Aluminum alloy surface impact-resistant white ceramic layer, preparation method and application thereof

Abstract

The present application relates to the technical field of micro-arc oxidation, and particularly relates to an aluminum alloy surface anti-impact white ceramic layer, a preparation method and application thereof. The method comprises the following steps: taking an aluminum alloy with a ZrO2 / Al2O3 porous ceramic layer as an anode, connecting the anode with a positive electrode of a bipolar pulse power supply, and performing micro-arc oxidation treatment in a soft spark discharge mode in an electrolyte without a whitening promoter to obtain an aluminum alloy surface anti-impact white ceramic layer. The present application improves the density of the anti-impact white ceramic layer and the anti-impact white ceramic layer / aluminum alloy substrate interface bonding strength through the synergistic effect of the soft spark discharge mode induced coating densification reconstruction and the interface electric field driven oxygen penetration, and at the same time, the anti-impact white ceramic layer is endowed with excellent anti-impact performance while maintaining high whiteness, thereby meeting the dual requirements of aesthetics and reliability of 3C electronic product shells.

Description

Technical Field

[0001] This invention relates to the field of micro-arc oxidation technology, specifically to an impact-resistant white ceramic layer on aluminum alloy surfaces, its preparation method, and its application. Background Technology

[0002] Aluminum alloys possess a distinct metallic texture, and compared to the dull visual effect of stainless steel, they are easier to precision-machine into high-end, aesthetically pleasing, and brightly glossy appearances. Furthermore, aluminum alloys offer numerous advantages, including high specific strength, excellent heat dissipation, fingerprint resistance, and antistatic properties, making them widely used in the manufacture of casings for 3C electronic products (including communication equipment, computers, and consumer electronics). However, aluminum itself has a very low standard electrode potential (-1.66V), making it a highly reactive metal, and aluminum alloys have low hardness, typically exhibiting poor wear resistance in engineering applications.

[0003] To improve surface properties, existing technologies typically employ micro-arc oxidation and anodic oxidation to modify aluminum alloy surfaces. Micro-arc oxidation, in particular, utilizes a high-voltage electric field to induce plasma discharge on the aluminum alloy surface, generating 10... 3 K~10 4 K's high-temperature ionization heat and 10 2 MPa~10 3 The shock wave pressure of MPa induces the formation of a ceramic layer mainly composed of Al2O3 on the surface of the aluminum alloy. Based on the principle of micro-arc in-situ oxidation with coloring technology, a ZrO2 / Al2O3 composite coating is constructed on the surface of the aluminum alloy by introducing zirconium salt as a whitening accelerator into the electrolyte, thereby significantly improving the whiteness performance of the micro-arc oxidation coating.

[0004] However, in existing micro-arc oxidation processes, the whiteness of the composite coating gradually increases with oxidation time. This thickening essentially relies on repeated breakdowns of the original film layer to achieve continuous oxidation of the inner layer. Each breakdown leaves discharge micropores in the film layer, and these micropores gradually increase in size with the accumulation of breakdowns. Therefore, when the oxidation time reaches the target whiteness, a large number of micron-sized defects inevitably exist inside the composite coating, severely weakening its density and impact resistance.

