WO2009146473A1

WO2009146473A1 - Process for producing nanostructured chromium layers on a substrate

Info

Publication number: WO2009146473A1
Application number: PCT/AT2008/000424
Authority: WO
Inventors: Vassilios Polydoros
Original assignee: Vassilios Polydoros
Priority date: 2008-06-03
Filing date: 2008-11-26
Publication date: 2009-12-10
Also published as: AT506076B1; AT506076A4

Abstract

Process for producing nanostructured chromium layers on a substrate, wherein the chromium layers are deposited by electroplating by means of direct-current from a bath comprising an aqueous solution of chromic acid and, if appropriate, foreign acids and the salts thereof, the following process steps being carried out: a) partial etching of the substrate; b) currentless latching; c) application of direct-current at a first current density for a first period in order to allow a desired chromium layer to grow; and d) application of pulsating direct-current at a second current density and a positive amplitude of the second current density, where the second current density is higher than the first current density, for a second period in order to form a nanostructure on the Cr surface.

Description

Process for producing nanostructured chromium layers on a substrate

The invention relates to a method for producing nanostructured chromium layers on a substrate, wherein the chromium layers are electrodeposited from a bath, comprising an aqueous solution of chromic acid and optionally foreign acids or their salts, by means of direct current.

In many areas of engineering, for example, machine components with special surface properties are needed. It is known,

Apply surface coatings by means of galvanic processes. The

Chromium plating is an important coating process. Chromium plating is used

Chromium electrolytically deposited from a mostly aqueous chromium solution by the substrate is connected as a cathode. Chromium layers can be applied as decorative layers or more often as corrosion protection. A big

Hardness, high roughness and a high number of carrying points as well as a structured layer is desirable.

In the hardening of metal surfaces by electrolytically applied chromium layers, the structure of the base material is not changed in contrast to the conventional thermal hardening process, but from aqueous solutions of chromic acid by electroplating an anchored on the base material and adherent layer of metallic chromium deposited. Due to the special nature of its microstructure and its crystal structure, this electrolytically precipitated chromium layer is characterized by high hardness and high hardness

Wear resistance, high resistance to mechanical wear, high surface smoothness, low coefficient of friction and low adhesiveness,

Resistance to chemical stress, resistance to tarnishing and higher temperatures and completely homogeneous connection and anchoring with the base material. In special fields of application, for example in the printing industry, it is necessary or desirable to produce a structured chromium-plated surface with certain functional properties while retaining the abovementioned properties.

In the printing industry, e.g. in the case of printing presses, the chrome-plated surface of the built-in color printing cylinders must have a morphological design (structuring).

Other methods for producing structured surfaces include, for example:

Dispersion Deposition Process: The process consists of embedding small particles simultaneously with the electrodeposition of the metallic matrix. The particles are held in suspension either by mechanical movement, by air turbulence or by adding suitable chemicals. With the help of this technique surface structures can be produced. The added phase (organic or inorganic foreign substances) causes the creation of a surface structure.

- Formation of surface structures by chemical or electrochemical etching processes (roughening of the surface and subsequent hard chrome plating).

Roughening by mechanical processing, e.g. Sand blasting and then hard chrome plating.

These methods involve the disadvantages of poor reproducibility and poor adhesion of the chromium layer.

DE 42 11 881 A1 describes a method for the electrochemical application of chromium layers on an object. In the method is by a

Initial impulse of the electrical size nucleation of the deposition material triggered and then by means of a successive pulse growth of the Abscheidematerialkeime brought about by addition of further deposition material. The individual pulses have a trapezoidal shape. Between the pulses is a de-energized state. Chromium layers with a roughness Rz = 9 μm and a load factor of 25% are achieved.

DE 43 34 122 C2 describes a method for the electrochemical application of a surface coating. The increase of the electrical current takes place in several stages, the current densities are in the range of 30-180 mA / cm ² . There are obtained layers with a roughness Rz = 8 microns and a carrying percentage of 25%.

The object of the invention is to provide a method for producing a structured chromium layer.

According to the invention, this is achieved by carrying out the following method steps: a) etching the substrate; b) electroless latching; c) applying direct current at a first current density for a first period of time to grow a desired chromium layer; d) applying a pulsating direct current having a second current density and a positive amplitude of the second current density, wherein the second current density is higher than the first current density, for a second time period to form a nanostructure on the Cr surface.

The etching removes any oxide layer on the substrate. The etching can be carried out, for example, by means of an acid or, preferably, by applying an anodic current. The growth of an electrolytic layer is significantly worse on an oxide layer than on pure metal. However, plastics can also be coated. The electroless latching serves to establish the equilibrium in the electrolyte solution. The electrolyte solution consists for example of an aqueous electrolyte having a chromic acid content of 180 g / l to 350 g / l, preferably 300 g / l, and 1% sulfuric acid. Salts such as SrSO ₄ may also be used. Any electrolyte known in the art may be used.

