A method for preparing a long-wear magnesium alloy micro-arc oxidation coating

Abstract

The present application belongs to the technical field of metal surface modification, and particularly relates to a preparation method of a long-wear-life magnesium alloy micro-arc oxidation coating. In the magnesium alloy micro-arc oxidation, a simple micro-arc oxidation electrolyte system composed of appropriate amounts of potassium fluozirconate and potassium fluoride is adopted, and the concentration ratio range thereof is optimized, so as to replace the traditional silicate, phosphate or aluminate system. By controlling the concentration of K2ZrF6 and KF in a specific range, the poor adhesion and peeling of the coating formed due to high concentration of K2ZrF6 are avoided, and the K2ZrF6 is not stable under alkaline conditions to form a colloid or a precipitate due to too high concentration of KF, so that the electrolyte system is stable and a relatively uniform coating is formed. The electrolyte system and the process of the present application are simple and low in cost, and are convenient for industrial application.

Description

Technical Field

[0001] This invention belongs to the field of metal surface modification technology, specifically relating to a method for preparing a long-wear-resistant magnesium alloy micro-arc oxidation coating. Background Technology

[0002] Magnesium alloys, as the lightest engineering metal structural materials, possess low density, high specific strength, and excellent damping properties, making them ideal materials for lightweighting. Currently, with the increasing demand for lightweighting, energy conservation, and emission reduction, magnesium alloys show great promise as a replacement for traditional steel and aluminum alloys in structural components. However, the high chemical reactivity (standard electrode potential of -2.37 V) and low hardness (approximately 50-100 HV) of magnesium alloys result in poor corrosion resistance and wear resistance, significantly limiting their application in harsh environments. Therefore, modification treatment is necessary before practical application.

[0003] Micro-arc oxidation (MAO) is a complex instantaneous high-temperature and high-pressure valve metal oxidation method developed from anodic oxidation. In a high-voltage and specific electrolyte system, it utilizes plasma generated by instantaneous high-voltage discharge to induce complex electrochemical, thermochemical, and plasma-chemical reactions on the valve metal surface. Through a cyclic discharge mechanism, a hard, dense ceramic coating with extremely strong adhesion to the substrate is grown in situ on the valve metal surface, significantly improving the corrosion and wear resistance of the metal surface. With its advantages of high efficiency, adjustable process, thick coating, strong adhesion, and high hardness, MAO is considered the most promising technology for improving the wear and corrosion resistance of magnesium and magnesium alloys.

[0004] However, while existing micro-arc oxidation technology improves the wear resistance of magnesium alloys to some extent, the intense discharge effect during micro-arc oxidation leads to porous coatings and microcracks. These characteristics of conventional micro-arc oxidation coatings, along with the inherently low hardness of the matrix oxide MgO, severely affect the wear life of the coating. Therefore, it is necessary to develop a method for preparing magnesium alloy coatings with a long wear life. Currently, conventional electrolyte systems mainly consist of silicates, phosphates, and aluminates. The electrolyte system contains 5-40 g / L Na₂SiO₃, 1-20 g / L NaOH, and 1-20 g / L KF. The electrical parameters are: positive voltage 200-600 V, negative voltage 1-50 V, power frequency 200-1000 Hz, duty cycle 4-40%, and micro-arc oxidation time 3-40 min. After hydrothermal treatment, the wear rate of the micro-arc oxidation coating prepared by this method after 20 min of friction is approximately 10%. -4 mm 3 ·N -1 ·m -1The order of magnitude. The hydrothermal reaction parameters are: rare earth salt content 1~50 g / L, organic matter content 1~30 g / L. The hydrothermal reaction temperature is 25~100℃, and the reaction time is 1~60 min. The prior art CN116607192A describes a method for preparing a corrosion-resistant and wear-resistant coating for ultra-low porosity magnesium alloy micro-arc oxidation, which includes the following steps: (1) cutting the magnesium alloy to obtain a sample; (2) grinding the sample, rinsing and drying it to obtain a magnesium alloy sample; (3) preparing an electrolyte solution: weighing the main film-forming agent, auxiliary film-forming agent, and additives; adding the main film-forming agent to deionized water, stirring it evenly with magnetic force, adding the auxiliary film-forming agent and stirring it with magnetic force, then adding the additives and stirring it with magnetic force until the solution is clear and transparent; adjusting the pH value of the above solution to obtain an electrolyte solution; (4) using the magnesium alloy sample in step (2) as the anode and the graphite plate or stainless steel plate as the cathode, performing micro-arc oxidation treatment in the electrolyte solution obtained in step (3) to obtain a micro-arc oxidation sample; (5) rinsing the micro-arc oxidation sample in step (4) and drying it with cold air to obtain a corrosion-resistant and wear-resistant coating. The main film-forming agent in step (3) includes one or two of potassium dihydrogen phosphate and sodium dihydrogen phosphate; the auxiliary film-forming agent includes one or two of sodium fluoride and potassium fluoride; and the additive includes one or more of potassium fluorotitanate, potassium fluorozirconate, and potassium fluorosilicate. The ratio of the main film-forming agent to deionized water in step (3) is (1-40) g / L; the ratio of the auxiliary film-forming agent to deionized water is (1-30) g / L; and the ratio of the additive to deionized water is (1-20) g / L. The wear rate of the resulting product is 1.89~3.31×10⁻⁶. -6 cm 3 / (N·m). Summary of the Invention

[0005] To further reduce wear rate, this invention provides a method for preparing a long-wear-resistant magnesium alloy micro-arc oxidation coating. This invention addresses technical problems in existing magnesium alloy micro-arc oxidation coatings, such as unstable electrolyte systems, insufficient wear resistance, and poor toughness, through optimization of electrical parameters and electrolyte composition. The result is a magnesium alloy micro-arc oxidation coating with long wear resistance and toughening effect.

