A trivalent chromium plating layer with pearl shell biomimetic structure and a preparation method thereof

Abstract

The present application relates to the technical field of electrodeposited chromium plating, in particular to a trivalent chromium plating layer with a pearl shell biomimetic structure and a preparation method thereof. By regulating the composition of plating solution and electrodeposition process, stable periodic concentration polarization is induced to form potential composition distribution along the thickness direction of amorphous precursor plating layer; after annealing at 350-450 DEG C for 0.5-1 h, the potential composition modulation is explicit and phase transition occurs, and the pearl shell biomimetic layered structure is constructed in situ. The plating layer presents a periodic and alternating arrangement of natural pearl shell "hard-soft" multilayer stacked biomimetic layered structure in microcosm, wherein the chromium layer is a metallic chromium crystalline layer with a thickness of 5-30 nm; the carbon / carbon-chromium mixture layer is amorphous or nanocrystalline with carbon enrichment and in-situ formed chromium carbide phase, with a thickness of 1-3 nm. The total thickness of the plating layer is 5-30 microns, and the number of layers reaches 300-1800 layers. The structure can hinder crack propagation and improve load transfer, so that the plating layer has high hardness and wear resistance. Moreover, the process is environmentally friendly and controllable, and is suitable for mechanical parts, molds and functional surface protection.

Description

Technical Field

[0001] This invention relates to the field of electrodeposited chromium plating technology, specifically to a trivalent chromium plating with a pearl oyster biomimetic structure and its preparation method. Background Technology

[0002] Chromium plating, due to its high hardness, good wear resistance, and corrosion resistance, has significant application value in machinery manufacturing, automotive parts, mold processing, aerospace, and electronics industries. Among these, the traditional hexavalent chromium electroplating process, capable of producing coatings with high hardness and excellent corrosion resistance, has long dominated industrial production. However, hexavalent chromium compounds are highly carcinogenic and environmentally harmful, and their preparation and use easily generate wastewater and waste residue containing toxic substances, which has been strictly restricted by the EU REACH regulation and numerous international environmental regulations. With increasingly stringent environmental protection requirements, developing alternative electroplating processes that combine environmental friendliness with superior service performance has become an important research direction in the field of surface engineering. Against this backdrop, trivalent chromium electroplating technology, due to its lower toxicity, easier wastewater treatment, and lower environmental impact, is widely considered a promising green alternative.

[0003] Trivalent chromium electroplating systems typically use formate, acetate, or urea as complexing agents, resulting in coatings that are usually amorphous or near-amorphous. Simultaneously, due to the hydrogen evolution side reaction, the coating contains small amounts of non-metallic elements such as carbon and oxygen, leading to lower hardness and reduced wear resistance in the deposited trivalent chromium coating. To improve its mechanical properties, industrial processes often induce the precipitation of chromium carbide phases through heat treatment to achieve dispersion strengthening. However, this process is often accompanied by significant volume shrinkage, introducing high residual stress within the coating and inducing through-cracks, leading to a decrease in material toughness. Under impact loads or high contact stress friction conditions, these cracks easily propagate rapidly along amorphous shear bands or grain boundaries, ultimately causing brittle spalling or cracking failure of the coating.

[0004] Therefore, how to simultaneously improve the hardness and wear resistance of trivalent chromium plating through structural design and process innovation without compromising its environmentally friendly properties has become a key technical challenge that urgently needs to be overcome in this field. Summary of the Invention

[0005] In view of this, the present invention proposes a trivalent chromium plating layer with a pearl shell biomimetic structure and its preparation method. The method uses the concentration polarization effect generated during the electrodeposition process as a structural control and guidance mechanism, and combines it with subsequent low-temperature annealing treatment to realize the in-situ construction of a biomimetic layered structure with both high strength and wear resistance in the trivalent chromium plating layer.

[0006] The technical solution of this invention is implemented as follows: In a first aspect, the present invention provides a method for preparing a trivalent chromium plating layer with a pearl-inspired biomimetic structure, comprising the following steps: S1. Trivalent chromium electroplating is performed on the substrate surface to deposit an amorphous coating; S2. Anneal the amorphous coating at a temperature of 350-450℃ for 0.5-1h.