[0005] Furthermore, the ZrO2 / Al2O3 composite coating prepared using conventional micro-arc oxidation processes exhibits primarily external growth behavior relative to the original aluminum alloy substrate surface, resulting in a limited thickness of the dense inner layer. Under sudden impact or mechanical shock, this composite coating is prone to large-area peeling, making it difficult to meet the practical application requirements of 3C electronic product casings. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides an impact-resistant white ceramic layer for aluminum alloy surfaces, its preparation method, and its application. This invention uses an aluminum alloy with a ZrO2 / Al2O3 porous ceramic layer as the anode. The anode is connected to the positive terminal of a bipolar pulsed power supply. Micro-arc oxidation is performed in an electrolyte under a soft spark discharge mode. This process induces densification and reconstruction of the ZrO2 / Al2O3 porous ceramic layer in the aluminum alloy, simultaneously driving oxygen ions to penetrate the interface between the porous ceramic layer and the aluminum alloy substrate and diffuse into the aluminum alloy substrate, forming a dense alumina layer, thus obtaining an impact-resistant white ceramic layer on the aluminum alloy surface. The preparation method of this invention simultaneously achieves densification and reconstruction of the ZrO2 / Al2O3 porous ceramic layer and interfacial electric field-driven oxygen infiltration. On the one hand, the low energy and high uniformity characteristics of soft spark discharge significantly reduce the original micropore size and increase the density of the porous ceramic layer; on the other hand, the high impedance layer induces an inter-electrode field strength to drive oxygen infiltration. 2- By penetrating the grain boundaries of the aluminum matrix, a dense, non-porous Al2O3 inner layer is grown in situ beneath the porous ZrO2 / Al2O3 ceramic layer, constructing a gradient bilayer structure of "outer layer densification and inner layer thickening." This structure fundamentally improves the stress distribution and energy dissipation capacity of the coating, significantly enhances the interfacial bonding strength between the impact-resistant white ceramic layer and the aluminum alloy substrate, and improves its impact resistance. It effectively solves the current technical challenge of achieving both high whiteness and impact resistance in white ceramic coatings on aluminum alloys used in 3C electronic product casings.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The first objective of this invention is to provide a method for preparing an impact-resistant white ceramic layer on an aluminum alloy surface, comprising the following steps: Using an aluminum alloy with a ZrO2 / Al2O3 porous ceramic layer as the anode, the anode is connected to the positive terminal of a bipolar pulsed power supply. Micro-arc oxidation is performed in a soft spark mode in an electrolyte without whitening accelerators. This process causes densification and reconstruction of the ZrO2 / Al2O3 porous ceramic layer, reducing the pore size of the microstructure. Simultaneously, oxygen ions are driven to pass through the interface between the ZrO2 / Al2O3 porous ceramic layer and the aluminum alloy substrate, diffusing into the aluminum alloy substrate. This electric field-driven oxygen infiltration process at the interface between the porous ceramic layer and the aluminum alloy substrate forms a dense alumina layer, resulting in an impact-resistant white ceramic layer on the aluminum alloy surface. During the densification and reconstruction process, the micropore size in the ZrO2 / Al2O3 porous ceramic is significantly reduced, and the density is increased. Typically, a bipolar power supply is required to induce a soft spark and initiate this densification and reconstruction process.

[0008] In an optional embodiment, the aluminum alloy with a ZrO2 / Al2O3 porous ceramic layer has a whiteness of 86-96 and a thickness of 15μm-80μm. Whiteness increases with thickness; when the whiteness reaches 89 or higher, the micro-arc oxidation coating exhibits an excellent white appearance. However, the upper limit of the thickness needs to be controlled to avoid a decrease in the impact resistance of the impact-resistant white ceramic layer due to excessive thickness of the micro-arc oxidation coating.

[0009] In an optional embodiment, the micro-arc oxidation treatment conditions for preparing the impact-resistant white ceramic layer on the aluminum alloy surface are: a forward output current density of 1 A / dm³. 2 ~15A / dm 2 The ratio R of negative output current density to positive output current density is 1.1~1.8, the output frequency is 100Hz~2000Hz, the duty cycle is 2%~20%, the processing time is 3min~20min, and the electrolyte temperature without whitening accelerator is 10℃~40℃. This parameter range aims to ensure a stable transition to the soft spark discharge mode while preventing energy overload that could lead to ablation or porosity of the impact-resistant white ceramic layer, thereby balancing the growth rate, density, and surface quality of the impact-resistant white ceramic layer by precisely controlling the energy input and electrolyte temperature.

[0010] In an optional embodiment, the electrolyte without whitening accelerator is composed of a main salt, a pH adjuster, an additive, and water, wherein the main salt is selected from at least one of sodium silicate, sodium dihydrogen phosphate, sodium hexametaphosphate, sodium pyrophosphate, or sodium hypophosphite; the pH adjuster is selected from sodium hydroxide or potassium hydroxide; and the additive is selected from at least one of disodium ethylenediaminetetraacetate, potassium citrate, potassium sodium tartrate, or ethylene glycol.

[0011] In the electrolyte without whitening accelerator, the mass concentration of the main salt is 10 g / L to 30 g / L, the mass concentration of the pH adjuster is 2 g / L to 10 g / L, and the mass concentration of the additive is 2 g / L to 10 g / L.

[0012] In an optional implementation, the micro-arc oxidation power source is selected from a unipolar pulse power source or a bipolar pulse power source.

[0013] In an optional embodiment, the aluminum alloy having a ZrO2 / Al2O3 porous ceramic layer is prepared according to the following steps: An aluminum alloy substrate is used as the anode and connected to the positive electrode of a micro-arc oxidation power supply. Micro-arc oxidation is performed in an electrolyte containing zirconium salt to grow a porous ceramic layer with ZrO2 / Al2O3 as the main phase in situ on the surface of the aluminum alloy, thus obtaining an aluminum alloy with a ZrO2 / Al2O3 porous ceramic layer.

