Sandwich-structured micro-arc oxidation film and its preparation method

Abstract

This invention relates to the field of micro-arc oxidation technology, specifically to a sandwich-structured micro-arc oxidation film and its preparation method. In a first aspect, a sandwich-structured micro-arc oxidation film is proposed, comprising a first film layer, a second film layer, and a third film layer stacked sequentially. Each of the first, second, and third film layers independently comprises γ-Al₂O₃ and α-Al₂O₃. In the first film layer, the content of α-Al₂O₃ is 40 wt%–60 wt%; in the second film layer, the content of γ-Al₂O₃ is 85 wt%–95 wt%; and in the third film layer, the content of α-Al₂O₃ is 40 wt%–60 wt%. This sandwich-structured micro-arc oxidation film exhibits high hardness and chemical stability, while also possessing excellent wear resistance and corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of micro-arc oxidation technology, specifically to a sandwich-structured micro-arc oxidation film and its preparation method. Background Technology

[0002] Easy-drilling aluminum alloy tubing (such as tubing used in oil extraction) requires high hardness, low ductility, and high strength to achieve efficient drilling. This unique combination of properties reduces drill bit cutting resistance, and the low ductility, which leads to brittle fracture, effectively prevents material from sticking to the drill bit, thus significantly improving drilling efficiency. Experimental data shows that the drilling speed of aluminum alloy tubing can reach 30 m / h, more than 12 times that of traditional steel tubing. However, in typical applications such as oil extraction and marine equipment, easy-drilling aluminum alloys often face abrasive wear, fretting wear, and chloride ion (Cl) degradation. - The severe coupling effect of corrosion. Therefore, preparing a protective film layer on its surface that is both wear-resistant and corrosion-resistant is the key to ensuring its long-term service quality.

[0003] Currently, micro-arc oxidation (MAO) can be used to prepare MAO coatings on easily drillable aluminum alloy pipes. This technology can grow MAO coatings with a thickness of 30 μm to 150 μm in situ on the aluminum alloy surface. The main phases are α-Al₂O₃ (hard phase) and γ-Al₂O₃ (tough phase), achieving a hardness of 1000–2000 HV, theoretically improving the wear resistance of the substrate by 5–8 times. However, MAO coatings are constrained by their instantaneous melting and rapid cooling mechanism due to spark discharge, resulting in the following drawbacks: the phases and structure within the MAO coating are uncontrollable, and the ratio of α-Al₂O₃ to γ-Al₂O₃ and their spatial arrangement within the coating cannot be precisely controlled. These issues make it difficult to achieve a synergistic balance between strength and toughness in MAO coatings. It is difficult to form an effective long-term corrosion barrier, nor can it maintain structural stability under complex dynamic loads. This leads to a situation where, in practical applications, the wear resistance, corrosion resistance, and impact resistance of MAO coatings often fluctuate, failing to achieve a true synergistic improvement. Summary of the Invention

[0004] This invention aims to at least partially solve one of the technical problems in related technologies. Therefore, one objective of this invention is to propose a sandwich-structured micro-arc oxidation film and a method for preparing the same. This preparation method can form a soft and hard film layer that combines wear resistance, corrosion resistance, and impact resistance.

[0005] In a first aspect of the present invention, a sandwich-structured micro-arc oxidation film layer is provided, the sandwich-structured micro-arc oxidation film layer comprising a first film layer, a second film layer and a third film layer stacked sequentially. The first film layer, the second film layer, and the third film layer each independently include γ-Al2O3 and α-Al2O3; In the first film layer, the content of α-Al2O3 is 40 wt% to 60 wt%; In the second film layer, the content of γ-Al2O3 is 85 wt% to 95 wt%; In the third film layer, the content of α-Al2O3 is 40 wt% to 60 wt%.

[0006] In a first aspect, this invention constructs a hard (high α)-soft (high γ)-hard (high α) gradient structure from the inside out by stacking a first, second, and third film layer and precisely controlling the phase content of α-Al2O3 and γ-Al2O3 in each layer: the first film layer is the bottom layer (high α), which has a strong bonding force with the substrate due to the high compatibility between α-Al2O3 and the substrate; the second film layer is the middle layer (high γ), which can absorb impact energy and alleviate stress concentration by utilizing the high toughness of γ-Al2O3, thereby improving the coating's resistance to dynamic loads; the third film layer is the surface layer, which has high hardness and chemical stability due to the high content of α-Al2O3, and also has excellent wear resistance and corrosion resistance. This micro-arc oxidation film layer completely solves the technical problems of brittle spalling, poor impact resistance, and the inability to simultaneously achieve wear resistance and corrosion resistance in traditional single-structure coatings or multi-layer coatings from the source design of matching phase composition and toughness.

[0007] According to an embodiment of the present invention, the ratio of the thickness of the first film layer, the thickness of the second film layer and the thickness of the third film layer is (3~4):(11~14):(3~4).

[0008] According to an embodiment of the present invention, the aluminum alloy includes at least one of 2-series aluminum alloys, 5-series aluminum alloys, 6-series aluminum alloys, and 7-series aluminum alloys.

[0009] A second aspect of the present invention provides a method for preparing a sandwich-structured micro-arc oxidation film, comprising the following steps: (1) Provide aluminum alloy substrate Prepare an alkaline composite electrolyte containing phosphates, silicates, alkaline regulators, buffers, and complexing agents; Na2WO4 is added to the alkaline composite electrolyte to serve as the first electrolyte; Na3PO4 is added to the alkaline composite electrolyte to serve as a second electrolyte; Na2WO4 and glycerol are added to the alkaline composite electrolyte to form a third electrolyte; (2) Performing a time-sequential micro-arc oxidation process Using the aluminum alloy substrate as the anode and the stainless steel plate as the cathode, a first micro-arc oxidation treatment is performed sequentially in the first electrolyte, a second micro-arc oxidation treatment is performed in the second electrolyte, and a third micro-arc oxidation treatment is performed in the third electrolyte, so as to form a first film layer, a second film layer and a third film layer stacked sequentially on the aluminum alloy substrate.

[0010] In a second aspect of this application, a layered phase control strategy was employed to achieve spatially customized arrangement of α-Al₂O₃ and γ-Al₂O₃ contents. The specific mechanism is as follows: The first film layer (inner hard layer) still follows the traditional high-temperature sintering mechanism, forming a dense α-Al₂O₃-rich hard layer at the substrate interface through the heat sink effect of the substrate, ensuring a high-strength metallurgical bond with the substrate. The second film layer (middle soft layer) introduces phosphate (PO₄²⁻)... 3- As a potent phase transition inhibitor, it significantly increases the activation energy of the transformation from γ-Al₂O₃ to α-Al₂O₃ at the chemical level; the α-Al₂O₃ content in the second film layer is actively suppressed, thereby constructing a highly toughened layer. During the formation of the third film layer (outer hard layer), the high viscosity of glycerol forms a robust viscous thermal shielding boundary layer on the anode surface, effectively blocking the direct quenching effect of external cold water; simultaneously, under the dielectric guidance of the bottom highly insulating film layer, the extremely high-energy plasma spark is forced to undergo violent re-ignition locally on the outermost surface. This artificially created reverse surface ultra-high temperature sintering not only induces a violent γ→α phase transition locally on the surface (making the α-Al₂O₃ content reach 40%~60%), but also protects the internal soft phase structure from high-pressure damage due to the aerogel-level thermal insulation effect of the porous soft layer in the middle. Through the above three-layer synergistic regulation, the second aspect of this application overcomes the inherent defects of the monotonically decreasing α-Al2O3 content along the growth direction and large fluctuations at both ends in the traditional micro-arc oxidation film layer, and realizes a customized phase distribution of α-rich bottom layer (high adhesion), γ-rich middle layer (high toughness), and α-rich surface layer (high wear resistance), which solves the technical problem of difficulty in balancing wear resistance and toughness in the prior art, and significantly improves the comprehensive protective performance of the coating under corrosion-wear coupling conditions.

[0011] According to an embodiment of the present invention, in the method for preparing the micro-arc oxidation film, the first micro-arc oxidation treatment includes: an initial voltage of 260 V to 270 V, increasing the initial voltage to 380 V to 390 V, a frequency of 900 Hz to 1000 Hz, and a duty cycle of 13% to 15%.

[0012] According to an embodiment of the present invention, in the method for preparing the micro-arc oxidation film, the second micro-arc oxidation treatment includes: a constant voltage of 460 V to 470 V, a frequency of 200 Hz to 300 Hz, and a duty cycle of 30% to 35%.