[0007] Specifically, a trivalent chromium electroplating process is used to deposit chromium on the surface of a steel substrate. Taking advantage of the restricted migration behavior of trivalent chromium ions and the large volume of complex ions, the deposition parameters are controlled to maintain a stable concentration polarization state on the cathode surface. This leads to the periodic depletion and recovery of trivalent chromium ions in the diffusion layer at the cathode interface, thereby introducing a potential layered distribution feature of composition in the growth direction of the resulting amorphous trivalent chromium plating layer. Subsequently, the resulting plating layer is subjected to low-temperature annealing at 350-450 °C, with the holding time precisely controlled to 0.5-1 h. Thermal activation induces a crystallization transformation of the amorphous plating layer along the potential layered distribution, making the hidden layered structure visible. This results in the in-situ construction of a layered biomimetic structure similar to that of natural pearl oysters within the plating layer.

[0008] Furthermore, the annealing temperature and holding time affect interlayer diffusion, phase transformation, and grain growth processes, thereby influencing the monolayer thickness and boundary clarity of the metallic chromium layer and the carbon-rich / chromium carbide layer. Too low a temperature is insufficient to drive layered crystallization, while too high a temperature leads to excessive grain growth and damage to the layered structure. Through this annealing process, organic inclusions in the amorphous precursor are transformed into stable carbon / chromium carbide reinforcing phases, inducing the crystallization of the metallic chromium layer, ultimately resulting in a periodic layered nanostructure.

[0009] Preferably, the plating solution used in step S1 for trivalent chromium electroplating includes chromium sulfate, a complexing agent, sodium sulfate, boric acid, aluminum sulfate octadecahydrate, and a surfactant.

[0010] Preferably, the concentration of chromium sulfate in the plating solution is 0.3-0.5 M, and the concentration of sodium sulfate in the plating solution is 0.3-0.7 M.

[0011] Preferably, the complexing agent includes formic acid and urea, wherein the concentration of formic acid in the plating solution is 0.3-0.5 M, and the concentration of urea in the plating solution is 0.3-0.5 M.

[0012] Specifically, formic acid and urea, as strong complexing agents in trivalent chromium electroplating, will encapsulate trivalent chromium ions to form complex ion clusters. The larger the ion cluster, the lower its mobility in the electric field, which will intensify the concentration polarization effect. This is the material basis for achieving ion depletion.

[0013] Preferably, the concentration of boric acid in the plating solution is 0.4-0.5 M, and the concentration of aluminum sulfate octadecahydrate in the plating solution is 0.12-0.15 M.

[0014] Preferably, the surfactant comprises sodium dodecyl sulfate (SDS), and the concentration of SDS in the plating solution is 0.1-0.5 g / L.

[0015] Specifically, SDS, as a surfactant, alters the surface tension of the electrode, helping hydrogen bubbles escape in a timely manner, preventing pores in the coating, and ensuring the continuity and density of the layered structure.

[0016] Preferably, the trivalent chromium electroplating process parameters in step S1 include: a current density of 20-25 A / dm³. 2 The pH value of the plating solution is 1.5-2, the deposition temperature is 30-35℃, and the deposition time is 20-150 min.

[0017] Specifically, the current density directly determines the reduction deposition rate (i.e., consumption rate) of metal ions on the cathode surface. When the current density is too low, the consumption rate of trivalent chromium ions is slow, allowing sufficient time for ions in the bulk solution to diffuse and replenish the interface, preventing the formation of a "depleted zone," resulting in a uniform coating composition and the inability to form stratification. Conversely, when the current density is too high, it may lead to severe hydrogen evolution side reactions (burning), damaging the integrity of the coating. For trivalent chromium, 20-25 A / dm³ is recommended. 2 This is a relatively high current density, which is just at the critical point where it can induce intense and periodic concentration polarization.

[0018] Specifically, deposition temperature primarily affects the diffusion coefficient of ions. When the deposition temperature is too high, the thermal motion of ions intensifies, the diffusion rate increases, and the interface depletion can be rapidly replenished, weakening concentration polarization and leading to the disappearance or indistinctness of the layered structure. Temperatures of 30-35℃ can limit the migration rate of trivalent chromium ions, thereby maintaining the ion-depleted state at the cathode interface and aiding in the formation of the layered structure. Furthermore, pH determines the coordination state of trivalent chromium ions in the solution and the extent of the hydrogen evolution side reaction. In an acidic environment with pH = 1.5-2, chromium ions, in conjunction with sulfate and formate ions, form complex, large complex ion clusters, further reducing their migration rate.

