A method for preparing a fluorine corrosion resistant micro-arc oxidation coating on an aluminum alloy surface

Abstract

A method for preparing a fluorine-resistant micro-arc oxidation coating on an aluminum alloy surface includes the following steps: forming a passivation film on the surface of an aluminum alloy substrate; rinsing and drying; and forming a fluorine-resistant micro-arc oxidation coating on the surface of the passivation film. This invention employs a two-step micro-arc oxidation method. The first step, micro-arc oxidation, forms a passivation film on the aluminum alloy surface, which prevents high-conductivity γ-doping. 3+ The inability to ignite an arc in the electrolyte leads to the failure of micro-arc oxidation; the second step of micro-arc oxidation involves doping the electrolyte with Y. 3+ Yttrium doping affects the efficiency of plasma discharge in the coating, reduces the formation of oxygen vacancies in the coating, thereby forming a more stable electron and ion transport discharge channel, improving the surface density of the MAO coating, reducing defect cracks, and altering the surface structure of MAO alumina. The corrosion products also exhibit higher mismatch and interface stability, thus improving the corrosion resistance of the MAO coating in a fluorine environment and providing good wear resistance.

Description

Technical Field

[0001] This invention relates to the field of material surface treatment technology, and in particular to a method for preparing a fluorine-resistant micro-arc oxidation coating on an aluminum alloy surface. Background Technology

[0002] Aluminum alloys are in high demand in the semiconductor equipment field due to their high strength-to-weight ratio, good wear and corrosion resistance, and excellent machinability. However, the semiconductor manufacturing process often uses highly corrosive fluorine-containing compounds, which significantly affect the service life of aluminum alloys. Micro-arc oxidation (MAO) technology can form a high-performance micro-arc oxidation coating (ceramic protective oxide film) on the metal surface in situ, which is one of the effective means to improve the corrosion resistance of aluminum alloys. However, due to the inherent instability of the discharge in MAO technology, micropores and microcracks inevitably form in the MAO coating. Furthermore, fluorine has a strong electronegativity, and the coating easily forms embrittled ionic fluorides after corrosion. Therefore, in a fluorine environment, the corrosion resistance and wear resistance of the MAO coating are poor. Summary of the Invention

[0003] Therefore, the purpose of this invention is to provide a method for preparing a fluorine-resistant micro-arc oxidation coating on an aluminum alloy surface, so as to improve the corrosion resistance and wear resistance of the micro-arc oxidation coating.

[0004] A method for preparing a fluorine-resistant micro-arc oxidation coating on an aluminum alloy surface includes the following steps:

[0005] Step S10: After the polished and dried aluminum alloy substrate is connected to the positive electrode of the primary micro-arc oxidation power supply, it is placed in the primary electrolyte for the first micro-arc oxidation to form a passivation film on the surface of the aluminum alloy substrate. The primary electrolyte includes sodium hexametaphosphate, sodium silicate and deionized water.

[0006] Step S11: After rinsing the aluminum alloy substrate containing the passivation film with deionized water, dry it at room temperature.

[0007] Step S12: After drying, the aluminum alloy substrate containing the passivation film is connected to the positive electrode of the secondary micro-arc oxidation power supply and then placed in the secondary electrolyte for a second micro-arc oxidation to form a fluorine-resistant micro-arc oxidation coating on the surface of the passivation film. The secondary electrolyte includes sodium hexametaphosphate, sodium silicate, yttrium nitrate and deionized water.

[0008] Further, in step S10, the concentration of sodium hexametaphosphate is 20–40 g / L, and the concentration of sodium silicate is 2–8 g / L.

[0009] Furthermore, in step S10, the first micro-arc oxidation adopts a constant current mode with a frequency of 400-800Hz, a positive and negative duty cycle of 10-30%, and a current density of 0.1-2A / dm³. 2 .

[0010] Furthermore, in step S10, the first micro-arc oxidation time is 1-4 min, and the thickness of the passivation film is 0.1-2 μm.

[0011] Furthermore, in step S11, the rinsing time is 1-5 minutes.

