Patent Application
20250066944
The present invention relates to a dispersion electrolyte for the electrodeposition of graphite-containing tin, nickel or tin-nickel layers, to a method for the electrodeposition of such a graphite-containing layer using the electrolyte, to a metal substrate coated with such a graphite-containing layer, and to the use of the dispersion electrolyte.
As a replacement for hard chrome layers, tin-nickel layers having comparable physical properties, in particular a high level of hardness, are described, for example in WO 2016/131916 A1. For the electrodeposition of such a tin-nickel layer, use was made of an electrolyte which, for ecological and health reasons, is much more advantageous than an electrolyte for hard chrome layers which uses chromium (VI) compounds and commonly also hexafluorosilicic acid, which can release toxic hydrogen fluoride at the typically low pH values.
Analogously to hard chrome layers, the tin-nickel layers described in WO 2016/131916 A1 have a high level of hardness, for example of HV 750 (HV=Vickers hardness) and higher. Furthermore, the hard tin-nickel layers are characterized by very good corrosion resistance to acids and bases, and can be machined using grinding, turning, milling and similar processes.
It is also possible to re-detach worn layers from a component, since the tin-nickel layer has a certain degree of brittleness.
Because of their properties, the hard tin-nickel layers, analogously to hard chrome layers, are commonly used as the “final layer”, i.e. the outer layer, of a component, primarily for protection of said component.
In principle, there is also a need for providing electronic components, for example a contact or an electrode, with a corrosion-resistant protective layer. In any case, recently, and in the course of increasing electrification, ever greater demands are placed on the sustainability of such components, since they should be reliable, long-lasting and as efficient as possible, i.e. as low-loss as possible. In addition, during the production thereof, substances and processes which are harmful to health and the environment should be dispensed with as far as possible.
Therefore, corrosion-resistant protective layers, for example for electric contacts and also for electrodes of batteries or fuel cells which, in addition to good corrosion resistance, also have low electric contact resistance (transitional resistance) and further improved tribological properties, in particular abrasion resistance, than conventional hard tin-nickel layers, are desirable. Therefore, there is a need for electrolytes by means of which layers having a combination of these advantageous properties can be electrolytically deposited.
It is also desirable to also improve these properties in tin layers and nickel layers which can also be used as protective layers against corrosion.
Thus, one object of the present invention consists in providing an electrolyte for the electrodeposition of tin layers, nickel layers or tin-nickel layers, having improved tribological properties and a low contact resistance, which electrolyte can be used safely and environmentally responsibly. Other objects of the invention consist in providing a method for the electrodeposition of such a tin layer, nickel layer or tin-nickel layer, a metal substrate coated with such a tin layer, nickel layer or tin-nickel layer, and a use of the electrolyte.
These objects are achieved by the dispersion electrolyte for the electrodeposition of graphite-containing tin layers, graphite-containing nickel layers or graphite-containing tin-nickel layers, a method for the electrodeposition of such a graphite-containing layer using the electrolyte, a metal substrate coated with such a graphite-containing layer, and by the use of the dispersion electrolyte, as stated in the independent claims. Preferred configurations are given in the dependent claims and below.
The figures show:
The present invention provides a dispersion electrolyte for the electrodeposition of graphite-containing tin layers, graphite-containing nickel layers or graphite-containing tin-nickel layers. The dispersion electrolyte comprises:
The dispersion electrolyte according to the invention has a pH of 4 to 7. The dispersion electrolyte is also abbreviated below to “electrolyte”.
The inventors have found, surprisingly, that the dispersion electrolyte according to the invention enables the electrodeposition of graphite particles together with the metal layer, thereby forming a graphite-containing layer. Furthermore, it was found, surprisingly, that integrating graphite particles into the tin, nickel or tin-nickel layer made it possible to improve the contact resistance (transitional resistance) and the tribological properties, in particular the abrasion resistance, compared to a corresponding layer without graphite particles. The improved tribological properties are also accompanied by increased durability of the coating, because of the reduced brittleness. The dispersion electrolyte thus makes it possible to form protective layers which make it possible, because of the lower contact resistance, to reduce electric energy losses and thus to prolong the running time, i.e. the service life, such that the components overall are more sustainable. Furthermore, the graphite-containing layer has very good corrosion resistance with respect to acids and bases. The electrolyte thus makes it possible to produce reliable, low-loss protective layers against corrosion.
It is thus possible, using the dispersion electrolyte, to achieve this advantageous combination of properties in one layer. To date, it was at best possible to obtain this combination of properties using a plurality of layers produced separately.
