A sintered neodymium-iron-boron composite coating with high bonding force and high hydrogen resistance and a preparation method thereof

Abstract

The application discloses a sintered neodymium-iron-boron composite coating with high bonding force and high hydrogen resistance and a preparation method thereof. The method comprises the following steps: (a) substrate pretreatment, wherein surface oxides are removed by energy gradient plasma sputtering cleaning; (b) sputtering an Al layer under an Ar atmosphere and oxidizing under O2 plasma assistance, thereby generating an intermetallic compound anchoring layer of FeAl3 / NdAl2 with a thickness of 1.0-1.5 nm in situ; (c) sequentially depositing 6 layers of Al2O3 layers to form an alternating stack with a total thickness of 20-30 nm, wherein the first, third and fifth layers are prepared by directly sputtering an Al2O3 ceramic target, and the second, fourth and sixth layers are prepared by first sputtering an Al layer and then completely converting by O2 oxidation; (d) low-temperature annealing to fully form Al-O-Fe bonds at the interface; and (e) cooling to room temperature. The application adopts a multi-dimensional in-situ closed-loop monitoring system to realize accurate process control, and is suitable for surface protection of sintered neodymium-iron-boron permanent magnets in hydrogen energy equipment.

Description

Technical Field

[0001] This invention relates to the field of surface engineering technology for rare earth permanent magnet materials, specifically to a method for preparing a composite coating on the surface of a sintered NdFeB permanent magnet. Background Technology

[0003] Although traditional electroplated Zn-Ni protective layers have good corrosion resistance, the catalytic Ni element they contain accelerates the dissociation of H2 molecules into active H atoms, causing hydrogen to diffuse rapidly along the Ni grain boundaries. This results in a sharp drop in coercivity (Hcj) within 100 hours, and the adhesion is usually only around 18.5 MPa, which is difficult to meet the long-term service requirements of hydrogen energy equipment.

[0004] In recent years, Al / Al2O3 composite coatings have attracted attention due to their excellent barrier properties. Chinese patent CN111304611B discloses a method for preparing a highly corrosion-resistant protective coating on the surface of NdFeB magnets. By detecting and controlling plasma characteristic parameters, an alternating deposition process forms an Al / Al2O3 multilayer film structure, effectively reducing micro-defects in the coating and improving the corrosion resistance of the NdFeB magnets. Although this method solves problems such as low deposition rate and difficulty in controlling the structure, the hydrogen permeability of the coating still reaches 3.5 × 10⁻² in hydrogen environments. 4 mol·m⁻¹·s⁻¹·Pa⁻ 0 · 5 The coating has an intrinsic hydrogen content of about 0.25 ppm, resulting in an Hcj retention rate of only 92.1% after 1000 hours of aging, and an adhesion of about 62.7 MPa, which fails to fully meet the stringent requirements of hydrogen energy equipment.

[0005] Chinese patent CN117089810B discloses a corrosion-resistant aluminum-based multilayer coating for the surface of sintered NdFeB magnets and its preparation method. The method uses high-density plasma excitation to prepare the aluminum-based barrier layer, effectively avoiding "target poisoning" during the deposition process and improving the coating's corrosion resistance. However, this method does not provide a specific numerical range for the plasma density or a precise control method, and lacks specific design for hydrogen environment protection. The batch stability index Cpk is only 1.71, and the hydrogen permeability is approximately 1.3 × 10⁻². 4 mol·m⁻¹·s⁻¹·Pa⁻ 0 · 5 The coating has an intrinsic hydrogen content of approximately 0.18 ppm and an adhesion of approximately 74.2 MPa, indicating that there are still shortcomings in the consistency control during industrial production.

[0006] Chinese patent CN110098044B discloses a composite modification method for surface protection of NdFeB magnets. This method employs high-power pulsed magnetron sputtering and plasma immersion ion implantation to prepare a composite modified coating, improving the surface corrosion resistance of NdFeB magnets. However, this method is an open-loop static control method with fixed process parameters, lacking in-situ monitoring methods. It cannot address process deviations caused by raw material batch fluctuations and equipment aging, and it does not involve verification of hydrogen barrier performance under hydrogen conditions, resulting in poor batch consistency.

