Silver-plated material, terminal for electric contact, and method for producing silver-plated material

Abstract

There is provided a silver-plated product having a silver plating layer on a base material, wherein a surface of the base material is copper or a copper alloy; the silver plating layer contains selenium; and Vickers hardness in an initial state and Vickers hardness after heating at 100°C for 168 hours are both 120 HV or more, raising gloss level of the silver plating layer to 1.0 or more.

Description

Technical Field

[0001] The present invention relates to a silver-plated product, a terminal for electrical contact, and a method for manufacturing the silver-plated product.Background Art

[0002] Conventionally, as materials for contacts and terminal parts of connectors and switches, plated materials are used in which a base material such as copper, copper alloy, or stainless steel, which is relatively inexpensive and has excellent corrosion resistance and mechanical properties, is plated with tin, silver, gold, etc., depending on required properties such as electrical properties and solderability.

[0003] Tin-plated materials obtained by plating the base material such as copper, a copper alloy, or stainless steel with tin are inexpensive, but have poor corrosion resistance in a high-temperature environment. Further, gold-plated materials obtained by plating these base materials with gold have excellent corrosion resistance and high reliability, but are expensive. On the other hand, silver-plated products obtained by plating these base materials with Ag (silver) are less expensive than gold-plated materials and have better corrosion resistance than tin-plated materials.

[0004] However, the silver-plated products are soft and prone to wear, so when used as materials for connection terminals, etc., they are prone to adhesion and wear due to insertion, removal, and sliding. Further, when the connection terminal is inserted, the surface is scraped off, increasing a coefficient of friction and resulting in a high insertion force.

[0005] To solve these problems, a technique has been proposed in which the hardness of the silver-plated product is improved by incorporating elements such as selenium (Se) into a silver plating (see, for example, Patent Literatures 1 and 2).Citation ListPatent Literature

[0006] [Patent Document 1] Republished WO2018 / 181190 [Patent Document 2] JP 2015-110833 A Summary of the InventionProblem to be Solved by the Invention

[0007] The silver-plated products of Patent Literatures 1 and 2 described above contain selenium and therefore have excellent hardness. Due to use of a silver plating solution containing 70 mg / L or less of selenium, the plated material of Patent Literature 2 has a predetermined hardness before a heat resistance test, enabling to maintain a predetermined hardness even after a heat resistance test in which the material is heated at 50°C for a predetermined time. The heat resistance of the plated material refers to the heat resistance against hardness of the plated material.

[0008] In recent years, there has been a growing demand for the plated-materials for automobiles and other vehicles that are used outdoors for a long period of time, for example, for high-voltage terminals of electric vehicles such as EVs and HEVs. It is required that a predetermined hardness can be maintained even when heated to a temperature higher than 50°C, for example, 100°C, for a predetermined period of time. In this regard, the silver-plated product of Patent Literature 2 described above sometimes fails to maintain high hardness when heated at 100°C.

[0009] Further, the silver-plated product is generally required to have a high gloss level in terms of appearance.

[0010] An object of the present invention is to provide a silver-plated product with a silver plating layer having a high hardness in an initial state, heat resistance at 100°C, and a high gloss level, and to provide a manufacturing technique for the same.Means for solving the Problem

[0011] The present inventors have produced a silver-plated product by significantly increasing a selenium concentration in a silver plating bath in order to increase a selenium content in the silver-plated product. Then, although it was found that the heat resistance of the silver-plated product at a high temperature is likely to improve, it was also found that the surface of the plating layer is roughened, whitened, resulting in low gloss level. Therefore, the present inventors investigated a method for suppressing surface roughening and whitening caused by selenium while increasing the selenium concentration in the silver plating bath. As a result, they found that it was effective to add a sulfonic acid group-containing polymer to the silver plating bath used to form the silver plating layer. As shown in the examples below, as a result of forming the silver plating layer in the presence of these additives, surface roughening and whitening of the silver plating layer can be suppressed, while increasing the selenium concentration in the silver plating bath to a specific range, resulting in achieving a high gloss level.

[0012] The aspects of the present invention created based on the above findings are as follows.

[0013] A first aspect of the present invention is a silver-plated product having a silver plating layer on a base material, wherein a surface of the base material is copper or a copper alloy; the silver plating layer contains selenium; and Vickers hardness in an initial state and Vickers hardness after heating at 100°C for 168 hours are both 120 HV or more, raising gloss level of the silver plating layer to 1.0 or more.

