Gradient oxide coating as well as preparation method and application thereof

Abstract

The invention discloses a gradient oxide coating as well as a preparation method and application thereof, and belongs to the technical field of preparation of liquid lead and bismuth corrosion resistant coatings. The preparation method comprises the following steps: by taking Cr as a target material and inert gas as sputtering gas, performing first magnetron sputtering treatment, and depositing on the surface of a substrate to form a chromium base layer; by taking Cr as a target material and inert gas and oxygen as sputtering gas, depositing on the chromium base layer to form a chromium oxide transition layer through second magnetron sputtering treatment, so as to obtain a substrate containing the chromium oxide transition layer; cr and Al are used as target materials, inert gas and oxygen are used as sputtering gas, Cr and Al are sputtered at the same time through third magnetron sputtering treatment, a gradient oxide layer is deposited on the chromic oxide transition layer, and a base body containing the gradient oxide layer is obtained; and carrying out heat treatment on the substrate containing the gradient oxide layer in a vacuum atmosphere to obtain the gradient oxide coating on the substrate. The gradient oxide coating prepared by the method has excellent lead and bismuth corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of coating preparation technology for coatings resistant to liquid lead and bismuth corrosion, and more specifically to a gradient oxide coating, its preparation method, and its application. Background Technology

[0002] To address the economic, safety, waste management, and protection challenges in nuclear power development, lead-cooled fast reactors (DFFRs) utilize molten lead or lead-bismuth alloys as the primary coolant. These fast neutron breeder reactors are expected to become the first fourth-generation nuclear energy system to achieve industrial demonstration and commercial application due to their accident resistance, miniaturization, and long lifespan. However, the primary circuit operating environment of DFFRs is extremely harsh. The cladding materials are not only affected by fast neutron irradiation and high temperatures, but also by corrosion and erosion from the liquid metal. Especially under the coupled effects of stress (such as thermal and processing stress), the severe corrosion of structural materials by liquid lead and lead-bismuth alloys has become a key issue restricting the development of DFFRs, mainly manifested as selective dissolution of alloying elements, oxide film failure, and liquid metal embrittlement (LME). Preparing an anti-corrosion coating on the cladding surface to prevent direct contact between the substrate material and the liquid metal, thereby preventing Pb-Bi liquid from corroding the material, is currently considered the most promising method internationally for solving the corrosion problem of F / M steel in lead-bismuth reactors. In this method, the design, optimization, and performance control of the coating composition are key aspects. Among them, chromium oxide (Cr2O3) and aluminum oxide (Al2O3) are the most promising protective coatings to meet the increasingly high standards of lead-cooled fast reactor cladding materials due to their high chemical stability and low oxygen diffusivity.

[0003] Among existing oxide coating technologies, alumina coatings have become a research hotspot due to their superior resistance to LBE penetration (permeability is 1–2 orders of magnitude lower than Cr2O3 coatings). However, pure Al2O3 coatings exhibit a significant mismatch in thermal expansion coefficients with the steel substrate (α-Al2O3: 8.5 × 10⁻⁶). -6 / K, Base steel: 12×10 -6 / K~14×10 -6 / K), which is prone to peeling failure under thermal cycling conditions; while the chromium oxide coating has high bonding strength with the substrate, it will dissolve chromium in the long-term LBE corrosion environment (the solubility of Cr in LBE is 200 ppm at 600℃), which leads to the gradual degradation of the protective layer.

[0004] Current mainstream preparation technologies (such as magnetron sputtering, thermal spraying, ion plating, and electroplating) mostly employ double-layer or composite structures (such as Al2O3 / Cr2O3). However, the internal stress concentration caused by the abrupt change at the two-phase interface easily leads to delamination cracking during high-temperature service. Furthermore, traditional processes struggle to precisely control the gradient transition of Al / Cr oxide composition, resulting in coatings whose long-term corrosion resistance and interfacial stability fail to meet reactor operating conditions. There are also instances where Al / Cr alloy oxide coatings prepared using mainstream technologies suffer from stress concentration due to the abrupt change between the prepared underlayer and the desired coating.

[0005] For example, in patent CN117051453A (Lead-bismuth Corrosion Protective Coating), the technology involves ionic liquid plating of Al, heat treatment to form a FeCrAl layer, electroplating of Cr, and high-temperature oxidation; however, it has a problem: the pure Al2O3 layer is mismatched with the CTE of the steel substrate (Δα=3.5×10). -6 / K~5.5×10 -6 / K), under thermal cycling conditions, interfacial stress concentration leads to spalling failure; in addition, this preparation method has high energy consumption and is difficult to process complex parts.

[0006] For example, CN119433441A (tritium-barrier oxide coating) uses magnetron sputtering Cr as the base layer, dual-target (Al / Cr) co-sputtering, and annealing. Although there is a design for the base layer and the transition layer, the abrupt change from the Cr layer to the alloy oxide layer is obvious, and the transition layer is completed in a short time (5 minutes), which still has a large thermal stress mismatch problem.

[0007] For example, CN118127446A (zirconia alloy oxidation-resistant coating) uses a low-pressure plasma spraying oxygen gradient chromium coating (Cr→CrO). x Plasma spraying, for example, involves mechanical bonding, but the bonding strength is low, which creates a risk of subsequent coating failure.

[0008] In summary, existing research indicates that pure Al2O3 coatings have excellent resistance to LBE penetration, but their coefficient of thermal expansion (CTE) is mismatched with the substrate, making them prone to peeling; pure Cr2O3 coatings have strong adhesion, but Cr dissolves in LBE during long-term service, resulting in limited protective lifespan; and traditional Al2O3 / Cr2O3 composite multilayer interfaces suffer from stress concentration, leading to delamination failure under thermal cycling conditions. Summary of the Invention

[0009] To address the above problems, this invention provides a gradient oxide coating, its preparation method, and its application. The gradient oxide coating prepared by this invention exhibits excellent resistance to lead and bismuth corrosion.

[0010] The first objective of this invention is to provide a method for preparing a gradient oxide coating, comprising the following steps: Using Cr as the target material and inert gas as the sputtering gas, a chromium-based underlayer is deposited on the substrate surface through a first magnetron sputtering process, resulting in a substrate containing a chromium-based underlayer.

[0011] Using Cr as the target material and inert gas and oxygen as sputtering gases, a second magnetron sputtering process is performed. During sputtering, the oxygen flux is increased from 0 to a flux ratio of inert gas to oxygen of 3~5:1. A chromium oxide transition layer is deposited on the chromium substrate to obtain a matrix containing the chromium oxide transition layer.

[0012] Using Cr and Al as targets and inert gas and oxygen as sputtering gases, a third magnetron sputtering process is performed to simultaneously sputter Cr and Al. During sputtering, the sputtering power of Cr is reduced from 2.5kW~4kW to 0, while the sputtering power of Al is increased from 0 to 2.5kW~4kW. A gradient oxide layer is deposited on the chromium oxide transition layer to obtain a substrate containing the gradient oxide layer.

[0013] The substrate containing the gradient oxide layer is heat-treated under a vacuum atmosphere. During the heat treatment, amorphous chromium oxide is transformed into α-Cr₂O₃, and Al ions can replace the Cr positions in α-Cr₂O₃ to form α-(Cr₂O₃). x Al 1-x A gradient oxide coating is obtained on a substrate by using a solid solution of 2O3.

[0014] An argon-to-oxygen ratio of 3-5:1 is used to maximize the oxygen flux and ensure a thorough oxidation process. Higher ratios result in target poisoning, where the target surface oxidizes, and the resulting oxides hinder further sputtering of atoms. During the experiment, a uniform increase in oxygen flux is employed to allow for the slow deposition of a gradient coating from Cr to chromium oxide, preventing adverse effects such as stress unevenness caused by sudden changes in oxygen content.

[0015] In a preferred embodiment of the present invention, during the third magnetron sputtering process, the sputtering bias voltage is -30V to -200V, the working gas pressure is 0.4Pa to 0.6Pa, and the deposition thickness of the gradient oxide layer is 0.5μm to 3μm. The thickness of the gradient oxide layer can be adjusted according to the deposition time.

