A low-strength metal surface hard protective coating and its preparation method and application

Abstract

The present invention provides a hard protective coating on a low-strength metal surface, and a preparation method and application thereof, and relates to the technical field of material surface protection. The present invention deposits a hard protective coating on a low-strength metal surface by electroplating and / or chemical plating methods, and the hard protective coating is a nickel-based multilayer coating. Starting from the low-strength metal surface, the hardness of the nickel-based multilayer coating increases gradually layer by layer. The present invention constructs a "mechanical spring" by preparing a hardness gradient coating on the low-strength metal surface, thereby alleviating the problem of soft-hard mechanical mismatch between the low-strength metal surface and the hard protective coating, and the nickel-based coating has the advantages of good corrosion resistance and strong wear resistance; and the electroplating or chemical plating technology is mature, and the prepared nickel-based coating has good bonding strength and a long service life. The preparation method provided by the present invention can improve the corrosion resistance of the protective coating while alleviating the film-base bonding problem caused by the soft-hard mechanical mismatch between the low-strength metal and the hard protective coating.

Description

Technical Field

[0001] The invention relates to the technical field of material surface protection, and in particular to a low-strength metal surface hard protective coating and a preparation method and application thereof. Background Art

[0002] Motion systems are always accompanied by friction and wear. According to relevant statistics, about 1 / 3 of the world's disposable energy is consumed by friction, and the resulting wear also causes about 60% of equipment damage or failure. Lubrication and surface strengthening technology is one of the effective means to reduce friction and wear. Low-strength materials, such as aluminum alloys and magnesium alloys, are widely used in modern industry due to their light weight, low density, and good ductility. Although low-strength materials have excellent performance, they are usually problematic to use directly as structural component materials due to their low hardness, high friction coefficient, and high chemical activity. However, the uniqueness of the properties of some low-strength metal materials makes them irreplaceable in specific applications. In view of this, it is of great significance to treat the surface of low-strength metal materials to cope with harsh friction conditions and extend their service life.

[0003] Generally speaking, the wear resistance of the material surface is positively correlated with its hardness property. Based on this, in order to obtain excellent wear resistance, researchers are committed to preparing ultra-high hardness wear-resistant coatings. However, due to many problems such as high internal stress in the coating, fatigue crack initiation, and poor bonding strength, the protection of low-strength materials by ultra-hard coatings has failed to achieve the expected results in actual engineering applications. Among them, solving the hardness mismatch problem between the high-hardness coating and the low-hardness substrate and achieving a good film-base bonding have become the key to achieving good protective effects of hard protective coatings on the surface of low-strength metal materials, and this problem has not yet been effectively solved. Summary of the invention

[0004] In view of this, the present invention aims to provide a low-strength metal surface hard protective coating and its preparation method and application. The preparation method provided by the present invention can solve the problem of soft-hard mismatch between the low-strength metal substrate and the hard protective coating, and the film-base combination is good.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a method for preparing a low-strength hard protective coating on a metal surface, comprising the following steps:

[0007] A hard protective coating is deposited on a low-strength metal surface by electroplating and / or chemical plating. The hard protective coating is a nickel-based multilayer coating. Starting from the low-strength metal surface, the hardness of the nickel-based multilayer coating increases gradually layer by layer.

[0008] Preferably, the deposition of the hard protective coating is: electroplating and depositing a first hardness nickel-based coating and a second hardness nickel-based coating on the low-strength metal surface in sequence; chemically depositing a third hardness nickel-based coating and a fourth hardness nickel-based coating on the surface of the second hardness nickel-based coating in sequence.

[0009] Preferably, the composition of the electroplating solution used for electroplating and depositing the first hardness nickel-based coating includes: 180-220 g / L nickel chloride, 130-170 mL / L hydrochloric acid, and the solvent is water; the pH value of the electroplating solution is 4.0-6.0.

[0010] Preferably, the composition of the electroplating solution used for the electroplating deposition of the second hardness nickel-based coating includes: 40-50 g / L nickel chloride, 260-300 g / L nickel sulfate, 30-40 g / L boric acid, 1.6-2.0 g / L hard nanoparticles, 0.05-1 g / L surfactant, and the solvent is water; the pH value of the electroplating solution is 4.0-6.0; the hard nanoparticles include SiC, TiO 2 、ZnO、Al 2 O 3 、Si 3 N 4 and ZrO 2 One or more of the .

[0011] Preferably, the deposition temperature of the electroplating deposition of the first hardness nickel-based coating or the electroplating deposition of the second hardness nickel-based coating is independently 50-60° C., the deposition time is independently less than 10 min, and the deposition current density is independently 2-8 A / dm 2 .

[0012] Preferably, the chemical plating solution used for chemically depositing the third hardness nickel-based coating includes: 25-35 g / L nickel sulfate, 20-30 g / L sodium hypophosphite, 13-17 g / L sodium acetate, 18-22 g / L trisodium citrate, 8-12 mL / L lactic acid, 10-14 mg / L thiourea, and the solvent is water; the pH value of the chemical plating solution is 4.0-6.0.

[0013] Preferably, the chemical plating solution used for chemically depositing the fourth hardness nickel-based coating includes: 34-38 g / L nickel sulfate, 26-30 g / L sodium hypophosphite, 32-36 g / L sodium acetate, 10-14 mL / L lactic acid, 3-5 mg / L thiourea, and the solvent is water; the pH value of the chemical plating solution is 4.0-6.0.