[0007] In summary, existing technologies for surface protection of sintered NdFeB permanent magnets have the following shortcomings: First, the adhesion between the coating and the substrate is insufficient, making it difficult to meet the thermal cycling requirements of hydrogen energy equipment; second, the high hydrogen permeability leads to severe coercivity attenuation; third, the intrinsic hydrogen content of the coating is too high, affecting long-term service performance; and fourth, the process lacks in-situ monitoring and precise control methods, resulting in poor batch stability and making it difficult to meet the requirements of industrial production. Therefore, there is an urgent need to develop a method for preparing sintered NdFeB composite coatings with high adhesion, high hydrogen barrier properties, and good batch stability to meet the stringent requirements of hydrogen energy equipment for permanent magnet surface protection. Summary of the Invention

[0008] To address the aforementioned shortcomings, this invention proposes a method for preparing a composite coating on the surface of sintered NdFeB permanent magnets, thereby solving problems such as insufficient adhesion, high hydrogen permeability, severe intrinsic hydrogen contamination of the coating, and poor batch stability in the existing protective layers on the surface of sintered NdFeB permanent magnets.

[0009] This invention provides the following technical solution: a method for preparing a sintered Nd2Fe2O3 composite coating with high bonding strength and high hydrogen barrier properties, comprising the steps of surface treatment of a sintered Nd2Fe2O3 magnet substrate and deposition of a coating thereon, including the following steps: (a) substrate pretreatment: sintering Nd2Fe2O3 magnet substrate... 14 Magnet B is placed in a magnetron sputtering apparatus, and the vacuum is evacuated to ≤5×10⁻⁻⁻⁻⁻⁴ ... 4(a) Plasma sputtering cleaning in a high-purity Ar atmosphere to remove surface oxides and adsorbed water; (b) Growth of intermetallic compound anchoring layer: Al layer is sputtered and deposited in an Ar atmosphere, followed by oxidation under O2 plasma assistance, so that its surface reacts with the Fe / Nd atoms of the substrate to generate FeAl3 / NdAl2 intermetallic compound anchoring layer in situ; (c) On the anchoring layer obtained in step (b), six Al2O3 layers are deposited sequentially: the first, third and fifth layers are obtained by sputtering Al2O3 ceramic target, and the second, fourth and sixth layers are obtained by sputtering Al layer and oxidizing under O2 plasma assistance to completely convert it into Al2O3 layer, finally forming an alternating stacked composite coating with a total thickness of 20-30 nm; (d) Low temperature stress relaxation: annealing at 180-220℃ under Ar protection for 0.5-1.5 hours to fully form the Al-O-Fe bond at the interface and eliminate residual stress; (e) Cooling to room temperature to obtain the high-adhesion and high-hydrogen-barrier composite coating.

[0010] Preferably, the plasma sputtering cleaning in step (a) is an energy gradient cleaning: the ion energy is 400-600 eV in the 0-120 second stage to strip the surface oxides, and the ion energy is reduced to 150-250 eV in the 120-300 second stage to passivate the new surface.

[0011] Preferably, the O2 plasma density in step (b) is (1.15-1.25)×10¹ 0 cm⁻³, the oxidation time is dynamically adjusted according to the Al layer thickness until the coating refractive index reaches the standard value of amorphous Al₂O₃, so as to ensure that the Al layer is completely converted into Al₂O₃.

[0012] Preferably, in step (c), a closed-loop temperature control is used to maintain the magnet surface temperature at 50-60°C to ensure that Al2O3 is in an amorphous state.

[0013] Preferably, the annealing atmosphere in step (d) is a mixture of Ar + 3-8 ppm H2 to promote the formation of Al-O-Fe bonds at the interface.

[0014] The present invention also provides a composite coating prepared by the above method, characterized in that it consists of a 1.0-1.5 nm thick FeAl3 / NdAl2 anchoring layer at the substrate / coating interface and six Al2O3 layers thereon, with a total thickness of 20-30 nm.

[0015] The present invention also provides a sintered NdFeB permanent magnet having the above-mentioned composite coating on its surface.

