Patent Application
20210156044
The present invention relates to a method for forming reduced-size crystal grains of a plating film.
Composite plating in which small particles have been eutectically deposited in a plating metal film has been known. For example, Japanese Patent Application Publication No. 2008-214667 mentions a zinc-nanocarbon composite plating. In the above composite plating, a zinc plating film is formed on a plating object by using a zinc plating solution into which nanocarbon, and polyacrylamide as a dispersion agent for the nanocarbon, have been added.
Japanese Patent Application Publication No. 2008-214667 also mentions that it is preferable that nanocarbon be present in the zinc plating film and that the amount of nanocarbon added into the zinc plating solution be 0.5 to 5.0 g/L. Furthermore, Japanese Patent Application Publication No. 2008-214667 indicates that because part of the nanocarbon is exposed out of the zinc plating film, a zinc plating film that exhibits excellent sliding characteristics can be made.
It is generally considered that, as in the technology described in Japanese Patent Application Publication No. 2008-214667, incorporation of nanocarbon into a plating film will reform the surface of the plating film. For example, the incorporation of nanocarbon into the plating film is considered to harden the plating film and improve anti-abrasion properties associated with sliding.
Actually, however, it is not that the plating film is made hard but that the nanocarbon particles in a surface layer are hard. Thus, the anti-abrasion property of the plating film is not a simple property that depends only on the hardness of the plating film, but may be affected by a combination of various other elements such as the surface roughness (sliding property), the lubricity of the plating, and the toughness and crystal grain size of plating metal.
In practice, even in the case of a plating metal having high hardness and good sliding property (a plating metal having small crystal grains), because of the hard plating surface, once sliding results in a plating surface (contact surface) having a chip (flaw) formed by galling or the like, the flaw sharply increases the friction coefficient of the plating surface. As a result, the plating surface is further damaged and abrasion rapidly progresses. The above phenomenon is more likely to occur on plating metals that have high hardness but low toughness (e.g. plating metals having brittle grain boundaries and weak binding force). On the other hand, in the case of a plating metal having relatively low hardness, chipping does not occur but its low hardness makes the wear rate high and cannot bring about high anti-abrasion property.
Therefore, it cannot generally be said that incorporation of nanocarbon into a plating film will reform the surface of the plating film. Furthermore, in the case where nanocarbon is incorporated into a plating film, it is very difficult to uniformly disperse the nanocarbon in the plating film or precisely control the amount of the nanocarbon contained in the plating film. In addition, since nanocarbon is a nonconductor, use of a nanocarbon-incorporated plating film on an electrical contact point makes the electrical contact resistance unstable and also greatly increases electrical contact resistance.
In view of the problems outlined above, it is an object of the present invention to provide a crystal grain size reduction method for a plating film which is capable of reforming the surface of the plating film without substantial incorporation of nanocarbon into the plating film.
As a result of vigorous study to solve the aforementioned problems, the present inventor found that crystal grains of a plating film can be reduced in size by causing nanocarbon to function as if it was a catalyst, while avoiding incorporation of nanocarbon into the plating film, thereby realizing some aspects associated with the present invention. Specifically, to solve the foregoing problems, a representative construction of a crystal grain size reduction method for a plating film according to the present invention is characterized by performing electroplating in a condition where ions of a plating metal, a nanocarbon, and an anion based surfactant as a dispersion agent for dispersing the nanocarbon have been blended in a plating solution.
According to the above-described construction, since the dispersion agent is blended in the plating solution, the nanocarbon is dispersed in the plating solution with molecules of the dispersion agent adsorbed to the nanocarbon. Due to the use of an anion based surfactant as a dispersion agent, the nanocarbon dispersed in the plating solution is not readily incorporated into the surfaces of parts to be plated (plating objects) that are connected to the negative electrode. On the surfaces of the plating objects, epitaxial growth of the plating metal proceeds to form crystal grains. The nanocarbon affects the epitaxial growth of the plating metal so as to reduce the size of the crystal grains of the plating film. Although the behavior during this process is not entirely known, it can be hypothesized that, due to the Brownian motion in the plating solution, nanocarbon particles come into contact with and exert forces to crystal grains, thereby reducing the size of the crystal grains. Thus, the present invention realizes reform of the surface of a plating film by reducing the size of crystal grains of the plating film without substantial incorporation of nanocarbon into the plating film.
It is appropriate that the nanocarbon be positively charged when in a state of mixture with the plating solution. It is hypothesized that the positively charged nanocarbon in the plating solution (despite molecules of the anion based surfactant being adsorbed to nanocarbon particles) is attracted to the surface of the part to be plated that is connected to the negative electrode. Due to attraction to the surface of the part to be plated, the nanocarbon particles can certainly come into contact with and exert forces to crystal grains of the plating film, reliably reducing the size of the crystal grains of the plating film.
