WO2024116940A1

WO2024116940A1 - Production method for silver coating material, silver coating material, and energizing component

Info

Publication number: WO2024116940A1
Application number: PCT/JP2023/041654
Authority: WO
Inventors: 悠太郎平井; 健太郎荒井; 陽介佐藤; 恵理土屋
Original assignee: Ｄｏｗａメタルテック株式会社
Priority date: 2022-11-29
Filing date: 2023-11-20
Publication date: 2024-06-06
Also published as: JP2024077764A

Abstract

[Problem] To provide a silver coating material which has favorable peeling resistance of a silver coating layer in a severely bent part and also has favorable durability against fine sliding wear. [Solution] This production method for a silver coating material comprises: a lower silver plating step for forming a lower silver-plated layer on a material using a silver plating solution not containing benzothiazoles and derivatives thereof; an upper silver plating step for forming an upper silver-plated layer on the lower silver-plated layer by means of an electroplating method using a silver plating solution containing at least one substance selected from among benzothiazoles and derivatives thereof; and a heat treatment step for holding the lower silver-plated layer and the upper silver-plated layer in the temperature range of 250-400ºC for 3-60 seconds.

Description

Method for manufacturing silver-coated material, silver-coated material and current-carrying component

The present invention relates to a method for producing a silver-coated material that is useful as a material for contacts and terminal parts such as connectors, switches, and relays used in automotive and consumer electrical wiring. It also relates to a silver-coated material that can be obtained by the production method, and current-carrying parts that use the silver-coated material. Here, "silver-coated material" refers to a material on which a silver-coated layer is formed. A "silver-coated layer" is a layer of silver formed on the surface of a material, and examples of this include a silver layer consisting of one or more silver-plated layers, and a silver layer obtained by subjecting such a silver layer to heat treatment.

　Traditionally, materials used for contacts and terminal parts of connectors and switches are relatively inexpensive materials with excellent corrosion resistance and mechanical properties, such as copper or copper alloys and stainless steel, plated with tin, silver, gold, etc. depending on the required properties, such as electrical properties and solderability. Of these, tin-plated materials are inexpensive but have poor corrosion resistance in high-temperature environments. Gold-plated materials have excellent corrosion resistance and are highly reliable, but are expensive. On the other hand, silver-plated materials have the advantage of being cheaper than gold-plated materials and having better corrosion resistance than tin-plated materials.

Materials for contacts and terminal parts of connectors and switches are required to be resistant to wear caused by the insertion and removal of connectors and the sliding of switches. However, because silver-plated materials are soft and easily worn, there are problems with using silver-plated materials as materials for connection terminals, etc., as they can easily become worn due to adhesion caused by insertion and removal or sliding, or the surface can be scraped off when the connection terminal is inserted, increasing the coefficient of friction and increasing the insertion force.

The applicant disclosed in Patent Document 1 a method for obtaining silver-plated products with better abrasion resistance than conventional methods. The method involves using a plating solution containing a specified amount of benzothiazoles or their derivatives.

JP 2022-48959 A

By following the method disclosed in Patent Document 1, it is possible to significantly improve the wear resistance of the silver plating layer compared to conventional methods. However, it was found that the silver plating material obtained by the method of Patent Document 1 does not necessarily have sufficient peel resistance in areas that have been subjected to severe bending, and there is room for improvement. In addition, there is an increasing demand for improved durability against micro-friction wear for silver-coated materials used in vibrating environments, such as automotive terminals and connectors.

The present invention aims to provide a silver-coated material that has good resistance to peeling of the silver-coated layer in severely bent areas and also has good durability against micro-sliding abrasion.

As a result of their investigations, the inventors have found that the above object can be achieved by forming a silver plating layer containing carbon and sulfur by electroplating on a normal silver plating layer to which no carbon or sulfur has been added using a silver plating solution containing benzothiazoles or their derivatives, and then constructing a modified silver coating layer derived from the multi-layer silver plating layer by heat treatment. This specification discloses the following invention.

[1] a lower silver plating step of forming a lower silver plating layer on a material using a silver plating solution that does not contain benzothiazoles and their derivatives;
an upper silver plating step of forming an upper silver plating layer on the lower silver plating layer by electroplating using a silver plating solution containing one or more substances selected from benzothiazoles and derivatives thereof;
a heat treatment step of holding the lower silver plating layer and the upper silver plating layer at a temperature range of 250 to 400° C. for 3 to 60 seconds;
A method for producing a silver-coated material comprising the steps of:
[2] The method for producing a silver-coated material described in [1] above, wherein the average thickness of the lower silver plating layer is 0.06 to 3.0 μm.
[3] The method for producing a silver-coated material according to the above [1] or [2], wherein the upper silver plating layer has an average thickness of 0.3 to 10.0 μm.
[4] The method for producing a silver-coated material according to any one of the above [1] to [3], wherein the silver plating solution used in the upper silver plating step contains one or more substances selected from benzothiazoles and their derivatives at a concentration of 0.01 to 0.80 mol/L.
[5] The method for producing a silver-coated material according to any one of [1] to [4] above, wherein in the upper silver plating step, the one or more substances selected from benzothiazoles and derivatives thereof are one or more substances selected from mercaptobenzothiazole and derivatives thereof.
[6] The method for producing a silver-coated material according to any one of [1] to [4] above, wherein in the upper silver plating step, the one or more substances selected from benzothiazoles and derivatives thereof are one or more substances selected from benzothiazoles and alkali metal salts thereof.
[7] The method for producing a silver-coated material according to any one of [1] to [6] above, wherein the material subjected to the lower silver plating step has a copper or copper alloy base material.
[8] The method for producing a silver-coated material according to any one of [1] to [7] above, wherein the material subjected to the lower silver plating step has a nickel plating layer on the surface on which the lower silver plating layer is to be formed.
[9] The method for producing a silver-coated material according to any one of [1] to [8] above, wherein the lower silver plating layer comprises a silver strike plating layer and a silver plating layer thereon.
[10] A silver-coated material having a silver coating layer formed on the surface of a material having a copper or copper alloy as a base material, wherein, in a depth direction element concentration profile of the silver coating layer by XPS (X-ray photoelectron spectroscopy), the cumulative sputtering time at a depth position where the atomic ratio of silver decreases to 1/2 of the maximum value is t ₀ (minutes), and on the silver profile curve, the maximum point of the atomic ratio of silver in a region (the outermost surface side of the sample) where the cumulative sputtering time is shorter than t ₀ /2 is point P ₁ , the maximum point of the atomic ratio of silver in a region (the center side of the sample) where the cumulative sputtering time is longer than t ₀ /2 is point P ₂ , and the minimum point of the atomic ratio of silver between points P ₁ and P ₂ is point Q, the ratio Ag(Q)/Ag(P ₁ ) of the atomic ratio of silver at point Q to the atomic ratio of silver at point P ₁ is 0.90 or less, and the ratio Ag(Q)/Ag(P ₁ ₎ of the atomic ratio of silver at point P ₂ is 0.90 or less. a ratio Ag(Q)/Ag( _P2 ) of 0.90 or less, a C/Ag atomic ratio at a depth position corresponding to point Q of 0.15 or more, and a C/Ag atomic ratio at a depth position corresponding to point _P2 of 0.10 or less.
[11] The silver-coated material according to [10] above, wherein the average crystallite size of silver in the silver-coating layer is 20 nm or more.
[12] An electrical component using the silver-coated material according to [10] or [11] above as a material.