[0014] The zirconium-containing electrolyte is composed of a main salt, a zirconium-containing ion salt, an auxiliary solubilizer, and water. The main salt has a mass concentration of 10 g / L to 40 g / L, the whitening accelerator zirconium-containing ion salt has a mass concentration of 5 g / L to 30 g / L, and the auxiliary solubilizer has a mass concentration of 2 g / L to 15 g / L. The main salt is selected from at least one of sodium silicate, sodium dihydrogen phosphate, sodium hexametaphosphate, sodium pyrophosphate, or sodium hypophosphite. The auxiliary solubilizer is selected from at least one of oxalic acid, cinnamic acid, citric acid, or phosphoric acid. The auxiliary solubilizer is used to improve the solubility of the zirconium-containing ion salt in the electrolyte.

[0015] In an optional embodiment, when preparing the aluminum alloy with a ZrO2 / Al2O3 porous ceramic layer, the micro-arc oxidation treatment conditions are: the electrolyte temperature containing zirconium salt is 10℃~40℃; the forward output current density is 1A / dm³. 2 ~15A / dm 2 The output frequency is 100Hz~2000Hz, the duty cycle is 2%~20%, and the processing time is 10min~60min. This parameter range is designed to balance the growth rate, density, and surface quality of the micro-arc oxidation coating by precisely controlling the energy input and electrolyte temperature, ensuring stable discharge while preventing energy overload that could lead to ablation or porosity of the coating.

[0016] In an optional embodiment, when preparing the aluminum alloy with the ZrO2 / Al2O3 porous ceramic layer, the aluminum alloy substrate is also pretreated. The pretreatment operation is as follows: the aluminum alloy substrate is polished, degreased, alkaline washed, acid washed, cleaned to remove surface oil, and dried for later use.

[0017] A second objective of this invention is to provide an impact-resistant white ceramic layer on an aluminum alloy surface prepared by the above-described method. The impact-resistant white ceramic layer comprises an aluminum alloy substrate and an impact-resistant white ceramic layer on the surface of the aluminum alloy substrate; the impact-resistant white ceramic layer has a double-layer structure, with an inner layer being a dense alumina layer and an outer layer being a densified and reconstructed ZrO2 / Al2O3 layer.

[0018] In an optional embodiment, the whiteness of the impact-resistant white ceramic layer is 86~96 and the thickness is 20μm~110μm; a thickness lower or higher than this range will affect the whiteness of the impact-resistant white ceramic layer.

[0019] A third objective of this invention is to provide the application of the aforementioned impact-resistant white ceramic layer in the surface protection of 3C electronic product casings.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention provides a method for preparing an impact-resistant white ceramic layer on an aluminum alloy surface. Using an aluminum alloy with a ZrO2 / Al2O3 porous ceramic layer as the anode, the anode is connected to the positive terminal of a bipolar pulse power supply. Micro-arc oxidation is performed in an electrolyte without whitening accelerators under soft spark mode, causing densification and reconstruction of the ZrO2 / Al2O3 porous ceramic layer. This reduces the pore size of the ZrO2 / Al2O3 porous ceramic layer and simultaneously drives oxygen ions to pass through the interface between the ZrO2 / Al2O3 porous ceramic layer and the aluminum alloy substrate, diffusing into the aluminum alloy substrate. This achieves an electric field-driven oxygen infiltration process at the interface between the porous ceramic layer and the aluminum alloy substrate, forming a dense alumina layer, thus obtaining an impact-resistant white ceramic layer on the aluminum alloy surface.

[0021] This invention utilizes a soft spark discharge mode for micro-arc oxidation, simultaneously achieving densification and reconstruction of micropore defects within the ZrO2 / Al2O3 porous ceramic layer, and directional penetration of oxygen ions into the deep layers of the aluminum alloy substrate. This results in the growth of a dense alumina layer beneath the ZrO2 / Al2O3 porous ceramic layer. This dual mechanism of "structural optimization and interface strengthening" significantly enhances the bonding strength between the impact-resistant white ceramic layer and the aluminum alloy substrate, and substantially improves impact resistance without altering whiteness. It fundamentally overcomes the inherent defects of traditional micro-arc oxidation coatings: their thickening process relies on repeated breakdown of the film layer, resulting in numerous residual micron-level discharge channels within the coating. Furthermore, the outward growth mode limits the thickness of the dense inner layer, making it highly susceptible to large-area peeling under sudden impacts or mechanical collisions.