It is thus possible, with the dispersion electrolyte according to the invention, to achieve the electrodeposition (i.e. to electrolytically deposit) graphite-containing tin layers, graphite-containing nickel layers or graphite-containing tin-nickel layers having advantageous properties on a metal substrate. In this context, “graphite-containing” means that graphite is present in the form of graphite particles that can be spectroscopically detected. The graphite-containing layer thus denotes a graphite particle-containing layer and is a composite layer.
For a “tin layer”, substantially (e.g. at least 95 wt %, in particular at least 98 wt %) only tin atoms are deposited as metal. The underlying electrolyte then contains Sn2+ ions but virtually none, or no, Ni2+ ions. Analogously, a “nickel layer” denotes a layer for which substantially (e.g. at least 95 wt %, in particular at least 98 wt %) only nickel atoms are deposited as metal. The underlying electrolyte then contains Ni2+ ions but virtually none, or no, Sn2+ ions. For a “tin-nickel layer”, substantially (e.g. at least 95 wt %, in particular at least 98 wt %) only tin and nickel atoms are deposited as metals. Accordingly, the electrolyte must contain Sn2+ and Ni2+ ions.
The dispersion electrolyte is very flexible to handle and makes it possible to choose the conditions for electrodeposition from a broad range. Because the dispersion electrolyte has a pH of 4 to 7, i.e. is only weakly acidic, it can be used safely without forming hazardous hydrofluoric acid (hydrogen fluoride, HF). At lower pH values, i.e. below 3.5, there would be increasing formation of hydrogen fluoride if the electrolyte contained fluoride ions.
The dispersion electrolyte contains Sn2+ and/or Ni2+ ions; i.e. dissolved tin and/or nickel salts. In principle, all suitable tin and/or nickel salts can be used. Preference is given to using chlorine salts, i.e. SnCl2·nH2O and NiCl2·nH2O, because these are inexpensive and can be readily handled, and the chloride ions (CI) increase the conductivity of the electrolyte. Use can be made of anhydrous salts or salts with water of crystallization, which is indicated by the additionally specified “nH2O”, with n typically being 0 to 6. SnCl2·2H2O and/or NiCl2·6H2O are preferably used for the electrolyte, because these salts can be stored and are inexpensive.
Regardless of whether the dispersion electrolyte is used for the deposition of a graphite-containing tin layer or a graphite-containing tin-nickel layer, the electrolyte contains Sn2+ ions in particular at a concentration of 5 to 45 g/L, preferably 20 to 30 g/L and more preferably 23 to 27 g/L. For a graphite-containing nickel layer and a graphite containing tin-nickel layer, the concentration of the Ni2+ ions is in particular 10 to 65 g/L, preferably 50 to 65 g/L and more preferably 50 to 60 g/L.
According to a preferred embodiment, a graphite-containing tin-nickel layer is deposited with a molar ratio of tin to nickel of 1:1, such that the nickel content, relative to the metals tin and nickel, is approximately 35 wt %. As explained below, the nickel content can be modified by the process conditions, such that the graphite-containing tin-nickel layer can contain 30 to 40 wt %, preferably 30 to 38 wt %, more preferably 32 to 37 wt %, and even more preferably 35 wt % nickel relative to the metals tin and nickel. For such a graphite-containing tin-nickel layer, the electrolyte preferably contains Sn2+ ions at a concentration of 20 to 30 g/L, and more preferably 23 to 27 g/L, and Ni2+ ions at a concentration of preferably 50 to 65 g/L, and more preferably 50 to 60 g/L. The nickel content in the electrolyte is typically higher than the tin content, because nickel is less noble than tin and therefore is not so readily deposited. In this embodiment, the weight ratio of Sn2+ ions: Ni2+ ions is preferably 1:1 to 1:4, more preferably 1:2 to 1:3.
Surprisingly, it was found that the nickel content in the graphite-containing tin-nickel layer can also be considerably reduced in comparison to the above-described typical 1:1 ratio. According to this embodiment, the nickel content, relative to the metals tin and nickel, is 10 to 20 wt %, preferably 13 to 18 wt % and even more preferably 14 to 17 wt %. In order to achieve the comparatively low nickel content, the weight ratio of tin to nickel in the electrolyte should be 8:1 or more, in particular 10:1 or more.
5 to 200 g/L of graphite particles are dispersed in the dispersion electrolyte according to the invention, leading to the abovementioned effects. The content of graphite particles in the electrolyte is preferably 20 to 150 g/L, and more preferably 40 to 100 g/L.
Generally, a lower content of graphite particles is used if the dispersion electrolyte also has a low content of Sn2+ ions and/or Ni2+ ions. Analogously, at a high content of Sn2+ ions and/or Ni2+ ions, a higher quantity of graphite particles is preferably used. In the dispersion electrolyte, the Ni:graphite weight ratio is preferably 1:0.5 to 1:2, more preferably 1:0.5 to 1:1, and/or the Sn:graphite weight ratio is preferably 1:0.5 to 1:5, more preferably 1:1.5 to 1:2.5. The graphite content of the graphite-containing layer can also be modified by the concentration of graphite in the electrolyte.