It is appropriate that the particle diameter of the nanocarbon be 2.6±0.5 nm. With the particle diameter of the nanocarbon being in this range, the nanocarbon particles in the plating solution certainly undergo Brownian motion and, when contacting crystal grains of the plating film, exert on the crystal grains appropriate forces that reduces the size of the crystal grains. It is hypothesized that the size reduction of the crystal grains is insufficient when the particle diameter of the nanocarbon is above the aforementioned range because the Brownian motion of the nanocarbon particles is not sufficient and therefore cannot exert appropriate forces on the crystal grains. Further, it is also hypothesized that the crystal grain size reduction becomes insufficient when the nanocarbon particle diameter is below the above range because, despite occurrence of Brownian motion, the small masses of the nanocarbon particles cannot exert to the crystal grains a force sufficient for size reduction of the crystal grains.
It is appropriate that the amount of the nanocarbon added into the plating solution be less than or equal to 0.2 g/L. By setting the amount of the nanocarbon added to a small amount that is less than or equal to 0.2 g/L, the nanocarbon can be prevented from being incorporated into the plating film.
It is appropriate that the plating metal be silver (Ag), nickel (Ni), tin (Sn), or gold (Au). Accordingly, a plating solution that is neutral or weakly acidic may be used.
According to the present invention, it is possible to provide a crystal grain size reduction method for a plating film which is capable of reforming a surface of a plating film without substantial incorporation of nanocarbon into the plating film.
Exemplary embodiments of the present invention will be described in detail hereinafter with reference to the accompanying drawings. The dimensions, materials, and other concrete numerical values mentioned in conjunction with the following exemplary embodiments are merely illustrative for the sake of easy understanding of the invention and not intended to limit the scope of the invention unless otherwise indicated. In the description and drawings, elements substantially the same in function and construction are indicated by the same reference characters and are not redundantly described. Furthermore, elements and the like that are not directly relevant to the present invention are omitted from graphical representation.
Blended in the plating solution 104 are ions of a plating metal 112, a nanocarbon 114, and a dispersion agent 116. The plating metal 112, as shown in this example is a monovalent cation of silver (Ag).
The dispersion agent 116 used in this example is an anion based surfactant. As illustrated in
As for the nanocarbon 114, for example, the amount added to the plating solution 104 was set to 0.2 g/L, and the particle diameter of the nanocarbon 114 was set to 2.6±0.5 nm. Furthermore, the nanocarbon particles 114 in a mixture with the plating solution 104 are positively charged. The plating solution 104 is neutral because the plating metal 112 is silver (Ag).
When a plating process is started in the plating apparatus 100 by applying, from the electricity source 110, a voltage between the negative electrode 106 and the positive electrode 108, then epitaxial growth of the plating metal 112 progresses on the surface of a plating object 120 connected to the negative electrode 106, so that crystal grains of the plating metal 112 form. As a result, the surface of the plating object 120 has on its surface a plating film 122 as indicated by hatching in
Observation of the microscopic photographs of the plating films 122 and 122A reveals that the crystal grains of the plating film 122 are clearly smaller in size than the crystal grains of the plating film 122A. Therefore, it is clear that the crystal grain size reduction method of this exemplary embodiment is capable of reducing the size of the crystal grains (forming nanocrystal grains) of the plating film 122. Table 1, presented below, compares the carbon contents of the plating films 122 and 122A.
As depicted in Table 1, the carbon content of the plating film 122 according to this exemplary embodiment in which the nanocarbon 114 was added was substantially the same as the carbon content of the plating film 122A of the comparative example in which the nanocarbon 114 was not added. Thus, it is clear that the plating film 122 formed by the crystal grain size reduction method of the exemplary embodiment did not substantially incorporate the nanocarbon 114.
Thus, in the crystal grain size reduction method according to the exemplary embodiment, size reduction of the crystal grains of the plating film 122 is achieved by the nanocarbon 114 functioning as if it was a catalyst, without substantial incorporation of the nanocarbon into the plating film 122. This phenomenon will be discussed below.
First of all, the nanocarbon 114 dispersed in the plating solution 104 is not readily incorporated into the plating film 122 on the surface of the plating object 120 that is connected to the negative electrode 106 because an anion based surfactant is used as the dispersion agent 116. In addition, since the amount of the nanocarbon 114 added is as small as 0.2 g/L, incorporation of the nanocarbon 114 into the plating film 122 does not easily occur in the first place. Thus, in the conditions as indicated above, the nanocarbon 114 was, actually, negligibly incorporated into the plating film 122.
