High-gloss hard gold coating with nanoscale roughness and preparation method of high-gloss hard gold coating

Abstract

The invention provides a high-gloss hard gold coating with nanoscale roughness and a preparation method of the high-gloss hard gold coating, and relates to the technical field of gold coatings. The roughness Ra of a hard gold plating layer is smaller than or equal to 15 nm, the glossiness is not lower than 600 GU, the hardness is 160-175 HV, and after annealing is conducted at the temperature of 300 DEG C, the hardness is reduced by not exceeding 20 HV. According to the electroplating method adopting the cyanide-free gold salt electroplating solution, raw material components comprise a gold source, a main complexing agent, an auxiliary complexing agent, a brightening and leveling auxiliary agent, a stress relieving auxiliary agent, conducting salt and a pH regulator.

Description

Technical Field

[0001] The invention belongs to the technical field of gold plating, and relates to a hard gold plating layer with high gloss and nanometer-level roughness and a preparation method thereof. Background Art

[0002] Low-roughness gold coatings offer excellent infrared reflectivity, electrical conductivity, chemical stability, and effective resistance to signal attenuation, making them suitable for high-reliability conductive and coating materials. For example, ultra-smooth surfaces in high-frequency millimeter-wave antennas for 5G / 6G communication devices significantly reduce electromagnetic wave scattering. Gold reflectors for high-power lasers effectively reduce energy absorption, and high-gloss gold coatings on infrared windows of optical sensors enhance gas detection sensitivity. High-hardness gold coatings offer excellent wear resistance, reducing the thickness of the coating during device operation.

[0003] Cyanide-free gold plating solutions are environmentally friendly, but the hardness of the gold coating obtained is generally low while meeting the requirements of low roughness. Chinese patent CN118256968A discloses a cyanide-free gold plating solution, comprising a cyanide-free gold salt, a complexing agent, a buffer, a surfactant, a stabilizer, a grain refiner, a hardener, and an antioxidant. The roughness of the gold coating obtained is low, but the coating has poor gloss and a matte surface, and the original hardness is 90-120HV. After annealing (280°C, 30min), its hardness is only maintained at 60-80HV. Chinese patent CN114934302A discloses a cyanide-free gold plating solution, comprising a gold source, a conductive salt, a buffer, an auxiliary complexing agent, an additive, and a cerium salt. However, the additives of the cyanide-free gold plating solution contain at least one of non-environmentally friendly antimony compounds, arsenic compounds, and thallium compounds, and the hardness of the prepared gold coating is between 50-110HV.

[0004] Therefore, the prior art urgently needs to solve the problem of providing a gold plating layer with lower roughness and higher hardness. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention provides a hard gold plating layer with high gloss and nanometer-level roughness and a preparation method thereof.

[0006] The technical solutions of the present invention are as follows:

[0007] A high-gloss hard gold coating with nanometer-scale roughness, wherein the roughness Ra of the hard gold coating is less than or equal to 15 nm and the hardness is 160-175 HV;

[0008] After annealing at 300° C., the hardness of the hard gold plating layer decreases by no more than 20 HV.

[0009] Preferably, the thickness of the hard gold plating layer is 1-20 μm, and the glossiness of the hard gold plating layer is not less than 600 GU.

[0010] Preferably, the annealing time does not exceed 1 hour, the annealing is performed in a vacuum, an inert gas atmosphere or a reducing gas atmosphere, and the hardness reduction after the annealing does not exceed 20 HV.

[0011] A method for preparing a hard gold coating with high gloss and nanometer-level roughness, wherein the hard gold coating is plated using a cyanide-free gold salt electroplating solution;

[0012] The raw material components of the cyanide-free gold salt electroplating solution include: a gold source, a main complexing agent, an auxiliary complexing agent, a brightening and leveling agent, a stress relieving agent, a conductive salt and a pH regulator;

[0013] The pH of the cyanide-free gold salt electroplating solution is 5.5-7.0;

[0014] The hard gold plating layer has a roughness Ra≤15nm and a hardness of 160-175HV.