In this specification, the notation "n1 to n2" indicating a numerical range means "greater than or equal to n1 and less than or equal to n2." Here, n1 and n2 are numerical values that satisfy n1 < n2.

The present invention makes it possible to provide a silver-coated material that has good resistance to peeling of the silver-coated layer in severely bent areas and also has good durability against micro-sliding wear. Therefore, the present invention contributes to improving the reliability of current-carrying parts such as connectors used as in-vehicle parts that are exposed to vibrations.

FIG. 2 is a schematic diagram illustrating a cross-sectional structure of an embodiment of a material to be subjected to a heat treatment step in the method for producing a silver-coated material of the present invention. FIG. 2 is a schematic diagram illustrating a cross-sectional structure of an embodiment of a material after a heat treatment step in the method for producing a silver-coated material of the present invention. Element concentration profile in the depth direction by XPS for the silver coating layer of the test material obtained in Comparative Example 1. 13 is an element concentration profile in the depth direction by XPS for the silver coating layer of the test material obtained in Example 3.

FIG. 1 shows a schematic cross-sectional structure of an embodiment of a material to be subjected to a heat treatment step in the manufacturing method of the silver-coated material of the present invention. A lower silver plating layer 20 and an upper silver plating layer 30 are formed on a material 10. The "material" referred to here means a material equivalent to the "material to be plated" for forming the lower silver plating layer 20. The material 10 in the illustrated example has a base plating layer 2 on a base material 1.

Examples of the substrate 1 include copper or a copper alloy, stainless steel, aluminum or an aluminum alloy, an iron-nickel alloy, and a nickel-based alloy. Considering the use of the current-carrying part, a material having a substrate of copper or a copper alloy is preferable. The undercoat plating layer 2 is an effective layer for ensuring sufficient adhesion of the silver coating layer to the substrate, and examples of the undercoat plating layer include a plating layer made of copper, nickel, or an alloy thereof. From the viewpoint of heat resistance, a nickel plating layer is preferable.

The lower silver plating layer 20 is a silver layer to which no carbon or sulfur has been added (unavoidable traces of carbon and sulfur are permitted). The lower silver plating layer 20 may be formed by multiple silver plating processes. In the illustrated example, the lower silver plating layer 20 is composed of a silver strike plating layer 3 and a silver plating layer 4 formed thereon. The silver strike plating layer 3 is a very thin electrolytic silver plating layer with an average thickness of, for example, 0.001 to 0.05 μm or 0.001 to 0.02 μm, and is a silver film formed as necessary as a base treatment for forming the main silver plating layer. The upper silver plating layer 30 is a silver plating layer to which carbon and sulfur have been added, and can be formed by the method described below.

Figure 2 shows a schematic example of a cross-sectional structure observable by a scanning electron microscope (SEM) of an embodiment of a material after a heat treatment step in the manufacturing method of the silver-coated material of the present invention. A silver coating layer 40 derived from adjacent silver layers is formed by the heat treatment. When the material with the cross-sectional structure shown in Figure 1 is subjected to heat treatment, as shown in Figure 2, there is a base plating layer 2 and a silver coating layer 40 on the substrate 1. The silver plating layer 40 is derived from a lower silver plating layer 20 consisting of a silver strike plating layer 3 and a silver plating layer 4, and an upper silver plating layer 30.

[Lower silver plating process]
First, a lower silver plating layer is formed on the above-mentioned material (material to be plated) using a silver plating solution that does not contain benzothiazoles and their derivatives. The term "silver plating solution that does not contain benzothiazoles and their derivatives" is a provision for distinguishing the lower silver plating step from the upper silver plating step described below. That is, in the lower silver plating step, a silver plating layer can be formed by a conventionally known method. It is also possible to carry out the lower silver plating step by a plurality of plating steps, such as a silver strike plating step and a normal silver plating step. The amount of carbon and sulfur mixed in the lower silver plating layer is sufficiently tolerable up to a level equivalent to that of an electric silver plating layer using a general cyan-based silver plating solution (e.g., a glossy silver plating solution, a matte silver plating solution, etc.). Specifically, the concentration of one or more substances selected from benzothiazoles and their derivatives in the silver plating solution is, for example, in the range of 0.001 mol/L or less, and it is preferable that the C/Ag atomic ratio representing the ratio of the number of carbon and silver atoms in the silver plating layer is 0.020 or less, and the S/Ag atomic ratio representing the ratio of the number of sulfur and silver atoms is 0.003 or less. Generally, an electrolytic silver plating method using a cyanide-based silver plating solution may be used for both the formation of the silver strike plating layer and the formation of the silver plating layer thereon. The use of a silver plating solution using a complexing agent other than a cyanide compound is not excluded. The average thickness of the lower silver plating layer is preferably set in the range of 0.06 to 3.0 μm, and more preferably in the range of 0.1 to 1.0 μm. Considering economic efficiency, it is preferably set in the range of 0.5 times or less the average thickness of the upper silver plating layer described below.

