A gradient coating on the surface of a large shaft

Abstract

The present invention discloses a gradient coating on the surface of a large shaft, wherein the gradient coating comprises an adhesive layer, a hardness layer, a transition layer and a lubricating layer sequentially formed on the surface of the shaft; the gradient coating is prepared by the following method: raw materials for preparing an adhesive layer, a first slurry for preparing the hardness layer, a second slurry for preparing the transition layer and a third slurry for preparing the lubricating layer are prepared respectively; the raw materials for the adhesive layer comprise metals Ni and Cr, the first slurry comprises a YSZ material, the second slurry comprises a composite material composed of YSZ and WS2, and the third slurry comprises a WS2 material; a plasma spraying process is used to spray the raw materials for preparing an adhesive layer, the first slurry, the second slurry and the third slurry respectively on the surface of the shaft to sequentially prepare the adhesive layer, the hardness layer, the transition layer and the lubricating layer; the hardness layer of the YSZ material can provide high hardness for the surface coating, and the lubricating layer of WS2 provides lubricating performance with a low friction coefficient.

Description

[0001] This application is a divisional application of the invention patent application with the application date of December 16, 2020, application number 2020114825132, and invention name "Gradient coating on the surface of large shaft parts and its preparation method". Technical Field

[0002] The invention relates to the technical field of surface coatings, and in particular to a gradient coating on the surface of a large shaft. Background Art

[0003] Shafts rotate with supporting components to transmit motion, torque, or bending moment. The fundamental performance requirement for shafts is "hard on the outside and tough on the inside." This means that through specific surface treatment processes, high surface hardness is achieved while maintaining high toughness in the core, resulting in high wear resistance. This high surface hardness is crucial for bearing heavy loads.

[0004] Wear between shafts and supports is the primary form of damage. Reducing the coefficient of friction is the most direct way to mitigate wear, reducing motion resistance and thus energy loss. To reduce the coefficient of friction, lubricants are typically used to form an oil film, lowering the friction coefficient between the shaft and support, thereby reducing surface scratches caused by friction and wear. However, under heavy loads, this oil film is destroyed, and in low-temperature, high-temperature, or corrosive service environments, the lubricant deteriorates and loses its lubricating effect. Therefore, achieving a dry, low-friction surface in a non-lubricated environment has become an important method for reducing friction and wear.

[0005] The basic structure of self-lubricating materials is lamellar. They carry loads in the normal direction, but have low transverse shear strength and are easily deformed, resulting in a low coefficient of friction. Such materials include graphite, molybdenum disulfide, tungsten disulfide, cubic boron nitride, calcium fluoride, and diamond-like carbon coatings (DLC).

[0006] At present, there are two levels of problems in the preparation of high-hardness, low-friction self-lubricating coatings. First, how to obtain high-quality, high-hardness, low-friction self-lubricating coatings on large-sized workpieces at low cost. PVD plasma deposition technology requires a high vacuum chamber and is not suitable for large-sized workpieces, such as nuclear power emergency diesel engine camshafts (length greater than 2.4m), heavy machinery hydraulic cylinder piston rods (length 1-3m) and other large-sized shafts. Secondly, the ion deposition rate makes its production efficiency low and the coating thickness is limited (a few microns), and the cost of the final product is high. Laser cladding has low production efficiency and high cost. Therefore, for large workpieces, plasma spraying is a more practical process. For example, the Chinese invention patent with application number 201610990809.2, patent name: A self-lubricating cylinder piston, discloses a technical solution for preparing self-lubricating coatings for shafts by plasma spraying. The self-lubricant used is graphite and cubic boron nitride, and the low-friction phase and ceramic phase are mechanically mixed, and then ordinary plasma spraying is used for molding. However, there are several issues: the self-lubricants used are graphite and cubic boron nitride, which have a higher friction coefficient than WS2. Furthermore, conventional plasma spraying can be difficult for conveying small particles, resulting in uneven coatings and a loose structure. Conventional shaft surface coatings typically use a high-hardness ceramic coating, which does not match the thermal expansion coefficient of the substrate, resulting in poor bonding and easy detachment. Summary of the Invention

[0007] In view of this, in order to overcome the defects of the prior art, the purpose of the present invention is to provide a gradient coating on the surface of a large shaft, which can achieve the effect of a dry low friction coefficient on the coating surface, and the gradient design achieves a hard outer and tough inner structure with strong bonding force between each layer.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] A gradient coating on the surface of a large shaft component, the gradient coating comprising an adhesive layer, a hardness layer, a transition layer and a lubricating layer sequentially formed on the surface of the shaft component, the adhesive layer being made of Ni and Cr, the hardness layer being made of YSZ material, the lubricating layer being made of WS2, and the transition layer being made of a composite material comprising YSZ and WS2.

