Coating for universal joint pin and preparation method of coating

Abstract

The invention discloses a coating for a cross shaft and a preparation method of the coating, and belongs to the technical field of surface coatings. The coating comprises a bottom layer, a middle layer and a surface layer; the bottom layer contains nickel-phosphorus alloy and nano TiO2, so that a compact passive film can be formed, and the salt spray corrosion resistance is improved; the middle layer contains tungsten carbide-cobalt alloy and nano Mo, and high-load wear and fretting wear are resisted through cooperation of a hard phase and a binding phase; the surface layer contains zirconium oxide, nano Al2O3, graphene, nano SiC and micron WC, and the functions of impact resistance, high-temperature oxidation resistance and self-lubricating are achieved through a micro-nano composite structure. The preparation method comprises the steps of sand blasting pretreatment, chemical plating of the bottom layer, plasma spraying of the middle layer, preparation of the surface layer through the electron beam evaporation-magnetron sputtering composite technology and vacuum heat treatment. Through hierarchical work division of bottom layer corrosion resistance, salt mist resistance, middle layer wear resistance, surface layer impact resistance, high temperature resistance and the like, a gradient protection system is formed on the surface of the universal joint pin, and the service life of the universal joint pin is effectively prolonged.

Description

Technical Field

[0001] The invention relates to the technical field of cross shaft coatings, in particular to a coating for a cross shaft and a preparation method thereof. Background Art

[0002] Cross shafts are widely used in transmission systems of automobiles, construction machinery, and other applications. During operation, there is frequent relative rotation and sliding between the journal and bearing, and the contact area between the two faces multiple extreme working conditions:

[0003] Complex load impact: Starting, braking, and bumpy roads (such as potholes and speed bumps) can generate instantaneous impacts of more than three times the rated load. Traditional coatings (such as chrome plating) are prone to cracking and peeling due to their high brittleness.

[0004] Multi-media corrosion: Winter snow melting agents (containing NaCl) or coastal areas can cause chloride ion penetration. The salt spray resistance of existing coatings is often less than 500 hours, which can easily cause substrate corrosion.

[0005] Fretting wear and lubrication failure: During low-speed steering or idling, the journal and bearing experience fretting wear of a certain amplitude, and the bonding phase of traditional coatings (such as cobalt phase) is easily eroded. After lubricating oil leaks, the friction coefficient of the coating without self-lubricating function rises sharply, exacerbating the wear of the mating parts.

[0006] High-temperature performance degradation: When driving at high speed for a long time (continuous climbing, full load), the operating temperature of the cross shaft can reach 400-500℃. The existing coating (such as ordinary ceramic layer) will cause the hardness to decrease due to high-temperature oxidation, and the wear resistance will be significantly reduced.

[0007] In the existing technology, a single chrome plating layer, ordinary ceramic coating, etc. cannot simultaneously meet the coordinated requirements of wear resistance, corrosion resistance, high temperature resistance and impact resistance, resulting in a reduced service life of the cross shaft. Therefore, it is urgent to develop a multi-gradient protective coating system. Summary of the Invention

[0008] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a coating for a cross shaft and a preparation method thereof. The coating is given excellent wear resistance, corrosion resistance, high temperature resistance and salt spray resistance through a gradient setting of a bottom layer, an intermediate layer and a surface layer.

[0009] The object of the present invention is achieved through the following technical solutions:

[0010] A coating for a cross shaft, comprising a base layer, an intermediate layer and a surface layer;

[0011] The bottom layer includes nickel-phosphorus alloy and nano-TiO2;

[0012] The intermediate layer includes tungsten carbide-cobalt alloy and nano Mo;

[0013] The surface layer includes zirconium oxide, nano-Al2O3, nano-graphene sheets, nano-SiC, and micron WC.