The following are examples of preferable plating conditions for forming the main silver plating layer (the portion excluding the silver strike plating, which corresponds to reference numeral 4 in the example of FIG. 1) that constitutes the lower silver plating layer 20.
For example, electrolytic silver plating can be performed in a silver plating solution made of an aqueous solution containing potassium silver cyanide (K[Ag(CN) ₂ ]) and potassium cyanide (KCN) under conditions set at a current density of 1 to 10 A/ ^dm2 and a current application time of 1 to 90 seconds. The silver plating solution can contain additives such as brighteners (e.g., selenium) as necessary. The silver plating layer 4 obtained under such conditions is preferably a silver plating layer containing 99.0 mass% or more of Ag.

In addition, even if a silver plating layer is formed using a silver plating solution that does not contain benzothiazoles or its derivatives, if the silver plating layer is very thin, for example, a silver film consisting only of a silver strike plating layer, with an average thickness of 0.05 μm or less, it is not considered to fall under the "lower silver plating layer" (reference numeral 20 in the example in Figure 1) that is the subject of this invention.

[Top silver plating process]
Next, a silver plating layer containing carbon and sulfur is formed on the lower silver plating layer 20. It is preferable to form a silver plating layer in which the C/Ag atomic ratio, which represents the ratio of the number of carbon and silver atoms, is, for example, 0.050 to 0.200 and the S/Ag atomic ratio, which represents the ratio of the number of sulfur and silver atoms, is, for example, 0.005 to 0.050, in measurement data by XPS (X-ray photoelectron spectroscopy) for the vicinity of the center of the thickness of the lower silver plating layer 20. Such an upper silver plating layer 30 containing carbon and sulfur can be formed, for example, by an electric silver plating method as follows.

(Plating solution for upper silver plating process)
As the silver plating solution, it is preferable to use a cyanide-containing silver plating solution. As for the cyanide-containing substance and silver-containing substance, which are the main components of the cyanide-containing silver plating solution, conventionally known substances can be used. For example, an aqueous solution containing potassium silver cyanide (K[Ag(CN) ₂ ]) or silver cyanide (AgCN) and potassium cyanide (KCN) or sodium cyanide (NaCN) is suitable.

In the present invention, one or more substances selected from benzothiazoles and their derivatives are used as additives to the plating solution. This is the same as the technology of Patent Document 1. Benzothiazole (C ₇ H ₅ NS) is a heterocyclic compound having a benzene skeleton and a thiazole skeleton. The benzothiazole is preferably a benzothiazole having a mercapto group (-SH), such as 2-mercaptobenzothiazole. In addition, as derivatives of benzothiazoles, sodium 2-mercaptobenzothiazole (sodium mercaptobenzothiazole (SMBT)), zinc-2-mercaptobenzothiazole, 5-chloro-2-mercaptobenzothiazole, 6-amino-2-mercaptobenzothiazole, 6-nitro-2-mercaptobenzothiazole, 2-mercapto-5-methoxybenzothiazole, etc. can be used. Of these benzothiazole derivatives, alkali metal salts of benzothiazoles are preferred, and for example, sodium salts of benzothiazoles such as sodium 2-mercaptobenzothiazole (sodium mercaptobenzothiazole (SMBT)) are preferred.

When electrolytic silver plating is performed by adding benzothiazoles such as mercaptobenzothiazole or their alkali metal salts (preferably sodium salts) as organic additives to a silver plating solution, components derived from the organic additives (components containing carbon and sulfur) are incorporated into the silver plating layer that is formed, and the components derived from the organic additives are contained in the silver coating layer formed by the heat treatment described below, which is thought to improve wear resistance. It is also thought that the lubricating effect of the components derived from the organic additives can reduce the friction coefficient of the surface layer. When the silver coating material is used as a material for connection terminals, etc., the reduction in the friction coefficient exerts an effect of suppressing the occurrence of adhesion due to insertion/removal and sliding, and this is also thought to be effective in improving wear resistance. Since the components derived from the organic additives are contained in the silver coating layer even after the heat treatment described below, the above-mentioned effect of improving wear resistance is maintained. In addition, the use of mercaptobenzothiazole or its derivatives makes it easier to incorporate the components of the organic additives into the silver plating layer, which is preferable.

The concentration of free cyanide in the silver plating solution can be set, for example, in the range of 3 to 70 g/L, preferably 10 to 70 g/L, and even more preferably 15 to 60 g/L. The concentration of free cyanide in the silver plating solution can be determined by diluting the silver plating solution with water, adding an aqueous solution of potassium iodide, and dripping an aqueous solution of silver nitrate until the silver plating solution becomes cloudy, and then measuring the amount of the dripped solution.

The concentration of one or more substances selected from benzothiazoles and their derivatives in the silver plating solution can be set, for example, in the range of 0.01 to 0.80 mol/L, preferably 0.015 to 0.35 mol/L, more preferably 0.03 to 0.3 mol/L, and even more preferably 0.07 to 0.25 mol/L.

The concentration of silver in the silver plating solution can be set, for example, in the range of 15 to 150 g/L, and more preferably in the range of 30 to 120 g/L. The concentration of potassium silver cyanide or silver cyanide in the silver plating solution can be set, for example, in the range of 30 to 220 g/L, and more preferably in the range of 50 to 200 g/L. The concentration of potassium cyanide or sodium cyanide in the silver plating solution can be set, for example, in the range of 30 to 150 g/L, and more preferably in the range of 35 to 145 g/L, and even more preferably in the range of 38 to 110 g/L. The concentration of benzothiazoles or their alkali metal salts in the silver plating solution can be set, for example, in the range of 15 to 70 g/L, and may be controlled in the range of 20 to 50 g/L.