[0014] Preferably, the deposition temperature of the chemical plating deposition of the third hardness nickel-based coating or the chemical plating deposition of the fourth hardness nickel-based coating is independently 85-90° C., and the deposition time is independently less than 90 minutes.

[0015] The present invention provides a low-strength metal surface hard protective coating obtained by the preparation method described in the above technical solution. Starting from the low-strength metal surface, the hard protective coating is a nickel-based hardness gradient progressive multi-layer coating.

[0016] The present invention provides the application of the low-strength metal surface hard protective coating described in the above technical solution in friction and wear protection.

[0017] The present invention provides a method for preparing a hard protective coating on a low-strength metal surface, comprising the following steps: depositing a hard protective coating on the low-strength metal surface by electroplating and / or chemical plating, wherein the hard protective coating is a nickel-based multilayer coating, and the hardness of the nickel-based multilayer coating increases gradually layer by layer starting from the low-strength metal surface. The hard-soft mismatch problem is mainly caused by the huge difference in mechanical properties between the substrate material and the hard coating material. Under working conditions, the collision and extrusion between the parts cause the workpiece to undergo elastic deformation. Due to the difference in mechanical properties between the film substrate, there is a serious mismatch between the stress and strain of the substrate and the coating at the bonding surface, which is manifested as coating peeling, crack initiation, and ultimately protection failure. The present invention can alleviate the hard-soft mechanical mismatch problem between the low-strength metal surface and the hard protective coating by preparing a hardness gradient coating on the low-strength metal surface to construct a "mechanical spring", and the nickel-based coating has the advantages of good corrosion resistance and strong wear resistance; and the electroplating or chemical plating technology is mature, the prepared nickel-based coating has good bonding strength, uniform coating thickness, and long service life. Therefore, the preparation method provided by the present invention can alleviate the film-base bonding problem caused by the soft-hard mechanical mismatch between the low-strength metal and the hard protective coating, while also improving the corrosion resistance of the protective coating; and the required equipment is simple, the process is simple, and it is easy to realize large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a high-magnification scanning electron microscope image of a cross section of a hardness gradient nickel-based multilayer coating prepared in Example 1;

[0019] Figure 2 This is a cross-sectional hardness indentation diagram of the hardness gradient nickel-based multilayer coating prepared in Example 1;

[0020] Figure 3 This is a statistical diagram of the cross-sectional hardness of the hardness gradient nickel-based multilayer coating prepared in Example 1;

[0021] Figure 4 This is a graph showing the XRD analysis results of the hardness gradient nickel-based multilayer coating prepared in Example 1;

[0022] Figure 5 This is an EDS analysis result diagram of the chemical plating layer in the hardness gradient nickel-based multi-layer coating prepared in Example 1. Figure 5(a) is an EDS analysis result diagram of the chemical plating layer formed in the chemical plating solution D, and (b) is an EDS analysis result diagram of the chemical plating layer formed in the chemical plating solution C;

[0023] Figure 6 This is a simulation diagram of the interface stress distribution of the hardness gradient nickel-based multilayer coating prepared in Example 1. Figure 6 (a) to (h) represent the stress distribution diagram of the lower surface of NiP-2 layer, the stress distribution diagram of the lower surface of NiP-1 layer, the stress distribution diagram of the lower surface of Ni-SiC layer, the stress distribution diagram of the lower surface of Ni layer, the stress distribution diagram of the upper surface of NiP-1 layer, the stress distribution diagram of the upper surface of Ni-SiC layer, the stress distribution diagram of the upper surface of Ni layer, and the stress distribution diagram of the upper surface of substrate, respectively;

[0024] Figure 7 This is a simulation diagram of the interface stress distribution of the nickel-based coating prepared in Comparative Example 1. Figure 7 (a) to (b) are the stress distribution diagrams of the lower surface of the NiP-2 layer and the upper surface of the substrate in Comparative Example 1, respectively. DETAILED DESCRIPTION

[0025] The present invention provides a method for preparing a low-strength hard protective coating on a metal surface, comprising the following steps:

[0026] A hard protective coating is deposited on a low-strength metal surface by electroplating and / or chemical plating. The hard protective coating is a nickel-based multilayer coating. Starting from the low-strength metal surface, the hardness of the nickel-based multilayer coating increases gradually layer by layer.

[0027] In the present invention, unless otherwise specified, the raw materials involved are commercially available products well known to those skilled in the art.

[0028] The present invention has no special requirements for the low-strength metal, and low-strength metals familiar to those skilled in the art can be used, such as aluminum alloys, magnesium alloys, pure metals (such as pure iron), carbon steel, nickel alloys, etc. Before depositing the nickel-based multilayer coating, the present invention preferably pre-treats the low-strength metal; the pre-treatment preferably includes grinding and roughening and ultrasonic cleaning performed in sequence. In the present invention, the grinding and roughening is preferably performed using 800-mesh sandpaper, and the grinding and roughening removes oxides on the metal surface and forms a surface texture, which is beneficial to improve the bonding force between the metal surface and the coating; the ultrasonic cleaning preferably includes ultrasonic cleaning in an organic solvent and ultrasonic cleaning in an acidic solution in sequence, and the organic solvent is preferably petroleum ether, and the acidic solution is preferably 0.1 mol / L dilute hydrochloric acid, and the impurities and oxides on the metal surface are further removed by the ultrasonic cleaning and fully activated.