[0022] The ZrO2 / Al2O3 porous ceramic layer and the oxygen film together constitute a high-resistivity layer with a high total resistance. During the micro-arc oxidation process, water oxidation occurs on the anode surface to generate O2 (4OH-). - (-4e=2H2O+O2↑) Some oxygen escapes from the electrolyte as bubbles, while the rest remains on the anode surface due to the confinement of the high-resistivity ceramic layer and the surface tension of the electrolyte, forming a local oxygen film covering the ceramic layer / electrolyte interface. This oxygen film, in series with the ZrO2 / Al2O3 porous ceramic layer, forms a composite high-resistivity layer, which significantly blocks OH-. - Contact with anolyte metal ions causes OH- - It is mainly consumed through the self-discharge oxygen evolution mechanism. To maintain the electron flux balance in the external circuit, the anode aluminum metal undergoes oxidation and dissolution under the influence of the inter-electrode electric field to generate Al. 3+ Al 3+ O that passes through the ceramic layer under the drive of an electric field 2- The reaction generates Al2O3, thereby realizing the oxygen permeation process driven by the electric field.

[0023] Under the influence of an electric field, although other anions in the electrolyte (such as phosphate and silicate) can also migrate directionally and penetrate the initial porous ceramic layer to reach the coating / substrate interface, their interaction with Al... 3+ The reactivity is significantly lower than that of O 2- Therefore, Al 3+ Priority and O 2- The reaction produces the Al₂O₃ phase. Simultaneously, O… 2- The diameter is much smaller than the discharge micropores in the micro-arc oxidation coating and the grain boundary width of aluminum metal, enabling it to penetrate into the anode aluminum substrate under the drive of the inter-electrode electric field and interact with Al. 3+ The process directly forms crystalline Al2O3 ceramics, thereby generating an electric field-driven oxygen infiltration process. Since this reaction process avoids severe breakdown of the ZrO2 / Al2O3 porous ceramic layer, it helps to achieve dense growth of the coating.

[0024] 2. The impact-resistant white ceramic layer on the aluminum alloy surface of the present invention has a double-layer structure (the outer layer is a densely reconstructed ZrO2 / Al2O3 layer, and the inner layer is a dense and non-porous Al2O3 layer), with high whiteness, low porosity, high bonding strength with the substrate, and excellent impact resistance. When applied to the outer shell of 3C electronic products, it successfully solves the technical bottleneck of the difficulty in achieving both high whiteness and impact resistance in the shell of 3C electronic products.

[0025] 0. The preparation method of this invention combines the advantages of high performance and low cost, and can meet the economic and functional requirements of consumer electronics exterior parts in industrial applications. At the same time, this preparation method is highly flexible and adaptable to various shapes, applicable not only to various types of aluminum alloys, but also to the processing of complex-shaped 3C electronic product casing exterior parts. Attached Figure Description

[0026] Figure 1 The image shows the SEM cross-sectional morphology of the white ceramic coating on the aluminum alloy surface of Comparative Example 1.

[0027] Figure 2 The cross-sectional morphology and corresponding elemental distribution diagram of the white ceramic coating on the aluminum alloy surface of Comparative Example 1 are shown. In the diagram, a is the morphology diagram, b is Al, c is O, d is Zr, e is Si, and f is P.

[0028] Figure 3 The images are photographs and SEM cross-sectional images of the white ceramic coating on the aluminum alloy surface of Comparative Example 1 after a drop hammer impact test. (a) is a photograph, and (b) is an SEM cross-sectional image.

[0029] Figure 4The cross-sectional morphology and corresponding elemental distribution diagram of the white ceramic coating on the aluminum alloy surface of Comparative Example 2 are shown. In the diagram, a is the morphology diagram, b is Al, c is O, d is Si, and e is Zr.

[0030] Figure 5 The images show SEM cross-sectional morphology images of the impact-resistant white ceramic layer on the aluminum alloy surface of Example 1 at different magnifications, where (a) has a scale bar of 100 μm and (b) has a scale bar of 50 μm.

[0031] Figure 6 The image shows the cross-sectional morphology and corresponding elemental distribution of the impact-resistant white ceramic layer on the aluminum alloy surface in Example 1. In the image, a is the morphology diagram, b is Al, c is O, d is Zr, e is Si, and f is P.

