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

Abstract

This silver-plated material comprises a silver-plated layer on a base material. The surface of the base material is copper or a copper alloy. The silver-plated layer contains selenium. The Vickers hardness of the silver-plated material in the initial state and the Vickers hardness of the silver-plated material after being heated at 100°C for 168 hours are both 120 HV or more. The glossiness of the silver plated layer is 1.0 or more.

Description

Silver-plated product, terminal for electrical contact, and method for manufacturing silver-plated product

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

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

[0003] Tin-plated materials, which are obtained by plating tin on a substrate such as copper or a copper alloy or stainless steel, are inexpensive but have poor corrosion resistance in high-temperature environments. Gold-plated materials, which are obtained by plating gold on these substrates, have excellent corrosion resistance and high reliability, but are expensive. On the other hand, silver-plated materials, which are obtained by plating Ag (silver) on these substrates, are cheaper than gold-plated materials and have better corrosion resistance than tin-plated materials.

[0004] However, because silver-plated materials are soft and prone to wear, when used as materials for connection terminals and the like, they are prone to adhesion and wear due to insertion, removal, and sliding.Furthermore, when the connection terminal is inserted, the surface is scraped off, increasing the 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 a silver-plated product is improved by incorporating elements such as selenium (Se) into the silver plating (see, for example, Patent Documents 1 and 2).

[0006] Republished WO2018 / 181190 Patent Publication No. 2015-110833

[0007] The silver-plated products of Patent Documents 1 and 2 described above have excellent hardness due to the inclusion of selenium. The plated product of Patent Document 2 uses a silver plating solution containing 70 mg / L or less of selenium, and has a predetermined hardness before a heat resistance test, and is able to maintain the predetermined hardness even after the heat resistance test in which the product is heated at 50°C for a predetermined time. The heat resistance of the plated product refers to the heat resistance of the plated product in terms of its hardness.

[0008] In recent years, there has been a demand for improved heat resistance in plated materials for automobiles and other vehicles that are used outdoors for long periods of time, such as high-voltage terminals for electric vehicles (EVs), HEVs, and other electric vehicles. These materials are required to maintain a predetermined hardness even when heated to temperatures higher than 50°C, for example, 100°C, for a predetermined period of time. In this regard, the silver-plated material disclosed in Patent Document 2 mentioned above sometimes failed to maintain a high hardness when heated to 100°C.

[0009] In addition, silver-plated products are generally required to have high gloss in terms of appearance.

[0010] An object of the present invention is to provide a silver-plated product in which the silver-plated layer has high hardness in the initial state, heat resistance at 100°C, and high gloss, and a manufacturing technique for the same.

[0011] The present inventors have produced silver-plated products by significantly increasing the selenium concentration in the silver plating bath to increase the selenium content of the silver-plated product. They found that the heat resistance of the silver-plated product at high temperatures tended to improve, but the surface of the plating layer became rough and whitened, resulting in a decrease in gloss. Therefore, the present inventors investigated methods for suppressing the surface roughening and whitening caused by selenium while increasing the selenium concentration in the silver plating bath. As a result, they found that adding a sulfonic acid group-containing polymer to the silver plating bath used to form the silver plating layer is effective. As shown in the examples below, when a silver plating layer is formed in the presence of these additives, the surface roughness of the silver plating layer can be reduced, whitening can be suppressed, and high gloss can be achieved, even when the selenium concentration in the silver plating bath is increased to a specific range.

[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 comprising a silver plating layer on a substrate, wherein the surface of the substrate is copper or a copper alloy, the silver plating layer contains selenium, and the silver plating layer has a Vickers hardness of 120 HV or more in an initial state and a Vickers hardness after heating at 100°C for 168 hours, and a gloss of 1.0 or more.

[0014] A second aspect of the present invention is 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 first or second aspect, wherein the preferred orientation plane of the silver plating layer in an initial state is the {111} plane.

[0016] A fourth aspect of the present invention is the electrolytic capacitor according to any one of the first to third aspects, further comprising a base layer containing nickel between the substrate and the silver plating layer.