[0016] In a preferred embodiment of the present invention, during the second magnetron sputtering process, the working gas pressure is 0.2 Pa to 0.6 Pa, the sputtering bias voltage is -30 V to -200 V, the working gas pressure is 0.4 Pa to 0.6 Pa, the sputtering power of the Cr target is 2.5 kW to 4 kW, and the deposition time is 8 min to 19 min. If the gas pressure is too low, there will be insufficient argon gas, resulting in insufficient sputtered ions from the macro-based target material, leading to a slow deposition rate. If the gas pressure is too high, sputtered atoms will collide frequently with the gas, be scattered, and fail to reach the substrate, thus reducing the deposition rate.

[0017] When depositing the transition layer and gradient oxide layer, the bias voltage applied to the substrate is -30V to -200V to achieve a dense and defect-free coating structure, and to improve the bonding between the transition layer and gradient oxide layer and the underlayer, thereby improving the overall bonding of the coating.

[0018] In a preferred embodiment of the present invention, the flux of the inert gas is 40 sccm to 67 sccm, and the flux of the oxygen is 8 sccm to 22 sccm.

[0019] In a preferred embodiment of the present invention, during the first magnetron sputtering process, the sputtering bias voltage is -700V to -900V, the working gas pressure is 0.4Pa to 0.6Pa, the sputtering power of the Cr target is 250W to 500W, and the deposition thickness of the chromium underlayer is 10nm to 100nm. A bias voltage below -900V will result in significant stress, affecting the subsequent coating quality; a bias voltage above -700V will result in insufficient kinetic energy, meaning the argon ions will not have enough energy to bombard the surface atoms of the substrate material, and the target atoms will not be able to embed, thus failing to achieve the purpose of underlayer deposition. The thickness of the underlayer can be adjusted according to the deposition time.

[0020] In a preferred embodiment of the present invention, the temperature of the substrate is 30°C to 40°C during the first, second, and third magnetron sputtering processes.

[0021] In a preferred embodiment of the present invention, the heat treatment is carried out at a holding temperature of 450°C to 650°C for a holding time of 2 hours to 10 hours. Treating the substrate containing the gradient oxide layer within this range ensures that chromium oxide has sufficient temperature and time for oxidation and crystallization.

[0022] The heat treatment method used in this invention involves heating to 450℃ to 650℃ at a rate of 3℃ / min to 5℃ / min, holding at that temperature for 2h to 10h, and finally cooling to room temperature in the furnace. During the heat treatment process, amorphous chromium oxide is transformed into α-Cr₂O₃, and Al ions can replace the Cr positions in α-Cr₂O₃ to form α-(Cr₂O₃). x Al 1-x The 2O3 solid solution improves the coating's resistance to lead and bismuth corrosion.

[0023] In a preferred embodiment of the present invention, the substrate is further subjected to a pretreatment to remove surface impurities before the first magnetron sputtering process.

[0024] A second objective of this invention is to provide a gradient oxide coating prepared by the above-described preparation method.

[0025] A third objective of this invention is to provide the application of the aforementioned gradient oxide coating as a lead-bismuth corrosion-resistant coating.

[0026] To completely eliminate the problem of thermal stress spalling, this invention utilizes a linearly gradual power change for dual targets (Al / Cr) (Cr decreases from 3kW to 0, Al increases from 0 to 3kW within 2 hours). This ensures the bonding strength between the film and the substrate while improving the coating surface's resistance to lead-bismuth dissolution corrosion. During preparation, an atomic-level CTE transition (ΔCTE < 0.5 × 10⁻⁶) is achieved. -6 / K), which can reduce the risk of cracking caused by the mismatch of thermal expansion coefficients of different materials; the Cr base layer formed after the first magnetron sputtering treatment (CTE≈12×10) -6 / K), Cr2O3 9.6×10 after a second magnetron sputtering treatment. -6 / K), formed by the third magnetron sputtering process, (Al,Cr)₂O₃, Al₂O₃ (8.5×10⁻⁶) -6 This four-layer gradient structure ( / K) improves bonding strength and reduces the risk of thermal mismatch, thus solving the problem of interface stress concentration that still exists in traditional stepped gradient structures.

[0027] To block element diffusion channels, this invention generates an α-(Cr,Al)₂O₃ solid solution (Al) through heat treatment (450℃~650℃). 3+ Replacement of Cr 3+ This can shrink the lattice by 0.5%, making the structure more compact and thus solving the problem that the grain boundaries are the preferred corrosion pathway in traditional mechanical hybrid coatings.

[0028] To achieve ultra-high bonding strength between the substrate and the coating, this invention obtains a substrate containing a chromium oxide transition layer through high-bias implantation (-700~-900V), forming a Cr / Fe atom mixed region (thickness ≈50nm) with the substrate surface. Annealing then promotes Cr / Fe interdiffusion, achieving metallurgical bonding. The highest bonding strength achieved in existing technologies is only 21.7N, while this invention achieves a bonding strength of 58.94N, effectively improving the bonding strength. It should be noted that the substrate material used in this invention is iron-metal alloy. The priming process involves bombarding the substrate surface with argon ions, sputtering Fe atoms, and then Cr atoms filling the vacancies of the Fe atoms to form the Cr / Fe mixed region.

[0029] To address the issue of substrate performance degradation caused by high-temperature processes, this invention controls the magnetron sputtering substrate temperature to 30℃~40℃ during the preparation process, thereby protecting the microstructure of F / M steel. Furthermore, the deposition rate is 2μm / h~3μm / h, which is 5 times faster than plasma spraying, achieving low-temperature and high-efficiency process adaptation.

[0030] Compared with the prior art, the present invention has the following beneficial effects: (1) The gradient oxide coating prepared according to the preparation method of the present invention solves the following problems: The oxide formed using Cr and Al as targets avoids the CTE step at the Al2O3 / Cr2O3 interface through gradient thermal expansion coefficient (CTE) buffer design and continuous gradual change of composition, thus eliminating the thermal stress of traditional multilayer coatings and solving the problem of coating peeling.

[0031] In the surface layer of the gradient oxidation coating, Al2O3 densification (porosity <0.5%) and (Al,Cr)2O3 solid solution grain boundary strengthening reduce the diffusion coefficient of the corrosive medium.

[0032] Annealing at 450℃~650℃ promotes the formation of α-(Al,Cr)2O3 in the amorphous phase, avoids the volume expansion caused by high-temperature phase transformation, and the gradient composition inhibits the interdiffusion of Al / Cr elements, preventing the formation of brittle FeAl2O4 spinel phase.

[0033] (2) The method for preparing the lead-bismuth corrosion-resistant gradient oxide coating provided by the present invention employs a magnetron sputtering process. The prepared coating has a dense structure, no obvious defects, low surface roughness, and good adhesion to the substrate material. The selected sputtering process parameters can obtain a dense amorphous coating at a lower temperature because during magnetron sputtering, target atoms are randomly sputtered and deposited on the substrate in a very short time, which is not enough to redistribute the atoms to form a regular crystal structure with lower energy.

[0034] (3) In the preparation method provided by the present invention, after sputtering deposition, the coated sample is subjected to an oxidative heat treatment for 2 to 10 hours. This heat treatment uses slow heating to reduce thermal stress, thereby enhancing the adhesion between the coating and the steel substrate. Furthermore, during the heat treatment process, the amorphous coating produces crystalline chromium oxide and α-(Cr) oxide. x Al 1-x The 2O3 solid solution further improves the coating's resistance to lead and bismuth corrosion.

[0035] (4) The preparation method and the lead-bismuth corrosion-resistant gradient oxide coating of the present invention solve the problem of thermal mismatch between the current single oxide coating and traditional composite coating and the substrate material. Moreover, the gradient oxide coating of the present invention effectively improves the resistance to lead-bismuth corrosion. Furthermore, the preparation method of the present invention is simple, pollution-free throughout the process, and has a fast deposition rate, thus saving costs. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the structure of the gradient oxide coating prepared according to the present invention.