[0014] In this solution, the synergistic effects of the coating layers are as follows:

[0015] Bottom layer: Nickel-phosphorus alloy provides basic adhesion and corrosion resistance for the bottom layer. Nano-TiO2 forms a dense passivation film by filling the grain boundaries of nickel-phosphorus alloy, effectively resisting salt spray corrosion, thereby effectively blocking the penetration of chloride ions in coastal areas or snow-melting agent environments.

[0016] Middle layer: Tungsten carbide as the hard phase plays the main role in wear resistance, and cobalt as the bonding phase gives the middle layer toughness. The two form a hard phase-bonding phase skeleton structure that can withstand instantaneous impact when the car is bumpy.

[0017] Under low-speed steering or idling conditions, the cross shaft and bearings experience fretting wear. The cobalt phase of the tungsten carbide-cobalt alloy in the intermediate layer is easily eroded, forming wear debris accumulation and accelerating coating failure. To address this issue, nano-Mo is introduced into the intermediate layer. Its low coefficient of friction forms a solid solution with the cobalt phase, increasing the hardness of the cobalt phase. The layered sliding properties of nano-Mo also reduce the wear scar depth of the intermediate layer under low-speed steering fretting wear conditions, thus preventing accelerated failure caused by wear debris accumulation.

[0018] surface layer:

[0019] Zirconia provides high temperature stability and base hardness.

[0020] When a car frequently encounters bumpy roads (such as potholes and speed bumps), the cross shaft can be subjected to transient impact loads that can reach over three times its rated load. Existing zirconia ceramics are highly brittle and prone to microcracks under alternating impacts, potentially causing spalling after long-term use. To address this, the introduction of nano-Al2O3 into the surface layer refines the zirconia grains through a "pinning effect," effectively improving impact toughness and resisting the transient impacts of bumpy roads.

[0021] When automotive lubricants leak or age, the surface zirconia ceramic loses its self-lubricity, causing a sudden increase in the dry friction coefficient, which in turn leads to rapid wear of the mating parts (bearings). To address this issue, nanographene sheets are introduced into the surface layer to effectively reduce the dry friction coefficient through interlayer sliding, thus resolving the wear failure problem caused by lubricant leaks.

[0022] When a car is driven at high speed for extended periods (e.g., continuously climbing a hill or fully loaded), the cross-axle operating temperature can reach 400-500°C. The graphene on the surface is susceptible to oxidation in the air, leading to loss of the self-lubricating phase. To address this, nano-SiC is introduced into the surface layer. Its excellent stability below 600°C allows it to form an interpenetrating network with graphene, physically blocking oxygen from contact with the graphene. This effectively improves graphene retention at high temperatures and maintains long-term self-lubrication.

[0023] Micron WC forms a micro-nano composite structure with nano Al2O3, nano SiC, etc., which effectively reduces the surface peeling surface and avoids the exposure of the middle layer under sand and gravel impact conditions.

[0024] In summary, the present invention forms a gradient protection system on the surface of the cross shaft through the hierarchical division of labor of bottom layer anti-corrosion and salt spray resistance → middle layer wear resistance → surface layer impact resistance, high temperature resistance, and resistance to lubrication failure, thereby effectively improving the service life of the cross shaft.

[0025] Preferably, the bottom layer has a thickness of 5-10 μm; in this layer, the mass ratio between the nickel-phosphorus alloy and nano-TiO2 is (98-100):(0.5-1).

[0026] Preferably, the thickness of the intermediate layer is 35-55 μm; in this layer, the mass ratio of the tungsten carbide-cobalt and nano-Mo is (85-105):(3-5).

[0027] Preferably, the thickness of the surface layer is 10-25 μm; in this layer, the mass ratio of the zirconium oxide, nano-Al2O3, micron WC, nano-graphene sheet and nano-SiC is (85-90):(5-10):(1-2):(0.6-0.8):(0.3-0.4).

[0028] Preferably, rare earth elements are introduced into the bottom layer.