[0010] The hardness layer of the YSZ material can provide high hardness for the surface coating, and the lubricating layer of WS2 provides lubrication performance with a low friction coefficient; the gradient structure can ensure continuous performance changes, and the bonding strength between the coatings is high, achieving a hard outside and tough inside effect.

[0011] In some embodiments of the present invention, the material of the bonding layer is NiCr metal alloy material with excellent strength and plasticity matching; the material of the hardness layer is YSZ material, which is a ceramic material that is easy to achieve strength and toughness at the nanoscale; the material of the lubricating layer is high-temperature resistant self-lubricating WS2 material; the material of the transition layer is a composite material containing YSZ and WS2.

[0012] In some embodiments of the present invention, the raw material for preparing the bonding layer is NiCr alloy powder, the mass fraction of Cr in the alloy powder is 40-60%, preferably 50%, that is, the material used is Ni-50Cr. The particle size of the alloy powder is 5-50 μm.

[0013] According to some preferred embodiments of the present invention, the YSZ (yttria-stabilized zirconia) material is composed of ZrO2 and Y2O3. The mass fraction of Y2O3 in the YSZ material is 6-10%, preferably 8%. The 8% Y2O3 stabilizer can improve the stability of the tetragonal ZrO2 phase. The particle size of the YSZ material powder used to prepare the hardness layer is 300-1000 nm.

[0014] According to some preferred embodiments of the present invention, the first slurry for preparing the hardness layer is formed by dispersing YSZ material powder in a first solvent. The mass fraction of YSZ material in the first slurry is 15-20%, which can improve the stability of the suspension during transportation.

[0015] According to some preferred embodiments of the present invention, the mass ratio of WS2 to YSZ in the composite material is (1.3-2):3, which can improve the bonding performance between the hardness layer and the lubricating layer. The raw material powder particle size of the composite material used to prepare the transition layer is 100-1500nm. The raw materials of the composite material are also in a powder state, that is, the powder used to prepare the transition layer is formed by mixing WS2 powder and YSZ material powder. During preparation, the WS2 powder and YSZ material powder are mixed and ground uniformly in a high-energy ball mill according to proportion.

[0016] According to some preferred embodiments of the present invention, the second slurry used to prepare the transition layer is formed by dispersing WS2 powder and YSZ material powder in a second solvent. The mass fraction of the WS2 / YSZ mixed powder material in the second slurry is 20-30%, which can improve the stability of the suspension during transportation.

[0017] According to some preferred embodiments of the present invention, the raw material for preparing the lubricating layer is WS2 powder, and the particle size distribution of WS2 powder is 100-300 nm. The fine particle size is easier to transport through slurry.

[0018] According to some preferred embodiments of the present invention, the third slurry used to prepare the lubricating layer is formed by dispersing WS2 powder in a third solvent. The mass fraction of WS2 in the third slurry is 20-30%, which can improve the stability of the suspension during transportation.

[0019] In some preferred embodiments of the present invention, the first solvent, the second solvent, and the third solvent are preferably ethanol. The first slurry, the second slurry, and the third slurry are prepared by using ethanol as a powder carrier, adding the corresponding raw material powders to the ethanol, and continuously stirring in a magnetic stirrer to mix uniformly to prepare the slurries.

[0020] According to some preferred embodiments of the present invention, the hardness layer has a thickness of 150-200 μm, ensuring high hardness of the shaft surface. The composite transition layer has a thickness of 50-80 μm, effectively improving the bonding performance between the hardness layer and the lubricating layer. The lubricating layer has a thickness of 20-50 μm, ensuring that the lubricating layer performs a lubricating function and reduces surface wear.

[0021] A method for preparing a gradient coating on the surface of a large shaft as described above comprises the following steps: roughening a substrate to obtain a surface roughness Ra of 5-10 μm. The appropriate roughness improves the bonding performance between the bonding layer and the substrate. Plasma spraying is then used to sequentially form an adhesive layer, a hardness layer, a transition layer, and a lubricating layer on the surface of the substrate.