[0016] The beneficial effects of this invention are: (1) Achieve atomic-level interfacial bonding, significantly improving adhesion: This application utilizes a unique "energy gradient sputtering cleaning" method to remove oxides and impurities from the substrate surface. Based on this, an intermetallic compound anchoring layer of FeAl3 / NdAl2 is grown in situ with O2 plasma assistance. This anchoring layer alters the traditional physical adsorption mode of coatings, achieving atomic-level metallurgical bonding between the coating and the NdFeB substrate. This results in significantly higher coating adhesion compared to existing single-layer Al coatings, perfectly meeting the anti-peeling requirements of hydrogen energy equipment under thermal cycling conditions.

[0017] (2) Constructing a "maze-like" blocking structure significantly reduces hydrogen permeability: This application innovatively employs an alternating process of "direct sputtering of target material" and "plasma oxidation after pure aluminum deposition" to construct an alternating stacked composite coating composed of six layers of amorphous Al2O3. This multilayer amorphous structure completely eliminates the easily hydrogen-conducting grain boundary channels in traditional metallic crystalline coatings, forming a dense "labyrinthine" hydrogen barrier and exhibiting extremely excellent hydrogen barrier performance.

[0018] (3) Overcoming the problem of intrinsic hydrogen pollution, with extremely low magnetic performance degradation during long-term service: This application breaks with convention by introducing a mixed atmosphere of trace amounts of hydrogen during the annealing stress relaxation stage. This trace hydrogen environment not only effectively removes residual free oxygen within the system and promotes the dense formation of high-strength Al-O-Fe bonds at the interface, but also controls the intrinsic hydrogen content within the coating to a low level. This solves the problem of traditional protective layers easily experiencing a "cliff-like" decline in magnetic properties under hydrogen conditions. Attached Figure Description

[0019] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments: Figure 1 This is a schematic diagram of the process flow of the preparation method of the present invention; Figure 2 is a comparison diagram of surface morphology and elemental distribution between the embodiments of the present invention and the comparative examples; Figure 2(a) shows the surface morphology and elemental distribution of Comparative Example 1, and Figure 2(b) shows the surface morphology and elemental distribution of Example 1. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0021] The raw material is N42SH sintered NdFeB magnet (Φ10 mm×2 mm, Hcj=14.2 kOe, Br=1.32 T).

[0022] Comparative Example 1 (Single-layer Al sputtering) Process flow: Only step (a) substrate pretreatment and part of step (b) Al target sputtering are performed, without O2 oxidation and subsequent alternating Al2O3 deposition, as detailed below: Step (a) Substrate pretreatment: Place the N42SH sintered NdFeB magnet (Φ10 mm × 2 mm) in the vacuum chamber of the magnetron sputtering system and evacuate to 3.2 × 10⁻ 4 High-purity Ar (H₂O ≤ 0.08 ppm) was introduced at a flow rate of 50 sccm and a pressure of 0.3 Pa. The RF power supply was started with a bias voltage of −500 V, and cleaning was performed for 5 minutes. After cleaning, the surface oxide layer decreased from 5.2 nm to 0.78 nm.

[0023] Al layer sputtering: After cleaning, maintain the Ar atmosphere and start the DC magnetron sputtering power supply for the Al target, sputtering continuously for 150 seconds to deposit a monolayer Al coating approximately 20 nm thick. No O2 is introduced during sputtering, and no plasma oxidation occurs; the Al layer remains in a metallic state.

[0024] Cooling: Turn off the sputtering power supply and allow the furnace to cool to room temperature before removing the sample.

[0025] Surface morphology and elemental distribution: As shown in Figure 2(a), the macroscopic photograph shows that the coating surface has a silvery-white metallic luster. Optical microscopy images show obvious scratches and uneven areas on the surface. Scanning electron microscopy images show that the coating surface is rough, with numerous microcracks and pores, resulting in poor density. EDS elemental surface scans show uneven Al distribution, with Nd and Fe elements exposed in some areas, indicating incomplete coating coverage. No O element was detected, confirming that the Al layer was not oxidized.

[0026] Performance testing: Adhesion 62.1 MPa (scratch test, critical load 38.9 N); Hydrogen permeability 8.5 × 10⁻² 4 mol·m⁻¹·s⁻¹·Pa⁻ 0 · 5 (Gleeble test, 120℃, 0.1 MPa H2); intrinsic hydrogen content of coating is 0.15 ppm (SIMS); Hcj retention rate after 1000h aging is 88.3% (14.2 kOe before aging → 12.54 kOe after aging).

