EP1738000A2

EP1738000A2 - Production of a structured hard chromium layer and production of a coating

Info

Publication number: EP1738000A2
Application number: EP05700700A
Authority: EP
Inventors: Rudolf Linde; Stefan Dürdoth; Wolfgang Stuckert
Original assignee: Federal Mogul Burscheid GmbH
Current assignee: Federal Mogul Burscheid GmbH
Priority date: 2004-04-21
Filing date: 2005-01-05
Publication date: 2007-01-03
Anticipated expiration: 2025-01-05
Also published as: WO2005108648A3; JP2007533852A; JP4542134B2; WO2005108648A2; BRPI0506445B1; US8110087B2; BRPI0506445A; EP1738000B1; PT1738000T; US20080060945A1; DE102004019370B3

Abstract

The invention relates to a method for producing a structured hard chromium layer, during which chromium from an electrolyte is deposited onto a workpiece, which contains: a) a Cr(VI) compound in a quantity corresponding to between 50 and 300 g/l of chromic anhydride; b) 0.5 g/l to 10 g/l sulfuric acid, and; c) 5 g/l to 15 g/l aliphatic sulfonic acid having 1 to 6 carbon atoms. The electrolyte contains essentially no compounds from the group consisting of ammonium molybdate, alkaline molybdate, alkaline earth molybdate, ammonium vanadate, alkaline vanadate and alkaline earth vanadate, ammonium zirconate, alkaline zirconate and alkaline earth zirconate, and is processed with a cathodic efficiency of 12 % or less. The invention also relates to a method for producing a coating, to a structured hard chromium layer, a coating and an electrolyte.

Description

Production of a structured hard chrome layer and production of a coating

The invention relates to a method for producing a structured hard chrome layer on a workpiece, a method for producing a coating with a structured hard chrome layer, structured hard chrome layers and coatings obtainable thereby and an electrolyte for carrying out the method.

Hard chrome layers produced electrochemically are not only used for the decorative finishing of surfaces. Rather, hard chrome layers are also applied as functional coatings to conductive and non-conductive workpieces, for example in order to perform a protective function or to favorably influence the surface properties. Typical applications are therefore protective layers made of hard chrome to reduce corrosion, wear or friction, as well as structured hard chrome layers on printing rollers to facilitate wetting with printing inks or on starch, embossing and deep-drawing tools to optimize manufacturing processes in industry.

EP 0 196 053 A2 and DE 34 02 .554 A1 each describe methods for the electrodeposition of hard chrome on metallic surfaces from an aqueous electrolyte containing chromic acid, sulfuric acid or sulfate and a sulphonic acid, with cathodic flow extraction ≥ 20% being used. The composition of the electrolyte is intended to rule out the risk of a disturbing etching of the surface to be coated. However, no structures are created in the hard chrome layer.

Another electrochemical method for the deposition of hard chrome layers on workpieces is known from US Pat. No. 5,196,108. The electrolyte used contains a molybdenum anion, so that it is possible to work with a high cathodic current efficiency. Structuring the hard chrome layer is not the goal of this process. An electrochemical method for producing structured hard chrome layers is known, for example, from DE 44 32 512 A1. The structure of the hard chrome layer is made possible by adding salts, such as salts of the elements selenium or tellurium, to the electrolyte. However, the layers produced have a spherical structure with spherical shapes with a size between less than 1 μm and several μm. This often results in a non-uniform spherical structure of the hard chrome layer, which is not suitable for all applications.

It would therefore be desirable to produce structured hard chrome layers that have a uniform structure and bring about improvements in the tribological properties of the workpiece, such as reduced wear and, in the case of insufficient lubrication, favorable emergency running properties.

The object of the invention is therefore to provide a method for producing a structured hard chrome layer with which the disadvantages of the prior art are overcome.

According to the invention, this object is achieved by a method for producing a structured hard chrome layer, chrome being deposited from an electrolyte onto a workpiece which contains:

(a) Cr (VI) compound in an amount corresponding to 50 g / l to 300 g / l chromic anhydride;

(b) 0.5 g / l to 10 g / l sulfuric acid;

(c) 5 g / l to 15 g / l aliphatic sulphonic acid having 1 to 6 carbon atoms;

wherein the electrolyte contains essentially no compound selected from ammonium, alkali and alkaline earth molybdate, ammonium, alkali and alkaline earth vanadate and ammonium, alkali and alkaline earth zirconate and is operated with a cathodic current efficiency of 12% or less.