[0022] According to some preferred embodiments of the present invention, the bonding layer is produced using an atmospheric plasma spraying process, while the hardness layer, transition layer, and lubricating layer are produced using a suspension plasma spraying process. Specifically, the hardness layer, transition layer, and lubricating layer are produced using slurry liquid feeding and axial powder feeding suspension plasma spraying to produce the gradient coating.

[0023] In some preferred embodiments of the present invention, the method for preparing a gradient coating on the surface of a large shaft specifically comprises the following steps:

[0024] 1) Cleaning and roughening of the shaft substrate: pneumatic sand blasting is used, using quartz sand with a particle size of 0.1-0.8 mm. The sand blasting parameters are: pressure of 0.5-0.6 MPa, and sand blasting distance of 150-180 mm. Finally, the roughness R a The values range from 5-10 μm.

[0025] 2) Preparation of the bonding layer: Common commercial Ni-50Cr powder with a particle size distribution of 5-50 μm was used; conventional plasma spraying was performed to a coating thickness of 50-80 μm. The spraying parameters were: operating current 580-620 A, operating voltage 60-65 V, powder feed rate 45-50 g / min, spraying distance 120-150 mm, and spray gun movement speed 300-350 mm / min.

[0026] 3) Preparation of the Hardness Layer: Using the first slurry described above, suspension plasma spraying was used to achieve a coating thickness of 150-200 μm. Spraying parameters were: operating current 500-540 A, operating voltage 50-55 V, liquid feed rate 45-50 mL / min, spray distance 80-90 mm, and spray gun travel speed 500-550 mm / min. Suspension plasma spraying can overcome the difficulty of feeding fine particles, resulting in a uniform, dense coating.

[0027] 4) Preparation of the transition layer: Using the aforementioned second slurry, suspension plasma spraying was performed to a coating thickness of 50-80 μm. Spraying parameters were: operating current 400-440 A, operating voltage 50-55 V, liquid feed rate 45-50 mL / min, spray distance 100-120 mm, and spray gun travel speed 600-650 mm / min. Suspension plasma spraying can overcome the difficulty of feeding fine particles, resulting in a uniform, dense coating.

[0028] 5) Preparation of the Lubricating Layer: Using the third slurry described above, suspension plasma spraying was performed to a coating thickness of 20-50 μm. Spraying parameters were: operating current 400-440 A, operating voltage 50-55 V, liquid feed rate 35-40 mL / min, spray distance 100-120 mm, and spray gun travel speed 600-650 mm / min. Suspension plasma spraying can overcome the difficulty of feeding fine particles, resulting in a uniform, dense coating.

[0029] Due to the adoption of the above technical solutions, compared with the existing technology, the benefits of the present invention are: the gradient coating on the surface of the large shaft of the present invention is achieved by designing a gradient coating of bonding layer, hardness layer, transition layer and lubrication layer. The hardness layer of YSZ material can provide high hardness for the surface coating, and the lubrication layer of WS2 provides lubrication performance with low friction coefficient, and the performance changes continuously, and the bonding strength between the coatings is relatively high, achieving the effect of hard outside and tough inside. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0031] Figure 1 Schematic cross-sectional view of a gradient coating on the surface of a large shaft according to an embodiment of the present invention;

[0032] Among them: substrate-1, bonding layer-2, hardness layer-3, transition layer-4, lubricating layer-5. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0034] Example 1

[0035] See attached Figure 1 In this embodiment, the shaft substrate is 410 martensitic stainless steel, and the surface gradient coating on the shaft substrate includes an adhesive layer, a hardness layer, a transition layer and a lubricating layer in sequence.

[0036] The preparation of the surface gradient coating in this embodiment includes the following steps:

[0037] 1) Cleaning and roughening of the shaft base

[0038] Pneumatic sand blasting was used, using quartz sand with a particle size of 0.6 mm. The blasting parameters were: pressure of 0.53 MPa, blasting distance of 160 mm. Finally, the surface roughness R a The value is 6μm.

[0039] 2) Preparation of adhesive layer

[0040] The raw material used to prepare the bonding layer is Ni-50Cr powder, and the powder particle size is 10 μm.