[0029] Frequent car starts and stops (such as at city traffic lights), and the cross bearing is subjected to alternating stress. This can easily lead to stress concentration in the bottom layer, which can cause microcracks at the interface between the bottom layer and the steel matrix. To address this issue, the introduction of rare earth elements into the bottom layer can refine the bottom layer's grain size, reduce stress concentration at grain boundaries, and effectively alleviate the problem of interface cracking.

[0030] Preferably, the rare earth element is selected from any one or more of yttrium, lanthanum and cerium.

[0031] In addition, to achieve the above-mentioned object, the present invention also provides a method for preparing a coating for a cross shaft, comprising the following steps:

[0032] S1. Surface treatment of the cross shaft substrate;

[0033] S2. The bottom layer is prepared by chemical plating process;

[0034] S3. The intermediate layer is prepared by plasma spraying process;

[0035] S4. The surface layer is prepared by a composite process of electron beam evaporation and magnetron sputtering;

[0036] S5. Vacuum heat treatment.

[0037] In this solution, a chemical plating process is used when preparing the bottom layer to form a uniform coating on the surface of the cross shaft, thereby improving the uniformity of nano-TiO2 dispersion.

[0038] When preparing the intermediate layer, high-temperature plasma spraying is used to fully melt the tungsten carbide-cobalt alloy and nano-Mo, thereby improving the density of the coating.

[0039] When preparing the surface layer, electron beam evaporation is used to ensure the purity of zirconia ceramics, and magnetron sputtering is used to promote the uniform distribution of nano-Al2O3, graphene, nano-SiC, etc., to avoid component segregation caused by a single process.

[0040] Finally, vacuum heat treatment can eliminate the residual stress in each layer, promote the diffusion of nanophase, and improve the interlayer bonding strength.

[0041] Preferably, step S1 includes sandblasting.

[0042] Preferably, in step S5, the vacuum degree is greater than 1×10 -3 Pa, heat to 400-500℃ and keep warm for 2-3h.

[0043] The beneficial effects of the present invention are:

[0044] Based on the working conditions of the existing cross shaft, the present invention provides a coating composed of a bottom layer, an intermediate layer and a surface layer. The coating has high temperature resistance, corrosion resistance and wear resistance. The functions of each layer in the coating are as follows:

[0045] Bottom layer: Nickel-phosphorus alloy provides basic adhesion and corrosion resistance for the bottom layer. Nano-TiO2 forms a dense passivation film by filling the grain boundaries of nickel-phosphorus alloy, effectively resisting salt spray corrosion, thereby effectively blocking the penetration of chloride ions in coastal areas or snow-melting agent environments.

[0046] Intermediate layer: Tungsten carbide, as the hard phase, provides primary wear resistance, while cobalt, as the binder phase, imparts toughness to the intermediate layer. The two form a hard-binder skeleton structure that can withstand the transient impacts of vehicle jolting. Nano-Mo forms a solid solution with the cobalt phase, increasing its hardness. The layered sliding properties of nano-Mo also reduce the wear scar depth of the intermediate layer under low-speed steering fretting wear conditions, preventing accelerated failure caused by wear debris accumulation.

[0047] Surface layer: Zirconia provides high-temperature stability and basic hardness. Al2O3, through the "pinning effect", refines the zirconium oxide grains, effectively improving impact toughness and resisting instantaneous impacts from bumpy roads. Nanographene sheets effectively reduce the dry friction coefficient through interlayer sliding, solving the problem of wear failure caused by lubricating oil leakage. Nano-SiC has excellent stability below 600°C and can form an interpenetrating network structure with graphene, which can physically block the contact between oxygen and graphene, effectively improve the graphene retention rate at high temperatures, and maintain long-term self-lubrication. Micron WC forms a micro-nano composite structure with nano-Al2O3, nano-SiC, etc., which effectively reduces the surface peeling surface and avoids exposure of the intermediate layer under sand and gravel impact conditions.