[0006] This invention optimizes the concentration of the fluorozirconate and potassium fluoride composite electrolyte system and incorporates electrolyte substances into the coating during the micro-arc oxidation process. This improves the microstructure and performance of the micro-arc oxidation coating, resulting in a coating with a longer wear life suitable for its application.

[0007] This invention requires no additional reagents and utilizes an extremely simple electrolyte system to obtain a product with excellent wear resistance and toughness.

[0008] In this invention, a simple electrolyte system is used to facilitate precise control during the micro-arc oxidation process and to provide good product performance stability.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A method for preparing a long-wear-resistant magnesium alloy micro-arc oxidation coating includes the following steps:

[0011] Step 1: Surface pretreatment of magnesium alloy substrate: The magnesium alloy substrate was successively ground with SiC sandpaper from coarse to fine, then residual grinding debris was removed from the substrate surface by ultrasonic treatment with anhydrous ethanol, followed by drying with a blower to obtain the pretreated magnesium alloy. The roughness Sa of the pretreated sample is ≤0.2µm.

[0012] Step 2: Preparation of electrolyte: Dissolve K2ZrF6 and KF separately in deionized water and stir. After the two solutions are fully dissolved, mix them and make up to a fixed volume to obtain the electrolyte.

[0013] Step 3: Micro-arc oxidation treatment: After micro-arc oxidation treatment in a prepared electrolyte, the pretreated magnesium alloy is rinsed and dried with deionized water and anhydrous ethanol to obtain a micro-arc oxidation coating with a long wear resistance life.

[0014] Furthermore, the magnesium alloy substrate selected in step 1 is AZ91D, the sandpaper used for grinding is 600 mesh, 800 mesh, and 1200 mesh, and the ultrasonic time is 8 minutes.

[0015] Further, in step 2, the concentration of K2ZrF6 is 0.03M, and the concentration of KF is 0.02M~0.15M, preferably 0.02M~0.05M or 0.14~0.15M. In this invention, K2ZrF6 serves as the source of Zr element in the coating. Zr element participates in the reaction to generate ZrO2. The concentration of K2ZrF6 determines the performance of the coating. If the concentration of K2ZrF6 is too high, hydrolysis will occur, which will reduce the stability of the solution and the quality of the coating. If the concentration is too low, the content of Zr element is too small and the coating performance is insufficient. KF can improve the conductivity of the solution and activate the magnesium alloy surface. The F⁻ in KF can dissociate with the ZrF6 released from K2ZrF6. 2- To form a dynamic equilibrium, stabilize the solution and prevent premature hydrolysis, and may participate in or catalyze the film-forming reaction, but too high a concentration will reduce the solubility of K2ZrF6. Therefore, controlling the concentration of K2ZrF6 and KF within a reasonable range (about 1 / 20) can improve the performance and film-forming efficiency of the coating while avoiding the impact of too high or too low concentrations on the stability of the electrolyte and the quality of the coating.

[0016] Further, the specific steps of step three are as follows: pour the prepared electrolyte into the electrolytic cell, use stainless steel as the cathode, and place the treated magnesium alloy substrate as the anode in the electrolyte. The process parameters of the micro-arc oxidation treatment are as follows: constant voltage mode is adopted, the power supply voltage is 450V~480V, preferably 450V, the duty cycle is 20~40%, preferably 20%, the pulse frequency is 500Hz~800Hz, preferably 500Hz, the micro-arc oxidation treatment time is 30 min~60 min, the electrolyte is continuously stirred by magnetic stirring during the treatment, and the electrolyte temperature is controlled at 20℃~25℃ by the circulating water system.

[0017] The coating obtained by this invention has a wear resistance life exceeding 500 min; the wear rate is less than or equal to 4 × 10⁻⁶. -5 mm 3 ·N -1 ·m -1 .

[0018] The optimized coating has a wear resistance life exceeding 1600 min; the wear rate is less than or equal to 9.4 × 10⁻⁶ min. -6 mm 3 ·N -1 ·m -1 After further optimization, the resulting coating has a wear resistance life exceeding 2160 min; the wear rate is less than or equal to 4.0 × 10⁻⁶. -6 mm 3 ·N -1 ·m -1 .

[0019] Beneficial effects

[0020] This invention provides a method for preparing a long-wear-resistant magnesium alloy micro-arc oxidation coating, which has the following significant advantages compared with the prior art:

[0021] (1) The method for preparing a magnesium alloy micro-arc oxidation coating with long wear resistance provided by the present invention can accelerate the dissolution rate by dissolving K2ZrF6 and KF respectively. The Zr element introduced by an appropriate amount of K2ZrF6 replaces the matrix oxide to form a high-hardness wear-resistant phase ZrO2 during the micro-arc oxidation process, which greatly improves the wear resistance of the coating. An appropriate amount of KF can increase the conductivity of the electrolyte, affect the spark discharge, and reduce the pore size and surface roughness. The formation of MgF2 and the introduction of the reinforcing phase can increase the coating thickness and hardness, thereby enhancing the wear resistance of the coating. Combining the high hardness (Mohs hardness 8.5-9) and wear resistance of ZrO2, as well as its excellent fracture toughness and phase transformation toughening effect, the discharge behavior and growth process during the micro-arc oxidation process are controlled by adjusting the voltage, duty cycle, frequency and other electrical process parameters in the micro-arc oxidation technology. This changes the phase composition, component distribution and microstructure of the coating, and forms a magnesium alloy micro-arc oxidation coating rich in ZrO2 in situ on the magnesium alloy surface, which greatly improves the wear resistance of the coating (more than 2160 min).