[0012] Further, in step S12, the concentration of sodium hexametaphosphate is 25–40 g / L, the concentration of sodium silicate is 2–10 g / L, and the concentration of yttrium nitrate is 1–5 mol% / L.

[0013] Furthermore, in step S12, the second micro-arc oxidation adopts a constant current mode with a frequency of 400-800Hz, a positive and negative duty cycle of 10-30%, and a current density of 0.1-2A / dm³. 2 .

[0014] Furthermore, in step S12, the second micro-arc oxidation time is 6-10 min, and the thickness of the fluorine-resistant micro-arc oxidation coating is 5-25 μm.

[0015] Furthermore, during the micro-arc oxidation process, the temperatures of the primary electrolyte and the secondary electrolyte are maintained at 20-50°C.

[0016] Further, in step S10, the polishing method includes:

[0017] In a water-cooled environment, the surface of the aluminum alloy substrate was polished by sequentially using SiC sandpaper with grits of 800, 1200, 2000, and 3000.

[0018] The surface of the aluminum alloy substrate is polished a second time using a cloth coated with polishing paste.

[0019] Compared to existing technologies, this invention employs a two-step micro-arc oxidation method. The first step, micro-arc oxidation, forms a passivation film on the aluminum alloy surface, thereby preventing high-conductivity Y doping. 3+ The inability to ignite an arc in the electrolyte leads to the failure of micro-arc oxidation; the second step of micro-arc oxidation involves doping the electrolyte with Y. 3+ This process influences the efficiency of plasma discharge in the coating, reduces the formation of oxygen vacancies, and thus creates more stable electron and ion transport discharge channels, improving the surface density of the MAO coating and reducing defects and cracks. Simultaneously, yttrium doping alters the formation of the Al4Y2O9 structure in MAO alumina. Compared to the alumina structure, the YF bonds in the corrosion products are stronger than the Al-F bonds, and the Al4Y2O9 / Na ratio is also improved.1.5 Y 2.5 F9 exhibits higher mismatch and interface stability, which can slow down the embrittlement of ionic crystals and inhibit crack propagation, thereby improving the corrosion resistance of MAO coatings in fluorine environments and providing good wear resistance. Attached Figure Description

[0020] Figure 1 This is a comparison graph of voltage changes over time in Embodiment 1 of the present invention;

[0021] Figure 2 This is a comparison chart of average pore size and average porosity in Embodiment 1 of the present invention;

[0022] Figure 3 This is a comparison diagram of electrochemical corrosion in Example 1 of the present invention;

[0023] Figure 4 This is a comparison chart of average wear rates in Embodiment 1 of the present invention;

[0024] Figure 5 The present invention is based on Example 1, with Y modified. 3+ Comparison of corrosion current and corrosion voltage with different concentrations added.

[0025] The following detailed description, in conjunction with the accompanying drawings, will further illustrate the present invention. Detailed Implementation

[0026] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Several embodiments of the invention are illustrated in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

[0027] An embodiment of the present invention provides a method for preparing a fluorine-resistant micro-arc oxidation coating on an aluminum alloy surface, comprising the following steps:

[0028] Step S10: After the polished and dried aluminum alloy substrate is connected to the positive electrode of the primary micro-arc oxidation power supply, it is placed in the primary electrolyte for the first micro-arc oxidation to form a passivation film on the surface of the aluminum alloy substrate. The primary electrolyte includes sodium hexametaphosphate, sodium silicate and deionized water.

[0029] It should be noted that the purpose of performing micro-arc oxidation in this step is to avoid the formation of Y atoms in subsequent steps. 3+ Excessive conductivity affects the arc initiation and discharge of micro-arc oxidation.

[0030] Specifically, in this step, sodium hexametaphosphate and sodium silicate are dissolved in deionized water and stirred thoroughly to obtain a micro-arc oxidation pretreatment electrolyte. The concentration of sodium hexametaphosphate is 20–40 g / L, and the concentration of sodium silicate is 2–8 g / L. The first micro-arc oxidation is performed in constant current mode with a frequency of 400–800 Hz, a positive and negative duty cycle of 10–30%, and a current density of 0.1–2 A / dm³. 2 The first micro-arc oxidation takes 1-4 minutes, and the passivation film thickness is 0.1-2 μm.