The application of direct current at a first current density for a first period of time serves to grow a desired chromium layer. In this step, depending on the length of the application of direct current islands of chromium or a continuous layer of chromium, the so-called base layer.

The application of pulsating direct current with a second current density and a positive amplitude of the second current density, wherein the second current density is higher than the first current density, for a second period of time serves to form a nanostructure on the Cr surface. Thus, a higher current density is applied, from which pulses with an even higher current density are applied. By the pulsation, a thermal relaxation (recovery phenomenon) of the electrolyte occurs during the rapid jump of the current from high to low level. The macroprofile of the surface is transferred to the microprofile. As a result, the surface is morphologically designed. The roughness and wearing points depend on the number of repetitions of the working cycle.

In one embodiment of the present invention, the amplitude in step d) may correspond to 10-100% of the second current density. This has proven in empirical experiments as the most advantageous size of the pulses.

In another embodiment of the present invention, the pulsating DC current can be applied according to the following scheme: 0.1-5 s at the second current density, 0.1-10 s at the amplitude of the second current density. This sequence of pulses deposits a uniform chromium layer on the precoated substrate which has the desired surface texture requirements.

In one embodiment of the invention, the second period of time in step d) may be 5-20 minutes. During this time, the desired layers are obtained.

In a further embodiment of the invention, the first period of time in step c) may be about 1-5 minutes, thereby producing an island-like Cr layer, and electroless snapping occurs between steps c) and d), thereby dissolving small Cr islands , Due to the short time span in step c), chromium is deposited only like an island. There is not enough time to form a continuous layer. There are larger and smaller islands. The subsequent electroless snap the smaller islands dissolve through the acid in the electrolysis solution. The remaining larger islands can also be referred to as "domes." More chromium is subsequently deposited on these domes, and their size therefore continues to increase without any connection of the individual domes.

In one embodiment of the present invention, the first time period in step c) may be about 10-20 minutes, thereby producing a continuous Cr adhesion layer on the substrate. This time is sufficient to deposit enough chromium to create a continuous layer. Onto this continuous layer, which is also referred to as an adhesive layer, a structured chromium layer is subsequently deposited.

In another embodiment of the present invention, the first current density may be 20-40 A / dm ² . This current density gives a base layer of chromium which is outstandingly suitable for further deposition.

In one embodiment of the invention, the second current density may be> 50 A / dm ² . This current density is effective to provide a suitably thick chromium layer with the appropriate roughness apply.

In one embodiment of the invention, the method at 38-43 ⁰ C can be performed. This temperature has proven to be extremely effective in obtaining layers which are best suited for practical applications, which layers have the desired roughness, load-bearing and other properties.

In one embodiment of the invention, after step d), step f) of applying direct current can be carried out with a first current density. The microstructured chromium layer produced in the preceding steps is again smoothed by this further chromium plating step; he takes a micro-cracked chrome plating.

In another embodiment of the invention, after step d), a step e) may be performed in which an anodic current is applied. At the tips of the growth crystals, there is a collapse of the electric field lines and thus an increase in the current density. This leads to the formation of dendrites and irregularities in the surface. By applying an anodic current, peaks on the structure are re-dissolved, resulting in a smoother, dendrite-free, dome-like surface.

In another embodiment of the invention, after step d), step e), as defined above, and step f), as defined above, can be carried out. By performing the two steps, a very smooth layer is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a current density-time diagram of a method according to the invention. The numerical characters indicate times in the process, while the letters mean the following process steps: A: activation (etching) of the surface; B:

Pulse area; C: Resolution of dendrites and irregularities on the Surface, D: DC chrome plating, production of a microcracked hard chrome plating; E: currentless latching.

Fig. 2 describes a current density-time diagram of another method according to the invention. The numeric characters indicate times in the process.

The present invention will now be explained in more detail by way of non-limiting examples.

EXAMPLES

example 1

A substrate, for example a printing cylinder or a dampening cylinder, is introduced into an electrolyte having a chromic acid content of 180 g / l to 350 g / l, preferably 300 g / l, and 1% sulfuric acid. Salts such as SrSO ₄ may also be used. An anode consisting for example of lead or placed titanium is also introduced into the electrolyte. The bath is heated to 40 +/- 1 ⁰ C. If necessary, the cylinder must be prepared accordingly. It is pre-ground to a predetermined extent, with the surface being free of pores, cracks, doublings, voids, inclusions and the like. The roughness is preferably in the range of 0.5 <Rz <3 μm. Then it is degreased and hanged 2-3 minutes to heat balance in the bathroom. The arrangement of the anodes in the electrolysis bath may also be important. For example, to allow uniform chrome plating of the cylinder, the anodes may be placed in an anode ring around the cylinder to allow chrome plating from many locations around the cylinder simultaneously. Also, the substrate may be moved relative to the anodes so as to avoid localization of the chrome plating at one location. Subsequently, current is applied, as shown in Fig. 1. The individual times are given in the following Table 1. T (0, 1) means the time interval between times 0 and 1 in FIG. 1, T (1, 2) the time interval between times 1 and 2 in Fig. 1, etc.