Next, it is hypothesized that, because the nanocarbon particles 114 are positively charged in the plating solution 104, the molecules of the anion based surfactant adsorbed to the nanocarbon particles 114 do not prevent the nanocarbon particles 114 from being attracted to the surface of the plating object 120 connected to the negative electrode 106, so that the nanocarbon particles 114 can affect the epitaxial growth of the plating metal 112.
Although the behavior of the nanocarbon particles 114 during this process is not entirely known, it can be hypothesized that, due to the Brownian motion in the plating solution 104, the nanocarbon particles 114 come into contact with and exert forces to crystal grains, thereby achieving size reduction of the crystal grains. Specifically, it can be hypothesized that the positively charged nanocarbon 114 in the plating solution 104 is attracted to the surface of the plating object 120 thereby definitely coming into contact with, and exerting forces on, crystal grains of the plating film, so that the crystal grains of the plating film can be definitively reduced in size.
Furthermore, since the particle diameter of the nanocarbon 114 is set within the range of 2.6±0.5 nm, the particles of the nanocarbon 114 in the plating solution 104 certainly undergo Brownian motion, so that as nanocarbon particles 114 come into contact with crystal grains of the plating film, the nanocarbon particles 114 exert to the crystal grains appropriate forces that reduce the size of the crystal grains. It is hypothesized that size reduction of the crystal grains becomes insufficient if the particle diameter of the nanocarbon 114 is above the aforementioned range because the Brownian motion of the nanocarbon particles is not sufficient and therefore cannot exert appropriate forces on the crystal grains. On the other hand, it is further hypothesized that crystal grain size reduction becomes insufficient when the particle diameter of the nanocarbon 114 is below the range is speculated so that, despite occurrence of the Brownian motion, the small masses of the nanocarbon particles cannot exert, on the crystal grains, a force sufficient to reduce the size of the crystal grains.
Plating object 120 may be provided with plating film 122 that can be used as an electrical contact. Therefore, it is desirable for the plating film 122 to have a low electric resistivity (contact resistance). Furthermore, since the plating object 120 may be repeatedly inserted into a socket or the like, it is also desirable that the plating film 122 have high durability (i.e., greater anti-abrasion properties that may be associated with sliding).
A crystal structure of a metal will be described with reference to
The metal can be viewed as a crystal grain aggregate which includes crystal grains and grain boundaries that surround the crystal grains (defects of crystals or impurities) and in which crystal grains are bound to each other at grain boundaries. The abrasion of metal caused by sliding occurs in two different ways: crystal grains themselves undergo transgranular fracture; and boundaries fracture and the metal chips and erodes in block units of crystal grains. In this exemplary embodiment, it is an object to restrain the grain boundary fracture in which the metal chips off grain by grain and therefore increase the durability. In metal, when grain boundary fracture involves erosion of large crystal grains, the chipped-off volume, that is, the amount of erosion, is large; on the other hand, erosion of a small crystal grain means a small amount of erosion. Furthermore, in metal, since crystal grains are bound together at grain boundaries, it is hypothesized that the greater the strength of grain boundaries and the binding force thereof, the less easily erosion occurs. Therefore, crystal structure features of a metal that facilitate realization of a highly durable plating film are that the crystal grains are small and that the binding force at grain boundaries that binds crystal grains together is strong.
In the plating film 122 illustrated in
Moreover, it is generally considered that metal becomes harder as the crystal grains are made smaller. In this regard, the plating film 122A of the comparative example, in which the crystal grains 124A were not reduced in size, had a Vickers hardness of 90 to 110 Hv. On the other hand, the plating film 122 of the exemplary embodiment, in which the crystal grains 124 were reduced in size, had a Vickers hardness of 100 to 110 Hv, making it clear that size reduction of the crystal grains 124 did not make the plating film 122 harder. Therefore, good running-in property (lubricity) of the contact surface (which is a characteristic of silver (Ag)) of the plating metal 112 can be maintained, so that the surface of the plating film 122 (the contact surface at the time of sliding) is smooth and the friction coefficient does not considerably change even after repeated sliding. Thus, the durability of the plating film 122 can be increased.
Next, the electrical contact resistance of the plating film 122 will be described. It is considered that when the crystal grains of metal are made smaller, grain boundaries generally increase, so that the electrical contact resistance increases. However, in the plating film 122 according to the exemplary embodiment, although the crystal grains 124 are small, the contact resistance is not high but about 3×10−6 to about 3.5×10−6 Ω·cm. Incidentally, the contact resistance of a super hard silver plating that has substantially the same crystal grain diameter is as high as greater than or equal to 8×10−6 Ω·cm. A reason for this is speculated to be that because the size reduction of the crystal grains 124 of the plating film 122 does not involve the alloying with a different metal, such as antimony (Sb), or because no adsorbent organic luster is used in the plating film 122, the grain boundaries 126 contain only a small amount of impurities.
