COMPOSITE MATERIAL, METHOD FOR PRODUCING THE COMPOSITE MATERIAL, AND TERMINAL

Description

TECHNICAL FIELD

The present invention relates to a composite material in which a predetermined composite coating is provided on a base material, a method for producing the same, etc., and particularly, relates to a composite material used as a material for sliding electrical contact parts such as switches and connectors, and a method for producing the same.

DESCRIPTION OF RELATED ART

Conventionally, a silver (Ag) plated material with silver plating applied to a conductive material, is used as a material for sliding electrical contact parts such as switches and connectors, to prevent oxidation of the conductive material such as copper (Cu) and a copper alloy due to heating during a sliding process.

However, silver plating is soft and easily worn, and generally has a high friction coefficient, thus involving a problem that it easily peels off due to sliding. In order to solve this problem, there is a method of improving wear resistance by applying a coating of a composite material onto a conductive material by electroplating, in which the composite material is composed of a silver matrix where graphite particles that are one of the carbon particles such as graphite and carbon black that have excellent wear resistance and lubricity are dispersed (for example, see Patent documents 1 and 2).

Also, the applicant performed research aimed at providing a composite material with excellent wear resistance, and achieved a composite material having a composite coating (AgC layer) with a small silver crystallite size and therefore being hard and having excellent wear resistance by performing electroplating using a silver plating solution containing specific components, and disclosed this composite material in Patent document 3.

PRIOR ART DOCUMENT

Patent Document

• [Patent Document 1] Japanese Patent Application Publication No. H9-7445
• [Patent Document 2] Japanese Patent Application Publication No. 2007-16250
• [Patent Document 3] WO2021/261066 Pamphlet

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

As described above, the composite material (composite coating) disclosed in Patent document 3 has excellent wear resistance.

However, further investigation by the present inventors has revealed that the technique disclosed in Patent document 3 does not provide sufficient smoothness to the surface of the composite coating, and for example, when the composite material is bent into the shape of a terminal using a mold, the convex portion of the surface of the composite coating comes into strong contact with the mold, and stress is concentrated in this portion and shedding of this portion may occur. If this happens, silver that has been shed could become a contamination (contamination source) and contaminate equipment during use of the composite material by a user (such as during bending).

The present invention was made under the above circumstances, and an object of the present invention is to provide a composite material in which a composite coating containing carbon particles in a silver layer is provided on a base material, which is the composite material having excellent wear resistance and being suppressed from shedding of silver from the composite coating (AgC layer) during bending.

Means for Solving the Problem

As a result of an extensive research by the present inventors to achieve the above object, it has been found that a composite material having excellent wear resistance and being able to suppress shedding of silver from the composite coating during bending, can be obtained by forming a composite coating by pulse plating using a specific plating solution similar to that described in Patent document 3. Thus, the present invention has been completed.

That is, the present invention is as follows.

• • [1] A composite material in which a composite coating composed of a silver layer containing carbon particles is provided on a base material, the composite coating having a silver crystallite size of more than 40 nm and not more than 70 nm and having an arithmetic mean roughness Ra (μm) of 2.0 μm or less.
  • [2] The composite material according to [1], wherein the base material is composed of Cu or a Cu alloy.
  • [3] The composite material according to [1] or [2], wherein a surface of the composite coating has a Vickers hardness of 90 or more.
  • [4] The composite material according to any one of [1] to [3], wherein the composite coating has a silver crystallite size of 50 to 66 nm.
  • [5] The composite material according to any one of [1] to [4], wherein an underlayer composed of at least one selected from the group consisting of Cu, Ni, Sn, and Ag is provided between the base material and the composite coating.
  • [6] A method for producing a composite material, the method comprising: forming a composite coating composed of a silver layer containing carbon particles on a base material by performing electroplating in a silver plating solution containing the carbon particles and containing a compound A represented by the following general formula (I),

• •  (In general formula (I), m is an integer from 1 to 5;
    • Ra is a carboxyl group;
    • Rb is an aldehyde group, a carboxyl group, an amino group, a hydroxyl group, or a sulfonic acid group;
    • Rc is hydrogen or an arbitrary substituent;
    • when m is 2 or more, a plurality of Rb's may be the same or different;
    • when m is 3 or less, a plurality of Rc's may be the same or different; and
    • Ra and Rb may each independently be bonded to a benzene ring via a divalent group composed of at least one selected from the group consisting of —O— and —CH₂—), wherein pulse plating is performed as the electroplating.
  • [7] The method for producing a composite material according to [6],
    • wherein during the pulse plating, alternate repeat is performed to a composite coating deposition step of depositing silver on the base material to form a silver matrix containing the carbon particles and a composite coating dissolution step of dissolving a part of the silver matrix of the composite coating, and the following conditions are adopted for this pulse plating,
    • the composite film deposition step: applying a current at a current density of 1.5 to 5.0 A/dm²;
    • the composite coating dissolution step: applying a current at a current density of −12 to −2.5 A/dm²;
    • wherein alternate repeat count of the composite coating deposition step and the composite coating dissolution step is 300 to 10,000 times, and
    • a ratio (Tp/Td) is 2 to 8, which is the ratio of time period Tp for applying a current in the composite coating deposition step with respect to time period Td for applying a current in the composite coating dissolution step.
  • [8] The method for producing a composite material according to [6] or [7], wherein a concentration of the carbon particles in the silver plating solution is 10 g/L or more and 150 g/L or less.
  • [9] A terminal for electrical contacts, the terminal comprising the composite material according to any one of [1] to [5] as a constituent material.

Advantage of the Invention

The present invention provides a composite material having a composite coating provided on a base material, the composite coating containing carbon particles in a silver layer and having excellent wear resistance and suppressing a shedding of silver from the composite coating during bending.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a test for evaluating shedding of silver during bending according to an example.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment of the present invention will be described.

[1. Method for Producing a Composite Material]

In the method for producing a composite material of the present invention, pulse plating is performed as electroplating in a silver plating solution containing carbon particles and specific compounds, thereby forming a composite coating on a base material, the composite coating containing carbon particles in a silver layer. Each step of the method for producing the composite material will now be described.

<<1-1. Base Material>>

As a base material on which the composite coating is provided, it is preferable to use a material that can be plated with silver and has an electrical conductivity required for sliding contact parts such as switches and connectors, and further, from the viewpoint of a cost, Cu (copper) and Cu alloys are preferable as the constituent material. As the Cu alloy, from the viewpoints of electrical conductivity, strength, etc., an alloy composed of Cu and 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 inevitable impurities is preferable. An amount of Cu in the Cu alloy is preferably 85 mass % or more, and more preferably 92 mass % or more (the amount of Cu is preferably 99.95 mass % or less).