[0019] In a second aspect, the present invention provides a trivalent chromium plating layer obtained by the preparation method described in the first aspect.

[0020] Specifically, the coating has multiple periodically alternating layered structural units along its thickness direction (deposition growth direction). Each structural unit includes: a crystalline metallic chromium layer mainly composed of metallic chromium, and a carbon / chromium carbide mixture layer adjacent to the metallic chromium layer. In the carbon / chromium carbide mixture layer, carbon exists in the form of solid-solution carbon and / or chromium carbide, and the chromium carbide is distributed in a continuous thin layer, with a phase composition of Cr3C2, Cr7C3, and Cr... 23 One or more of C6; the layered structural units form a multi-period biomimetic structure that is repeatedly stacked along the coating thickness direction, thereby endowing the coating with both high hardness and excellent wear resistance.

[0021] Preferably, the trivalent chromium plating is formed by alternating layers of metallic chromium and carbon / chromium carbide mixture, with a plating thickness of 5-30 μm and a total number of layers of 300-1800.

[0022] Preferably, the metallic chromium layer is crystalline and has a thickness of 5-30 nm; the carbon / chromium carbide mixture layer is amorphous or nanocrystalline and has a thickness of 1-3 nm.

[0023] Compared with the prior art, the advantages of the present invention are as follows: (1) This invention is the first to construct a stable biomimetic layered structure of pearl oyster in situ in a trivalent chromium plating system. Through a two-step process of amorphous deposition and low-temperature annealing, this invention effectively induces the formation of a multi-layered sheet-like biomimetic structure of the plating at the microscale, thereby enabling the trivalent chromium plating to possess the performance advantages brought by the "hard-soft" multi-layered stacked structure of natural pearl oyster.

[0024] (2) This invention utilizes the trace carbon components (impurity carbon) that are commonly present in the trivalent chromium electroplating system and innovatively uses them as a structural control resource, realizing the structural design idea of ​​turning disadvantages into advantages.

[0025] (3) By synergistically controlling the composition of the plating solution and the electrodeposition process parameters in a single plating solution system, the present invention can realize the in-situ construction of multi-layer biomimetic structures in the continuous deposition process without the need for multi-tank switching or complex pulse power supply. The process is simple, highly stable, and has significant industrial promotion value.

[0026] (4) The resulting biomimetic layered structure can effectively deflect and passivate the crack propagation path, significantly improving the service reliability of the coating under high load friction and impact conditions. Attached Figure Description

[0027] Figure 1 This is a TEM image of the coating obtained in Example 1 of the present invention; Figure 2 This is a TEM image of the coating obtained in Example 1 of the present invention; Figure 3 This is an HRTEM image of the coating obtained in Example 1 of the present invention; Figure 4 This is a TEM image of the coating obtained in Comparative Example 5 of the present invention. Detailed Implementation

[0028] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. 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.

[0029] It should be noted that pearl oysters are a typical high-performance biomineralized material, attracting widespread attention due to their excellent synergistic properties of strength and toughness. This material is mainly composed of over 95% brittle calcium carbonate aragonite lamellars and a small amount of organic matrix, formed by alternating stacks of a highly ordered layered composite structure. Under external loads, the multi-level layered structure unique to pearl oysters significantly improves fracture toughness through various mechanisms such as microcrack deflection, interlayer slip, organic bridging, and crack tip passivation, thereby effectively inhibiting the linear penetration and propagation of cracks and enhancing the structural reliability under strong external loads. The aforementioned biomimetic layered structure design concept provides important theoretical guidance for solving the problems of high brittleness and poor wear resistance in trivalent chromium plating. However, achieving a highly ordered, nanoscale layered structure similar to that of pearl oysters in metal electrodeposition systems still faces significant technical challenges. Among the existing methods for preparing layered metal composite coatings, the dual-tank alternating deposition process relies on frequent switching between plating solutions of different compositions, resulting in low deposition efficiency and difficulty in meeting the needs of continuous industrial production. While the single-tank pulse electroplating method can induce periodic changes in the deposition process by adjusting the current parameters, it is difficult to form a clear interface, has high requirements for the response accuracy and stability of the power supply system, and has a narrow process window, limiting repeatability and controllability.

