EP1565596B1 - Production of structured hard chrome layers - Google Patents

Description

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

Electrochemically or galvanically deposited chromium layers have been considered as prior art for many years in functional and decorative applications and applications for coating electrically conductive and non-conductive workpieces.

The use of structured electrochemically generated hard chrome layers has become increasingly important in recent years and especially recently. Above all, the use of functional structures shows a strong upward trend. Typical applications are in the coating of printing rollers for their better wetting with paint, in rolls for embossing sheets, to improve the deep-drawing process for the auto industry and also in the storage for protection against wear and the reduction of friction.

However, the structured layers used in the prior art all have a pronounced spherical layer structure. The size of the spherical shapes varies between less than 1 micron and several microns. The degree of coverage or the density of the spherical layer structure is more or less variable depending on the method.

Thus, structured chromium layers according to DE 33 077 48 A1, DE 42 11 881 A1 and DE 43 34 122 A1 are produced by varying the direct electrical currents, the structure of the structures being influenced by variation of current ramp and current pulse heights.

According to DE 44 32 512 A1, spherical structures are also produced. Here, however, their growth is made possible by the addition of salts of the elements selenium or tellurium to the chromium plating electrolyte.

According to DE 19828545 C1, the addition of 2-hydroxyethanesulfonate ions to the electrolyte in combination with its temperature variation and the variation of the applied direct electrical current also results in spherical structures.

However, all of these listed chromium coatings have the same, more or less pronounced spherical characteristics, which is not suitable for all applications. The layers are partially uneven and the process for their deposition is subject to some not sure to control mechanisms.

From WO 02/38835 A1 a process for the electrolytic coating of materials is known, wherein a chromium alloy is deposited from an electrolyte containing at least chromic acid, sulfuric acid, a isopolyanionen ausbildendes metal, a short-chain aliphatic sulfonic acid, its salt and / or their halogen derivative and Contains fluorides. The formation of a structured hard chrome layer is not mentioned in this prior art.

It is therefore an object of the present invention to provide a process for producing a structured hard chromium layer which does not have the disadvantages of the prior art.

• (a) Cr(VI)-Verbindung in einer Menge, die 50 g/l bis 600 g/l Chromsäureanhydrid entsprechen;
• (b) 0,5 g/l bis 10 g/l Schwefelsäure;
• (c) 1 g/l bis 20 g/l aliphatische Sulfonsäure mit 1 bis 6 Kohlenstoffatomen und
• (d) 10 g/l bis 200 g/l mindestens einer einen dichten Kathodenfilm ausbildende Verbindung ausgewählt unter Ammonium-, Alkali- und Erdalkalimolybdat, Ammonium-, Alkali- und Erdalkalivanadat und Ammonium-, Alkali- und Erdalkalizirkonat,

wobei mit einer Rathodischen Stromausbeute von 12% oder weigher gearbeitet wird.According to the invention, this is achieved by a method for producing a structured hard chromium layer, wherein chromium is deposited from an electrolyte on a workpiece, which contains:

• (a) Cr (VI) compound in an amount corresponding to 50 g / l to 600 g / l of chromic anhydride;
• (b) 0.5 g / l to 10 g / l sulfuric acid;
• (c) 1 g / l to 20 g / l of aliphatic sulfonic acid having 1 to 6 carbon atoms and
• (d) 10 g / l to 200 g / l of at least one dense cathode film forming compound selected from ammonium, alkali and alkaline earth molybdate, ammonium, alkali and alkaline earth metal and ammonium, alkali and alkaline earth zirconate,

working with a Rathodische current efficiency of 12% or weigher.

The method according to the invention is ideally suited for use in the production of structured hard chrome layers.

By means of the method according to the invention, a structured hard chrome layer is obtained by targeted influencing of the cathode film forming during the electrochemical deposition of metals, wherein the layer is formed in the shape of cups and / or labyrinth-like and / or columnar.

If salts are dissolved in water, they dissociate into cations and anions. These dissociated ions surround simultaneously with a hydration shell, i. Water molecules (as dipoles) attach themselves to the cations or anions. During hydration, the charge of the ions is not changed. If a hydrated metal ion then begins to flow under the influence of current, it will be in the vicinity of the cathode in a boundary region between the electrolyte and the cathode.