The type of graphite used is in principle not limited; use can thus be made of natural graphite or synthetic graphite types. The median (d50) particle size of the graphite particles is typically in the range from 20 nm to 20 μm, preferably 1 to 10 μm, and more preferably 1.5 to 8 μm. The particle size is determined by means of laser diffraction according to ISO 13320:2020, e.g. using a HELOS (Helium-Neon Laser Optical System) spectrometer. Generally, the particle size should be less than or equal to the desired layer thickness of the graphite-containing layer. The particle size can be adjusted by conventional milling and/or screening processes, by means of which particles which are too large or small can be separated off. Suitable graphite particles are also commercially available.
Smaller graphite particles, in particular having a median particle size of 20 nm to 0.5 μm, in relatively hard layers, tend to result in a moderate improvement in the tribological properties, and also in lower contact resistances. Graphite particles having a median particle size of 1.5 to 8 μm tend to result in very good tribological properties and in a significant reduction in the contact resistance, albeit with a somewhat lower level of hardness than comparable nickel or tin-nickel layers without graphite particles.
In order to obtain efficient dispersion of the graphite particles in the electrolyte and uniform integration of the graphite particles in the graphite-containing layer, the dispersion electrolyte according to the invention contains an anionic dispersant. The anionic dispersant typically has sulfate groups (—OSO3—), sulfonate groups (—SO3—), carboxylate groups (—CO2—) or carboxy groups (—CO2H) which can then be present as anions in aqueous solution. Alkali metal ions, preferably Na+, and ammonium anions (NH4+) are generally used as counterions, because they result in good water solubility, increase conductivity and do not negatively influence electrodeposition. Sulfate groups and sulfonate groups are in dissociated form at the pH of the dispersion electrolyte, i.e. they are not in the protonated form. In contrast, carboxy groups and carboxylate groups can be in equilibrium with one another.
The anionic dispersant is preferably at least one selected from the group consisting of sulfate compounds having an alkyl group, an aralkyl group or an aromatic group in each case having 6 to 24 carbon atoms; sulfonate compounds having an alkyl group, an aralkyl group or an aromatic group in each case having 6 to 24 carbon atoms; and polymers containing carboxylate or carboxy groups. Combinations of different anionic dispersants can be used. A “sulfate compound” denotes herein an organic sulfate group-containing compound. Analogously, a “sulfonate compound” is understood to mean an organic sulfonate group-containing compound.
The anionic dispersant is particularly preferably at least one selected from the group consisting of sulfate compounds having an alkyl group having 6 to 24 carbon atoms; aromatic sulfonate compounds, wherein the underlying aromatic group in each case has 6 to 14 carbon atoms; and polymers containing poly(meth)acrylic acid and salts thereof.
Very particularly preferably, the dispersants are sulfate compounds having an alkyl group having 6 to 24 carbon atoms selected from the group consisting of fatty alcohol sulfates, fatty alcohol polyether sulfates, fatty alcohol aryl polyether sulfates and combinations thereof, and/or polymers having aromatic sulfonate groups, wherein the underlying aromatic groups in each case have 6 to 14 carbon atoms and wherein the aromatic sulfonate groups are preferably derived from phenylsulfonic acid, phenolsulfonic acid or naphthylsulfonic acid. In this case, the alkyl group having 6 to 24 carbon atoms originates from the fatty alcohol. Polyether groups are in particular polyethylene glycol groups. Polymers having aromatic sulfonate groups are for example condensates of aromatic sulfonate compounds, for example phenylsulfonic acid, phenolsulfonic acid or naphthylsulfonic acid, with formaldehyde. Fatty alcohol sulfates and fatty alcohol polyglycol ether sulfates containing an alkyl group having 6 to 20 carbon atoms, in particular 8 to 18 carbon atoms, are more preferably present as sulfate compounds.
Furthermore, a combination of at least one of the preferred sulfate compounds and at least one of the preferred sulfonate compounds is particularly preferred as anionic dispersant. These are preferably used in a weight ratio of 1:10 to 10:1, in particular 1:8 to 3:1.
Examples of anionic dispersants are Sokalan® (BASF SE, poly(meth)acrylate-containing polymer or salts thereof), e.g. Sokalan® SR, sodium-phenolsulfonic acid condensates, sodium-phenylsulfonic acid condensates, sodium-naphthalenesulfonic acid condensates, Disponil® APE (BASF SE, alkyl polyglycol ether sulfate) and fatty alcohol sulfates having 6 to 20 carbon atoms, for example 2-ethylhexyl sulfate (e.g. sodium metasulfate), lauryl sulfate, oleyl sulfate, stearyl sulfate and sulfates of mixed fatty alcohols, in particular the corresponding sodium salts thereof.