[0022] (2) This invention employs a simple micro-arc oxidation electrolyte system composed of potassium fluorozirconate (K2ZrF6) and potassium fluoride (KF) in appropriate amounts and proportions for the micro-arc oxidation of magnesium alloys, and optimizes its concentration ratio range to replace traditional silicate, phosphate, or aluminate systems. By controlling the concentrations of K2ZrF6 and KF within a specific range, high concentrations of K2ZrF6 are avoided, which leads to poor coating adhesion and detachment. Excessive KF concentration causes K2ZrF6 to become unstable under alkaline conditions, forming colloids or precipitates. This ensures the stability of the electrolyte system while forming a relatively uniform coating. The electrolyte system and process of this invention are simple and low in cost. Attached Figure Description Figure 1 These are SEM images of the surface microstructure of the micro-arc oxidation coatings in Example 4 and Comparative Example 1 of the present invention.

[0023] Figure 2 These are SEM images of the cross-sectional microstructure of the micro-arc oxidation coatings in Example 4 and Comparative Example 1 of this invention.

[0024] Figure 3 These are 3D morphology images of the micro-arc oxidation coatings of Example 4 and Comparative Example 1 of the present invention.

[0025] Figure 4 These are XRD morphology images of the micro-arc oxidation coatings of Example 4 and Comparative Example 1 of the present invention;

[0026] Figure 5 These are friction curves of the products obtained in Examples 1-5 and Comparative Examples 1, 2, 5, and 6 of this invention;

[0027] Figure 6 These are SEM images of the wear traces of the micro-arc oxidation coatings in Example 4 and Comparative Example 1 of this invention.

[0028] Figure 7 These are 3D morphology images of the wear marks of the micro-arc oxidation coatings in Example 4 and Comparative Example 1 of the present invention.

[0029] Figure 1 Composed of (a) and (b), where (a) is a SEM image of the surface microstructure of the micro-arc oxidation coating obtained in Example 4, and (b) is a SEM image of the surface microstructure of the micro-arc oxidation coating obtained in Comparative Example 1; from Figure 1 As can be seen from the comparison, the surface of the coating of the present invention has a pancake-like structure and the pore size of the coating is relatively small.

[0030] Figure 2 Composed of (a) and (b), where (a) is a cross-sectional microstructure SEM image of the micro-arc oxidation coating obtained in Example 4, and (b) is a cross-sectional microstructure SEM image of the micro-arc oxidation coating obtained in Comparative Example 1; from Figure 2 As can be seen from the comparison, the coating is uneven, with many cracks and pores, while the coating of the present invention is more uniform overall, and there are almost no cracks in the coating.

[0031] Figure 3 Composed of (a) and (b), where (a) is a 3D morphology image of the micro-arc oxidation coating obtained in Example 4, and (b) is a 3D morphology image of the micro-arc oxidation coating obtained in Comparative Example 1; from Figure 3 As can be seen, the roughness Sa=4.92µm of Comparative Example 1 is lower than that of the present invention, which has a roughness Sa=2.7µm and a more uniform coating surface.

[0032] from Figure 4 It can be seen that the coating phase of Comparative Example 1 is mainly composed of Mg, MgO and Mg2SiO4 phases, while the coating phase of this study is mainly composed of Mg, MgO, MgF2 and ZrO2. Compared with Comparative Example 1, the coating of this invention contains the hard phase ZrO2.

[0033] Figure 5 Composed of (a) and (b), where (a) is the friction curve of the products obtained in Examples 1-5; and (b) is the friction curve of the products obtained in Comparative Examples 1, 2, 5, and 6; from Figure 5 It can be seen that the coatings of the comparative examples mostly fail in about 5 minutes, with the coating of Comparative Example 1 failing in 20 minutes. The wear resistance life of the present invention is the best, exceeding 2160 minutes, which is much higher than that of Comparative Example 1, and the life is more than 100 times longer.

[0034] Figure 6 Composed of (a) and (b), where (a) is a SEM image of the wear track microstructure of the product obtained in Example 4; and (b) is a SEM image of the wear track microstructure of the product obtained in Comparative Example 1; from Figure 6It can be seen that the wear marks exhibit similar morphological structures. After friction, the present invention exhibits a more complete structure and is less affected by wear.

[0035] Figure 7 Composed of (a) and (b), where (a) is a 3D morphology image of the wear marks on the product obtained in Example 4; and (b) is a 3D morphology image of the wear marks on the product obtained in Comparative Example 1; from Figure 7 It can be seen that the wear depth of the present invention is shallower than that of Comparative Example 1. Detailed Implementation

[0036] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0037] In this invention, the micro-arc oxidation power supply used is a WHD-20 type micro-arc oxidation power supply, and the substrate used is AZ91D magnesium alloy.

[0038] The wear resistance test of the product is as follows: The wear resistance of the coating is evaluated by friction and wear test. The wear life of the coating is tested by using a high temperature friction and wear tester under dry friction conditions, with the wear material being a Si3N4 ball with a diameter of 4mm. The parameters are: load of 8N, rotation speed of 200 r / min, and friction radius of 2mm.