[0014] A second aspect of the present invention is the silver-plated product according to the first aspect, wherein a ratio of the Vickers hardness after heating to the Vickers hardness in the initial state is 95% or more.

[0015] A third aspect of the present invention is the silver-plated product according to the first or second aspect, wherein a preferential orientation plane of the silver plating layer in the initial state is a {111} plane.

[0016] A fourth aspect of the present invention is the silver-plated product according to any one of the first to third aspects, wherein a nickel-containing underlayer is provided between the base material and the silver plating layer.

[0017] A fifth aspect of the present invention is a terminal for electrical contact, which is made of the silver-plated product according to any one of the first to fourth aspects.

[0018] A sixth aspect of the present invention is a method for manufacturing a silver-plated product, including: forming a silver plating layer on a base material whose surface is copper or copper alloy, wherein in the forming the silver plating layer, a silver plating bath is used, which contains silver cyanide or potassium silver cyanide, cyanide salt, a selenium compound, and a sulfonic acid group-containing polymer, and in which a ratio of a concentration of selenium derived from the selenium compound to a concentration of the cyanide salt is exceeding 0.1%.

[0019] A seventh aspect of the present invention is the method for manufacturing a silver-plated product according to the sixth aspect, wherein a concentration of selenium derived from the selenium compound is 85 mg / L or more and 200 mg / L or less.Advantageous Effects of Invention

[0020] According to the present invention, the silver plating layer can have hardness in the initial state and heat resistance at 100°C, and can also have high gloss level.Description of Embodiments

[0021] Embodiments of the present invention will be described below. In this specification, the initial state refers to a state within a few days after the silver-plated product is formed and before it is heated for heat resistance evaluation. In this specification, the term "to" indicates a value greater than or equal to a specified value and less than or equal to a specified value.(1) Silver plating bath

[0022] First, a silver plating bath for forming the silver plating layer of the silver-plated product will be described.

[0023] The silver plating bath of this embodiment contains silver cyanide or potassium silver cyanide, cyanide salt, a selenium compound, and a sulfonic acid group-containing polymer.

[0024] For supply of silver in the silver plating bath, silver cyanide or potassium silver cyanide can be used. Commercially available silver cyanide or potassium silver cyanide can be used, and among them, potassium silver cyanide is preferably used. The concentration of silver cyanide or potassium silver cyanide in the silver plating bath is not particularly limited as long as it is capable of forming a silver plating layer. However, if the silver concentration is too low, a film deposition rate will be slow, and if it is too high, a large amount of silver will be consumed due to carry-out of the plating solution, which is uneconomical. From the viewpoint of a silver plating layer formation rate and reducing a cost, it is advisable to adjust the concentration of silver cyanide or potassium silver cyanide so that the silver concentration in the silver plating bath is, for example, 40 to 110 g / L. For example, in the case of potassium silver cyanide, the concentration may be set to 74 to 203 g / L.

[0025] As the cyanide salt, known components can be used, such as potassium cyanide and sodium cyanide. Among these, potassium cyanide is preferably used. The concentration of the cyanide salt is preferably set in accordance with the selenium concentration so that the concentration of selenium derived from the selenium compound described below is in a range exceeding 0.1% relative to the concentration of the cyanide salt. For example, from the viewpoint of favorably forming the silver plating layer, the concentration of the cyanide salt is preferably 70 to 160 g / L, more preferably 75 to 125 g / L. When the concentration of the cyanide salt is less than 70 g / L, an anode current efficiency may decrease, making it difficult to dissolve silver at the anode. On the other hand, when the concentration of the cyanide salt exceeds 160 g / L, the selenium concentration will be adjusted to exceed 0.1% relative to the concentration of the cyanide salt, resulting in an excessively high selenium concentration, which involves a risk that the surface of the silver plating layer is roughened, resulting in whitening. The concentration of the cyanide salt corresponds to the amount of the cyanide salt added to the silver plating bath.

[0026] As the selenium compound, a known component used as an additive for adding selenium to a cyanide-based silver plating bath can be used, and for example, potassium selenocyanate, potassium selenate, or selenium dioxide can be used. The selenium compound is a component that acts to improve hardness and heat resistance of the silver plating layer.