The base material is preferably used for terminal applications (as a composite material with a composite coating formed thereon) as described below, in which the base material itself may have a shape for that application, or the base material may have a flat shape (such as a plate shape) and then once it is formed into a composite material, it may be molded into the shape required for the intended use in some cases. In order to obtain the effects of the present invention, the base material preferably has a flat shape. In this case, long, flat-shaped materials can be electroplated together (continuously). This exhibits excellent productivity for composite materials. Further, if the arithmetic mean roughness Ra (μm) of the base material is large, the arithmetic mean roughness Ra (μm) of the composite coating formed thereon may also be large, and therefore it is preferable that the Ra of the base material is small, specifically, 0.5 μm or less. The Ra of the base material is preferably 0.05 to 0.2 μm.

<<1-2. Formation of an Underlayer>>

In the method for producing a composite material of the present invention, an underlayer may be formed on the base material, and the underlayer may be subjected to electroplating, which will be described later. The underlayer is formed for the purpose of preventing the copper of the base material from diffusing into and reaching the plating surface and oxidizing, which would cause a deterioration in the electrical conductivity of the composite material, and for the purpose of improving the adhesion of the composite coating. The constituent metal of the underlayer is at least one metal or an alloy selected from the group consisting of Cu, Ni, Sn, and Ag. The underlayer may be a single layer of Cu, Ni, Sn, Ag, or an alloy thereof, or a combination of these (a layered structure), and the underlayer may be formed on an entire surface of the base material or only on a part of it, depending on the application of the composite material to be produced.

The method for forming the underlayer is not particularly limited, and the underlayer can be formed by electroplating using a plating solution containing ions of the above-described constituent metals in a known manner, or by sequentially laminating layers composed of each metal that constitutes a target alloy layer and then applying reflow (heat treatment) thereto. From the viewpoint of a wastewater treatment cost, it is preferable that the plating solution does not substantially contain cyanide compounds.

<<1-3. Ag Strike Plating>>

Before forming the composite coating on the base material, it is preferable to form a very thin intermediate layer by Ag strike plating to improve adhesion between the base material and the composite coating. When the underlayer is formed on the base material, it is preferable to perform Ag strike plating on the underlayer to improve adhesion between the underlayer and the composite coating. As a method for performing Ag strike plating, any conventionally known method can be used without any particular limitation as long as it does not impair the effects of the present invention. It is preferable that the plating solution used in Ag strike plating does not substantially contain cyanide compounds in terms of a wastewater treatment cost.

<<1-4. Electroplating (Pulse Plating)>>

In the method for producing a composite material of the present invention, a composite coating containing carbon particles in a silver layer is formed on the base material by pulse-plating the above-described base material as electroplating in a specific silver plating solution after forming an underlayer and/or an intermediate layer by Ag strike plating as necessary.

<1-4-1. Silver Plating Solution>

The silver plating solution contains silver ions, a specific compound A, and carbon particles.

(1-4-1-1. Silver Ions)

The silver plating solution contains silver ions. The silver concentration in this silver plating solution is preferably 5 to 150 g/L, more preferably 10 to 120 g/L, and most preferably 20 to 100 g/L, from the viewpoints of the rate of formation of the composite coating and of suppressing unevenness in the appearance of the composite coating.

(1-4-1-2. Compound A)

Next, compound A is represented by the following general formula (I).

In the general formula (I), m is an integer of 1 to 5; Ra is a carboxyl group; Rb is an aldehyde group, a carboxyl group, an amino group, a hydroxyl group, or a sulfonic acid group; Rc is hydrogen or an arbitrary substituent, and Ra and Rb may each independently be bonded to a benzene ring via a divalent group composed of at least one selected from the group consisting of —O— and —CH₂—. Examples of the divalent group include —CH₂—CH₂—O—, —CH₂—CH₂—CH₂—O—, and (—CH₂—CH₂—O—) n (n is an integer of 2 or more).

It is considered that compound A is adsorbed on the surface of deposited silver and inhibits the growth of silver crystals, thereby reducing a silver crystallite size in the composite coating that is formed by electroplating. This results in a composite material having excellent hardness and therefore excellent wear resistance.

Further, in the above general formula (I), when m is 2 or more, a plurality of Rb's may be the same or different, and when m is 3 or less, a plurality of Rc's may be the same or different. Regarding Rc, the “arbitrary substituent” includes an alkyl group having 1 to 10 carbon atoms, an alkylaryl group, an acetyl group, a nitro group, a halogen group, and an alkoxyl group having 1 to 10 carbon atoms.

The concentration of compound A in the silver plating solution is preferably 2 to 250 g/L, and more preferably 3 to 200 g/L, from the viewpoints of suppressing unevenness in the appearance of the composite coating and appropriately controlling the silver crystallite size in the composite coating to be formed.

(1-4-1-3. Carbon Particles)

The silver plating solution contains carbon particles. When the silver plating solution contains carbon particles, the carbon particles are entrapped in the silver matrix when the composite coating (silver plating film) is formed on the base material by electroplating. The inclusion of the carbon particles in the composite coating increases the wear resistance of the composite material. From the viewpoint of exerting such a function, carbon particles are preferably graphite particles. The volume-based cumulative 50% particle size (D50) of the carbon particles, measured using a laser diffraction/scattering particle size distribution analyzer, is preferably 0.5 to 15 μm, and more preferably 1 to 10 μm, from the viewpoint of ease of entrapment in the silver plating film. Further, the shape of the carbon particles is not particularly limited and may be substantially spherical, flaky, or amorphous. However, a flaky shape is preferable because it can improve the wear resistance of the composite material by smoothing the surface of the composite coating. “The carbon particles being entrapped in the silver matrix” includes not only the case where the carbon particles are completely embedded in the silver matrix, but also the case where only a portion of the carbon particle is included in the silver matrix and a portion of the carbon particle is exposed outside the silver matrix.

Further, the amount of the carbon particles in the silver plating solution is preferably 10 to 150 g/L, more preferably 15 to 120 g/L, and most preferably 30 to 100 g/L, from the viewpoint of the wear resistance of the composite material obtained by forming the composite coating on the base material using a silver plating solution, and because the amount of carbon particles that can be introduced into the composite coating is limited.

