Preparation method of self-repairing magnesium alloy / fluorocarbon composite material

Abstract

The application belongs to the field of advanced metal material surface protection and composite material technology, and specifically relates to a preparation method of a self-repairing magnesium alloy / fluorocarbon composite material. The method comprises the following steps: (1) sequentially performing polishing, oil removal, pickling activation and fluoride conversion treatment on the magnesium alloy substrate to form a magnesium fluoride transition layer on the surface of the substrate; (2) in-situ growing a layered double metal hydroxide film layer on the surface of the magnesium fluoride transition layer and loading an inhibitor to prepare a self-repairing functional intermediate layer; and (3) depositing a fluorocarbon polymer film on the surface of the self-repairing functional intermediate layer by using a plasma enhanced chemical vapor deposition technology to obtain the self-repairing magnesium alloy / fluorocarbon composite material. The method significantly improves the corrosion resistance and interface bonding strength of the material and prolongs the service life of the magnesium alloy component.

Description

Technical Field

[0001] This invention belongs to the field of advanced metal material surface protection and composite material technology, specifically a method for preparing self-healing magnesium alloy / fluorocarbon composite material. Background Technology

[0002] Magnesium alloys, as the lightest commercially available metallic structural materials, possess high specific strength, high specific stiffness, excellent damping properties, and good biocompatibility, demonstrating enormous application potential in fields such as automotive lightweighting, aerospace weight reduction, and biomedical implants (e.g., bone screws and plates). However, magnesium is chemically extremely reactive, with a low standard electrode potential (-2.37 V), and its naturally formed oxide film (MgO / Mg(OH)2) is loose and porous, failing to provide an effective protective barrier like the oxide film on aluminum alloy surfaces. This makes magnesium alloys highly susceptible to severe electrochemical corrosion, and even catastrophic failure, in corrosive environments containing chloride and sulfate ions, significantly limiting their widespread application.

[0003] Currently, the main technologies used for protecting magnesium alloys include chemical conversion coatings, anodizing, micro-arc oxidation, and organic coatings. Among these, chromate conversion coatings were once widely used due to their excellent protective performance, but the toxicity of hexavalent chromium ions and environmental pollution issues have led to their gradual ban. While chromium-free conversion coatings (such as phosphate, permanganate, and rare earth conversion coatings) are environmentally friendly, their protective performance still needs improvement. Micro-arc oxidation (MAO) technology can generate high-hardness ceramic films in situ, significantly improving corrosion resistance, but these films often contain micropores and cracks, which can easily become rapid diffusion channels for corrosive media, leading to film peeling and substrate corrosion.

[0004] Organic coatings are the most commonly used protective method, effectively isolating corrosive media. However, traditional organic coatings are mostly physical shielding layers. Once subjected to mechanical damage (such as scratches or impacts) or the formation of microcracks during service, the coating will lose its protective ability, and corrosion will concentrate at the damaged site and spread rapidly, leading to premature failure of the substrate. In addition, the high chemical reactivity of magnesium alloy surfaces makes the interfacial bonding between the coating and the substrate less stable, easily resulting in peeling and detachment.

[0005] To address the problem of corrosion failure caused by coating damage, the concept of self-healing coatings has emerged. Self-healing coatings can automatically repair damaged areas and restore their protective function when damage or corrosion occurs, either by releasing embedded corrosion inhibitors or through the material's own chemical reactions. Current self-healing strategies mainly include microcapsule-type, nano-container-type, and intrinsic self-healing coatings. For example, layered bimetallic hydroxides (LDHs), as layered nanomaterials, can adsorb and release corrosion inhibitors (such as phosphates and vanadates) through ion exchange mechanisms, constructing a protective layer with "smart response" characteristics on magnesium alloy surfaces, exhibiting excellent self-healing properties. Research shows that novel self-healing glassy Ce... x Both Oᵧ films and multilayer coatings with loading inhibitors can effectively inhibit the corrosion of magnesium alloys. However, single self-healing functional layers often suffer from low mechanical strength and poor hydrophobicity, making it difficult to meet the long-term protection requirements in complex environments.