(Silver plating conditions for the upper silver plating process)
The electrolytic silver plating for forming the upper silver plating layer 30 is preferably performed at a liquid temperature of 15 to 50°C, more preferably 18 to 47°C. The current density of this electrolytic silver plating can be set in the range of, for example, 0.5 to 12 A/ ^dm2 , and more preferably 0.5 to 10 A/ ^dm2 . In order to efficiently form a good silver plating layer with few defects, it is preferable to ensure a current density of 2 A/dm2 or ^more , and more preferably 3 A/ ^dm2 or more. The plating time may be set according to the application so that the average thickness of the upper silver plating layer 30 formed by this electrolytic silver plating is, for example, in the range of 0.3 to 10.0 μm, more preferably 0.5 to 3.0 μm.

[Heat treatment process]
As disclosed in Patent Document 1, when electrolytic silver plating is performed using a silver plating solution containing benzothiazoles or their derivatives as additives, the wear resistance of the silver coating layer can be significantly improved. On the other hand, the peel resistance of the silver coating layer at the severely bent portion is lower than that of conventional general silver-plated materials. According to the inventors' study, by subjecting the silver coating layer composed of the adjacent lower silver plating layer 20 and upper silver plating layer 30 as described above to a heat treatment in which the silver coating layer is held at a temperature range of 250 to 400°C for 3 to 60 seconds, the peel resistance of the silver coating layer 40 at the severely bent portion can be restored to the same level as or better than that of conventional general silver-plated materials while maintaining the improved wear resistance.

The lower silver plating layer 20 is a normal silver plating layer with a high silver concentration and a low carbon concentration. On the other hand, the upper silver plating layer 30 has a relatively lower silver concentration than the lower silver plating layer 20 due to the introduction of carbon and sulfur. When the above-mentioned heat treatment is performed on a silver coating layer consisting of these different types of silver plating layers adjacent to each other, a silver coating layer 40 with a new structure is formed by atomic diffusion. This new structure exhibits a unique silver concentration distribution in which the silver concentration is high both in the region closest to the surface in the depth direction of the silver coating layer and in the region closest to the substrate, and the silver concentration is low in the region between them. In addition, the existence of a region with a high silver concentration and a low carbon concentration is maintained near the substrate in the silver coating layer 40. A typical element distribution in such a silver coating layer is shown in Figure 4 described below.

The mechanism by which a silver coating layer with such an element concentration distribution overcomes the decrease in peel resistance of the silver coating layer in severely bent areas is currently unclear, but this may be due to the presence of an area with high silver concentration and low carbon concentration near the substrate in the silver coating layer after heat treatment.

The heat treatment conditions are such that the silver coating layer after the upper silver plating process is held in a temperature range of 250 to 400 ° C for 3 to 60 seconds, more preferably 3 to 30 seconds. In this heat treatment, a heat pattern is adopted in which the maximum temperature T _MAX of the silver coating layer is in the range of 250 to 400 ° C, and the time during which the temperature of the silver coating layer is 250 ° C or more and Tmax (° C) or less is in the range of 3 to 60 seconds, more preferably 3 to 30 seconds. If the maximum temperature Tmax is too low or the holding time at 250 to 400 ° C is too short, there is a risk that the effect of improving the peel resistance of the silver coating layer at the severe bending part cannot be sufficiently obtained. If the maximum temperature Tmax is too high or the holding time at 250 to 400 ° C is too long, it is disadvantageous in maintaining a stable high level of wear resistance, especially durability against fretting wear. This heat treatment can be performed in an air atmosphere. In actual product manufacturing sites, the heat treatment conditions can be controlled appropriately by determining in advance through preliminary experiments the heat curve (temperature change over time) of the silver coating layer according to the thickness of the substrate in the heating equipment to be used.

[Silver-coated material]
The silver-coated material according to the present invention obtained through the above-mentioned lower silver plating process, upper silver plating process and heat treatment process has a significantly improved durability against fretting wear, and the peeling resistance of the silver coating layer at the site subjected to severe bending is as excellent as that of a general silver-plated material. As described above, the silver coating layer has a high silver concentration both in the region near the outermost surface in the depth direction and in the region near the substrate, and the silver concentration is low in the region between them. In addition, carbon in the silver coating tends to be concentrated toward the center in the depth direction, and in particular, a region with a high silver concentration and a low carbon concentration is formed near the substrate of the silver coating layer. More specifically, in the silver coating layer of the silver-coated material which is a preferred embodiment of the present invention, when the cumulative sputtering time at a depth position where the atomic percentage of silver decreases to 1/2 of the maximum value in a depth direction element concentration profile of the silver coating layer by XPS (X-ray photoelectron spectroscopy) is t ₀ (minutes), the maximum point of the atomic percentage of silver in a region (the outermost surface side of the sample) where the cumulative sputtering time is shorter than _{t 0} _/ 2 on the silver profile curve is point P ₁ _, the maximum point of the atomic percentage of silver in a region (the center side of the sample) where the cumulative sputtering time is longer than t 0 / _{2 is point P 2} _, and the minimum point of the atomic percentage of silver between points P ₁ and P _{2 is point Q, the ratio Ag(Q)/Ag(P 1} ₎ of the atomic percentage of silver at point _Q to the atomic percentage of silver at point P ₁ is 0.90 or less, and the ratio Ag(Q)/Ag(P ₂ ) is 0.90 or less, the C/Ag atomic ratio at the depth position corresponding to point Q is 0.15 or more, and the C/Ag atomic ratio at the depth position corresponding to point _P2 is 0.10 or less.