[0015] Preferably, the main complexing agent is selected from one or a combination of sodium sulfite and ammonium sulfite, and the concentration of the main complexing agent is 10-150 g / L.

[0016] Preferably, the brightening and smoothing agent is selected from one or a combination of two or more of imidazole salts and pyridinium salts; the concentration of the brightening and smoothing agent is 3-20 mg / L;

[0017] Preferably, the imidazole salt is selected from N-substituted imidazole salts;

[0018] Preferably, the pyridinium salt is selected from N-substituted pyridinium salts.

[0019] More preferably, the structure of the imidazole salt is shown in the following formula (1):

[0020]

[0021] Among them, R1 - is an organic anion and / or an inorganic anion, m=2-5.

[0022] More preferably, the structure of the pyridinium salt is shown in the following formula (2):

[0023]

[0024] Among them, R2 - is an organic anion and / or an inorganic anion, and n=2-5.

[0025] Preferably, the stress relief aid is selected from one or a combination of organic sulfonic acid and its salts, organic sulfuric acid and its salts, and organic phosphoric acid and its salts;

[0026] The concentration of the stress reliever is 10-60 mg / L.

[0027] Preferably, the auxiliary complexing agent is a short peptide containing a cysteinyl structure, and the concentration of the auxiliary complexing agent is 1-10 g / L;

[0028] The plating is pulse plating.

[0029] The beneficial effects of the present invention are:

[0030] (1) The hard gold coating of the present invention presents an ultra-smooth metal surface with a roughness Ra≤15nm, a glossiness not less than 600GU, a high reflectivity, and a hardness of 160-175HV. The hardness changes little after annealing.

[0031] (2) The present invention adopts cyanide-free gold plating solution, and by adjusting the auxiliary complexing agent and adding a brightening and leveling agent and a stress relieving agent, a gold plating layer with both low roughness and high hardness is achieved. Moreover, the hardness of the high-hardness gold plating layer changes little after annealing. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a picture of the text on the paper reflected after the gold plating layer obtained in Example 2 is erected.

[0033] Figure 2 This is the SEM image of the gold plating obtained in Example 2. DETAILED DESCRIPTION

[0034] The technical solution of the present invention is further illustrated and described below through specific implementation methods.

[0035] On the one hand, the present invention provides a high-gloss hard gold coating with nano-scale roughness, wherein the roughness Ra of the hard gold coating is less than or equal to 15 nm and the hardness is 160-175 HV;

[0036] After annealing at 300°C, the hardness of the hard gold coating decreases by no more than 20HV.

[0037] The hard gold coating of the present invention has a low roughness (Ra) of 15 nm or less, presenting an ultra-smooth metal surface. Its hardness can reach 160-175 HV, achieving both low roughness and high hardness. Furthermore, the hard gold coating of the present invention has good stability, with the hardness reduction after heat treatment not exceeding 20 HV. For example, the hardness reduction can be 20 HV, 18 HV, 16 HV, 15 HV, 13 HV, 12 HV, 11 HV, 10 HV, 9 HV, 8 HV, 7 HV, etc.

[0038] In some embodiments, the thickness of the hard gold plating layer is 1-20 μm, and the glossiness of the hard gold plating layer is not less than 600 GU. For example, the thickness of the hard gold plating layer can be any value among 1 μm, 3 μm, 5 μm, 6 μm, 7 μm, 8 μm, 10 μm, 12 μm, 13 μm, 15 μm, 16 μm, 18 μm, 20 μm, etc., or any value in between. Furthermore, the thickness of the hard gold plating layer can be 1-10 μm. For example, the glossiness of the hard gold plating layer can be any value among 600 GU, 620 GU, 630 GU, 650 GU, etc., or any value in between.