[0048] In summary, the present invention forms a gradient protection system on the surface of the cross shaft through the hierarchical division of labor of bottom layer anti-corrosion and salt spray resistance → middle layer wear resistance → surface layer impact resistance, high temperature resistance, and resistance to lubrication failure, thereby effectively improving the service life of the cross shaft. DETAILED DESCRIPTION

[0049] Example 1

[0050] S1. Pretreatment: The clean cross shaft substrate is sandblasted (using 80 mesh corundum sand, pressure 0.4-0.5 MPa) to make the surface roughness of the cross shaft reach Ra4-5.0 μm, thereby obtaining a pretreated cross shaft.

[0051] S2. Preparation of the bottom layer:

[0052] S21. Prepare the plating solution: The plating solution consists of nickel sulfate (NiSO4·6H2O, 26 g / L), sodium hypophosphite (22 g / L), sodium citrate (16 g / L), nano-TiO2 (particle size 20-50 nm, 0.08 g / L) and deionized water.

[0053] S22. Process parameters: The pH value of the plating solution is controlled at 4.5-5.5, the temperature is maintained at 85-90°C, the pretreated cross shaft is placed in the plating solution, the plating time is 70 minutes, and a bottom layer with a thickness of 8μm is formed on the surface of the cross shaft.

[0054] S3. Preparation of the middle layer:

[0055] S31. Preparation of spray powder: tungsten carbide-cobalt alloy powder (mass ratio of tungsten carbide to cobalt is 85:10) and nano-Mo (particle size 40-60 nm) are mixed in a mass ratio of 100:3.5, and then ball milled (ball-to-material ratio is 10:1, rotation speed is 300 r / mi) for 4.5 h to obtain spray powder.

[0056] S32. Spraying parameters: plasma spraying power is 40 kW, spraying distance is 130 mm, powder feeding rate is 25 g / min, working gases are argon (flow rate is 45 L / min) and hydrogen (flow rate is 6 L / min), and an intermediate layer with a thickness of 42 μm is formed on the surface of the bottom layer.

[0057] S4. Preparation of surface layer:

[0058] S41. Preparation of composite powder: Zirconia ceramic powder, nano-Al2O3 (particle size 50-100nm), micron WC particles (particle size 1-3μm), nano-graphene sheets (thickness 1-3nm, lateral size 5-10μm, sheet defect rate <5%) and nano-SiC (particle size 30-50nm) are mixed in a mass ratio of 85:7:1.5:0.8:0.4, and then spray granulated (inlet temperature 220℃, outlet temperature 100℃) to prepare a composite powder with a particle size of 50-100μm.

[0059] S42. Electron beam evaporation: Place the composite powder into the crucible of the electron beam evaporation equipment and control the vacuum degree at 2×10 -3 , the electron beam power is 13kW, the deposition rate is 0.7nm / s, and it is deposited on the surface of the intermediate layer. When the deposition thickness reaches 16μm, the electron beam evaporation is stopped.

[0060] S43. Magnetron sputtering: Start the magnetron sputtering equipment, use nano-graphene sheets and nano-SiC as targets, sputtering power of 300W, sputtering time of 12 minutes, and form a sputtering layer with a thickness of 4 μm on the surface of the electron beam evaporation layer, and finally obtain a surface layer with a total thickness of 20 μm.

[0061] S5. Overall heat treatment:

[0062] The cross shaft with the prepared coating was placed in a vacuum furnace for heat treatment, and the vacuum degree was maintained at 1×10 -3 Pa, heat to 500℃, keep warm for 2h, and then cool to room temperature with the furnace.

[0063] Example 2

[0064] S1. Pretreatment: The clean cross shaft substrate is sandblasted (using 80 mesh corundum sand, pressure 0.4-0.5 MPa) to make the surface roughness of the cross shaft reach Ra4-5.0 μm, thereby obtaining a pretreated cross shaft.