[0013] According to an embodiment of the present invention, in the method for preparing the micro-arc oxidation film, the third micro-arc oxidation treatment includes: an initial voltage of 360 V to 370 V, increasing the initial voltage to 520 V to 530 V, a frequency of 900 Hz to 1000 Hz, and a duty cycle of 13% to 15%.

[0014] According to an embodiment of the present invention, in the method for preparing the micro-arc oxidation film, the concentration of Na2WO4 is 10 g / L to 15 g / L.

[0015] According to an embodiment of the present invention, in the preparation method of the micro-arc oxidation film, the concentration of Na3PO4 is 8 g / L to 12 g / L.

[0016] According to an embodiment of the present invention, in the method for preparing the micro-arc oxidation film, the concentration of Na2WO4 is 12 g / L to 20 g / L, and the concentration of glycerol is 25 ml / L to 30 ml / L.

[0017] According to an embodiment of the present invention, in the method for preparing the micro-arc oxidation film, step (2) further includes a first transition treatment and a second transition treatment; the first transition treatment is to continue micro-arc oxidation treatment in the first electrolyte for 2 min to 5 min after the first micro-arc oxidation treatment is completed, with a voltage of 400 V to 410 V, a frequency of 490 Hz to 510 Hz, and a duty cycle of 30% to 35%.

[0018] According to an embodiment of the present invention, in the method for preparing the micro-arc oxidation film, the second transition treatment is to continue micro-arc oxidation treatment in the second electrolyte for 2 min to 5 min after the second micro-arc oxidation treatment is completed, with a voltage of 400 V to 410 V, a frequency of 490 Hz to 510 Hz, and a duty cycle of 30% to 35%.

[0019] According to an embodiment of the present invention, in the method for preparing the micro-arc oxidation film, the alkaline composite electrolyte contains sodium hexametaphosphate as the phosphate, sodium silicate as the silicate, sodium hydroxide as the alkalinity regulator, sodium tetraborate and boric acid as the buffer, and disodium ethylenediaminetetraacetate as the complexing agent; the concentration of sodium silicate is 20 g / L to 30 g / L; the concentration of sodium hydroxide is 1 g / L to 5 g / L; the concentration of sodium tetraborate is 1 g / L to 3 g / L; the concentration of boric acid is 3 g / L to 4 g / L; and the concentration of disodium ethylenediaminetetraacetate is 0.5 g / L to 1 g / L.

[0020] According to an embodiment of the present invention, in the method for preparing the micro-arc oxidation film, the temperature of the electrolyte in step (2) for the first micro-arc oxidation treatment, the second micro-arc oxidation treatment, the third micro-arc oxidation treatment, the first transition treatment, and the second transition treatment is 20°C to 50°C; and an AC power supply is used to perform micro-arc oxidation in constant current mode.

[0021] According to an embodiment of the present invention, in the method for preparing the micro-arc oxidation film, the aluminum alloy substrate includes at least one of 2-series aluminum alloys, 5-series aluminum alloys, 6-series aluminum alloys, and 7-series aluminum alloys.

[0022] In a third aspect of the present invention, a micro-arc oxidation film layer prepared according to the preparation method of the second aspect of the present application is provided, wherein the content of α-Al2O3 in the first film layer is 40 wt% to 60 wt%; the content of γ-Al2O3 in the second film layer is 85 wt% to 95 wt%; and the content of α-Al2O3 in the third film layer is 40 wt% to 60 wt%.

[0023] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0024] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 These are the XRD patterns of Example 2 and Comparative Example 4; Figure 2 These are hardness distribution diagrams of the films in Examples 1-4 and Comparative Examples 1-4; Figure 3 These are electrochemical polarization curves of the films in Examples 1-4 and Comparative Examples 1-4; Figure 4 It is the friction coefficient of the film layer in Examples 1-4 and Comparative Examples 1-4. Detailed Implementation

[0025] Embodiments of the present invention are described in detail below. Examples of these embodiments are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0026] In a first aspect of the present invention, a sandwich-structured micro-arc oxidation film layer is provided, the sandwich-structured micro-arc oxidation film layer comprising a first film layer, a second film layer and a third film layer stacked sequentially. The first film layer, the second film layer, and the third film layer each independently include γ-Al2O3 and α-Al2O3; In the first film layer, the content of α-Al2O3 is 40 wt% to 60 wt%; In the second film layer, the content of γ-Al2O3 is 85 wt% to 95 wt%; In the third film layer, the content of α-Al2O3 is 40 wt% to 60 wt%.

[0027] In a first aspect, this invention constructs a hard (high α)-soft (high γ)-hard (high α) gradient structure from the inside out by stacking a first film layer, a second film layer, and a third film layer, and precisely controlling the phase content of α-Al2O3 and γ-Al2O3 in each layer: the first film layer is the bottom layer (high α), which has a strong bonding force with the substrate due to the high compatibility between α-Al2O3 and the substrate; the second film layer is the middle layer (high γ), which can absorb impact energy and alleviate stress concentration by utilizing the high toughness of γ-Al2O3, thereby improving the coating's resistance to dynamic loads; the third film layer is the surface layer, which has high hardness and chemical stability due to the high content of α-Al2O3, and also has excellent wear resistance and corrosion resistance. This micro-arc oxidation film layer of aluminum alloy, from the source design of matching phase composition and toughness, completely solves the technical problems of brittle spalling, poor impact resistance, and the inability to simultaneously achieve wear resistance and corrosion resistance inherent in traditional single-structure coatings or multi-layer coatings. Actual tests show that the aluminum alloy of this invention can extend the service life by more than three times under simulated oilfield corrosion and wear conditions, breaking through the limitations of aluminum alloy pipes under extreme conditions (high Cl). - Application bottlenecks under conditions of high wear and dynamic loads.

[0028] It should be noted that in the first film layer, the content of α-Al₂O₃ is 40 wt%–60 wt%, and the remaining portion (40 wt%–60 wt%) is mainly γ-Al₂O₃, along with unavoidable impurities (such as trace doping from electrolyte elements, incompletely oxidized aluminum, and pores). Similarly, the remaining portion in the second film layer is α-Al₂O₃, along with unavoidable impurities. The remaining portion (40 wt%–60 wt%) in the third film layer is mainly γ-Al₂O₃, along with unavoidable impurities.

[0029] In the first film layer, if the content of α-Al2O3 is less than 40 wt%, the bottom layer cannot provide sufficient rigid support, and is prone to microscopic plastic deformation under complex loads. It is also difficult to fully utilize the high-strength metallurgical bonding advantage with the aluminum alloy substrate, thus reducing the overall load-bearing capacity of the coating. If the content of α-Al2O3 is greater than 60 wt%, the brittleness of the bottom layer increases significantly. Furthermore, due to the large difference in thermal expansion coefficients between α-Al2O3 and the aluminum alloy substrate, excessively high α-phase content will generate extremely large residual thermal stress at the interface, which can easily lead to early cracking or even peeling of the film layer at the substrate interface.

[0030] In the second film layer, if the content of γ-Al2O3 is less than 85 wt%, the intermediate layer is not tough enough and cannot effectively play a soft elastic buffering role to absorb external impact energy. It is difficult to alleviate the stress concentration between the upper and lower high hardness layers, resulting in a significant decrease in the overall dynamic load resistance of the coating. If the content of γ-Al2O3 is greater than 95 wt%, the intermediate layer is too soft due to its low hardness. It is prone to yielding and collapse when subjected to high normal loads, causing the surface hard film layer to lose effective mechanical support, thus triggering a failure mode similar to the crushing of an eggshell.

[0031] In the third film layer, if the content of α-Al2O3 is less than 40 wt%, the surface hardness cannot meet the design requirements (it is difficult to break through 2000 HV), its chemical stability and scratch resistance are insufficient, and it is difficult to resist the intrusion of high wear and corrosive media under the extreme working conditions of simulated oilfields, which seriously shortens the service life of the coating. If the content of α-Al2O3 is greater than 60 wt%, the brittleness of the surface layer increases sharply. Under dynamic load or solid particle impact, microcracks are very easy to be generated on the surface and propagate rapidly, eventually leading to large-area brittle peeling failure of the surface film layer.

[0032] It is worth noting that, in the embodiments of this application, the contents of α-Al2O3 and γ-Al2O3 were independently measured using thin film XRD.