[0027] From the viewpoint of realizing the hardness in the initial state and heat resistance at 100°C in the silver plating layer described below, the concentration of the selenium compound is in a range such that the concentration of selenium derived from the selenium compound exceeds 0.1% relative to the concentration of the cyanide salt. It is preferable that the ratio of the selenium concentration derived from selenium compound in the unit of g / L to the cyanide salt concentration in the unit of g / L is 0.11% or more and 0.3% or less, and it is more preferable that the ratio of the concentration of selenium derived from the selenium compound to the concentration of the cyanide salt is 0.2% or less. Here, the concentration of selenium derived from the selenium compound (hereinafter also simply referred to as the selenium concentration) is a value calculated by dividing the concentration of the selenium compound by its molecular weight and multiplying the result by the atomic weight of selenium. If the ratio is 0.1% or less, there is a risk that the silver plating layer has low heat resistance at 100°C, and if the ratio exceeds 0.3%, there is a risk that the silver plating layer has low gloss level and it cannot be prevented. By adjusting the selenium concentration to a predetermined ratio with respect to the cyanide salt concentration, and adding a sulfonic acid group-containing polymer, which will be described later, the silver plating layer has high gloss level and improved hardness in the initial state and heat resistance at 100°C.

[0028] The selenium concentration in the silver plating bath is preferably 85 mg / L or more and 300 mg / L or less, more preferably 85 mg / L or more and 200 mg / L or less, and even more preferably 100 mg / L or more and 200 mg / L or less. For example, when potassium selenocyanate is used as the selenium compound, the concentration of potassium selenocyanate may be set to 155 mg / L or more and 548 mg / L or less in order to achieve the above-described selenium concentration in the silver plating bath. When the selenium concentration is less than 85 mg / L, there is a risk of low heat resistance of the silver plating layer at 100°C. On the other hand, the selenium concentration exceeding 300 mg / L lowers the gloss level of the silver plating layer even when a sulfonic acid group-containing polymer is used, involving a risk of whitening.

[0029] The sulfonic acid group-containing polymer is a compound that acts to suppress surface roughening caused by selenium when forming a silver plating layer and to achieve high gloss level. The sulfonic acid group-containing polymer is preferably an aromatic sulfonic acid polymer. As the aromatic sulfonic acid polymer, a naphthalene sulfonate formaldehyde condensate or polystyrene sodium sulfonate can be suitably used, and for example, sodium salt of β-naphthalenesulfonic acid formalin condensate (for example, "Demol N (registered trademark)" manufactured by Kao Corporation) can be used.

[0030] The concentration of the sulfonic acid group-containing polymer is not particularly limited, but from the viewpoint of suppressing surface roughness of the silver plating layer and achieving high gloss level, the concentration of the sulfonic acid group-containing polymer is preferably 0.1 g / L or more and 10 g / L or less.

[0031] The silver plating bath may contain other additives in addition to the above components. Other additives that can be used include a conventionally known component such as a hardener such as a carbon sulfide compound (carbon disulfide, etc.) and an inorganic sulfur compound (sodium thiosulfate, etc.), and conductive salt such as potassium carbonate.

[0032] When antimony is contained as a component in the other additive, the antimony concentration is preferably lower than the selenium concentration in the silver plating bath, and more preferably, no antimony compound is contained as other additive.

[0033] Further, the silver plating bath contains a solvent, and water is preferable as the solvent because it dissolves the other component contained in the silver plating bath and has a small environmental impact. As the solvent, water is preferably used, and a mixed solvent of water and alcohol may also be used.(2) Method for manufacturing a silver-plated product

[0034] Next, a method for manufacturing a silver-plated product according to this embodiment will be described. The method for manufacturing a silver-plated product includes a step of forming a silver plating layer on a base material whose surface is a copper or copper alloy. Before the step of forming the silver plating layer, there may be a step of forming another layer on the base material. Hereinafter, the silver-plated product will be described in which an underlayer, a strike plating layer, and a silver plating layer are formed in this order on a base material. However, the underlayer and the strike plating layer are not necessarily formed and may be formed as needed.(Preparation step)

[0035] First, a base material to be plated is prepared.