[0006] Fluorocarbon coatings are hailed as the "king of coatings" due to their extremely low surface energy, excellent chemical inertness, and outstanding weather and corrosion resistance. The high-energy CF bonds (approximately 485 kJ / mol) in fluorocarbon coatings endow them with exceptional stability. However, existing fluorocarbon coating preparation techniques (such as spraying and brushing) often struggle to form defect-free and firmly bonded films on magnesium alloy surfaces. Literature indicates that while fluorocarbon polymer films prepared by microwave-assisted deposition (MS) or plasma-enhanced chemical vapor deposition (PECVD) exhibit high hydrophobicity (water contact angle up to 107.3°), random defects often exist within the coating. These defects can become initiation points for corrosive media penetration, ultimately leading to coating failure. Furthermore, the difference in thermal expansion coefficients between fluorocarbon materials and the metal substrate, as well as interfacial compatibility issues, also limit their direct application. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention proposes a method for preparing self-healing magnesium alloy / fluorocarbon composites. The core idea is to first optimize the composition of the magnesium alloy matrix to enhance its intrinsic corrosion resistance; second, to construct a self-healing intermediate layer (such as LDH or a modified conversion film) containing corrosion inhibitors on the matrix surface as a "corrosion sentinel"; and finally, to deposit a dense, defect-free fluorocarbon polymer top layer on the surface of the intermediate layer using low-temperature plasma-enhanced chemical vapor deposition (PECVD) technology as a "physical shield." This composite structure not only utilizes the excellent isolation properties of the fluorocarbon coating but also endows the material with active self-healing capabilities through the intermediate layer, achieving synergistic protection of "passive shielding" and "active repair," effectively solving the problems of easy failure and difficult repair of traditional coatings. This invention aims to solve the problems of short protective life, inability to repair damage, and poor interfacial adhesion of existing magnesium alloy surface coatings. By designing a special "self-healing intermediate layer + fluorocarbon top layer" composite structure, the corrosion resistance and interfacial adhesion strength of the material are significantly improved, extending the service life of magnesium alloy components.

[0008] The technical solution adopted in this invention is as follows: Step 1: Composition design and pretreatment of magnesium alloy matrix Matrix composition design: Mg-Al-Zn alloys (such as AZ91D) or Mg-RE-Zn-Zr alloys (such as WE43) are selected. Specifically, to optimize corrosion resistance, Mg-Gd-Y-Zn-Zr alloys are recommended, with the following composition range (mass percentage): Gd 8.0-12.0 wt%, Y 2.0-4.0 wt%, Zn 0.5-1.5 wt%, Zr 0.3-0.8 wt%, with the balance being Mg and unavoidable impurities. The content of impurity elements Fe, Cu, and Ni should be controlled below 0.005 wt%. The addition of rare earth elements (Gd, Y) can purify grain boundaries, forming a dense passivation film and significantly reducing the corrosion rate.

[0009] Surface pretreatment: Grinding and polishing: The substrate surface was ground sequentially with 400#, 800#, 1200# and 2000# SiC sandpaper to remove oxide scale and macroscopic defects. Then, diamond polishing paste was used to polish to a mirror finish with a surface roughness Ra ≤ 0.2 μm.

[0010] Degreasing: Ultrasonic cleaning in acetone or anhydrous ethanol for 10-15 minutes to remove surface oil.

[0011] Acid washing activation: Immerse the sample in a 5-10% dilute nitric acid or dilute sulfuric acid solution for 30-60 seconds to remove the natural oxide film and activate the surface.

[0012] Fluoride conversion pretreatment: To enhance the adhesion between the subsequent coating and the substrate, a fluorination treatment is performed in a solution containing hydrofluoric acid (HF) or ammonium bifluoride (NH4HF2) to form a dense magnesium fluoride (MgF2) transition layer. The thickness of this layer is controlled at 1~2 μm, serving both as a corrosion barrier and improving the interfacial bonding strength.

[0013] Step 2: Preparation of the self-healing intermediate layer Layered bimetallic hydroxide or corrosion inhibitor-loaded conversion films are grown in situ on the pretreated substrate surface using hydrothermal or chemical bath deposition methods.