[0041] Ordinary atmospheric plasma spraying process was used for spraying, and the obtained bonding layer thickness was 80 μm. The spraying parameters were: working current 600 A, working voltage 60 V, powder feeding rate 45 g / min, spraying distance 140 mm, and spray gun moving speed 300 mm / min.

[0042] 3) Preparation of hardness layer

[0043] The raw material used to prepare the hardness layer is YSZ material, specifically ZrO2-8wt% Y2O3. The powder particle size of the YSZ material is 500nm.

[0044] Preparation of the first slurry: YSZ material powder is dispersed in ethanol. In this embodiment, the mass fraction of the YSZ material in the first slurry is 15%.

[0045] Using the above-mentioned first slurry, suspension plasma spraying is adopted to obtain a hardness layer with a thickness of 175 μm; the spraying parameters are: working current 510A, working voltage 52V, liquid feed rate 46mL / min, spraying distance 80mm, and spray gun moving speed 500mm / min.

[0046] 4) Preparation of transition layer

[0047] The transition layer is made from a composite material of WS2 and YSZ, which improves the bonding between the hardness layer and the lubricating layer. The weight ratio of WS2 to YSZ in the composite material is 1.5:3. The particle size of the composite material's raw powder is 800 nm.

[0048] Prepare the second slurry: First, mix the WS2 powder and YSZ material powder in a high-energy ball mill and grind them uniformly. Then, disperse the mixed powder in ethanol. The mass fraction of the WS2 / YSZ mixed powder in the second slurry is 20%.

[0049] The above-mentioned second slurry was used and suspension plasma spraying was adopted, with a coating thickness of 65 μm; the spraying parameters were: working current 420 A, working voltage 52 V, liquid feeding rate 48 mL / min, spraying distance 105 mm, and spray gun moving speed 620 mm / min.

[0050] 5) Preparation of lubricating layer

[0051] The raw material used to prepare the lubricating layer is WS2 powder, and the particle size distribution of WS2 powder is 200nm.

[0052] Preparation of the third slurry: WS2 powder was dispersed in ethanol to form a 20% mass fraction of WS2 in the third slurry.

[0053] The third slurry mentioned above was used for suspension plasma spraying, with a coating thickness of 30 μm; the spraying parameters were: working current 420 A, working voltage 52 V, liquid feed rate 40 mL / min, spraying distance 105 mm, and spray gun moving speed 620 mm / min.

[0054] Example 2

[0055] See attached Figure 1 In this embodiment, the shaft substrate is 410 martensitic stainless steel, and the surface gradient coating on the shaft substrate includes an adhesive layer, a hardness layer, a transition layer and a lubricating layer in sequence.

[0056] The preparation of the surface gradient coating in this embodiment includes the following steps:

[0057] 1) Cleaning and roughening of the shaft base

[0058] Pneumatic sand blasting was used, using quartz sand with a particle size of 0.2 mm. The blasting parameters were: pressure of 0.53 MPa, blasting distance of 160 mm. Finally, the surface roughness R a The value is 8μm.

[0059] 2) Preparation of adhesive layer

[0060] The raw material used to prepare the bonding layer is Ni-50Cr powder, and the powder particle size is 30 μm.

[0061] Ordinary atmospheric plasma spraying process was used for spraying, and the obtained bonding layer thickness was 80 μm. The spraying parameters were: working current 600 A, working voltage 60 V, powder feeding rate 45 g / min, spraying distance 140 mm, and spray gun moving speed 300 mm / min.

[0062] 3) Preparation of hardness layer

[0063] The raw material used to prepare the hardness layer is YSZ material, specifically ZrO2-8wt% Y2O3. The powder particle size of the YSZ material is 1000nm.

[0064] Preparation of the first slurry: YSZ material powder is dispersed in ethanol. In this embodiment, the mass fraction of the YSZ material in the first slurry is 20%.

[0065] Using the above-mentioned first slurry, suspension plasma spraying is adopted to obtain a hardness layer with a thickness of 195 μm; the spraying parameters are: working current 535 A, working voltage 54 V, liquid feed rate 46 mL / min, spraying distance 90 mm, and spray gun moving speed 520 mm / min.

[0066] 4) Preparation of transition layer

[0067] The transition layer is made from a composite material of WS2 and YSZ, which improves the bonding between the hardness layer and the lubricating layer. The weight ratio of WS2 to YSZ in the composite material is 2:3. The particle size of the composite material's raw powder is 1000 nm.