[0031] Further, in step S10, the polishing method includes:

[0032] In a water-cooled environment, the surface of the aluminum alloy substrate was polished by sequentially using SiC sandpaper with grits of 800, 1200, 2000, and 3000.

[0033] The surface of the aluminum alloy substrate is polished a second time using a cloth coated with polishing paste until it achieves a mirror finish. Avoid dry friction on the aluminum alloy surface in a water environment, and then dry and store it at room temperature.

[0034] Step S11: After rinsing the aluminum alloy substrate containing the passivation film with deionized water, dry it at room temperature.

[0035] Specifically, in this step, the rinsing time is 1-5 minutes.

[0036] Step S12: After drying, the aluminum alloy substrate containing the passivation film is connected to the positive electrode of the secondary micro-arc oxidation power supply and then placed in the secondary electrolyte for a second micro-arc oxidation to form a fluorine-resistant micro-arc oxidation coating on the surface of the passivation film. The secondary electrolyte includes sodium hexametaphosphate, sodium silicate, yttrium nitrate and deionized water.

[0037] Specifically, in this step, the concentration of sodium hexametaphosphate is 25-40 g / L, the concentration of sodium silicate is 2-10 g / L, and the concentration of yttrium nitrate is 1-5 mol% / L. The second micro-arc oxidation uses a constant current mode with a frequency of 400-800 Hz, a positive and negative duty cycle of 10-30%, and a current density of 0.1-2 A / dm³. 2 The second micro-arc oxidation time is 6-10 min, and the thickness of the fluorine-resistant micro-arc oxidation coating is 5-25 μm.

[0038] This invention tested the electrochemical performance (fluorine corrosion resistance) of aluminum alloy and micro-arc oxidation coating in 0.1 mol / L NaF solution using an electrochemical workstation; and tested the wear performance of aluminum alloy and micro-arc oxidation coating using a rotating ball-and-disc friction tester. The friction pair consisted of alumina balls (6 mm in diameter), the wear test lasted 30 minutes, the load was 2 N, and the average rotation speed was 200 rpm. The experiments showed that after Y...3+ The corrosion current density of the doped micro-arc oxidation coating is significantly reduced, and the wear rate is also reduced.

[0039] It should be noted that this invention employs a two-step micro-arc oxidation method. During the micro-arc oxidation process, the temperatures of the primary and secondary electrolytes are maintained between 20-50°C. Specifically, the first step of micro-arc oxidation forms a passivation film on the aluminum alloy surface, which prevents high-conductivity Y doping. 3+ The inability to ignite an arc in the electrolyte leads to the failure of micro-arc oxidation; the second step of micro-arc oxidation involves doping the electrolyte with Y. 3+ This process influences the efficiency of plasma discharge in the coating, reduces the formation of oxygen vacancies, and thus creates more stable electron and ion transport discharge channels, improving the surface density of the MAO coating and reducing defects and cracks. Simultaneously, yttrium doping alters the formation of the Al4Y2O9 structure in MAO alumina. Compared to the alumina structure, the YF bonds in the corrosion products are stronger than the Al-F bonds, and the Al4Y2O9 / Na ratio is also improved. 1.5 Y 2.5 F9 exhibits higher mismatch and interface stability, which can slow down the embrittlement of ionic crystals and inhibit crack propagation, thereby improving the corrosion resistance of MAO coatings in fluorine environments and providing good wear resistance.

[0040] The following is a specific example: Example 1

[0041] In an aqueous environment, aluminum alloys were polished to a mirror finish using SiC sandpaper of 800, 1200, 2000, and 3000 grits, and a cloth coated with polishing paste, respectively. The polished alloys were then dried and stored at room temperature.

[0042] Dissolve 35g sodium hexametaphosphate and 4g sodium silicate in 1L of deionized water and stir thoroughly to obtain micro-arc oxidation pretreatment electrolyte.