[0033] In some embodiments, the thickness ratio of the first film layer, the second film layer, and the third film layer in this aluminum alloy is (3~4):(11~14):(3~4). This thickness ratio establishes a good model for mechanical transmission and stress dissipation. The first and third film layers, while ensuring strong interfacial bonding and high surface wear resistance, effectively suppress the residual stress accumulation and brittle cracking risk caused by the high α phase; while the second film layer, occupying a larger thickness, provides sufficient elastic deformation and buffer space. This thin-hard-thick-soft-thin-hard spatial distribution achieves a synergistic mechanical effect of surface wear resistance, middle layer energy absorption, and bottom layer foundation consolidation.

[0034] In some embodiments, the total thickness of the micro-arc oxidation film in the aluminum alloy is H; the thickness of the first film layer is 15% to 20% of the total thickness H; the thickness of the second film layer is 55% to 70% of the total thickness H; and the thickness of the third film layer is 15% to 20% of the total thickness H. Thus, the three layers work synergistically to avoid the performance defects of a single-structure coating that is either hard and brittle or soft and weak.

[0035] Optionally, the total thickness H of the micro-arc oxidation film layer is 45 μm to 60 μm. This allows the hard-soft-hard gradient structure to be fully formed, with each layer functioning synergistically, thereby endowing the micro-arc oxidation film layer with excellent anti-stripping ability, impact resistance, and long-term protective performance, significantly extending the service life of aluminum alloy tubes under extreme working conditions.

[0036] Furthermore, the thickness of the first film layer is 8 μm to 12 μm, which can form a dense α-Al2O3-rich bonding layer, achieving a high-strength metallurgical bond with the aluminum alloy substrate.

[0037] Furthermore, the second film layer has a thickness of 30 μm to 36 μm, forming a tough buffer layer rich in γ-Al2O3, which can fully absorb impact energy and alleviate stress concentration.

[0038] Furthermore, the third film layer has a thickness of 8 μm to 12 μm, forming a high-hardness α-Al2O3-rich wear-resistant and corrosion-resistant layer that directly resists wear and corrosion.

[0039] In some embodiments, the aluminum alloy includes at least one of 2-series, 5-series, 6-series, and 7-series aluminum alloys. Therefore, the micro-arc oxidation film and its preparation method of the present invention are applicable to aluminum alloy substrates with different composition systems, exhibiting good substrate versatility. Whether for 5-series (Al-Mg) and 6-series (Al-Mg-Si) aluminum alloys requiring high corrosion resistance, or for 2-series (Al-Cu) and 7-series (Al-Zn-Mg-Cu) aluminum alloys requiring higher strength, a hard-soft-hard gradient structure film can be formed on their surface through the same sequential oxidation process, achieving a synergistic improvement in wear resistance, corrosion resistance, and impact resistance, thereby meeting the application requirements under different working conditions.

[0040] Furthermore, the 2-series aluminum alloys include at least one of 2024 and 2A12.

[0041] Furthermore, the 5-series aluminum alloys include at least one of 5052 and 5083.

[0042] Furthermore, 6-series aluminum alloys include at least one of 6061 and 6063.

[0043] Furthermore, 7-series aluminum alloys include at least one of 7075 and 7A04.

[0044] It is worth noting that the sandwich structure micro-arc oxidation film layer in this application refers to a series of hard and soft film layers stacked sequentially, that is, the middle film layer has the lowest hardness. This application does not mean that the film layers on both sides of the middle layer are completely the same.

[0045] In a second aspect of the present invention, a method for preparing a sandwich-structured micro-arc oxidation film is provided, comprising the following steps: S1: Provides an aluminum alloy matrix; optionally, the aluminum alloy includes at least one of 2-series, 5-series, 6-series, and 7-series aluminum alloys.

[0046] S2: An alkaline composite electrolyte containing phosphates, silicates, an alkaline regulator, a buffer, and a complexing agent is prepared. Phosphates and silicates, as the main film-forming agents, participate in the discharge reaction, generating an in-situ Al2O3-based ceramic layer on the aluminum alloy surface. The film growth is uniform and dense. The alkaline regulator is used to control the discharge, improve conductivity, and ensure stable arc initiation. The buffer suppresses violent arcs, prevents localized ablation, and ensures uniform distribution of discharge sparks. The complexing agent complexes metal impurity ions in the solution, extending the electrolyte's lifespan and maintaining process repeatability. This alkaline composite electrolyte exhibits good additive compatibility, facilitating the subsequent introduction of sodium tungstate (inducing α-Al2O3) or sodium phosphate (inducing γ-Al2O3) to achieve precise construction of a hard-soft-hard gradient structure. In summary, this alkaline composite electrolyte provides a stable and adjustable film-forming platform for subsequent time-sequential control and is the fundamental electrolyte for realizing sandwich structure coatings.

[0047] Optionally, in the alkaline composite electrolyte, the phosphate includes sodium hexametaphosphate, the silicate includes sodium silicate, the alkalinity regulator includes sodium hydroxide, the buffer includes sodium tetraborate and boric acid, and the complexing agent includes disodium ethylenediaminetetraacetate.

[0048] Furthermore, in the alkaline composite electrolyte, the concentration of sodium hexametaphosphate can be 20 g / L to 28 g / L; for example, the concentration of sodium hexametaphosphate can be 20 g / L, 22 g / L, 23 g / L, 25 g / L, 28 g / L, etc.

[0049] Furthermore, the concentration of sodium silicate is 20 g / L to 30 g / L. For example, the concentration of sodium silicate can be 20 g / L, 22 g / L, 25 g / L, 28 g / L, or 30 g / L.

[0050] Furthermore, the concentration of sodium hydroxide is 1 g / L to 5 g / L. For example, the concentration of sodium hydroxide can be 1 g / L, 2 g / L, 3 g / L, 4 g / L, 5 g / L, etc.

[0051] Furthermore, the concentration of sodium tetraborate is 1 g / L to 3 g / L, and the concentration of boric acid is 3 g / L to 4 g / L. For example, the concentration of sodium tetraborate can be 1 g / L, 2 g / L, or 3 g / L, and the concentration of boric acid can be 3 g / L, 3.5 g / L, or 4 g / L, etc.

[0052] Furthermore, the concentration of disodium ethylenediaminetetraacetate is 0.5 g / L to 1 g / L. For example, the concentration of disodium ethylenediaminetetraacetate is 0.5 g / L, 0.8 g / L, 1 g / L, etc.

[0053] S3: Perform a time-sequential micro-arc oxidation process; in some embodiments, adding different substances to the alkaline composite electrolyte is beneficial for forming a time-sequential film layer.

[0054] Optionally, Na2WO4 is added to the alkaline composite electrolyte as the first electrolyte; thus, the addition of Na2WO4 to the first electrolyte can promote the formation of α-Al2O3, forming a dense and hard inner layer.

[0055] Furthermore, Na2WO4 is added to the alkaline composite electrolyte to a concentration of 10 g / L to 15 g / L. For example, the concentration in the alkaline composite electrolyte can be 10 g / L, 12 g / L, 13 g / L, 15 g / L, etc.

[0056] Optionally, Na3PO4 is added to the alkaline composite electrolyte as a second electrolyte; thereby, Na3PO4 in the second electrolyte induces the formation of γ-Al2O3, forming a tough intermediate layer.

[0057] Furthermore, Na3PO4 is added to the alkaline composite electrolyte to achieve a concentration of 8 g / L to 12 g / L. For example, the concentration in the alkaline composite electrolyte can be 8 g / L, 9 g / L, 10 g / L, 12 g / L, etc.

[0058] Optionally, Na2WO4 and glycerol are added to the alkaline composite electrolyte as a third electrolyte; thus, the combination of Na2WO4 and glycerol in the third electrolyte further strengthens the outer α-Al2O3 hard layer. The synergistic effect of the three electrolytes achieves a gradient distribution of the α / γ phases along the thickness direction.

[0059] Furthermore, Na₂WO₄ and glycerol are added to the alkaline composite electrolyte, such that the concentration of Na₂WO₄ is 12 g / L to 20 g / L, and the concentration of glycerol is 25 ml / L to 30 ml / L. For example, the concentration of Na₂WO₄ can be 12 g / L, 15 g / L, 18 g / L, 20 g / L, etc.; and the concentration of glycerol can be 25 ml / L, 27 ml / L, 28 ml / L, 30 ml / L, etc.

[0060] In some embodiments, an aluminum alloy substrate can be subjected to a sequential micro-arc oxidation process in a first electrolyte, a second electrolyte, and a third electrolyte. Using the aluminum alloy substrate as the anode and a stainless steel plate as the cathode, a first micro-arc oxidation treatment is performed sequentially in the first electrolyte, a second micro-arc oxidation treatment in the second electrolyte, and a third micro-arc oxidation treatment in the third electrolyte, so as to form a first film layer, a second film layer, and a third film layer sequentially stacked on the aluminum alloy substrate.