The plating film 122 described in
Thus, it was made clear that the plating film 122 formed by the crystal grain size reduction method according to the exemplary embodiment was lower in contact resistance and higher in durability than the plating film 122A of the comparative example, in which nanocarbon 114 was not added into the plating solution 104. That is, in the crystal grain size reduction method according to the exemplary embodiment, reform of the surface of the plating film 122 is realized by reducing the size of the crystal grains of the plating film 122 without substantial incorporation of the nanocarbon 114 into the plating film 122 although the nanocarbon 114 is added into the plating solution 104.
Examples and comparative examples in which different amounts of the nanocarbon 114 were added will be described below. Table 2 describes Examples 1 and 2 and Comparative Examples 1 and 2. In Examples 1 and 2, the amounts of the nanocarbon 114 added were 0.1 g/L and 0.2 g/L, respectively. In Comparative Example 1, the amount of the nanocarbon 114 added was zero, that is, no nanocarbon 114 was added. In Comparative Example 2, the amount of the nanocarbon 114 added was 0.3 g/L.
As mentioned in Table 2, in Comparative Example 1 with no nanocarbon 114 added into the plating solution 104, the size of the crystal grains of the plating film was “Large”, the anti-abrasion property (durability) thereof was “No good”, and the volume resistance (electrical resistance) thereof was “Low”. In Comparative Example 2 with the amount of the nanocarbon 114 added being greater than or equal to 0.3 g/L, the size of the crystal grains of the plating film was “Intermediate”, the anti-abrasion property thereof was “Acceptable”, and the volume resistance thereof was “High”. In contrast, in both Examples 1 and 2 with the amount of the nanocarbon 114 added being less than or equal to 0.2 g/L, the size of the crystal grains of the plating film was “Quite small”, the anti-abrasion property thereof was “Acceptable”, and the volume resistance thereof was “Low”. Therefore, it is clear that if the amount of the nanocarbon 114 added is less than or equal to 0.2 g/L, the size of the crystal grains of the plating film 122 can be made quite small and the anti-abrasion property thereof can be made high and, furthermore, the volume resistance thereof does not increase but remains low despite the quite small size of the crystal grains.
Observation of the microscopic photographs of the plating films 128 and 128A reveals that the crystal grains of the plating film 128 are clearly smaller than the crystal grains of the plating film 128A. Therefore, it is clear that the crystal grain size reduction method according to the another exemplary embodiment is able to reduce the size of the crystal grains of the plating film 128. Table 3, presented below, indicates results of a sliding test of the plating films 128 and 128A.
The plating film 128A of the comparative example was formed by blending nickel sulfamate in the plating solution 104 and omitting addition of the nanocarbon 114 into the plating solution 104. As indicated in Table 3, the plating film 128A was subjected to repeated sliding with a load of 50 g and was destroyed when the number of sliding cycles reached, averagely, 425.4.
On the other hand, the plating film 128 according to the another exemplary embodiment was formed by blending nickel sulfamate in the plating solution 104 and adding the nanocarbon 114 into the plating solution 104. As indicated in Table 3, the plating film 128 was destroyed when the number of sliding movements reached, averagely, 523.2. This clarifies that the plating film 128 was more durable than the plating film 128A of the another comparative example.
Therefore, by the size reduction method according to this exemplary embodiment, reform of the surfaces of the plating films 122 and 128 can be realized by making the crystal grains of the plating films 122 and 128 quite small without incorporation of the nanocarbon 114 into the plating films 122 and 128, respectively.
Incidentally, although in the foregoing exemplary embodiments of the invention, the plating metal 112 is silver (Ag) or nickel (Ni) as an example, these examples are non-limiting. The plating metal 112 may also be tin (Sn) or gold (Au). In such cases, it is hypothesized that the crystal grains of the plating film can also be made quite small to reform the surface of the plating film by causing the nanocarbon 114 to function as if the nanocarbon 114 was a catalyst, while avoiding incorporation of the nanocarbon 114 into the plating film.
While the exemplary embodiments of the invention have been described with reference to the drawings, it should be apparent that the invention is not limited by the foregoing examples or the like. It should be understood that a person having ordinary skill in the art can conceive various changes and modifications within the scope described in the appended claims and that such changes and modifications belong to the technical scope of the present invention.
The invention can be utilized as a method for forming reduced-size crystal grains of a plating film.
RESISTANCE VALUE (mΩ)
RESISTANCE VALUE (mΩ)
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-240928 | Dec 2017 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/046031 | 12/14/2018 | WO | 00 |