The method according to the invention is used to produce structured hard chrome layers which are cup-shaped and / or labyrinth-like and / or column-shaped. This is achieved by deliberately influencing the cathode film formed during the electrochemical deposition, which is explained below. The electrolytes used in galvanic processes contain salts that dissociate in anions and cations in the aqueous environment. This creates a hydration shell around the dissociated ions. During an electrochemical deposition, hydrated metal ions of the electrolyte migrate to the workpiece to be coated, which is connected as a cathode. The so-called cathode film is located in the border area between the electrolyte and the cathode directly on the surface of the cathode. If a hydrated metal ion gets into this phase boundary, it absorbs electrons from the cathode and is thereby aligned in the diffusion zone.

An electrochemical double layer, the "Helmholtz double layer", is formed under this diffusion zone and directly on the cathode surface. This consists of an electrically charged zone at the interface between the electrolyte and the cathode and is approximately a few atomic or molecular layers thick. Electrons or directed dipole molecules are involved. Since the "Helmholtz double layer" is positively charged on one side and negatively on the other side, it behaves on the cathode like a plate capacitor with a very small plate spacing.

In order for the metal ion to get onto the workpiece surface and to be built into a growth site on the surface of the workpiece, it must overcome the cathode film. This process can be influenced by a suitable choice of the deposition conditions, such as chemical composition of the electrolyte, temperature, hydrodynamics and electrical current strength. In order to form uniformly thick metal layers on the workpiece, the deposition conditions for the electrolyte are selected so that the permeability of the cathode film to the metal ion is as uniform as possible.

If the element chromium is to be deposited from an aqueous electrolyte on a workpiece, it is in a strongly acidic solution as a negatively charged hydrogen dichromate complex. Chromium has oxidation level 6 therein, although small amounts of chromium (III) compounds can also be present.

If such a solution is electrolyzed, a solid film is formed on the cathode, which prevents chromium deposition. Only hydrogen is produced which, due to its small radius, can pass through the solid cathode film, in contrast to the large hydrogen dichromations. It is only through the addition of foreign ions, such as sulfate and chloride, that the cathode film becomes permeable to the chromipene and the chromium is deposited over various oxidation levels (see "Chemistry for Electroplating", Leutze Verlag, 2nd edition, 1993). According to the invention, the use of an electrolyte with a chromium (VI) compound in an amount which corresponds to 50 to 300 g / l, preferably 50 to 150 g / l, chromic anhydride, 0.5 to 10 g / l sulfuric acid and 5 g / 1 to 15 g / l aliphatic sulphonic acid with 1 to 6 carbon atoms to form a cathode film with a very dense barrier layer. If a suitably high coating current density is applied, the barrier layer breaks through, which leads to the formation of a chromium layer of uneven layer thickness on the workpiece, with a cathodic current efficiency of 12% or less.

Structured hard chrome layers with cup-shaped and / or labyrinth-like and / or column-shaped structures are thus produced without the use of additives which favor the formation of the barrier layer of the cathode film. It is therefore possible to dispense with compounds which promote the formation of a dense cathode film, such as, for example, ammonium, alkali and alkaline earth molybdate, ammonium, alkali and alkaline earth vanadate and ammonium, alkali and alkaline earth zirconate.

The cathodic current yield of 12% or less ensures the formation of the structured hard chrome layer in the method according to the invention, since the structuring of the hard chrome layer is not obtained with a higher current yield.

The structured hard chrome layers produced with the method according to the invention are formed more uniformly than the structured hard chrome layers of the prior art due to the cup-shaped and / or labyrinth-like and / or columnar structure. Structured hard chrome layers obtainable by the process according to the invention are very suitable for coating piston rings, in particular those of internal combustion engines. Layers produced according to the invention have not only high corrosion resistance but also excellent tribological properties, such as good sliding properties and wear and seizure resistance, particularly in the case of inadequate lubrication. Furthermore, the hard chrome layers obtained according to the invention can be used for many decorative and functional applications. The surface topography of the hard chrome layers produced according to the invention enables, for example, a high absorption capacity for light and heat radiation when using solar collectors. Furthermore, the special structure of the hard chrome layers according to the invention enables better absorption capacity for liquids. Gas cushions can also be built up well on the structured surface.