[0061] Furthermore, the first micro-arc oxidation process includes: an initial voltage of 260 V to 270 V, increasing the initial voltage to 380 V to 390 V, a frequency of 900 Hz to 1000 Hz, and a duty cycle of 13% to 15%. Thus, by employing a low initial voltage increase rate of 260 V to 380 V, combined with a high frequency and low duty cycle, a uniform and dense α-Al₂O₃-rich layer can be formed on the substrate surface. For example, in the first micro-arc oxidation process, the initial voltage can be 260 V, 265 V, 270 V, etc.; the initial voltage can be increased to 380 V, 385 V, 390 V, etc.; the frequency can be 900 Hz, 950 Hz, 1000 Hz, etc.; and the duty cycle can be 13%, 14%, 15%, etc.

[0062] Optionally, the voltage can be increased from 260 V to 380 V at a rate of 10 V / min. This slow voltage increase avoids excessive initial discharge shock, which is conducive to the formation of a dense underlayer that bonds with the substrate metallurgically, providing high bonding strength (>45MPa) and a basic corrosion-resistant barrier for subsequent coatings.

[0063] Furthermore, the second micro-arc oxidation process includes: a constant voltage of 460 V to 470 V, a frequency of 200 Hz to 300 Hz, and a duty cycle of 30% to 35%, accounting for 60% of the total processing time. This stage maintains a stable discharge state, forming a porous, tough layer of 30 μm to 35 μm. The high duty cycle and moderate frequency allow for sufficient expansion of the discharge channels, resulting in a soft structure rich in γ-Al₂O₃, which effectively absorbs impact energy, alleviates stress concentration, and improves the coating's resistance to dynamic loads. For example, the constant voltage can be 460 V, 465 V, or 470 V; the frequency can be 200 Hz, 250 Hz, or 300 Hz; the duty cycle can be 30%, 32%, 34%, or 35%; optionally, the second micro-arc oxidation process accounts for 60% of the total processing time.

[0064] Furthermore, the third micro-arc oxidation process includes: an initial voltage of 360 V to 370 V, which is then increased to 520 V to 530 V; a frequency of 900 Hz to 1000 Hz; and a duty cycle of 13% to 15%. This generates a high-hardness, dense, α-Al₂O₃-rich outer layer within the higher voltage range. For example, in the third micro-arc oxidation process, the initial voltage can be 360 ​​V, 365 V, 370 V, etc.; it can be increased to 520 V, 525 V, 530 V, etc.; and the frequency can be 900 Hz, 950 Hz, 1000 Hz, etc.

[0065] This application forcibly alters the natural growth trajectory of the film by abruptly switching electrical parameters in three stages (first micro-arc oxidation treatment, second micro-arc oxidation treatment, and third micro-arc oxidation treatment), and constructs a sandwich gradient structure of hard (bottom)-soft (middle)-hard (surface) in situ using MAO technology. The ceramic coating of this application possesses both wear resistance and impact resistance. This application solves the problem that the natural growth pattern of traditional micro-arc oxidation films is that the inner layer is hard and the outer layer is soft, resulting in the surface layer being extremely easy to wear and the overall film being hard and brittle.

[0066] Optionally, an initial voltage of 360 V is used, which is increased to 520 V at a rate of 5 V / min. This reduces the thermal shock of spark discharge to the formed intermediate layer.

[0067] In some embodiments, the time ratios of the first, second, and third micro-arc oxidation treatments can be determined based on the total thickness of the target film, the required functional thickness of each layer, and the response characteristics of the substrate material. For example, if it is necessary to improve the impact toughness of the coating, the time ratio of the second micro-arc oxidation treatment can be appropriately increased (e.g., from 60% to 65%), and the time ratios of the first or third stages can be reduced accordingly (e.g., each reduced by 2.5%) to thicken the γ-Al2O3-rich middle soft layer; if it is necessary to further improve the surface wear resistance, the time ratio of the third micro-arc oxidation treatment can be increased (e.g., from 20% to 25%) to thicken the outer hard layer. Furthermore, for different aluminum alloy substrates (e.g., 2-series and 7-series), since their growth rates for micro-arc oxidation differ, the time ratios can be fine-tuned to ensure that the absolute thickness of each layer reaches the design range (inner layer 8 μm~12 μm, middle layer 30 μm~36 μm, outer layer 8 μm~12 μm).

[0068] In this embodiment, the three parameters of voltage, frequency, duty cycle, and time ratio form a time sequence matching of low-energy nucleation, medium-energy toughening, and high-energy hardening. This, combined with the staged electrolyte additives, precisely constructs a hard-soft-hard sandwich gradient structure, thereby simultaneously achieving comprehensive performance of high bonding strength, high toughness, high wear resistance, and high corrosion resistance. This solves the technical problem that traditional single coatings cannot simultaneously achieve both wear resistance and impact resistance.

[0069] In a preferred embodiment, the sequential micro-arc oxidation process in S3 further includes a first transition treatment and a second transition treatment. The first transition treatment involves continuing micro-arc oxidation in the first electrolyte for 2 minutes at a voltage of 400 V, a frequency of 500 Hz, and a duty cycle of 30% after the first micro-arc oxidation treatment. The second transition treatment involves continuing micro-arc oxidation in the second electrolyte for 2 minutes at a voltage of 400 V, a frequency of 500 Hz, and a duty cycle of 30%. Thus, the transition treatment uses intermediate electrical parameters (400 V / 500 Hz / 30%) to ensure a smooth transition of discharge energy, effectively releasing accumulated interfacial stress and preventing microcracks or film peeling caused by stress concentration. Simultaneously, the transition treatment maintains the continuity of the discharge state, creating a gradual transition zone of composition and structure between adjacent layers, rather than an abrupt interface, thereby significantly improving interlayer bonding strength and ensuring the structural integrity of the overall coating.

[0070] In some embodiments, in the method for preparing the micro-arc oxidation film, the electrolyte temperatures for the first micro-arc oxidation treatment, the second micro-arc oxidation treatment, the third micro-arc oxidation treatment, the first transition treatment, and the second transition treatment in step (2) of S3 are independently set to 20°C to 50°C; and an AC power supply is used to perform micro-arc oxidation in constant current mode. Thus, by using AC constant current mode, the power supply automatically adjusts the voltage to maintain the set current, keeping the discharge energy constant at each stage, avoiding process drift caused by resistance changes, and thereby precisely controlling the growth rate, thickness, and α / γ phase ratio of each film layer.

[0071] In traditional micro-arc oxidation processes, the film generally exhibits a phase distribution pattern of hard inside and soft outside along the growth direction: micro-arc discharge mainly initiates at the interface between the substrate and the initial oxide film. The aluminum alloy substrate, acting as a large heat sink with relatively slow thermal conductivity, allows the internal region (closer to the substrate) to maintain extremely high local temperatures for a long time. This provides sufficient thermodynamic driving force and time for the phase transformation from γ-Al₂O₃ to the high-hardness α-Al₂O₃, thus forming a dense and hard inner layer. Meanwhile, the outermost surface of the coating is always in direct contact with the room-temperature liquid electrolyte. Frequent rapid condensation (quenching) causes the molten alumina to be directly frozen into a metastable, porous, and relatively low-hardness γ-Al₂O₃ structure before it can crystallize and rearrange. Therefore, along the film growth direction, from the inside out, the α-Al₂O₃ content gradually decreases while the γ-Al₂O₃ content gradually increases, resulting in a macroscopically uneven phase distribution, making it difficult to achieve a synergistic match between wear resistance and toughness.