[0036] The base material is preferably the one that can be silver plated and has an electrical conductivity required for sliding electrical contact parts such as switches and connectors. The surface of the base material is a copper or copper alloy to be silver plated. The material other than that of the surface is selected depending on an intended use, and an entire base material may be copper or a copper alloy. As the material for forming the surface of the base material, pure copper can be used as copper, for example, and the purity of the pure copper can be 3N to 5N. From the viewpoint of electrical conductivity and strength, the copper alloy is preferably an alloy composed of Cu, and at least one element selected from the group consisting of Si (silicon), Fe (iron), Mg (magnesium), P (phosphorus), Ni (nickel), Sn (tin), Co (cobalt), Zn (zinc), Be (beryllium), Pb (lead), Te (tellurium), Ag (silver), Zr (zirconium), Cr (chromium), Al (aluminum), and Ti (titanium), and inevitable impurities. Brass, bronze, etc., may also be used. The amount of Cu in the copper alloy may be 60 mass % or more.

[0037] As will be described later, the base material is preferably used for terminals as an intended use (as a silver-plated product on which a silver plating layer is formed), but the base material itself may have a shape for such a use, or the base material may be flat (flat shape, etc.) and formed into the shape for such a use after becoming a silver-plated product.(Underlayer forming step)

[0038] The underlayer may be formed on the base material before the silver plating layer is formed, as needed. The underlayer can prevent the component of the base material, such as copper, from diffusing to a plating surface and oxidizing, which would cause the conductivity of the silver-plated product to deteriorate, and can also improve the adhesion of the silver plating layer.

[0039] The constituent metal of the underlayer may be at least one metal or alloy selected from the group consisting of Cu, Ni, Sn, and Ag. For example, when the base material is composed of copper and in order to prevent copper in the base material from diffusing to the surface of the silver plating layer and deteriorating its conductivity, it is preferable to form an underlayer containing Ni. Further, for example, when the base material is composed of a copper alloy containing zinc, such as brass, and in order to prevent zinc in the base material from diffusing to the silver plating layer surface, it is preferable to form an underlayer containing Cu. Further, for example, in order to improve the adhesion of the silver plating layer to the base material, it is preferable to form an underlayer containing Ag.

[0040] The underlayer may be a single layer composed of Cu, Ni, Sn, Ag or an alloy thereof, or a layer composed of a combination of these (a laminate structure), and the underlayer may be formed on the entire surface of the base material or on only a part of it, depending on the intended use of the silver-plated product to be manufactured.

[0041] The thickness of the underlayer is not particularly limited, but from the viewpoint of its function and cost, it is preferably 0.1 to 2 µm, and more preferably 0.2 to 1.5 µm.

[0042] The method for forming the underlayer is not particularly limited, and it can be formed by electroplating using a plating bath containing ions of the above-described constituent metals using a known method, or by sequentially laminating layers composed of each metal that constitutes a desired alloy layer and then applying reflow (heat treatment) thereto.(Strike plating step)

[0043] A strike plating layer may be formed on the base material or the underlayer, which can improve adhesion between the base material and the silver plating layer. The strike plating layer can be formed by a conventionally known method, for example, Ag strike plating. The thickness of the strike plating layer may be thinner than the other layers, for example, 5 nm or more.(Silver plating layer forming step)

[0044] Next, the silver plating layer is formed on the base material (for example, on the strike plating layer) using the silver plating bath described above.

[0045] Specifically, first, a cathode and an anode are immersed in the silver plating bath, and a current is passed through them to perform silver plating. Here, the base material, which is the material to be plated, serves as the cathode, and, for example, a silver electrode plate that dissolves to provide silver ions serves as the anode.

[0046] In this embodiment, since the silver plating bath contains a predetermined concentration of selenium, the silver plating layer can be formed to contain selenium. At this time, as will be described later, the silver plating layer is designed to have a predetermined hardness in the initial state and heat resistance at 100°C. In addition, by adding the sulfonic acid group-containing polymer to the silver plating bath and performing silver plating in its presence, surface roughness can be suppressed and high gloss level can be maintained, although increase of the selenium concentration relative to the cyanide salt concentration in the silver plating bath is likely to cause surface roughening and low gloss level.

[0047] Further, by using the above-described silver plating bath, it is preferable to form the silver plating layer having a predetermined hardness so that the preferential orientation plane on the surface is the {111} plane. With a configuration in this manner, the silver plating layer can have a high hardness, with suppressed increase in contact resistance. Here, the preferential orientation plane is evaluated in the above-described initial state.