[0017] A fifth aspect of the present invention is a terminal for electrical contacts, 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 producing a silver-plated product, comprising the step of forming a silver plating layer on a substrate having a surface of copper or a copper alloy, wherein the step of forming the silver plating layer uses a silver plating bath containing silver cyanide or silver potassium cyanide, a cyanide salt, a selenium compound, and a sulfonate group-containing polymer, and wherein the ratio of the concentration of selenium derived from the selenium compound to the concentration of the cyanide salt is more than 0.1%.

[0019] A seventh aspect of the present invention is the sixth aspect, wherein the concentration of selenium derived from the selenium compound is 85 mg / L or more and 200 mg / L or less.

[0020] According to the present invention, the silver plating layer can have initial hardness and heat resistance at 100° C., and can also have improved gloss.

[0021] This embodiment will be described below. In this specification, the initial state refers to the 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 symbol "to" indicates a value greater than or equal to a predetermined value and less than or equal to a predetermined value.

[0022] (1) Silver Plating Bath First, the 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, a cyanide salt, a selenium compound, and a sulfonic acid group-containing polymer.

[0024] Silver cyanide or potassium silver cyanide can be used to supply silver to the silver plating bath. Commercially available silver cyanide or potassium silver cyanide can be used. Among these, potassium silver cyanide is preferred. The concentration of silver cyanide or potassium silver cyanide in the silver plating bath is not particularly limited as long as a silver plating layer can be formed; however, if the silver concentration is too low, the film formation rate decreases, while if it is too high, the silver consumption due to carryover from the plating solution increases, which is uneconomical. From the viewpoint of the formation rate of the silver plating layer and cost reduction, 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 should be 74 to 203 g / L.

[0025] Known cyanide salts can be used, such as potassium cyanide and sodium cyanide. Among these, potassium cyanide is preferred. The concentration of the cyanide salt is preferably set to match the selenium concentration so that the selenium concentration derived from the selenium compound described below exceeds 0.1% relative to the concentration of the cyanide salt. For example, from the perspective of favorably forming a silver plating layer, the cyanide salt concentration is preferably 70 to 160 g / L, more preferably 75 to 125 g / L. If the cyanide salt concentration is less than 70 g / L, the anode current efficiency may decrease, making it difficult for the anode silver to dissolve. On the other hand, if the cyanide salt concentration exceeds 160 g / L, adjusting the selenium concentration to exceed 0.1% relative to the cyanide salt concentration may result in an excessively high selenium concentration, which may roughen the surface of the silver plating layer and cause whitening. Note that the cyanide salt concentration corresponds to the amount of cyanide salt added to the silver plating bath.

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

[0027] From the viewpoint of realizing the initial hardness 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 selenium concentration derived from the selenium compound exceeds 0.1% relative to the concentration of the cyanide salt. The ratio of the selenium concentration derived from the selenium compound (g / L) to the cyanide salt (g / L) is preferably 0.11% or more and 0.3% or less, and more preferably 0.2% or less. Here, the selenium concentration derived from the selenium compound (hereinafter simply referred to as selenium concentration) is calculated by multiplying the concentration of the selenium compound divided by its molecular weight by the atomic weight of selenium. If the ratio is 0.1% or less, the silver plating layer may have poor heat resistance at 100°C. If the ratio exceeds 0.3%, a decrease in gloss may not be suppressed. By adjusting the selenium concentration to a predetermined ratio relative to the cyanide salt concentration and adding a sulfonic acid group-containing polymer described below, it is possible to obtain a high gloss, improve the initial hardness, and improve heat resistance at 100°C.

[0028] The selenium concentration in the silver plating bath is preferably 85 mg / L to 300 mg / L, more preferably 85 mg / L to 200 mg / L, and even more preferably 100 mg / L to 200 mg / L. For example, when potassium selenocyanate is used as the selenium compound, the concentration of potassium selenocyanate is preferably 155 mg / L to 548 mg / L to achieve the above selenium concentration in the silver plating bath. If the selenium concentration is less than 85 mg / L, the heat resistance of the silver plating layer at 100°C may be reduced. On the other hand, if the selenium concentration exceeds 300 mg / L, the gloss of the silver plating layer may be reduced and whitening may occur, even if a sulfonic acid group-containing polymer is used.