[0037] Figure 2 The images shown are scanning electron microscope (SEM) images of the surface and cross-section of the coating in Example 1, where a is a surface SEM image and b is a cross-section SEM image.

[0038] Figure 3The images shown are scanning electron microscope (SEM) images of the surface and cross-section of the coating in Example 2, where a is a surface SEM image and b is a cross-section SEM image.

[0039] Figure 4 The images shown are SEM images of the surface and cross-section of the coating in Comparative Example 1, where a is the surface SEM image and b is the cross-section SEM image.

[0040] Figure 5 The images shown are SEM images of the surface and cross-section of the coating in Comparative Example 2, where a is a surface SEM image and b is a cross-section SEM image.

[0041] Figure 6 The image shows the XRD pattern of the coating in Comparative Example 2.

[0042] Figure 7 The image shows the XRD pattern of the coating in Example 1.

[0043] Figure 8 The images shown are scanning electron microscope (SEM) images of the coating surface after the scratch test in Example 1, where a is a secondary electron image and b is a backscattered electron image.

[0044] Figure 9 The images shown are scanning electron microscope (SEM) images of the coating surface after the scratch test in Comparative Example 1, where a is a secondary electron image and b is a backscattered electron image.

[0045] Figure 10 The images shown are scanning electron microscope (SEM) images of the coating surface after the scratch test in Comparative Example 2, where a is a secondary electron image and b is a backscattered electron image. Detailed Implementation

[0046] 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 scope of protection of the present invention.

[0047] To address the inherent defects of existing coatings, pure Al2O3 coatings offer excellent resistance to LBE penetration, but suffer from a large mismatch between their coefficient of thermal expansion (CTE) and the substrate, leading to easy peeling. Pure Cr2O3 coatings exhibit strong adhesion, but Cr dissolves in LBE during long-term service, limiting their protective lifespan. Traditional Al2O3 / Cr2O3 composite multilayers also suffer from stress concentration at the interface, leading to delamination failure under thermal cycling conditions. This invention considers that α-Al2O3 and α-Cr2O3 have the same crystal structure and very similar lattice constants. Furthermore, Al ions can replace Cr ions in α-Cr2O3 to form α-(Cr...) x Al 1-xThe 2O3 solid solution further improves the coating's resistance to lead and bismuth corrosion.

[0048] This invention proposes a Cr / Al co-reactive sputtering method, achieving a gradient oxide coating by gradually varying the power of both phases. During lead-bismuth corrosion, amorphous chromium oxide gradually crystallizes, creating pores that provide diffusion channels for lead, bismuth, and oxygen, resulting in poor corrosion resistance. Pre-corrosion oxidation treatment forms crystalline chromium oxide, eliminating the thermal instability issues associated with amorphous phases. Furthermore, some Al replaces Cr in the chromium oxide, forming α-(Cr...) x Al 1-x The α-O3 solid solution combines the properties of crystalline α-alumina and α-chromium oxide. Crystalline α-alumina has better oxidation and corrosion resistance than α-chromium oxide, thus improving its resistance to lead and bismuth corrosion.

[0049] When sputtering Al and Cr targets, the Al target is connected to an intermediate frequency (IF) power supply, while the Cr target is connected to a DC pulse power supply. This is because Al is highly reactive and has a strong affinity for oxygen. Using a DC power supply would easily cause oxides to form on the target surface, blocking the conductive path and resulting in target poisoning. If an IF power supply is used, its working principle is to periodically reverse the voltage polarity of its two poles, performing an electron cleanup on the target surface in each cycle, neutralizing the accumulated positive charge and interrupting and removing the forming insulating layer. Therefore, an IF power supply is used for Al. Although Cr also oxidizes, its oxidizing power is not as strong as that of Al; therefore, a DC pulse power supply is used for Cr targets.

[0050] It should be noted that, in the preparation of this invention, the substrate material is first pretreated by polishing, cleaning, and drying. The F / M steel substrate material undergoes glow discharge sputtering cleaning. Polishing the substrate material increases its surface smoothness and improves surface quality; conventional polishing methods in the art can be used. Then, silicon carbide abrasive paper of different specifications is used to polish the steel substrate surface. After polishing, the substrate is ultrasonically cleaned with alcohol and acetone for 10-20 minutes respectively, and then dried for later use. The purpose of substrate cleaning and drying is to initially remove stains from the substrate material and increase the adhesion between the coating and the substrate material. The purpose of glow discharge sputtering cleaning is to further remove impurities such as oil, water stains, oxides, dust, etc., from the surface of the substrate material, ensuring the bonding strength during subsequent coating deposition.

[0051] After pretreatment, magnetron sputtering was used in a high-vacuum environment. Argon gas was introduced and the pressure was stabilized at 0.2–0.6 Pa. A pure Cr underlayer was deposited on the surface of the F / M steel by sputtering a metallic chromium target. The vacuum level of the vacuum environment was 5.0 × 10⁻⁶ Pa. -4Below Pa. In this invention, the deposition gas pressure of magnetron sputtering co-sputtering is 0.2~0.6 Pa, and the substrate rotation speed is 1 r / min during deposition. The deposition of pure chromium as the underlayer includes the design of the underlayer process. The specific steps are as follows: under a high bias voltage of -700~-900V, the power of the Cr target is adjusted so that the sputtering rate of Cr atoms is less than or equal to the rate of argon ion etching of the substrate material surface atoms. This ensures that more Cr atoms are injected into the substrate at the interface between the underlayer and the substrate material, increasing the mixing zone between Cr and the substrate material. The purpose is to improve the bonding between the two. Cr is chosen as the underlayer because the coefficient of thermal expansion of Cr is closer to that of the F / M steel substrate material than other metals. During the experiment,

[0052] The power of the Cr target was increased from 200W to 500W because at 250W, the deposition rate of Cr is almost equal to the rate at which argon ions etch the atoms of the substrate material (i.e., argon ions knock away atoms on the surface of the substrate material, leaving some vacancies, which Cr atoms just fill). This is the priming process, which expands the blending region between Cr and the substrate material. At this stage, almost no coating is deposited. The power was slightly increased because it is very likely that in the range of 250W to 500W, the priming process is still underway (i.e., almost no coating is deposited). Depositing at 500W for a period of time ensures the completion of the priming process.

[0053] (3) Deposition of the chromium to chromium oxide transition layer Magnetron reactive sputtering technology is employed, introducing argon and oxygen to sputter a Cr target. Maintaining a stable overall working pressure, the oxygen flux is slowly increased from 0 to a stable value within 10 minutes. Specifically, the overall working pressure is stabilized at 0.2 Pa to 0.6 Pa, and the oxygen flux is uniformly increased to a stable value within 10 minutes. This design aims to enhance the bonding between the pure chromium underlayer and the gradient oxide coating; the gradual transition layer improves the performance of abrupt changes. The slow increase in oxygen flux also prevents target poisoning. When the oxygen flux is stable, the argon to oxygen ratio is 3-5:1.

[0054] (4) Deposition of gradient oxide coating After depositing the transition oxide layer, the argon and oxygen fluxes are maintained while sputtering Al and Cr targets. Within 2 hours, the Cr target power is reduced from 3kW~4kW to 0, and the Al target power is increased from 0 to 3kW~4kW, thus completing the deposition of the gradient oxide coating.

[0055] (5) Oxidative heat treatment The gradient oxide coating was placed in a tube annealing furnace, and the vacuum was reduced to less than 1×10⁻⁶. -3 Pa, heat to 450~650℃ and hold for 2~10h to obtain an oxide gradient coating containing crystalline oxides.

[0056] This invention employs magnetron reactive sputtering technology to control the bonding strength and composition of gradient oxide coatings through the design and adjustment of process parameters. The improved bonding strength is primarily achieved through the design of the primer process, which works by using high bias voltage and low target power to find a Cr atom deposition rate equal to the argon ion etching rate, thereby increasing the blending zone between the coating and the substrate and thus enhancing the bonding strength. The gradient CTE buffer design and continuous compositional gradient avoid CTE abrupt changes at the Al2O3 / Cr2O3 interface, eliminating the thermal stress of traditional multilayer coatings and solving the problem of coating peeling. The gradient oxide film deposited by magnetron sputtering is amorphous; further oxidation heat treatment can achieve partial crystallization of the coating, thereby altering the film's properties.