The anionic dispersant is used in the dispersion electrolyte at a concentration of 1 to 25 g/L. It is preferably present in the electrolyte at a concentration of 2 to 20 g/L, preferably 4 to 10 g/L. Typically, a smaller quantity of anionic dispersant is used for a lower concentration of graphite particles and, analogously, a larger quantity is used for a higher concentration of graphite particles.
As stated above, the anionic dispersant enables effective dispersion of the graphite particles in the electrolyte and uniform incorporation of the graphite particles in the graphite-containing layer. Advantageously, it is not necessary to disperse the graphite particles beforehand; rather, they can generally be introduced into the electrolyte in pulverulent form. Advantageously, to this end, it is not necessary to homogenize the electrolyte in an ultrasound bath, thereby reducing the preparatory effort and making it possible to use the dispersion electrolyte economically and on a large scale. Furthermore, it is also possible to dispense with phosphates and pyrophosphates in the dispersion electrolyte according to the invention, thereby improving the solubility of the tin and/or nickel salts.
A further component of the dispersion electrolyte is the complexing agent, which serves to maintain the Ni2+ and/or Sn2+ salts stably in solution and to mobilize them, such that the graphite-containing layer can be formed uniformly and with a good deposition rate. In principle, any known complexing agent used for tin and nickel electrolytes can be used as complexing agent. Preferably, the complexing agent is a chelating, water-soluble, organic compound having at least three functional groups selected from amino groups, carboxy groups and carboxylate groups. Preferably, at least two of the functional groups are amino groups. The amino groups are selected from primary, secondary and tertiary amino groups. More preferably, the complexing agent contains one or more secondary and/or tertiary amino groups. The functional groups provided for coordination are typically spaced apart from one another by 2 or 3 carbon atoms, such that a stable chelate complex can form.
Particularly preferably, the complexing agent is at least one selected from the group consisting of EDTA, DETA, DOTA and DOTATOC. EDTA stands for ethylenediaminetetraacetic acid. DETA denotes diethylenetriamine. DOTA stands for 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid. DOTATOC denotes a DOTA-derived complexing agent in which a DOTA molecule is bonded via an amine bond to the N-terminus of an octapeptide (in particular Phe-Cys-Tyr-Lys-Thr-Cys-Thr).
The quantity of complexing agent used is based above all on the concentration of the Sn2+ and/or Ni2+ ions in the dispersion electrolyte. Customarily, the complexing agent is present in the electrolyte at a concentration of 5 to 70 g/L, preferably 10 to 65 g/L, more preferably 40 to 60 g/L.
The dispersion electrolyte additionally contains a conducting salt. A conducting salt is understood to mean a water-soluble salt that increases the conductivity of the electrolyte. Preferably, the conducting salt is at least one selected from the group consisting of sodium chloride, potassium chloride, ammonium chloride, sodium acetate, potassium acetate, ammonium acetate, ammonium fluoride, ammonium bifluoride, sodium fluoride and potassium fluoride. The electrolyte preferably contains at least one conducting salt selected from ammonium fluoride, ammonium bifluoride and ammonium acetate. Advantageously, the conducting salt also increases the solubility of the tin and/or nickel salts.
Preferably, a dispersion electrolyte for the electrodeposition of graphite-containing tin-nickel layers, in particular at a nickel content of 32 to 37 wt %, e.g. 35 wt %, contains at least one fluoride (F−)-containing conducting salt. It can be combined with other conducting salts, but does not have to be. It is assumed that the fluoride ions stabilize a binuclear complex of tin and nickel and can thereby support the formation of a very constant alloying ratio of the tin-nickel layers at the molar ratio of 1:1.
The conducting salt is typically present in the electrolyte at a concentration of 5 to 70 g/L, preferably 10 to 65 g/L, more preferably 40 to 60 g/L.
According to a preferred embodiment, the dispersion electrolyte comprises:
In this case, the anionic dispersant is at least one selected from the group consisting of sulfate compounds having an alkyl group having 6 to 24 carbon atoms; aromatic sulfonate compounds, wherein the underlying aromatic group in each case has 6 to 14 carbon atoms; and polymers containing poly(meth)acrylic acid and salts thereof; preferably, it is a combination of at least one of the sulfate compounds and at least one of the sulfonate compounds, for example in the abovementioned ratio.