[0043] The dried aluminum alloy was connected to the positive electrode of the micro-arc oxidation power supply and placed in the micro-arc oxidation pretreatment electrolyte. A constant current mode was used with a frequency of 600 Hz, a positive and negative duty cycle of 20%, and a current density of 1 A / dm³. 2 The time was 210s, and a passivation film was obtained on the aluminum alloy surface;

[0044] Then, 35g of sodium hexametaphosphate, 4g of sodium silicate, and 2.5mol% yttrium nitrate were dissolved in 1L of deionized water and stirred to obtain the micro-arc oxidation formal electrolyte.

[0045] The aluminum alloy was rinsed with deionized water for 3 minutes to obtain a passivation film, and then dried at room temperature.

[0046] The dried aluminum alloy with a passivation film was connected to the positive electrode of the micro-arc oxidation power supply and placed in the formal electrolyte. A constant current mode was used with a frequency of 600 Hz, a positive and negative duty cycle of 20%, and a current density of 1 A / dm³. 2 The micro-arc oxidation process lasted 450 seconds, after which a fluorine-resistant micro-arc oxidation coating was obtained on the surface. The electrolyte temperature was maintained at 25°C throughout the micro-arc oxidation process.

[0047] The formal electrolyte obtained in this embodiment is doped with Y 3+ Micro-arc oxidation coating, undoped Y 3+ The comparison of voltage variation over time, average pore size, and average porosity of the micro-arc oxidation coating is shown in the figure. Figure 1 , Figure 2 The formal electrolyte obtained in this embodiment is doped with Y. 3+ Micro-arc oxidation coating, undoped Y 3+ The comparison charts of micro-arc oxidation coating, electrochemical testing of aluminum alloy in a fluorine environment, and average wear rate are shown below. Figure 3 , Figure 4 Y-doped 3+ The final voltage of the post-micro-arc oxidation coating decreases ( Figure 1 Y-doped 3 + The micro-arc oxidation coating has a lower average pore size and average porosity, resulting in higher coating density. Figure 2 Y-doped 3+ The micro-arc oxidation coating exhibited the lowest corrosion current density in 0.1 mol / L NaF ( Figure 3 It has the highest resistance to fluorine corrosion. Figure 4 Showing doped Y 3+ The micro-arc oxidation coating has the lowest average wear rate and better wear resistance. Figure 5 This indicates that as Y 3 + As the concentration increases, the corrosion current density of the MAO coating decreases, and its resistance to fluorine corrosion improves. However, excessively high concentrations can lead to insufficient plasma discharge, preventing molten oxides from filling the pores in time, increasing the defect pore size, and reducing the resistance to fluorine corrosion. Example 2

[0048] In an aqueous environment, the aluminum alloy was polished to a mirror finish using SiC sandpaper of 800, 1200, 2000, and 3000 grit, and a cloth coated with polishing paste, respectively. Then, it was dried and stored at room temperature.

[0049] Dissolve 20g sodium hexametaphosphate and 6g sodium silicate in 1L of deionized water and stir thoroughly to obtain micro-arc oxidation pretreatment electrolyte.

[0050] The dried aluminum alloy was connected to the positive electrode of the micro-arc oxidation power supply and placed in the micro-arc oxidation pretreatment electrolyte. A constant current mode was used with a frequency of 400 Hz, a positive and negative duty cycle of 20%, and a current density of 1.5 A / dm³. 2 The time was 210s, and a passivation film was obtained on the aluminum alloy surface;

[0051] Then, 25g sodium hexametaphosphate, 6g sodium silicate, and 2mol% yttrium nitrate were dissolved in 1L of deionized water and stirred to obtain the micro-arc oxidation formal electrolyte.

[0052] The aluminum alloy was rinsed with deionized water for 3 minutes to obtain a passivation film, and then dried at room temperature.

[0053] The dried aluminum alloy with a passivation film was connected to the positive electrode of the micro-arc oxidation power supply and placed in the formal electrolyte. A constant current mode was used with a frequency of 400 Hz, a positive and negative duty cycle of 20%, and a current density of 1.5 A / dm³. 2 The micro-arc oxidation process lasted 450 seconds, after which a fluorine-resistant micro-arc oxidation coating was obtained on the surface. The electrolyte temperature was maintained at 25°C throughout the micro-arc oxidation process. Example 3

[0054] In an aqueous environment, the aluminum alloy was polished to a mirror finish using SiC sandpaper of 800, 1200, 2000, and 3000 grit, and a cloth coated with polishing paste, respectively. Then, it was dried and stored at room temperature.