[0068] Prepare the second slurry: First, mix the WS2 powder and YSZ material powder in a high-energy ball mill and grind them uniformly. Then, disperse the mixed powder in ethanol. The mass fraction of the WS2 / YSZ mixed powder in the second slurry is 30%.

[0069] The above-mentioned second slurry was used and suspension plasma spraying was adopted, with a coating thickness of 75 μm; the spraying parameters were: working current 440 A, working voltage 54 V, liquid feed rate 48 mL / min, spraying distance 110 mm, and spray gun moving speed 630 mm / min.

[0070] 5) Preparation of lubricating layer

[0071] The raw material used to prepare the lubricating layer is WS2 powder, and the particle size distribution of WS2 powder is 300nm.

[0072] Preparation of the third slurry: WS2 powder was dispersed in ethanol to form a 30% mass fraction of WS2 in the third slurry.

[0073] The third slurry mentioned above was used for suspension plasma spraying, with a coating thickness of 40 μm; the spraying parameters were: working current 430 A, working voltage 52 V, liquid feed rate 40 mL / min, spraying distance 115 mm, and spray gun moving speed 620 mm / min.

[0074] Comparative Example 1

[0075] The shaft substrate and YSZ hardness layer in this comparative example are the same as those in Example 1, except that there is no transition layer and lubricating layer in this comparative example. The corresponding preparation method and parameters are similar to those in Example 1.

[0076] Comparative Example 2

[0077] The shaft substrate and WS2 self-lubricating layer in this comparative example are the same as those in Example 1. The difference is that there is no YSZ hardness layer and transition layer in this comparative example. The corresponding preparation method and parameters are similar to those in Example 1.

[0078] Testing and Results

[0079] The porosity, bonding strength, hardness and friction coefficient of the coatings in Examples 1-2 and Comparative Examples 1-2 were tested according to the methods specified in ASTM E2109-01 (2014), HB5476-1991, HB5486-1991 and ASTM D3702-94 (2009). The test results are shown in the following table:

[0080] Table 1 Test results

[0081]

[0082]

[0083] As can be seen from the above table, the test results of the gradient structure protective coatings prepared in Example 1 and Example 2 in terms of porosity, bonding strength, hardness and friction coefficient are better than those in Comparative Examples 1 and 2.

[0084] Although the surface of Comparative Example 1 is hardened by the YSZ coating, its friction coefficient is too high. During wear, the YSZ easily cracks, causing rapid damage to the surface structure, and failing to protect the shaft and extend its life. Comparative Example 2 has a lubricating WS2 coating on the surface, but lacks a hardness layer and transition layer between the substrate and the surface, resulting in low surface hardness. Although the self-lubricating effect reduces the friction coefficient, the WS2 is easily damaged under high loads. At the same time, there is a mismatch in plastic deformation between the WS2 coating and the substrate, which leads to interlayer cracking and destruction, and loses the WS2 self-lubricating effect.

[0085] By designing a smaller nanoscale powder particle size and utilizing liquid phase delivery, this invention achieves a larger design space for coating performance through particle size and composition design. This results in higher hardness, toughness, and wear resistance. The coating's layer-by-layer characteristics result in a more uniform microstructure and more complete melting. Smaller particles result in thinner layers, denser coatings, and finer microstructures. By controlling thickness, interlayer stress concentration is reduced, minimizing differences in plastic deformation and linear expansion coefficient between the shaft substrate and the heterogeneous self-lubricating layer.

[0086] The large-scale shaft surface gradient coating of the present invention and its preparation method are suitable for large-size shafts such as nuclear power emergency diesel engine camshafts (length greater than 2.4 meters) and heavy machinery hydraulic cylinder piston rods (length 1-3 meters). The "hard outside and tough inside" structure of the shaft is achieved through a continuous gradient structure, and a dry low-friction coefficient surface is achieved through a self-lubricating coating on the outermost layer. It has the following advantages: the process is easy to implement, and a high-hardness, low-friction self-lubricating coating can be achieved on large-sized shafts. Compared with the PVD process which is limited by the size of the workpiece, plasma spraying can relatively easily achieve a high-hardness, low-friction self-lubricating coating on large-sized shafts; the gradient coating system has good performance, and a continuous gradient structure and a high-hardness, low-friction surface are achieved on the surface of the shaft; suspension liquid phase feeding is used to achieve nano-scale powder feeding plasma spraying, and the ratio of ceramic and self-lubricating powder is regulated to achieve a continuous gradient structure; at the same time, a self-lubricating layer with a thickness of microns is formed on the surface; the suspension powder feeding method is efficient, and the suspension liquid phase axial feeding is used, so that the powder particles can directly enter the center of the plasma flame flow, directly heated, evenly heated, with low stress, and greatly improved powder feeding rate and deposition efficiency; and the nano-level powder is transported smoothly.