The amounts of components (a) to (c) given above relate to the electrolyte. In the present case, an electrolyte is understood to mean aqueous solutions which are electrically conductive due to dissociated ions. Cr0 ₃ is preferably used for component (a), ie the Cr (VI) compound, since it is particularly suitable for electrolytic deposition.

As component (c), i.e. as aliphatic sulphonic acid, preference is given to using methanesulfonic acid, ethanesulfonic acid, methanedisulfonic acid or ethanedisulfonic acid, which has proven to be particularly advantageous for the formation of the advantageous decorative and functional properties of the hard chrome layer produced.

In one embodiment, the electrolyte can be substantially free of fluorides. The latter often complicate the formation of the structured hard chrome layer. Therefore only so much fluoride is tolerable in the electrolyte that does not affect the deposition of the structured hard chrome layer. It has proven to be advantageous if there are no more than 0.1 g / l fluoride in the electrolyte.

In addition, conventional catalysts for chromium deposition, such as S0 ₄ ^{2 "} and / or CI ^" _{, can be contained} in the electrolyte in conventional amounts.

According to the invention, structured hard chrome layers are deposited on workpieces using the method described above. The term “workpiece” means metallic or non-metallic objects that are to be provided with a structured hard chrome layer. In the case of a non-metallic object, this is coated with a thin metal film before the structured hard chrome layer is applied in order to make it electrically conductive ,

In order to deposit the structured hard chrome layer on the workpiece, the latter is switched as a cathode and immersed in the electrolyte. Then a direct current, e.g. a pulsating direct current with a frequency of up to 1000 Hz. During the deposition of the chromium, the temperature is kept at 45 ° C. to 95 ° C., preferably 55 ° C. The longer the deposition is carried out, the greater the layer thickness of the hard chrome layer.

In the method according to the invention, a current density of 20 A / dm ² to 200 A / dm ² can be used. This range of current density leads to the deposition of particularly favorably structured hard chrome layers. The higher the current density selected, the denser the protruding areas of the surface of the hard chrome layer according to the invention. In a preferred embodiment of the method according to the invention, a second layer is deposited before and / or after the structured hard chrome layer is deposited. Thus, several layers can be applied to the workpiece, for example a metal layer made of a conventional electrolyte on the structured hard chrome layer according to the invention. Furthermore, the two layers can consist of different materials, and if a conventional metal layer is applied to the structured hard chrome layer, an improved anchoring of the conventional metal layer is made possible.

Furthermore, a conventional hard chrome layer or a structured hard chrome layer according to the invention can in each case be deposited with deposits as the second layer, wherein the deposits can consist of aluminum oxide, diamond and / or boron nitride of the hexagonal type. The materials mentioned are suspended in the electrolyte used for this. The deposits lead to a further improvement in the tribological properties.

In addition, in a particularly favorable embodiment of the invention, a hard chrome layer according to the invention is electrolytically applied to a conventional hard chrome layer with a uniform layer thickness. This leads to a so-called graded structured hard chrome layer, in which the corrosion protection is ensured by the conventional hard chrome layer of uniform layer thickness, while the structured hard chrome layer according to the invention improves the tribological properties of the workpiece.

The invention also relates to a method for producing a coating, wherein chromium is deposited on a workpiece to form a structured hard chrome layer, and a composition is applied to the structured hard chrome layer which contains epoxy resin, a solid lubricant, a hard material or mixtures thereof. The structured hard chrome layer can be a structured hard chrome layer produced according to the invention. The epoxy resin serves as a binder in order to hold the solid lubricant and / or the hard material in the depressions of the structured hard chrome layer. MoS ₂ , bomitride, preferably the hexagonal type of boron nitride, or Teflon, or a mixture of two or more of these substances, is particularly suitable as the solid lubricant. Examples of hard materials are microscale diamond, aluminum oxide, Si ₃ N, BC, SiC or a mixture of two or more of these materials.