This so-called cathode film is located directly on the surface of the workpiece to be coated, since the workpiece is electrically negatively connected. In this phase boundary, the metal ions present in the electrolyte are first aligned by receiving electrons which are offered by the cathode from the electric current. Immediately on the workpiece surface under the diffusion zone is an electrochemical double layer, also called "Helmholtz double layer". It is a name for the approximately some atomic or molecular layers thick electrically charged zone at the interface electrolyte / electrode. Their formation is dependent on ions, electrons or directed dipole molecules. It is positive on the one hand and negatively charged on the other, and behaves like a plate capacitor with extremely low plate spacing. The resulting metal atom is now on the workpiece surface. His condition is not yet comparable to that of an atom in the metal interior. Only when a so-called growth site is present, the resulting atoms arrange themselves in the existing metal lattice.

Usually, the deposition conditions of the electrolytes, such as chemical composition, temperature, hydrodynamics and electric current, are chosen so that a uniform coverage of the base material takes place with the metal to be deposited. This means that the cathode film is influenced by this measure so that its permeability to the ions present is as uniform as possible.

The element chromium is present in aqueous solution, compared to most electrochemically depositable elements, as negatively charged complexion mainly as hydrodichromate in strongly acidic solution.

The chromium in this complex has the oxidation state 6. In addition, smaller amounts of chromium (III) compounds are contained in electrolytes.

If such an aqueous solution is electrolyzed, a solid film is formed on the cathode which prevents chromium deposition. It only produces hydrogen, which can pass through the solid cathode film because of its small radius, but not the large Hydgendgendichromationen. Only by the addition of foreign anions, e.g. Sulfate and chloride, the cathode film for chromium ions permeable and it comes through different oxidation states for the deposition of chromium (see "Chemistry for electroplating" Leutze Verlag, 2nd edition, 1993).

By the addition of at least one dense cathode film forming compounds to the electrolyte, the formation of the cathode film is controlled so that it becomes permeable to chromium ions, so that initially forms a very dense barrier layer, which then durchschlägt depending on the applied electrical coating current density and the metal structure of different thickness or layer thickness can arise. In this way, structured chromium layers which are cup-shaped and / or labyrinth-like and / or columnar are obtained.

The chromium layer obtained by the method according to the invention has a high resistance to wear and corrosion, excellent sliding properties and seizure resistance as well as an aesthetically favorable appearance, which is hardly achieved by any other coating. Due to the cup-shaped and / or labyrinth-like and / or columnar structured hard chrome layer, it can be used for many functional or decorative applications. For example, the special structure of the layer ensures better absorption of liquids. Furthermore, the construction of a gas cushion allows as well as an improved anchoring possibility for einzulagemde there substances, such as plastics, dyes, metals, ceramics, electronic components, the body's own tissue as implant coating, can be achieved. Furthermore, this surface structure allows specific optical effects, such as high adsorption capacity for light and heat radiation when using solar collectors, as well as decorative applications in the field of design.

The term "electrolyte" in the context of the present invention means aqueous solutions whose electrical conductivity is due to electrolytic dissociation in ions. Accordingly, the electrolyte in addition to the components (a) to (d) and optionally further present additives as the remainder of water.

The above amounts of components (a) to (d) refer to the electrolyte.

As component (a) preferably CrO _{3 is} used, which has proven to be particularly favorable for the electrolytic deposition of chromium.

An aliphatic sulfonic acid preferably used as component (c) is methanesulfonic acid. This has proven to be particularly favorable for the formation of the structured hard chrome layers with the above properties.

As the alkali ions for the component (d), Li ⁺ , Na ⁺ and K ⁺ can be used. Examples of alkaline earth ions are Mg ²⁺ and Ca ²⁺ . In a preferred embodiment, component (d) (NH ₄ ) ₆ Mo ₇ O ₂₄ .4H ₂ O, which has been found to be particularly favorable for the formation of the patterned hard chrome layer having the above properties.