[0065] S2. Preparation of the bottom layer:

[0066] S21. Prepare the plating solution: The plating solution consists of nickel sulfate (NiSO4·6H2O, 26 g / L), sodium hypophosphite (22 g / L), sodium citrate (16 g / L), nano-TiO2 (particle size 20-50 nm, 0.1 g / L) and deionized water.

[0067] S22. Process parameters: The pH value of the plating solution is controlled at 4.5-5.5, the temperature is maintained at 85-90°C, the pretreated cross shaft is placed in the plating solution, the plating time is 65 minutes, and a bottom layer with a thickness of 7 μm is formed on the surface of the cross shaft.

[0068] S3. Preparation of the middle layer:

[0069] S31. Preparation of spray powder: Tungsten carbide-cobalt alloy powder (mass ratio of tungsten carbide to cobalt is 88:12) and nano-Mo (particle size 40-60 nm) are mixed in a mass ratio of 100:4, and then ball milled (ball-to-material ratio is 10:1, speed 300 r / min) for 4.5 h to obtain spray powder.

[0070] S32. Spraying parameters: plasma spraying power is 40 kW, spraying distance is 130 mm, powder feeding rate is 25 g / min, working gases are argon (flow rate is 45 L / min) and hydrogen (flow rate is 6 L / min), and an intermediate layer with a thickness of 50 μm is formed on the surface of the bottom layer.

[0071] S4. Preparation of surface layer:

[0072] S41. Preparation of composite powder: Zirconia ceramic powder, nano-Al2O3 (particle size 50-100nm), micron WC particles (particle size 1-3μm), nano-graphene sheets (thickness 1-3nm, lateral size 5-10μm, sheet defect rate <5%) and nano-SiC (particle size 30-50nm) are mixed in a mass ratio of 88:5:2:0.6:0.3, and then spray granulated (inlet temperature 220℃, outlet temperature 100℃) to prepare a composite powder with a particle size of 50-100μm.

[0073] S42. Electron beam evaporation: Place the composite powder into the crucible of the electron beam evaporation equipment and control the vacuum degree at 2×10 -3 Pa, the electron beam power is 13kW, the deposition rate is 0.7nm / s, and it is deposited on the surface of the intermediate layer. When the deposition thickness reaches 13μm, the electron beam evaporation is stopped.

[0074] S43. Magnetron sputtering: Start the magnetron sputtering equipment, use nano-graphene sheets and nano-SiC as targets, sputtering power of 300 W, sputtering time of 12 minutes, and form a sputtering layer with a thickness of 5 μm on the surface of the electron beam evaporation layer, and finally obtain a surface layer with a total thickness of 18 μm.

[0075] S5. Overall heat treatment:

[0076] The cross shaft with the prepared coating was placed in a vacuum furnace for heat treatment, and the vacuum degree was maintained at 1×10 -3 Pa, heat to 500℃, keep warm for 2h, and then cool to room temperature with the furnace.

[0077] Example 3

[0078] Based on Example 1, lanthanum is introduced into the bottom layer.

[0079] The bottom layer is prepared as follows:

[0080] S21. Prepare the plating solution: The plating solution consists of nickel sulfate (NiSO4·6H2O, 26 g / L), sodium hypophosphite (22 g / L), sodium citrate (16 g / L), nano-TiO2 (particle size 20-50 nm, 0.08 g / L), lanthanum nitrate (La(NO3)3·6H2O, 0.05 g / L) and deionized water.

[0081] S22. Process parameters: The pH value of the plating solution is controlled at 4.5-5.5, the temperature is maintained at 85-90°C, the pretreated cross shaft is placed in the plating solution, the plating time is 70 minutes, and a bottom layer with a thickness of 8μm is formed on the surface of the cross shaft.

[0082] The remaining steps are the same as in Example 1.

[0083] Comparative Example 1

[0084] Compared with Example 1, no intermediate layer is provided, that is, the surface layer (thickness is the same as Example 1) is directly prepared after the bottom layer (thickness is the same as Example 1), and the remaining steps and parameters are the same as Example 1.