[0027] The single-layer Al coating, lacking a FeAl3 / NdAl2 anchoring layer, exhibits physical adsorption at the interface with the substrate, resulting in an adhesion strength of only 62.1 MPa. In contrast, the metallic Al layer, with its crystalline structure and numerous grain boundaries, allows for rapid diffusion of hydrogen atoms along these boundaries, resulting in a hydrogen permeability as high as 8.5 × 10⁻². 4 mol·m⁻¹·s⁻¹·Pa⁻ 0 · 5The coating surface is rough and contains microcracks, which accelerates corrosion in a hydrogen environment. After 1000 hours of aging, the coercivity retention rate is only 88.3%.

[0028] Example 1 Step (a) Substrate pretreatment: Place the magnet in a vacuum chamber and evacuate to 3.2 × 10⁻ 4 High-purity Ar (H₂O ≤ 0.08 ppm) was introduced at a flow rate of 50 sccm and a pressure of 0.3 Pa. The RF power supply (150 W) was turned on, with a bias voltage of −500 V, and cleaning was performed for 5 minutes. After cleaning, the surface oxide layer decreased from 5.2 nm to 0.78 nm (XPS measurement).

[0029] Step (b) Anchor layer growth: Al target sputtering (power 80 W, 12 s, thickness 2.10 nm) → O2 plasma oxidation (pressure 0.8 Pa, power 100 W, 45 s, density 1.20 × 10¹) 0 The anchoring layer thickness was 1.22 nm as observed by HRTEM, and it was composed of FeAl3 (65%) and NdAl2 (35%).

[0030] Step (c) 6-layer Al2O3 deposition: Layers 1 / 3 / 5 were sputtered using an Al2O3 ceramic target (power 120 W, 30 s, thickness 3.80 nm, temperature 55 °C); Layers 2 / 4 / 6 were sputtered using an Al target (12 s, 2.10 nm) followed by O2 oxidation (45 s) to convert to Al2O3 (thickness 2.80 nm). The total thickness was 25.03 nm (measured by AFM).

[0031] Step (d) Annealing: 200℃, Ar + 5 ppm H2, 1 hour. XPS analysis showed that the Al-O-Fe bond ratio increased from 78.2% to 89.3%, and the residual stress decreased from −320 MPa to −85 MPa.

[0032] Step (e) Cooling: Cool the furnace to room temperature at a rate of 2°C / min.

[0033] Surface morphology and elemental distribution: As shown in Figure 2(b), the macroscopic photograph shows that the coating surface is a uniform light gray. The optical microscope photograph shows that the surface is smooth and flat, without obvious defects. The high-magnification scanning electron microscope photograph shows that the coating has a dense granular microstructure with uniform particle size and no cracks or pores. The EDS elemental surface scan shows that Al, O, Nd and Fe elements are uniformly distributed, and O element is uniformly distributed throughout the entire coating area, confirming that the Al2O3 layer is completely formed and fully covered.

[0034] Performance testing: Adhesion 85.3 MPa (scratch test, critical load 53.6 N); Hydrogen permeability 4.2 × 10⁻²5 mol·m⁻¹·s⁻¹·Pa⁻ 0 · 5 (Gleeble test, 120℃, 0.1 MPa H2); intrinsic hydrogen content of coating is 0.028 ppm (SIMS); Hcj retention rate after 1000h aging is 95.3% (14.2 kOe before aging → 13.52 kOe after aging).

[0035] Comparison table: Technical Advantages Analysis: Compared to the single-layer Al coating in Comparative Example 1, the alternating Al / Al2O3 composite coating of Example 1 of this invention comprehensively surpasses the other coatings in four key performance indicators. Adhesion is improved by 37.4%, thanks to the atomic-level interfacial bonding achieved by the FeAl3 / NdAl2 anchoring layer (lattice mismatch 1.15%, Al-O-Fe bond energy 450 kJ / mol). Hydrogen permeability is reduced by 95.1%, due to the labyrinthine hydrogen-blocking channels formed by the six layers of amorphous Al2O3 (H diffusion coefficient in amorphous Al2O3 is 1.2 × 10⁻¹). 8 m² / s, far lower than the 5×10⁻¹ m² / s of crystalline Al grain boundaries. 6 The intrinsic hydrogen content was reduced by 81.3%, thanks to the three-stage atmosphere gradient annealing (Ar + 5ppm H2 promotes Al-O-Fe bond formation and removes residual oxygen). The coercivity retention rate was improved by 7.0 percentage points, demonstrating that the method of this invention significantly improves the long-term service stability of the coating in a hydrogen environment.

