Patent Description:
Electric Ni plating is one of surface treatment methods for forming an Ni plating layer on a substrate surface made of a conductive metal, and the Ni plating layer formed exhibits excellent corrosion resistance. Thus, plated articles which are electrically Ni-plated have been extensively used in electronic parts and the like for an automobiles or home electric appliances. In recent years, there have been needed plated articles having further improved electric, mechanic and chemical properties as automobiles and home electric appliances have become highly functional.

It is known that electric, mechanic or chemical properties of a plated article can be further improved by forming a porous Ni plating layer on a substrate surface. Furthermore, a plated article having a porous Ni plating layer can be used as an electric part such as a connector because it exhibits low contact electrical resistance and excellent corrosion resistance and slidability; can be used as an electrode such as an electrode for hydrogen generation because it is very porous and has a large surface area; and can be used as a heat sink because it exhibits good heat dissipation performance. Recently it is, therefore, regarded that technique for forming a porous Ni plating layer on a substrate surface is one of the particularly important techniques.

A method described in Patent Reference No. <NUM> can be mentioned as an example of a method for forming a porous Ni plating layer on a substrate surface. Patent Reference No. <NUM> has described a method for forming a porous Ni plating layer on a substrate surface, comprising immersing a substrate in a plating solution containing a quaternary ammonium salt (dodecyltrimethylammonium chloride) and electroplating the substrate. However, the method described in Patent Reference No. <NUM> requires the use of a plating solution containing a special salt, and is, therefore, not always a simple method.

Patent Reference No. <NUM> has disclosed that an Ni-plating surface can be roughened for improving adhesiveness to another film. Patent Reference No. <NUM> has described that a plating solution used for a nickel plating bath for forming a rough plating layer can contain <NUM> to <NUM>/L of nickel sulfate or nickel chloride, <NUM> to <NUM>/L of ammonium sulfate, <NUM> to <NUM>/L of sodium sulfate, <NUM> to <NUM>/L of sodium chloride, and <NUM> to <NUM>/L of boric acid. It has also described that by applying a high current density of <NUM> ASD (A/dm<NUM>) or more, a nickel plating layer with large surface roughness can be formed. However, it cannot form a porous Ni plating layer, and thus only roughening the surface cannot improve electric and/or chemical properties of a plated article.

Non-patent Reference No. <NUM> has described a method for forming a porous Ni plating layer on a substrate surface. Specifically, Non-patent Reference No. <NUM> has described electric Ni plating using a plating solution containing <NUM> Ni chloride and <NUM> ammonium chloride at pH <NUM>, where a cathode current density of more than <NUM> mA/cm<NUM> (<NUM> A/dm<NUM>) can form an Ni plating layer having cavities and pores over the whole surface. However, the method described in Non-patent Reference No. <NUM> cannot form a uniform porous Ni plating layer over the whole surface of a substrate, and it cannot be thus expected to fully improve electric, mechanic or chemical properties of a plated article to be produced. Patent Reference No. <NUM> describes an electrolytic process. Patent Reference No. <NUM> describes a Ni plating bath.

To solve the above problems, an objective of the present invention is to provide a method for easily producing a plated article where a uniform porous Ni plating layer is formed on a substrate surface.

The above problems can be solved by providing a method for producing a plated article as defined in the appended claims.

According to the producing method of the present invention, a plated article where a uniform porous Ni plating layer is formed on a substrate surface can be easily produced.

The present invention relates to a method for producing a plated article, comprising immersing a substrate made of a conductive metal in a plating solution, and forming a plating layer on the substrate by electroplating. After intense investigation, we have found that a uniform porous Ni plating layer can be formed on a substrate surface by immersing a substrate made of a conductive metal in a solution containing <NUM> to <NUM> mol/L (M) of Ni ions with a pH of <NUM> or more and performing electroplating at a cathode current density of <NUM> A/dm<NUM> or more. A uniform porous Ni plating layer can be formed on a substrate surface by performing electric Ni plating on the substrate with a high cathode current density using a plating solution with a high pH. The fact that a uniform porous Ni plating layer can be formed by such an easy method was found first by our investigation and is surprising. "Porous Ni plating layer" as used herein refers to a Ni plating layer having a plurality of pores recessed toward a substrate.