[0029] In the present invention, the deposited hard protective coating is preferably: a first hardness nickel-based coating and a second hardness nickel-based coating are electroplated on the low-strength metal surface in sequence; a third hardness nickel-based coating and a fourth hardness nickel-based coating are chemically deposited on the surface of the second hardness nickel-based coating in sequence.

[0030] The present invention preferably electroplates and deposits a first hardness nickel-based coating on a low-strength metal surface. In the present invention, the composition of the electroplating solution (hereinafter referred to as electroplating solution A) used for electroplating and depositing the first hardness nickel-based coating preferably includes: 180-220 g / L of nickel chloride, 130-170 mL / L of hydrochloric acid, and water as a solvent; the pH value of the electroplating solution A is preferably 4.0-6.0. In the present invention, the concentration of nickel chloride in the electroplating solution A is preferably 200 g / L, the concentration of hydrochloric acid is preferably 150 mL / L, and the hydrochloric acid is preferably 20% hydrochloric acid by mass; the water is preferably deionized water. The present invention has no special requirements for the preparation method of the electroplating solution A, and the components can be mixed evenly; the pH value of the electroplating solution A is adjusted by adding acid-base reagents, such as sulfuric acid or ammonia water. In the present invention, the deposition temperature of the electroplating deposition of the first hardness nickel-based coating is preferably 50-60°C, more preferably 55°C; the deposition time is preferably less than 10 minutes, more preferably 2-5 minutes; the deposition current density is preferably 2-8A / dm 2 , more preferably 4 to 5 A / dm 2 In the present invention, the specific operation of electroplating and depositing the first hardness nickel-based coating on the surface of the low-strength metal is: the low-strength metal is used as a workpiece to be plated and connected to the negative electrode of the power supply as a cathode, and the pure nickel plate is connected to the positive electrode of the power supply as an anode. The assembled cathode and anode are placed in an electroplating solution A with a temperature of 50 to 60°C, and the power supply current is adjusted to 2 to 8A / dm 2 The low-strength metal surface is electroplated at a current density of . During the electroplating process of the low-strength metal surface, the nickel ions in the electroplating solution A are reduced to metallic nickel, thereby forming a nickel coating with uniform thickness, low hardness and good plasticity on the low-strength metal surface, that is, the first hardness nickel-based coating.

[0031] After depositing the first hardness nickel-based coating, the present invention preferably electroplates and deposits a second hardness nickel-based coating on the surface of the first hardness nickel-based coating. In the present invention, the composition of the electroplating solution (hereinafter referred to as electroplating solution B) used for electroplating and depositing the second hardness nickel-based coating preferably includes: 40-50 g / L nickel chloride, 260-300 g / L nickel sulfate, 30-40 g / L boric acid, 1.6-2.0 g / L hard nanoparticles, 0.05-1 g / L surfactant, and the solvent is water; the pH value of the electroplating solution B is preferably 4.0-6.0. In the present invention, the concentration of nickel chloride in the electroplating solution B is preferably 45 g / L, the concentration of nickel sulfate is preferably 280 g / L, the concentration of boric acid is preferably 35 g / L, the concentration of hard nanoparticles is preferably 1.8 g / L, and the concentration of surfactant is preferably 0.5 g / L. In the present invention, the nickel chloride and nickel sulfate are the main salts, providing nickel ions required for the coating; the role of the boric acid is to maintain the pH environment of the electroplating solution stable; the hard nanoparticles preferably include SiC, TiO 2 、ZnO、Al 2 O 3 、Si 3 N 4 and ZrO 2One or more of the hard nanoparticles, the particle size of the hard nanoparticles is preferably 10 to 100 nm, and the role of the hard nanoparticles is to disperse in the cemented carbide material to play a dispersion toughening effect; the surfactant preferably includes one or more of cetyltrimethylammonium bromide (CTAB), sodium dodecylbenzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS), and the surfactant can effectively disperse the hard nanoparticles and alleviate the problem of particle agglomeration. In the present invention, the preparation method of the electroplating solution B preferably includes the following steps: mixing nickel chloride, nickel sulfate, boric acid and water, and separating a small amount from the obtained first mixed system; adding a surfactant and hard nanoparticles to the small amount, and performing ultrasonic vibration dispersion and magnetic stirring dispersion on the obtained second mixed system in turn to obtain a hard nanoparticle pre-dispersion system; adding the hard nanoparticle pre-dispersion system to the remaining part of the first mixed system, adjusting the pH value to 4.0 to 6.0 after mixing, to obtain the electroplating solution B. In the present invention, the time of the ultrasonic vibration dispersion is preferably 60 to 90 minutes, the rotation speed of the magnetic stirring dispersion is preferably 400 to 600 rpm, and the time is preferably 60 to 90 minutes; the present invention uses the ultrasonic vibration dispersion and magnetic stirring dispersion to achieve pre-dispersion of the hard nanoparticles under the action of a surfactant, thereby facilitating the uniform dispersion of the hard nanoparticles in the entire chemical plating solution B; the pH value is preferably adjusted by adding an acid-base reagent, such as sulfuric acid or ammonia water. In the present invention, the deposition temperature of the electroplated second hardness nickel-based coating is preferably 50 to 60°C, more preferably 55°C; the deposition time is preferably less than 10 minutes, more preferably 2 to 5 minutes; the deposition current density is preferably 2 to 8A / dm 2 , more preferably 4 to 5 A / dm 2 In the present invention, the specific operation of electroplating and depositing the second hardness nickel-based coating is the same as the specific operation of electroplating and depositing the first hardness nickel-based coating. The metal part deposited with the first hardness nickel-based coating is used as the workpiece to be plated, and the electroplating solution A is replaced with the electroplating solution B. During the electroplating process, the hard nanoparticles are introduced into the electroplating solution B, thereby forming a Ni-hard nanoparticle toughness coating with a dispersion strengthening effect on the surface of the first hardness nickel-based coating, that is, the second hardness nickel-based coating.