Further, it is preferable to remove a lipophilic organic matter adsorbed on the surface of the carbon particles by subjecting the carbon particles to an oxidation treatment before they are introduced into the plating solution. Such a lipophilic organic matter includes aliphatic hydrocarbons such as alkanes and alkenes, and aromatic hydrocarbons such as alkylbenzenes. As the oxidation treatment of carbon particles, in addition to wet oxidation treatment, dry oxidation treatment using O₂ gas, etc., can be used, but from the viewpoint of mass productivity, it is preferable to use the wet oxidation treatment, which allows carbon particles having a large surface area to be uniformly treated. As a method for the wet oxidation treatment, a method of suspending carbon particles in water and then adding an appropriate amount of an oxidizing agent, can be used. The oxidizing agent that can be used includes nitric acid, hydrogen peroxide, potassium permanganate, potassium persulfate, sodium perchlorate, etc. It is considered that the lipophilic organic matter adhered to the carbon particles is oxidized by the added oxidizing agent to become easily soluble in water and is appropriately removed from the surface of the carbon particles. Further, after the wet oxidation treatment, filtration and further cleaning of the carbon particles with water can further enhance the effect of removing the lipophilic organic matter from the surface of the carbon particles. Due to the oxidation treatment of the carbon particles, the lipophilic organic matter such as aliphatic and aromatic hydrocarbon can be removed from the surface of the carbon particles, and according to an analysis of 300° C. heated gas, the gas generated by heating the carbon particles after oxidation treatment at 300° C. contains almost no lipophilic aliphatic hydrocarbons such as alkanes and alkenes, or lipophilic aromatic hydrocarbons such as alkylbenzenes. Even when the carbon particles after oxidation treatment contain small amounts of aliphatic or aromatic hydrocarbons, the carbon particles can be uniformly dispersed in the silver plating solution used in the present invention, but it is preferable that the carbon particles do not contain any hydrocarbons having a molecular weight of 160 or more, and that the gas generation intensity of hydrocarbons having molecular weights of less than 160 in carbon particles heated at 300° C. (purge-and-trap gas chromatograph mass spectrometry intensity) is 5,000,000 or less.

(1-4-1-4. Complexing Agent)

The silver plating solution used in the present invention preferably contains a complexing agent. The complexing agent forms a complex with silver ions in the silver plating solution, increasing their stability as ions, which increases the solubility of silver in the solvent contained in the plating solution.

A wide variety of complexing agents having the above-described functions can be used, but from the viewpoint of the stability of the complex to be formed, compounds having a sulfonic acid group are preferable. Examples of the compounds having a sulfonic acid group include alkylsulfonic acids having 1 to 12 carbon atoms, alkanolsulfonic acids having 1 to 12 carbon atoms, and hydroxyarylsulfonic acids. Specific examples of these compounds s include methanesulfonic acid, 2-propanolsulfonic acid, and phenolsulfonic acid.

The amount of the complexing agent in the silver plating solution is preferably 30 to 200 g/L, and more preferably 50 to 120 g/L, from the viewpoint of stabilizing the silver ions.

(1-4-1-5. Other Additives)

As other additives, for example, the silver plating solution used in the present invention may contain brighteners, hardeners, and conductive salts. Examples of the hardeners include carbon sulfide compounds (such as carbon disulfide), inorganic sulfur compounds (such as sodium thiosulfate), organic compounds (sulfonates), selenium compounds, tellurium compounds, and metals from group 4B or 5B of a periodic table. The conductive salt may be potassium hydroxide, etc.

(1-4-1-6. Solvent)

The solvent contained in the silver plating solution is mainly water. Water is preferable because it has excellent solubility for (complexed) silver ions and other components contained in the plating solution, and it places little strain on the environment. Further, a mixed solvent of water and alcohol may be used as the solvent.

<1-4-2. Pulse Plating Conditions>

In the present invention, pulse plating is performed as electroplating using the silver plating solution described above. The pulse plating performed in the present invention is a plating process in which the sign of the applied current is periodically (alternately) reversed. This type of pulse plating alternates between the formation of the plating film (deposition of metal silver while entrapping the carbon particles) and dissolution, resulting in the formation of a smooth composite coating with little unevenness. Further, due to the function of compound A, the silver crystallite size in the composite coating is suppressed to be small.

The inventors of the present invention have investigated the reason why the surface smoothness of the composite coating is insufficient in the technique disclosed in Patent document 3, and have concluded as follows. When the silver plating solution contains a specific additive (a benzoic acid-based compound) as in Patent document 3, it is considered that plating progresses, with the additive adsorbed on the carbon particles. It can be considered that on the surface of the composite coating being formed, carbon particles are present, that are entrapped in a silver matrix with some part exposed, the additive is adsorbed on the carbon particles, and Ag is deposited and grows on the exposed portions of the carbon particles where the additive is adsorbed. Normally, Ag is deposited on the Ag matrix that is being formed, but it is considered that the additive adsorbed on the carbon particles becomes a starting point for Ag deposition.

As the deposition of Ag progresses in this manner, the parts that grow from the Ag deposited on the carbon particles become convex parts, that is, it is considered that a composite coating is formed that has an uneven, non-smooth surface. During the formation of the silver matrix containing carbon particles (this is called a composite coating deposition step) during the pulse plating of the present invention, it is considered that the same thing as described above occurs because the silver plating solution contains a specific benzoic acid compound, as in Patent document 3. However, due to the step of dissolving a portion of the silver matrix by the negative current of the pulse plating (hereinafter referred to as the composite film dissolution step), both the Ag deposited on the carbon particles, which is the starting point for the formation of the above-described convex parts, and the silver matrix formed by the Ag deposited and grown on the base material, are dissolved. Further, regarding the growth of Ag deposited on the carbon particles and the growth of the silver matrix, the latter is considered to be faster. It is considered that due to alternate repeat of such a composite coating dissolution step and composite coating deposition step, a composite coating having a smooth surface is formed in the present invention. Hereinafter, an embodiment of the pulse plating according to the present invention will be described in detail.

(1-4-2-1. Cathode and Anode)

The base material to be electroplated is a cathode. What dissolves to provide silver ions, e.g. a silver electrode plate is an anode.

(1-4-2-2. Current Density)

The cathode and the anode are immersed in a silver plating solution (plating bath), and a current is applied to perform silver plating. Regarding the plating current here, the sign of the current is set as positive during the composite coating deposition step, and the sign of the current is set as negative during the composite coating dissolution step.

In the composite coating deposition step, the current density is preferably 1.5 to 5.0 A/dm², and more preferably 2.0 to 4.0 A/dm², from the viewpoints of the rate of formation of the composite coating and of suppressing unevenness in the appearance of the composite coating. In the composite coating dissolution step, the current density is preferably −12 to −2.5 A/dm², and more preferably −11 to −6 A/dm² from the viewpoint of the productivity of the composite coating in an entire pulse plating and the smoothness of the composite coating to be formed.

From the viewpoint of forming a smooth composite coating and the productivity of the composite coating, an absolute value of the ratio (Ed/Ef) of the current density Ed in the composite coating dissolution step with respect to the current density Ef in the composite coating deposition step is preferably 1 to 10, more preferably 1.5 to 7, and particularly preferably 2 to 5. The ratio (Ed/Ef) may be varied within the above range for each alternate repeat of the two steps during the pulse plating.