[0032] Figure 7 The images show the appearance and SEM cross-sectional morphology of the aluminum alloy surface anti-impact white ceramic layer of Example 1 after a drop hammer impact test. (a) is the appearance image and (b) is the SEM cross-sectional morphology. Detailed Implementation

[0033] The technical solution of the present invention will be clearly and completely described below with reference to the data in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0034] It should be noted that the technical terms used in this invention are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased on the market or prepared by existing methods.

[0035] In existing technologies, when preparing white ceramic coatings using the conventional one-step micro-arc oxidation method, the coating thickness mainly relies on repeated penetration of the original film layer to achieve continuous oxidation of the inner layer. This process causes the size of residual discharge micropores to increase with the extension of oxidation time, forming a large number of micron-sized micropore defects. At the same time, the coating mainly exhibits external growth behavior relative to the original substrate surface, with a limited thickness of the dense inner layer. The resulting ZrO2 / Al2O3 composite coating has a typical "loose outer layer and dense inner layer" bilayer structure. During service, corrosive media can easily penetrate into the substrate through the porous outer layer, and when the coating is subjected to sudden impact or mechanical impact, the interface between the porous outer layer and the dense inner layer can easily become a crack propagation path, leading to large-area peeling of the coating. This makes it difficult to meet the practical application requirements of 3C electronic product casings that combine high whiteness and impact resistance.

[0036] To address the problems existing in the prior art, the present invention provides a method for preparing an impact-resistant white ceramic layer on an aluminum alloy surface, comprising the following steps: using an aluminum alloy with a ZrO2 / Al2O3 porous ceramic layer as the anode, connecting the anode to the positive electrode of a bipolar pulse power supply, and performing micro-arc oxidation treatment in an electrolyte without whitening accelerator under soft spark mode, causing the ZrO2 / Al2O3 porous ceramic layer to undergo densification and reconstruction, while simultaneously driving oxygen ions to pass through the interface between the ZrO2 / Al2O3 porous ceramic layer and the aluminum alloy substrate, and diffuse into the aluminum alloy substrate to form a dense alumina layer, thereby obtaining an impact-resistant white ceramic layer on the aluminum alloy surface.

[0037] This invention significantly improves the density of the impact-resistant white ceramic layer and the interfacial bonding strength between the impact-resistant white ceramic layer and the aluminum alloy substrate through the synergistic effect of soft spark discharge mode-induced coating densification reconstruction and interfacial electric field-driven oxygen permeation. While maintaining high whiteness, it endows the impact-resistant white ceramic layer with excellent impact resistance, meeting the dual requirements of aesthetics and reliability for 3C electronic product casings.

[0038] Furthermore, this invention employs a two-step micro-arc oxidation synergistic control strategy. By switching the electrolyte system (zirconium salt-containing electrolyte → electrolyte without whitening accelerator) and changing the discharge mode (micro-arc discharge → soft spark discharge), the whiteness, thickness, and densification of the ZrO2 / Al2O3 porous ceramic layer are independently controllable. This avoids problems such as uneven element distribution and narrow process window caused by the competitive deposition of zirconium salt and basic electrolyte in single-step processes, which is the preferred technical route of this invention.

[0039] In this embodiment of the invention, the solute of the electrolyte without whitening accelerator is composed of Na3PO4·12H2O, Na2SiO3, NaOH and ethylene glycol, and the mass concentration of Na3PO4·12H2O in the electrolyte without whitening accelerator is 10 g / L, the mass concentration of Na2SiO3 is 7 g / L, the mass concentration of NaOH is 3 g / L, and the volume concentration of ethylene glycol is 20 mL / L.

[0040] The solutes in the zirconium salt-containing electrolyte consist of NaH2PO4·2H2O, Na2SiO3, K2ZrF6, and H3PO4. The mass concentrations of NaH2PO4·2H2O, Na2SiO3, K2ZrF6, and H3PO4 in the zirconium salt-containing electrolyte are 18 g / L, 3 g / L, 10 g / L, and 7 g / L, respectively.