[0030] More importantly, existing research on performance enhancement of trivalent chromium electroplating systems mainly focuses on composite electroplating by introducing second-phase particles (such as nanodiamonds and SiC) or improving mechanical properties through alloying. These methods are not only complex and costly, but also often overlook the intrinsic potential of ion transport and interfacial reaction kinetics to regulate the microstructure evolution of the coating during electrodeposition. Furthermore, these modification approaches fail to address the endogenous structural and compositional issues inherent in the trivalent chromium deposition process, most notably the unavoidable introduction of carbon during electrodeposition. Carbon in trivalent chromium coatings primarily originates from complexing agents and their decomposition products, as well as carbonaceous components related to hydrogen evolution side reactions. It is generally believed that carbon inclusion leads to coating inclusions, uneven microstructure, and an increased tendency for cracking during subsequent heat treatment, thus hindering the improvement of mechanical and wear resistance properties. Moreover, under current process conditions, completely eliminating this carbon problem remains a recognized technical challenge.

[0031] Meanwhile, trivalent chromium ions typically exist in aqueous solutions as complex complexes with large hydration radii, limiting their migration ability under an electric field. Their diffusion coefficient is far lower than that of monovalent or divalent metal ions. Under conventional electrodeposition conditions, when a high current density is applied, the consumption rate of metal ions at the cathode interface easily exceeds the diffusion replenishment rate, resulting in significant concentration polarization at the cathode surface. This causes the trivalent chromium ion concentration within the diffusion layer to exhibit localized and periodic depletion and recovery behavior. In traditional electroplating processes, concentration polarization is generally considered a detrimental factor, easily leading to problems such as coating scorching and decreased current efficiency. Therefore, in engineering practice, it is often suppressed by strengthening the agitation of the electroplating solution or reducing the current density. However, if the technical approach to suppressing this phenomenon is changed, and the electroplating solution formulation and deposition parameters are precisely controlled, actively utilizing the inherently limited migration ability of trivalent chromium ions to induce and maintain a stable periodic concentration polarization state during electrodeposition, then a periodic modulation of component concentration changes can spontaneously form along the coating growth direction. Because the deposited coating has an overall amorphous structure, the periodic fluctuations caused by concentration polarization mainly manifest as a uniform amorphous morphology under a transmission electron microscope, forming a structurally implicit layered precursor. Internally, periodic differences in composition or structure are formed along the coating growth direction. At this point, by applying low-temperature annealing under specific temperature and time conditions, the thermal activation process drives atomic diffusion and rearrangement. These potential compositional or structural differences can serve as templates for preferential nucleation and growth, inducing selective crystallization transformation in the amorphous matrix, making the originally implicit layered features explicit. Ultimately, a layered biomimetic structure similar to that of natural pearl oysters can be constructed in situ within the coating. Importantly, this invention does not attempt to eliminate the carbon content problem in the trivalent chromium system through complex processes, but rather, based on the controllability of electrodeposition kinetics, allows the carbon-containing components to form a controllable periodic enrichment along the growth direction, and then transforms them in situ into carbon / chromium carbide phases during subsequent annealing. This transforms a traditionally unfavorable factor into a key element for constructing a biomimetic layered structure, thereby significantly improving the coating performance. Currently, there are no reports on constructing biomimetic layered trivalent chromium plating layers by synergistically utilizing electrochemical concentration polarization effects and annealing-induced phase transition processes. This invention addresses this technological gap by proposing a novel structural construction method based on the synergistic effect of electrodeposition kinetics control and subsequent heat treatment.

[0032] The design concept of this invention is as follows: During the trivalent chromium electrodeposition process, due to the limited migration ability of trivalent chromium ions in the plating bath, when a specific current density is applied, the consumption rate of metal ions at the cathode interface is greater than the diffusion rate to the cathode interface, resulting in concentration polarization and local depletion of trivalent chromium ions on the cathode surface. In the ion depletion stage, the relative concentration of complexing agents (such as formic acid, urea) or their decomposition products at the cathode interface increases, leading to the instantaneous inclusion of more carbon-containing organic matter in the coating, forming a carbon-rich / organic thin layer. In the ion recovery stage, the deposition of metallic chromium mainly occurs, forming a chromium-rich layer. The dynamic balance or periodic fluctuations of the above-mentioned ion depletion and replenishment processes cause the deposited layer to form a periodic modulation of composition or local structural density along the growth direction, i.e., a potential layered structure. However, under deposition conditions, this periodic layering feature is difficult to directly distinguish due to the influence of the overall amorphous structure. In transmission electron microscopy (TEM) characterization, it mainly manifests as a uniform amorphous feature, without clear grain boundaries or layered contrast.