[0039] Example 1

[0040] Pretreatment of the substrate: The AZ91D magnesium alloy sheet was cut into cylinders with a diameter of 20mm and a thickness of 2mm by laser cutting. The cut AZ91D magnesium alloy substrate was then ground sequentially with SiC sandpaper of 600 grit, 800 grit and 1200 grit from coarse to fine. After grinding until there were no obvious scratches on the surface, the residual grinding debris on the substrate surface was removed by ultrasonic treatment with anhydrous ethanol and then dried by a blower to obtain the pretreated magnesium alloy.

[0041] Electrolyte preparation: 17.02 g of K₂ZrF₆ and 2.32 g of KF were weighed separately using a balance. The weighed samples were dissolved separately in 1500 ml and 500 ml of deionized water, respectively, and stirred until fully dissolved. The two solutions were then mixed and brought to a final volume to obtain the electrolyte. The concentration of K₂ZrF₆ in the resulting mixed electrolyte was 0.03 M, and the concentration of KF was 0.02 M. The pH of the electrolyte was measured to be 6.50 using a pH meter, and the conductivity was measured to be 7.99 mS / cm using a conductivity meter.

[0042] Micro-arc oxidation treatment: The prepared electrolyte is poured into an electrolytic cell. Stainless steel is used as the cathode, and the treated magnesium alloy substrate is placed in the electrolyte as the anode. A constant voltage mode is used, with a power supply voltage of 450V, a duty cycle of 20%, a pulse frequency of 500Hz, and a micro-arc oxidation treatment time of 30 minutes. During the treatment, the electrolyte is continuously stirred by magnetic stirring, and the electrolyte temperature is controlled at 20℃~25℃ through a circulating water system. The resulting coating has a wear resistance life exceeding 1500 minutes. The wear rate is 3.99×10⁻⁶. -5 mm 3 ·N -1 ·m -1 .

[0043] Example 2

[0044] Pretreatment of the substrate: The AZ91D magnesium alloy sheet was cut into cylinders with a diameter of 20mm and a thickness of 2mm by laser cutting. The cut AZ91D magnesium alloy substrate was then ground sequentially with SiC sandpaper of 600 grit, 800 grit and 1200 grit from coarse to fine. After grinding until there were no obvious scratches on the surface, the residual grinding debris on the substrate surface was removed by ultrasonic treatment with anhydrous ethanol and then dried by a blower to obtain the pretreated magnesium alloy.

[0045] Electrolyte preparation: 17g of K₂ZrF₆ and 5.81g of KF were weighed separately using a balance. The weighed samples were dissolved in 1500ml and 500ml of deionized water, respectively, and stirred until fully dissolved. The two solutions were then mixed and brought to a final volume to obtain the electrolyte. The concentration of K₂ZrF₆ in the resulting mixed electrolyte was 0.03M, and the concentration of KF was 0.05M. The pH of the electrolyte was measured to be 7.20 using a pH meter, and the conductivity was measured to be 11.05 mS / cm using a conductivity meter.

[0046] Micro-arc oxidation treatment: The prepared electrolyte was poured into an electrolytic cell. Stainless steel was used as the cathode, and the treated magnesium alloy substrate was placed in the electrolyte as the anode. A constant voltage mode was adopted, with a power supply voltage of 450V, a duty cycle of 20%, a pulse frequency of 500Hz, and a micro-arc oxidation treatment time of 60 min. During the treatment, the electrolyte was continuously stirred by magnetic stirring, and the electrolyte temperature was controlled at 20℃~25℃ by a circulating water system. After rinsing and drying with deionized water and anhydrous ethanol, the magnesium alloy was obtained with a micro-arc oxidation coating on the surface that has a long wear resistance life. The wear resistance life of the obtained coating exceeded 1100 min, and the wear rate was 3.12×10⁻⁶. -5 mm 3 ·N -1 ·m -1 .

[0047] Example 3

[0048] Pretreatment of the substrate: The AZ91D magnesium alloy sheet was cut into cylinders with a diameter of 20mm and a thickness of 2mm by laser cutting. The cut AZ91D magnesium alloy substrate was then ground sequentially with SiC sandpaper of 600 grit, 800 grit and 1200 grit from coarse to fine. After grinding until there were no obvious scratches on the surface, the residual grinding debris on the substrate surface was removed by ultrasonic treatment with anhydrous ethanol and then dried by a blower to obtain the pretreated magnesium alloy.

[0049] Electrolyte preparation: 17g of K₂ZrF₆ and 11.62g of KF were weighed separately using a balance. The weighed samples were dissolved in 1500ml and 500ml of deionized water, respectively, and stirred until fully dissolved. The two solutions were then mixed and brought to a final volume to obtain the electrolyte. The concentration of K₂ZrF₆ in the resulting mixed electrolyte was 0.03M, and the concentration of KF was 0.10M. The pH of the electrolyte was measured to be 7.43 using a pH meter, and the conductivity was measured to be 14.61 mS / cm using a conductivity meter.