[0048] The plating conditions for forming the silver plating layer include, for example, current density, plating temperature, stirring speed, plating time, etc., and each of these may be adjusted as follows.

[0049] The current density in silver plating is preferably 0.5 to 10 A / dm 2< , more preferably 1 to 10 A / dm 2< , and even more preferably 3 to 8 A / dm 2< , from the viewpoint of the rate of deposition of the silver plating layer and from the viewpoint of suppressing unevenness in the appearance of the silver plating layer.

[0050] The temperature of the silver plating bath (plating temperature) is preferably 15 to 50°C, more preferably 20 to 45°C, from the viewpoints of plating production efficiency and preventing excessive evaporation of the solution.

[0051] The speed of stirring the silver plating bath with a stirrer or a stirring blade is preferably 300 to 700 rpm, more preferably 350 to 650 rpm, from the viewpoint of achieving uniform plating.

[0052] The silver plating time (time for applying current) can be adjusted appropriately depending on a desired thickness of the silver plating layer, and is typically in a range of 25 to 1800 seconds.

[0053] The area to be plated may be an entire surface of the base material or a part of the surface of the base material, depending on an intended use of the silver-plated product to be manufactured.

[0054] In this way, the silver-plated product is obtained.(3) Silver-plated product

[0055] Next, explanation will be given for the silver-plated product obtained by the above-described method for manufacturing a silver-plated product.

[0056] The silver-plated product of this embodiment is a silver-plated product having a silver plating layer on a base material. The silver-plated product may be configured to include one or more of an underlayer or a strike plating layer between the surface of the base material and the silver plating layer, and it is also preferable that the silver-plated product is a member having an underlayer, a strike plating layer, and a silver plating layer in this order on the surface of the base material.

[0057] As described above, the base material has a surface to be plated formed from copper or a copper alloy. The material other than the above material of the surface of the base material is selected depending on the intended use. The entire base material may be composed of copper or a copper alloy.

[0058] The underlayer is formed as needed, and as described above, is formed on the surface of the base material by, for example, electroplating, and is configured to contain at least one metal or alloy selected from the group consisting of Cu, Ni, Sn, and Ag. It is preferable to use Ni for the underlayer.

[0059] As described above, the strike plating layer is formed by, for example, Ag strike plating before forming the silver plating layer. The strike plating layer may be formed thinly and integrated with the silver plating layer formed thereon. That is, the strike plating layer may be indistinguishable from the silver plating layer.

[0060] The silver plating layer of this embodiment is formed using the above-described silver plating bath. The silver plating layer may be formed by laminating the silver plating layer on the surface of the base material interposing the underlayer or the strike plating layer. The silver plating layer may be formed on the entire surface of the base material, or may be formed on only a part of its surface.

[0061] The silver plating layer is formed using the silver plating bath of the present invention. This allows the silver-plated product to have a predetermined hardness in the initial state and heat resistance at 100°C. In addition, the silver plating layer is formed by depositing silver in the presence of the sulfonic acid group-containing polymer using the silver plating bath containing the sulfonic acid group-containing polymer, and therefore although it contains a relatively large amount of selenium, it is configured to suppress roughening of the surface and achieve high gloss level.

[0062] Specifically, the silver-plated product has a Vickers hardness of 120 HV or more in the initial state described above. Further, the silver-plated product has a Vickers hardness of 120 HV or more after being heated at 100°C for 168 hours. That is, the silver-plated product has a Vickers hardness of 120 HV or more both before and after heating, maintains a predetermined hardness even after heating to 100°C, and has high heat resistance at 100°C. Here, the Vickers hardness is a value measured using a hardness tester as will be described later in the examples.

[0063] It is also preferable that the silver-plated product has high heat resistance at 100°C and a hardness fluctuation rate before and after heating is suppressed to a low level. Specifically, it is preferable that the ratio of the Vickers hardness after heating to the Vickers hardness in the initial state is high, for example, 95% or more. This allows the silver-plated product to maintain high hardness and desired wear resistance even under a severe environmental temperature without being affected thereby.

[0064] The silver plating layer has a gloss level of 1.0 or more. The gloss level here is a value measured using a densitometer as will be described later in the Examples. The gloss level is likely to be low as the surface of the silver plating layer is roughened and whitened. In this embodiment, the roughening of the surface and the resulting whitening are suppressed, raising the gloss level of the silver plating layer to 1.0 or higher. The gloss level is a value in the initial state of the silver plating layer.