A method of determining whether the above-mentioned provisions of the present invention are satisfied will be described with reference to FIG. 4, which shows an element concentration profile in the depth direction by XPS for a silver coating layer obtained in Example 3 (heat treatment conditions: maximum temperature of 350° C., holding time in the temperature range of 250° C. to the maximum temperature of 10 seconds) described later. In this example, the cumulative sputtering time t ₀ by argon at the depth position where the atomic percentage of silver decreases to 1/2 of the maximum value is 47 minutes, and t ₀ /2 is 23.5 minutes. In FIG. 4, the maximum atomic percentage point of silver in the region (the outermost surface side of the sample) where the cumulative sputtering time is less than t ₀ /2 on the silver profile curve is indicated as P ₁ , and the maximum atomic percentage point of silver in the region (the center side of the sample) where the cumulative sputtering time is greater than t ₀ /2 is indicated as P ₂ . In addition, the minimum atomic percentage point of silver between points P ₁ and P ₂ is indicated as Q. In this example, the ratio Ag(Q)/Ag( _P1 ) of the atomic ratio of silver at point Q to the atomic ratio of silver at point _P1 is 0.824, which satisfies the above-mentioned regulation of the present invention of "0.90 or less". The ratio Ag(Q)/Ag( _P2 ) of the atomic ratio of silver at point _P2 to the atomic ratio of silver at point _P2 is 0.820, which also satisfies _the above-mentioned regulation of the present invention of "0.90 or less". The C/Ag atomic ratio at the depth position corresponding to point Q is 0.278, which satisfies the above-mentioned regulation of the present invention of "0.15 or more". The C/Ag atomic ratio at the depth position corresponding to point _P2 is 0.054, which also satisfies the above-mentioned regulation of the present invention of "0.10 or less".

In addition, the silver-coated material according to the present invention preferably has an average crystallite diameter of 20 nm or more in the silver coating layer obtained by heat treatment. It is believed that by not making the silver crystal grains excessively fine, plastic deformation of the silver plating layer occurs moderately easily, which is particularly advantageous for improving the peel resistance of the silver coating layer containing carbon and sulfur in areas subjected to severe bending processing. There is no particular upper limit for the average crystallite diameter, but it may be, for example, 120 nm or less. The average thickness of the silver coating layer in the silver-coated material according to the present invention is preferably 0.5 to 5 μm, and more preferably 0.7 to 3 μm, as measured, for example, with a fluorescent X-ray film thickness gauge.

A typical form of the silver-coated material according to the present invention is a sheet material (silver-coated metal sheet material) having a silver coating layer on at least one surface of the metal plate. The sheet thickness can be, for example, 0.05 to 3.5 mm, and is more preferably 0.1 to 3.0 mm. Here, "sheet material" refers to a sheet-like metal material. Thin sheet-like metal materials are sometimes called "foils," and such "foils" are also included in the "sheet material" referred to here. Long sheet-like metal materials wound into a coil are also included in the "sheet material." The thickness of the sheet-like metal material is called the "sheet thickness."

[Electrically-carrying parts]
The above silver-coated material can be processed by a known method to obtain current-carrying parts such as connectors, switches, relays, etc., and is particularly applicable to high-voltage parts. In the current-carrying parts using the silver-coated material according to the present invention, it is effective that the above-mentioned silver-coated layer has a structure that constitutes a part that can slide against a contacting material.

[Example 1]
(Preprocessing)
A rolled plate of 67 mm x 50 mm x 0.3 mm made of oxygen-free copper (C1020, 1/2H) was prepared as the substrate. This substrate was used as the cathode and the stainless steel plate was used as the anode in an alkaline degreasing solution, and electrolytic degreasing was performed for 30 seconds at a voltage of 5 V. The substrate was then rinsed with water and immersed in a 3% aqueous sulfuric acid solution for 15 seconds for pickling. The substrate, whose surface had been cleaned in this way, was subjected to the following steps in order to produce a silver-coated material.

(Nickel base plating process)
In a dull nickel plating solution consisting of an aqueous solution containing 540 g/L of nickel sulfamate tetrahydrate, 25 g/L of nickel chloride, and 35 g/L of boric acid, electroplating was performed for 70 seconds under conditions of a solution temperature of 50° C. and a current density of 7 A/dm ² while stirring at 500 rpm with a stirrer, using the pretreated substrate as the cathode and a nickel electrode plate as the anode, to form a dull nickel undercoat plating layer on the substrate. The thickness of the undercoat nickel plating layer was measured at the center of the surface of this plate sample with a fluorescent X-ray film thickness meter (SFT-110A, manufactured by Hitachi High-Tech Science Corporation) and found to be 1 μm.

(Lower silver plating process)
Silver strike plating step In a silver strike plating solution consisting of an aqueous solution containing 3 g/L of potassium silver cyanide (K[Ag(CN) ₂ ]) and 90 g/L of potassium cyanide (KCN), the plate material sample on which the above-mentioned undercoat nickel plating layer was formed was used as the cathode, and a platinum-coated titanium electrode plate was used as the anode, and electroplating was carried out at room temperature (25°C) with a current density of 2.0 A/ ^dm2 for 10 seconds while stirring with a stirrer at 500 rpm, forming a silver strike plating layer with an average thickness of about 0.01 μm. The silver strike plating solution was then thoroughly washed away by rinsing with water.

Silver plating step In a silver plating solution consisting of an aqueous solution containing 175 g/L of potassium silver cyanide (K[Ag(CN) ₂ ]), 95 g/L of potassium cyanide (KCN), and further containing an amount of potassium selenocyanate (KSeCN) such that the selenium concentration was 37 mg/L, the plate material sample on which the silver strike plating layer was formed was used as the cathode, and the silver electrode plate was used as the anode, and electroplating was performed for 4 seconds under conditions of a solution temperature of 18°C and a current density of 7 A/ ^dm2 while stirring at 500 rpm with a stirrer, to form a silver plating layer. The thickness of the lower silver plating layer consisting of the silver plating layer and the silver strike plating layer formed here was measured at the center of the surface of this plate material sample by the fluorescent X-ray thickness meter and was found to be 0.2 μm.