[0032] After electroplating and depositing the second hardness nickel-based coating, the present invention preferably chemically deposits a third hardness nickel-based coating on the surface of the second hardness nickel-based coating. In the present invention, the composition of the chemical plating solution (hereinafter referred to as chemical plating solution C) used for chemically depositing the third hardness nickel-based coating preferably includes: 25-35 g / L nickel sulfate, 20-30 g / L sodium hypophosphite, 13-17 g / L sodium acetate, 18-22 g / L trisodium citrate, 8-12 mL / L lactic acid, 10-14 mg / L thiourea, and the solvent is water; the pH value of the chemical plating solution C is preferably 4.0-6.0. In the present invention, the concentration of nickel sulfate in the chemical plating solution C is preferably 30 g / L, the concentration of sodium hypophosphite is preferably 25 g / L, the concentration of sodium acetate is preferably 14.8 g / L, the concentration of trisodium citrate is preferably 20 g / L, the concentration of lactic acid is preferably 10 mL / L, and the concentration of thiourea is preferably 12 mg / L. In the present invention, the nickel sulfate and sodium hypophosphite are the main salt and the reducing agent, respectively. The former provides nickel ions, and the latter can reduce nickel ions to metallic nickel. The sodium acetate acts as a buffer to stabilize the acid and base, keep the pH value of the system fluctuating within a certain range, and effectively avoid the reaction process. + The generation of lactic acid affects the pH value of the system; the lactic acid acts as a complexing agent to form a relatively stable chelate with the nickel ions in the chemical plating solution, thereby reducing the concentration of free nickel ions in the system. The addition of lactic acid can control the deposition rate to ensure that the coating is uniform and fine and the plating solution is stable, and forms a binary composite complexing agent with trisodium citrate, which has a strong complexing ability for nickel ions in the chemical plating solution; the thiourea acts as a stabilizer to prevent the chemical plating solution from spontaneous decomposition. The present invention has no special requirements for the preparation method of the chemical plating solution C, and it is sufficient to mix the components evenly; the pH value of the chemical plating solution C is adjusted by adding acid and alkali reagents, such as sulfuric acid or ammonia water. In the present invention, the deposition temperature of the chemically plated third hardness nickel-based coating is preferably 85-90°C, more preferably 89°C; the deposition time is preferably less than 90 minutes, more preferably 15-60 minutes. In the present invention, the metal deposited with the second hardness nickel-based coating is placed in a chemical plating solution C to perform chemical plating on the surface of the second hardness nickel-based coating; during the chemical plating process, the nickel ions in the chemical plating solution C are reduced to nickel by sodium hypophosphite, and the phosphorus element and part of the metallic nickel form a covalent state, thereby forming a corrosion-resistant and high-hardness nickel-phosphorus alloy coating, namely, the third hardness nickel-based coating.

[0033] After chemically depositing the third hardness nickel-based coating, the present invention preferably chemically deposits the fourth hardness nickel-based coating on the surface of the third hardness nickel-based coating. In the present invention, the composition of the chemical plating solution (hereinafter referred to as chemical plating solution D) used for chemically depositing the fourth hardness nickel-based coating preferably includes: 34-38 g / L nickel sulfate, 26-30 g / L sodium hypophosphite, 32-36 g / L sodium acetate, 10-14 mL / L lactic acid, 3-5 mg / L thiourea, and the solvent is water; the pH value of the chemical plating solution D is preferably 4.0-6.0. In the present invention, the concentration of nickel sulfate in the chemical plating solution D is preferably 36 g / L, the concentration of sodium hypophosphite is preferably 28 g / L, the concentration of sodium acetate is preferably 34 g / L, the concentration of lactic acid is preferably 12.4 mL / L, and the concentration of thiourea is preferably 4 mg / L. In the present invention, the functions of the nickel sulfate, sodium hypophosphite, sodium acetate, lactic acid and thiourea are the same as those in the above technical solution, and are not repeated here. The present invention has no special requirements for the preparation method of the chemical plating solution D, and the components can be mixed evenly; the pH value of the chemical plating solution D is adjusted by adding an acid-base reagent, such as sulfuric acid or ammonia water. In the present invention, the deposition temperature of the chemical plating deposition of the fourth hardness nickel-based coating is preferably 85-90°C, more preferably 89°C; the deposition time is preferably less than 90 minutes, more preferably 15-60 minutes. In the present invention, the metal deposited with the third hardness nickel-based coating is placed in the chemical plating solution D to chemically plate the surface of the third hardness nickel-based coating. Compared with the chemical plating solution C, the complexing ability of the chemical plating solution D is weak, and there are more free nickel ions in the chemical plating solution. Therefore, the phosphorus content in the coating is low, and the resulting coating is a low-phosphorus nickel-phosphorus alloy coating with higher hardness, that is, the fourth hardness nickel-based coating.