According to a particularly preferred embodiment, the dispersion electrolyte comprises:
In this case, the anionic dispersant is preferably at least one selected from the group consisting of fatty alcohol sulfates and fatty alcohol polyether sulfates in which the alkyl group of the fatty alcohol portion has 6 to 24 carbon atoms, in particular 6 to 20 carbon atoms and preferably 8 to 18 carbon atoms, and polymers having aromatic sulfonate groups, wherein the underlying aromatic groups in each case have 6 to 14 carbon atoms and wherein the aromatic sulfonate groups are preferably derived from phenylsulfonic acid, phenolsulfonic acid or naphthylsulfonic acid. Particularly preferably, in this case, the at least one anionic dispersant is a combination of fatty alcohol sulfate or fatty alcohol polyglycol ether sulfate containing an alkyl group having 6 to 20 carbon atoms, in particular 8 to 18 carbon atoms, and a phenylsulfonic acid polymer (phenylsulfonic acid condensate), a phenolsulfonic acid polymer (phenolsulfonic acid condensate) or a naphthylsulfonic acid polymer (naphthylsulfonic acid condensate), for example in the abovementioned ratio.
Furthermore, the dispersion electrolyte can contain customary additives as are known from conventional electrolytes for the electrodeposition of tin, nickel and tin-nickel layers. In addition to the above-described anionic dispersant, other dispersants can also be added to the electrolyte. However, preferably only the anionic dispersant is used.
The dispersion electrolyte can be formed by mixing or dissolving the tin and/or nickel salts and the other components, i.e. the conducting salt, the complexing agent, the anionic dispersant, a graphite powder and water. The graphite powder can be added as a solid, without having to disperse it beforehand or having to treat the mixture with ultrasound for dispersing. When forming the electrolyte, stirring is preferably carried out and, if required, gentle heating is carried out, in order to accelerate the dissolution of the components. The pH can for example be adjusted by adding hydrochloric acid or sodium hydroxide, ammonia, potassium hydroxide or an aqueous solution thereof. The dispersion electrolyte according to the invention can advantageously be produced on a large scale and is thus suitable for industrial applications. The electrolyte can be stored, and it is possible to use stirring to re-disperse any graphite particles that might have settled out.
Furthermore, the present invention provides a method for the electrodeposition of a graphite-containing tin layer, a graphite-containing nickel layer or a graphite-containing tin-nickel layer. The method comprises the electrodeposition of the graphite-containing layer on a metal substrate using the dispersion electrolyte according to at least one of the embodiments described herein. The method is carried out at temperatures in the range from 50 to 85° C., preferably 55 to 70° C. The dispersion electrolyte is heated to this temperature.
The method can be carried out using all customary apparatuses for the electrodeposition of tin-nickel layers. If the electrolyte contains fluoride salts, plastic vessels are more suitable than glass vessels. Typically, the electrolyte is thoroughly mixed during the deposition, for example is stirred.
The metal substrate is connected as the cathode. This can for example be a metal component, a component for a contact, a switch or an electrode. Preferably, the graphite-containing layer is generated as the final layer, i.e. as the outer layer, on the metal substrate. “Metal substrate” is also understood to mean a metal layer which can be coated electrolytically, i.e. for example a premetallized plastic. Examples of metals are, inter alia, copper, nickel, noble metals such as palladium or platinum, steel, stainless steel, brass and bronze. The surface to be coated can be purified beforehand using customary methods, for example can be degreased.
Nickel anodes can for example be used as anode. This makes it possible to control the content of nickel ions in the electrolyte in a simple manner. It is also possible to use a plurality of anodes.
Furthermore, the method enables great flexibility in carrying out the method. Preferably, deposition is carried out at a current density of 0.1 to 10 A/dm2, preferably 0.5 to 5 A/dm2. The current density can be used, for example, to influence the content of graphite and, in the case of a graphite-containing tin-nickel layer, the content of nickel. In principle, a higher current density leads to a higher graphite content in the graphite-containing layer. Current densities of 5 A/dm2 and more make it possible to increase the nickel content of a graphite-containing tin-nickel layer.
It has also been found that a higher tin concentration in the electrolyte tends to lead to a lower nickel content in a graphite-containing tin-nickel layer. Likewise, a lower nickel concentration in the electrolyte can somewhat reduce the nickel content in the graphite-containing tin-nickel layer. For example, a tin/nickel ratio of 8:1 and higher, in particular 10:1 and higher, makes it possible to set a nickel content of 10 to 20 wt % in the graphite-containing tin-nickel layer.
In some cases, the pH of the electrolyte can also influence the nickel content of a graphite-containing tin-nickel layer. A higher pH tends to lead to a somewhat higher nickel content.
In principle, the method can be used to generate a graphite-containing layer of any desired layer thickness. Typically, the graphite-containing layer is deposited with a layer thickness of 4 to 30 μm, preferably 5 to 20 μm and more preferably 5 to 12 μm. The layer thickness is measured by means of X-ray fluorescence spectrometry according to DIN EN ISO 3497 (2001-12), for example using a Fischerscope XDAL X-ray fluorescence spectroscope.