[0055] Dissolve 30g sodium hexametaphosphate and 2g sodium silicate in 1L of deionized water and stir thoroughly to obtain micro-arc oxidation pretreatment electrolyte.

[0056] The dried aluminum alloy was connected to the positive electrode of the micro-arc oxidation power supply and placed in the micro-arc oxidation pretreatment electrolyte. A constant current mode was used with a frequency of 600 Hz, a positive and negative duty cycle of 20%, and a current density of 0.5 A / dm³. 2 The time was 180s, and a passivation film was obtained on the aluminum alloy surface;

[0057] Then, 30g of sodium hexametaphosphate, 2g of sodium silicate, and 1.5mol% yttrium nitrate were dissolved in 1L of deionized water and stirred to obtain the micro-arc oxidation formal electrolyte.

[0058] The aluminum alloy was rinsed with deionized water for 3 minutes to obtain a passivation film, and then dried at room temperature.

[0059] The dried aluminum alloy with a passivation film was connected to the positive electrode of the micro-arc oxidation power supply and placed in the formal electrolyte. A constant current mode was used with a frequency of 600 Hz, a positive and negative duty cycle of 20%, and a current density of 0.5 A / dm³. 2The micro-arc oxidation process lasted 480 seconds, after which a fluorine-resistant micro-arc oxidation coating was obtained on the surface. The electrolyte temperature was maintained at 25°C throughout the micro-arc oxidation process. Example 4

[0060] In an aqueous environment, the aluminum alloy was polished to a mirror finish using SiC sandpaper of 800, 1200, 2000, and 3000 grit, and a cloth coated with polishing paste, respectively. Then, it was dried and stored at room temperature.

[0061] Dissolve 40g sodium hexametaphosphate and 8g sodium silicate in 1L of deionized water and stir thoroughly to obtain micro-arc oxidation pretreatment electrolyte.

[0062] The dried aluminum alloy was connected to the positive electrode of the micro-arc oxidation power supply and placed in the micro-arc oxidation pretreatment electrolyte. A constant current mode was used with a frequency of 400Hz, a positive and negative duty cycle of 20%, and a current density of 2A / dm³. 2 The time was 240s, and a passivation film was obtained on the aluminum alloy surface;

[0063] Then, 40g sodium hexametaphosphate, 8g sodium silicate, and 4mol% yttrium nitrate were dissolved in 1L of deionized water and stirred to obtain the micro-arc oxidation formal electrolyte.

[0064] The aluminum alloy was rinsed with deionized water for 3 minutes to obtain a passivation film, and then dried at room temperature.

[0065] The dried aluminum alloy with a passivation film was connected to the positive electrode of the micro-arc oxidation power supply and placed in the formal electrolyte. A constant current mode was used with a frequency of 400Hz, a positive and negative duty cycle of 20%, and a current density of 2A / dm³. 2 The micro-arc oxidation process lasted 420 seconds, after which a fluorine-resistant micro-arc oxidation coating was obtained on the surface. The electrolyte temperature was maintained at 25°C throughout the micro-arc oxidation process.

[0066] Table 1 shows the comparison of various parameters in the micro-arc oxidation process in Examples 1-4.

[0067]

[0068] From the appendix Figures 1-5 Based on the textual descriptions in Examples 1-4 and Table 1, it can be concluded that: First, changes in the electrolyte concentration in the electrolyte significantly alter the plasma discharge efficiency, which in turn affects the growth of the film. When the electrolyte concentration is too high, micro-arc oxidation becomes difficult to initiate, leading to insufficient plasma discharge and preventing the molten oxide from filling the pores in time, thus increasing the defect pore size. When the electrolyte concentration is too low, the oxide film growth is uneven, and the film quality deteriorates. In summary, both excessively high and low electrolyte concentrations adversely affect the fluorine corrosion resistance of the oxide film.