[0029] The sulfonic acid group-containing polymer is a compound that acts to suppress surface roughening caused by selenium and improve the gloss of the silver plating layer when it is formed. 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, a sodium salt of β-naphthalene sulfonate-formalin condensate (e.g., "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 improving the gloss, 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, such as conventionally known hardeners including carbon sulfide compounds (such as carbon disulfide) and inorganic sulfur compounds (such as sodium thiosulfate), and conductive salts such as potassium carbonate.

[0032] When the other additives contain antimony as a component, it is preferable that the antimony concentration is lower than the selenium concentration in the silver plating bath, and more preferably, no antimony compounds are contained as other additives.

[0033] The silver plating bath also contains a solvent. Water is preferred as the solvent because it dissolves the other components contained in the silver plating bath and has a low environmental impact. Water is preferably used as the solvent, but a mixed solvent of water and alcohol may also be used.

[0034] (2) Manufacturing Method of Silver-Plated Product 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-plated layer on a substrate whose surface is made of copper or a copper alloy. Before the step of forming the silver-plated layer, a step of forming another layer on the substrate may be performed. Below, a silver-plated product in which an underlayer, a strike-plated layer, and a silver plating are formed on the substrate in this order will be described. However, the formation of the underlayer and strike-plated layer is not necessary and may be performed as needed.

[0035] (Preparation Step) First, a substrate to be plated is prepared.

[0036] The substrate is preferably made of a material that can be silver-plated and has the conductivity required for sliding electrical contact components such as switches and connectors. The surface of the substrate to be silver-plated is copper or a copper alloy. Materials other than the surface are selected depending on the application. The entire substrate may be copper or a copper alloy. Pure copper, for example, can be used as the surface material of the substrate, and the purity of the pure copper can be 3N to 5N. From the viewpoints of conductivity and strength, the copper alloy is preferably an alloy composed of Cu, at least one 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 unavoidable impurities. Brass or bronze may also be used. The Cu content in the copper alloy may be 60 mass % or more.

[0037] As described below, the substrate is preferably used for terminal applications (as a silver-plated material on which a silver plating layer is formed), but the substrate itself may have a shape suitable for such applications, or the substrate may have a flat shape (such as a flat plate shape) and be formed into the shape for application after becoming a silver-plated material.

[0038] (Underlayer Formation Step) If necessary, an underlayer may be formed on the substrate before the formation of the silver plating layer. The underlayer can prevent the components of the substrate, such as copper, from diffusing to the plating surface and oxidizing, thereby reducing the electrical conductivity of the silver-plated product, and can 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, if the substrate is made of copper and copper in the substrate is to be prevented from diffusing to the surface of the silver plating layer, thereby deteriorating the conductivity, it is preferable to form an underlayer containing Ni. Furthermore, if the substrate is made of a copper alloy containing zinc, such as brass, and zinc in the substrate is to be prevented from diffusing to the surface of the silver plating layer, it is preferable to form an underlayer containing Cu. Furthermore, if the adhesion of the silver plating layer to the substrate is to be improved, it is preferable to form an underlayer containing Ag.

[0040] The underlayer may be a single layer made of Cu, Ni, Sn, Ag, or an alloy thereof, or a layer made of a combination of these (a laminate structure), and the underlayer may be formed on the entire surface of the substrate or on only a part of it, depending on the 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-mentioned constituent metals using a known method, or by stacking layers made of each metal that constitutes the desired alloy layer in order and then reflowing (heat treating).

[0043] (Strike Plating Step) A strike plating layer may be formed on the substrate or the above-described underlayer. The strike plating layer can improve adhesion between the substrate 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.