Table 1: Sequence of chrome plating

The pulsing of the direct current takes place in steps T (10, 11) and T (12, 13), which are repeated until a total time T (8, 15) of 10 minutes has elapsed. After the first pulse (10-14 in Fig. 1), the next pulse, i. the time 14 = time 10. The last time 13 of the pulse sequence corresponds to time 15 in Fig. 1. In step T (18,19), prominent peaks are greatly resolved from the structure.

It became a continuous Cr layer with dome-like design of the surface, wherein the average height of the hemispheres is about 28-30 microns, and with a roughness Rz (determined with drum tester T1000) of 28 μm and a number of carrying points of 61 / cm.

Example 2

A substrate, for example a printing cylinder or a dampening cylinder, is introduced into an electrolyte having a chromic acid content of 180 g / l to 350 g / l, preferably 300 g / l, and 1% sulfuric acid. Salts such as SrSO ₄ may also be used. An anode consisting for example of lead or platinized titanium is also introduced into the electrolyte. The cylinder is pretreated, for example, as described in Example 1. The anode arrangement may be as described in Example 1. The bath is heated to 40 +/- 1 ⁰ C. Subsequently, current is applied as shown in FIG. The individual times are given in the following Table 2. T (0,1) means the time between times 0 and 1 in Fig. 2, T (1, 2) means the time between times 1 and 2 in Fig. 2, and so on.

Table 2: Sequence of chrome plating

In area 0-4 an activation (etching) of the surface takes place. The oxide films are removed for proper adhesion of the subsequently deposited chromium layer. In the range 4-6, a local nucleation occurs, ie the formation of active centers. The current density as well as the time in the range 5-6 are chosen so that only a part of the surface of the cylinder is covered with stable nuclei. In the range 7-8, the dissolution of the thermodynamically unstable nuclei (crystals) on the surface, which are released dissolved in the electrolyte solution takes place. In the range 8-10 the stabilization of the active centers takes place. The time T (9,10) corresponds to approximately 5 to 7 times the time T (5,6). In the 10-12 range, the growth of the resulting active sites occurs because the environment of the active sites is depleted of atoms because no further nucleation nuclei are formed. The current density in the range 11-12 corresponds to about 2.5 to 3 times the current density at 5-6. In area 13-14, the resulting microstructure on the surface of the dome formed is smoothed by this further chrome plating step. The temperature in step 13-14 must be kept constant at 40 ^° C.

An island-like Cr layer was obtained. The one roughness Rz (measured by means of drum tester T1000) was 31 μm and the number of carrying points was 56 / cm.

Claims

1. A process for the production of nanostructured chromium coatings on a substrate, wherein the chromium layers are deposited galvanically from a bath, comprising an aqueous solution of chromic acid and optionally foreign acids or salts thereof, by means of direct current, characterized in that the following process steps are carried out: a) Etching the substrate; b) electroless latching; c) applying direct current at a first current density for a first period of time to grow a desired chromium layer; d) applying a pulsating direct current having a second current density and a positive amplitude of the second current density, wherein the second current density is higher than the first current density, for a second time period to form a nanostructure on the Cr surface.

2. The method according to claim 1, characterized in that the amplitude in step d) corresponds to 10 - 100% of the second current density.

3. The method according to any one of the preceding claims, characterized in that the pulsating direct current is applied according to the following scheme:

0.1-5 s at the second current density,

0.1-10 s at the amplitude of the second current density.

4. The method according to any one of the preceding claims, characterized in that the second period in step d) is 5-20 min.

5. The method according to any one of claims 1 to 4, characterized in that the first time period in step c) is about 1-5 minutes, whereby an island-like Cr layer is generated, and between step c) and d), an electroless locking takes place , whereby small Cr islands are dissolved.

6. The method according to any one of claims 1 to 4, characterized in that the first time period in step c) is about 10-20 minutes, whereby a continuous Cr-adhesion layer is formed on the substrate.

7. The method according to any one of the preceding claims, characterized in that the first current density 20-40 A / dm ² .

8. The method according to any one of the preceding claims, characterized in that the second current density> 50 A / dm ² .

9. The method according to any one of the preceding claims, characterized in that the method is carried out at 38-43 ⁰ C.

10. The method according to any one of the preceding claims, characterized in that after step d) step f) application of direct current is performed with a first current density.

11. The method according to any one of claims 1 to 9, characterized in that after step d), a step e) is performed, in which an anodic current is applied.

12. The method according to any one of claims 1 to 9, characterized in that after step d) step e), as defined in claim 11, and step f), as defined in claim 10, is performed.