[0014] LDH layer preparation: Prepare a mixed salt solution containing Mg²⁺ (e.g., Mg(NO₃)₂) and Al³⁺ (or Zn²⁺, NO₃⁻), and adjust the pH to 9-11. Place the pretreated magnesium alloy sample in a reaction vessel and carry out a hydrothermal reaction at 100-150°C for 12-24 hours to grow an LDH film in situ on its surface.

[0015] Corrosion inhibitor loading: The sample with grown LDH is immersed in a solution containing a corrosion inhibitor (such as sodium phosphate, benzotriazole BTA, or rare earth salt Ce(NO3)3) for ion exchange or adsorption treatment for 2-4 hours, allowing the corrosion inhibitor molecules or ions to insert into the LDH interlayer or adsorb onto the surface of the plate. When the coating is damaged and comes into contact with corrosive media, the LDH layer releases the corrosion inhibitor, which reacts with Mg²⁺ or OH⁻ to form a sparingly soluble precipitate, sealing the damaged area and achieving self-repair.

[0016] Drying and curing: Remove the sample, wash it with deionized water, and vacuum dry it at 60~80°C.

[0017] Step 3: Plasma-enhanced chemical vapor deposition (PECVD) for fluorocarbon top layer A defect-free fluorocarbon polymer film was deposited on the surface of a self-healing intermediate layer using PECVD technology.

[0018] Gas source selection: Use fluorine-containing precursor gases, such as hexafluoroethane (C2F6), tetrafluoromethane (CF4), or octafluorocyclobutane (C4F8).

[0019] Process parameter optimization: Background vacuum: Evacuate to a chamber bottom pressure of 5×10⁻ 4 Pa is used to remove impurities, gases, and moisture from the chamber.

[0020] Plasma pretreatment: Argon gas is introduced at a flow rate of 50~100 sccm, and the radio frequency power supply is turned on to perform glow discharge (power 100~200 W, time 5~10 min) to sputter clean and activate the surface of the intermediate layer, remove impurities adsorbed on the surface, increase surface active sites, and improve the coating adhesion.

[0021] Deposition process: RF power: Controlled between 50 and 150 W. Excessive power may lead to over-crosslinking of the film and internal stress, while insufficient power will result in slow deposition rate and insufficient film density.

[0022] Substrate temperature: controlled between 100 and 250°C. Magnesium alloys have a low melting point; excessively high temperatures may lead to substrate degradation or interlayer decomposition. Low-temperature PECVD processes are not only suitable for temperature-sensitive magnesium substrates, but can also obtain thin films with specific microstructures by adjusting parameters.

[0023] Working pressure: Controlled between 10 and 30 Pa.

[0024] Gas flow rate: The precursor gas flow rate is 20~50 sccm. Hydrogen or argon can be introduced as a dilution gas to adjust the F / C ratio and structure of the film.

[0025] Deposition time: 10–30 min, with film thickness controlled between 500 nm and 2 μm. Studies have shown that coatings with superior morphology and performance can be obtained when the deposition time is 10–15 min.

[0026] Post-annealing treatment: After deposition, in-situ annealing is performed under vacuum or inert atmosphere protection at a temperature of 200~300°C for 30~60 min to eliminate internal stress in the film, improve density, and reduce micropores and pinhole defects.

[0027] Beneficial effects: The invention employs a synergistic protection mechanism, constructing a dual protection system consisting of a self-healing intermediate layer and a fluorocarbon top layer. The fluorocarbon top layer provides an excellent physical barrier, effectively blocking the penetration of water, oxygen, and corrosive ions; the self-healing intermediate layer responds to corrosion signals when the coating is damaged, releasing corrosion inhibitors to actively repair defects, achieving a synergistic protection effect of "1+1>2".

[0028] Excellent interfacial bonding strength: Through fluoride conversion pretreatment and plasma surface activation during PECVD, a strong chemical bond or mechanical interlocking structure is formed between the magnesium alloy matrix, the intermediate layer, and the fluorocarbon layer. The magnesium fluoride transition layer significantly improves interfacial compatibility and solves the problem of easy peeling of the fluorocarbon coating. The interfacial bonding strength is expected to reach over 50 MPa.