This coating structure not only improves the general wear properties, it also results in excellent emergency running properties when using MoS _2. gouge the workpiece in the event of insufficient lubrication. Particularly if boron nitride is contained in the composition, the coating is self-lubricating so that, depending on the application, the use of additional lubricants can be dispensed with. If a mixture of two or more of the solid lubricants mentioned is used in the composition that is applied to the structured hard chrome layer, the above-mentioned favorable tribological properties add up.

The invention further comprises a structured hard chrome layer, as can be obtained by one of the above methods.

The invention also relates to a coating which can be obtained by the above process for producing a coating.

The invention furthermore relates to an electrolyte for carrying out the process according to the invention for producing a structured hard chrome layer, comprising

(a) Cr (VI) compound in an amount corresponding to 50 to 300 g / l of chromic anhydride;

(b) 0.5 g / l to 10 g / l sulfuric acid;

(c) 5 g / l to 15 g / l aliphatic sulphonic acid having 1 to 6 carbon atoms;

wherein the electrolyte contains essentially no compound selected from ammonium, alkali and alkaline earth molybdate, ammonium, alkali and alkaline earth vanadate and ammonium, alkali and alkaline earth zirconate.

The electrolyte according to the invention, which can preferably contain the Cr (VI) compound in an amount which corresponds to 50 to 150 'g / l chromic acid anhydride, is used in particular for the electrodeposition of the structured hard chrome layers described in more detail above on workpieces.

The invention is explained in more detail in the following examples with reference to the figures, but without restricting it to them.

Figures 1 to 10 show photographs of the hard chrome layers from Examples 1 to 4. Example 1 :

The following aqueous electrolyte is produced to produce a conventional hard chrome layer:

Chromic anhydride Cr0 ₃ 250 g / l

Sulfuric acid H ₂ S0 ₄ 2.5 g / l.

The article to be coated is immersed in the electrolyte after a customary pretreatment. At 55 ° C with a current density of 40 A / dm ² chrome for 30 minutes

Part of the goods separated.

The resulting article has a conventional shiny and evenly formed chrome layer, as shown in Fig. 1.

Example 2: An electrolyte according to the invention is used for the formation of structured hard chrome layers according to the invention, which contains:

Chromic anhydride Cr0 ₃ 200 g / l

Sulfuric acid H ₂ S0 ₄ 3 g / l

Methanesulfonic acid CH ₃ SO ₃ H (70%) 9 ml / l. Structured hard chrome layers according to the invention are deposited on workpieces at a temperature of 70 ° C., a cathodic current efficiency of 10% and an exposure time of 30 minutes. For the photographs shown in FIGS. 2 to 6, the current densities are varied as follows: FIG. 2: 30 A / dm ^2; 3: 40 A / dm ² ; Fig. 4: 50 A / dm ² ; Fig. 5: 60 A / dm ² ; Fig. 6: 70 A / dm ² . Typical surface structures emerge with structure valleys that appear dark in the photographs, ie depressions.

If the current density is left constant and the electrolyte constituents are changed instead, there is also an influence on the structure formation, which, however, leads to structures which are comparable to those of FIGS. 2 to 6.

Example 3:

Conventional hard chrome layers with aluminum oxide deposits and structured hard chrome layers according to the invention are alternately deposited on one part of the goods. For the latter an electrolyte is used, the chromic anhydride Cr0 ₃ 100 g / l

Sulfuric acid H ₂ S0 ₄ 3.5 g / l

Methanesulfonic acid CH ₃ S0 ₃ H (70%) 6 ml / l contains. The structured hard chrome layers are deposited at a temperature of 60 ° C., a cathodic current efficiency of 10% and a current density of 80 A / dm ^{2 for} 30 minutes. A total of six layers are applied alternately with and without deposits. 7 and 8 show a typical cross section of these graded structured chrome layers in different magnifications. Corrosion protection is ensured by the conventional hard chrome layers, while the favorable tribological properties result from the structured hard chrome layers according to the invention. Instead of aluminum oxide, diamond or hexagonal boron nitride can also be incorporated.

The resulting graded structured hard chrome layers can be treated further as described in Example 4 in order to support the self-lubricating properties of the surface.

Example 4: In a structured hard chrome layer according to the invention produced according to Example 2 on a workpiece, the structure valleys or depressions in the surface are filled with a mixture of epoxy resin and boron nitride of the hexagonal type. The photographs of FIGS. 9 and 10 illustrate the filling of the depressions in the hard chrome layer. The resulting coating has excellent self-lubricating properties. Depending on the application, the additional use of additional lubricants can also be dispensed with.