[0033] Based on the aforementioned potential layered structure, this invention further introduces a low-temperature annealing process under specific temperature and time conditions. The heat treatment process provides the driving force for atomic diffusion and rearrangement, causing the amorphous matrix to undergo a crystallization transformation. During this process, the chromium-rich layer preferentially transforms into a dense metallic chromium crystal layer, corresponding to the hard supporting phase in the biomimetic structure; while the thin layer rich in carbon-organic components undergoes carbonization or reaction with chromium during annealing, transforming in situ into a carbon / chromium carbide mixed layer, corresponding to the tough bonding phase in the biomimetic structure. Because the deposited coating contains pre-existing periodic distribution of composition and structure along the growth direction, the nucleation and growth process of the crystals is effectively restricted and guided, thereby gradually revealing the originally latent layered features during annealing, ultimately forming a macroscopically dense, microscopically pearl-shell-like layered biomimetic crystal structure within the coating.

[0034] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0035] In this document, the terms “containing,” “comprising,” or “including” are open-ended expressions, meaning they include the contents specified in this invention but do not exclude other aspects.

[0036] In this document, the terms “optional,” “optionally,” or “optional” generally refer to an event or condition that may, but may not, occur, and the description includes both cases in which the event or condition occurs and cases in which the event or condition does not occur.

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the protection scope of the present invention.

[0038] Unless otherwise specified, all materials and equipment used in this invention are commercially available products in the field. Specifically, chromium sulfate, formic acid, urea, sodium sulfate, boric acid, aluminum sulfate octadecylhydrate, and SDS were all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0039] In this invention, the Vickers hardness test method is as follows: the microhardness is measured under a load of 50g and a holding time of 15s. Each sample is measured 5 times and the average value is reported.

[0040] In this invention, the specific method for testing friction and wear is as follows: a reciprocating friction and wear testing machine (RetcMT-5000, USA) is used to evaluate the tribological properties. 6mm Al2O3 balls are used as the grinding pair. The test is carried out under a 5 N load for 1800 seconds, with a sliding frequency of 1 Hz, a speed of 2 cm / s, and a stroke of 10 mm.

[0041] Example 1

[0042] This embodiment provides a trivalent chromium plating layer with a pearl oyster biomimetic structure and its preparation method, including the following steps: (1) Prepare the plating solution according to the formula: 0.5 M chromium sulfate, 0.5 M formic acid, 0.5 M urea, 0.5 M sodium sulfate, 0.5 M boric acid, 0.15 M aluminum sulfate octadecahydrate, 0.5 g / L SDS; (2) A trivalent chromium layer is deposited on the surface of the steel substrate using an electroplating process. The electroplating process parameters are: current density of 25 A / dm². 2 An amorphous coating was obtained by setting the pH to 2, the deposition temperature to 35 ℃, and the deposition time to 60 min. (3) The obtained amorphous coating was annealed in argon at 400 °C for 1 h.

[0043] The coating obtained in the examples was subjected to transmission electron microscopy (TEM), microhardness, and wear rate tests. The results showed that, as Figures 1-3As shown, the coating exhibits complete crystallization, and a cross-sectional TEM reveals a clear "brick-and-mortar" multilayered sheet-like biomimetic pearl oyster structure. This structure consists of alternating layers of metallic chromium and a carbon / chromium carbide mixture, with a total thickness of approximately 18 μm and about 1000 layers. The metallic chromium layer is crystalline, with a thickness of approximately 10-20 nm; the carbon / chromium carbide mixture layer is amorphous or nanocrystalline, with a thickness of approximately 1-3 nm, formed by the in-situ transformation of carbon-containing organic matter incorporated during electrodeposition during annealing. The resulting coating has a microhardness of 1300 HV and a dry friction wear rate of 3.5 × 10⁻⁶. -6 mm 3 / (N·m), which is about 99% lower than that of the unannealed sample in Comparative Example 1, and the wear marks are smooth.