[0041] To enable those skilled in the art to more clearly understand the technical solution of the present invention, the following will provide a detailed description in conjunction with specific embodiments: Example 1 A method for preparing an impact-resistant white ceramic layer on an aluminum alloy surface includes the following steps: S1. Conventional Micro-arc Oxidation Treatment: The aluminum alloy is sequentially polished, degreased, alkaline washed, and acid-washed to remove surface oil and then dried, resulting in a pre-treated aluminum alloy. Using a micro-arc pulse power supply, the pre-treated aluminum alloy is connected to the anode of the power supply and subjected to micro-arc oxidation treatment in a zirconium salt-containing electrolyte at 35°C. This forms a micro-arc oxidation coating on the surface of the pre-treated aluminum alloy, resulting in an aluminum alloy with a ZrO2 / Al2O3 porous ceramic layer. The thickness of the ZrO2 / Al2O3 porous ceramic layer is 60 μm, and its whiteness is 90. The micro-arc oxidation treatment conditions are: forward output current density of 2 A / dm³. 2 The output frequency is 500Hz, the duty cycle is 20%, and the processing time is 25min.

[0042] S2. Micro-arc oxidation treatment under soft spark discharge mode: After washing the aluminum alloy with a porous ZrO2 / Al2O3 ceramic layer, it is connected to the anode of a bipolar pulse power supply and subjected to micro-arc oxidation treatment in a soft spark mode in an electrolyte without whitening accelerator at 35°C. After cleaning and drying, an impact-resistant white ceramic layer is obtained on the aluminum alloy surface. The thickness of the impact-resistant white ceramic layer on the aluminum alloy surface is 80 μm, and the whiteness is 90. The micro-arc oxidation treatment conditions are: forward output current density of 5 A / dm³. 2 The ratio R of negative output current density to positive output current density is 1.2, the output frequency is 500Hz, the duty cycle is 20%, and the processing time is 15min.

[0043] Comparative Example 1 A method for preparing a white ceramic coating on an aluminum alloy surface, using only conventional micro-arc oxidation treatment, includes the following steps: After polishing, degreasing, alkaline washing, and acid washing to remove surface oil, the aluminum alloy was dried to obtain a pretreated aluminum alloy. Using a micro-arc pulse power supply, the pretreated aluminum alloy was connected to the anode of the power supply and subjected to micro-arc oxidation treatment in an electrolyte containing zirconium salt at 35°C. This formed a micro-arc oxidation coating on the surface of the pretreated aluminum alloy, resulting in an aluminum alloy with a ZrO2 / Al2O3 porous ceramic layer, i.e., a white ceramic coating on the aluminum alloy surface. The thickness of the ZrO2 / Al2O3 porous ceramic layer was 50 μm, and its whiteness was 88. The micro-arc oxidation treatment conditions were: a forward output current density of 2 A / dm³. 2 The output frequency is 500Hz, the duty cycle is 20%, and the processing time is 20min.

[0044] Comparative Example 2 A method for preparing a white ceramic coating on an aluminum alloy surface employs a single-step micro-arc oxidation process, using cathodic polarization to control the discharge behavior. When the ZrO2 / Al2O3 ceramic layer on the aluminum anode surface grows to 60 μm, the plasma discharge mode gradually evolves from micro-arc discharge to soft spark discharge. The method includes the following steps: S1. After polishing, degreasing, alkaline washing, and acid washing of the aluminum alloy in sequence, surface oil stains are removed and the alloy is dried to obtain a pretreated aluminum alloy. Using a micro-arc pulse power supply, the pretreated aluminum alloy is connected to the anode of the micro-arc pulse power supply and subjected to micro-arc oxidation treatment in a zirconium salt-containing electrolyte at 35°C to form a micro-arc oxidation coating on the surface of the aluminum alloy, resulting in a white ceramic coating on the aluminum alloy surface. The conditions for the micro-arc oxidation treatment are: forward output current density of 2A / dm³. 2 The ratio R of negative output current density to positive output current density is 1.2, the output frequency is 500Hz, the duty cycle is 20%, and the processing time is 40min.

[0045] S2. After the reaction proceeds for 25 minutes, the anodic plasma discharge mode changes from micro-arc discharge to soft spark discharge.

[0046] observe Figure 1 It was found that the white ceramic coating (Comparative Example 1) prepared by conventional micro-arc oxidation treatment has a large number of discharge micropores inside, and the dense inner layer is relatively thin.

[0047] observe Figure 3 The results showed that the white ceramic coating prepared by conventional micro-arc oxidation treatment exhibited severe failure behavior during drop hammer tests: significant pitting occurred at the impact point, and large-scale peeling of the surrounding coating also occurred. Figure 3 As shown in 'a'; further observation Figure 3 The cross-sectional morphology of section b shows that the porous outer layer in the peeled area has completely detached from the substrate, exposing the dense inner layer underneath. This phenomenon indicates that the bonding force between the porous outer layer and the dense inner layer is weak, and under the action of impact stress, it becomes the preferred path for crack propagation, which can lead to the overall peeling of the micro-arc oxidation coating.