[0085] Comparative Example 2

[0086] Compared with Example 1, nano-Mo is not added to the middle layer, and the remaining steps and parameters are the same as Example 1.

[0087] Comparative Example 3

[0088] Compared with Example 1, no micron WC particles were added to the surface layer, and the remaining steps and parameters were the same as Example 1.

[0089] Comparative Example 4

[0090] Compared with Example 3, lanthanum nitrate [La(NO3)3·6H2O] was replaced with cerium nitrate [Ce(NO3)3·6H2O]. The remaining steps and parameters were the same as those in Example 3.

[0091] Experimental example:

[0092] By simulating the actual working conditions of the cross shaft, the wear resistance, corrosion resistance, high temperature resistance and impact resistance of the coatings in Examples 1-3 and Comparative Examples 1-4 were tested, thereby verifying the performance differences of different coating formulations and structures.

[0093] The experimental samples are coated cross shafts prepared by the methods of Examples 1-3 and Comparative Examples 1-4, and the base material is 40Cr steel (size Φ30mm×50mm, journal diameter Φ15mm).

[0094] (1) Wear resistance test.

[0095] A pin-on-disc friction and wear tester was used to simulate the friction conditions between a cross shaft and a bearing. The experimental conditions were: a load of 500 N, a rotational speed of 1000 r / min, and a friction time of 10 hours. The friction medium consisted of automotive lubricant (initial state) and no lubrication (simulating lubricant failure). An electronic balance (accuracy 0.1 mg) was used to measure the mass difference before and after wear, and the wear loss was calculated. The experimental results are shown in Table 1.

[0096] Table 1:

[0097] sample Wear amount with lubrication (mg) Wear loss without lubrication (mg) Example 1 7.9 24.8 Example 2 7.8 24.3 Example 3 6.5 20.1 Comparative Example 1 35.6 89.0 Comparative Example 2 15.4 42.3 Comparative Example 3 10.4 30.2 Comparative Example 4 7.9 28.3

[0098] (2) Corrosion resistance test.

[0099] The neutral salt spray corrosion test was conducted on the sample (except the coating area, the rest of the surface was sealed with epoxy resin). The experimental conditions were: 5% NaCl solution, temperature 35℃, salt spray deposition rate 1-2mL / (h·80cm 2 The test time was 1500 hours, and the corrosion area percentage was calculated using Image-Pro Plus software. The experimental results are shown in Table 2.

[0100] Table 2:

[0101]

[0102]

[0103] (3) High temperature resistance test.

[0104] The samples were placed in a 450°C furnace for 100 hours. After cooling, the surface hardness and friction coefficient were measured. The experimental results are shown in Table 3.

[0105] Table 3:

[0106] sample Surface hardness after high temperature treatment (HV) Dry friction coefficient after high temperature treatment Example 1 1277 0.37 Example 2 1271 0.36 Example 3 1321 0.35 Comparative Example 1 989 0.64 Comparative Example 2 1105 0.45 Comparative Example 3 1207 0.40 Comparative Example 4 1231 0.39

[0107] (4) Impact resistance test.

[0108] A drop-weight impact tester was used to simulate the impact conditions experienced by a car on a bumpy road. The hammer was a 10 mm Ø 45 steel ball. The impact point was the center of the cross-shaft journal (the area most susceptible to stress on the coating). The impact energy was 10 J and the number of impacts was 1000. After impact, the number of cracks was observed using an Olympus BX51 microscope (500x magnification), and the spalling area was measured using a 3D profilometer. The experimental results are shown in Table 4.