[0039] In some embodiments, the annealing time does not exceed 1 hour, and the annealing is performed in a vacuum, an inert gas atmosphere, or a reducing gas atmosphere. The hardness reduction after annealing does not exceed 15 HV. For example, the annealing is performed in a vacuum, an inert gas atmosphere, or a reducing gas atmosphere, and the annealing time can be any value among 10 minutes, 15 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes, 60 minutes, etc., or any value in between. This can prevent oxidation of the hard gold coating during annealing, thereby affecting the performance of the hard gold coating. For example, annealing at 300°C for 30 minutes in a nitrogen atmosphere can reduce the hardness of the hard gold coating to 10 HV, 11 HV, etc.

[0040] On the other hand, the present invention also provides a method for preparing a hard gold coating with high gloss and nanometer-level roughness, wherein the hard gold coating is plated using a cyanide-free gold salt electroplating solution;

[0041] The raw material components of the cyanide-free gold salt electroplating solution include: a gold source, a main complexing agent, an auxiliary complexing agent, a brightening and leveling agent, a stress relieving agent, a conductive salt, and a pH adjuster; the pH of the cyanide-free gold salt electroplating solution is 5.5-7.0;

[0042] The obtained hard gold plating has a roughness Ra≤15nm and a hardness of 160-175HV.

[0043] The gold source of the cyanide-free gold salt electroplating solution is not particularly limited. For example, it can be sodium gold sulfite and / or ammonium gold sulfite. The concentration of the gold source can be 5-25 g / L, or further, 5-15 g / L, based on the gold content.

[0044] In some embodiments, the main complexing agent is selected from one or a combination of sodium sulfite and ammonium sulfite, and the concentration of the main complexing agent is 10-150 g / L. For example, the concentration can be any value of 10 g / L, 20 g / L, 30 g / L, 40 g / L, 50 g / L, 60 g / L, 70 g / L, 80 g / L, 90 g / L, 100 g / L, 110 g / L, 120 g / L, 130 g / L, 140 g / L, 150 g / L, etc. Further, the concentration of the main complexing agent can be 15-110 g / L.

[0045] In some embodiments, the brightening and leveling agent is selected from one or a combination of two or more of imidazole salts and pyridinium salts; the concentration of the brightening and leveling agent is 3-20 mg / L. In the present invention, the use of pyridinium salts and / or imidazole salts as brightening and leveling agents adsorbs on the surface of the plated substrate where the current density is high, preventing gold ions from depositing in high current areas without affecting reduction in low current areas, thereby promoting the production of a low-roughness, high-gloss hard gold coating. For example, the concentration of the brightening and leveling agent can be 3 mg / L, 4 mg / L, 5 mg / L, 6 mg / L, 7 mg / L, 8 mg / L, 9 mg / L, 10 mg / L, 11 mg / L, 12 mg / L, 13 mg / L, 14 mg / L, 15 mg / L, 16 mg / L, 17 mg / L, 18 mg / L, 19 mg / L, 20 mg / L, etc.

[0046] In some embodiments, the imidazole salt is selected from N-substituted imidazole salts, and further, the imidazole salt has a structure as shown in the following formula (1):

[0047]

[0048] Among them, R1 - is an organic anion and / or an inorganic anion, and m=2 to 5. For example, the imidazolium salt can be 1-butyl-3-methylimidazolium thiocyanate, 1-butyl-3-methylimidazolium bromide, 1-butyl-3-methylimidazolium chloride, 1-propyl-3-methylimidazolium chloride, 1-propyl-3-methylimidazolium sulfate, 1-propyl-3-methylimidazolium acetate, etc.

[0049] In some embodiments, the pyridinium salt is selected from N-substituted pyridinium salts. Further, the structure of the pyridinium salt is shown in the following formula (2):

[0050]

[0051] Among them, R2 - is an organic anion and / or an inorganic anion, and n = 2 to 5. For example, the pyridinium salt may be butylpyridinium bromide, butylpyridinium chloride, butylpyridinium sulfate, butylpyridinium maleate, propylpyridinium bromide, butylpyridinium acetate, and the like.