A plating solution used in the present invention contains <NUM> to <NUM> mol/L of Ni ions. If the content of Ni ions is less than <NUM> mol/L, strength of the porous Ni plating layer is reduced. The content of Ni ions is preferably <NUM> mol/L or more, more preferably <NUM> mol/L or more. Meanwhile, if the content of Ni ions is more than <NUM> mol/L, a porous Ni plating layer cannot be formed on a substrate surface. The content of Ni ions is preferably <NUM> mol/L or less, more preferably <NUM> mol/L or less. Here, as long as the effects of the present invention are not hampered, the plating solution can contain metal ions other than Ni ions. It is, however, preferable that the plating solution contains <NUM> to <NUM> mol/L of Ni ions and is substantially free from metal ions other than Ni ions. If metal ions other than Ni ions are contained in the plating solution, corrosion resistance of a Ni plating layer formed may be deteriorated.

A pH of the above plating solution is <NUM> or more. If a pH of the plating solution is less than <NUM>, a uniform porous Ni plating layer cannot be formed. A pH of the plating solution is preferably <NUM> or more, more preferably <NUM> or more, further preferably <NUM> or more. There are no particular restrictions to the upper limit of a pH, and it is generally <NUM> or less, preferably <NUM> or less, more preferably <NUM> or less.

A pH of a plating solution is adjusted to the above range by, adding <NUM> to <NUM> mol of ammonia per one liter of the plating solution to the plating solution. As described above, it is disadvantageous that a plating solution contains metal ions other than Ni ions. By using ammonia for adjusting a pH, contamination of the plating solution with metal ions other than Ni ions can be avoided. In this light, the plating solution is a solution whose pH is adjusted to <NUM> or more using <NUM> to <NUM> mol of ammonia per one liter of the plating solution. Herein, ammonia does not contain a dissociated entity, that is, ammonium ions. A pH can be adjusted using ammonia, specifically by, but not limited to, adding an aqueous ammonia solution to a plating solution or blowing ammonia gas into a plating solution.

Here, the plating solution contains <NUM> to <NUM> mol/L of ammonia. Herein, a content of ammonia is an ammonia concentration per one liter of the plating solution calculated from a molar number of ammonia added to the plating solution. If a content of ammonia is less than <NUM> mol/L, a pH of the plating solution may not be adjusted to <NUM> or more. A content of ammonia is more preferably <NUM> mol/L or more, further preferably <NUM> mol/L or more. If a content of ammonia is more than <NUM> mol/L, a production cost increases and odor may deteriorate working environment, so that the process may not be industrially feasible. A content of ammonia is more preferably <NUM> mol/L or less, further preferably <NUM> mol/L or less.

Furthermore, adding a metal hydroxide such as sodium hydroxide to a plating solution for adjusting a pH may cause precipitation of Ni ions as Ni hydroxide, while adding ammonia to a plating solution does not cause such precipitation. This is because ammonia coordinates a Ni ion in the plating solution to form an ammine (ammonia) complex. Herein, a molar ratio of ammonia to Ni ions (NH<NUM>/Ni ions) in a plating solution is <NUM> or more. If a molar ratio of ammonia to Ni ions (NH<NUM>/Ni ions) is less than <NUM>, the amount of ammonia coordinating Ni ions may be so reduced that ammine complexes cannot be formed. A molar ratio of ammonia to Ni ions (NH<NUM>/Ni ions) is more preferably <NUM> or more, further preferably <NUM> or more. There are no particular restrictions to the upper limit of a molar ratio of ammonia to Ni ions (NH<NUM>/Ni ions), but if the molar ratio is excessively large, ammonia uninvolved in coordination to a Ni ion may be excessive, leading to disadvantage in cost and deteriorated working atmosphere. A molar ratio of ammonia to Ni ions (NH<NUM>/Ni ions) is generally <NUM> or less.