[0072] To address the aforementioned technical challenges, this application breaks through the inherent phase distribution patterns of traditional micro-arc oxidation. Through a layered phase control strategy, it achieves spatially customized arrangement of α-Al₂O₃ and γ-Al₂O₃ content. The specific mechanism is as follows: The first film layer (inner hard layer) still follows the traditional high-temperature sintering mechanism, forming a dense α-Al₂O₃-rich hard layer at the substrate interface through the heat sink effect of the substrate, ensuring a high-strength metallurgical bond with the substrate. The second film layer (middle soft layer) introduces phosphate (PO₄²⁻)... 3- As a potent phase transition inhibitor, it significantly increases the activation energy of the transformation from γ-Al₂O₃ to α-Al₂O₃ at the chemical level. Simultaneously, the coarse porous structure formed by constant-voltage high-energy discharge and the resulting rapid electrolyte backflow quenching effect firmly lock the crystal phase in a high-content energy-absorbing γ-Al₂O₃ state (85%~95%). The α-Al₂O₃ content in this layer is actively suppressed, thus constructing a highly toughened layer. The third film layer (outer hard layer) is achieved by increasing the breakdown voltage limit to 520V and adding glycerol. The high viscosity of glycerol forms a robust viscous thermal shielding boundary layer on the anode surface, effectively blocking the direct quenching effect of external cold water. Simultaneously, under the dielectric guidance of the underlying highly insulating film layer, the extremely high-energy plasma spark is forced to re-ignite violently on the outermost surface. This artificially created reverse surface ultra-high temperature sintering not only induces a violent γ→α phase transformation in the local surface layer (resulting in an α-Al2O3 content of 40%~60%), but also protects the internal soft phase structure from high-pressure damage due to the aerogel-level thermal insulation effect of the porous soft layer in the middle. Through the synergistic regulation of the above three layers, this application overcomes the inherent defects of the monotonically decreasing α-Al2O3 content along the growth direction and large fluctuations at both ends in traditional micro-arc oxidation films. It achieves a customized phase distribution with a bottom layer rich in α (high adhesion), a middle layer rich in γ (high toughness), and a surface layer rich in α (high wear resistance), solving the technical problem of difficulty in balancing wear resistance and toughness in the prior art, and significantly improving the comprehensive protective performance of the coating under corrosion-wear coupling conditions.

[0073] A third aspect of the present invention provides a micro-arc oxidation film layer prepared according to the preparation method of the second aspect of this application. In the first film layer, the content of α-Al₂O₃ is 40 wt% to 60 wt%; for example, the content of α-Al₂O₃ can be 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, etc. In the second film layer, the content of γ-Al₂O₃ is 85 wt% to 95 wt%; for example, the content of γ-Al₂O₃ can be 85 wt%, 88 wt%, 90 wt%, 93 wt%, 95 wt%, etc. In the third film layer, the content of α-Al₂O₃ is 40 wt% to 60 wt%. For example, the content of α-Al₂O₃ can be 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, etc.

[0074] Optionally, the range of α-Al₂O₃ and γ-Al₂O₃ content in the first, second, and third films is less than 4 wt%. The preparation method of this application can precisely control the content of the target phase in the first, second, and third films. For example, if the α-Al₂O₃ content in the first film is 50 wt%, it means that the α-Al₂O₃ content fluctuates between 48 wt% and 52 wt%, i.e., the minimum content is not less than 48 wt%, the maximum content is not more than 52 wt%, and the range is ≤ 4 wt%.

[0075] It should be noted that in the embodiments of this application, α-rich means that the content of α-Al2O3 in the film layer is more than 40 wt%; γ-rich means that the content of γ-Al2O3 in the film layer is more than 85 wt%.

[0076] The present invention will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way.

[0077] Example 1

[0078] Step 1: Surface Pretreatment The substrate material used in this embodiment is 6061 free-drilling aluminum alloy. First, the aluminum alloy sample was sequentially wet-polished using 200#, 400#, 800#, 1200#, and 2000# silicon carbide sandpaper until the surface was smooth and free of obvious scratches, with a surface roughness Ra ≤ 0.2 μm. Then, the polished sample was placed in anhydrous ethanol and cleaned using an ultrasonic cleaner (100 W) for 10 minutes to remove surface debris, oil, and impurities. After removal, it was rinsed repeatedly three times with deionized water and allowed to air dry naturally in clean air at room temperature for 20 minutes before use.

[0079] Step 2: Preparation of alkaline composite electrolyte The alkaline composite electrolyte comprises the following components and concentrations: sodium hexametaphosphate 24 g / L, sodium silicate 30 g / L, sodium hydroxide 5 g / L, sodium tetraborate 3 g / L, boric acid 4 g / L, and disodium ethylenediaminetetraacetate (EDTA-2Na) 1 g / L. The electrolyte is stirred at 300 r / min for 15 min using a magnetic stirrer to obtain a homogeneous and transparent alkaline composite electrolyte. Corresponding functional additives are added in stages according to the needs of different subsequent stages.

[0080] Step 3: Three-stage time-sequential micro-arc oxidation treatment The reaction was conducted in constant current mode with a fixed electrode spacing of 8 cm and an electrolyte temperature of 35°C. The total reaction time was 60 min, which was divided into three stages with transition treatments between the stages.

[0081] Step 301: First micro-arc oxidation treatment (for forming the first film layer) The initial voltage was 260 V, which was uniformly increased to 380 V at a rate of 10 V / min; the frequency was 1000 Hz, the duty cycle was 15%, and the duration was 12 min, accounting for 20% of the total duration. Na₂WO₄ was added to the alkaline composite electrolyte to a concentration of 10 g / L, serving as the first electrolyte. The reaction generated a dense α-Al₂O₃-rich first film with a thickness of 8 μm, achieving metallurgical bonding with the aluminum alloy substrate.

[0082] First transition processing Voltage 400 V, frequency 500 Hz, duty cycle 30%, duration 2 min; maintain the composition of the first electrolyte.

[0083] Step 302: Second micro-arc oxidation treatment (for forming the second film layer) The reaction was carried out at a constant voltage of 460 V, a frequency of 300 Hz, a duty cycle of 30%, and a duration of 36 min, accounting for 60% of the total duration. Na3PO4 was added to the alkaline composite electrolyte to make its concentration 8 g / L, which served as the second electrolyte; the reaction generated a second γ-Al2O3-rich film with a thickness of 30 μm.

[0084] Second transition processing Voltage 400 V, frequency 500 Hz, duty cycle 30%, duration 2 min. Maintain the composition of the second electrolyte. Further release interlayer stress, providing a smooth, defect-free transition interface for the third micro-arc oxidation process.

[0085] Step 303: Third micro-arc oxidation treatment (for forming the third film layer) The initial voltage was 360 V, which was increased uniformly to 520 V at a rate of 5 V / min; the frequency was 1000 Hz, the duty cycle was 15%; and the reaction time was 12 min, accounting for 20% of the total reaction time. Na₂WO₄ (12 g / L) and glycerol (25 ml / L) were added to the alkaline composite electrolyte as a third electrolyte. The reaction produced a third α-Al₂O₃-rich film with a thickness of 8 μm.

[0086] Step 4, Post-processing After the micro-arc oxidation reaction is completed, the voltage is rapidly reduced to 0 V and the power supply is cut off. The sample is removed and the surface is rinsed three times with deionized water to remove residual electrolyte. Then it is placed in clean air at room temperature to air dry naturally, finally obtaining a hard-soft-hard sandwich-like micro-arc oxidation film with a total thickness of approximately 46 μm.

[0087] Example 2

[0088] Step 1: Surface Pretreatment The substrate material used in this embodiment is 6061 easily drillable aluminum alloy. The pretreatment process is exactly the same as in Example 1: the sample is successively wet-polished with 200#, 400#, 800#, 1200#, and 2000# silicon carbide sandpaper until the surface is smooth, free of obvious scratches, and the surface roughness Ra ≤ 0.2 μm. Then, the polished sample is placed in anhydrous ethanol and cleaned with an ultrasonic cleaner (100 W power) for 10 min to remove surface debris, oil, and impurities. After removal, it is rinsed repeatedly with deionized water 3 times and allowed to air dry naturally in clean air at room temperature for 20 min for later use.

[0089] Step 2: Preparation of alkaline composite electrolyte The alkaline composite electrolyte comprises the following components and concentrations: sodium hexametaphosphate 20 g / L, sodium silicate 25 g / L, sodium hydroxide 3 g / L, sodium tetraborate 2 g / L, boric acid 3.5 g / L, and disodium ethylenediaminetetraacetate (EDTA-2Na) 0.8 g / L. The electrolyte is stirred at 300 r / min for 15 min using a magnetic stirrer to obtain a homogeneous and transparent alkaline composite electrolyte. Corresponding functional additives are added in stages according to the needs of different subsequent stages.

[0090] Step 3: Three-stage time-sequential micro-arc oxidation treatment The process employs an AC constant current mode, with a fixed electrode spacing of 8 cm and an electrolyte temperature of 35 ℃. The total process time is 64 min (including two transition treatments), and it is divided into the following three stages, with transition treatments set between the stages.

[0091] Step 301: First micro-arc oxidation treatment (for forming the first film layer) The initial voltage was 260 V, which was uniformly increased to 380 V at a rate of 10 V / min; the frequency was 1000 Hz, the duty cycle was 15%, and the duration was 10 min. 12 g / L Na₂WO₄ was added to the alkaline composite electrolyte as the first electrolyte. The reaction generated a dense α-Al₂O₃-rich first film with a thickness of 9 μm, achieving metallurgical bonding with the aluminum alloy substrate.