The electrolyte specified above, in a particularly preferred embodiment, is substantially free of fluorides. Here are understood as fluorides both simple and complex fluorides. If fluorides are present in the electrolyte, the formation of the structured hard chromium layer is disturbed. Accordingly, the term "substantially no fluorides" means that so much fluoride is tolerable in the electrolyte that the formation of the patterned hard chromium layer is unaffected. The amount of fluorides that are tolerable can be readily determined by those skilled in the art. It has proved to be favorable if not more than 0.1 g / l is present in the electrolyte.

The electrolyte may further contain conventional catalysts which promote chromium deposition, such as SO ₄ ^2- and / or Cl ^- . These can be present in the electrolyte in conventional amounts.

With the method according to the invention structured hard chrome layers, as described in more detail above, formed on workpieces. The term "workpiece" is understood to mean objects of any type that are to be provided with a structured chrome layer. These may be metallic or non-metallic objects. Should a non-metallic object structured hard chrome layer are formed, it is first made electrically conductive by applying a thin metal film.

To form the structured hard chromium layer on the workpiece, it is switched cathodically and immersed in the electrolyte. A direct current, for example a pulsating direct current with a frequency up to 1000 Hz, is applied to the workpiece. The temperature for the deposition of the chromium may be 45 ° C to 95 ° C, in particular about 55 ° C. The duration of the deposition is chosen as a function of the desired thickness of the structured hard chromium layer, the layer becoming thicker the longer the deposition takes place.

In a preferred embodiment of the present invention with a current density of 20 A / dm ² to 200 A / worked dm ^2. As a result, particularly favorable structures of the hard chrome layer are obtained. The higher the current density is chosen, the denser the protruding areas of the patterned hard chrome layer.

The cathodic current efficiency in the production of the structured hard chromium layer according to the method of the invention is 12% or less. If the current efficiency is higher, the desired structuring of the hard chrome layer is not obtained.

Multiple layers may be applied to the workpiece, whereby the aforementioned patterned hard chromium layers and layers formed from conventional electrolytes may be alternately deposited one on top of the other. For example, it is possible first to apply to a workpiece the structured hard chrome layer obtainable by the process according to the invention and then a layer selected from a conventional chromium layer, black chrome layer, copper, nickel or tin layer. Furthermore, a conventional chromium, copper and / or nickel layer can first be applied to the workpiece and then the hard chrome layer described in more detail above can be deposited thereon.

Directly onto the hard chrome layer obtainable by the process according to the invention, it is possible to apply further coatings which are not chromium-containing, such as copper, nickel, tin, zinc, ceramic, plastic, solid lubricants, dyes.

The subject matter of the present invention is furthermore a structured hard chrome layer, as obtainable by the process according to the invention described in more detail above.

The structured hard chrome layer is - in contrast to the hard chrome layers of the prior art, which have a pronounced spherical layer structure - cup-shaped and / or labyrinth-like and / or columnar. The structured hard chromium layer according to the invention has the advantages described in connection with the method according to the invention.

The structured hard chrome layer according to the invention can be used to coat a variety of workpieces, such as piston rings, cylinders, pistons, bolts, camshafts, gaskets, composite materials, valves, bearings to protect against wear and to reduce friction, pressure cylinder for better wetting with colors, embossing rollers for better deep-drawing processes for the automotive industry, solar technology, decorative applications, medical technology, microtechnology and microelectronics.

The present invention is explained in more detail in the following examples with reference to the figures, but without limiting it thereto.

FIGS. 1 to 8 show photographs according to the hard chrome layers of Examples 1 to 8.

Example 1:

A conventional chromium electrolyte of the following basic composition was prepared chromic anhydride CrO ₃ 250 g / l sulfuric acid H ₂ SO ₄ 2.5 g / l

A part of the product is introduced into the electrolyte according to customary proviso and at 40 ° C. with 40 A / dm ² for 30 min. coated.

The product coated under these conditions has, after the treatment, a conventional glossy uniform chromium layer, cf. Fig. 1.