[0034] The nickel-based coating formed by electroplating is usually of low hardness, while the nickel-based coating formed by chemical plating is of high hardness. The present invention preferably first deposits the first hardness nickel-based coating and the second hardness nickel-based coating of low hardness on the surface of the low-strength metal by electroplating, and then continues to deposit the third hardness nickel-based coating and the fourth hardness nickel-based coating of high hardness by chemical plating; the composition of the treatment liquid will affect the internal composition, structure and performance of the coating. The present invention specially prepares electroplating liquid A, electroplating liquid B, chemical plating liquid C and chemical plating liquid D by adjusting the components and their proportions, so that after being treated by the electroplating liquid A, electroplating liquid B, chemical plating liquid C and chemical plating liquid D, a multilayer coating with increased hardness gradient can be stacked layer by layer on the surface of the low-strength metal. In the present invention, the hardness of each nickel-based coating can be adjusted within a certain range by adjusting the component content of each treatment liquid according to actual needs, and the thickness of each nickel-based coating can be adjusted by adjusting the current density and deposition time of electroplating or by adjusting the deposition temperature, pH value and deposition time of chemical plating according to actual needs. The specific hardness gradient law and thickness combination of each nickel-based coating can be combined as needed. In the embodiment of the present invention, the thickness of the first hardness nickel-based coating and the second hardness nickel-based coating are independently preferably 0.5-20 μm, and the thickness of the third hardness nickel-based coating and the fourth hardness nickel-based coating are independently preferably 0.5-30 μm.

[0035] The present invention constructs a "mechanical spring" by preparing a hardness gradient coating on the surface of a low-strength metal, thereby alleviating the soft-hard mechanical mismatch problem between the low-strength metal surface and the hard protective coating (manifested as failure problems such as coating cracks and peeling), and can effectively improve the coating's bearing capacity and enhance the coating's reliability; and the nickel-based coating has the advantages of good corrosion resistance and strong wear resistance; and the electroplating or chemical plating technology is mature, the prepared nickel-based coating has good bonding strength, uniform coating thickness, and long service life. Therefore, the preparation method provided by the present invention can alleviate the film-based bonding problem caused by the soft-hard mechanical mismatch between the low-strength metal and the hard protective coating, while also improving the corrosion resistance of the protective coating.

[0036] The present invention provides a low-strength metal surface hard protective coating obtained by the preparation method described in the above technical solution, starting from the low-strength metal surface, the hard protective coating is a nickel-based hardness gradient progressive multi-layer coating. In the present invention, the low-strength metal surface hard protective coating preferably includes a first hardness nickel-based coating, a second hardness nickel-based coating, a third hardness nickel-based coating and a fourth hardness coating compounded on the low-strength metal surface in sequence; the hardness of the first hardness nickel-based coating, the second hardness nickel-based coating, the third hardness nickel-based coating and the fourth hardness nickel-based coating are preferably 190-220HV, 400-460HV, 590-660HV and 790-811HV respectively; the phosphorus content of the third hardness nickel-based coating and the fourth hardness nickel-based coating is preferably 3-12wt.%, and has good corrosion resistance and wear resistance. The hard protective coating provided by the present invention has excellent film-base bonding while having good corrosion resistance, realizing long-term protection of parts made of low-strength materials, and can be effectively used in the field of friction and wear protection.

[0037] The low-strength metal surface hard protective coating provided by the present invention, its preparation method and application are described in detail below in conjunction with the embodiments, but they should not be understood as limiting the protection scope of the present invention.

[0038] Comparative Example 1

[0039] The preparation method of the hard protective coating on the low-strength metal surface is as follows:

[0040] (1) Substrate surface pretreatment: Provide a substrate (industrial pure iron, hardness of about 130 HV), grind and roughen the surface of the substrate (using 800-mesh sandpaper), clean the substrate with deionized water, and then place it in petroleum ether for ultrasonic cleaning for 5 min. Take it out and clean it and place it in a 0.1 mol / L dilute hydrochloric acid solution for ultrasonic cleaning for 5 min.

[0041] (2) Chemical plating solution configuration: Chemical plating solution D is composed of 36 g / L nickel sulfate, 28 g / L sodium hypophosphite, 34 g / L sodium acetate, 12.4 mL / L lactic acid, and 4 mg / L thiourea. The solvent is deionized water, and the pH value is 5.1.

[0042] (3) Chemical plating of the substrate: After the temperature of the chemical plating solution reaches a certain level, the substrate is placed in a chemical plating solution D (temperature 89° C.) for plating for 15 min to form a chemical plating layer (referred to as NiP-2 layer).

[0043] The cross-sectional hardness measurement results showed that the coating hardness was 805 HV, and the thickness measurement results showed that the thickness was 5 μm.