[0092] First transition processing Voltage 400 V, frequency 500 Hz, duty cycle 30%, duration 2 min. Maintain the composition of the first electrolyte.

[0093] Step 302: Second micro-arc oxidation treatment (for forming the second film layer) Duration: 38 min. Constant voltage 460 V, frequency 300 Hz, duty cycle 30%. 10 g / L Na3PO4 was added to the alkaline composite electrolyte as a second electrolyte. The reaction produced a γ-Al2O3-rich second film with a thickness of 34 μm.

[0094] Second transition processing The voltage was 400 V, the frequency was 500 Hz, the duty cycle was 30%, and the duration was 2 min, maintaining the composition of the second electrolyte. This further released interlayer stress, providing a smooth and defect-free transition interface for the third micro-arc oxidation process.

[0095] Step 303: Third micro-arc oxidation treatment (for forming the third film layer) The initial voltage was 360 V, which was increased uniformly to 520 V at a rate of 5 V / min; the frequency was 1000 Hz, the duty cycle was 15%, and the duration was 10 min. 15 g / L Na₂WO₄ and 28 ml / L glycerol were added to the alkaline composite electrolyte as a third electrolyte, and a 9 μm thick α-Al₂O₃-rich third film was formed.

[0096] Step 4, Post-processing After the micro-arc oxidation reaction is completed, the voltage is rapidly reduced to 0 V and the power supply is cut off. The sample is removed and the surface is rinsed three times with deionized water to remove residual electrolyte. Then it is placed in clean air at room temperature to dry naturally, finally obtaining a hard-soft-hard sandwich-like micro-arc oxidation film with a total thickness of 52 μm (9 μm + 34 μm + 9 μm).

[0097] Example 3

[0098] Step 1: Surface Pretreatment The substrate material used in this embodiment is 7075 easily drillable aluminum alloy. The pretreatment process is the same as in Example 1: the sample is successively wet-polished with 200#, 400#, 800#, 1200#, and 2000# silicon carbide sandpaper until the surface is smooth, free of obvious scratches, and the surface roughness Ra ≤ 0.2 μm. Then, the polished sample is placed in anhydrous ethanol and cleaned with an ultrasonic cleaner (100 W power) for 10 min to remove surface debris, oil, and impurities. After removal, it is rinsed repeatedly with deionized water 3 times and allowed to air dry naturally in clean air at room temperature for 20 min for later use.

[0099] Step 2: Preparation of alkaline composite electrolyte The alkaline composite electrolyte comprises the following components and concentrations: sodium hexametaphosphate 28 g / L, sodium silicate 30 g / L, sodium hydroxide 5 g / L, sodium tetraborate 3 g / L, boric acid 4 g / L, and disodium ethylenediaminetetraacetate (EDTA-2Na) 1 g / L. The electrolyte is stirred at 300 r / min for 15 min using a magnetic stirrer to obtain a homogeneous and transparent alkaline composite electrolyte. Corresponding functional additives are added in stages according to the needs of different subsequent stages.

[0100] Step 3: Three-stage time-sequential micro-arc oxidation treatment The process employs an AC constant current mode, with a fixed electrode spacing of 8 cm and an electrolyte temperature of 35℃. The total process time is 66 min (including two transition treatments), and it is divided into the following three stages, with transition treatments set between the stages.

[0101] Step 301: First micro-arc oxidation treatment (for forming the first film layer) Duration: 12 min. Initial voltage: 260 V, increased uniformly to 380 V at a rate of 10 V / min; frequency: 1000 Hz, duty cycle: 15%. 15 g / L Na₂WO₄ was added to the alkaline composite electrolyte as the first electrolyte. The reaction generated a dense α-Al₂O₃-rich first film with a thickness of 12 μm, achieving metallurgical bonding with the aluminum alloy substrate.

[0102] First transition processing Voltage 400 V, frequency 500 Hz, duty cycle 30%, duration 2 min. Maintain the composition of the first electrolyte.

[0103] Step 302: Second micro-arc oxidation treatment (for forming the second film layer) The reaction was carried out at a constant voltage of 460 V, a frequency of 300 Hz, a duty cycle of 30%, and a duration of 36 min. 12 g / L Na3PO4 was added to the alkaline composite electrolyte as a second electrolyte. The reaction produced a γ-Al2O3-rich second film with a thickness of 36 μm.

[0104] Second transition processing Voltage 400 V, frequency 500 Hz, duty cycle 30%, duration 2 min. Maintain the composition of the second electrolyte. Further release interlayer stress, providing a smooth, defect-free transition interface for the third micro-arc oxidation process.

[0105] Step 303: Third micro-arc oxidation treatment (for forming the third film layer) The initial voltage was 360 V, which was increased uniformly to 520 V at a rate of 5 V / min; the frequency was 1000 Hz, the duty cycle was 15%, and the reaction time was 12 min. 20 g / L Na₂WO₄ and 30 ml / L glycerol were added to the alkaline composite electrolyte as a third electrolyte. The reaction produced a 12 μm thick α-Al₂O₃-rich third film.

[0106] Step 4, Post-processing After the micro-arc oxidation reaction is completed, the voltage is rapidly reduced to 0 V and the power supply is cut off. The sample is removed and the surface is rinsed three times with deionized water to remove residual electrolyte. Then it is placed in clean air at room temperature to dry naturally, finally obtaining a hard-soft-hard sandwich-like micro-arc oxidation film with a total thickness of 60 μm (12 μm + 36 μm + 12 μm).

[0107] Example 4

[0108] Step 1: Surface Pretreatment The substrate material used in this embodiment is 7075 easily drillable aluminum alloy. The pretreatment process is exactly the same as in Example 1: the sample is successively wet-polished with 200#, 400#, 800#, 1200#, and 2000# silicon carbide sandpaper until the surface is smooth, free of obvious scratches, and the surface roughness Ra ≤ 0.2 μm. Then, the polished sample is placed in anhydrous ethanol and cleaned with an ultrasonic cleaner (100 W power) for 10 min to remove surface debris, oil, and impurities. After removal, it is rinsed repeatedly with deionized water 3 times and allowed to air dry naturally in clean air at room temperature for 20 min for later use.

[0109] Step 2: Preparation of alkaline composite electrolyte The alkaline composite electrolyte comprises the following components and concentrations: sodium hexametaphosphate 24 g / L, sodium silicate 28 g / L, sodium hydroxide 5 g / L, sodium tetraborate 3 g / L, boric acid 4 g / L, and disodium ethylenediaminetetraacetate (EDTA-2Na) 1 g / L. The electrolyte is stirred at 300 r / min for 15 min using a magnetic stirrer to obtain a homogeneous and transparent alkaline composite electrolyte. Corresponding functional additives are added in stages according to the needs of different subsequent stages.

[0110] Step 3: Three-stage time-sequential micro-arc oxidation treatment The process employed an AC constant current mode, with a fixed electrode spacing of 8 cm and an electrolyte temperature of 35 ℃. The total process time was 60 min (including two transition treatments, each 2 min). The durations of the first, second, and third micro-arc oxidation treatments were 12 min, 36 min, and 12 min, respectively, accounting for 20%, 60%, and 20% of the total time, as detailed below: Step 301: First micro-arc oxidation treatment (for forming the first film layer) The initial voltage was 260V, which was uniformly increased to 380V at a rate of 10V / min; the frequency was 1000 Hz, the duty cycle was 15%, and the duration was 12 min. Na₂WO₄ at a concentration of 12 g / L was added to the alkaline composite electrolyte as the first electrolyte. The reaction generated a dense α-Al₂O₃-rich first film with a thickness of 9 μm, achieving metallurgical bonding with the aluminum alloy substrate.

[0111] First transition processing Voltage 400 V, frequency 500 Hz, duty cycle 30%, duration 2 min. Maintains the composition of the first electrolyte. Used to release interfacial stress generated during the growth of the first film layer.

[0112] Step 302: Second micro-arc oxidation treatment (for forming the second film layer) The reaction was carried out at a constant voltage of 460V, a frequency of 300 Hz, a duty cycle of 30%, and a duration of 36 min. 10 g / L Na3PO4 was added to the alkaline composite electrolyte as a second electrolyte. The reaction produced a γ-Al2O3-rich second film with a thickness of 32 μm.

[0113] Second transition processing Voltage 400 V, frequency 500 Hz, duty cycle 30%, duration 2 min. Maintain the composition of the second electrolyte. Further release interlayer stress.