[0052] In some embodiments, a stress relief agent can reduce the internal stress of the hard gold coating, improve the uniformity, wear resistance, and stability of the hard gold coating on various device shapes, and effectively maintain the hardness of the gold coating during annealing. The stress relief agent can be selected from one or a combination of organic sulfonic acids and their salts, organic sulfuric acids and their salts, and organic phosphoric acids and their salts, such as sodium saccharin-N-3-propanesulfonate, 2-naphthalenesulfonic acid, sodium dodecyl sulfate, dodecylbenzenesulfonic acid, sodium dodecylbenzenesulfonate, and sodium dodecylsulfonate.

[0053] The concentration of the stress reliever is 10-60 mg / L. For example, the concentration can be any value among 10 mg / L, 15 mg / L, 20 mg / L, 25 mg / L, 30 mg / L, 35 mg / L, 40 mg / L, 45 mg / L, 50 mg / L, 55 mg / L, 60 mg / L, etc. Further, the concentration of the stress reliever can be 25-40 mg / L.

[0054] In some embodiments, the auxiliary complexing agent is a short peptide containing a cysteinyl structure, and the concentration of the auxiliary complexing agent is 1-10g / L. For example, the short peptide containing a cysteinyl structure can be cysteinyl alanine, cysteinyl aspartic acid, cysteinyl lysine, and cysteinyl tyrosine, etc., which contain amino, carboxyl, sulfhydryl, amide, etc. groups, have a good complexing effect on gold, and are more effective as auxiliary complexing agents. For example, the concentration of the auxiliary complexing agent can be any value among 1g / L, 2g / L, 3g / L, 4g / L, 5g / L, 6g / L, 7g / L, 8g / L, 9g / L, 10g / L, etc., or any value therebetween.

[0055] The conductive salt may be an inorganic salt, such as sodium chloride, potassium chloride, sodium nitrate, etc. The concentration of the conductive salt may be 10-70 g / L, or further, 15-45 g / L.

[0056] The pH adjuster can be an acid or an alkali to adjust the pH of the cyanide-free gold salt electroplating solution to 5.5-7.0. The acid can be sulfuric acid, sodium dihydrogen phosphate, tartaric acid, malic acid, citric acid, etc. The alkali can be sodium hydroxide, sodium carbonate, sodium phosphate, sodium monohydrogen phosphate, etc.

[0057] The plating is pulse plating, such as a single pulse plating method, a double pulse method, etc. Further, the pulse plating is a single pulse plating.

[0058] For example, a plating method of the cyanide-free gold salt electroplating solution of the present invention can be as follows: the plating solution temperature is adjusted to 45-60°C, the anode used for electroplating is a Pt-coated titanium mesh, and the cathode is the workpiece to be plated. The distance between the anode and cathode is 1-2.5 cm, the duty cycle is 10-40%, the frequency is 500-1200 Hz, and the current density is 20-60 mA / cm 2 In the electroplating process, the plating solution adopts the method of cathode movement or magnetic stirring, and the electroplating is completed when the required thickness is reached.

[0059] The hard gold coating of the present invention has low roughness, high gloss, high hardness and good high temperature resistance. It can be plated on the surface of metal nickel, metal copper, etc. as a coating material, and is used in signal transmitters such as high-end communication equipment, high-power and sensor devices.

[0060] The technical solution of the present invention is further described and illustrated below based on various embodiments. Unless otherwise specified, the parts in the following embodiments are parts by weight.

[0061] Example 1

[0062] The raw material components of the cyanide-free gold salt electroplating solution include: gold source, main complexing agent, auxiliary complexing agent, brightening and leveling auxiliary agent, stress relieving auxiliary agent, conductive salt and pH regulator.