(1-4-2-3. Pulse Waveform, Alternate Repeat Count of the Deposition Step and the Dissolution Step, and Time Allocation of Both Steps)

There is no particular limitation in the waveform when performing pulse plating, and for example, a sine wave or a square wave can be adopted. The alternate repeat count of the composite coating deposition step and the composite coating dissolution step can be appropriately adjusted depending on the thickness of the composite coating to be formed and the allocation of the time for each of the two steps, but can be set to, for example, 300 to 10,000 times (meaning that, when the alternate repeat of the composite coating deposition step and the dissolution step is counted as one cycle, this repeat is performed 300 to 10,000 times). The repeat count is preferably 800 to 6,000 times.

Further, the ratio (Tp/Td) of the time period Tp for applying a current in each composite film deposition step with respect to the time period Td for applying a current in each composite film dissolution step is preferably 2 to 8, and more preferably 3 to 6, from the viewpoint of achieving both the productivity and smoothness of the composite coating. Due to the above-described alternate repeat of the two steps, the composite coating is formed, and the ratio (Tp/Td) may be varied within the above-described range for each alternate repeat of the above two steps (one composite coating deposition step and one composite coating dissolution step are considered as one step).

Further, the time period Tp is preferably 0.2 to 2 seconds, and more preferably 0.4 to 1.2 seconds. The time period Td is preferably 0.05 to 1 second, and more preferably 0.1 to 0.4 seconds. The time periods Tp and Td may also be varied within the above-described ranges for each alternate repeat of the composite coating deposition step and the dissolution step.

(1-4-2-4. Temperature, Stirring, and Plating Target Area)

The temperature (plating temperature) of the plating bath (silver plating solution) when performing pulse plating 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. In this case, the stirring speed of the plating bath is preferably 200 to 550 rpm, and more preferably 350 to 500 rpm, from the viewpoint of performing uniform plating. A plating target area may be an entire surface layer of the base material or a part of the surface layer of the base material, depending on the application of the composite material to be produced.

<<1-5. Partial Removal of Carbon Particles from the Surface of the Composite Coating>>

By the electroplating (pulse plating) described above, the composite coating is formed on the base material. In the surface of this composite coating, there are carbon particles that are entrapped and embedded in the silver matrix and are difficult to shed, and there are carbon particles that are adhered to the surface rather than being entrapped in the silver matrix and are therefore more likely to shed. The latter can contaminate equipment, for example during bending of the composite material. Therefore, it is preferable to remove such carbon particles by cleaning. One cleaning method is to subject the surface of the composite coating to ultrasonic cleaning. The ultrasonic cleaning is preferably performed at 20 to 100 kHz for 1 to 300 seconds. Another cleaning method is electrolytic cleaning. In this case, the electrolytic cleaning is preferably performed at 1 to 30 A/dm² for 10 to 300 seconds.

[2. Composite Material]

Hereinafter, an embodiment of the composite material of the present invention will be described. The composite material is a composite material in which a composite coating composed of a silver layer containing carbon particles is formed on a base material, the composite coating having a silver crystallite size of more than 40 nm and 70 nm or less and having an arithmetic mean roughness Ra (μm) of 2.0 μm. This composite material can be produced, for example, by the method for producing the composite material of the present invention. Each component of this composite material will now be described.

<<2-1. Base Material>>

The base material is similar to those described above for the method for producing a composite material of the present invention. That is, Cu (copper) and a Cu alloy are suitable as the constituent materials of the base material, and the Cu alloy is preferably an alloy made of Cu and 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 inevitable impurities, from the viewpoint of electrical conductivity and strength.

<<2-2. Underlayer>>

An underlayer may be formed between the base material and the composite coating for various purposes. The constituent metal of the underlayer is at least one metal or alloy selected from the group consisting of Cu, Ni, Sn, and Ag. For example, in order to prevent copper in the base material from diffusing into and reaching the surface of the composite coating, thereby causing a deterioration in electrical conductivity, it is preferable to form an underlayer composed of Ni. When the base material is a copper alloy containing zinc, such as brass, it is preferable to form an underlayer composed of Cu in order to prevent the zinc in the base material from diffusing into and reaching the surface of the composite coating. For the purpose of improving the adhesion of the composite coating to the base material, it is preferable to form an underlayer composed of Ag. The thickness of the underlayer is not particularly limited, but from the viewpoints of function and cost, it is preferably 0.1 to 2 μm, and more preferably 0.2 to 1.5 μm. Further, terminals of electric and electronic components are often composed of Sn-plated or reflow Sn-plated materials including Cu or Ni undercoat (having a layered structure of Cu undercoat, Ni undercoat, Sn undercoat from the base material side), and such a layered underlayer may also be formed in the present invention. Accordingly, in the present invention, the composite coating may have a single layer of Cu, Ni, Sn, Ag or an alloy thereof or a combination of these layers (layered structure) as undercoat, and also, different layers may be formed in different locations as follows: for example, a composite coating as defined in the present invention is formed on the electrical contact part of the base material (the underlayer may or may not be formed) and a reflow Sn-plated underlayer is formed on a wire crimping part (without forming the composite coating).

<<2-3. Composite Coating>>

The composite coating provided on the base material is composed of a silver layer containing carbon particles. In this silver layer, carbon particles are dispersed (preferably substantially uniformly) in a matrix composed of silver. When Ag strike plating is performed before forming the composite coating, an intermediate layer resulting from this strike plating exists between the base material (or the underlayer described below) and the composite coating, but in many cases this intermediate layer is so thin that it cannot be distinguished from the composite coating. The composite coating may be formed on an entire surface, or on only a portion of the surface of the base material.

<2-3-1. Crystallite Size and Vickers Hardness>

The silver crystallite size in the composite coating in the embodiment of the composite material of the present invention is relatively small, more than 40 nm and 70 nm or less. Such a small crystallite size results in a composite coating with high hardness due to a Hall-Petch relationship (in general, the smaller the crystal grains of a metal material, the stronger it is). High hardness makes the composite coating less susceptible to wear, improving the wear resistance of the composite material. Although the hardness may be slightly lower than that of the composite coating of the composite material disclosed in Patent document 3, the composite coating of the composite material of the present invention also has a wear resistance that does not pose any practical problem. When the crystallite size is too small, the composite coating will have high hardness and excellent wear resistance, but will have many crystal grain boundaries, which will easily cause diffusion of constituent metals such as Cu from the base material into the composite coating when heated, resulting in reduced electrical conductivity (poor reliability). For this reason, in the composite material of the present invention, the composite coating has a silver crystallite size of more than 40 nm. From the viewpoint of achieving both wear resistance and reliability, the crystallite size is preferably 45 to 68 nm, and more preferably 50 to 66 nm.