[0048] Table 1 shows the elemental composition of white ceramic coatings on aluminum alloy surfaces prepared under different conditions. Combination Figure 2 As shown in Table 1, the white ceramic coating (Comparative Example 1) prepared by conventional micro-arc oxidation treatment is mainly composed of Al, O, and Zr elements, and also contains small amounts of Si and P elements. The elements are uniformly distributed in the coating, and the mass fraction of Zr element is 21.3 wt%.

[0049] like Figure 4As shown in Table 1, the white ceramic coating (Comparative Example 2) prepared by a one-step "micro-arc to soft spark" micro-arc oxidation process mainly consists of Al, O, Zr, Si, and trace amounts of P. Among them, Si is enriched in the outer layer, while Zr is uniformly distributed with a mass fraction of 9.4 wt%, which limits the improvement in whiteness of the coating.

[0050] Combination Figure 6 As shown in Table 1, the impact-resistant white ceramic layer on the aluminum alloy surface (Example 1) prepared using a two-step micro-arc oxidation synergistic control strategy exhibits a distinct bilayer structure. The outer layer is mainly composed of Al, O, and Zr elements, and contains trace amounts of Si and P elements, with a Zr mass fraction of 19.4 wt%. The inner layer is mainly composed of Al and O elements, with a total content as high as 98.6 wt%. This result further demonstrates that the soft spark discharge mode is beneficial to the oxygen infiltration process driven by the electric field, and can promote the oxidation of O. 2- It penetrates into the aluminum matrix to form a dense, non-porous Al2O3 inner layer, thereby significantly improving the impact resistance of the white ceramic coating on the aluminum alloy surface.

[0051] Depend on Figure 5 The results showed that, compared with the white ceramic coating on the aluminum alloy surface prepared in Comparative Example 1, the porosity of the impact-resistant white ceramic layer on the aluminum alloy surface prepared in Example 1 was significantly reduced. The density of the ZrO2 / Al2O3 porous ceramic layer was significantly improved, and micropore defects were effectively healed.

[0052] Furthermore, compared to the interface between the ZrO2 / Al2O3 porous ceramic layer and the substrate formed in step S1, the newly formed ceramic layer under the soft spark discharge mode in step S2 mainly grows at the interface between the ZrO2 / Al2O3 porous ceramic layer and the aluminum substrate, entirely beneath the original ZrO2 / Al2O3 porous ceramic layer, exhibiting a typical internal growth mode. This newly formed layer has a dense structure with no obvious pores, forming a dense inner layer of the impact-resistant white ceramic layer on the aluminum alloy surface. This gradient structural optimization of outer layer densification and inner layer thickening fundamentally improves the stress distribution and energy dissipation capacity of the impact-resistant white ceramic layer, laying the structural foundation for its excellent impact resistance.

[0053] Depend on Figure 7 As shown in a, after undergoing a drop hammer impact test under the same conditions, the impact-resistant white ceramic layer prepared by this invention only forms impact pits on the surface of the impact-resistant white ceramic layer, while the impact-resistant white ceramic layer around the impact pits remains intact without any signs of peeling; further observation... Figure 7As shown in b, although the impact-resistant white ceramic layer developed obvious cracks due to instantaneous impact stress, the crack propagation was effectively suppressed and did not induce flaking or delamination failure. This fully demonstrates the excellent interfacial bonding strength between the impact-resistant white ceramic layer and the substrate, as well as the significant improvement in the toughness of the impact-resistant white ceramic layer itself after densification and reconstruction, which fully meets the tolerance requirements of 3C electronic product casings to sudden mechanical impacts.

[0054] It should be noted that when numerical ranges are involved in this invention, it should be understood that both endpoints of each numerical range, as well as any value between the two endpoints, can be selected. Since the steps and methods used are the same as in the embodiments, to avoid redundancy, this invention describes embodiments in optional implementations. Although preferred embodiments of the invention have been described, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this invention.