[0063] The gold source is ammonium gold sulfite, with a concentration of 5 g / L, calculated as gold element. The primary complexing agent is ammonium sulfite, with a concentration of 15 g / L. The auxiliary complexing agent is cysteinyl alanine, with a concentration of 10 g / L. The brightening and leveling agent is butylpyridinium bromide, with a concentration of 3 mg / L. The stress relief agent is sodium saccharin-N-3-propane sulfonate, with a concentration of 25 mg / L. The conductive salt is sodium chloride, with a concentration of 15 g / L. The pH adjuster is 10 wt% dilute sulfuric acid, and the pH of the plating solution is adjusted to 6.0.

[0064] The electroplating temperature was 45°C, the anode was a Pt-coated titanium mesh, and the cathode was a nickel sheet to be plated. The distance between the cathode and anode was 1.0 cm, the duty cycle was 10%, the frequency was 500 Hz, and the current density was 20 mA / cm 2 The electroplating process employed magnetic stirring, resulting in a gold coating with an average thickness of 10 μm. The gold coating was uniform and bright, with a roughness of Ra = 15 nm, a gloss of 628 GU, and a micro-Vickers hardness of 163 HV. After annealing at 300°C for 30 minutes in a nitrogen atmosphere, the micro-Vickers hardness of the coating was 151 HV, a decrease of 12 HV in hardness after annealing.

[0065] Example 2

[0066] The raw material components of the cyanide-free gold salt electroplating solution include: gold source, main complexing agent, auxiliary complexing agent, brightening and leveling auxiliary agent, stress relieving auxiliary agent, conductive salt and pH regulator.

[0067] The gold source is sodium gold sulfite, with a concentration of 10 g / L, calculated as gold element. The primary complexing agent is sodium sulfite, with a concentration of 45 g / L. The auxiliary complexing agent is cysteinyl aspartic acid, with a concentration of 7 g / L. The brightening and leveling agent is 1-butyl-3-methylimidazole thiocyanate, with a concentration of 6 mg / L. The stress relief agent is 2-naphthalenesulfonic acid, with a concentration of 30 mg / L. The conductive salt is sodium chloride, with a concentration of 25 g / L. The pH adjuster is a 10 wt% NaOH solution, and the pH of the plating solution is adjusted to 6.5.

[0068] The electroplating temperature was 50°C, the anode was a Pt-coated titanium mesh, and the cathode was a nickel sheet to be plated. The distance between the cathode and anode was 1.5 cm, the duty cycle was 20%, the frequency was 800 Hz, and the current density was 40 mA / cm 2 The electroplating process used a moving cathode method, resulting in a gold coating with an average thickness of 8μm. The gold coating was uniform and bright, with a roughness of Ra = 13nm, a gloss of 647GU, and a micro-Vickers hardness of 173HV. After annealing at 300°C for 30 minutes in a nitrogen atmosphere, the micro-Vickers hardness of the coating was 162HV, a decrease of 12HV after annealing.

[0069] The gold plating obtained in this embodiment reflects the text on the paper after it is erected, as shown in the attached figure. Figure 1 As shown, the upper golden color is the gold plating of the present invention, and the lower silvery white is the paper. It can be seen that the surface flatness of the gold plating of the present invention is very high, achieving a smooth mirror effect. The SEM picture of the gold plating obtained in this embodiment is shown in the attached figure. Figure 2 shown.

[0070] Comparative Example 1

[0071] The difference between this comparative example and Example 2 is that in Example 2, the single pulse electroplating is adjusted to constant current electroplating (i.e., not pulse electroplating), the distance between the cathode and anode is 1.5 cm, and the current density is 8 mA / cm 2 The remaining steps remain unchanged, and the roughness of the gold coating obtained is Ra = 75nm, the glossiness is 356GU, the micro-Vickers hardness of the coating is 135HV, and the micro-Vickers hardness of the coating after annealing at 300℃ for 30min is 117HV, with a hardness decrease of 18HV.