In the present invention, the silver crystallite size is determined by averaging (adding up and dividing by 2) crystallite sizes of the silver (111) and (222) in order to reduce bias due to the crystal plane. A more detailed method for measuring the crystallite size will be described in the examples.

As described above, the composite coating has a small silver crystallite size and therefore a high hardness. Specifically, the Vickers hardness Hv is preferably 90 or more, and more preferably 100 to 180. The method for measuring the Vickers hardness Hv will be described in detail in the examples.

<2-3-2. Arithmetic Mean Roughness Ra>

An arithmetic mean roughness Ra (μm) of the composite coating of the composite material of the present invention is 2.0 μm or less. In the composite material of the present invention having a composite coating with such a smooth surface, silver is effectively prevented from shedding from the composite coating during bending. The composite material having such a composite coating with a smooth surface, a small silver crystallite size, and excellent wear resistance can be produced by, for example, the method for producing the composite material of the present invention.

From the viewpoint of preventing silver from shedding from the composite coating during bending, the arithmetic mean roughness Ra of the composite coating is preferably 1.5 μm or less. When considering that it is practically difficult to produce a composite coating having a very small arithmetic mean roughness Ra, the Ra is more preferably 0.2 to 1.2 μm.

Further, it has been found that the thicker the composite coating of the composite material, the larger its arithmetic mean roughness Ra, and as a result of investigations by the present inventors, it has been found that the value obtained by dividing the arithmetic mean roughness Ra (μm) by the 0.5th power of the thickness of the composite coating (μm^0.5) can adequately calibrate the increase in arithmetic mean roughness Ra due to the thickness of the composite coating (it is easy to use as an adjustment index that eliminates the effect of the thickness of the composite coating on Ra). The value of the arithmetic mean roughness Ra (μm) of the composite material of the present invention divided by the 0.5th power (μm^0.5) of the thickness of the composite coating is preferably 0.05 to 0.5 (μm^0.5), more preferably 0.1 to 0.3 (μm^0.5), from the viewpoint of preventing silver from shedding from the composite coating during bending and because it is difficult to produce a composite coating having a very small arithmetic mean roughness Ra.

<2-3-3. Carbon Particles>

The carbon particles are the same as those described above regarding the method for producing the composite material of the present invention. That is, the carbon particles are preferably graphite particles, and the shape is not particularly limited and may be substantially spherical, scaly, or amorphous. A scaly shape is preferable because the wear resistance of the composite material can be improved by smoothing the surface of the composite coating.

From the viewpoint of the wear resistance of the composite material, the average primary particle size of the carbon particles is preferably 0.2 to 15 μm, and more preferably 0.4 to 10 μm. The average primary particle size is the average value of the long diameters of the particles, and the long diameter is defined as the length of a longest line that can be drawn within a particle (between two points on the outer periphery of the particle) in an image (plane) of a carbon particle in the composite coating of the composite material observed at an appropriate observation magnification. The long diameter is determined for 50 or more particles.

<2-3-4. Composition of Composite Coating and the Area Percentage of Carbon Particles>

The composite coating in the embodiment of the composite material of the present invention contains carbon particles as described above, and from the viewpoint of the wear resistance and electrical conductivity of the composite material, the content of carbons in the composite coating is preferably 1 to 50 mass %, more preferably 1.5 to 40 mass %, and even more preferably 2 to 35 mass %. The elemental composition of the composite coating in the embodiment of the composite material of the present invention typically consists essentially of silver and carbon.

Further, the percentage (area percentage) of the carbon particles on the surface of a composite coating containing carbon particles is an index of the wear resistance, and a certain degree of percentage is necessary from the viewpoint of wear resistance. On the other hand, if the percentage is too high, there is a problem in terms of electrical conductivity. From the viewpoint of a balance between the wear resistance and electrical conductivity, the area percentage is 5 area % or more and 80 area % or less, preferably 8 area % or more and 60 area % or less, more preferably 10 area % or more and 50 area % or less, and further preferably 22 area % or more and 40 area % or less. As explained in the description of the method for producing the composite material of the present invention, the carbon particles that are merely attached and easily shed may be present on the surface of the composite coating. In this case, the area percentage of carbon in the surface of the composite coating is obtained after the same ultrasonic cleaning treatment as explained in the section <<1-5. Partial removal of carbon particles from the surface of the composite coating>>. The method for measuring the area percentage will be described in detail in examples.

<2-3-5. Thickness of the Composite Coating>

The thickness of the composite coating is not particularly limited, but it is preferable that the thickness be a minimum value in terms of the wear resistance and electrical conductivity. Further, if the thickness is too large, the effect of the composite coating becomes saturated and a raw material cost increases. From the above viewpoints, the thickness of the composite coating is preferably 0.5 μm or more and 45 μm or less, more preferably 1 μm or more and 35 μm or less, and even more preferably 2 μm or more and 25 μm or less. Details of the method for measuring the thickness of the composite coating will be described in the examples.

<<2-4. Shedding of Silver from the Composite Coating During Bending>>

As described above, in the embodiment of the composite material of the present invention, the arithmetic mean roughness Ra of the surface of the composite coating is as small as 2.0 μm or less, and silver is prevented from shedding from the composite coating during bending. Specifically, in a test to evaluate silver shedding during bending in the examples described below, a carbon tape used for peeling was subjected to EDS analysis using an energy dispersive X-ray analyzer. Then, the EDS analysis reveals that the percentage of Ag is preferably 15 mass % or less, more preferably 10 mass % or less, even more preferably 7 mass % or less, and particularly preferably 4 mass % or less, based on 100 mass % of the total of the detected elements. It is difficult to reduce the percentage of Ag to 0, and it is usually 0.2 mass % or more.

[3. Terminal]

The composite material according to the embodiment of the present invention has excellent wear resistance and is less susceptible to shedding of silver from the composite coating during bending, and is therefore suitable as a constituent material for terminals for electrical contacts, particularly terminals (produced by bending) in electrical contact parts such as switches and connectors that undergo sliding during use.

EXAMPLE

Hereinafter, examples of the composite material and a method for producing the same according to the present invention will be described in detail.

Example 1

<Preparation of Carbon Particles: Oxidation Treatment>

80 g of flaky-shaped graphite particles (PAG-3000 manufactured by Nippon Graphite Industries Co., Ltd.) having an average particle size of 4.8 μm as carbon particles were added to 1.4 L of pure water, and the mixture was heated to 50° C. while stirring. An average particle size is a particle size whose cumulative value on a volume basis is 50%, measured using a laser diffraction/scattering type particle size distribution measuring device (Microtrac Bell MT3300 (LOW-WET MT3000II Mode)). Next, 0.6 L of a 0.1 mol/L aqueous solution of potassium peroxodisulfate (270 g/mol) as an oxidizing agent was gradually added dropwise to the mixture, and the mixture was then stirred for 2 hours to perform an oxidation treatment, followed by filtering through a filter paper, and a solid matter thus obtained was cleaned with water. The cleaning was repeated until the electrical conductivity of the filtrate after cleaning was 10 μS/cm or less.