[0109] Table 4:

[0110] sample Number of cracks <![CDATA[Spalling area (mm 2 )]]> Example 1 2 0.05 Example 2 2 0.04 Example 3 0 0.01 Comparative Example 1 13 2.6 Comparative Example 2 7 0.7 Comparative Example 3 5 0.3 Comparative Example 4 3 0.1

[0111] From Table 1-4 we can see that:

[0112] The cross-axle coatings prepared in Examples 1-3 of the present invention exhibit excellent wear resistance, corrosion resistance, high temperature resistance, and impact resistance through the synergistic effects of the base layer, intermediate layer, and surface layer. Example 3 exhibits the best performance due to the introduction of lanthanum. Comparative Examples 1-4 exhibit varying degrees of performance degradation due to the lack of key components or structures, further demonstrating the rationality of the coating formulation and structure of the present invention.

[0113] The specific comparison is as follows:

[0114] In Example 3, due to the introduction of lanthanum in the bottom layer, the grains are refined, and the wear resistance, corrosion resistance, high temperature resistance and impact resistance are all better than those of Example 1 and Example 2. In particular, the impact resistance is significantly improved, and there is no cracking or peeling.

[0115] In Comparative Example 1, no intermediate layer is provided, and the overall wear resistance and impact resistance of the coating are greatly reduced, indicating that the intermediate layer plays a key role in bearing load and friction.

[0116] In Comparative Example 2, nano-Mo was not added to the middle layer, and the fretting wear was aggravated and the wear resistance was reduced, which verified the importance of nano-Mo in improving the anti-fretting wear ability of the middle layer.

[0117] In Comparative Example 3, micron WC particles were not added to the surface layer, and the impact resistance and wear resistance were reduced, which demonstrated the role of micron WC in resisting the impact of sand and gravel.

[0118] In Comparative Example 4, cerium nitrate was used instead of lanthanum nitrate, and the performance was slightly lower than that of Example 3, indicating that lanthanum is superior to cerium in improving coating performance.

Claims

1. A coating for a cross shaft, comprising a bottom layer, an intermediate layer and a surface layer; characterized in that: The bottom layer includes nickel-phosphorus alloy and nano-TiO2; The intermediate layer includes tungsten carbide-cobalt alloy and nano Mo; The surface layer includes zirconium oxide, nano-Al2O3, nano-graphene sheets, nano-SiC, and micron WC.

2. The coating for a cross shaft according to claim 1, characterized in that: The thickness of the bottom layer is 5-10 μm; in the layer, the mass ratio between the nickel-phosphorus alloy and nano-TiO2 is (98-100):(0.5-1).

3. The coating for a cross shaft according to claim 1, characterized in that: The thickness of the intermediate layer is 35-55 μm; in this layer, the mass ratio of the tungsten carbide-cobalt and nano-Mo is (85-105):(3-5).

4. The coating for a cross shaft according to claim 1, characterized in that: The thickness of the surface layer is 10-25 μm; in this layer, the mass ratio of the zirconium oxide, nano-Al2O3, micron WC, nano-graphene sheet and nano-SiC is (85-90):(5-10):(1-2):(0.6-0.8):(0.3-0.4).

5. The coating for a cross shaft according to claim 1, characterized in that: Rare earth elements are introduced into the bottom layer.

6. The coating for a cross shaft according to claim 5, characterized in that: The rare earth element is selected from any one or more of yttrium, lanthanum and cerium.

7. The method for preparing a coating for a cross shaft according to any one of claims 1 to 6, characterized in that: The steps include: S1. Surface treatment of the cross shaft substrate; S2. The bottom layer is prepared by chemical plating process; S3. The intermediate layer is prepared by plasma spraying process; S4. The surface layer is prepared by a composite process of electron beam evaporation and magnetron sputtering; S5. Vacuum heat treatment.

8. The method for preparing a coating for a cross shaft according to claim 7, characterized in that: Step S1 includes sandblasting.

9. The method for preparing a coating for a cross shaft according to claim 8, characterized in that: In step S5, the vacuum degree is greater than 1×10 -3 Pa, heat to 400-500℃ and keep warm for 2-3h.

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