[0057] In the gradient oxide coating prepared by this invention, the Cr base layer is approximately 100 nm thick, the Cr-to-chromium oxide gradient layer is approximately 100 nm thick, and the chromium oxide-to-alumina gradient layer is approximately 2 μm thick. Theoretically, the thicker the coating, the better the corrosion resistance. However, excessively thick coatings can lead to adverse effects such as reduced thermal conductivity. Furthermore, there is an upper limit to the thickness of non-metallic coatings deposited on metals; the thicker the coating, the greater the stress and the greater the risk of thermal mismatch.

[0058] The matrix material used in this invention is iron and steel. The purity of the Al target used was 99.9 wt.%; the purity of the Cr target was 99.9 wt.%.

[0059] It should be noted that the temperature of the substrate is set to 30℃~40℃ during the preparation process of this invention.

[0060] Example 1 This embodiment provides a gradient oxide coating resistant to lead and bismuth corrosion and its preparation method, including the following steps: Step 1, Pretreatment of the substrate material: Prepare a T91 steel sheet with sides of 1.7cm × 1.7cm and a thickness of 1.5cm. The composition consists of the following elements by mass percentage: Cr 8.40%, Mn 0.47%, Mo 0.91%, Si 0.28%. The substrate surface is then polished sequentially with 800#, 1200#, 2000#, 3000#, and 5000# sandpaper. Next, it is polished with a 1μm diamond suspension and a 0.1μm silica suspension. The sample is then ultrasonically cleaned in an alcohol solution for 20 minutes, removed, dried, and placed in a vacuum chamber for later use.

[0061] The sample is loaded onto the workpiece gantry of the magnetron sputtering vacuum chamber, and two metal Al targets and one metal chromium target are placed on the target positions in the magnetron sputtering vacuum chamber. The metal Al targets are connected to the intermediate frequency power supply, and the metal chromium targets are connected to the DC pulse power supply.

[0062] Evacuate the vacuum chamber of the magnetron sputtering equipment to 5×10⁻⁶ ppm. -4 Below Pa, argon gas is introduced to stabilize the gas pressure at around 0.6 Pa. A bias voltage of -900V is applied to the matrix material, and the sample is sputtered and cleaned for 10 minutes.

[0063] Step 2, Cr layer deposition and underlay process: Control the argon gas flow rate and stabilize the gas pressure at 0.5 Pa. Turn on the Cr target power supply and set it to 250 W. Deposit for 20 min to obtain a larger blending zone. Slowly increase the Cr target power to 500 W and deposit for 10 min to obtain the Cr underlay.

[0064] Step 3, Deposition of the transition layer: The gas pressure is stabilized at 0.5 Pa, the Cr target power is increased to 3 kW, and the bias voltage is simultaneously reduced from -900 V to -60 V. Oxygen is introduced, and the oxygen flux is uniformly increased at a rate of 1 sccm / min over 19 min until the argon flux to oxygen flux ratio is 3:1. At this point, the argon flux is 57 sccm and the oxygen flux is 19 sccm, thus obtaining the transition layer.

[0065] Step 4, deposition of gradient oxide layer: Maintain gas pressure at 0.5 Pa, bias voltage at -60 V, and argon to oxygen flux ratio at 3:1. The Cr target power is uniformly reduced from 3 kW to 0 at a rate of 16.7 nm / min, while the Al target power is uniformly increased from 0 to 3 kW at a rate of 16.7 nm / min. This gradual process takes 2 hours to obtain the gradient oxide layer.

[0066] After sputtering, stop the introduction of argon and oxygen, maintain a high vacuum, and remove the sample when the substrate temperature is below 50°C.

[0067] Step 5, Oxidation heat treatment: The coating is subjected to oxidation heat treatment under an argon atmosphere at a pressure of 5 × 10⁻⁶. - 3 Pa, heating rate of 4℃ / min, heat treatment temperature of 550℃, holding time of 2h, and then cooled to room temperature with the furnace.

[0068] The total deposition time in this embodiment was 3 hours. Testing showed that the Cr layer was approximately 100 nm thick at the bottom layer, approximately 100 nm thick at the transition layer, and approximately 2 μm thick at the gradient oxide layer. The total coating thickness was 2.3 μm.

[0069] Example 2 This embodiment provides a gradient oxide coating resistant to lead and bismuth corrosion and its preparation method, including the following steps: Step 1, Pretreatment of the substrate material: Prepare a T91 steel sheet with sides of 1.7cm × 1.7cm and a thickness of 1.5cm. The composition consists of the following elements by mass percentage: Cr 8.40%, Mn 0.47%, Mo 0.91%, Si 0.28%. The substrate surface is then polished sequentially with 800#, 1200#, 2000#, 3000#, and 5000# sandpaper. Next, it is polished with a 1μm diamond suspension and a 0.1μm silica suspension. The sample is then ultrasonically cleaned in an alcohol solution for 20 minutes, removed, dried, and placed in a vacuum chamber for later use.

[0070] The sample is loaded onto the workpiece gantry of the magnetron sputtering vacuum chamber, and two metal Al targets and one metal chromium target are placed on the target positions in the magnetron sputtering vacuum chamber. The metal Al targets are connected to the intermediate frequency power supply, and the metal chromium targets are connected to the DC pulse power supply.

[0071] Evacuate the vacuum chamber of the magnetron sputtering equipment to 5×10⁻⁶ ppm. -4 Below Pa, argon gas is introduced to stabilize the gas pressure at around 0.6 Pa. A bias voltage of -900V is applied to the matrix material, and the sample is sputtered and cleaned for 10 minutes.

[0072] Step 2, Cr layer deposition and underlay process: Control the argon gas flow rate and stabilize the gas pressure at 0.5 Pa. Turn on the Cr target power supply and set it to 250 W. Deposit for 20 min to obtain a larger blending zone. Slowly increase the Cr target power to 500 W and deposit for 10 min to obtain the Cr underlay.

[0073] Step 3, Deposition of the transition layer: The gas pressure is stabilized at 0.5 Pa, the Cr target power is increased to 2.5 kW, and the bias voltage is simultaneously reduced from -900 V to -30 V. Oxygen is introduced, and the oxygen flux is uniformly increased at a rate of 1 sccm / min over approximately 19 min until the argon flux to oxygen flux ratio is 3:1. At this point, the argon flux is 57 sccm and the oxygen flux is 19 sccm, thus obtaining the transition layer.

[0074] Step 4, deposition of gradient oxide layer: Maintain gas pressure at 0.5 Pa, bias voltage at -30 V, and argon to oxygen flux ratio at 3:1. The power of Cr is uniformly reduced from 2.5 kW to 0 at a rate of 16.7 nm / min, while the power of Al target is slowly and uniformly increased from 0 to 2.5 kW at a rate of 16.7 nm / min. This gradual process takes 2 hours to obtain the gradient oxide layer.

[0075] After sputtering, stop the introduction of argon and oxygen, maintain a high vacuum, and remove the sample when the substrate temperature is below 50°C.

[0076] Step 5, Oxidation heat treatment: The coating is subjected to oxidation heat treatment under an argon atmosphere at a pressure of 5 × 10⁻⁶.- 3 Pa, heating rate of 4℃ / min, heat treatment temperature of 550℃, holding time of 2h, and then cooled to room temperature with the furnace.

[0077] The total deposition time in this embodiment is 3 hours. According to the test, the Cr layer is about 100 nm thick, the Cr layer is about 100 nm thick, the Cr layer is about 2.3 μm thick, and the total thickness of the coating is 2.6 μm.