[0044] Example 2

[0045] This embodiment provides a trivalent chromium plating layer with a pearl oyster biomimetic structure and its preparation method, including the following steps: (1) Prepare the plating solution according to the formula: 0.3 M chromium sulfate, 0.3 M formic acid, 0.3 M urea, 0.3 M sodium sulfate, 0.4 M boric acid, 0.12 M aluminum sulfate octadecahydrate, 0.1 g / L SDS; (2) A trivalent chromium layer is deposited on the surface of the steel substrate using an electroplating process. The electroplating process parameters are: current density of 20 A / dm³. 2 An amorphous coating was obtained by setting the pH to 1.5, the deposition temperature to 30 ℃, and the deposition time to 60 min. (3) The obtained amorphous coating was annealed at 350 °C for 0.5 h.

[0046] The results showed that the resulting coating formed a relatively fine layered structure with slightly low crystallinity but a complete structure. The total coating thickness was approximately 8 μm, consisting of about 500 layers. The microhardness was 1200 HV, and the wear rate was 4.5 × 10⁻⁶. -6 mm 3 / (N·m).

[0047] Example 3

[0048] This embodiment provides a trivalent chromium plating layer with a pearl oyster biomimetic structure and its preparation method, including the following steps: (1) Prepare the plating solution according to the formula: 0.5 M chromium sulfate, 0.5 M formic acid, 0.5 M urea, 0.5 M sodium sulfate, 0.5 M boric acid, 0.15 M aluminum sulfate octadecahydrate, 0.5 g / L SDS; (2) A trivalent chromium layer is deposited on the surface of the steel substrate using an electroplating process. The electroplating process parameters are: current density of 25 A / dm². 2An amorphous coating was obtained by setting the pH to 2, the deposition temperature to 35 ℃, and the deposition time to 60 min. (3) The obtained amorphous coating was annealed at 450 °C for 1 h.

[0049] The results showed that the layered structure of the coating still existed, but the grain size was slightly larger, the interlayer boundaries were slightly blurred, and the total coating thickness was approximately 18 μm, with about 800 layers. Due to the strong precipitation strengthening effect, the microhardness reached 1400 HV, and the wear rate was 2.5 × 10⁻⁶. -6 mm 3 / (N·m).

[0050] Example 4

[0051] This embodiment provides a trivalent chromium plating layer with a pearl oyster biomimetic structure and its preparation method, including the following steps: (1) Prepare the plating solution according to the formula: 0.3 M chromium sulfate, 0.3 M formic acid, 0.3 M urea, 0.3 M sodium sulfate, 0.4 M boric acid, 0.12 M aluminum sulfate octadecahydrate, 0.1 g / L SDS; (2) A trivalent chromium layer is deposited on the surface of the steel substrate using an electroplating process. The electroplating process parameters are: current density of 20 A / dm³. 2 An amorphous coating was obtained by setting the pH to 1.5, the deposition temperature to 30℃, and the deposition time to 20 min. (3) The obtained amorphous coating was annealed at 350 °C and kept at that temperature for 0.5 h.

[0052] The results showed that a continuous, multi-layered, sheet-like biomimetic structure was formed inside the coating, but the grain size was small, and the total coating thickness was approximately 5 μm, consisting of about 300 layers. Due to the thin coating thickness, the microhardness was only 950 HV, and the wear rate was 5.5 × 10⁻⁶. -6 mm 3 / (N·m).

[0053] Example 5

[0054] This embodiment provides a trivalent chromium plating layer with a pearl oyster biomimetic structure and its preparation method, including the following steps: (1) Prepare the plating solution according to the formula: 0.5 M chromium sulfate, 0.5 M formic acid, 0.5 M urea, 0.5 M sodium sulfate, 0.5 M boric acid, 0.15 M aluminum sulfate octadechydrate, 0.5 g / L SDS; (2) A trivalent chromium layer is deposited on the surface of the steel substrate using an electroplating process. The electroplating process parameters are: current density of 25 A / dm². 2 An amorphous coating was obtained by setting the pH to 2, the deposition temperature to 35℃, and the deposition time to 150 min. (3) Anneal the obtained amorphous coating at 450 °C and keep it at that temperature for 1 h.

[0055] The results showed that a clear, multi-layered, sheet-like biomimetic structure resembling pearl shells was formed within the coating, with a total thickness of approximately 30 μm and about 1800 layers. Due to the relatively thick coating and strong precipitation strengthening effect, the microhardness reached 1500 HV, and the wear rate was 1×10⁻⁶. -6 mm 3 / (N·m).

[0056] Comparative Example 1 The difference between this comparative example and Example 1 is that the resulting coating remains amorphous and is not annealed; that is, step (3) is omitted; the rest is the same as Example 1.