[0050] Micro-arc oxidation treatment: The prepared electrolyte was poured into an electrolytic cell. Stainless steel was used as the cathode, and the treated magnesium alloy substrate was placed in the electrolyte as the anode. A constant voltage mode was adopted, with a power supply voltage of 450V, a duty cycle of 20%, a pulse frequency of 500Hz, and a micro-arc oxidation treatment time of 60 min. During the treatment, the electrolyte was continuously stirred by magnetic stirring, and the electrolyte temperature was controlled at 20℃~25℃ by a circulating water system. After rinsing and drying with deionized water and anhydrous ethanol, the magnesium alloy was obtained with a micro-arc oxidation coating on the surface that has a long wear resistance life. The wear resistance life of the obtained coating exceeded 500 min, and the wear rate was 2.99×10⁻⁶. -5 mm 3 ·N -1 ·m -1 .

[0051] Example 4

[0052] Pretreatment of the substrate: The AZ91D magnesium alloy sheet was cut into cylinders with a diameter of 20mm and a thickness of 2mm by laser cutting. The cut AZ91D magnesium alloy substrate was then ground sequentially with SiC sandpaper of 600 grit, 800 grit and 1200 grit from coarse to fine. After grinding until there were no obvious scratches on the surface, the residual grinding debris on the substrate surface was removed by ultrasonic treatment with anhydrous ethanol and then dried by a blower to obtain the pretreated magnesium alloy.

[0053] Electrolyte preparation: 17.02 g of K₂ZrF₆ and 17.43 g of KF were weighed separately using a balance. The weighed samples were dissolved in 1500 ml and 500 ml of deionized water, respectively, and stirred until fully dissolved. The two solutions were then mixed and brought to a final volume to obtain the electrolyte. The concentration of K₂ZrF₆ in the resulting mixed electrolyte was 0.03 M, and the concentration of KF was 0.15 M. The pH of the electrolyte was measured to be 7.66 using a pH meter, and the conductivity was measured to be 18.65 mS / cm using a conductivity meter.

[0054] Micro-arc oxidation treatment: The prepared electrolyte was poured into an electrolytic cell. Stainless steel was used as the cathode, and the treated magnesium alloy substrate was placed in the electrolyte as the anode. A constant voltage mode was adopted, with a power supply voltage of 450V, a duty cycle of 20%, a pulse frequency of 500Hz, and a micro-arc oxidation treatment time of 60min. During the treatment, the electrolyte was continuously stirred by magnetic stirring, and the electrolyte temperature was controlled at 20℃~25℃ through a circulating water system. After rinsing and drying with deionized water and anhydrous ethanol, a micro-arc oxidation coating with a long wear resistance was obtained. The wear resistance of the obtained coating exceeded 2160min. The wear rate was 3.96×10⁻⁶. -6 mm 3 ·N -1 ·m -1 .

[0055] Example 5

[0056] Pretreatment of the substrate: The AZ91D magnesium alloy sheet was cut into cylinders with a diameter of 20mm and a thickness of 2mm by laser cutting. The cut AZ91D magnesium alloy substrate was then ground sequentially with SiC sandpaper of 600 grit, 800 grit and 1200 grit from coarse to fine. After grinding until there were no obvious scratches on the surface, the residual grinding debris on the substrate surface was removed by ultrasonic treatment with anhydrous ethanol and then dried by a blower to obtain the pretreated magnesium alloy.

[0057] Electrolyte preparation: 17.02 g of K₂ZrF₆ and 17.43 g of KF were weighed separately using a balance. The weighed samples were dissolved in 1500 ml and 500 ml of deionized water, respectively, and stirred until fully dissolved. The two solutions were then mixed and brought to a final volume to obtain the electrolyte. The concentration of K₂ZrF₆ in the resulting mixed electrolyte was 0.03 M, and the concentration of KF was 0.15 M. The pH of the electrolyte was measured to be 7.66 using a pH meter, and the conductivity was measured to be 18.65 mS / cm using a conductivity meter.

[0058] Micro-arc oxidation treatment: The prepared electrolyte was poured into an electrolytic cell. Stainless steel was used as the cathode, and the treated magnesium alloy substrate was placed in the electrolyte as the anode. A constant voltage mode was adopted, with a power supply voltage of 480V, a duty cycle of 20%, a pulse frequency of 500Hz, and a micro-arc oxidation treatment time of 60min. During the treatment, the electrolyte was continuously stirred by magnetic stirring, and the electrolyte temperature was controlled at 20℃~25℃ through a circulating water system. After rinsing and drying with deionized water and anhydrous ethanol, a micro-arc oxidation coating with a long wear resistance was obtained. The wear resistance of the obtained coating exceeded 1600min. The wear rate was 9.31×10⁻⁶. -6 mm 3 ·N -1 ·m -1 .

[0059] Comparative Example 1

[0060] To verify the improvement effect of the present invention on the micro-arc oxidation coating of existing micro-arc oxidation technology on the micro-arc oxidation structure and performance, the comparative example is a traditional silicate micro-arc oxidation coating.

[0061] Pretreatment of the substrate: The AZ91D magnesium alloy sheet was cut into cylinders with a diameter of 20mm and a thickness of 2mm by laser cutting. The cut AZ91D magnesium alloy substrate was then ground sequentially with SiC sandpaper of 600 grit, 800 grit and 1200 grit from coarse to fine. After grinding until there were no obvious scratches on the surface, the residual grinding debris on the substrate surface was removed by ultrasonic treatment with anhydrous ethanol and then dried by a blower to obtain the pretreated magnesium alloy.