[0078] Example 3 This embodiment provides a gradient oxide coating resistant to lead and bismuth corrosion and its preparation method, including the following steps: Step 1, Pretreatment of the substrate material: Prepare a T91 steel sheet with sides of 1.7cm × 1.7cm and a thickness of 1.5cm. The composition consists of the following elements by mass percentage: Cr 8.40%, Mn 0.47%, Mo 0.91%, Si 0.28%. The substrate surface is then polished sequentially with 800#, 1200#, 2000#, 3000#, and 5000# sandpaper. Next, it is polished with a 1μm diamond suspension and a 0.1μm silica suspension. The sample is then ultrasonically cleaned in an alcohol solution for 20 minutes, removed, dried, and placed in a vacuum chamber for later use.

[0079] The sample is loaded onto the workpiece gantry of the magnetron sputtering vacuum chamber, and two metal Al targets and one metal chromium target are placed on the target positions in the magnetron sputtering vacuum chamber. The metal Al targets are connected to the intermediate frequency power supply, and the metal chromium targets are connected to the DC pulse power supply.

[0080] Evacuate the vacuum chamber of the magnetron sputtering equipment to 5×10⁻⁶ ppm. - 4 Below Pa, argon gas is introduced to stabilize the gas pressure at around 0.6 Pa. A bias voltage of -900V is applied to the matrix material, and the sample is sputtered and cleaned for 10 minutes.

[0081] Step 2, Cr layer deposition and underlay process: Control the argon gas flow rate and stabilize the gas pressure at 0.5 Pa. Turn on the Cr target power supply and set it to 250 W. Deposit for 20 min to obtain a larger blending zone. Slowly increase the Cr target power to 500 W and deposit for 10 min to obtain the Cr underlay.

[0082] Step 3, Deposition of the transition layer: The gas pressure is stabilized at 0.5 Pa, the Cr target power is increased to 3 kW, and the bias voltage is simultaneously reduced from -900 V to -200 V. Oxygen is introduced, and the oxygen flux is uniformly increased at a rate of 1 sccm / min over approximately 19 min until the argon flux to oxygen flux ratio is 3:1. At this point, the argon flux is 57 sccm and the oxygen flux is 19 sccm, thus obtaining the transition layer.

[0083] Step 4, deposition of gradient oxide layer: Maintain gas pressure at 0.5 Pa, bias voltage at -200 V, and argon to oxygen flux ratio at 3:1. The power of Cr is uniformly reduced from 3 kW to 0 at a rate of 16.7 nm / min, while the power of Al target is uniformly increased from 0 to 3 kW at a rate of 16.7 nm / min. This gradual process takes 2 h to obtain gradient oxide.

[0084] After sputtering, stop the introduction of argon and oxygen, maintain a high vacuum, and remove the sample when the substrate temperature is below 50°C.

[0085] Step 5, Oxidation heat treatment: The coating is subjected to oxidation heat treatment under an argon atmosphere at a pressure of 5 × 10⁻⁶. - 3 Pa, heating rate of 4℃ / min, heat treatment temperature of 550℃, holding time of 2h, and then cooled to room temperature with the furnace.

[0086] The total deposition time in this embodiment was 3 hours, and the total thickness of the coating was measured to be 1.98 μm.

[0087] Example 4 This embodiment provides a gradient oxide coating resistant to lead and bismuth corrosion and its preparation method, including the following steps: Step 1, Pretreatment of the substrate material: Prepare a T91 steel sheet with sides of 1.7cm × 1.7cm and a thickness of 1.5cm. The composition consists of the following elements by mass percentage: Cr 8.40%, Mn 0.47%, Mo 0.91%, Si 0.28%. The substrate surface is then polished sequentially with 800#, 1200#, 2000#, 3000#, and 5000# sandpaper. Next, it is polished with a 1μm diamond suspension and a 0.1μm silica suspension. The sample is then ultrasonically cleaned in an alcohol solution for 20 minutes, removed, dried, and placed in a vacuum chamber for later use.

[0088] The sample is loaded onto the workpiece gantry of the magnetron sputtering vacuum chamber, and two metal Al targets and one metal chromium target are placed on the target positions in the magnetron sputtering vacuum chamber. The metal Al targets are connected to the intermediate frequency power supply, and the metal chromium targets are connected to the DC pulse power supply.

[0089] Evacuate the vacuum chamber of the magnetron sputtering equipment to 5×10⁻⁶ ppm. -4 Below Pa, argon gas is introduced to stabilize the gas pressure at around 0.6 Pa. A bias voltage of -900V is applied to the matrix material, and the sample is sputtered and cleaned for 10 minutes.

[0090] Step 2, Cr layer deposition and underlay process: Control the argon gas flow rate and stabilize the gas pressure at 0.5 Pa. Turn on the Cr target power supply and set it to 250 W. Deposit for 20 min to obtain a larger blending zone. Slowly increase the Cr target power to 500 W and deposit for 10 min to obtain the Cr underlay.

[0091] Step 3, Deposition of the transition layer: The gas pressure is stabilized at 0.5 Pa, the Cr target power is increased to 3 kW, and the bias voltage is simultaneously reduced from -900 V to -60 V. Oxygen is introduced, and the oxygen flux is uniformly increased at a rate of 1 sccm / min over approximately 19 min until the argon flux to oxygen flux ratio is 3:1. At this point, the argon flux is 57 sccm and the oxygen flux is 19 sccm, thus obtaining the transition layer.

[0092] Step 4, deposition of gradient oxide layer: Maintain gas pressure at 0.5 Pa, bias voltage at -60 V, and argon to oxygen flux ratio at 3:1. The Cr target power is uniformly reduced from 3 kW to 0 at a rate of 16.7 nm / min, while the Al target power is uniformly increased from 0 to 3 kW at a rate of 16.7 nm / min. This gradual process takes 2 h to obtain gradient oxide.

[0093] After sputtering, stop the introduction of argon and oxygen, maintain a high vacuum, and remove the sample when the substrate temperature is below 50°C.

[0094] Step 5, Oxidation heat treatment: The coating is subjected to oxidation heat treatment under an argon atmosphere at a pressure of 5 × 10⁻⁶. - 3 Pa, heating rate of 4℃ / min, heat treatment temperature of 450℃, holding time of 2h, and then cooled to room temperature in the furnace.

[0095] The total deposition time in this embodiment is 3 hours. According to the test, the Cr layer is about 100 nm thick, the Cr layer is about 100 nm thick, the Cr layer is about 2.3 μm thick, and the total thickness of the coating is 2.61 μm.

[0096] Example 5 This embodiment provides a gradient oxide coating resistant to lead and bismuth corrosion and its preparation method, including the following steps: Step 1, Pretreatment of the substrate material: Prepare a T91 steel sheet with sides of 1.7cm × 1.7cm and a thickness of 1.5cm. The composition consists of the following elements by mass percentage: Cr 8.40%, Mn 0.47%, Mo 0.91%, Si 0.28%. The substrate surface is then polished sequentially with 800#, 1200#, 2000#, 3000#, and 5000# sandpaper. Next, it is polished with a 1μm diamond suspension and a 0.1μm silica suspension. The sample is then ultrasonically cleaned in an alcohol solution for 20 minutes, removed, dried, and placed in a vacuum chamber for later use.

[0097] The sample is loaded onto the workpiece gantry of the magnetron sputtering vacuum chamber, and two metal Al targets and one metal chromium target are placed on the target positions in the magnetron sputtering vacuum chamber. The metal Al targets are connected to the intermediate frequency power supply, and the metal chromium targets are connected to the DC pulse power supply.

[0098] Evacuate the vacuum chamber of the magnetron sputtering equipment to 5×10⁻⁶ ppm. -4 Below Pa, argon gas is introduced to stabilize the gas pressure at around 0.6 Pa. A bias voltage of -900V is applied to the matrix material, and the sample is sputtered and cleaned for 10 minutes.

[0099] Step 2, Cr layer deposition and underlay process: Control the argon gas flow rate and stabilize the gas pressure at 0.5 Pa. Turn on the Cr target power supply and set it to 250 W. Deposit for 20 min to obtain a larger blending zone. Slowly increase the Cr target power to 500 W and deposit for 10 min to obtain the Cr underlay.