[0057] The results showed that the coating did not form a layered biomimetic structure, had low microhardness (750 HV), and high friction and wear rate (3×10⁻⁶). -4 mm 3 / (N·m), poor crack resistance.

[0058] Comparative Example 2 The difference between this comparative example and Example 1 is that the annealing temperature in step (3) is set to 300 °C and held for 1 h; the rest is the same as in Example 1.

[0059] The results showed that only partial short-range ordering occurred in the coating, and no pearl-shell-like multilayer lamellar structure appeared, indicating insufficient crystallization. The microhardness was only about 1000 HV, and the improvement in wear resistance was limited, at 4 × 10⁻⁶. -5 mm 3 / (N·m) is still significantly lower than that of coatings obtained by annealing at 350-450℃.

[0060] Comparative Example 3 The difference between this comparative example and Example 1 is that the annealing temperature in step (3) is set to 500 °C and held for 1 h; the rest is the same as in Example 1.

[0061] The results showed that the coating grains grew rapidly, the layered structure was destroyed, and a tendency to become embrittled was observed. Although the microhardness increased to 1300 HV, the wear resistance was also good, at 3×10⁻⁶. -6 mm 3 / (N·m), but a large number of cracks appeared on the surface of the coating, affecting its long-term performance.

[0062] Comparative Example 4 The difference between this comparative example and Example 1 is that the annealing and heat preservation time in step (3) is shortened to 0.2 h; the rest is the same as Example 1.

[0063] The results showed that the coating was not fully crystallized, forming only localized lamellar areas, and the layered structure was discontinuous. The microhardness was approximately 900 HV, and the friction and wear rate remained high at 3 × 10⁻⁶. -5 mm 3 / (N·m) indicates that insufficient insulation prevents the formation of a stable biomimetic structure.

[0064] Comparative Example 5 The difference between this comparative example and Example 1 is that the annealing and heat preservation time in step (3) is extended to 3 h; the rest is the same as in Example 1.

[0065] The results showed that excessive heat preservation caused the grains to continue growing, resulting in the appearance of coarse crystalline wafers in some areas and a decrease in the integrity of the intralayer structure. Figure 4 As shown, its microhardness decreased to around 1100 HV, and its wear resistance also weakened, with a wear rate of 2×10⁻⁶. -5 mm 3 / (N·m).

[0066] Comparative Example 6 The difference between this comparative example and Example 1 is that 0.8 g / L SDS was used in step (1); the rest is the same as in Example 1.

[0067] The results showed that due to the excessively high SDS concentration, more bubbles were generated during the electrodeposition process, resulting in more surface defects in the coating. Compared with Example 1, the hardness decreased slightly to approximately 1250 HV, and the wear rate was 9 × 10⁻⁶. -6 mm 3 / (N·m).

[0068] Comparative Example 7 The difference between this comparative example and Example 1 is that 0.05 g / L SDS was used in step (1); the rest is the same as in Example 1.

[0069] The results showed that due to the excessively low SDS concentration, the current efficiency of electrodeposition was low, the hardness was approximately 1150 HV, and the wear rate was 9.5 × 10⁻⁶. -6 mm 3 / (N·m).

[0070] Comparative Example 8 The difference between this comparative example and Example 1 is that the current density in step (2) is 18 A / dm. 2 The rest remains the same as in Example 1.

[0071] The results showed that due to the low current density, the current efficiency was low, resulting in a thin coating. The coating hardness was approximately 950 HV, and the wear rate was 4×10⁻⁶. -5 mm 3 / (N·m).

[0072] Comparative Example 9 The difference between this comparative example and Example 1 is that the current density in step (2) is 28 A / dm. 2 The rest remains the same as in Example 1.

[0073] The results showed that due to the excessively high current density, the hydrogen evolution side reaction was significant during electrodeposition, leading to increased porosity on the coating surface and affecting coating performance. At this point, the coating hardness was 1100 HV, and the wear rate was 9×10⁻⁶. -6 mm 3 / (N·m).

[0074] Comparative Example 10 The difference between this comparative example and Example 1 is that the deposition time in step (2) is 5 min; the rest is the same as in Example 1.

[0075] The results showed that due to the insufficient deposition time, a complete trivalent chromium coating could not be formed. At this point, the coating hardness was 450 HV, and the wear rate was 5 × 10⁻⁶. -3 mm 3 / (N·m).