[0036] Example 2 The difference from Example 1 is that step (a) uses energy gradient cleaning monitored by SEE.

[0037] Step (a) Energy Gradient Cleaning: First Stage (0-120 seconds): Ar flow rate 50 sccm, pressure 0.3 Pa, RF power 150 W, bias voltage −500 V, ion energy 500 eV. The secondary electron emission coefficient (SEE) was monitored in real time by a microchannel plate detector. The initial value was δ=0.85 (surface oxide coverage), which decreased to δ=0.22 at 120 seconds (close to 0.20 for pure Fe), indicating that the oxides had been sufficiently stripped. Second Stage (120-300 seconds): The bias voltage was reduced to −200 V, the ion energy was 200 eV, the SEE stabilized at δ=0.23±0.01, the surface roughness decreased from 18.5 nm before cleaning to 9.2 nm (AFM measurement), and the residual oxygen content decreased from 5.2 at.% to 0.8 at.% (XPS measurement).

[0038] Step (b) Anchor layer growth: Al target sputtering (power 80 W, 12 s, thickness 2.10 nm) → O2 plasma oxidation (pressure 0.8 Pa, power 100 W, 45 s, density 1.20 × 10¹) 0 The anchoring layer thickness was 1.22 nm as observed by HRTEM, and it was composed of FeAl3 (65%) and NdAl2 (35%).

[0039] Step (c) 6-layer Al2O3 deposition: Layers 1 / 3 / 5 were sputtered using an Al2O3 ceramic target (power 120 W, 30 s, thickness 3.80 nm, temperature 55 °C); Layers 2 / 4 / 6 were sputtered using an Al target (12 s, 2.10 nm) followed by O2 oxidation (45 s) to convert to Al2O3 (thickness 2.80 nm). The total thickness was 25.03 nm (measured by AFM).

[0040] Step (d) Annealing: 200℃, Ar + 5 ppm H2, 1 hour. XPS analysis showed that the Al-O-Fe bond ratio increased from 78.2% to 89.3%, and the residual stress decreased from −320 MPa to −85 MPa.

[0041] Step (e) Cooling: Cool the furnace to room temperature at a rate of 2°C / min.

[0042] Performance testing: Adhesion 88.2 MPa (scratch test, critical load 55.3 N); Hydrogen permeability 3.9 × 10⁻² 5 mol·m⁻¹·s⁻¹·Pa⁻ 0 · 5 (Gleeble test, 120℃, 0.1 MPa H2); intrinsic hydrogen content of coating is 0.025 ppm (SIMS); Hcj retention rate after 1000h aging is 95.8% (14.2 kOe before aging → 13.60 kOe after aging).

[0043] Technical Advantages Analysis: Compared to Example 1, energy gradient cleaning increased adhesion from 85.3 MPa to 88.2 MPa (+3.4%), decreased surface roughness from 11.8 nm to 9.2 nm (−22%), and reduced residual oxygen content from 1.2 at.% to 0.8 at.% (−33%). In the first stage, high-energy ions (500 eV) thoroughly stripped surface oxides, reducing the surface ester separation (SEE) from δ=0.85 to δ=0.22, close to the intrinsic value of pure Fe (0.20), demonstrating that the oxides were essentially removed. In the second stage, low-energy ions (200 eV) gently passivated the nascent surface, avoiding surface roughening caused by excessive sputtering, and controlling the surface roughness at 9.2 nm. The cleaner interface promoted the full growth of the FeAl3 / NdAl2 anchoring layer, increasing the Al-O-Fe bond density from 8.5 × 10¹. 4 cm⁻² increased to 9.2×10¹ 4 cm⁻², ultimately achieving a dual improvement in adhesion and hydrogen barrier performance.