[0087] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A gradient coating on the surface of a large shaft, wherein the length of the large shaft is greater than or equal to 1m, characterized in that: The gradient coating comprises an adhesive layer, a hardness layer, a transition layer and a lubricating layer sequentially formed on the surface of the shaft; the gradient coating is prepared by the following method: Prepare raw materials for preparing the bonding layer, a first slurry for preparing the hardness layer, a second slurry for preparing the transition layer, and a third slurry for preparing the lubricating layer respectively; The raw materials of the bonding layer include metal Ni and Cr, the first slurry includes YSZ material, the second slurry includes a composite material composed of YSZ and WS2, and the third slurry includes WS2 material; The raw material for preparing the bonding layer, the first slurry, the second slurry and the third slurry are sprayed on the surface of the shaft by a plasma spraying process to sequentially prepare the bonding layer, the hardness layer, the transition layer and the lubricating layer; The bonding layer is prepared by atmospheric plasma spraying process; The hardness layer, transition layer and lubricating layer are prepared by suspension plasma spraying process; The particle size of the raw material powder of the YSZ material in the first slurry is 300-1000 nm; the particle size of the raw material powder of the composite material in the second slurry is 100-1500 nm; the particle size distribution of the WS2 powder in the third slurry is 100-300 nm; The raw material for preparing the bonding layer is NiCr alloy powder, the mass fraction of Cr in the alloy powder is 40-60%, and the particle size distribution is 5-50 μm; The first slurry is formed by dispersing YSZ material powder in a first solvent, and the mass fraction of the YSZ material in the first slurry is 15-20%; The second slurry is formed by dispersing WS2 powder and YSZ material powder in a second solvent; The YSZ material in the first slurry and the second slurry consists of ZrO2 and Y2O3, and the mass fraction of Y2O3 in the YSZ material is 6-10%; The mass fraction of the WS2 / YSZ mixed powder material in the second slurry is 20-30%; the mass ratio of WS2 to YSZ in the composite material is 1.3-2:3; The third slurry is formed by dispersing WS2 powder in a third solvent, and the mass fraction of WS2 in the third slurry is 20-30%; The spraying parameters of the hardness layer are: working current 500-540A, working voltage 50-55V, liquid feeding rate 45-50mL / min, spraying distance 80-90mm, and spray gun moving speed 500-550mm / min.

2. The gradient coating according to claim 1, characterized in that The spraying parameters of the adhesive layer are: working current 580-620A, working voltage 60-65V, powder feeding rate 45-50g / min, spraying distance 120-150mm, and spray gun moving speed 300-350mm / min.

3. The gradient coating according to claim 1, characterized in that The spraying parameters of the transition layer are: working current 400-440A, working voltage 50-55V, liquid feeding rate 45-50mL / min, spraying distance 100-120mm, and spray gun moving speed 600-650mm / min.

4. The gradient coating according to claim 1, characterized in that The spraying parameters of the lubricating layer are: working current 400-440A, working voltage 50-55V, liquid feeding rate 35-40mL / min, spraying distance 100-120mm, and spray gun moving speed 600-650mm / min.

5. The gradient coating according to claim 1, characterized in that The thickness of the hardness layer is 150-200 μm, the thickness of the transition layer is 50-80 μm, and the thickness of the lubricating layer is 20-50 μm.

6. The gradient coating according to claim 1, characterized in that The preparation method further comprises the following steps: performing a roughening treatment on the surface of the shaft, and after the treatment, the roughness Ra value of the surface of the shaft is 5-10 μm.

7. The gradient coating according to claim 6, characterized in that The parameters of the texturing treatment are as follows: pneumatic sand blasting is used, quartz sand with a particle size of 0.1-0.8 mm is used, the sand blasting pressure is 0.5-0.6 MPa, and the sand blasting distance is 150-180 mm.

8. The gradient coating according to claim 1, characterized in that The first solvent, the second solvent or the third solvent is ethanol.

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