[0062] Electrolyte preparation: 69.83 g of Na₂SiO₃·9H₂O, 10.00 g of NaF, and 4 g of NaOH were weighed using a balance. The weighed samples were then stirred in deionized water until fully dissolved, and the solution was brought to a final volume to obtain the electrolyte. The resulting mixed electrolyte had a Na₂SiO₃·9H₂O concentration of 0.12 M, a NaF concentration of 0.24 M, and a NaOH concentration of 0.05 M. The pH of the electrolyte was measured to be 13.00 using a pH meter, and the conductivity was measured to be 32.7 mS / cm using a conductivity meter.

[0063] Micro-arc oxidation treatment: The prepared electrolyte was poured into an electrolytic cell. Stainless steel was used as the cathode, and the treated magnesium alloy substrate was placed in the electrolyte as the anode. A constant voltage mode was adopted, with a power supply voltage of 400V, a duty cycle of 20%, a pulse frequency of 500Hz, and a micro-arc oxidation treatment time of 60min. During the treatment, the electrolyte was continuously stirred by magnetic stirring, and the electrolyte temperature was controlled at 20℃~25℃ by a circulating water system. After rinsing and drying with deionized water and anhydrous ethanol, a micro-arc oxidation coating with a long wear resistance was obtained on the surface. The wear resistance of the obtained coating was 20min. The wear rate was 6.91×10⁻⁶. -4 mm 3 ·N -1 ·m -1 .

[0064] Comparative Example 2

[0065] Pretreatment of the substrate: The AZ91D magnesium alloy sheet was cut into cylinders with a diameter of 20mm and a thickness of 2mm by laser cutting. The cut AZ91D magnesium alloy substrate was then ground sequentially with SiC sandpaper of 600 grit, 800 grit and 1200 grit from coarse to fine. After grinding until there were no obvious scratches on the surface, the residual grinding debris on the substrate surface was removed by ultrasonic treatment with anhydrous ethanol and then dried by a blower to obtain the pretreated magnesium alloy.

[0066] Electrolyte preparation: Weigh 17 g of K₂ZrF₆ separately using a balance. Dissolve the weighed sample in 2000 ml of deionized water and stir. After both solutions are fully dissolved, mix them and bring the volume to a final volume to obtain the electrolyte. The concentration of K₂ZrF₆ in the resulting mixed electrolyte is 0.03 M. The pH of the electrolyte was measured to be 6.43 using a pH meter, and the conductivity was measured to be 7.52 mS / cm using a conductivity meter.

[0067] Micro-arc oxidation treatment: The prepared electrolyte was poured into an electrolytic cell. Stainless steel was used as the cathode, and the treated magnesium alloy substrate was placed in the electrolyte as the anode. A constant voltage mode was adopted, with a power supply voltage of 450V, a duty cycle of 20%, a pulse frequency of 500Hz, and a micro-arc oxidation treatment time of 60min. During the treatment, the electrolyte was continuously stirred by magnetic stirring, and the electrolyte temperature was controlled at 20℃~25℃ through a circulating water system. The wear resistance life of the resulting coating was less than 5min. During the micro-arc oxidation process, the solution became turbid, indicating that the electrolyte system was unstable and the performance of the prepared coating was unstable.

[0068] Comparative Example 3

[0069] Pretreatment of the substrate: The AZ91D magnesium alloy sheet was cut into cylinders with a diameter of 20mm and a thickness of 2mm by laser cutting. The cut AZ91D magnesium alloy substrate was then ground sequentially with SiC sandpaper of 600 grit, 800 grit and 1200 grit from coarse to fine. After grinding until there were no obvious scratches on the surface, the residual grinding debris on the substrate surface was removed by ultrasonic treatment with anhydrous ethanol and then dried by a blower to obtain the pretreated magnesium alloy.

[0070] Electrolyte preparation: 17.02 g of K2ZrF6 and 23.24 g of KF were weighed using a balance. The weighed samples were dissolved in 1500 ml and 500 ml of deionized water, respectively, and stirred. After the two solutions were fully dissolved, they were mixed and the volume was adjusted to obtain the electrolyte. The concentration of K2ZrF6 in the obtained mixed electrolyte was 0.03 M and the concentration of KF was 0.20 M. After prolonged stirring, particulate matter appeared in the solution, and the electrolyte system could not exist stably, indicating that micro-arc oxidation was not performed.

[0071] Comparative Example 4

[0072] Pretreatment of the substrate: The AZ91D magnesium alloy sheet was cut into cylinders with a diameter of 20mm and a thickness of 2mm by laser cutting. The cut AZ91D magnesium alloy substrate was then ground sequentially with SiC sandpaper of 600 grit, 800 grit and 1200 grit from coarse to fine. After grinding until there were no obvious scratches on the surface, the residual grinding debris on the substrate surface was removed by ultrasonic treatment with anhydrous ethanol and then dried by a blower to obtain the pretreated magnesium alloy.

[0073] Electrolyte preparation: 22.67 g of K₂ZrF₆ and 2.32 g of KF were weighed separately using a balance. The weighed samples were dissolved in 1500 ml and 500 ml of deionized water, respectively, and stirred until fully dissolved. The two solutions were then mixed and brought to a final volume to obtain the electrolyte. The concentration of K₂ZrF₆ in the resulting mixed electrolyte was 0.04 M, and the concentration of KF was 0.02 M. The pH of the electrolyte was measured to be 5.87 using a pH meter, and the conductivity was measured to be 9.61 mS / cm using a conductivity meter.