(Top silver plating process)
In a silver plating solution consisting of an aqueous solution containing 175 g/L of potassium silver cyanide (K[Ag(CN) ₂ ]), 95 g/L of potassium cyanide (KCN), and 30 g/L (=0.16 mol/L) of 2-mercaptobenzothiazole sodium (C ₇ H ₄ NNaS ₂ ) as a substance corresponding to benzothiazoles or its derivatives, electroplating was performed for 18 seconds under conditions of a solution temperature of 35° C. and a current density of 7 A/dm ^{2 while stirring at 500 rpm with a stirrer in a silver plating solution consisting of an aqueous solution containing 175 g/L of potassium silver cyanide (K[Ag(CN) 2 ]), 95 g/L of potassium cyanide (KCN), and 30 g/L (=0.16 mol/L) of 2-mercaptobenzothiazole sodium (C 7 H 4 NNaS 2 )} as a substance corresponding to benzothiazoles or its derivatives, with the plate material sample on which the lower silver plating layer was formed as described above as the cathode and the silver electrode plate as the anode, to form an upper silver plating layer. The concentration of free cyan in the silver plating solution was 38 g/L. The total thickness of the lower silver plating layer and the upper silver plating layer at the center of the surface of this plate material sample was measured by the fluorescent X-ray film thickness meter described above, and was found to be 1.2 μm. In this manner, a silver-coated material having a lower silver plating layer and an upper silver plating layer on both sides of the plate material was obtained.
The thickness of each silver plating layer is shown in Table 1. The thickness of the upper silver plating layer is the value obtained by subtracting the measured value of the lower silver plating layer from the measured value of the total thickness of the lower silver plating layer and the upper silver plating layer (the same applies to each of the following examples). In this example, the thickness of the upper silver plating layer is calculated to be 1.2 μm - 0.2 μm = 1 μm.

(Heat treatment process)
The silver coating layer of the plate sample obtained in the above upper silver plating process was subjected to heat treatment using the temperature control function of the tabletop hot stirrer. Specifically, the temperature of the tabletop hot stirrer was set to 300°C, and after the temperature stabilized at the set value, the plate sample was placed on the flat plate surface of the tabletop hot stirrer, and the silver coating layer on one side of the plate sample was brought into close contact with the plate surface of the tabletop hot stirrer. 30 seconds after the start of placement, the plate sample was removed from the plate surface of the tabletop hot stirrer and allowed to cool in air at room temperature. That is, the placement time was 30 seconds. In this experiment, heating was performed from one side surface, but a measurement of the heat curve in a separate preliminary experiment confirmed that the temperature rose rapidly to the surface opposite the plate surface of the tabletop hot stirrer, and the maximum temperature Tmax of the silver coating layers on both sides was almost equal to the set temperature of the tabletop hot stirrer, and the time during which the silver coating layers on both sides were held in a temperature range of 250°C or higher and Tmax (°C) or lower was almost the same as the placement time. Therefore, in this example, the time during which both silver coating layers are held in the temperature range of 250° C. or more and 300° C. or less (Tmax) can be regarded as 30 seconds.
In this way, a silver-coated material was obtained after the heat treatment process.

The obtained silver-coated material was used as a test material and subjected to the following tests.
(180° bending test)
After bending the test plate material 180°, the bent portion was bent back to the original plate shape, and the outer and inner surfaces of the bent portion were observed to check whether peeling of the silver coating layer occurred. In this test, a test piece in which no peeling (falling off) or lifting of the silver coating layer was observed on either the outer or inner surface of the bent portion was rated as ⊚ (excellent peeling resistance), a test piece in which no peeling (falling off) of the silver coating layer was observed on either the outer or inner surface of the bent portion but at least one of the silver coating layers was slightly lifted was rated as ◯ (good peeling resistance), and a test piece in which peeling (falling off) of the silver coating layer was observed on at least one of the outer and inner surfaces of the bent portion was rated as × (poor peeling resistance). A test piece with a rating of ◯ or higher was judged to pass.
The silver-coated material obtained in this example was rated as excellent.

(Minor friction durability test)
Two sheets of silver-coated material were prepared as test materials, one of which was indented (inner R = 1.5 mm) and used as an indenter, and the other was used as a flat evaluation sample. Using a precision sliding tester (Yamazaki Seiki Kenkyusho Co., Ltd., CRS-G2050-DWA), the indenter was pressed against the evaluation sample with a constant load (5 N) while continuing a fine sliding reciprocating motion (sliding distance 0.1 mm, sliding speed 0.2 mm/s), and the number of reciprocating sliding movements when the contact resistance exceeded 0.5 mΩ was taken as the durability number of the silver-coated layer. If the durability number under these conditions is 3500 times or more, it can be determined that the silver-coated layer has excellent wear resistance against fine sliding. Therefore, those with a durability of less than 3,500 cycles were rated as × (freak abrasion resistance: insufficient), those with a durability of 3,500 or more cycles but less than 5,000 cycles were rated as ○ (freak abrasion resistance: good), and those with a durability of 5,000 cycles or more were rated as ◎ (freak abrasion resistance: excellent), and those with a rating of ○ or higher were judged to pass.
The durability of the silver-coated material obtained in this example was 3,700 cycles, and was rated as "good."

(Contact resistance measurement)
The contact resistance of the silver-coated layer on the surface of the test material was measured using the above-mentioned precision sliding test device, and the result was that the contact resistance of the silver-coated material of this example was 0.23 mΩ.

(Measurement of crystallite size)
For the silver coating layer of the test material, based on the X-ray diffraction pattern measured by an X-ray diffractometer (Rigaku Corporation, fully automatic multipurpose horizontal X-ray diffractometer, Smart Lab) using Cu-Kα radiation, the crystallite diameters in the direction perpendicular to each crystal plane of the (111) plane, (200) plane, (220) plane and (311) plane of the silver crystal were calculated from the half-width of each peak by Scherrer's formula, and the average crystallite diameter was calculated by the weighted average of the crystallite diameters of each crystal plane, weighted by the orientation ratio of each crystal plane. Here, Scherrer's constant was set to 0.9400.
For the measurement of the half width, the (111) peak appearing at 2θ of approximately 38°, the (200) peak appearing at 2θ of approximately 44°, the (220) peak appearing at 2θ of approximately 64°, and the (311) peak appearing at 2θ of approximately 77° were used.
As the orientation ratio, a value (corrected intensity) obtained by correcting the intensities of the X-ray diffraction peaks of the (111) plane, the (200) plane, the (220) plane, and the (311) plane of the silver plating film based on an X-ray diffraction pattern obtained by scanning the scanning range 2θ/θ using a Cu tube and a Kβ filter method, by the respective relative intensity ratios (relative intensity ratios at the time of powder measurement) ((111):(200):(220):(311)=100:40:25:26) listed in JCPDS Card No. 40783 was used.
As a result, the average crystallite size of the silver coating layer in the silver-coated material obtained in this example was 60.2 nm.
The above results are shown in Table 1.