[0044] Example 1

[0045] The preparation method of the hard protective coating on the low-strength metal surface is as follows:

[0046] (1) Substrate surface pretreatment: Provide a substrate (industrial pure iron, hardness of about 130 HV), grind and roughen the surface of the substrate (using 800-mesh sandpaper), clean the substrate with deionized water, and then place the substrate in petroleum ether for ultrasonic cleaning for 5 minutes. Take out the substrate, clean it, and place it in a 0.1 mol / L dilute hydrochloric acid solution for ultrasonic cleaning for 5 minutes.

[0047] (2) Configuration of electroplating solution and chemical plating solution:

[0048] Electroplating solution A is composed of 200 g / L nickel chloride, 150 mL / L concentrated hydrochloric acid (using 20% ​​hydrochloric acid by mass), the solvent is deionized water, and the pH value is 4.5;

[0049] The electroplating solution B is composed of 45 g / L nickel chloride, 280 g / L nickel sulfate, 35 g / L boric acid, 1.8 g / L nano-SiC particles (particle size 50 nm), and 0.5 g / L hexadecyltrimethylammonium bromide (CTAB), and the solvent is deionized water, and the pH value is 4.5; the preparation method of the electroplating solution B is as follows: the surfactant CTAB and the nano-SiC particles are added to a small amount of the electroplating solution B, and the dispersion is performed by ultrasonic vibration dispersion and magnetic stirring at a speed of about 400 rpm for 60 minutes each, and then the obtained pre-dispersed nano-SiC particle system is added to the entire solution;

[0050] Chemical plating solution C is composed of 30 g / L nickel sulfate, 25 g / L sodium hypophosphite, 20 g / L trisodium citrate, 14.8 g / L sodium acetate, 10 mL / L lactic acid, and 12 mg / L thiourea, the solvent is deionized water, and the pH value is 4.8;

[0051] Chemical plating solution D is composed of 36 g / L nickel sulfate, 28 g / L sodium hypophosphite, 34 g / L sodium acetate, 12.4 mL / L lactic acid, and 4 mg / L thiourea. The solvent is deionized water, and the pH value is 5.1.

[0052] (3) Electroplating and chemical plating of the substrate: The substrate is connected to the negative electrode of the power supply as the workpiece to be plated, and the pure nickel plate is connected to the positive electrode of the power supply. The substrate is placed in electroplating solution A (temperature 55°C) and electroplating solution B (temperature 55°C) respectively for electroplating. The power supply current is adjusted to maintain the current density of 5A / dm for the two electroplatings. 2, wherein the electroplating time in electroplating solution A is 3 minutes, and the electroplating time in electroplating solution B is 4 minutes; after two electroplatings, two electroplated layers are formed on the surface of the substrate in sequence, namely the first hardness nickel-based coating (recorded as Ni layer) and the second hardness nickel-based coating (recorded as Ni-SiC layer). The electroplated samples are then placed in chemical plating solution C (temperature 89°C) and chemical plating solution D (temperature 89°C) in sequence, and plated for 15 minutes respectively, to form two chemical plating layers on the surface of the second hardness nickel-based coating in sequence, namely the third hardness nickel-based coating (recorded as NiP-1 layer) and the fourth hardness nickel-based coating (recorded as NiP-2 layer). After the above electroplating and chemical plating treatments, a hardness gradient nickel-based multilayer coating is formed on the surface of the base metal.

[0053] Figure 1 The high-power scanning electron microscope image of the cross section of the hardness gradient nickel-based multilayer coating prepared in Example 1 shows that the thickness of each layer of the hardness gradient multilayer coating prepared is 5 μm to 7 μm ( Figure 1 9.5 μm is the thickness of two layers of chemical plating), and the thickness of each layer is uniform.

[0054] Figure 2 and Figure 3 They are respectively the cross-sectional hardness indentation diagram and the cross-sectional hardness statistical diagram of the hardness gradient nickel-based multilayer coating prepared in Example 1, Figure 3 The horizontal axis number corresponds to Figure 2 The results show that from the substrate to the coating surface, the hardness is 133HV, 135V, 200HV, 443HV, 638HV, 805HV (where 133V and 135V are two test points on the substrate, i.e., the two test points numbered 5 and 6), and the coating achieves a gradient change in hardness.

[0055] Figure 4 and Figure 5 The XRD analysis result diagram of the hardness gradient nickel-based multilayer coating prepared in Example 1 and the EDS analysis result diagram of the chemical plating layer are shown in FIG. Figure 5 (a) is the EDS analysis result of the chemical plating layer formed in the chemical plating solution D, and (b) is the EDS analysis result of the chemical plating layer formed in the chemical plating solution C. Figure 1 The surface coating of the multi-layer coating shown is two layers of chemical plating, wherein the phosphorus content of the chemical plating layer formed in chemical plating solution D is 3.27wt%, and the phosphorus content of the chemical plating layer formed in chemical plating solution C is 7.53%.