[0044] (Silver Plated Layer Forming Step) Subsequently, a silver plated layer is formed on the substrate (for example, on the strike plated layer) using the silver plating bath described above.

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

[0046] In this embodiment, the silver plating bath contains a predetermined concentration of selenium, so that the silver plating layer can be formed to contain selenium. As described below, the silver plating layer has a predetermined initial hardness and heat resistance at 100°C. Furthermore, increasing the selenium concentration relative to the cyanide salt concentration in the silver plating bath tends to cause surface roughness and reduce gloss. However, adding a sulfonic acid group-containing polymer to the silver plating bath and performing silver plating in its presence can suppress surface roughness and maintain high gloss.

[0047] Furthermore, by using the above-mentioned silver plating bath, it is preferable to form a silver plating layer having a predetermined hardness and having a preferred orientation plane on the surface of the silver plating layer as the {111} plane. By forming the silver plating layer in this manner, it is possible to achieve high hardness and suppress an increase in contact resistance. Here, the preferred orientation plane is evaluated in the above-mentioned 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 for silver plating is 0.5 to 10 A / dm from the viewpoint of the speed of forming the silver plating layer and from the viewpoint of suppressing unevenness in the appearance of the silver plating layer. ２ is preferred, and 1 to 10 A / dm ２ More preferably, 3 to 8 A / dm ２ is more preferred.

[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 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 the desired thickness of the silver plating layer, but is typically in the range of 25 to 1800 seconds.

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

[0054] In this way, a silver-plated product is obtained.

[0055] (3) Silver-Plated Material Next, a silver-plated material obtained by the above-described method for producing a silver-plated material will be described.

[0056] The silver-plated product of this embodiment is a silver-plated product having a silver plating layer on a substrate. The silver-plated product may have a configuration including at least one of an underlayer or a strike plating layer between the surface of the substrate and the silver plating layer, and is also preferably a member having an underlayer, a strike plating layer, and a silver plating layer in this order on the surface of the substrate.

[0057] As described above, the surface of the substrate to be plated is made of copper or a copper alloy. The materials of the substrate other than the surface are selected depending on the application. The entire substrate may be made 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. Ni is preferably used 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. In other words, the strike plating layer may be indistinguishable from the silver plating layer.

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

[0061] The silver plating layer is formed using the silver plating bath of the present invention, which allows the silver-plated product to have a predetermined initial hardness and heat resistance at 100°C. Moreover, since the silver plating layer is formed by precipitating silver in the presence of the sulfonic acid group-containing polymer using a silver plating bath containing the sulfonic acid group-containing polymer, the silver plating layer is configured to have high gloss and suppress surface roughness despite containing a relatively large amount of selenium.

[0062] Specifically, the silver-plated product has a Vickers hardness of 120 HV or more in the initial state. Furthermore, the silver-plated product has a Vickers hardness of 120 HV or more after heating at 100°C for 168 hours. In other words, 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 at 100°C, and has high heat resistance at 100°C. Here, the Vickers hardness is a value measured using a hardness tester as described later in the examples.

[0063] Furthermore, it is preferable that the silver-plated product has high heat resistance at 100°C and that the rate of change in hardness before and after heating is suppressed to a low level. Specifically, the ratio of the Vickers hardness after heating to the Vickers hardness in the initial state is high, for example, preferably 95% or more. This allows the silver-plated product to maintain high hardness and desired wear resistance without being affected by harsh environmental temperatures.

[0064] The silver plating layer has a glossiness of 1.0 or more. Here, glossiness is a numerical value measured using a densitometer as described later in the Examples. The glossiness tends to decrease as the surface of the silver plating layer becomes rough and white. In this embodiment, surface roughness and the resulting whitening are suppressed, resulting in a glossiness of 1.0 or more for the silver plating layer. The glossiness is a numerical value for the initial state of the silver plating layer.