[0072] Comparative Example 2

[0073] This comparative example differs from Example 2 in that, in Example 2, the auxiliary complexing agent was changed from cysteinyl aspartic acid to sodium citrate, with the concentration remaining unchanged. The remaining steps remained unchanged, resulting in a gold coating with a roughness Ra of 28 nm, a glossiness of 415 GU, and a micro-Vickers hardness of 137 HV. After annealing at 300°C for 30 minutes, the micro-Vickers hardness of the coating was 117 HV, a decrease of 20 HV.

[0074] Comparative Example 3

[0075] This comparative example differs from Example 2 in that the brightening and leveling agent in Example 2 was replaced with a combination of vanillin and sodium propargyl sulfonate in a weight ratio of 2:3, replacing 1-butyl-3-methylimidazolium thiocyanate. The concentrations remained unchanged. The remaining steps remained unchanged. The resulting gold coating had a roughness Ra of 15 nm, a gloss of 498 GU, and a micro-Vickers hardness of 128 HV. After annealing at 300°C for 30 minutes, the micro-Vickers hardness of the coating was 107 HV, a decrease of 21 HV.

[0076] Comparative Example 4

[0077] The difference between this comparative example and Example 2 is that in Example 2, the stress relief agent 2-naphthalenesulfonic acid is not added, and the other steps remain unchanged. The roughness Ra of the obtained gold plating layer is 14 nm, the glossiness is 640 GU, the micro-Vickers hardness of the coating is 165 HV, and the micro-Vickers hardness of the coating after annealing at 300°C for 30 minutes is 126 HV, and the hardness decreases by 39 HV.

[0078] Example 3

[0079] The raw material components of the cyanide-free gold salt electroplating solution include: gold source, main complexing agent, auxiliary complexing agent, brightening and leveling auxiliary agent, stress relieving auxiliary agent, conductive salt and pH regulator.

[0080] The gold source is sodium gold sulfite, with a concentration of 10 g / L, calculated as gold element. The primary complexing agent is sodium sulfite, with a concentration of 80 g / L. The auxiliary complexing agent is cysteinyl lysine, with a concentration of 5 g / L. Brightening and leveling agents are butylpyridinium bromide and 1-hexyl-3-methylimidazolium trifluoromethanesulfonate, both at a concentration of 3 mg / L. The stress relief agent is sodium lauryl sulfate, with a concentration of 35 mg / L. The conductive salt is potassium chloride, with a concentration of 35 g / L. The pH adjuster is 10 wt% dilute sulfuric acid, and the pH of the plating solution is adjusted to 7.0.

[0081] The electroplating temperature was 60°C, the anode was a Pt-coated titanium mesh, and the cathode was a nickel sheet to be plated. The distance between the cathode and the cathode was 2 cm, the duty cycle was 30%, the frequency was 1000 Hz, and the current density was 50 mA / cm 2The electroplating process used a moving cathode method, resulting in a gold coating with an average thickness of 8μm. The gold coating was uniform and bright, with a roughness of Ra = 12nm, a gloss of 650GU, and a micro-Vickers hardness of 168HV. After annealing at 300°C for 30 minutes in a nitrogen atmosphere, the micro-Vickers hardness of the coating was 155HV, a decrease of 13HV in hardness after annealing.

[0082] Example 4

[0083] The raw material components of the cyanide-free gold salt electroplating solution include: gold source, main complexing agent, auxiliary complexing agent, brightening and leveling auxiliary agent, stress relieving auxiliary agent, conductive salt and pH regulator.

[0084] The gold source is ammonium gold sulfite, with a concentration of 15 g / L, calculated as gold element. The primary chelating agent is ammonium sulfite, with a concentration of 110 g / L. The auxiliary chelating agent is cysteinyl tyrosine, with a concentration of 1 g / L. The brightening and leveling agent is 1-hexyl-3-methylimidazolium trifluoromethanesulfonate, with a concentration of 20 mg / L. The stress relief agent is sodium lauryl sulfate, with a concentration of 40 mg / L. The conductive salt is sodium chloride, with a concentration of 45 g / L. The pH adjuster is 30 wt% dilute sulfuric acid, and the pH of the plating solution is adjusted to 6.0.