Claims

1. A method for preparing an impact-resistant white ceramic layer on an aluminum alloy surface, characterized in that, Includes the following steps: Using an aluminum alloy with a ZrO2 / Al2O3 porous ceramic layer as the anode, the anode is connected to the positive terminal of a bipolar pulse power supply. Micro-arc oxidation is performed in an electrolyte without whitening accelerator in soft spark mode, causing the ZrO2 / Al2O3 porous ceramic layer to undergo densification and reconstruction. This reduces the pore size of the ZrO2 / Al2O3 porous ceramic layer and drives oxygen ions to pass through the interface between the ZrO2 / Al2O3 porous ceramic layer and the aluminum alloy substrate, and diffuse into the aluminum alloy substrate to form a dense alumina layer, resulting in an impact-resistant white ceramic layer on the aluminum alloy surface.

2. The method according to claim 1, characterized in that, The conditions for micro-arc oxidation treatment are: forward output current density of 1 A / dm³. 2 ~15A / dm 2 The ratio R of negative output current density to positive output current density is 1.1~1.8, the output frequency is 100Hz~2000Hz, the duty cycle is 2%~20%, the electrolyte temperature without whitening accelerator is 10℃~40℃, and the micro-arc oxidation treatment time is 3min~20min.

3. The method according to claim 1, characterized in that, In aluminum alloys with ZrO2 / Al2O3 porous ceramic layers, the whiteness of the ZrO2 / Al2O3 porous ceramic layers is 86~96 and the thickness is 15μm~80μm.

4. The method according to claim 1, characterized in that, The electrolyte without whitening accelerator is composed of a main salt, a pH adjuster, an additive, and water. The main salt is selected from at least one of sodium silicate, sodium dihydrogen phosphate, sodium hexametaphosphate, sodium pyrophosphate, or sodium hypophosphite. The pH adjuster is selected from sodium hydroxide or potassium hydroxide. The additive is selected from at least one of disodium ethylenediaminetetraacetate, potassium citrate, potassium sodium tartrate, or ethylene glycol. In the electrolyte without whitening accelerator, the mass concentration of the main salt is 10 g / L to 30 g / L, the mass concentration of the pH adjuster is 2 g / L to 10 g / L, and the mass concentration of the additive is 2 g / L to 10 g / L.

5. The method according to claim 1, characterized in that, The aluminum alloy with the ZrO2 / Al2O3 porous ceramic layer is prepared according to the following steps: An aluminum alloy substrate is used as the anode and connected to the positive electrode of a micro-arc oxidation power supply. Micro-arc oxidation is performed in an electrolyte containing zirconium salt to grow a porous ceramic layer with ZrO2 / Al2O3 as the main phase in situ on the surface of the aluminum alloy substrate, thus obtaining an aluminum alloy substrate with a ZrO2 / Al2O3 porous ceramic layer. The zirconium-containing electrolyte is composed of a main salt, a zirconium-containing ion salt, an auxiliary solubilizer, and water. The main salt is selected from at least one of sodium silicate, sodium dihydrogen phosphate, sodium hexametaphosphate, sodium pyrophosphate, or sodium hypophosphite. The auxiliary solubilizer is selected from at least one of oxalic acid, cinnamic acid, citric acid, or phosphoric acid. In the zirconium-containing electrolyte, the mass concentration of the main salt is 10 g / L to 40 g / L, the mass concentration of the zirconium-containing ion salt is 5 g / L to 30 g / L, and the mass concentration of the auxiliary solubilizer is 2 g / L to 15 g / L.

6. The method according to claim 5, characterized in that, The conditions for micro-arc oxidation treatment are: forward output current density of 1 A / dm³. 2 ~15A / dm 2 The output frequency is 100Hz~2000Hz, the duty cycle is 2%~20%, the electrolyte temperature containing zirconium salt is 10℃~40℃, and the micro-arc oxidation treatment time is 10min~60min.

7. An impact-resistant white ceramic layer on an aluminum alloy surface prepared by the method according to any one of claims 1 to 6, characterized in that, The impact-resistant white ceramic layer on the aluminum alloy surface includes an aluminum alloy substrate and an impact-resistant white ceramic layer on the surface of the aluminum alloy substrate; the impact-resistant white ceramic layer has a double-layer structure, with an inner layer of alumina and an outer layer of densified and reconstructed ZrO2 / Al2O3.

8. The impact-resistant white ceramic layer on the aluminum alloy surface according to claim 7, characterized in that, The whiteness of the impact-resistant white ceramic layer is 86~96, and the thickness is 20μm~110μm.

9. The application of the impact-resistant white ceramic layer of claim 7 in the manufacture of housings for 3C electronic products.

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