Furthermore, the invention provides a coated metal substrate which can be obtained by the method described herein, using the dispersion electrolyte described herein. Accordingly, the metal substrate is coated with a graphite-containing tin layer, a graphite-containing nickel layer or a graphite-containing tin-nickel layer. The graphite-containing layer contains 0.1 to 8 wt % graphite relative to the total weight of the graphite-containing layer.
Preferably, the graphite-containing layer contains 0.5 to 3 wt % and more preferably 0.8 to 2.3 wt % graphite relative to the total weight of the graphite-containing layer.
As stated above, compared to a corresponding layer without graphite particles, the graphite-containing layer has a lower contact resistance and improved tribological properties.
According to one embodiment, the graphite-containing layer is a graphite-containing tin layer. Naturally, a tin layer is very soft and ductile. Surprisingly, it was found that, even in such a layer, the tribological properties, i.e. in particular the abrasion resistance, could be significantly increased. Likewise, it was found that the contact resistance could once again be significantly lowered. The graphite-containing tin layer is particularly suitable for applications in plug-in contacts and sliding contacts.
According to another embodiment, the graphite-containing layer is a graphite-containing nickel layer. Again, it was found that, by integrating graphite particles into the nickel layer made it possible to significantly improve the tribological properties and the contact resistance compared to a corresponding nickel layer without added graphite. As a result, the brittleness of the nickel layer is also reduced, giving it extended durability. At the same time, it was possible to obtain a good hardness of the graphite-containing nickel layer even without a subsequent hardening step.
According to a preferred embodiment, the graphite-containing nickel layer has a mean coefficient of friction of 0.4 or less, preferably 0.3 or less and even more preferably 0.2 or less, and/or a hardness of at least HV 300 (Vickers hardness), preferably at least HV 350 and more preferably at least HV 400. The mean coefficient of friction and the hardness are determined using the methods described below. In particular, these properties can be adjusted via the graphite content in the layer and also the size of the graphite particles.
The graphite-containing nickel layer is particularly suitable as a protective layer and for electrode coatings, plug-in connections and contact surfaces. The graphite-containing nickel layer can serve as a more inexpensive replacement for a tin-nickel layer if the requirements for corrosion resistance are not so high. Dense and well-adhering oxide layers form on freshly-deposited nickel layers, offering good protection against corrosion from dilute acids and bases. While conventional nickel layers have a high contact resistance due to the oxide layer, this property is improved in the nickel layers according to the invention through the incorporation of graphite.
According to another embodiment, the graphite-containing layer is a graphite-containing tin-nickel layer. The graphite-containing tin-nickel layer typically has a nickel content of 10 to 40 wt %. In this case, it is preferred for the nickel content to either be in the range from 13 to 18 wt % or in the range from 30 to 38 wt % relative to the metals tin and nickel.
Surprisingly, it was found that it was possible to electrolytically deposit graphite-containing tin-nickel layers having a proportion of nickel far below the otherwise customary molar ratio of 1:1 (approximately 35 wt %). At a nickel content of 13 to 18 wt %, in particular 14 to 17 wt %, relative to the metals tin and nickel, the graphite-containing tin-nickel layer has very good tribological properties and a low contact resistance. The increased incorporation of tin into the layer once again significantly lowers the contact resistance. The graphite-containing tin-nickel layers according to this embodiment are particularly suitable for plug-in connections, switching contacts and as a replacement for silver layers.
In another configuration, the nickel content of the graphite-containing tin-nickel layer is preferably 30 to 38 wt %, more preferably 32 to 37 wt %, e.g. 35 wt %, relative to the metals tin and nickel. Compared to the corresponding tin-nickel layer without graphite, the tribological properties were improved and the contact resistance was lowered. Surprisingly, it was found that these properties can also be combined with good hardness. Thus, the graphite-containing tin-nickel layer preferably has a hardness of HV 200 or more, more preferably of HV 300 or more.
The graphite-containing tin-nickel layer preferably has a mean coefficient of friction of 0.4 or less, preferably 0.2 or less.
The contact resistance of the graphite-containing tin-nickel layer in relation to a gold contact at 25° C. is in particular 50 mΩ (mOhm) or less, preferably 40 mΩ or less, and even more preferably 30 mΩ or less. The contact resistance is determined according to the method stated below.
The hardness, the mean coefficient of friction and the contact resistance of the tin-nickel layer can in particular be adjusted via the graphite content in the layer and also the size of the graphite particles.