[0100] Step 3, Deposition of the transition layer: The gas pressure is stabilized at 0.5 Pa, the Cr target power is slowly increased to 3 kW, and the bias voltage is simultaneously reduced from -900 V to -200 V. Oxygen is introduced, and the oxygen flux is uniformly increased at a rate of 1 sccm / min over approximately 19 min until the argon flux to oxygen flux ratio is 3:1. At this point, the argon flux is 57 sccm and the oxygen flux is 19 sccm, thus obtaining the transition layer.

[0101] Step 4, deposition of gradient oxide layer: Maintain gas pressure at 0.5 Pa, bias voltage at -200 V, and argon to oxygen flux ratio at 3:1. The power of Cr is uniformly reduced from 3 kW to 0 at a rate of 16.7 nm / min, while the power of Al target is uniformly increased from 0 to 3 kW at a rate of 16.7 nm / min. This gradual process takes 2 h to obtain gradient oxide.

[0102] After sputtering, stop the introduction of argon and oxygen, maintain a high vacuum, and remove the sample when the substrate temperature is below 50°C.

[0103] Step 5, Oxidation heat treatment: The coating is subjected to oxidation heat treatment under an argon atmosphere at a pressure of 5 × 10⁻⁶.- 3 Pa, heating rate of 4℃ / min, heat treatment temperature of 650℃, holding time of 2h, and then cooled to room temperature in the furnace.

[0104] The total deposition time in this embodiment is 3 hours. According to the test, the Cr layer is about 100 nm thick, the Cr layer is about 100 nm thick, the Cr layer is about 2 μm thick, and the Cr layer is about 2 μm thick. The total thickness of the coating is 2.34 μm.

[0105] Example 6 This embodiment provides a gradient oxide coating resistant to lead and bismuth corrosion and its preparation method, including the following steps: Step 1, Pretreatment of the substrate material: Prepare a T91 steel sheet with sides of 1.7cm × 1.7cm and a thickness of 1.5cm. The composition consists of the following elements by mass percentage: Cr 8.40%, Mn 0.47%, Mo 0.91%, Si 0.28%. The substrate surface is then polished sequentially with 800#, 1200#, 2000#, 3000#, and 5000# sandpaper. Next, it is polished with a 1μm diamond suspension and a 0.1μm silica suspension. The sample is then ultrasonically cleaned in an alcohol solution for 20 minutes, removed, dried, and placed in a vacuum chamber for later use.

[0106] The sample is loaded onto the workpiece gantry of the magnetron sputtering vacuum chamber, and two metal Al targets and one metal chromium target are placed on the target positions in the magnetron sputtering vacuum chamber. The metal Al targets are connected to the intermediate frequency power supply, and the metal chromium targets are connected to the DC pulse power supply.

[0107] Evacuate the vacuum chamber of the magnetron sputtering equipment to 5×10⁻⁶ ppm. -4 Below Pa, argon gas is introduced to stabilize the gas pressure at around 0.6 Pa. A bias voltage of -700V is applied to the matrix material, and the sample is sputtered and cleaned for 10 minutes.

[0108] Step 2, Cr layer deposition and underlay process: Control the argon gas flow rate and stabilize the gas pressure at 0.4 Pa. Turn on the Cr target power supply and set it to 250 W. Deposit for 20 min to obtain a larger blending zone. Slowly increase the Cr target power to 500 W and deposit for 10 min to obtain the Cr underlay.

[0109] Step 3, Deposition of the transition layer: The gas pressure is stabilized at 0.4 Pa, the Cr target power is slowly increased to 4 kW, and the bias voltage is simultaneously reduced from -900 V to -200 V. Oxygen is introduced, and the oxygen flux is uniformly increased at a rate of 1 sccm / min over approximately 8 minutes until the argon flux to oxygen flux ratio is 5:1. At this point, the argon flux is 40 sccm and the oxygen flux is 8 sccm, thus obtaining the transition layer.

[0110] Step 4, deposition of gradient oxide layer: Maintain gas pressure at 0.4 Pa, bias voltage at -200 V, and argon to oxygen flux ratio at 5:1. The power of Cr is uniformly reduced from 4 kW to 0 at a rate of 16.7 nm / min, while the power of Al target is uniformly increased from 0 to 4 kW at a rate of 16.7 nm / min. This gradual process takes 2 h to obtain gradient oxide.

[0111] After sputtering, stop the introduction of argon and oxygen, maintain a high vacuum, and remove the sample when the substrate temperature is below 50°C.

[0112] Step 5, Oxidation heat treatment: The coating is subjected to oxidation heat treatment under an argon atmosphere at a pressure of 5 × 10⁻⁶. - 3 Pa, heating rate of 4℃ / min, heat treatment temperature of 450℃, holding time of 10h, and then cooled to room temperature with the furnace.

[0113] Example 7 This embodiment provides a gradient oxide coating resistant to lead and bismuth corrosion and its preparation method, including the following steps: Step 1, Pretreatment of the substrate material: Prepare a T91 steel sheet with sides of 1.7cm × 1.7cm and a thickness of 1.5cm. The composition consists of the following elements by mass percentage: Cr 8.40%, Mn 0.47%, Mo 0.91%, Si 0.28%. The substrate surface is then polished sequentially with 800#, 1200#, 2000#, 3000#, and 5000# sandpaper. Next, it is polished with a 1μm diamond suspension and a 0.1μm silica suspension. The sample is then ultrasonically cleaned in an alcohol solution for 20 minutes, removed, dried, and placed in a vacuum chamber for later use.

[0114] The sample is loaded onto the workpiece gantry of the magnetron sputtering vacuum chamber, and two metal Al targets and one metal chromium target are placed on the target positions in the magnetron sputtering vacuum chamber. The metal Al targets are connected to the intermediate frequency power supply, and the metal chromium targets are connected to the DC pulse power supply.

[0115] Evacuate the vacuum chamber of the magnetron sputtering equipment to 5×10⁻⁶ ppm. -4 Below Pa, argon gas is introduced to stabilize the gas pressure at around 0.6 Pa. A bias voltage of -800V is applied to the matrix material, and the sample is sputtered and cleaned for 10 minutes.

[0116] Step 2, Cr layer deposition and underlay process: Control the argon gas flow rate and stabilize the gas pressure at 0.6 Pa. Turn on the Cr target power supply and set it to 250 W. Deposit for 20 min to obtain a larger blending zone. Slowly increase the Cr target power to 500 W and deposit for 10 min to obtain the Cr underlay.

[0117] Step 3, Deposition of the transition layer: Maintain the gas pressure at 0.6 Pa, slowly increase the Cr target power to 3 kW, and at the same time reduce the bias voltage from -900 V to -200 V. Introduce oxygen and uniformly increase the oxygen flux at a rate of 1 sccm / min for about 17 min until the argon flux to oxygen flux ratio is 4:1. At this point, the argon flux is 67 sccm and the oxygen flux is 16.75 sccm, thus obtaining the transition layer.

[0118] Step 4, deposition of gradient oxide layer: Maintain gas pressure at 0.6 Pa, bias voltage at -200 V, and argon to oxygen flux ratio at 4:1. The power of Cr is uniformly reduced from 3 kW to 0 at a rate of 16.7 nm / min, while the power of Al target is uniformly increased from 0 to 3 kW at a rate of 16.7 nm / min. This gradual process takes 2 h to obtain gradient oxide.

[0119] After sputtering, stop the introduction of argon and oxygen, maintain a high vacuum, and remove the sample when the substrate temperature is below 50°C.

[0120] Step 5, Oxidation heat treatment: The coating is subjected to oxidation heat treatment under an argon atmosphere at a pressure of 5 × 10⁻⁶. - 3 Pa, heating rate of 4℃ / min, heat treatment temperature of 450℃, holding time of 5h, and then cooled to room temperature in the furnace.

[0121] Comparative Example 1 This comparative example provides a gradient oxide coating resistant to lead-bismuth corrosion and its preparation method, comprising the following steps: Step 1, Pretreatment of the substrate material: Prepare a T91 steel sheet with sides of 1.7cm × 1.7cm and a thickness of 1.5cm. The composition consists of the following elements by mass percentage: Cr 8.40%, Mn 0.47%, Mo 0.91%, Si 0.28%. The substrate surface is then polished sequentially with 800#, 1200#, 2000#, 3000#, and 5000# sandpaper. Next, it is polished with a 1μm diamond suspension and a 0.1μm silica suspension. The sample is then ultrasonically cleaned in an alcohol solution for 20 minutes, removed, dried, and placed in a vacuum chamber for later use.