The carbon particles before and after the oxidation treatment were analyzed for the gas generated when heated to 300° C., using a purge and trap gas chromatograph mass spectrometer (a combined device of a JHS-100 manufactured by Japan Analytical Industry Co., Ltd., as a thermal desorption device and a GCMS QP-5050A manufactured by Shimadzu Co., Ltd., as a gas chromatograph mass spectrometer). Then, it was found that due to the above oxidation treatment, lipophilic aliphatic hydrocarbons (such as nonane, decane, and 3-methyl-2-heptene) and lipophilic aromatic hydrocarbons (such as xylene) that were adhered to the carbon particles, were removed.

<Ag Strike Plating>

A plate material was prepared, which was composed of a Cu—Ni—Sn—P alloy measuring 5.0 cm in length, 5.0 cm in width, and 0.2 mm in thickness. The plate material is NB-109EH manufactured by Dowa Metaltech Co., Ltd., and is a copper alloy plate material containing 1.0 mass % of Ni, 0.9 mass % of Sn, 0.05 mass % of P, with a balance being Cu and unavoidable impurities. Further, the arithmetic mean roughness Ra of the surface of this plate material, measured by the method described below, was 0.1 μm.

With this plate material used as a base material, and using this base material as a cathode and an iridium oxide mesh electrode plate (a titanium mesh material coated with iridium oxide) used as an anode, electroplating (silver strike plating) was performed at a liquid temperature of 25° C. and a current density of 5 A/dm² for 120 seconds, in a sulfonic acid-based silver strike plating solution (Dainsilver GPE-ST manufactured by Daiwa Kasei Co., Ltd., substantially free of cyanide compounds, silver concentration 3 g/L, methanesulfonic acid concentration 42 g/L) containing methanesulfonic acid as a complexing agent. Silver strike plating was applied to an entire surface of the base material.

<AgC Plating>

The carbon particles (graphite particles) that have been subjected to the above-described oxidation treatment were added to a sulfonic acid-based silver plating solution (Dainsilver GPE-HB manufactured by Daiwa Kasei Co., Ltd. (containing compound A corresponding to general formula (I) at a concentration of 4.2 g/L, with a solvent being mainly water)) containing methanesulfonic acid as a complexing agent and having a silver concentration of 30 g/L and a methanesulfonic acid concentration of 60 g/L, to prepare a carbon particle-containing sulfonic acid-based silver plating solution containing carbon particles at a concentration of 50 g/L, silver at a concentration of 30 g/L, and methanesulfonic acid at a concentration of 60 g/L. This silver plating solution is substantially free of Sb and cyanide compounds.

Next, with an Ag strike plated base material used as a cathode and a silver electrode plate used as an anode, pulse reverse plating was performed in the carbon particle-containing sulfonic acid-based silver plating solution at a temperature of 25° C. while stirring with a stirrer at 400 rpm (the composite coating deposition step at a current density of 3.0 A/dm² for 0.8 seconds and a composite coating dissolution step at a current density of −9.0 A/dm² for 0.2 seconds were counted as one step, and the repeat count of the step was 4,000 times) to obtain a composite material in which a composite coating (AgC plating coating) containing carbon particles in a silver layer was provided on the base material. The pulse reverse plating was performed using a bipolar power supply BP4610 manufactured by Noise Laboratory Co., Ltd., with a pulse waveform as a rectangular shape (rise and fall times of the waveform were about 4 μs).

<Ultrasonic Cleaning Treatment>

The surface of the composite coating of the obtained composite material was subjected to ultrasonic cleaning treatment at 28 kHz for 4 minutes using an ultrasonic cleaner (VS-100III manufactured by AS ONE Co., Ltd., output 100 W, tank interior dimensions: length 140 mm×width 240 mm×depth 100 mm), with water as a liquid medium, to obtain a composite material according to example 1.

The manufacturing conditions, etc., of the above composite materials are summarized in tables 1 below, along with the production conditions of examples 2 and 3 and comparative examples 1 to 3 described below.

	TABLE 1
	Condition	Example 1	Example 2	Example 3	Com. Ex. 1	Com. Ex. 2	Com. Ex. 3

Ag strike	Main components of	Ag concentration	3	3	3	3	3	3
plating	plating solution	(g/L)

	Methane sulfonic	42	42	42	42	42	42
	acid concentration
	(g/L)

Plating conditions

Current density

5

5

5

5

5

5

	(A/dm2)
	Plating	25	25	25	25	25	25
	temperature
	(° C.)
	Plating time	120	120	120	120	120	120
	(s)

AgC	Main components of	Ag concentration	30	30	30	30	30	30
plating	plating solution	(g/L)
		Methane sulfonic	60	60	60	60	60	60

			acid concentration
			(g/L)
			Presence or	Present	Present	Present	Present	Absent	Absent
			absence of
			compound A
			Concentration of	50	50	50	50	50	50
			carbon particles
			(g/L)
	Ag Plating	Direct	Current density	—	—	—	3	3	—
	conditions	current	(A/dm2)
			Plating				25	25
			temperature
			(° C.)
			Stirring speed				400	400
			(rpm)
			Plating time				600	300
			(s)
		Pulse	Deposition	3	3	3	—	—	3
			current density
			(A/dm2)
			Deposition	0.8	0.8	0.8			0.8
			treatment time
			period Tp
			(s)
			Dissolution	−9	−9	−9			−9
			current density
			(A/dm2)
			Dissolution	0.2	0.2	0.2			0.2
			treatment time
			period Td
			(s)
			Repeat count	4000	2000	1000			2000
			Plating	25	25	25			25
			temperature
			(° C.)
			Stirring speed	400	400	400			400
			(rpm)
			Tp/Td	4	4	4			4

Ultrasonic

Is there cleaning or not?

Yes

Yes

Yes

Yes

Yes

Yes

cleaning

*Com. Ex. = Comparative Example

The obtained composite material according to example 1 was evaluated as follows.

<Thickness of the Composite Coating>

The thickness of this composite coating (a circular area with a diameter of 0.2 mm at the center of a surface of 5.0 cm in length and 5.0 cm in width) was measured with a fluorescent X-ray thickness meter (FT9450 manufactured by Hitachi High-Tech Science Co., Ltd.). As a result, the thickness was 21.2 μm. It is difficult to detect C atoms (of carbon particles) with the fluorescent X-ray thickness meter, so the thickness is determined by detecting Ag atoms, but in the present invention, the thickness determined in this manner is regarded as the thickness of the composite coating.