[0114] Step 303: Third micro-arc oxidation treatment (for forming the third film layer) The initial voltage was 360 V, which was increased uniformly to 520 V at a rate of 5 V / min; the frequency was 1000 Hz, the duty cycle was 15%, and the reaction time was 12 min. A third electrolyte was prepared by adding 15 g / L Na₂WO₄ and 28 ml / L glycerol to the alkaline composite electrolyte. The reaction produced a 9 μm thick α-Al₂O₃-rich third film.

[0115] Step 4, Post-processing After the micro-arc oxidation reaction is completed, the voltage is rapidly reduced to 0 V and the power supply is cut off. The sample is removed, and the surface is rinsed three times with deionized water to remove residual electrolyte. It is then placed in clean air at room temperature to air dry naturally, finally obtaining a hard-soft-hard sandwich-like micro-arc oxidation film with a total thickness of 50 μm (9 μm + 32 μm + 9 μm).

[0116] Comparative Example 1 The other steps are the same as in Example 1, except that in step 3, step 302, the second micro-arc oxidation treatment directly uses the alkaline composite electrolyte as the second electrolyte, as detailed below:

[0117] Step 302: Second micro-arc oxidation treatment (for forming the second film layer) The reaction was carried out at a constant voltage of 460 V, a frequency of 300 Hz, a duty cycle of 30%, and a duration of 36 min, accounting for 60% of the total duration. The alkaline composite electrolyte was used as the second electrolyte to generate a γ-Al2O3-rich second film with a thickness of 30 μm.

[0118] Second transition processing Voltage 400 V, frequency 500 Hz, duty cycle 30%, duration 2 min. Maintain the composition of the second electrolyte. Further release interlayer stress, providing a smooth, defect-free transition interface for the third micro-arc oxidation process.

[0119] Comparative Example 2 The other steps are the same as in Example 1, except that in step 303, the third micro-arc oxidation treatment directly uses the alkaline composite electrolyte as the third electrolyte, as detailed below:

[0120] Step 303: Third micro-arc oxidation treatment (for forming the third film layer) The initial voltage was 360 V, which was increased uniformly to 520 V at a rate of 5 V / min; the frequency was 1000 Hz, the duty cycle was 15%; and the duration was 12 min, accounting for 20% of the total duration. An alkaline composite electrolyte was used as the third electrolyte. The reaction produced a third α-Al₂O₃-rich film with a thickness of 8 μm.

[0121] Comparative Example 3 The other steps are the same as in Example 1, except that the process parameters for the third micro-arc oxidation treatment in step 303 of step 3 are as follows: Step 303: Third micro-arc oxidation treatment (for forming the third film layer) The reaction was conducted at a constant voltage of 360 V, a frequency of 1000 Hz, and a duty cycle of 15%. The reaction duration was 12 min, accounting for 20% of the total reaction time. 12 g / L Na₂WO₄ and 25 ml / L glycerol were added to the alkaline composite electrolyte to form a third electrolyte. A third α-Al₂O₃-rich film with a thickness of 8 μm was formed.

[0122] Comparative Example 4 The other steps in Comparative Example 4 are the same as in Example 1, except for step 3, which is as follows: Step 3: Micro-arc oxidation treatment The pretreated aluminum alloy sample was used as the anode, and the 304 stainless steel plate was used as the cathode, with the electrode spacing controlled at 8 cm. The electrode assembly was immersed in the above-mentioned alkaline composite electrolyte, and the electrolyte temperature was controlled to be constant at 30℃. The AC constant current mode was adopted, and fixed process parameters were set: voltage 500 V, frequency 500 Hz, duty cycle 30%, and continuous micro-arc oxidation reaction for 60 min. The electrolyte was continuously stirred during the reaction to ensure uniform temperature and concentration.

[0123] [Test Example]

[0124] I. Interface Bond Strength Test

[0125] Test method: The scratch test method was adopted, and the test was conducted in accordance with GB / T 30707-2014 "Test method for adhesion of fine ceramic coatings - scratch method".

[0126] Test equipment: WS-2005 scratch tester (or similar model), equipped with an acoustic emission signal acquisition system.

[0127] Test conditions: diamond indenter (cone angle 120°, tip radius 0.2 mm); loading rate 20 N / min; scratch length 5 mm; maximum load 100 N.

[0128] Test Procedure: The coating samples prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were fixed on the test platform. The indenter slid uniformly across the coating surface and the load was continuously increased, while acoustic emission signals and friction force signals were collected simultaneously. The load at which the coating begins to peel off from the substrate is the critical load Lc, representing the interfacial bonding strength of the coating.

[0129] II. Film Hardness Test

[0130] Test method: Vickers microhardness test was conducted in accordance with GB / T 4340.1-2009 "Metallic materials Vickers hardness test - Part 1: Test method".

[0131] Testing equipment: HVS-1000 digital display microhardness tester.

[0132] Test conditions: test force 50 gf (0.49 N); holding time 15 s; the coating section (after inlaying, grinding and polishing) was tested, and points were marked from the outer surface to the inner layer, with a measurement point every 5 μm.

[0133] Test procedure: The Vickers hardness values ​​of the third film layer (outer layer), the second film layer (middle layer) and the first film layer (inner layer) were measured respectively, and the average value was calculated from 5 points in each region.

[0134] III. Electrochemical Corrosion Resistance Test

[0135] Test method: The Tafel curve method was adopted, referring to GB / T 24196-2009 "Electrochemical test methods for corrosion of metals and alloys - Guidelines for potentiostatic and potentiodynamic polarization measurement".

[0136] Test equipment: CHI660E electrochemical workstation (or similar model), three-electrode system: the coated sample is the working electrode (exposed area 1 cm²). 2 The saturated calomel electrode (SCE) is used as the reference electrode, and the platinum sheet is used as the auxiliary electrode.

[0137] Test conditions: The corrosive medium was 3.5 wt% NaCl solution (simulating seawater / oilfield brine), the solution temperature was 25±1℃, and the solution was stabilized at open circuit potential for 30 min before the test. The scanning potential range was -0.5 V ~ +1.0 V (relative to open circuit potential), and the scanning rate was 0.5 mV / s.

[0138] Test procedure: The polarization curves of Examples 1 to 4 and Comparative Examples 1 to 4 (traditional single MAO coating) were tested respectively. The self-corrosion potential (Ecorr) and self-corrosion current density (icorr) were obtained by Tafel extrapolation, and the polarization resistance and corrosion rate were calculated.

[0139] IV. Corrosion and Wear Performance Testing

[0140] Test method: The ball-disc pin-disc friction and wear test method is adopted.

[0141] Test equipment: MMQ-02 pin-disc friction and wear test bench.

[0142] Test conditions: The grinding material was a 6 mm diameter SiC ceramic ball; the constant load was 10 N, the rotation speed was 200 r / min, the friction diameter was 6 mm, and the wear time lasted for 20 minutes; the ambient temperature was 25±2℃, and the relative humidity was 50±5%. Corrosive medium: simulated oilfield produced fluid (containing 50,000 ppm Cl⁻, pH 6.5, temperature 60℃, 0.2 g / L KCl, 39.0 g / L NaCl, 6.8 g / L CaCl₂, 3.6 g / L MgCl₂·6H₂O, 0.2 g / L Na₂SO₄, 0.5 g / L NaHCO₃).

[0143] Test procedure: The coefficient of friction in the corrosive media of Examples 1-4 and Comparative Examples 1-4 (traditional single MAO coating) was tested respectively.

[0144] V. The relative contents of α-Al₂O₃ and γ-Al₂O₃ in the first, second, and third films were determined using X-ray diffraction (XRD) combined with Relative Infrared Spectrum (RIR) quantitative analysis. The testing equipment was a Bruker D8 Advance X-ray diffractometer (or similar equipment). The testing conditions were: Cu target Kα radiation (λ = 0.15406 nm), tube voltage 40 kV, tube current 40 mA, scanning range 2θ = 10°~90°, scanning step size 0.02°, and scanning speed 2° / min. Layer-by-layer testing was performed on the coating samples prepared in Examples 1-4 and Comparative Examples 1-4: a layer-by-layer grinding method was used (removing approximately 5-10 μm of thickness each time) to expose the target layer, followed by XRD scanning. MDI Jade 6.0 software was used for phase retrieval of the diffraction patterns, and the relative mass fractions of the α-Al₂O₃ and γ-Al₂O₃ phases were calculated using the RIR method. Three different locations were tested for each sample, and the average value was taken. The specific test results are shown in Table 1.