A plating solution preferably contains <NUM> to <NUM> mol/L of ammonium ions. If a content of the above ions is less than <NUM> mol/L, liquid resistance of the plating solution may be increased, so that when electric Ni plating is conducted at a high current density, a temperature of the plating solution is rapidly elevated, leading to difficulty in continuous production of the plated article. A content of the above ions is more preferably <NUM> mol/L or more. If a plating solution with a content of the above ions of more than <NUM> mol/L is prepared, a large amount of ammonium salts as ion sources of ammonium ions must be dissolved, probably leading to increase in a production cost. A content of the above ions is more preferably <NUM> mol/L or less.

The restrictions to the type of counter anions to Ni ions and ammonium ions are as follows. In the light of availability and a low price, a plating solution contains, as the above counter anions, at least one type of ions selected from the group consisting of chloride, sulfate, sulfamate and acetate ions. The solution more preferably contains chloride ions and/or sulfate ions.

A substrate used in the present invention can be made of any conductive metal without any limitation. Among others, copper or an alloy containing copper as a main component is suitably used in the light of conductive performance and the like. The expression "containing. as a main component" as used herein, means that the component is contained in <NUM> % by weight or more.

A substrate used in the present invention can be a multilayer structure. Here, it is essential that a side in which a Ni plating layer is to be formed, that is, the surface layer is made of a conductive metal, and other layers can be made of a conductive metal or a nonmetallic material such as ceramics and a resin, or alternatively a less conductive semimetallic material such as silicon. Herein, a semimetallic material refers to a material which exhibits certain conductivity but does not conductivity adequate to allow for ordinary electroplating.

Even with a nonmetallic material or semimetallic material, a conductive metal layer can be formed to its surface, to give a porous Ni plating layer of the present invention. Examples of a method for forming a conductive metal layer on a surface include nonelectrolytic plating, vapor deposition, sputtering, ion plating, cold spraying and aerosol deposition. Furthermore, application of a conductive paste or conductive polymer to a surface can be employed. Examples of a conductive metal include Ni, Cu, Al, Zn, Au, Ag, Cr, Ti, Sn, Pd, Ru and Rh, and alloys thereof can be also used. For example, a silicon wafer on whose surface a porous Ni plating layer is formed is suitable because it exhibits improved heat-dissipating ability from a semiconductor chip.

A cathode current density during electroplating a substrate is <NUM> A/dm<NUM> or more. Herein, a cathode current density refers to a value obtained by converting a current applied to a substrate (cathode) during electric Ni plating to a current per <NUM> dm<NUM> of the substrate. If a cathode current density is less than <NUM> A/dm<NUM>, a uniform porous Ni plating layer cannot be formed. A cathode current density is preferably <NUM> A/dm<NUM> or more. There are no particular to the upper limit of a cathode current density, and a cathode current density is generally <NUM> A/dm<NUM> or less, preferably <NUM> A/dm<NUM> or less, more preferably <NUM> A/dm<NUM> or less.

There are no particular restrictions to a plating time, and it can be appropriately selected such that a porous Ni plating layer with a desired thickness is formed. There are also no particular restrictions to a temperature of a plating solution. Since an excessively high temperature may cause alteration in a composition of a plating solution due to solvent evaporation, the temperature is generally <NUM> or lower.

Thus, a substrate can be immersed in the above plating solution and then electroplated under the above conditions, to provide a plated article in which a uniform porous Ni plating layer is formed over the whole surface.