[0044] Example 3 The difference from Example 1 is that the anchoring layer thickness in step (b) is optimized to 1.5 nm, and the total thickness of the alternating layers in step (c) is optimized to 30 nm.

[0045] Step (a) Substrate pretreatment: Place the magnet in a vacuum chamber and evacuate to 3.2 × 10⁻ 4 High-purity Ar (H₂O ≤ 0.08 ppm) was introduced at a flow rate of 50 sccm and a pressure of 0.3 Pa. The RF power supply (150 W) was turned on, with a bias voltage of −500 V, and cleaning was performed for 5 minutes. After cleaning, the surface oxide layer decreased from 5.2 nm to 0.78 nm (XPS measurement).

[0046] Step (b) Anchor layer growth (thickness optimized to 1.5 nm): Al target sputtering (power 80 W, 15 s, thickness 2.65 nm) → O2 plasma oxidation (pressure 0.8 Pa, power 100 W, 58 s, density 1.20 × 10¹) 0 The anchoring layer thickness was 1.48 nm as observed by HRTEM, consisting of FeAl3 (67%) and NdAl2 (33%). Compared to 1.22 nm in Example 1, the anchoring layer thickness increased by 21%, and the Al-O-Fe bond density increased from 8.5 × 10¹⁻¹. 4 cm⁻² increased to 9.8×10¹ 4 cm⁻²(+15.3%).

[0047] Step (c) 6-layer Al2O3 deposition (total thickness optimized to 30 nm): Layers 1 / 3 / 5 were sputtered using an Al2O3 ceramic target (power 120 W, 38 s, thickness 4.82 nm, temperature 55 °C); layers 2 / 4 / 6 were sputtered using an Al target (15 s, 2.65 nm) followed by O2 oxidation (58 s), converting to Al2O3 (thickness 3.53 nm). Total thickness = (4.82 × 3) + (3.53 × 3) = 30.05 nm (AFM measurement, deviation +0.17%). Compared to 25.03 nm in Example 1, the alternating stack thickness increased by 20%, the number of labyrinthine hydrogen barrier layers remained unchanged (still 6 layers), but the thickness of a single layer increased, and the hydrogen diffusion path lengthened from 25 nm to 30 nm.

[0048] Step (d) Annealing: 200℃, Ar + 5 ppm H2, 1 hour. XPS analysis showed that the Al-O-Fe bond ratio increased from 78.2% to 90.1%, and the residual stress decreased from −320 MPa to −75 MPa.

[0049] Step (e) Cooling: Cool the furnace to room temperature at a rate of 2°C / min.

[0050] Performance testing: Adhesion 85.0 MPa (scratch test, critical load 53.4 N); Hydrogen permeability 3.8 × 10⁻² 5 mol·m⁻¹·s⁻¹·Pa⁻ 0 · 5 (Gleeble test, 120℃, 0.1 MPa H2); intrinsic hydrogen content of coating is 0.022 ppm (SIMS); Hcj retention rate after 1000h aging is 95.2% (14.2 kOe before aging → 13.51 kOe after aging).

[0051] Technical advantages analysis: Compared to Example 1, the anchoring layer thickness was optimized from 1.22 nm to 1.48 nm (+21%), the Al-O-Fe bond density increased by 15.3%, and the adhesion remained at 85.0 MPa (essentially the same as 85.3 MPa in Example 1). The total thickness of the alternating layers was optimized from 25.03 nm to 30.05 nm (+20%), the hydrogen diffusion path was extended by 5 nm, and the hydrogen permeability increased from 4.2 × 10⁻². 5 Reduced to 3.8×10⁻² 5 mol·m⁻¹·s⁻¹·Pa⁻ 0 · 5(−9.5%). The thicker anchoring layer and the synergistic effect of the alternating layers reduced the intrinsic hydrogen content from 0.028 ppm to 0.022 ppm (−21.4%), and the coercivity retention slightly decreased from 95.3% to 95.2% (−0.1 percentage points, within the error range). This optimized scheme validates the rationality of the technical parameter range of 1.0–1.5 nm for the anchoring layer thickness and 20–30 nm for the total thickness of the alternating layers, providing process adjustment space for different application scenarios.

[0052] Example 4 The difference from Example 2 is that in step (a), the ion energy in the first stage of energy gradient cleaning is increased from 500 eV to 550 eV, and in the second stage it is decreased from 200 eV to 180 eV, which further optimizes the surface roughness and residual oxygen content.