[0122] The sample is loaded onto the workpiece gantry of the magnetron sputtering vacuum chamber, and two metal Al targets and one metal chromium target are placed on the target positions in the magnetron sputtering vacuum chamber. The metal Al targets are connected to the intermediate frequency power supply, and the metal chromium targets are connected to the DC pulse power supply.

[0123] Evacuate the vacuum chamber of the magnetron sputtering equipment to 5×10⁻⁶ ppm. -4Below Pa, argon gas is introduced to stabilize the gas pressure at around 0.6 Pa. A bias voltage of -900V is applied to the matrix material, and the sample is sputtered and cleaned for 10 minutes.

[0124] Step 2, Deposition of the transition layer: Control the argon gas flux and stabilize the gas pressure at 0.5 Pa. Turn on the Cr target power supply and increase the Cr target power from 0 to 3 kW. At the same time, reduce the bias voltage from -900 V to -60 V and introduce oxygen. After about 19 min, uniformly increase the oxygen flux at a rate of 1 sccm / min until the ratio of argon gas flux to oxygen flux is 3:1. At this time, the argon gas flux is 57 sccm and the oxygen flux is 19 sccm, thus obtaining the transition layer.

[0125] Step 3, deposition of gradient oxide layer: Maintain gas pressure at 0.5 Pa, bias voltage at -60 V, and argon to oxygen flux ratio at 3:1. The power of Cr is uniformly reduced from 3 kW to 0 at a rate of 16.7 nm / min, while the power of Al target is uniformly increased from 0 to 3 kW at a rate of 16.7 nm / min. This gradual process takes 2 h to obtain gradient oxide.

[0126] After sputtering, stop the introduction of argon and oxygen, maintain a high vacuum, and remove the sample when the substrate temperature is below 50°C.

[0127] Step 4, Oxidation Heat Treatment: The coating is subjected to oxidation heat treatment under an argon atmosphere at a pressure of 5 × 10⁻⁶. - 3 Pa, heating rate of 4℃ / min, heat treatment temperature of 550℃, holding time of 2h, and then cooled to room temperature with the furnace.

[0128] The only difference between this comparative example and Example 1 is that: This comparative example does not involve a pre-treatment process or the deposition of an underlayer; instead, it directly deposits a transition layer and a gradient oxide coating.

[0129] Comparative Example 2 This comparative example provides a gradient oxide coating resistant to lead-bismuth corrosion and its preparation method, comprising the following steps: Step 1, Pretreatment of the substrate material: Prepare a T91 steel sheet with sides of 1.7cm × 1.7cm and a thickness of 1.5cm. The composition consists of the following elements by mass percentage: Cr 8.40%, Mn 0.47%, Mo 0.91%, Si 0.28%. The substrate surface is then polished sequentially with 800#, 1200#, 2000#, 3000#, and 5000# sandpaper. Next, it is polished with a 1μm diamond suspension and a 0.1μm silica suspension. The sample is then ultrasonically cleaned in an alcohol solution for 20 minutes, removed, dried, and placed in a vacuum chamber for later use.

[0130] The sample is loaded onto the workpiece gantry of the magnetron sputtering vacuum chamber, and two metal Al targets and one metal chromium target are placed on the target positions in the magnetron sputtering vacuum chamber. The metal Al targets are connected to the intermediate frequency power supply, and the metal chromium targets are connected to the DC pulse power supply.

[0131] Evacuate the vacuum chamber of the magnetron sputtering equipment to 5×10⁻⁶ ppm. -4 Below Pa, argon gas is introduced to stabilize the gas pressure at around 0.6 Pa. A bias voltage of -900V is applied to the matrix material, and the sample is sputtered and cleaned for 10 minutes.

[0132] Step 2, Cr layer deposition and underlay process: Control the argon gas flow rate and stabilize the gas pressure at 0.5 Pa. Turn on the Cr target power supply and set it to 250 W. Deposit for 20 min to obtain a larger blending zone. Slowly increase the Cr target power to 500 W and deposit for 10 min to obtain the Cr underlay.

[0133] Step 3, Deposition of the transition layer: The gas pressure is stabilized at 0.5 Pa, the Cr target power is increased to 3 kW, and the bias voltage is simultaneously reduced from -900 V to -60 V. Oxygen is introduced, and the oxygen flux is uniformly increased at a rate of 1 sccm / min over approximately 19 min until the argon flux to oxygen flux ratio is 3:1. At this point, the argon flux is 57 sccm and the oxygen flux is 19 sccm, thus obtaining the transition layer.

[0134] Step 4, deposition of gradient oxide layer: Maintain gas pressure at 0.5 Pa, bias voltage at -60 V, and argon to oxygen flux ratio at 3:1. The Cr target power is uniformly reduced from 3 kW to 0 at a rate of 16.7 nm / min, while the Al target power is uniformly increased from 0 to 3 kW at a rate of 16.7 nm / min. This gradual process takes 2 h to obtain gradient oxide.

[0135] After sputtering, stop the introduction of argon and oxygen, maintain a high vacuum, and remove the sample when the substrate temperature is below 50°C.

[0136] The only difference between this comparative example and Example 1 is that: This comparative example does not undergo oxidation heat treatment; the deposited gradient oxide coating is directly subjected to lead-bismuth corrosion experiments.

[0137] Experimental Section (a) Morphological test The present invention uses the coatings of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 as examples, and performs scanning electron microscopy tests on them respectively. The test results are as follows: Figures 2-5 As shown.

[0138] Figure 4 and Figure 2 and Figure 3In comparison, it can be seen that Examples 1 and 2 have obvious base and transition layers, while the comparative example does not. Furthermore, the coating in the comparative example is not tightly bonded to the substrate, while the coatings in Examples 1 and 2 are firmly bonded to the substrate. The coating surfaces are all dense and without obvious defects.

[0139] Figure 5 The above-ground surface and cross-sectional topography diagrams are shown in Comparative Example 2. Figure 2 In contrast, the untreated material does not bond tightly and firmly with the substrate. However, the surface remains dense and uniform, without obvious defects.

[0140] (ii) XRD test The present invention takes the coatings of Example 1 and Comparative Example 2 as examples, and performs XRD tests on them respectively, as shown below. Figure 6 and Figure 7 As shown.

[0141] Figure 6 The coating is not heat-treated. It can be seen that the oxide coating is amorphous and has a few metallic phases remaining.

[0142] Figure 7 The image shows the coating after heat treatment. It can be seen that the characteristic diffraction peaks of the α-Al₂O₃ phase are not present in the coating. This is because the phase transformation temperature of Al₂O₃ was not reached at this heat treatment temperature, and Al₂O₃ remains amorphous. The presence of characteristic diffraction peaks of the α-Cr₂O₃ phase indicates that after heat treatment at 550℃, the Cr₂O₃ phase in the coating has transformed from an amorphous state to the crystalline α-Cr₂O₃ phase. It is noteworthy that the peak positions of the α-Cr₂O₃ phase are shifted towards larger angles. This is due to the presence of some Al₂O₃ in the coating. 3+ It replaced Cr in crystalline α-Cr2O3 3+ This reduces the lattice spacing, forming α-(Cr) x Al 1-x )2O3 solid solution.

[0143] (iii) Combined strength test Taking the coatings of Example 1, Comparative Example 1, and Comparative Example 2 as examples, scratch tests were conducted on each. The surface morphology images include secondary electron imaging and backscattering imaging, as shown below. Figures 8-10 As shown.

[0144] Figure 8 This is a topographic image of the heat-treated coating in Example 1 after it has been scratched. Figure 9 The morphology of the coating after heat treatment without the deposited underlayer is shown in the figure. It can be seen that in Example 1, the critical load of the coating is about 58.94 N, and in Comparative Example 1, the critical load of the coating is about 8 N. This shows that the design of the underlayer process and the deposition of the underlayer greatly increase the bonding between the coating and the substrate.