The graphite-containing layers formed with the electrolyte according to the invention are suitable, as described above, for various applications, in particular electronic components having low contact resistance and, because of the tribological properties and resistance to corrosion from acids and bases, with good durability. The invention also provides the use of the dispersion electrolyte described herein for producing an electronic component having a graphite-containing tin layer, a graphite-containing nickel layer or a graphite-containing tin-nickel layer. Preferably, the dispersion electrolyte is used to produce a graphite-containing tin-nickel layer as a protective layer or final layer on a contact or an electrode. The electrodes can for example be used in batteries, electrolyzers or fuel cells. Further fields of application are electric connector technologies and catalyst layers.
The present invention is illustrated below by means of examples. It is not limited to these examples.
The following measurement methods were used.
The layer thickness was measured using a Fischerscope XDAL X-ray fluorescence spectroscope according to DIN EN ISO 3497 (2001-12). This method is used for layer thicknesses up to 20 μm. For layer thicknesses of more than 20 μm, the layer thickness is determined by a microscope on the cross section of the layer. A mean measurement value is stated.
The composition of the alloys (i.e. tin and nickel) was also analyzed by X-ray fluorescence spectroscopy using the abovementioned device according to DIN EN ISO 3497 (2001-12). The measurement values are obtained with an accuracy of +2 wt % or better. The corresponding content is given relative to the metals in the deposited layer.
The graphite content was determined by means of glow discharge optical emission spectroscopy (GDOES) according to DIN ISO 11505 (2018-02) over the cross-section of the graphite-containing layer. The content is stated relative to the entire graphite-containing layer.
The investigations regarding tribology and mean coefficient of friction were determined as pin-on-disk tribology according to the test specification SOP 4CP1 by the “Forschungsinstitut Edelmetalle+Metallchemie (fem)”, Schwäbisch Gmünd (Germany). A CSEM pin-on-disk tribometer machine was used as the test device. The counterbody was a ball having a diameter of 6 mm, of the alloy 100 Cr6, material number 1.3505. The following parameters were used:
The mean coefficient of friction was determined in a long-duration test with 10,000 cycles (revolutions). The mean coefficient of friction (μ) is dimensionless.
The contact resistance was determined at 25° C. in a 4-point measurement in relation to gold contacts, according to the specification MIL-DTL-81706B (ContRes-ConCoat-001 of the “Forschungsinstituts Edelmetalle+Metallchemie (fem)”, Schwäbisch Gmünd (Germany)) at a contact pressure of up to 2 N. Keithley Instruments INC. Model 2410, 4393118, C34 was used as the measuring device.
The hardness was tested according to DIN EN ISO 14577-1 (2015-11) on the surface, optionally after polishing (i.e. conditioning) the surface, of the respective deposited layer. The Vickers hardness is stated using the unit “HV”. The 0.005 test force used corresponds to 0.049 N (multiplied by a proportionality factor of 0.102). The Vickers hardness is dimensionless.
The hydrogen fluoride emission was determined using a Dräger accuro device, using Dräger hydrogen fluoride test tubes 0.5/a for a content of 0.5 to 15 ppm.
The MAK value (2001) for hydrogen fluoride is 2 ppm.
Steel sheets (DC03) of 50×120×1-2 mm were used as metal substrate. The pre-treatment was carried out as follows:
Slotoclean AK 160 (contains NaOH and disodium metasilicate), SLOTOCLEAN EL DCG (contains NaOH, disosium metasilicate and sodium carbonate), and SLOTOCLEAN BEF 30 (contains but-2-yne-1,4-diol and isotridecanol ethoxylated) are products from the company Dr. Ing. Max Schlötter.
The following reagents, inter alia, were used for the experiments.
VP 11 2571: Bath additive from the company Dr. Ing. Max Schlötter, containing ammonium bifluoride as conducting salt (15 to 20 wt %) and a polyamine as complexing agent (15 to 20 wt %).
VP 11 2572: Bath additive from the company Dr. Ing. Max Schlötter, containing, as anionic dispersant, a sodium salt of a polymer of an aromatic sulfonic acid (20 to 25 wt %).
VP 11 2573: Bath additive from the company Dr. Ing. Max Schlötter, containing, as anionic dispersant, a sodium sulfate of a fatty alcohol having 6 to 20 carbon atoms (15 to 20 wt %).
Culmo AN 11-1 additive: Bath additive from the company Dr. Ing. Max Schlötter, containing 2-propylheptanol, ethoxylated, propoxylated (15 to 25 wt %).
Tin bath additive SAT 31 1: Bath additive from the company Dr. Ing. Max Schlötter, containing 2-propylheptanol, ethoxylated, propoxylated (15 to 25 wt %) and 1,2-dihydroxybenzene (1 to 7 wt %).
Nickel bath additive SLOTONIK M: Bath additive from the company Dr. Ing. Max Schlötter, containingC12-14 alkyl ether sulfate with EO sodium salt (3 to 5 wt %).