[0065] The silver plating layer preferably has a predetermined hardness and is configured so that a preferential orientation plane in its initial state is the {111} plane. If the preferential orientation plane of the silver plating layer is, for example, the {200} plane, there is a risk that the hardness of the silver plating layer becomes unstable due to heating or the passage of time, and if the preferential orientation plane is the {220} plane, there is a risk of low hardness or low gloss level of the silver plating layer. In this regard, when the preferential orientation plane is the {111} plane, high hardness, heat resistance, and high gloss level can be achieved for the silver-plated product. In this embodiment, as will be shown in the examples described later, by adding the sulfonic acid group-containing polymer to the silver plating bath and performing silver plating in the presence of the polymer, the silver plating layer can be stably oriented in the {111} plane.

[0066] The thickness of the silver plating layer is not particularly limited, but from the viewpoints of wear resistance, reliability, and conductivity, it is preferable that the thickness be as small as possible. If the thickness is too large, the effect of the silver plating layer becomes saturated and the cost of raw materials increases. From the above viewpoints, the thickness of the silver plating layer is preferably 0.1 µm to 45 µm, more preferably 0.5 µm to 35 µm, and even more preferably 1.5 µm to 25 µm. The thickness of the silver plating layer can be measured using a fluorescent X-ray film thickness meter, and details of a measurement method will be given in the Examples section.(4) Terminal

[0067] The silver-plated product according to this embodiment is suitable as a constituent material for terminals for electrical contacts, particularly terminals in electrical contact parts that slide during use, such as switches and connectors.Examples

[0068] Examples of the silver-plated product and the method for manufacturing the same according to the present invention will be described in detail below.(1) Fabrication of the silver-plated product

[0069] First, a pure copper metal base material (purity 99.96% or higher) measuring 67 mm × 50 mm × 0.3 mm was prepared as a material to be plated (base material), and this material to be plated and a SUS plate were placed in an alkaline degreasing solution, and electrolytic degreasing was performed for 30 seconds at a voltage of 5 V, with the material to be plated as the cathode and the other SUS plate as the anode, followed by pickling in 3% sulfuric acid for 15 seconds. 15 second water rinse was performed between each operation.

[0070] Next, in a nickel plating solution consisting of pure water, 540 g / L of nickel sulfamate tetrahydrate, 25 g / L of nickel chloride, and 35 g / L of boric acid, electroplating was performed at a current density of 7 A / dm 2< and a solution temperature of 50°C until a nickel film thickness reached 1 µm while stirring with a magnetic stirrer at 500 rpm, with the material to be plated as the cathode and the SK nickel electrode plate as the anode, to form a nickel underlayer on the base material.

[0071] Next, in a strike silver plating solution consisting of pure water, 3 g / L of silver potassium cyanide and 90 g / L of potassium cyanide, electroplating was performed at a current density of 2 A / dm 2< for 10 seconds while stirring at 500 rpm with a stirrer, with the material to be plated as the cathode and a platinum-coated titanium electrode plate as the anode, to form a strike plating layer composed of silver having a thickness of 0.01 µm on the underlayer.

[0072] Next, the silver plating bath was prepared by adding potassium silver cyanide to pure water, followed by adding potassium cyanide as a cyanide salt, potassium selenocyanate as a selenium compound, and an aromatic sulfonic acid polymer, which is a type of sulfonic acid group-containing polymer "Demol N" (sodium salt of β-naphthalenesulfonic acid formalin condensate manufactured by Kao Corporation) or sodium polystyrene sulfonate (hereinafter simply referred to as PSS), and changing the concentrations of each as shown in Tables 1 and 2 below. Using this silver plating bath, electroplating was performed to the strike plating layer, with the material to be plated as the cathode and a silver electrode plate with a purity of 99.99% or higher as the anode, while stirring at 500 rpm with a stirrer, at a current density and silver plating bath temperature shown in Tables 1 and 2 below, until a silver plating layer with a thickness of 5 µm was formed, and after silver plating, the plated material was washed with water for 15 seconds and dried with an air gun to obtain the silver-plated product of this example and comparative example.