[0065] Furthermore, it is preferable that the silver plating layer has a predetermined hardness and is configured so that its preferred orientation plane in its initial state is the {111} plane. If the preferred orientation plane of the silver plating layer is, for example, the {200} plane, the hardness of the silver plating layer may become unstable due to heating or the passage of time, and if the preferred orientation plane is the {220} plane, the hardness or gloss of the silver plating layer may be reduced. In this regard, by having the preferred orientation plane be the {111} plane, the silver-plated product can achieve high hardness, heat resistance, and high gloss. In this embodiment, as shown in the examples described below, by adding a 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 abrasion resistance, reliability, and conductivity, a minimum thickness is preferable. Furthermore, if the thickness is excessively large, the effect of the silver plating layer saturates, and raw material costs increase. 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 the measurement method are given in the Examples section.

[0067] (4) Terminals 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.

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

[0069] (1) Preparation of silver-plated material First, a pure copper metal substrate (purity 99.96% or higher) measuring 67 mm × 50 mm × 0.3 mm was prepared as the material to be plated (substrate), and this material to be plated and a SUS plate were placed in an alkaline degreasing solution, and electrolytic degreasing was performed at a voltage of 5 V for 30 seconds with the material to be plated as the cathode and another SUS plate as the anode, followed by pickling in 3% sulfuric acid for 15 seconds. Note that water rinsing was performed for 15 seconds 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, the material to be plated was used as the cathode and the SK nickel electrode plate as the anode, and the plating was conducted at a current density of 7 A / dm while stirring at 500 rpm with a magnetic stirrer. ２ Electroplating was carried out at a solution temperature of 50° C. until the nickel film thickness reached 1 μm, thereby forming a nickel underlayer on the substrate.

[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, the material to be plated was used as the cathode and a platinum-coated titanium electrode plate as the anode, and the plating was conducted at a current density of 2 A / dm while stirring at 500 rpm with a stirrer. ２ Electroplating was carried out at a temperature of 1000 V for 10 seconds to form a strike-plated layer of silver having a thickness of 0.01 μm on the underlayer.

[0072] Next, a silver plating bath was prepared by adding potassium cyanide as the cyanide salt, potassium selenocyanate as the selenium compound, and either "Demol N" (manufactured by Kao Corporation, sodium salt of β-naphthalenesulfonic acid formalin condensate) or polystyrene sodium sulfonate (hereinafter simply referred to as PSS) as an aromatic sulfonic acid polymer, which is a type of sulfonic acid group-containing polymer, with the concentrations appropriately adjusted as shown in Tables 1 and 2 below. Using this silver plating bath, electroplating was performed on the strike-plated layer until the thickness of the silver plating layer reached 5 μm, with the workpiece used 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 the current density and silver plating bath temperature shown in Tables 1 and 2 below. After silver plating, the workpiece was rinsed with water for 15 seconds and dried with an air gun, thereby obtaining silver-plated materials of this example and comparative example.

[0073] The thickness of the silver plating layer was measured at the center of the 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 the measured value was taken as the film thickness. When the film thickness was measured, it was confirmed that it was 5 μm in all cases.

[0074] In Tables 1 and 2, the Ag concentration is the silver concentration (i.e., the silver concentration derived from the concentration of potassium silver cyanide), which is the silver equivalent of the concentration of added potassium silver cyanide using its molecular weight and the atomic weight of silver. The KCN concentration is the concentration of added potassium cyanide. The Se concentration is the selenium concentration (i.e., the selenium concentration derived from the concentration of potassium selenocyanate), which is calculated by dividing the concentration of added potassium selenocyanate by its molecular weight and multiplying it by the atomic weight of selenium. Furthermore, Se / KCN is the ratio of the selenium concentration, converted from mg / L to g / L, to the potassium cyanide concentration, which is g / L. These values ​​are shown in Tables 1 and 2, respectively.

[0075]

[0076]

[0077] (2) Evaluation Methods The initial hardness, hardness after heating, gloss, and crystal orientation of each silver-plated product were evaluated. Each evaluation method is described below.