[Example 2]
A silver-coated material was produced in the same manner as in Example 1, except that in the heat treatment step, the temperature of the tabletop hot stirrer was set to 350° C. and the time of placement on the tabletop hot stirrer was 5 seconds.
The silver-coated material obtained in this example was rated as ⊚ for peel resistance in a 180° bending test, and rated as ⊚ for fretting wear resistance in a fretting durability test (endurance count: 8200 times). The contact resistance was 0.23 mΩ, and the average crystallite diameter of the silver coating layer was 25.0 nm.

[Example 3]
A silver-coated material was produced in the same manner as in Example 1, except that in the heat treatment step, the set temperature of the tabletop hot stirrer was 350° C. and the time of placement on the tabletop hot stirrer was 10 seconds.
The silver-coated material obtained in this example was rated as ⊚ for peel resistance in a 180° bending test, and rated as ⊚ for fretting wear resistance in a fretting durability test (10,000 cycles). The contact resistance was 0.18 mΩ, and the average crystallite diameter of the silver coating layer was 88.8 nm.

[Comparative Example 1]
The silver-coated material was used as a test material after the upper silver plating step was completed in the same manner as in Example 1, without carrying out the heat treatment step.
The silver-coated material obtained in this example was rated x for peel resistance in a 180° bending test, and rated ⊚ for fretting wear resistance in a fretting durability test (endurance count 6000 times). The contact resistance was 0.29 mΩ, and the average crystallite diameter of the silver coating layer was 16.6 nm.
It is clear that without heat treatment, the peel resistance of the silver coating layer at the site subjected to severe bending is not improved.

[Comparative Example 2]
A silver-coated material was produced in the same manner as in Example 1, except that in the lower silver plating step, only a silver strike plating layer was formed and the formation of a main silver plating layer was omitted, and the heat treatment step was omitted.
The silver-coated material obtained in this example was rated x for peel resistance in a 180° bending test, and rated ◯ for fretting wear resistance in a fretting durability test (durability of 4,800 cycles).
This silver-coated material corresponds to the technology disclosed in Patent Document 1. In this case, it is found that the peel resistance of the silver-coated layer is poor at the portion subjected to severe bending.

[Comparative Example 3]
A silver-coated material was produced in the same manner as in Example 1, except that in the lower silver plating step, only a silver strike plating layer was formed and the formation of a main silver plating layer was omitted, and in the heat treatment step, the set temperature of the tabletop hot stirrer was 300° C. and the time for which the material was placed on the tabletop hot stirrer was 10 seconds.
The silver-coated material obtained in this example was rated as ◯ for peel resistance in a 180° bending test, and rated as × for fretting wear resistance in a fretting durability test (durability of 1800 cycles).
The silver strike plating layer formed in this example is very thin, with an average thickness of about 0.01 μm, and is therefore not considered to be the lower silver plating layer. In the case where the lower plating layer was not formed, it was not possible to achieve both the improvement in peel resistance and the improvement in fretting wear resistance at the site subjected to severe bending, even when the heat treatment process was performed.

[Comparative Example 4]
A silver-coated material was produced in the same manner as in Example 1, except that the lower silver plating step and the upper silver plating step were changed to the silver plating step described below, and the heat treatment step was omitted.

(Silver plating process)
A silver strike plating layer was formed in the same manner as in Example 1.
Next, in a silver plating solution containing 175 g/L potassium silver cyanide (K[Ag(CN) ₂ ]), 95 g/L potassium cyanide (KCN), and an amount of potassium selenocyanate (KSeCN) such that the selenium concentration was 69 mg/L, electroplating was performed with the plate material sample on which the silver strike plating layer was formed as the cathode and the silver electrode plate as the anode under conditions of a solution temperature of 18°C, a current density of 5 A/ ^dm2 , and a current application time of 120 seconds while stirring at 500 rpm with a stirrer, to form a silver plating layer. The thickness of the silver coating layer consisting of the silver plating layer and the silver strike plating layer formed here was measured at the center of the surface of this plate material sample by the fluorescent X-ray thickness meter and found to be 5 μm.
This silver coating layer is not a plating layer intended to introduce carbon or sulfur, but a known bright silver plating layer, and therefore, for convenience, the thickness is shown in the "Lower Silver Plating Layer" column in Table 1 (the same applies to Comparative Example 5 described below).

The silver-coated material obtained in this example was rated as ⊚ for peel resistance in a 180° bending test, and rated as x for fretting wear resistance in a fretting durability test (3000 cycles). The contact resistance was 0.20 mΩ, and the average crystallite diameter of the silver coating layer was 27.8 nm.
It can be seen that when the plating solution does not contain a silver coating layer obtained by using benzothiazoles or their derivatives, the fretting wear resistance is poor.

[Comparative Example 5]
A silver-coated material was produced in the same manner as in Comparative Example 4, except that a silver plating solution containing potassium selenocyanate (KSeCN) in an amount giving a selenium concentration of 37 mg/L was used, and the current density during electroplating was changed from 5 A/ ^dm2 to 7 A/ ^dm2 and the current application time was changed from 120 seconds to 90 seconds. The thickness of the silver coating layer formed here, consisting of the silver plating layer and the silver strike plating layer, was measured at the center of the surface of this sheet material sample by the above-mentioned fluorescent X-ray film thickness meter and was found to be 5 μm.
The silver-coated material obtained in this example was rated as ⊚ for peel resistance in a 180° bending test, and rated as x for fretting wear resistance in a fretting durability test (endurance count 1500 times). The contact resistance was 0.18 mΩ, and the average crystallite diameter of the silver coating layer was 75.0 nm.
As in Comparative Example 4, this example also shows that when the plating solution does not contain a silver coating layer obtained by using benzothiazoles or their derivatives, the fretting wear resistance is poor.