<Vickers Hardness Hv of the Surface of the Composite Coating>

The Vickers hardness Hv of the surface of the composite coating was measured using a microhardness tester (Mitutoyo HM221) by applying a load of 0.01 N to a flat portion of the composite material for 15 seconds in accordance with JIS Z2244, and an average value of three measurements was used. As a result, the Vickers hardness was 124 Hv.

In the case of examples 2 and 3 and comparative example 3 described later, the thickness of the composite coating was too thin to measure the hardness by the above method, so the Vickers hardness was not measured. Regarding comparative examples 1 and 2 described later, the thickness of the composite coating is also thin, but for the purpose of measuring Vickers hardness, a composite material was prepared by changing only the plating time from comparative examples 1 and 2 (the plating time was 1200 seconds for both comparative examples 1 and 2). For the obtained composite material having a composite coating thickness of about 20 μm, the Vickers hardness was measured by the above method.

<Area Percentage of the Carbon Particles in the Surface of the Composite Coating>

The surface of the composite coating was observed using a tabletop microscope (TM4000 Plus manufactured by Hitachi High-Tech Co., Ltd.) at an accelerating voltage of 5 kV and a magnification of 1000 times, and a backscattered electron composition (COMPO) image (one field of view) was binarized using GIMP 2.10.10 (image analysis software), and the area percentage of carbon particles in the surface of the composite coating was calculated. Specifically, when a highest brightness among all pixels is 255 and a lowest brightness is 0, a gradation is binarized such that pixels with a brightness of 127 or less are black and pixels with a brightness of more than 127 are white. Then, the image is separated into a silver portion (white portion) and a carbon particle portion (black portion), and the ratio Y/X of the number of pixels Y in the carbon particle portion with respect to the number of pixels X in an entire image was calculated as the area percentage (%) of the carbon particles in the surface. As a result, the area percentage of the carbon particles was 33%.

<Silver Crystallite Size in the Composite Coating>

The surface of the composite coating was subjected to X-ray diffraction measurement (Cu Kα ray tube, tube voltage: 30 kV, tube current: 10 mA, step width: 0.01°, scanning range: 20=0° to 155°, scanning speed: 5°/min, measurement time: about 31 min, (111) peak: 20=38.5 to 39.3°, (222) peak: 20=81.9 to) 82.8° using an X-ray diffractometer (RINT-2000 manufactured by Rigaku Co., Ltd.) in accordance with JIS H7805: 2005. From the peaks of the detected crystal planes (111) and (222) of silver, the full width at half maximum (FWHM) was obtained using X-ray analysis software (PDXL manufactured by Rigaku Co., Ltd.), and the crystallite size in each crystal plane of silver was calculated using Scherrer's formula. In order to reduce bias due to the crystal plane, an average value of the crystallite sizes of the crystal planes (111) and (222) of silver was taken as a silver crystallite size. As a result, the crystallite size was 63.3 nm.

The Scherrer formula is as follows:

D = K ⁢ λ / ( β ⁢ cos ⁢ θ )

• • D: crystallite size
  • K: Scherrer constant, taken as 0.9
  • λ: X-ray wavelength, 1.54 Å for CuKα rays
  • β: full width at half maximum (FWHM) (rad)
  • θ: measurement angle (deg)

<Arithmetic Mean Roughness Ra of the Composite Coating>

An image was taken at 1000× magnification using a laser microscope (Keyence VKX-110), and the arithmetic mean roughness Ra that is a parameter that represents surface roughness (over an entire observation surface of the composite coating), was calculated using an analysis application (Keyence Corporation VK-HIXA version 3.8.0.0) based on JIS B 0601 (2001). As a result, the arithmetic mean roughness Ra was 1.00 μm. Further, the calculated value of the arithmetic mean roughness Ra (μm) of the composite material of the present invention divided by the 0.5th power (μm^0.5) was 0.22 (μm^0.5).

<Composition of the Composite Coating>

The composite coating was observed using a tabletop electron microscope (TM4000 Plus manufactured by Hitachi High-Technologies Co., Ltd.) at 1000× magnification and at an accelerating voltage of 15 kV, and in this observation area (one visual field), EDX analysis was performed using an energy dispersive X-ray analyzer (AztecOne manufactured by Oxford Instruments Co., Ltd.) attached to the above-described tabletop microscope, and the detected elements were taken as the composition of the composite coating. As a result, no elements other than Ag and C were detected, and it was found that the composite coating was essentially composed of Ag and C.

<Evaluation of the Wear Resistance>

A flat test piece measuring 2.0 cm in width×3.0 cm in length was cut out from the composite material obtained in example 1.

On the other hand, the following convex indenter was prepared for sliding against the flat test piece (plate).

The copper alloy sheet used as the raw material in example 1 was subjected to press processing (so-called indent processing) with an inner diameter of 1.0 mm. Using this processed product as a base material, silver strike plating was performed in the same manner as in example 1. The silver strike plating was applied to an entire surface of the base material.

Next, with the Ag strike plated base material used as a cathode and the silver electrode plate used as an anode, electroplating (current density 1.2 A/dm² for 37.5 minutes) was performed at a temperature of 15° C. in an Ag—Sb alloy plating solution (cyanide bath) containing 10 mass % of silver sodium cyanide, 30 mass % of sodium cyanide, and 50 mL/L of Nissin Bright N (brightener, containing 6 mass % of diantimony trioxide) (manufactured by Nissin Chemical Industries, Co., Ltd.), while stirring with a stirrer at 400 rpm. As a result, an indented test piece was obtained, on which an AgSb plating layer containing 2 mass % Sb, having a Vickers hardness (Hv) of 180 and a thickness of 20 μm is formed.

This indented test piece was used as a convex indenter in the following sliding wear test.

The wear resistance was evaluated by performing a wear test to check a wear state of the indented test piece and the flat test piece by pressing the indented test piece against the flat test piece with a constant load (5N) using a sliding wear tester (CRS-G2050-DWA manufactured by Yamazaki Seiki Laboratory Co., Ltd.), with the protruding portion of the indented test piece contacting the flat test piece and continuing a reciprocating sliding motion (sliding distance 10 mm (i.e. 20 mm per reciprocating motion), sliding speed 10 mm/s). As a result, after 20,000 reciprocating sliding motions, the center of the sliding marks on the indented test piece (indenter) and the flat test piece (plate) were observed with a microscope (VHX-1000 manufactured by Keyence Co., Ltd.) at a magnification of 200 times, and it was confirmed that the (brown) base material (copper alloy sheet material) was not exposed in any case. As a result, it was found that the composite material of example 1 has an excellent wear resistance.