[0056] Figure 6 This is a simulation diagram of the interface stress distribution of the hardness gradient nickel-based multilayer coating prepared in Example 1. Figure 6(a) to (h) respectively represent the stress distribution diagram of the lower surface of the NiP-2 layer, the stress distribution diagram of the lower surface of the NiP-1 layer, the stress distribution diagram of the lower surface of the Ni-SiC layer, the stress distribution diagram of the lower surface of the Ni layer, the stress distribution diagram of the upper surface of the NiP-1 layer, the stress distribution diagram of the upper surface of the Ni-SiC layer, the stress distribution diagram of the upper surface of the Ni layer, and the stress distribution diagram of the upper surface of the substrate. Figure 7 This is a simulation diagram of the interface stress distribution of the nickel-based coating prepared in Comparative Example 1. Figure 7 (a) to (b) are the stress distribution diagrams of the lower surface of the NiP-2 layer and the upper surface of the substrate in Comparative Example 1, respectively. Figure 6 and Figure 7 It can be seen that compared with Comparative Example 1, Example 1 can effectively reduce the stress difference between adjacent coatings and reduce the stress value of the substrate surface, thereby effectively alleviating the soft-hard mechanical mismatch between the low-strength metal and the hard protective coating.

[0057] Example 2

[0058] The preparation method of the hard protective coating on the low-strength metal surface is as follows:

[0059] (1) Substrate surface pretreatment: Provide a substrate (industrial pure iron, hardness of about 130 HV), grind and roughen the surface of the substrate (using 800-mesh sandpaper), and then use deionized water to clean the substrate. Then, place the substrate in petroleum ether for ultrasonic cleaning for 5 minutes, take out the substrate for cleaning, and place it in a 0.1 mol / L dilute hydrochloric acid solution for ultrasonic cleaning for 5 minutes.

[0060] (2) Configuration of electroplating solution and chemical plating solution:

[0061] Electroplating solution A is composed of 200 g / L nickel chloride, 150 mL / L concentrated hydrochloric acid (using 20% ​​hydrochloric acid by mass), the solvent is deionized water, and the pH value is 4.5;

[0062] Plating solution B consists of 45g / L nickel chloride, 280g / L nickel sulfate, 35g / L boric acid, 2.0g / L nano-Si 3 N 4 The preparation method of electroplating solution B is as follows: the surfactant SDBS is mixed with nano-Si 3 N 4 The particles were added to a small amount of electroplating solution B and dispersed by ultrasonic vibration and magnetic stirring at a speed of about 600 rpm for 90 minutes each. 3 N 4 The particles are added to the entire solution;

[0063] Chemical plating solution C is composed of 30 g / L nickel sulfate, 25 g / L sodium hypophosphite, 20 g / L trisodium citrate, 14.8 g / L sodium acetate, 10 mL / L lactic acid, and 12 mg / L thiourea, the solvent is deionized water, and the pH value is 4.8;

[0064] Chemical plating solution D is composed of 36 g / L nickel sulfate, 28 g / L sodium hypophosphite, 34 g / L sodium acetate, 12.4 mL / L lactic acid, and 4 mg / L thiourea. The solvent is deionized water, and the pH value is 5.1.

[0065] (3) Electroplating and chemical plating of the substrate: The substrate is connected to the negative electrode of the power supply as the workpiece to be plated, and the pure nickel plate is connected to the positive electrode of the power supply. The substrate is placed in electroplating solution A (temperature 55°C) and electroplating solution B (temperature 55°C) respectively for electroplating. The power supply current is adjusted to maintain the current density of 5A / dm for the two electroplatings. 2 , wherein the electroplating time in electroplating solution A is 3 minutes, and the electroplating time in electroplating solution B is 3 minutes; then the electroplated samples are placed in chemical plating solution C (temperature 89°C) and chemical plating solution D (temperature 89°C) in turn, and plated for 30 minutes respectively to form a chemical plating layer. After the above electroplating and chemical plating treatments, a hardness gradient nickel-based multilayer coating is formed on the surface of the base metal.

[0066] In the hardness gradient multilayer coating prepared in Example 2, the thickness of each electroplated layer is 5 μm to 7 μm, the thickness of each chemically plated layer is 9 μm to 11 μm, and the thickness of each layer is uniform. The cross-sectional hardness measurement results show that the hardness of the multilayer coating prepared in Example 2 is 220 HV (the first coating in contact with the substrate), 417 HV, 598 HV, and 795 HV, respectively, and the coating achieves a gradient hardness change.

[0067] Example 3

[0068] The preparation method of the hard protective coating on the low-strength metal surface is as follows:

[0069] (1) Substrate surface pretreatment: Provide a substrate (industrial pure iron, hardness of about 130 HV), grind and roughen the surface of the substrate (using 800-mesh sandpaper), and then use deionized water to clean the substrate. Then, place the substrate in petroleum ether for ultrasonic cleaning for 5 minutes, take out the substrate for cleaning, and place it in a 0.1 mol / L dilute hydrochloric acid solution for ultrasonic cleaning for 5 minutes.