[0074] Micro-arc oxidation treatment: The prepared electrolyte is poured into an electrolytic cell. Stainless steel is used as the cathode, and the treated magnesium alloy substrate is placed in the electrolyte as the anode. A constant voltage mode is adopted, with a power supply voltage of 450V, a duty cycle of 20%, a pulse frequency of 500Hz, and a micro-arc oxidation treatment time of 60min. During the treatment, the electrolyte is continuously stirred by magnetic stirring, and the electrolyte temperature is controlled at 20℃~25℃ through a circulating water system. The resulting coating peeled off; excessively high concentrations of K2ZrF6 resulted in poor adhesion of the formed coating.

[0075] Comparative Example 5

[0076] Pretreatment of the substrate: The AZ91D magnesium alloy sheet was cut into cylinders with a diameter of 20mm and a thickness of 2mm by laser cutting. The cut AZ91D magnesium alloy substrate was then ground sequentially with SiC sandpaper of 600 grit, 800 grit and 1200 grit from coarse to fine. After grinding until there were no obvious scratches on the surface, the residual grinding debris on the substrate surface was removed by ultrasonic treatment with anhydrous ethanol and then dried by a blower to obtain the pretreated magnesium alloy.

[0077] Electrolyte preparation: 11.34 g of K₂ZrF₆ and 17.43 g of KF were weighed separately using a balance. The weighed samples were dissolved separately in 1500 ml and 500 ml of deionized water, respectively, and stirred until fully dissolved. The two solutions were then mixed and brought to a final volume to obtain the electrolyte. The concentration of K₂ZrF₆ in the resulting mixed electrolyte was 0.02 M, and the concentration of KF was 0.15 M. The pH of the electrolyte was measured to be 7.83 using a pH meter, and the conductivity was measured to be 17.48 mS / cm using a conductivity meter.

[0078] Micro-arc oxidation treatment: The prepared electrolyte was poured into an electrolytic cell. Stainless steel was used as the cathode, and the treated magnesium alloy substrate was placed in the electrolyte as the anode. A constant voltage mode was used, with a power supply voltage of 450V, a duty cycle of 20%, a pulse frequency of 500Hz, and a micro-arc oxidation treatment time of 15 minutes. During the treatment, the electrolyte was continuously stirred by magnetic stirring, and the electrolyte temperature was controlled at 20℃~25℃ through a circulating water system. The resulting coating had a wear resistance life of less than 5 minutes. Due to the low wear resistance life of the coating, it was quickly worn through during friction. The wear process exceeding the wear resistance life was mainly due to the magnesium alloy substrate; therefore, the wear rate was not tested.

[0079] Comparative Example 6

[0080] Pretreatment of the substrate: The AZ91D magnesium alloy sheet was cut into cylinders with a diameter of 20mm and a thickness of 2mm by laser cutting. The cut AZ91D magnesium alloy substrate was then ground sequentially with SiC sandpaper of 600 grit, 800 grit and 1200 grit from coarse to fine. After grinding until there were no obvious scratches on the surface, the residual grinding debris on the substrate surface was removed by ultrasonic treatment with anhydrous ethanol and then dried by a blower to obtain the pretreated magnesium alloy.

[0081] Electrolyte preparation: Weigh 17.02g of K2ZrF6 and 17.43g of KF using a balance. Dissolve the weighed samples in 1500ml and 500ml of deionized water respectively and stir. After the two solutions are fully dissolved, mix them and make up to a final volume to obtain the electrolyte. The concentration of K2ZrF6 in the resulting mixed electrolyte is 0.03M and the concentration of KF is 0.15M.

[0082] Micro-arc oxidation treatment: The prepared electrolyte was poured into an electrolytic cell. Stainless steel was used as the cathode, and the treated magnesium alloy substrate was placed in the electrolyte as the anode. A constant voltage mode was adopted, with a power supply voltage of 400V, a duty cycle of 20%, a pulse frequency of 500Hz, and a micro-arc oxidation treatment time of 60min. During the treatment, the electrolyte was continuously stirred by magnetic stirring, and the electrolyte temperature was controlled at 20℃~25℃ through a circulating water system. After rinsing and drying with deionized water and anhydrous ethanol, a micro-arc oxidation coating with a long wear resistance life was obtained on the surface. The wear resistance life of the obtained coating was less than 3min. Due to the low wear resistance life of the coating, the coating was quickly worn through during friction. The wear process exceeding the wear resistance life was the magnesium alloy substrate, so the wear rate was not tested.

[0083] The micro-arc oxidation coating with long wear resistance obtained in Example 4 was selected for analysis, characterization and performance testing of the micro-arc oxidation coating in Comparative Example 1. Figure 1 These are SEM images of the surface microstructure of the micro-arc oxidation coatings in Example 4 and Comparative Example 1 of this invention. Figure 2 These are SEM images of the cross-sectional microstructure of the micro-arc oxidation coatings of Example 4 and Comparative Example 1. A comparison of the surface and cross-sectional microstructure of the prepared coatings shows that, compared to the coating of Comparative Example 1, which exhibits a porous surface structure, microcracks, and uneven coating, with obvious cracks or pores, the coating of this invention has a pancake-like surface structure with fewer pores. Furthermore, the coating is more uniform, with a thickness of approximately 30 μm, lower than the 40 μm of the coating prepared in Comparative Example 1.

[0084] The surface of the coating was characterized and tested using a white light interferometer. Figure 3 These are 3D morphology images of the micro-arc oxidation coatings of Example 4(a) and Comparative Example 1(b) of the present invention. The roughness of the coating in this study is 2.70, which is lower than that of Comparative Example 1 (4.92), indicating that the surface of the coating in this study is more uniform.