[0029] Defect-free coating preparation: Optimized PECVD process parameters (low temperature deposition, appropriate power and pressure) combined with post-annealing treatment effectively reduce random defects and micropores inside the fluorocarbon film, solving the problem that fluorocarbon coatings prepared by traditional methods are prone to defects that lead to early failure.

[0030] Environmentally friendly and long-lasting: The prepared composite materials do not contain harmful substances such as chromium and have an ultra-long service life, meeting the requirements of green manufacturing. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the layered structure of the self-healing magnesium alloy / fluorocarbon composite material prepared in this invention; Figure 2 for Figure 1 A magnified view of a portion of the interface area. Detailed Implementation

[0032] like Figure 1 , 2 As shown, the present invention will be further described in detail below with reference to specific embodiments.

[0033] Example 1: Preparation and Pretreatment of AZ91D Magnesium Alloy Matrix Matrix material: Commercial AZ91D magnesium alloy ingot is selected, with the following chemical composition (mass fraction %): Al 8.5-9.5, Zn 0.45-0.90, Mn ≥ 0.17, Si ≤ 0.10, Fe ≤ 0.004, Cu ≤ 0.025, Ni ≤ 0.001, and the balance is Mg.

[0034] Sample preparation: Wire-cut samples with dimensions of 20 mm × 20 mm × 5 mm.

[0035] Surface polishing: Wet polishing was carried out using 400#, 800#, 1200# and 2000# metallographic sandpaper in sequence, followed by polishing with W2.5 diamond polishing paste until the surface roughness Ra = 0.1 μm.

[0036] Degreasing: Ultrasonic cleaning in acetone for 15 minutes, then rinse with anhydrous ethanol and dry with hot air.

[0037] Acid washing and activation: Immerse in a mixed acid solution (containing 10 g / L NaOH and 100 g / L Na2CO3) at room temperature for 5 min, then rinse with deionized water.

[0038] Fluoride pretreatment: The sample was immersed in a 40% hydrofluoric acid solution for 10 seconds, then removed and ultrasonically cleaned with deionized water. This step forms a dense MgF2 film on the substrate surface, with a thickness of approximately 1.5 μm. This film not only serves as a corrosion barrier but also improves the adhesion strength between subsequent coatings and the substrate.

[0039] Example 2: Preparation of a self-healing LDH intermediate layer Solution preparation: Prepare 100 mL of a mixed salt solution containing 0.1 M Mg(NO3)2 and 0.05 M Al(NO3)3, and adjust the pH to 10.0 with dilute NaOH solution to obtain a clear and transparent hydrothermal growth solution.

[0040] Hydrothermal reaction: The sample treated in Example 1 was placed vertically in a stainless steel reactor lined with polytetrafluoroethylene, and the growth solution was poured in. After sealing, it was placed in a forced-air drying oven and reacted at a constant temperature of 120°C for 18 h.

[0041] Corrosion inhibitor loading: After the reaction was completed and the sample was allowed to cool naturally, it was removed and the surface powder was washed off with deionized water. The sample was then immersed in a 0.1 M Na3PO4 solution and stirred at 60°C for 3 h. The ion exchange properties of LDH were utilized to insert phosphate ions (PO4³⁻) into the interlayer. Phosphate is a highly efficient corrosion inhibitor for magnesium alloys, capable of forming insoluble Mg3(PO4)2 precipitates at corroded sites, achieving self-repair.

[0042] Drying: The sample was removed and dried in a vacuum drying oven at 80°C for 2 hours to obtain a white LDH / PO4 composite film. The film thickness was approximately 5–8 μm.

[0043] Example 3: Deposition of fluorocarbon top layer by PECVD Equipment preparation: Use a radio frequency (RF) PECVD deposition system at a frequency of 13.56 MHz.

[0044] Sample loading and vacuuming: The sample with the LDH interlayer prepared in Example 2 was placed on the deposition platform of the PECVD chamber. The vacuum pump was started, and the chamber was evacuated until the bottom pressure reached 5 × 10⁻⁻⁻⁶. 4 Pa.