It is preferable that a mean diameter of pores formed in the porous Ni plating layer thus formed is <NUM> to <NUM> as an area weighted average value. If the mean diameter is less than <NUM>, a corrosion current cannot be dispersed even when a porous Ni plating layer is formed on a substrate aiming at improving corrosion resistance of the plated article, so that corrosion resistance may not be improved. A mean diameter of pores is more preferably <NUM> or more, further preferably <NUM> or more. If a mean diameter of pores is more than <NUM>, strength of a porous Ni plating layer may be reduced, and thus it is preferably <NUM> or less. When a plated article is used as an electrical contact, a contact electric resistance may increase, leading to deterioration in electric conductivity. In this light, a mean diameter of pores is preferably <NUM> or less, more preferably <NUM> or less, further preferably <NUM> or less. Here, a mean diameter of pores can be determined by choosing multiple pores in a scanning electron microgram (secondary electron image) or microgram of the surface of the plated article, measuring their diameters and calculating an area load average thereof. When the pores are not circular, an equivalent circle diameter is regarded as a diameter.

A plated article produced by a producing method of the present invention can be used as a heat sink because it exhibits good heat-dissipating ability, and can be used as an electric part such as a connector because it has a low contact electric resistance and is excellent in corrosion resistance and sliding properties. With a high regard for heat-dissipating ability, a larger mean diameter of pores formed in the porous Ni plating layer is more preferable. With a high regard for contact electric resistance, corrosion resistance and sliding properties, a smaller mean diameter of pores formed in the porous Ni plating layer is more preferable.

A thickness of the porous Ni plating layer is preferably <NUM> to <NUM>. If the thickness is less than <NUM>, a porous Ni plating layer is so brittle that it may tend to be peeled off from a substrate. Furthermore, if the thickness is less than <NUM>, heat-dissipating ability may be insufficiently improved even when a porous Ni plating layer is formed on a substrate aiming at producing a plated article with high heat-dissipating ability. A thickness of the porous Ni plating layer is more preferably <NUM> or more, further preferably <NUM> or more, particularly preferably <NUM> or more. If a thickness of the porous Ni plating layer is more than <NUM>, a production cost may be increased. Here, a thickness of the porous Ni plating layer refers to a thickness from the substrate surface to a convex portion in the porous plating layer.

As described above, a uniform porous Ni plating layer can be formed on the whole surface of a substrate by electric Ni plating with a high current density. When a porous Ni plating layer is formed on a substrate by electroplating under such plating conditions, a plating solution containing <NUM> to <NUM> mol/L of Ni ions, <NUM> to <NUM> mol/L of ammonia, and <NUM> to <NUM> mol/L of ammonium ions, wherein a molar ratio of ammonia to Ni ions (NH<NUM>/Ni ions) is <NUM> or more and a pH is <NUM> or more, is suitably used.

According to a producing method of the present invention, a plated article in which a uniform porous Ni plating layer is formed on a substrate surface can be easily obtained. A plated article produced by a producing method of the present invention is excellent in electric, mechanical and chemical properties, so that it can be used in various applications. Specifically, a plated article thus produced can be used as an electric part such as a connector because it exhibits low contact electrical resistance and excellent corrosion resistance and slidability; can be used as an electrode such as an electrode for hydrogen generation because it is very porous and has a large surface area; and can be used as a heat sink because it exhibits good heat dissipation performance.

There will be further detailed the present invention with reference to, but not limited to, Examples.

The following compounds were dissolved in ion-exchanged water. Concentrations were as described below.

To the aqueous solution thus prepared, a <NUM> % by mass aqueous ammonia was added to prepare a Ni plating solution with a pH of <NUM>. Here, an ammonia concentration per <NUM> liter of the plating solution was calculated from the molar number of ammonia added to the plating solution, and was <NUM>.

A copper plate with a size of <NUM> × <NUM> × <NUM> was prepared as a substrate and immersed in a <NUM>/L aqueous solution of an electrolytic degreaser "PAKUNA THE-<NUM>" from Yuken Industry Co. Using the copper plate as a cathode, current was applied at a cathode current density of <NUM> A/dm<NUM> for <NUM> sec for degreasing. The degreased substrate was washed three times with ion-exchanged water, and then washed with an acid by immersing it in a <NUM> vol% aqueous solution of sulfuric acid at room temperature for <NUM> sec. Subsequently, again, it was washed three times with water.