<Measurement of element concentration profile of silver coating layer by XPS>
For reference, the results of element concentration profile measurement in the depth direction by XPS (X-ray photoelectron spectroscopy) for the silver coating layers obtained in Comparative Example 1 (no heat treatment) and Example 3 (heat treatment conditions: maximum temperature of 350° C., holding time in the temperature range of 250° C. or higher and maximum temperature of 10 seconds) are shown below. The measurement was performed as follows.

From the outermost surface of the silver coating layer of the test material, the element concentration profile in the depth direction was measured by XPS for each of the elements C, O, S, Ag, Ni, and Cu in Comparative Example 1, and for each of the elements C, K, O, N, S, Ag, and Ni in Example 3.
The X-ray photoelectron spectrometer used was a PHI5000 VersaProbeIII manufactured by ULVAC-PHI, Inc. The measurement was performed under the following conditions: ultimate vacuum: 10 ⁻⁷ Pa, excitation source: monochromatic AlKα, output: 25 W, acceleration voltage: 15 kV, beam size: 100 μmΦ, incidence angle: 90 deg, and irradiation with an electron beam from an electron neutralization gun at an emission current: 20 μA, bias voltage: 1.0 V, and acceleration voltage: 30.0 V, and with argon ions from an argon ion gun at an ion species: Ar ⁺ , acceleration voltage: 0.11 kV, and emission current: 7 mA, with a photoelectron take-off angle: 45 deg, number of integrations: 5, integration time: 40 ms (20 ms×2), pass energy: 140 eV, and measurement energy interval: 0.25 eV/step.
Surface sputtering for analysis in the depth direction was performed using an argon ion gun under the following conditions: ion species: Ar ⁺ , acceleration voltage: 4 kV, emission current: 20 mA, sweep area: 2.7 mm × 2.7 mm, sputtering rate: 20 nm/min ( _SiO2 equivalent). The intervals of sputtering time to adjust to each measurement depth were 1 min intervals until the cumulative sputtering time reached 20 min, and 4 min intervals thereafter in each example.
The spectral species used to determine the atomic concentration were the 3d orbital binding energy peak (Ag3d) for Ag, the 1s orbital binding energy peak (C1s) for C, and the 2p orbital binding energy peak (S2p) for S, and the Shirley method was used for background processing.

3 and 4 show element concentration profiles in the depth direction by XPS for Comparative Example 1 and Example 3, respectively. The above-mentioned points _P1 , _P2 , and Q are shown in the figures. Table 2 summarizes the evaluation based on these element concentration profiles in the depth direction.

REFERENCE SIGNS LIST 1 Substrate 2 Undercoat plating layer 3 Silver strike plating layer 4 Silver plating layer 10 Material 20 Lower silver plating layer 30 Upper silver plating layer 40 Silver coating layer

Claims

a lower silver plating step of forming a lower silver plating layer on the base material using a silver plating solution that does not contain benzothiazoles and their derivatives;
an upper silver plating step of forming an upper silver plating layer on the lower silver plating layer by electroplating using a silver plating solution containing one or more substances selected from benzothiazoles and derivatives thereof;
a heat treatment step of holding the lower silver plating layer and the upper silver plating layer at a temperature range of 250 to 400° C. for 3 to 60 seconds;
A method for producing a silver-coated material comprising the steps of:
The method for manufacturing a silver-coated material according to claim 1, wherein the average thickness of the lower silver plating layer is 0.06 to 3.0 μm.
The method for manufacturing a silver-coated material according to claim 1, wherein the average thickness of the upper silver plating layer is 0.3 to 10.0 μm.
The method for producing a silver-coated material according to claim 1, wherein the silver plating solution used in the upper silver plating step contains one or more substances selected from benzothiazoles and their derivatives at a concentration of 0.01 to 0.80 mol/L.
The method for producing a silver-coated material according to claim 1, wherein in the upper silver plating process, the one or more substances selected from benzothiazoles and their derivatives are one or more substances selected from mercaptobenzothiazole and its derivatives.
The method for producing a silver-coated material according to claim 1, wherein in the upper silver plating step, the one or more substances selected from benzothiazoles and their derivatives are one or more substances selected from benzothiazoles and their alkali metal salts.
The method for manufacturing a silver-coated material according to claim 1, wherein the material subjected to the lower silver plating process has a base material of copper or a copper alloy.
The method for manufacturing a silver-coated material according to claim 1, wherein the material subjected to the lower silver plating process has a nickel plating layer on the surface on which the lower silver plating layer is formed.
The method for manufacturing a silver-coated material according to claim 1, wherein the lower silver plating layer is composed of a silver strike plating layer and a silver plating layer thereon.
A silver-coated material having a silver coating layer formed on the surface of a material having a copper or copper alloy substrate, wherein, in a depth direction element concentration profile of the silver coating layer by XPS (X-ray photoelectron spectroscopy), a cumulative sputtering time at a depth position where the atomic ratio of silver decreases to 1/2 of the maximum value is t 0 (minutes), a maximum point of the atomic ratio of silver in a region (the outermost surface side of the sample) where the cumulative sputtering time is shorter than t 0 /2 on a silver profile curve is point P 1 , a maximum point of the atomic ratio of silver in a region (the center side of the sample) where the cumulative sputtering time is longer than t 0 /2 is point P 2 , and a minimum point of the atomic ratio of silver between points P 1 and P 2 is point Q, a ratio Ag(Q)/Ag(P 1 ) of the atomic ratio of silver at point Q to the atomic ratio of silver at point P 1 is 0.90 or less, and a ratio Ag(Q)/Ag(P 1 ) of the atomic ratio of silver at point P 2 is 0.90 or less. a ratio Ag(Q)/Ag( P2 ) of 0.90 or less, a C/Ag atomic ratio at a depth position corresponding to point Q of 0.15 or more, and a C/Ag atomic ratio at a depth position corresponding to point P2 of 0.10 or less.
The silver-coated material according to claim 10, wherein the average crystallite size of the silver in the silver-coated layer is 20 nm or more.
An electrically conductive part made using the silver coating material described in claim 10 or 11.