[0085] The electroplating temperature was 60°C, the anode was a Pt-coated titanium mesh, and the cathode was a nickel sheet to be plated. The distance between the cathode and anode was 2 cm, the duty cycle was 40%, the frequency was 1200 Hz, and the current density was 60 mA / cm 2 The electroplating process used a moving cathode method, resulting in a gold coating with an average thickness of 5μm. The gold coating was uniform and bright, with a roughness of Ra = 15nm, a glossiness of 653GU, and a micro-Vickers hardness of 167HV. After annealing at 300°C for 30 minutes in a nitrogen atmosphere, the micro-Vickers hardness of the coating was 157HV, a decrease of 10HV in hardness after annealing.

[0086] As described above, the basic principles, main features, and advantages of the present invention are shown and described. Those skilled in the art should understand that the present invention is not limited to the above-described embodiments. The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. In other words, equivalent changes and modifications made within the scope of the present invention and the contents of the specification should still fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A high gloss hard gold coating with nano-scale roughness, characterized in that: The roughness of the hard gold plating layer is Ra≤15nm and the hardness is 160-175HV; After annealing at 300° C., the hardness of the hard gold plating layer decreases by no more than 20 HV.

2. The hard gold plating layer according to claim 1, characterized in that The thickness of the hard gold plating layer is 1-20 μm, and the glossiness of the hard gold plating layer is not less than 600 GU.

3. The hard gold plating layer according to claim 1, characterized in that The annealing time does not exceed 1 hour, the annealing is performed in vacuum, inert gas atmosphere or reducing gas atmosphere, and the hardness reduction after the annealing does not exceed 20HV.

4. A method for preparing a high-gloss hard gold coating with nano-scale roughness, characterized in that: The hard gold plating layer is plated using a cyanide-free gold salt electroplating solution; The pH of the cyanide-free gold salt electroplating solution is 5.5-7.0; The raw material components of the cyanide-free gold salt electroplating solution include: a gold source, a main complexing agent, an auxiliary complexing agent, a brightening and leveling agent, a stress relieving agent, a conductive salt and a pH regulator; The hard gold plating layer has a roughness Ra≤15nm and a hardness of 160-175HV.

5. The method for preparing a hard gold plating layer according to claim 4, wherein: The main complexing agent is selected from one or a combination of sodium sulfite and ammonium sulfite, and the concentration of the main complexing agent is 10-150 g / L.

6. The method for preparing a hard gold plating layer according to claim 4, wherein: The brightening and smoothing auxiliary agent is selected from one or a combination of two or more of imidazole salts and pyridinium salts; the concentration of the brightening and smoothing auxiliary agent is 3-20 mg / L; Preferably, the imidazolium salt is selected from N-substituted imidazolium salts; preferably, the pyridinium salt is selected from N-substituted pyridinium salts.

7. The method for preparing a hard gold plating layer according to claim 6, wherein: The structure of the imidazole salt is shown in the following formula (1): Among them, R1 - is an organic anion and / or an inorganic anion, m=2-5.

8. The method for preparing a hard gold plating layer according to claim 6, wherein: The structure of the pyridinium salt is shown in the following formula (2): Among them, R2 - is an organic anion and / or an inorganic anion, and n=2-5.

9. The method for preparing a hard gold plating layer according to claim 4, wherein: The stress relief aid is selected from one or a combination of organic sulfonic acid and its salts, organic sulfuric acid and its salts, and organic phosphoric acid and its salts; The concentration of the stress reliever is 10-60 mg / L.

10. The method for preparing a hard gold plating layer according to claim 4, characterized in that: The auxiliary complexing agent is a short peptide containing a cysteinyl structure, and the concentration of the auxiliary complexing agent is 1-10 g / L; The plating is pulse plating.

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