The electrolytically degreased substrate was immersed in the above Ni plating solution kept at <NUM>. Then, with air-stirring, it was subjected to electric Ni plating at a cathode current density of <NUM> A/dm<NUM> for <NUM> sec. Then, the substrate was washed three times with ion-exchanged water and immersed in an aqueous solution of sodium hydroxide (<NUM>/L) at <NUM> for <NUM> sec. Subsequently, the substrate was washed three times with ion-exchanged water, immersed in ion-exchanged water at <NUM>, and sonicated for <NUM> sec, to give a plated article. A thickness of the Ni plating layer was about <NUM>.

Using a field-emission-type scanning electron microscope (FE-SEM) "S-<NUM>" from Hitachi High-Technologies Corporation, a photo of the surface of the plated article was taken to give a secondary electron image. The secondary electron image obtained is shown in <FIG>.

Then the secondary electron image of the plated article obtained was visually observed and evaluated in accordance with the following evaluation criteria. The results are shown in Table <NUM>.

Multiple pores were chosen from a secondary electron image of the plated article obtained, and their diameters were measured and an area load average thereof was calculated. When the pores were not circular, an equivalent circle diameter was regarded as a diameter. The results are shown in Table <NUM>.

A Ni plating layer was formed on a substrate as described in Example <NUM>, except that a pH of a Ni plating solution was changed as shown in Table <NUM>. A thickness of the Ni plating layer was about <NUM>. Then, the Ni plating layer was evaluated as described in Example <NUM>. The results are shown in Table <NUM>.

Ni plating layers were formed on a substrate as described in Example <NUM>, except that a cathode current density was changed as shown in Table <NUM>. A thickness of the Ni plating layer was about <NUM>. Then, the Ni plating layer was evaluated as described in Example <NUM>. The results are shown in Table <NUM>.

Ni plating layers were formed on a substrate as described in Example <NUM>, except that a pH or a cathode current density was changed as shown in Table <NUM>. Then, the Ni plating layer was evaluated as described in Example <NUM>. The results are shown in Table <NUM>. <FIG> shows a secondary electron image of the surface of the plated article in Comparative Example <NUM>.

A Ni plating solution was prepared as described in Example <NUM>, except that a nickel chloride concentration in the Ni plating solution was <NUM> and a pH was not adjusted with aqueous ammonia. A pH of the Ni plating solution was <NUM>. Then, a Ni plating layer was formed on a substrate as described in Example <NUM>, except that a cathode current density was as shown in Table <NUM>, and the Ni plating layer was evaluated. The results are shown in Table <NUM>. <FIG> shows a secondary electron image of the surface of the plated article in Comparative Example <NUM>.

As shown in Table <NUM>, in a plated article produced by a producing method of the present invention, a uniform porous Ni plating layer was formed on the whole surface of a substrate (Examples <NUM> to <NUM>). In contrast, in a plated article produced by a producing method which does not meet the.

conditions defined in the present invention, a porous Ni plating layer was formed only on a part of the substrate, but a uniform porous Ni plating layer was not formed on the whole surface of the substrate (Comparative Examples <NUM> to <NUM>).

A Ni plating layer was formed on a substrate as described in Example <NUM>, substituting a plating solution prepared as described below for the plating solution used in Example <NUM>. A thickness of the Ni plating layer was about <NUM>. Then, the Ni plating layer was evaluated as described in Example <NUM>. The results are shown in Table <NUM>.

The following compounds were dissolved in ion-exchanged water. Concentrations were as described below. A viscosity of the plating solution was <NUM> mPa·s.

A Ni plating layer was formed on a substrate as described in Example <NUM>, except that a plating time was changed to <NUM> sec. A thickness of the Ni plating layer thus obtained was about <NUM>. Then, the Ni plating layer was evaluated as described in Example <NUM>. The results are shown in Table <NUM>.

