the anode surface temperature exceeds the boiling point of the electrolyte, and the electrolyte vaporizes immediately or before contact with it, and the plasma electrochemical reaction cannot be carried out. As a result, the polishing effect decreases.

Fig. 8.

The influence of voltage on the effect of electrolytic plasma polishing stainless steel.

The influence of electrolyte temperature on surface roughness and material removal rate

The stability of the gas layer on the anode surface during electrolytic plasma polishing is mainly influenced by the electrolyte temperature, which also affects the conductivity, surface tension, and viscosity of the electrolyte. In addition, an increase in the electrolyte temperature also influences the critical voltage, which is the decisive factor in determining whether the transition from the negative differential resistance region to the stable polishing region can be achieved. A typical feature of electrolytic plasma polishing is that when entering the processing state, the anode workpiece will be surrounded by a stable gas layer and the current will be reduced to a constant minimum range. It was found that polishing could not be performed when the electrolyte temperature was 70 ℃ or less during the experiment, because the current transitioned to the heating mode without forming a stable gas layer. In this study, three gradients of 70 ℃, 80 ℃ and 90 ℃ (error ± 3 ℃) were selected to study the effect of temperature on surface roughness and material removal rate, and the results are shown in Fig. 9. As can be seen from Fig. 9 (a), the value of roughness Ra shows a trend of decreasing and then increasing with the increase of electrolyte temperature, and the polishing effect is better when the electrolyte temperature is 80 ℃, and Ra decreases from the initial 0.451 μm to 0.075 μm. As shown in Fig. 9 (b), the polishing effect is reduced when the electrolyte temperature is 90 ℃, the material removal rate decreases with the increase of electrolyte temperature, and the material removal rate decreases approximately 50% for every 10 ℃ of the increase in temperature. It is due to the higher temperature of the electrolyte, the thicker the gas layer around the anode, hindering the direct contact between the solution and the workpiece, and the increased resistance of the system circuit, leading to a reduction in current, which adversely affects the polishing effect and material removal rate. As the temperature rises, the viscosity of the electrolyte decreases, and the bubbles generated on the anode surface are more easily dislodged, which is conducive to reducing the surface roughness value of stainless steel. At the same time, high temperature also promotes the hydrolysis of NH⁴⁺ ions in the electrolyte and the rapid decomposition of volatile products such as ammonia, thus affecting the stability of the gas layer as well as exacerbating electrolyte consumption. Elevated electrolyte temperatures result in the production of more ammonia, which in turn increases the complexity and uncertainty of the gas composition in the gas film. This may have an effect on the stability of the film. A decrease in the concentration of ammonium ions in the electrolyte affects the conductivity and pH of the electrolyte, which can cause the rate of material removal to decrease with increasing temperature.

Fig. 9.

The influence of electrolyte temperature on the effect of electrolytic plasma polishing stainless steel.

Conclusion

Electrolytic plasma polishing can be used to polish complex shapes of metal parts as a new polishing technology, which represents a development direction for polishing technology. This technique is objective and precise, making it an ideal choice for achieving a high-quality finish on metal parts. This study investigates the evolution of the gas layer during electrolytic plasma polishing, using the thermodynamic conditions of gas layer formation as the theoretical basis. The influence of voltage and electrolyte temperature on the evolution of the gas layer and polishing effect is examined using an orthogonal experimental method. The main conclusions are as follows:

(1) The vapor-gaseous envelope is created by the merging of numerous microbubbles that form on the surface of the anode workpiece. The microscopic elevated position is more likely to reach the superheat required for bubble nucleation of bubbles under the charge aggregation effect, which preferentially generates bubbles and forms the gas layer.

(2) The generation and stable maintenance of the vapour-gaseous envelope is primarily determined by the voltage and electrolyte temperature. The voltage-current characteristic curve indicates that the voltage in the range of 180–380 V is the stability zone for electrolytic plasma polishing of stainless steel polishing. The thickness of the gas layer is affected by the electrolyte temperature, which influences the critical bubble radius. The higher the temperature, the greater the thickness of the gas layer.

(3) The optimal conditions for gas layer stability and substance transport and diffusion occur when the voltage is 300 V and the electrolyte temperature is 80 ℃. This is due to the most favorable temperature gradient at the interface between the anode and the electrolyte. It is important to note that this statement is based on objective evaluations and not subjective opinions.

(4) The best polishing parameters were found to be a voltage of 300 V, resulting in a decrease of roughness Ra from the initial 0.451 μm to 0.076 μm with a material removal rate of 1.45 μm/min, and a temperature of 80 ℃, resulting in a decrease of roughness Ra from the initial 0.451 μm to 0.075 μm with a material removal rate of 1.55 μm/min.

Author contributions

Gangqiang Ji and Liyun Wu wrote the main manuscript text and Longfei Ma prepared Figs. 4, 5, 6, 7 and 8. All authors reviewed the manuscript.

Data availability

All datasets used and analyzed in this study herein are available upon reasonable request from the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.