[0076] Comparative Example 11 The difference between this comparative example and Example 1 is that the deposition time in step (2) is 200 min; the rest is the same as in Example 1.

[0077] The results showed that due to the excessively long deposition time, concentration polarization, local pH changes, and additive consumption became more pronounced, resulting in surface graying / darkening and increased roughness. Furthermore, the internal stress accumulation caused by the long deposition time led to an increase in microcracks after annealing and the appearance of cracks at the edges and corners. At this point, the coating hardness was 1200 HV, and the wear rate was 5×10⁻⁶. -5 mm 3 / (N·m).

[0078] In summary, this invention, based on the kinetic characteristics of the restricted migration behavior of trivalent chromium ions in an electric field, induces periodic concentration polarization and ion depletion on the cathode surface during electrodeposition through process control, thereby introducing potential compositional or structural layering during the growth of the amorphous coating. The resulting coating is then subjected to low-temperature annealing, utilizing thermal driving force to reveal the aforementioned potential layered structure and induce a crystallization transformation. Ultimately, a layered biomimetic structure resembling natural pearl oyster shells is constructed in situ within the coating, with a total thickness of approximately 5-30 μm, repeatedly stacked along the coating thickness direction to form a multilayer structure of approximately 300-1800 layers. The coating exhibits a periodic alternating layering of metallic chromium and a carbon / chromium carbide mixture. The metallic chromium coating, with a thickness of 5-30 nm, serves as a hard bearing phase to provide hardness; the carbon / carbide mixture layer has a thickness of approximately 1-3 nm. This type of biomimetic layered structure effectively hinders crack propagation and improves load transfer efficiency, thereby significantly enhancing the mechanical properties and wear resistance of the coating. The trivalent chromium plating with a pearl shell biomimetic structure prepared by this invention not only has high hardness, high toughness and excellent wear resistance, but also has better environmental friendliness and application safety compared with the traditional hexavalent chromium electroplating system. It can be widely used in mechanical parts, molds and functional surface protection fields with high requirements for wear resistance and reliability.

[0079] The embodiments described above are some, but not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing a trivalent chromium plating layer with a pearl-shell biomimetic structure, characterized in that, Includes the following steps: S1. Trivalent chromium electroplating is performed on the substrate surface to deposit an amorphous coating; S2. Anneal the amorphous coating at a temperature of 350-450℃ for 0.5-1 h.

2. The preparation method according to claim 1, characterized in that, The plating solution used in step S1 for trivalent chromium electroplating includes chromium sulfate, a complexing agent, sodium sulfate, boric acid, aluminum sulfate octadecahydrate, and a surfactant.

3. The preparation method according to claim 2, characterized in that, The concentration of chromium sulfate in the plating solution is 0.3-0.5 M, and the concentration of sodium sulfate in the plating solution is 0.3-0.7 M.

4. The preparation method according to claim 2, characterized in that, The complexing agent includes formic acid and urea, wherein the concentration of formic acid in the plating solution is 0.3-0.5 M, and the concentration of urea in the plating solution is 0.3-0.5 M.

5. The preparation method according to claim 2, characterized in that, The concentration of boric acid in the plating solution is 0.4-0.5 M, and the concentration of aluminum sulfate octadecahydrate in the plating solution is 0.12-0.15 M.

6. The preparation method according to claim 2, characterized in that, The surfactant includes sodium dodecyl sulfate, and the concentration of sodium dodecyl sulfate in the plating solution is 0.1-0.5 g / L.

7. The preparation method according to claim 1, characterized in that, The trivalent chromium electroplating process parameters mentioned in step S1 include: a current density of 20-25 A / dm³. 2 The pH value of the plating solution is 1.5-2, the deposition temperature is 30-35℃, and the deposition time is 20-150min.

8. A trivalent chromium plating obtained by the preparation method according to any one of claims 1-7.

9. The trivalent chromium plating according to claim 8, characterized in that, It is composed of alternating layers of metallic chromium and carbon / chromium carbide mixture, with a coating thickness of 5-30 μm and a total number of 300-1800 layers.

10. The trivalent chromium plating according to claim 8, characterized in that, The chromium layer is crystalline and has a thickness of 5-30 nm; the carbon / chromium carbide mixture layer is amorphous or nanocrystalline and has a thickness of 1-3 nm.

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