[0070] (2) Electroplating and chemical plating solution configuration:

[0071] Electroplating solution A is composed of 200 g / L nickel chloride, 150 mL / L concentrated hydrochloric acid (using 20% ​​hydrochloric acid by mass), the solvent is deionized water, and the pH value is 4.5;

[0072] Plating solution B consists of 45g / L nickel chloride, 280g / L nickel sulfate, 35g / L boric acid, 2g / L nano ZrO 2 The preparation method of electroplating solution B is as follows: the surfactant SDS is mixed with nano ZrO 2 The particles were added to a small amount of electroplating solution B and dispersed by ultrasonic vibration and magnetic stirring at a speed of about 500 rpm for 90 minutes each. 2 The particle system is added to the entire solution;

[0073] Chemical plating solution C is composed of 30 g / L nickel sulfate, 25 g / L sodium hypophosphite, 20 g / L trisodium citrate, 14.8 g / L sodium acetate, 10 mL / L lactic acid, and 12 mg / L thiourea, the solvent is deionized water, and the pH value is 4.8;

[0074] Chemical plating solution D is composed of 36 g / L nickel sulfate, 28 g / L sodium hypophosphite, 34 g / L sodium acetate, 12.4 mL / L lactic acid, and 4 mg / L thiourea. The solvent is deionized water, and the pH value is 5.1.

[0075] (3) Electroplating and chemical plating of the substrate: The substrate is connected to the negative electrode of the power supply as the workpiece to be plated, and the pure nickel plate is connected to the positive electrode of the power supply. The substrate is placed in electroplating solution A (temperature 55°C) and electroplating solution B (temperature 55°C) respectively for electroplating. The power supply current is adjusted to maintain the current density of 4A / dm 2 , wherein the electroplating time in electroplating solution A is 2 minutes, and the electroplating time in electroplating solution B is 2 minutes; then the electroplated samples are placed in chemical plating solution C (temperature 89°C) and chemical plating solution D (temperature 89°C) in turn, and plated for 60 minutes respectively to form chemical plating layers. After the above electroplating and chemical plating treatments, a hardness gradient nickel-based multilayer coating is formed on the surface of the base metal.

[0076] In the hardness gradient multilayer coating prepared in Example 3, the thickness of each electroplated layer is 3 μm to 5.5 μm, the thickness of each chemically plated layer is 18 μm to 21 μm, and the thickness of each layer is uniform. The cross-sectional hardness measurement results show that the hardness of the prepared multilayer coating is 190 HV (the first coating in contact with the substrate), 403 HV, 591 HV, and 811 HV, respectively, and the coating achieves a gradient hardness change.

[0077] It can be seen from the above embodiments that the present invention can alleviate the soft-hard mechanical mismatch problem between the low-strength metal surface and the hard protective coating by preparing a hardness gradient coating on the low-strength metal surface to construct a "mechanical spring".

[0078] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a low-strength hard protective coating on a metal surface, characterized in that: The following steps are involved: Depositing a hard protective coating on the low-strength metal surface by electroplating and / or chemical plating, wherein the hard protective coating is a nickel-based multilayer coating, and the hardness of the nickel-based multilayer coating increases gradually layer by layer starting from the low-strength metal surface; Depositing the hard protective coating comprises: electroplating and depositing a first hardness nickel-based coating and a second hardness nickel-based coating on the low-strength metal surface in sequence; On the surface of the second hardness nickel-based coating, a third hardness nickel-based coating and a fourth hardness nickel-based coating are sequentially deposited by chemical plating; The composition of the electroplating solution used for electroplating and depositing the first hardness nickel-based coating is: 180-220 g / L nickel chloride, 130-170 mL / L hydrochloric acid, and the solvent is water; the pH value of the electroplating solution is 4.0-6.0; The composition of the electroplating solution used for electroplating and depositing the second hardness nickel-based coating is: 40-50 g / L nickel chloride, 260-300 g / L nickel sulfate, 30-40 g / L boric acid, 1.6-2.0 g / L hard nanoparticles, 0.05-1 g / L surfactant, and the solvent is water; the pH value of the electroplating solution is 4.0-6.0; The deposition temperature of the electroplating deposition of the first hardness nickel-based coating or the electroplating deposition of the second hardness nickel-based coating is independently 50-60° C., the deposition time is independently less than 10 minutes, and the deposition current density is independently 2-8 A / dm 2 ; The chemical plating solution used in the chemical plating deposition of the third hardness nickel-based coating is composed of: 25-35 g / L nickel sulfate, 20-30 g / L sodium hypophosphite, 13-17 g / L sodium acetate, 18-22 g / L trisodium citrate, 8-12 mL / L lactic acid, 10-14 mg / L thiourea, and the solvent is water; the pH value of the chemical plating solution is 4.0-6.0; The chemical plating solution used for chemically depositing the fourth hardness nickel-based coating is composed of: 34-38 g / L nickel sulfate, 26-30 g / L sodium hypophosphite, 32-36 g / L sodium acetate, 10-14 mL / L lactic acid, 3-5 mg / L thiourea, and the solvent is water; the pH value of the chemical plating solution is 4.0-6.

0.

2. The preparation method according to claim 1, characterized in that: The hard nanoparticles include one or more of SiC, TiO2, ZnO, Al2O3, Si3N4 and ZrO2.

3. The preparation method according to claim 1, characterized in that: The deposition temperature of the chemical plating deposition of the third hardness nickel-based coating or the chemical plating deposition of the fourth hardness nickel-based coating is independently 85-90° C., and the deposition time is independently less than 90 minutes.

4. The low-strength metal surface hard protective coating obtained by the preparation method according to any one of claims 1 to 3, wherein the hard protective coating is a nickel-based hardness gradient multi-layer coating starting from the low-strength metal surface.

5. Application of the low-strength metal surface hard protective coating as claimed in claim 4 in friction and wear protection.

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