[0069] Secondly, power supply parameters and micro-arc oxidation treatment time significantly affect the film structure. Higher current density usually accelerates the coating growth rate, but may also lead to localized overheating, affecting the uniformity and quality of the coating. Frequency, on the other hand, affects the frequency and intensity of discharge; an appropriate frequency helps control the duration and intensity of discharge, improves the density of the coating, and reduces the formation of pores.

[0070] Third, the pretreatment time determines the thickness of the passivation film, thus affecting the second-step micro-arc oxidation process. If the pretreatment passivation time is too short, the passivation film will be too thin, preventing the second-step micro-arc oxidation process from discharging and causing the micro-arc oxidation process to fail. If the passivation time is too long, the passivation film resistance will be too high, resulting in a more intense second-step micro-arc oxidation process, more defects in the coating, and poor resistance to fluorine corrosion.

[0071] In summary, the parameters described in Example 1 can achieve optimal fluorine corrosion resistance by comprehensively considering plasma discharge growth and coating structure.

[0072] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

[0073] In this specification, the various embodiments are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. Furthermore, the above-described embodiments merely illustrate several implementation methods of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A method for preparing a fluorine-resistant micro-arc oxidation coating on an aluminum alloy surface, characterized in that, Includes the following steps: Step S10: After the polished and dried aluminum alloy substrate is connected to the positive electrode of the primary micro-arc oxidation power supply, it is placed in the primary electrolyte for the first micro-arc oxidation to form a passivation film on the surface of the aluminum alloy substrate. The primary electrolyte is sodium hexametaphosphate, sodium silicate and deionized water. The concentration of sodium hexametaphosphate is 20-40 g / L, the concentration of sodium silicate is 2-8 g / L, the first micro-arc oxidation time is 1-4 min, and the thickness of the passivation film is 0.1-2 μm. Step S11: After rinsing the aluminum alloy substrate containing the passivation film with deionized water, dry it at room temperature. Step S12: After drying, the aluminum alloy substrate containing the passivation film is connected to the positive electrode of the secondary micro-arc oxidation power supply and then placed in the secondary electrolyte for a second micro-arc oxidation to form a fluorine corrosion resistant micro-arc oxidation coating on the surface of the passivation film. The secondary electrolyte includes sodium hexametaphosphate, sodium silicate, yttrium nitrate, and deionized water. The concentration of sodium hexametaphosphate is 25-40 g / L, the concentration of sodium silicate is 2-10 g / L, the concentration of yttrium nitrate is 1-5 mol% / L, the second micro-arc oxidation time is 6-10 min, and the thickness of the fluorine corrosion resistant micro-arc oxidation coating is 5-25 μm.

2. The method for preparing a fluorine-resistant micro-arc oxidation coating on an aluminum alloy surface according to claim 1, characterized in that, In step S10, the first micro-arc oxidation adopts a constant current mode with a frequency of 400-800Hz, a positive and negative duty cycle of 10-30%, and a current density of 0.1-2A / dm³. 2 .

3. The method for preparing a fluorine-resistant micro-arc oxidation coating on an aluminum alloy surface according to claim 1, characterized in that, In step S11, the rinsing time is 1-5 minutes.

4. The method for preparing a fluorine-resistant micro-arc oxidation coating on an aluminum alloy surface according to claim 1, characterized in that, In step S12, the second micro-arc oxidation is performed in constant current mode with a frequency of 400-800Hz, a positive and negative duty cycle of 10-30%, and a current density of 0.1-2A / dm³. 2 .

5. The method for preparing a fluorine-resistant micro-arc oxidation coating on an aluminum alloy surface according to any one of claims 1 to 4, characterized in that, During the micro-arc oxidation process, the temperatures of the primary electrolyte and the secondary electrolyte are maintained at 20-50℃.

6. The method for preparing a fluorine-resistant micro-arc oxidation coating on an aluminum alloy surface according to any one of claims 1 to 4, characterized in that, In step S10, the polishing method includes: In a water-cooled environment, the surface of the aluminum alloy substrate was polished by sequentially using SiC sandpaper with grits of 800, 1200, 2000, and 3000. The surface of the aluminum alloy substrate is polished a second time using a cloth coated with polishing paste.

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