[0045] Plasma cleaning: Argon gas is introduced into the chamber at a flow rate of 80 sccm, and the working pressure is adjusted to 5 Pa. The RF power supply is turned on and the power is set to 150 W. The sample surface is cleaned by glow discharge for 10 min to remove adsorbed moisture and organic contaminants, while generating a large number of surface-active free radicals.

[0046] Deposition parameters: Gas: Introduce precursor C4F8 gas at a flow rate of 30 sccm.

[0047] Pressure: Adjust the needle valve to maintain the working pressure of the chamber at 20 Pa.

[0048] Power: Set the RF power to 100 W.

[0049] Substrate temperature: The deposition platform was heated to 150°C. This temperature is below the tolerance limit of the LDH layer and the magnesium alloy substrate, and is beneficial for improving the adsorption and reaction rates of gas molecules.

[0050] Time: Deposition time 15 min.

[0051] Post-annealing: After deposition, the RF power supply and gas are turned off, and the film is allowed to cool naturally to room temperature under vacuum conditions. This process is equivalent to a low-temperature annealing, which helps to eliminate internal stress in the film.

[0052] Example 4: Performance Testing and Characterization Surface morphology and composition analysis: The surface and cross-sectional morphology of the coating were observed using scanning electron microscopy (SEM). The results showed that the fluorocarbon top layer had a smooth and dense surface, without obvious micropores or cracks. Cross-sectional observation revealed a distinct four-layer structure of "matrix / MgF2 / LDH / fluorocarbon," with clear interfaces and tight bonding between the layers. Energy dispersive spectroscopy (EDS) confirmed the enrichment of fluorine and carbon elements on the surface, and the distribution of magnesium, aluminum, and phosphorus in the intermediate layers.

[0053] Water contact angle test: The hydrophilicity and hydrophobicity of the coating surface were tested using a contact angle meter. The results showed that after PECVD fluorocarbon treatment, the water contact angle of the composite material surface increased significantly from 61.5° (literature value) of the AZ91D matrix to 110°~115°, exhibiting excellent superhydrophobic properties. This is beneficial for reducing the adhesion of water film on the surface and delaying corrosion.

[0054] Interface bonding strength test: Referring to GB / T 228.1 and related coating bond strength testing standards, the interfacial bond strength between the coating and the substrate was tested using the tensile method. The sample was glued to a paired tensile bar with epoxy resin for tensile testing. The test results show that the average interfacial bond strength of the composite material prepared in this invention is 52 MPa, significantly higher than that of traditional sprayed fluorocarbon coatings (typically <10 MPa). The introduction of the MgF2 transition layer acts as a "bridge," effectively enhancing the interfacial bonding force.

[0055] Electrochemical corrosion performance test: Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves were performed in a 3.5 wt.% NaCl solution using an electrochemical workstation to evaluate the corrosion resistance and self-healing ability of the material.

[0056] Tafel polarization curves: measuring corrosion potential and corrosion current density. Untreated AZ91D magnesium alloy exhibits a high corrosion current density. In contrast, the composite material prepared in this invention shows a significant positive shift in corrosion potential and a reduction in corrosion current density by 2-3 orders of magnitude. This indicates that the fluorocarbon layer greatly enhances the energy barrier for the corrosion reaction.

[0057] EIS Testing: Long-term immersion experiments were conducted to simulate the self-healing process of the coating after damage. Initially, due to the shielding effect of the fluorocarbon layer, the impedance value |Z| was extremely high at 0.01 Hz. After artificial scratching, the impedance decreased initially, but as the immersion time increased (e.g., after 24 hours), the impedance gradually increased again because the phosphate ions released from the LDH interlayer reacted with corrosion products to form a poorly soluble film that blocked the scratch channels. The capacitive arc radius in the Nyquist plot showed a trend of first decreasing and then increasing over time after scratching, demonstrating the material's excellent self-healing properties.