Example 2:

Ammonium molybdate (NH ₄ ) ₆ Mo ₇ O ₂₄ .4H ₂ O 100 g / l and methanesulfonic acid 4 g / l are additionally added to the electrolyte according to Example 1. A product is coated according to the conditions described in Example 1. The product described thus has a structured chromium layer after the treatment. This chrome layer has a shiny appearance on the protruding surface portions (supporting portion), and brown-colored cathode film is obtained in the recesses of the structure (Fig. 2).

Example 3:

A product is coated according to the conditions of Example 2. However, it is carried out instead of a coating current density of 40 A / dm ² with 20 A / dm ² .

The product thus coated has a structured chromium layer after the treatment. The proportion of the protruding, glossy surface areas (supporting portion) is lower and the proportion of the deep areas is larger compared to the structural layer of Example 2 (FIG. 3).

Example 4:

A product is coated according to the conditions of Example 2. However, instead of a coating current density of 40 A / dm ² , 60 A / dm ² is used.

The product thus coated has a structured chromium layer after the treatment. The proportion of the protruding, glossy surface areas (carrying portion) is greater and the proportion of the deep areas is lower compared to the structural layer of Example 2 (FIG. 4).

Example 5:

A product is coated according to the conditions of Example 2. The product thus coated has a structured chromium layer after the treatment. In a conventional chromium electrolyte from Example 1 is now on this patterned chromium layer with chromium at 55 ° C and 50 A / dm ² for 120 min. further coated. The thus coated article has a significant increase in the structural height as compared with Example 2 (Fig. 5).

This graded layer has metallurgical properties on the surface such as conventional chrome and is additionally structured.

The advantage of this layer structure is due to the fact that the profile height of the structural layer can be varied within a wide range, which is limited by the exclusive deposition according to Examples 2-4 by their low film growth rate.

Example 6:

A product is coated according to the conditions of Example 2. The product thus coated has a structured chromium layer after the treatment. In a conventional black chromium electrolyte, a black chromium oxide-containing layer is now deposited on this structured chromium layer.

The thus coated article has a uniform, deep black surface with a very high light index (Figure 6).

Example 7:

A product is coated according to the conditions of Example 2. The product thus coated has a structured chromium layer after the treatment. In a conventional tin electrolyte, a tin layer is now deposited on this patterned chromium layer, of sufficient thickness to fill the recesses of the structural chrome layer with tin.

The product thus coated has a surface which also has very good sliding properties with high wear resistance (FIG. 7).

Example 8:

A product is coated according to the conditions of Example 1 with a conventional chrome layer.

Subsequently, a structured chromium layer is applied with the conditions from Example 2 to the chromium layer from Example 1.

The structured chromium layer represents an entry layer for the conventional chromium layer and, depending on the tribological application, leads to an improvement of the layer system (FIG. 8).

Claims (7)

1. Method for manufacturing a structured hard chrome layer whereby chromium is deposited on a workpiece from an electrolyte which contains:

   (a) Cr(VI) compound in a quantity corresponding to 50 g/l to 600 g/l chromic acid anhydride;

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

   (c) 1 g/l to 20 g/l aliphatic sulphonic acid with 1 to 6 carbon atoms, and

   (d) 10 g/l to 200 g/l of at least one of a compound forming a dense cathode film selected from ammonium, alkaline and earth-alkaline molybdate, ammonium, alkaline and earth-alkaline vanadate and ammonium, alkaline and earth-alkaline zirconate,

   whereby work is carried out with a cathodic current efficiency of 12% or less.
2. Method according to claim 1, whereby the Cr(VI) compound is CrO₃.
3. Method according to one of the preceding claims, whereby the aliphatic sulphonic acid is methane sulphonic acid.
4. Method according to one of the preceding claims, whereby the compound forming a dense cathode film is (NH₄)₆Mo₇O₂₄ 4H₂O.
5. Method according to one of the preceding claims, whereby the electrolyte substantially contains no fluorides.
6. Method according to one of the preceding claims, whereby the work is carried out with a current density of 20 A/dm² to 200 A/dm².
7. Structured hard chrome layer obtained according to the method in line with one of claims 1 to 6.

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