[0073] The thickness of the silver plating layer was measured at a center of a sample using a fluorescent X-ray film thickness meter (FT-110A manufactured by Hitachi High-Tech Science Corporation) with a collimator diameter of 0.2 mm and a measurement time of 10 seconds, and a measured value was taken as a film thickness. Measurement of the film thickness reveals that it was 5 µm in all cases.

[0074] Further, the Ag concentration in Tables 1 and 2 is the silver concentration obtained by converting the concentration of added potassium silver cyanide into silver using its molecular weight and the atomic weight of silver (i.e., the silver concentration derived from the concentration of potassium silver cyanide). The KCN concentration is the concentration of added potassium cyanide, and the Se concentration is the selenium concentration calculated by dividing the concentration of the added potassium selenocyanate by its molecular weight and multiplying the result by the atomic weight of selenium (i.e., the selenium concentration derived from the potassium selenocyanate concentration). The Se / KCN ratio represents the ratio of a selenium concentration (converted from mg / L to g / L) to a potassium cyanide concentration (in g / L). These values are shown in Tables 1 and 2, respectively. Example12345678Potassium silver cyanide [g / L]110110110175175175175175Potassium cyanide [g / L]1201201209595959595Potassium selenocyanate [mg / L]255334334278185185185185Demol N [g / L]1010101010.1--Silver plating bath compositionPSS [g / L]------0.11KCN concentration [g / L]1201201209595959595Se concentration [mg / L]140183183152101101101101Se / KCN0.12%0.15%0.15%0.16%0.11%0.11%0.11%0.11%Ag concentration [g / L]6060609595959595Silver plating conditionsBath temperature [°C]3030353025252525Current density [A / dm 2< ]55755555EvaluationVickers hardness in initial state (A) [HV]132131136132131131130129Vickers hardness after heating (B) [HV]126127129129126126129131(B) / (A)95%97%95%98%96%97%99%101%Gloss level1.741.691.721.721.761.761.391.40Preferential orientation plane{111}{111}{111}{111}{111}{111}{111}{111} [Table 2] Comparative example123456Potassium silver cyanide [g / L]175175175110110110Potassium cyanide [g / L]95959570120120Potassium selenocyanate [mg / L]100100185164255334Silver plating bath compositionDemol N [g / L]------PSS [g / L]------KCN concentration [g / L]95959570120120Se concentration [mg / L]555510190140183Se / KCN0.06%0.06%0.11%0.13%0.12%0.15%Ag concentration [g / L]959595606060Silver plating conditionsBath temperature [°C]181825303030Current density [A / dm 2< ]675955Vickers hardness in initial state (A) [HV]126129108113117136Vickers hardnessafter heating (B) [HV]114115125-113129Evaluation(B) / (A)90%89%116%-97%95%Gloss level1.491.500.120.230.090.05Preferential orientation plane{111}{111}{220}{220}{220}{220} (2) Evaluation method

[0075] The resulting silver-plated product was evaluated for the hardness in the initial state, hardness after heating, gloss level, and crystal orientation. Each evaluation method is described below.(Hardness in the initial state)

[0076] The hardness of the silver-plated product was measured by Vickers hardness using a micro Vickers hardness tester ("Micro Vickers Hardness Tester HM-221" manufactured by Mitutoyo Corporation). Here, an indentation was made on the fabricated silver-plated product with a load of 10 gf, a lowering time of 3 seconds, a holding time of 10 seconds, and a rising time of 3 seconds, to measure the Vickers hardness. Then, measurements were taken at arbitrary five points on the silver-plated product, and maximum and minimum values were discarded to calculate an average value at three points, from which the Vickers hardness was calculated as the hardness in the initial state.(Hardness after heating)

[0077] The hardness after heating was measured by heating the fabricated silver-plated product in the atmosphere at 100°C for 168 hours in a thermostatic chamber, and then air-cooling the material. The Vickers hardness of the sample after heating was then measured in the same manner as for the hardness in the initial state.(Gloss level)

[0078] The gloss level of the silver plating layer in the initial state was measured. The gloss level of the silver plating layer was measured in a direction parallel to a rolling direction of the material using a densitometer ("Densitometer ND-1" manufactured by Nippon Denshoku Industries Corporation), and the obtained value was taken as the gloss level.(Crystal orientation)