<Evaluation Test for Silver Shedding During Bending>

A bending test piece having a width of 10 mm was cut out from the obtained composite material so that a longitudinal direction was TD (direction perpendicular to a rolling direction) and a width direction was LD (rolling direction), and a 90° W bending test was performed in accordance with JIS H3130 for the bending test piece, with LD set as a bending axis (Bad Way bending (B. W. bending)) and a bending radius R set as 0.2 mm.

The 90° W bending test will be described with reference to FIG. 1. FIG. 1 is a schematic cross-sectional view illustrating a test piece T that has been sandwiched between an upper jig J_UP and a lower jig J_DOWN, and in which a peak M and a valley V have been created. When the test piece T is sandwiched between the upper jig J_UP and the lower jig J_DOWN and bent, loads applied to the upper and lower jigs were monitored so as not to apply further load to the bent test piece T by the upper and lower jigs.

After this test, carbon tape C (carbon double-sided tape for SEM 7322: width 12 mm manufactured by Nisshin EM Co., Ltd.) was stuck to the surface FUP of the test piece T that had been in contact with the upper jig J_UP, and the tape was then manually pulled vertically upward to perform peeling. The carbon tape C was stuck to the test piece T so that it protruded 1 mm on both sides in a width direction.

In the surface FUP that contacted the upper jig J_UP of the test piece T in the carbon tape C after peeling, observation was performed on a position which is 2 mm in the longitudinal direction toward a center position in the width direction corresponding to a bent portion that becomes a peak portion M, starting from the center position in a width direction corresponding to a bent portion that becomes a valley portion V of the test piece T, using a tabletop electron microscope (TM4000 Plus manufactured by Hitachi High-Technologies Co., Ltd.) at an accelerating voltage of 15 kV and a magnification of 100 times.

Then, an EDS analysis was performed to an observation area (one visual field) of this carbon tape C using an energy dispersive X-ray analyzer (AztecOne manufactured by Oxford Instruments Co., Ltd.) attached to the above-described tabletop microscope. As a result, Ag and C were detected, and an amount of Ag was 3.80 mass %.

The evaluation results of the above composite material are summarized in table 2 below, together with the evaluation results of examples 2 and 3 and comparative examples 1 to 3 described below.

	TABLE 2
				Comparative	Comparative	Comparative
	Example 1	Example 2	Example 3	example 1	example 2	example 3

Physical	Thickness of	21.2	9.8	5.4	10.0	5.0	5.5
properties	composite coating
	(μm)
	Vickers hardness	124	—	—	160	70	—
	(Hv)
	Area percentage of	33	26	28	14	30	22
	carbon particles
	(%)
	Silver crystallite size	63.3	63.0	60.0	11	78.8	80.6
	(nm)
	Arithmetic mean	1.00	0.70	0.40	2.20	0.50	1.20
	roughness
	Ra(μm)
	Ra/(composite	0.22	0.22	0.17	0.70	0.22	0.51
	coating thickness)0.5
	(μm0.5)
	Composition of	Essentially	Essentially	Essentially	Essentially	Essentially	Essentially
	composite coating	silver and	silver and	silver and	silver and	silver and	silver and
		carbon	carbon	carbon	carbon	carbon	carbon

Characteristics	Wear	Base	Not	Not	Not	Not	Exposed	Exposed
	resistance	material	exposed	exposed	exposed	exposed
		exposure
		of Plate
		Base	Not	Not	Not	Not	Exposed	Exposed
		material	exposed	exposed	exposed	exposed
		exposure
		of Indenter

	Shedding of silver	3.80	3.40	1.50	38.20	0.30	1.90
	during bending
	(mass %)

Example 2

A composite material according to example 2 was fabricated in the same manner as in example 1, except that the repeat count of the composite coating deposition step and the composite coating dissolution step in the pulse reverse plating was 2000 times. The obtained composite material was subjected to various evaluations in the same manner as in example 1.

Example 3

A composite material according to example 3 was fabricated in the same manner as in example 1, except that the repeat count of the composite coating deposition step and the composite coating dissolution step in the pulse reverse plating was 1,000 times. The obtained composite material was subjected to various evaluations in the same manner as in example 1.

Comparative Example 1

A composite material according to comparative example 1 was fabricated in the same manner as in example 1, except that the following normal electroplating was performed instead of the pulse reverse plating, and various evaluations were performed to this composite material in the same manner as in example 1.

Ag strike plating was performed to the base material in the same manner as in example 1. With an Ag strike plated base material used as a cathode and a silver electrode plate used as an anode, electroplating was performed in the same sulfonic acid-based silver plating solution containing carbon particles as used in example 1, at a temperature of 25° C. and a current density of 3 A/dm² for 600 seconds while stirring with a stirrer at 400 rpm, to obtain a composite material according to comparative example 1.

The composite coating of the composite material of comparative example 1 had a small silver crystallite size, a high Vickers hardness, and excellent wear resistance, but has a large arithmetic mean roughness Ra in its surface, resulting in large silver shedding during bending.

Comparative Example 2

A composite material was prepared in the same manner as in comparative example 1, except that the current application time was 300 seconds, and as the silver plating solution, a sulfonic acid-based silver plating solution containing carbon particles was used, which was obtained by adding carbon particles (graphite particles) that had been subjected to the same oxidation treatment as in example 1 to a sulfonic acid-based silver plating solution containing methanesulfonic acid at a concentration of 100 g/L and having a silver concentration of 30 g/L (Dainsilver GPE-PL manufactured by Daiwa Kasei Co., Ltd. containing carbon particles (containing no compound A corresponding to general formula (1), solvent is water)), with a plating time set as 300 seconds.

The obtained composite material of comparative example 2 was subjected to various evaluations in the same manner as in example 1. As a result, the composite coating of the composite material had a large silver crystallite size, and therefore had a low Vickers hardness, resulting in poor wear resistance. On the other hand, there was little shedding of silver during bending.

Comparative Example 3

A composite material according to comparative example 3 was fabricated in the same manner as in example 1, except that the used silver plating solution was a sulfonic acid-based silver plating solution containing carbon particles similar to that used in comparative example 2 and the repeat count of the composite film deposition step and the composite film dissolution step in the pulse reverse plating was 2000 times.

This composite material was subjected to various evaluations in the same manner as in example 1. As a result, the composite material had a large silver crystallite size and therefore was poor in wear resistance, but there was little shedding of silver during bending.

DESCRIPTION OF SIGNS AND NUMERALS

• • T Test piece
  • C Carbon tape
  • M Peak
  • V Valley
  • FUP Surface in contact with upper jig
  • J_UP Upper jig
  • J_DOWN Lower jig