[0145] Table 1

[0146] Continued from Table 1

[0147] Note: Comparative Example 4 in Table 1 is a single film layer, where the α-Al2O3 content is the average content of the entire film layer, and the thickness is the overall thickness of the film layer.

[0148] The hard-soft-hard sandwich structure micro-arc oxidation films prepared in Examples 1-4 allow for precise control of the phase composition and thickness distribution of the film. The α-Al2O3 content in the first and third layers is stably controlled at 40wt%~60wt%, while the γ-Al2O3 content in the second layer reaches 85wt%~95wt%. The thickness ratio of the three layers meets the gradient design requirement of (3~4):(11~14):(3~4). Mechanical property test results show that the interfacial bonding strength between the film and the aluminum alloy substrate in Examples 1-4 is 82 N~87 N, and the surface microhardness can reach up to 2068 HV. In contrast, Comparative Example 4 uses a traditional single micro-arc oxidation process, resulting in a film without a gradient phase structure, disordered α-Al2O3 distribution, an interfacial bonding strength of only 79 MPa, and a surface hardness as low as 831 HV. Comparative Examples 1-3, due to the lack of key electrolyte additives or process parameter optimization, show a significant decrease in surface hardness and interlayer bonding stability. The above data demonstrates that this invention, through the synergistic regulation of phased time-series electrical parameters and stepped electrolyte components, combined with inter-stage transition treatment, can effectively release interfacial stress, enhance interlayer bonding strength, and construct a mechanical gradient structure with high bonding strength in the bottom layer, high toughness in the middle layer, and high hardness in the surface layer. This fundamentally solves the technical defects of traditional micro-arc oxidation films, which are hard and brittle and have insufficient bonding strength.

[0149] The corrosion resistance and tribological wear performance test results showed that the self-corrosion current density of the films in Examples 1-4 was as low as 2.550 × 10⁻⁶. -7 ~3.701×10 -7 A / cm 2 The self-corrosion potential shifts significantly to the positive value, with an average friction coefficient of only 0.190~0.210. Corrosion resistance is improved by an order of magnitude compared to traditional films, and tribological properties are greatly optimized. In Comparative Example 4, the self-corrosion current density of a single film reaches as high as 7.107 × 10⁻⁶. -6 A / cm 2 The coefficient of friction reached 0.334, and the corrosion resistance and wear resistance were far inferior to those of Examples 1-4. Comparative Examples 1-3, due to the incomplete construction of the sandwich phase structure, showed varying degrees of attenuation in both corrosion resistance and wear resistance. These results fully demonstrate that the hard-soft-hard gradient structure of this invention can simultaneously achieve a synergistic improvement in corrosion resistance, wear resistance, and impact resistance, effectively solving the industry problem of brittle peeling of the protective film and the trade-off in performance of easily drillable aluminum alloys under extreme working conditions, and significantly improving the long-term service stability of aluminum alloy pipes used in oil fields.

[0150] In the description of this specification, references to the terms "an embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0151] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A sandwich-structured micro-arc oxidation film, characterized in that, The sandwich-structured micro-arc oxidation film layer includes a first film layer, a second film layer, and a third film layer stacked sequentially. The first film layer, the second film layer, and the third film layer each independently comprise γ-Al2O3 and α-Al2O3; In the first film layer, the content of α-Al2O3 is 40 wt% to 60 wt%; In the second film layer, the content of γ-Al2O3 is 85 wt% to 95 wt%; In the third film layer, the content of α-Al2O3 is 40 wt% to 60 wt%; The ratio of the thickness of the first film layer, the thickness of the second film layer and the thickness of the third film layer is (3~4):(11~14):(3~4).

2. The sandwich-structured micro-arc oxidation film layer according to claim 1, characterized in that, The sandwich-structured micro-arc oxidation film layer uses an aluminum alloy as the substrate, and the aluminum alloy includes at least one of 2-series aluminum alloys, 5-series aluminum alloys, 6-series aluminum alloys, and 7-series aluminum alloys.

3. A method for preparing a sandwich-structured micro-arc oxidation film, characterized in that, Includes the following steps: (1) Provide an aluminum alloy substrate; Prepare an alkaline composite electrolyte containing phosphates, silicates, alkaline regulators, buffers, and complexing agents; Na2WO4 is added to the alkaline composite electrolyte to form a first electrolyte; Na3PO4 is added to the alkaline composite electrolyte to serve as a second electrolyte; Na2WO4 and glycerol are added to the alkaline composite electrolyte to form a third electrolyte; (2) Performing a time-sequential micro-arc oxidation process Using the aluminum alloy substrate as the anode and the stainless steel plate as the cathode, a first micro-arc oxidation treatment is performed sequentially in the first electrolyte, a second micro-arc oxidation treatment is performed in the second electrolyte, and a third micro-arc oxidation treatment is performed in the third electrolyte, so as to form a first film layer, a second film layer and a third film layer stacked sequentially on the aluminum alloy substrate. The first micro-arc oxidation process includes: an initial voltage of 260 V to 270 V, increasing the initial voltage to 380 V to 390 V, a frequency of 900 Hz to 1000 Hz, and a duty cycle of 13% to 15%; and / or The second micro-arc oxidation process includes: a constant voltage of 460 V~470 V, a frequency of 200 Hz~300 Hz, and a duty cycle of 30%~35%; and / or The third micro-arc oxidation process includes: an initial voltage of 360 V to 370 V, which is then increased to 520 V to 530 V, a frequency of 900 Hz to 1000 Hz, and a duty cycle of 13% to 15%.

4. The method for preparing the sandwich-structured micro-arc oxidation film layer according to claim 3, characterized in that, In the first electrolyte, the concentration of Na2WO4 is 10 g / L to 15 g / L; and / or In the second electrolyte, the concentration of Na3PO4 is 8 g / L to 12 g / L; and / or In the third electrolyte, the concentration of Na2WO4 is 12 g / L to 20 g / L, and the concentration of glycerol is 25 ml / L to 30 ml / L.

5. The method for preparing the sandwich-structured micro-arc oxidation film layer according to claim 3, characterized in that, Step (2) also includes a first transition process and a second transition process; The first transition treatment is to continue micro-arc oxidation treatment in the first electrolyte for 2 min to 5 min after the first micro-arc oxidation treatment is completed, with a voltage of 400 V to 410 V, a frequency of 490 Hz to 510 Hz, and a duty cycle of 30% to 35%. The second transition treatment is to continue micro-arc oxidation treatment in the second electrolyte for 2 min to 5 min after the second micro-arc oxidation treatment is completed, with a voltage of 400 V to 410 V, a frequency of 490 Hz to 510 Hz, and a duty cycle of 30% to 35%.

6. The method for preparing the sandwich-structured micro-arc oxidation film layer according to claim 3, characterized in that, In the alkaline composite electrolyte, the phosphate includes sodium hexametaphosphate, the silicate includes sodium silicate, the alkalinity regulator includes sodium hydroxide, the buffer includes sodium tetraborate and boric acid, and the complexing agent includes disodium ethylenediaminetetraacetate. In the alkaline composite electrolyte, the concentration of sodium hexametaphosphate is 20 g / L to 28 g / L; In the alkaline composite electrolyte, the concentration of sodium silicate is 20 g / L to 30 g / L; In the alkaline composite electrolyte, the concentration of sodium hydroxide is 1 g / L to 5 g / L; In the alkaline composite electrolyte, the concentration of sodium tetraborate is 1 g / L to 3 g / L, and the concentration of boric acid is 3 g / L to 4 g / L. In the alkaline composite electrolyte, the concentration of disodium ethylenediaminetetraacetate is 0.5 g / L to 1 g / L.

7. The method for preparing the sandwich-structured micro-arc oxidation film layer according to claim 5, characterized in that, In step (2), the electrolyte temperature for the first micro-arc oxidation treatment, the second micro-arc oxidation treatment, the third micro-arc oxidation treatment, the first transition treatment, and the second transition treatment is 20℃~50℃; and all micro-arc oxidation is performed using an AC power supply in constant current mode.

8. The method for preparing the sandwich-structured micro-arc oxidation film layer according to claim 3, characterized in that, The aluminum alloy matrix includes at least one of 2-series aluminum alloys, 5-series aluminum alloys, 6-series aluminum alloys, and 7-series aluminum alloys.

9. A sandwich-structured micro-arc oxidation film layer prepared by the preparation method according to any one of claims 3 to 8, characterized in that, In the first film layer, the content of α-Al2O3 is 40 wt% to 60 wt%; In the second film layer, the content of γ-Al2O3 is 85 wt% to 95 wt%; In the third film layer, the content of α-Al2O3 is 40 wt% to 60 wt%.

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