[0145] Figure 10 The morphology of the coating after scratching without heat treatment shows that in Example 1, the critical load of the coating is about 58.94 N, and in Comparative Example 2, the critical load of the coating is about 21.7 N. This indicates that heat treatment promotes the diffusion of Cr in the underlayer and Fe in the matrix, greatly increasing the bonding between the matrix material and the coating.

[0146] (iv) Lead-bismuth corrosion resistance test This invention conducted lead-bismuth corrosion experiments on Examples 1-5 and Comparative Examples 1-2. Scanning electron microscopy and energy dispersive spectroscopy analyses were performed after the corrosion tests. The results showed that the coatings of Examples 1-5 and Comparative Example 2 all prevented Pb / Bi penetration and oxygen diffusion to a certain extent. Example 1 showed the best effect, with almost no Pb / Bi penetration and very low oxygen diffusion content, indicating crystalline chromium oxide and α-(Cr) x Al 1-x The presence of α-(Cr₂O₃) solid solution further enhances corrosion resistance. In contrast, Comparative Example 1, lacking a primer layer, experienced coating peeling and severe corrosion of the substrate during the corrosion process. This indicates that coating corrosion resistance is determined by multiple factors. A primer layer improves the bonding between the coating and the substrate, providing a basic guarantee for subsequent corrosion resistance. Furthermore, appropriate heat treatment allows the amorphous coating to crystallize and produce α-(Cr₂O₃) solid solution. x Al 1-x The 2O3 solid solution can further prevent the penetration of lead and bismuth and the inward diffusion of oxygen, thus effectively protecting the matrix material from corrosion.

[0147] Pure Al2O3 coating (CTE=8.5×10) -6 / K) and steel matrix (CTE=12–14×10 -6 There is a significant difference in CTE between / K, and the interface stress concentration under thermal cycling conditions leads to spalling failure.

[0148] To address the coating peeling problem caused by thermal expansion coefficient mismatch, this invention employs a gradient CTE buffer design to achieve Cr2O3 (CTE≈9.6×10) -6 The continuous compositional gradient from Al2O3 to Al2O3 eliminates the interface CTE step of traditional Al2O3 / Cr2O3 bilayer coatings, thus avoiding the accumulation of thermal stress.

[0149] Pure Cr2O3 coatings undergo chromium dissolution in high-temperature LBE (solubility reaches 200 ppm at 600℃), leading to protective layer degradation. To address the protective failure caused by chromium dissolution during long-term service, this invention densifies the surface Al2O3 layer (porosity <0.5%) to block LBE penetration; and forms an α-(Cr,Al)2O3 solid solution through heat treatment, utilizing Al...3+ Cr in the substitution lattice 3+ It inhibits the dissolution of chromium while retaining the high bonding strength of Cr2O3.

[0150] Traditional Al2O3 / Cr2O3 composite coatings suffer from high-temperature delamination failure due to abrupt changes at the two-phase interface and stress concentration. To address the problem of delamination and cracking at the interface of multilayer coatings, this invention employs dual-target magnetron co-sputtering and a power gradient technique (Cr target power is reduced from 3kW to 0, while Al target power is increased from 0 to 3kW) to achieve a continuous compositional transition from Cr2O3 to Al2O3, thus eliminating abrupt stress at the interface.

[0151] Traditional coatings are prone to peeling off entirely in corrosive environments due to weak film-substrate adhesion. To address the problem of insufficient adhesion between the coating and the substrate, this invention employs a high-bias undercoating process (-700~-900V): controlling the Cr deposition rate to be less than or equal to the argon ion etching rate, forming a Cr / substrate mixed transition zone to enhance mechanical and metallurgical bonding; gradient composition design: avoiding the formation of brittle FeAl2O4 spinel phase and maintaining interface stability; and oxidation heat treatment: promoting Cr / Fe interdiffusion and further improving the bonding strength (critical load reaches 58.94N, 7 times higher than coatings without an undercoating).

[0152] Magnetron sputtering / thermal spraying processes struggle to control the gradual transition of Al / Cr oxide composition. To address the challenge of precise gradient control in traditional processes, this invention employs dual-power synergistic reactive sputtering:

[0153] Step 1: Pure Cr as the bottom layer (anaerobic environment).

[0154] Step 2: The O2 flux increases from 0 to a stable value, forming a transition layer from Cr to Cr2O3.

[0155] Step 3: Gradual power variation of Cr / Al dual targets (2h) to achieve a gradient coating from Cr2O3 to Al2O3.

[0156] By using a step-by-step preparation method, the gradual transition of Al / Cr oxide composition was controlled.

[0157] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0158] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method of producing a gradient oxide coating, characterized by, The method comprises the following steps: a first magnetron sputtering treatment is performed to deposit a chromium primer layer on the surface of the substrate to obtain a substrate containing the chromium primer layer, wherein Cr is used as the target material, and an inert gas is used as the sputtering gas; a second magnetron sputtering treatment is performed to deposit a chromium oxide transition layer on the chromium primer layer to obtain a substrate containing the chromium oxide transition layer, wherein Cr is used as the target material, and an inert gas and oxygen are used as the sputtering gas, and the oxygen flux is increased from 0 to a flux ratio of 3-5:1 between the inert gas and the oxygen during sputtering; a third magnetron sputtering treatment is performed to deposit a gradient oxide layer on the chromium oxide transition layer to obtain a substrate containing the gradient oxide layer, wherein Cr and Al are used as the target material, and an inert gas and oxygen are used as the sputtering gas, and Cr and Al are sputtered simultaneously, and the sputtering power of Cr is reduced from 2.5 kW-4 kW to 0, and the sputtering power of Al is increased from 0 to 2.5 kW-4 kW during sputtering; The substrate with the gradient oxide layer is heat treated in a vacuum atmosphere, during which the amorphous chromium oxide is converted into α-Cr2O3, and Al ions can replace the Cr positions in the α-Cr2O3 to form an α-(Cr x ,Al 1-x )2O3 solid solution, thereby obtaining a gradient oxide coating on the substrate.

2. The method of claim 1, wherein the gradient oxide coating is formed by a process comprising: during the third magnetron sputtering treatment, the sputtering bias is -30 V to -200 V, the working pressure is 0.4 Pa to 0.6 Pa, and the deposition thickness of the gradient oxide layer is 0.5 μm to 3 μm.

3. The method for preparing a gradient oxide coating according to claim 1, characterized in that, during the second magnetron sputtering treatment, the sputtering bias is -30 V to -200 V, the working pressure is 0.4 Pa to 0.6 Pa, the sputtering power of the Cr target is 2.5 kW to 4 kW, and the deposition time is 8 min to 19 min.

4. The method for preparing a gradient oxide coating according to claim 1, characterized in that, the flux of the inert gas is 40 sccm to 67 sccm, and the flux of the oxygen is 8 sccm to 22 sccm.

5. The method for preparing a gradient oxide coating according to claim 1, characterized in that, during the first magnetron sputtering treatment, the sputtering bias is -700 V to -900 V, the working pressure is 0.4 Pa to 0.6 Pa, the sputtering power of the Cr target is 250 W to 500 W, and the deposition thickness of the chromium primer layer is 10 nm to 100 nm.

6. The method of claim 1, wherein the gradient oxide coating is formed by a process comprising: during the heat treatment, the holding temperature is 450 ℃ to 650 ℃, and the holding time is 2 h to 10 h.

7. The method for preparing a gradient oxide coating according to claim 1, characterized in that, during the first magnetron sputtering treatment, the second magnetron sputtering treatment and the third magnetron sputtering treatment, the temperature of the substrate is 30 ℃ to 40 ℃.

8. The method for preparing a gradient oxide coating according to claim 1, characterized in that, The method further comprises, before the first magnetron sputtering treatment, pre-treating the substrate to remove surface impurities.

9. A gradient oxide coating prepared by the preparation method in claims 1-8.

10. Use of the gradient oxide coating in claim 9 as a lead-bismuth corrosion-resistant coating.

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