[0053] Step (a) Optimized energy gradient cleaning: First stage (0-100 seconds): Ar flow rate 50 sccm, pressure 0.3 Pa, RF power 150 W, bias voltage −550 V, ion energy 550 eV. The secondary electron emission coefficient (SEE) was monitored in real time by a microchannel plate detector. The initial value was δ=0.85 (surface oxide coverage), which decreased to δ=0.21 at 100 seconds (closer to 0.20 for pure Fe), indicating a 17% increase in oxide stripping rate (compared to 120 seconds in Example 2). Second stage (100-300 seconds): Bias voltage is reduced to −180 V, ion energy is 180 eV, SEE stabilizes at δ=0.22±0.01, surface roughness decreases from 18.5 nm before cleaning to 8.0 nm (AFM measurement, a further reduction of 13% compared to 9.2 nm in Example 2), and residual oxygen content decreases from 5.2 at.% to 0.5 at.% (XPS measurement, a reduction of 38% compared to 0.8 at.% in Example 2).

[0054] Step (b) Anchor layer growth: Al target sputtering (power 80 W, 12 s, thickness 2.10 nm) → O2 plasma oxidation (pressure 0.8 Pa, power 100 W, 45 s, density 1.20 × 10¹) 0 The anchoring layer thickness was 1.24 nm as observed by HRTEM, consisting of FeAl3 (66%) and NdAl2 (34%). A cleaner interface (0.5 at.%) promoted sufficient anchoring layer growth, with the Al-O-Fe bond density increasing from 9.2 × 10¹ in Example 2. 4 cm⁻² increased to 9.5×10¹ 4 cm⁻²(+3.3%).

[0055] Step (c) 6-layer Al2O3 deposition: Layers 1 / 3 / 5 were sputtered using an Al2O3 ceramic target (power 120 W, 30 s, thickness 3.80 nm, temperature 55 °C); Layers 2 / 4 / 6 were sputtered using an Al target (12 s, 2.10 nm) followed by O2 oxidation (45 s) to convert to Al2O3 (thickness 2.80 nm). The total thickness was 25.03 nm (measured by AFM).

[0056] Step (d) Annealing: 200℃, Ar + 5 ppm H2, 1 hour. XPS analysis showed that the Al-O-Fe bond ratio increased from 78.2% to 90.5%, and the residual stress decreased from −320 MPa to −78 MPa.

[0057] Step (e) Cooling: Cool the furnace to room temperature at a rate of 2°C / min.

[0058] Performance testing: Adhesion 88.0 MPa (scratch test, critical load 55.2 N); Hydrogen permeability 3.7 × 10⁻² 5 mol·m⁻¹·s⁻¹·Pa⁻ 0 · 5 (Gleeble test, 120℃, 0.1 MPa H2); intrinsic hydrogen content of coating is 0.024 ppm (SIMS); Hcj retention rate after 1000h aging is 95.6% (14.2 kOe before aging → 13.57 kOe after aging).

[0059] Technical Advantages Analysis: Compared to Example 2, the optimized energy gradient cleaning increased the high-energy ion energy in the first stage from 500 eV to 550 eV, improving the oxide stripping rate by 17% (achieving SEE=0.21 in 100 seconds vs. 120 seconds). In the second stage, the low-energy ion energy decreased from 200 eV to 180 eV, further reducing the surface roughness from 9.2 nm to 8.0 nm (−13%), and the residual oxygen content from 0.8 at.% to 0.5 at.% (−38%). The cleaner and smoother interface promoted the full growth of the FeAl3 / NdAl2 anchoring layer, increasing the Al-O-Fe bond density from 9.2 × 10¹. 4 cm⁻² increased to 9.5×10¹ 4The residual oxygen content was reduced by 3.3% (cm⁻²), and the adhesion remained stable at 88.0 MPa. The significant reduction in residual oxygen content (0.5 at.%) reduced oxygen defects in the coating, inhibiting hydrogen trapping and diffusion at defects, reducing the intrinsic hydrogen content to 0.024 ppm, and improving coercivity retention to 95.6%. This optimized scheme validated the technical parameter range of 400-600 eV for the first stage and 150-250 eV for the second stage of energy gradient cleaning, providing process guidance for obtaining an ultra-clean interface (residual oxygen ≤0.5 at.%, surface roughness ≤8 nm).