Example 5:

A workpiece which is covered with a structured hard chrome layer produced according to Example 2 is treated with a mixture of epoxy resin and MoS _{2 in} such a way that the depressions in the chrome layer are filled with the mixture. The epoxy resin serves as a binder to fix the MoS ₂ in the wells and partly on the elevations. This results in good wear properties as well as excellent emergency running properties if the workpiece is insufficiently lubricated. In addition, there is improved corrosion behavior compared to the untreated structured hard chrome layer.

Example 6:

The depressions of a structured hard chrome layer produced according to Example 2 on a product part are filled with a mixture of epoxy resin and microscale diamond, ie diamond granules with a size in the μm range. Here too, compared to the non-filled structured hard chrome layer, significantly improved wear properties and a significantly more favorable corrosion behavior can be seen. Example 7:

A workpiece produced according to Example 5 is additionally treated with a mixture from Example 6. The resulting coating has greatly improved tribological properties compared to Examples 5 and 6, e.g. excellent self-lubrication, as well as a more favorable corrosion behavior compared to the untreated structured hard chrome layer.

Claims

claims

1. Method for producing a structured hard chrome layer, wherein chrome is deposited from an electrolyte onto a workpiece which contains:

(b) 0.5 g / l to 10 g / l sulfuric acid;

(c) 5 g / l to 15 g / l aliphatic sulphonic acid having 1 to 6 carbon atoms;

wherein the electrolyte contains essentially no compound selected from ammonium, alkali and alkaline earth molybdate, ammonium, alkali and alkaline earth metal adanate and ammonium, alkali and alkaline earth metal zirconate and is operated with a cathodic current efficiency of 12% or less.

2. The method according to claim 1, wherein the Cr (VI) compound is Cr0 ₃ .

3. The method according to any one of the preceding claims, wherein the aliphatic sulphonic acid is selected from methanesulfonic acid, ethanesulfonic acid, methanedisulfonic acid and ethanedisulfonic acid.

4. The method according to any one of the preceding claims, wherein the electrolyte contains essentially no fluorides.

5. The method according to any one of the preceding claims, wherein a current density of 20 A / dm ² to 200 A / dm ² is used.

6. The method according to claim 1, wherein a second layer is deposited before and / or after the deposition of the structured hard chrome layer.

7. The method according to claim 6, wherein the structured hard chrome layer and the second layer consist of different materials.

8. The method according to claim 6 or 7, wherein a hard chrome layer of uniform layer thickness is deposited as the second layer.

9. A method for producing a coating, wherein chrome is deposited on a workpiece to form a structured hard chrome layer, and on the structured

Hard chrome layer is applied a composition containing epoxy resin and a solid lubricant, a hard material or mixtures thereof.

10. The method according to claim 9, wherein the structured hard chrome layer is produced by the method according to one of claims 1 to 8.

11. The method according to claim 9 or 10, wherein MoS ₂ , boron nitride, Teflon or a mixture thereof is used as the solid lubricant.

12. The method of claim 9 to 11, wherein the hard material microscale diamond, aluminum oxide, Si ₃ N ₄ , BC, SiC or a mixture thereof is used.

13. Structured hard chrome layer, obtainable by the process according to one of claims 1 to 8.

14. Coating obtainable by the process according to one of claims 9 to 12.

15. Electrolyte for performing the method according to one of claims 1 to 12, comprising:

(b) 0.5 g / l to 10 g / l sulfuric acid;

(c) 5 g / l to 15 g / l aliphatic sulphonic acid having 1 to 6 carbon atoms; wherein the electrolyte contains essentially no compound selected from ammonium, alkali and alkaline earth molybdate, ammonium, alkali and alkaline earth vanadate and ammonium, alkali and alkaline earth zirconate.

16. The electrolyte according to claim 15, wherein the Cr (VI) compound is Cr0 ₃ .

17. Electrolyte according to claim 15 or 16, wherein the aliphatic sulphonic acid is selected from methanesulfonic acid, htansulfonic acid, methane disulfonic acid or ethane disulfonic acid.

18. Electrolyte according to one of claims 15 to 17, wherein the electrolyte contains essentially no fluorides.