[0058] Self-repair efficiency assessment The self-healing efficiency was calculated using a formula based on electrochemical test data. The protection efficiency was calculated by comparing the corrosion current density of the scratched sample during the initial immersion period and after repair. The results showed that the repair efficiency could reach over 95% within immersion time of 24–48 hours.

[0059] in conclusion: This invention addresses the bottleneck problem of magnesium alloys' susceptibility to corrosion by innovatively proposing a composite material preparation technology that integrates "physical shielding" and "active repair." Fluoride pretreatment enhances interfacial bonding, an LDH interlayer imparts self-healing functionality, and a defect-free fluorocarbon top layer is prepared using a PECVD process with optimized parameters. Electrochemical and interfacial bonding strength tests demonstrate that this composite material exhibits excellent corrosion resistance, high interfacial bonding strength, and good self-healing efficiency, providing a reliable technical solution for the long-term service of magnesium alloys in harsh environments. It possesses significant engineering application value and broad market prospects.

Claims

1. A method for preparing a self-healing magnesium alloy / fluorocarbon composite, characterized in that, Includes the following steps: (1) The magnesium alloy substrate is subjected to grinding and polishing, degreasing, pickling and activation and fluoride conversion treatment in sequence to form a magnesium fluoride transition layer on the substrate surface; (2) A layered bimetallic hydroxide film is grown in situ on the surface of the magnesium fluoride transition layer and a corrosion inhibitor is loaded to prepare a self-healing intermediate layer. (3) A fluorocarbon polymer film was deposited on the surface of the self-healing functional intermediate layer by plasma enhanced chemical vapor deposition (PECVD) technology to obtain a self-healing magnesium alloy / fluorocarbon composite material.

2. The method for preparing a self-healing magnesium alloy / fluorocarbon composite according to claim 1, characterized in that, The fluoride conversion treatment in step (1) uses hydrofluoric acid or ammonium hydrogen fluoride solution, and the thickness of the magnesium fluoride transition layer formed is 0.5 to 2.0 μm.

3. The method for preparing a self-healing magnesium alloy / fluorocarbon composite according to claim 1, characterized in that, The self-healing intermediate layer described in step (2) is grown in situ using a hydrothermal method, with a thickness of 5–10 μm.

4. The method for preparing a self-healing magnesium alloy / fluorocarbon composite according to claim 1, characterized in that, In step (2), the interlayer loading corrosion inhibitor is one or more of phosphate, benzotriazole or rare earth salt.

5. The method for preparing a self-healing magnesium alloy / fluorocarbon composite according to claim 1, characterized in that, The PECVD deposition process parameters in step (3) are: RF power 50-150W, substrate temperature 100-250℃, working pressure 10-30Pa, and deposition time 10-30min.

6. The method for preparing a self-healing magnesium alloy / fluorocarbon composite according to claim 1, characterized in that, After the deposition is completed in step (3), an annealing process is also included in the vacuum or inert atmosphere, with an annealing temperature of 200-300℃.

7. The method for preparing a self-healing magnesium alloy / fluorocarbon composite according to claim 1, characterized in that, The grinding in step (1) is done by grinding with 400# to 2000# SiC sandpaper in stages; the degreasing is done by ultrasonic cleaning with acetone or anhydrous ethanol; the pickling is done by soaking in 5 to 10% dilute nitric acid or dilute sulfuric acid.

8. The method for preparing a self-healing magnesium alloy / fluorocarbon composite according to claim 3, characterized in that, In step (2), the reaction solution used in the hydrothermal method is a mixed salt solution containing Mg²⁺ and Al³⁺ or Zn²⁺, and the pH value is adjusted to 9-11.

9. The method for preparing a self-healing magnesium alloy / fluorocarbon composite according to claim 1, characterized in that, In step (3), before PECVD deposition, argon gas is introduced for plasma pretreatment. The pretreatment power is 100-200W and the time is 5-10min.

10. A self-healing magnesium alloy / fluorocarbon composite material prepared by the method according to any one of claims 1 to 9, characterized in that, The material has a layered structure of "matrix / magnesium fluoride transition layer / LDH intermediate layer / fluorocarbon top layer", with an interfacial bonding strength ≥50MPa and a water contact angle ≥110°.

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