[0060] The above description only illustrates the preferred embodiments of the present invention and should not be construed as limiting the scope of the claims. The present invention is not limited to the above embodiments, and variations in its specific structure are permitted. All modifications made within the scope of the independent claims of this invention are also within the scope of protection of this invention.

Claims

1. A method for preparing a sintered NdFeB composite coating with high adhesion and high hydrogen barrier properties, comprising the steps of surface treatment of a sintered NdFeB magnet substrate and deposition of a coating thereon, characterized in that, Includes the following steps: (a) Matrix pretreatment: sintered Nd2Fe 14 Magnet B is placed in a magnetron sputtering apparatus, and the vacuum is evacuated to ≤5×10⁻⁻⁻⁻⁻⁴ ... 4 Pa, high-purity Ar atmosphere is introduced for plasma sputtering cleaning to remove surface oxides and adsorbed water; (b) Growth of intermetallic compound anchoring layer: An Al layer is sputtered and deposited in an Ar atmosphere, and then oxidized with the assistance of O2 plasma to react with the Fe / Nd atoms of the substrate on its surface, thereby generating an FeAl3 / NdAl2 intermetallic compound anchoring layer in situ with a thickness of 1.0-1.5 nm. (c) On the anchoring layer obtained in step (b), six Al2O3 layers are deposited sequentially: layers 1, 3, and 5 are obtained by direct sputtering of Al2O3 ceramic target; layers 2, 4, and 6 are obtained by first sputtering pure Al layer and then oxidizing it under O2 plasma assistance to completely transform it into Al2O3 layer; finally, an alternating stacked composite coating with a total thickness of 20-30 nm is formed. (d) Low-temperature stress relaxation: Annealing at 180-220℃ under Ar protection for 0.5-1.5 hours to fully form Al-O-Fe bonds at the interface and eliminate residual stress; (e) Cool to room temperature to obtain the high-adhesion, high-hydrogen-barrier composite coating.

2. The preparation method according to claim 1, characterized in that, The plasma sputtering cleaning described in step (a) is an energy gradient cleaning: the ion energy is 400-600 eV in the 0-120 second stage to strip the surface oxides, and the ion energy is reduced to 150-250 eV in the 120-300 second stage to passivate the new surface.

3. The preparation method according to claim 1, characterized in that, The O2 plasma density in step (b) is (1.15-1.25)×10¹ 0 cm⁻³, the oxidation time is dynamically adjusted according to the Al layer thickness until the coating refractive index reaches the standard value of amorphous Al₂O₃, so as to ensure that the Al layer is completely converted into Al₂O₃.

4. The preparation method according to any one of claims 1, characterized in that, In step (c), a closed-loop temperature control is used to maintain the magnet surface temperature at 50-60℃ to ensure that Al2O3 is in an amorphous state.

5. The preparation method according to any one of claims 1, characterized in that, In step (d), the annealing atmosphere is a mixture of Ar + 3-8 ppm H2 to promote the formation of Al-O-Fe bonds at the interface, and the proportion of Al-O-Fe bonds after annealing is ≥89%.

6. A sintered NdFeB composite coating with high adhesion and high hydrogen barrier properties, characterized in that: The composite coating consists of, from the inside out, a FeAl3 / NdAl2 intermetallic compound anchoring layer with a thickness of 1.0-1.5 nm and alternating Al2O3 layers with a total thickness of 20-30 nm, wherein the alternating layers are composed of 6 Al2O3 layers.

7. The composite coating according to claim 6, characterized in that, The composite coating has the following performance indicators: adhesion ≥80 MPa, hydrogen permeability <4.5×10⁻² 5 mol·m⁻¹·s⁻¹·Pa⁻ 0 · 5 The coating has an intrinsic hydrogen content of ≤0.03ppm and a coercivity retention rate of ≥94.5% after aging for 1000 hours at 120℃ and 0.1 MPa H2.

8. A sintered NdFeB permanent magnet, characterized in that, Its surface has the high-adhesion, high-hydrogen-barrier composite coating as described in claim 7.

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