Composite Material, Method for Producing the Composite Material, and Terminal

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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

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.

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 —CH—. Examples of the divalent group include —CH—CH—O—, —CH—CH—CH—O—, and (—CH—CH—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.

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 Ogas, 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.

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.

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.

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.

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.

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

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/dm, and more preferably 2.0 to 4.0 A/dm, 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/dm, and more preferably −11 to −6 A/dmfrom 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.

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.

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/dmfor 10 to 300 seconds.

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.