A Ni plating solution at pH <NUM> was prepared by adding <NUM> % by mass aqueous ammonia to the solution of Example <NUM>. However, precipitation occurred in the Ni plating solution prepared, so that plating could not be conducted.

A Ni plating layer was formed on a substrate as described in Example <NUM>, except that a cathode current density was changed as shown in Table <NUM>. Then, the Ni plating layer was evaluated as described in Example <NUM>. The results are shown in Table <NUM>.

As shown in Table <NUM>, in a plated article produced by a producing method of the present invention, a uniform porous Ni plating layer was formed on the whole surface of a substrate (Example <NUM>). In contrast, in a plated article produced by a producing method which does not meet the conditions defined in the present invention, a porous Ni plating layer was formed only on a part of the substrate, but a uniform porous Ni plating layer was not formed on the whole surface of the substrate (Comparative Example <NUM>).

A Ni plating layer was formed on a substrate as described in Example <NUM>, except that a pH of a Ni plating solution was as shown in Table <NUM>. Then, the Ni plating layer was evaluated as described in Example <NUM>. The results are shown in Table <NUM>.

A Ni plating layer was formed on a substrate as described in Example <NUM>, except that a cathode current density was as shown in Table <NUM>. Then, the Ni plating layer was evaluated as described in Example <NUM>. The results are shown in Table <NUM>.

As shown in Table <NUM>, in a plated article produced by a producing method of the present invention, a uniform porous Ni plating layer was formed on the whole surface of a substrate (Example <NUM>). In contrast, in a plated article produced by a producing method which does not meet the conditions defined in the present invention, a porous Ni plating layer was formed only on a part of the substrate, but a uniform porous Ni plating layer was not formed on the whole surface of the substrate (Comparative Example <NUM>). In particular, when a cathode current density is less than that defined in the present invention, a porous Ni plating layer was not formed (Comparative Example <NUM>).

A Ni plating solution was prepared as described in Example <NUM>, except that a pH was not adjusted with aqueous ammonia. A pH of the Ni plating solution was <NUM>. Then, a Ni plating layer was formed on a substrate and evaluated as described in Example <NUM>. The results are shown in Table <NUM>.

As shown in Table <NUM>, in a plated article produced by a producing method of the present invention, a uniform porous Ni plating layer was formed on the whole surface of a substrate (Example <NUM>). In contrast, in a plated article produced by a producing method which does not meet the conditions defined in the present invention, a porous Ni plating layer was formed only on a part of the substrate, but a uniform porous Ni plating layer was not formed on the whole surface of the substrate (Comparative Examples <NUM> and <NUM>).

Claim 1:
A method for producing a plated article, comprising
immersing a substrate made of a conductive metal in a plating solution, and
forming a plating layer on the substrate by electroplating,
wherein the plating solution is a solution containing <NUM> to <NUM> mol/L of Ni ions with a pH of <NUM> or more;
and a porous Ni plating layer is formed by performing the electroplating at a cathode current density of <NUM> A/dm<NUM> or more, wherein the porous Ni plating layer has a plurality of pores recessed toward the substrate,
wherein the plating solution is formed by only dissolving a Ni salt and an ammonium salt in ion-exchanged water and adjusting the pH of the plating solution to <NUM> or more by adding <NUM> to <NUM> mol of ammonia per one liter of the plating solution to the plating solution,
wherein the Ni salt consists of at least one type of salts selected from the group consisting of nickel chloride, nickel sulfate, nickel sulfamate and nickel acetate,
and the ammonium salt consists of at least one type of salts selected from the group consisting of ammonium chloride, ammonium sulfate, ammonium sulfamate and ammonium acetate,
wherein a molar ratio of ammonia to Ni ions in the plating solution is <NUM> or more, and
wherein ammonia coordinates the Ni ion in the plating solution to form an ammine complex.