[0079] A crystal orientation of the silver plating layer in the initial state was measured. The crystal orientation of the silver plating layer was evaluated using an X-ray diffraction (XRD) analyzer ("Smart Lab" fully automatic multipurpose horizontal X-ray diffractometer manufactured by Rigaku Corporation) using a Cu tube and Kβ filter method. Specifically, first, the X-ray diffraction pattern was obtained by scanning at a scanning speed of 50 deg / min, with θ being varied from 6 to 120 deg in a scanning range 2θ / θ. From the obtained X-ray diffraction pattern, X-ray diffraction peak intensities (X-ray diffraction peak intensities) of the {111}, {200}, {220}, and {311} planes of the silver plating layer were corrected by dividing them by the relative intensity ratios (relative intensity ratios in powder measurement) ({111}:{200}:{220}:{311}=100:40:25:26) listed in JCPDS Card No. 40783, and the plane orientation of the X-ray diffraction peak with the strongest corrected intensity was evaluated as the direction of crystal orientation (preferential orientation plane) of the silver plating layer.(3) Evaluation results

[0080] The results of each evaluation method are summarized in Tables 1 and 2 above.

[0081] As shown in Table 2, in Comparative Examples 1 and 2, the concentration of the selenium compound in the silver plating bath was low, with Se / KCN being 0.1% or less. Therefore, although the Vickers hardness in the initial state could be 120 HV or more, it was confirmed that after heating at 100°C, the hardness fell below 120 HV. That is, it was confirmed that in Comparative Examples 1 and 2, high heat resistance could not be achieved in the silver plating layer.

[0082] On the other hand, in Comparative Examples 3 to 5, the concentration of the selenium compound was increased compared to Comparative Examples 1 and 2, with Se / KCN exceeding 0.1%, but it was confirmed that the surface of the silver plating layer was roughened, causing whitening, resulting in significantly low gloss level.

[0083] Further, in Comparative Example 6, the concentration of the selenium compound was increased further than in the other Comparative Examples, with Se / KCN being 0.31%, and although high hardness in the initial state and heat resistance at 100°C were obtained, significant whitening and further low gloss level was confirmed.

[0084] As described above, it was confirmed that in Comparative Examples 1 to 6, the silver-plated product having high hardness in the initial state, heat resistance at 100°C, and high gloss level could not be obtained.

[0085] In contrast, in Examples 1 to 8, as shown in Table 1, the silver plating bath contained a sulfonic acid group-containing polymer, and the selenium compound was contained at a predetermined concentration with Se / KCN exceeding 0.1%. As a result, it was confirmed that, compared with Comparative Examples 1 to 6, high hardness in the initial state and heat resistance at 100°C were achieved, the Vickers hardness both in the initial state and after heating was 120 HV or higher, whitening was suppressed and high gloss level was achieved.

[0086] Further, it was confirmed that the fluctuation rate of hardness before and after heating could be decreased in Examples 1 to 8. Specifically, it was confirmed that the ratio of the Vickers hardness after heating to the Vickers hardness in the initial state could be 95% or more.

Claims

1. A silver-plated product having a silver plating layer on a base material, wherein a surface of the base material is copper or a copper alloy; the silver plating layer contains selenium; and Vickers hardness in an initial state and Vickers hardness after heating at 100°C for 168 hours are both 120 HV or more, raising gloss level of the silver plating layer to 1.0 or more.

2. The silver-plated product according to claim 1, wherein a ratio of the Vickers hardness after heating to the Vickers hardness in the initial state is 95% or more.

3. The silver-plated product according to claim 1 or 2, wherein a preferential orientation plane of the silver plating layer in the initial state is a {111} plane.

4. The silver-plated product according to claim 1 or 2, wherein a nickel-containing underlayer is provided between the base material and the silver plating layer.

5. A terminal for electrical contact, which is composed of the silver-plated product according to claim 1 or 2.

6. A method for manufacturing a silver-plated product, comprising: forming a silver plating layer on a base material whose surface is copper or copper alloy, wherein in the forming the silver plating layer, a silver plating bath is used, which contains silver cyanide or potassium silver cyanide, cyanide salt, a selenium compound, and a sulfonic acid group-containing polymer, and in which a ratio of a concentration of selenium derived from the selenium compound to a concentration of the cyanide salt is exceeding 0.1%.

7. The method for manufacturing a silver-plated product according to claim 6, wherein a concentration of selenium derived from the selenium compound is 85 mg / L or more and 200 mg / L or less.

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