[0085] The phase composition of the coating was analyzed and characterized by XRD. Figure 4 These are the XRD patterns of the micro-arc oxidation coatings of Example 4 and Comparative Example 1 of this invention. The coating phases in this study mainly consist of Mg, MgO, MgF2, and ZrO2. The coating phases in Comparative Example 1 mainly consist of Mg, MgO, and Mg2SiO4 phases. ZrO2 exhibits higher hardness and wear resistance compared to Mg2SiO4.

[0086] The wear resistance of the coating was evaluated by friction and wear test. The wear life of the coating was tested under dry friction conditions using a high-temperature friction and wear tester with Si3N4 balls with a diameter of 4 mm as the grinding material. The parameters were 8 N load, 200 r / min rotation speed, and 2 mm friction radius. Figure 5The figures show the friction curves of Example 4 and Comparative Example 1. The wear resistance life of the present invention exceeds 2160 min, while the coating of Comparative Example 1 fails in 20 min. The wear resistance life of the coating of the present invention is much higher than that of Comparative Example 1, exceeding 100 times.

[0087] The wear marks were analyzed and characterized by friction time. Figure 6 These are SEM images of the wear marks of the micro-arc oxidation coatings in Example 4(a) and Comparative Example 1(b) of the present invention. The wear marks show similar structures. Example 2 of the present invention shows a more complete structure after friction and is less worn.

[0088] The wear marks on the rubbed coating were characterized and analyzed using a white light interferometer. Figure 7 These are 3D morphological images of the wear marks of the micro-arc oxidation coatings of Example 4(a) and Comparative Example 1(b) of the present invention. The wear mark depth of the present invention is shallower than that of Comparative Example 1.

[0089]

[0090] The above-disclosed embodiments are merely a few examples of the present invention and should not be construed as limiting the scope of the present invention. Therefore, any equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for preparing a long-wear-resistant magnesium alloy micro-arc oxidation coating, characterized in that: Includes the following steps: Step 1: Surface pretreatment of magnesium alloy substrate: The magnesium alloy substrate is ground sequentially with SiC sandpaper from coarse to fine, and then the residual grinding debris on the substrate surface is removed by ultrasonic treatment with anhydrous ethanol and dried by a blower to obtain the pretreated magnesium alloy. The roughness of the pretreated sample is Sa≤0.2µm; Step 2: Preparation of electrolyte: Dissolve K2ZrF6 and KF in deionized water and stir until homogeneous to obtain the electrolyte; the concentration of K2ZrF6 in the electrolyte is 0.028~0.032M, and the concentration of KF is 0.02M~0.15M; Step 3: Micro-arc oxidation treatment: After micro-arc oxidation treatment in a prepared electrolyte, the pretreated magnesium alloy is rinsed and dried with deionized water and anhydrous ethanol to obtain a micro-arc oxidation coating with a long wear resistance life.

2. The method for preparing a long wear-resistant magnesium alloy micro-arc oxidation coating according to claim 1, characterized in that: The magnesium alloy matrix selected in step 1 is AZ91D.

3. The method for preparing a long-wear-resistant magnesium alloy micro-arc oxidation coating according to claim 1, characterized in that: The sandpaper grits are 600, 800, and 1200, and the ultrasonic treatment time is 8 minutes.

4. The method for preparing a long wear-resistant magnesium alloy micro-arc oxidation coating according to claim 1, characterized in that: In step 2, K2ZrF6 and KF are dissolved separately in deionized water and stirred. After the two solutions are stirred until fully dissolved, they are mixed and the volume is adjusted to obtain the electrolyte.

5. The method for preparing a long wear-resistant magnesium alloy micro-arc oxidation coating according to claim 1, characterized in that: In step 2, the concentration of K2ZrF6 is 0.03M, and the concentration of KF is 0.02M~0.05M or 0.14~0.15M.

6. The method for preparing a long wear-resistant magnesium alloy micro-arc oxidation coating according to claim 1, characterized in that: In step 3, the prepared electrolyte is poured into the electrolytic cell, with stainless steel as the cathode and the treated magnesium alloy substrate as the anode placed in the electrolyte. The process parameters for the micro-arc oxidation treatment are as follows: constant voltage mode, power supply voltage of 450V~480V, duty cycle of 20~40%, pulse frequency of 500Hz~800Hz, micro-arc oxidation treatment time of 30 min~60 min, continuous stirring of the electrolyte by magnetic stirring during the treatment, and control of the electrolyte temperature at 20℃~25℃ by the circulating water system.

7. The method for preparing a long wear-resistant magnesium alloy micro-arc oxidation coating according to claim 6, characterized in that: In step 3, the power supply voltage is 450V, the duty cycle is 20%, and the pulse frequency is 500Hz.

8. The method for preparing a long-wear-resistant magnesium alloy micro-arc oxidation coating according to claim 6, characterized in that: The resulting coating has a wear resistance life exceeding 500 minutes; Wear rate less than or equal to 4×10 -5 mm 3 ·N -1 ·m -1 .

9. The method for preparing a long wear-resistant magnesium alloy micro-arc oxidation coating according to claim 6, characterized in that: The resulting coating has a wear resistance life exceeding 1600 min; Wear rate less than or equal to 9.4 × 10 -6 mm 3 ·N -1 ·m -1 After further optimization, the wear resistance life of the resulting coating exceeded 2160 min. Wear rate less than or equal to 4.0 × 10 -6 mm 3 ·N -1 ·m -1 .

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