Synthetic graphite (40 nm) and highly-conductive synthetic graphite (40 nm) from Asbury Carbons, Inc.
For examples 1 to 7 and comparative examples 1 and 2 (“CE1” and “CE2”), tin-nickel electrolytes having the components of table 1 were placed in a plastic beaker. To this end, firstly, SnCl2·2H2O and NiCl2·6H2O were dissolved in a mixture of distilled water and the bath additives, with stirring (250 rpm) at 55° C. Then, for examples 1 to 7 and comparative example 2, the corresponding graphite powder was slowly added and dispersed with stirring (250 rpm). The median (d50) particle size of the graphite particles is given in brackets as the particle size in table 1. The pH was optionally adjusted by adding hydrochloric acid. The quantity of electrolyte was 2 L.
Steel sheets as described above were pretreated and then electrolytically coated under the conditions of table 1. The electrolyte was thoroughly mixed using a (40 mm) stirrer rod (250 rpm). Each of the steel rods was connected as cathode and immersed to a depth of 10 cm in the electrolyte. Two Ni electrodes (50×120×5 mm), arranged on both sides in parallel to the cathode and each at a distance of 4 cm, served as anodes. The immersion depth of the anodes was also 10 cm. The two anodes connected in parallel were connected in series with the electrolyte, the cathode and a source of direct current.
The nickel content and the graphite content were determined and the layer thickness of the deposited layer was measured.
In examples 1 to 7 and in comparative examples 1 and 2, tin-nickel layers were electrolytically deposited on the copper substrate with good adhesive strength. In examples 1 to 7, there was uniform deposition of the metals and uniform incorporation of graphite particles into the layer, regardless of the size of said particles. In comparative example 2, without the addition of an anionic dispersant, there was no visible graphite incorporated into the tin-nickel layer, and therefore the graphite content was significantly less than 0.1 wt %.
The tin-nickel layers formed in examples 1 to 7 and comparative examples 1 and 2 were investigated regarding their hardness, tribological properties (coefficient of friction) and contact resistance. To this end, the above-described methods were used. The results are summarized in table 1 above.
The customary tin-nickel layer from comparative example 1 is characterized by a high level of hardness. Furthermore, a contact resistance in relation to gold contacts of 75.7 mΩ and a mean coefficient of friction of 0.57 were measured. Because of the low, or absent, graphite incorporation, the physical properties of comparative example 1 were similar to those of comparative example 2.
In examples 1 to 7, it was possible to significantly improve both the tribological properties and the contact resistance of the tin-nickel layer compared to comparative examples 1 and 2. These examples therefore prove that the successful incorporation of graphite particles into the electrolytically deposited tin-nickel layer made it possible to improve durability and conductivity compared to a tin-nickel layer without incorporated graphite. Furthermore, it was noted that it was possible to obtain layers with a comparatively high level of hardness despite the incorporation of graphite.
It was also clear that lower mean coefficients of friction and contact resistances tended to be measured with larger graphite particles (examples 1 to 3). Smaller graphite particles and lower quantities of incorporated graphite tended to result in somewhat higher mean coefficients of friction, but also somewhat higher levels of hardness (examples 4 to 6). Example 7, with a nickel content of only 16 wt %, resulted in very good tribological properties, i.e. a low mean coefficient of friction, and also a low contact resistance. The high tin content of approximately 84 wt % was accompanied by a lower level of hardness.
Example 2 was repeated and the pH was set at 4.0 and the bath temperature at 60° C. The hydrogen fluoride emission was determined according to the above method.
The hydrogen fluoride emission was below the limit of detection and was thus less than 0.5 ppm. The dispersion electrolyte can therefore be used safely.
The procedure was carried out analogously to examples 1 and 7 and comparative example 1. For example 9 and comparative example 3 (CE3), no nickel salt was used, in order to produce a tin layer. Analogously, for example 10 and comparative example 4 (CE4), no tin salt was used, in order to produce a nickel layer. The electrolyte compositions, the conditions for electrodeposition, and the compositions of the layers formed, are summarized in table 2.
In comparative examples 3 and 4, conventional tin or nickel layers were deposited electrolytically. As shown in examples 9 and 10, it was also possible to successfully produce graphite-containing tin or nickel layers using the dispersion electrolyte according to the invention. Substantial adaptation of the dispersion electrolyte (aside from the nickel or tin salt) was not required to achieve this.
Furthermore, it was observed that, both in the naturally soft tin layers and in the harder and more brittle nickel layers, a real improvement in the tribological properties (lower mean coefficient of friction) and in the contact resistance was observed (table 2: example 9 compared to comparative example 3, and example 10 compared to comparative example 4).
Thus, it was also possible to improve durability and conductivity for electrolytically deposited tin or nickel layers.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/087846 | 12/30/2021 | WO |