[0078] (Hardness in the Initial State) The hardness of the silver-plated material 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 silver-plated material after production with a load of 10 gf, a fall time of 3 seconds, a hold time of 10 seconds, and a rise time of 3 seconds, and the Vickers hardness was measured. Then, measurements were taken at five arbitrary points on the silver-plated material, and the maximum and minimum values ​​were discarded to calculate the average of the three points, which was used to determine the Vickers hardness in the initial state.

[0079] (Hardness after heating) The hardness after heating was measured by heating the prepared silver-plated material in the atmosphere at 100°C for 168 hours in a thermostatic chamber, followed by air cooling. The Vickers hardness of the obtained heated sample was then measured using the same procedure as for the hardness in the initial state.

[0080] (Gloss) The gloss of the silver plating layer in the initial state was measured using a densitometer ("Densitometer ND-1" manufactured by Nippon Denshoku Industries Co., Ltd.) in a direction parallel to the rolling direction of the material, and the obtained value was taken as the gloss.

[0081] (Crystal Orientation) The 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 (Rigaku Corporation's "Smart Lab" fully automated multipurpose horizontal X-ray diffractometer) with a Cu tube and a Kβ filter method. Specifically, an X-ray diffraction pattern was obtained by scanning at a scanning speed of 50 deg / min, with θ varied from 6 to 120 deg within the scanning range 2θ / θ. From the obtained X-ray diffraction pattern, the X-ray diffraction peak intensities (intensities of X-ray diffraction peaks) of the {111}, {200}, {220}, and {311} planes of the silver plating layer were calculated using the JCPDS card no. The plane orientation of the X-ray diffraction peak with the strongest corrected intensity obtained by dividing the intensity by the relative intensity ratios (relative intensity ratios in powder measurement) ({111}:{200}:{220}:{311}=100:40:25:26) described in U.S. Pat. No. 40783 was evaluated as the crystal orientation direction (preferential orientation plane) of the silver plating layer.

[0082] (3) Evaluation Results The results of each evaluation method are summarized in Tables 1 and 2 above.

[0083] 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, and therefore, although the Vickers hardness in the initial state could be made 120 HV or more, it was confirmed that after heating at 100° C., it was below 120 HV. In other words, it was confirmed that Comparative Examples 1 and 2 could not achieve high heat resistance in the silver plating layer.

[0084] 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, whitening occurred, and the glossiness was significantly low.

[0085] In Comparative Example 6, the concentration of the selenium compound was increased even more 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, it was confirmed that whitening was significant and the glossiness was further reduced.

[0086] As described above, it was confirmed that in Comparative Examples 1 to 6, it was not possible to obtain a silver-plated product that had high hardness in the initial state, heat resistance at 100° C., and high gloss.

[0087] In contrast, in Examples 1 to 8, as shown in Table 1, a sulfonic acid group-containing polymer was added to the silver plating bath, 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, and the Vickers hardness both in the initial state and after heating was 120 HV or more, whitening was suppressed, and high gloss was obtained.

[0088] Furthermore, it was confirmed that the variation rate of hardness before and after heating could be reduced 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 made 95% or more.

Claims

1. A silver-plated product comprising a silver plating layer on a substrate, the surface of the substrate being copper or a copper alloy, the silver plating layer containing selenium, the initial Vickers hardness and the post-heating Vickers hardness after heating at 100°C for 168 hours both being 120 HV or more, and the gloss of the silver plating layer being 1.0 or more.

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

4. The silver-plated product according to claim 1 or 2, further comprising a nickel-containing underlayer between the substrate and the silver-plated layer.

5. A terminal for electrical contacts, comprising the silver-plated product according to claim 1 or 2.

6. A method for producing a silver-plated product, comprising the step of forming a silver plating layer on a substrate having a copper or copper alloy surface, wherein the step of forming the silver plating layer uses a silver plating bath containing silver cyanide or silver potassium cyanide, a cyanide salt, a selenium compound, and a sulfonate group-containing polymer, and the ratio of the concentration of selenium derived from the selenium compound to the concentration of the cyanide salt exceeds 0.1%.

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

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