WO1995009938A1

WO1995009938A1 - Process for the galvanic application of a surface coating

Info

Publication number: WO1995009938A1
Application number: PCT/EP1994/003314
Authority: WO
Inventors: Karl Müll
Original assignee: Heidelberger Druckmaschinen Aktiengesellschaft; Winterthurer Metallveredelung Aktiengesellschaft
Priority date: 1993-10-07
Filing date: 1994-10-01
Publication date: 1995-04-13
Also published as: BR9405631A; SK86195A3; SK281999B6; EP0722515B1; SI9420006A; FI103674B; CZ144795A3; JP3293828B2; GR3026689T3; EP0722515A1; SI9420006B; KR100332077B1; FI952774A; FI952774A0; CN1044395C; ES2114703T3; JPH09503550A; CA2172613C; DE59405190D1; PL309286A1

Abstract

The invention relates to a galvanic coating process to provide a structured surface coating on a workpiece with an electrically conductive surface and a device for implementing the process. Here, the object to be coated is the cathode in a galvanic bath. The process current is raised in steps during a nucleation phase (10, 11) in which the stepwise increase in the current results in the formation of a deposit of individual or adjacent bodies on the surface of the object. The process current is then kept constant during a ramp working period (12), resulting in the growth of the previously produced nuclei or bodies. The process may be cyclically repeated.

Description

Process for the galvanic application of a surface coating

DESCRIPTION

The invention relates to a method for electro-chemical (galvanic) application of a surface coating according to the German application with the file number DE 42 11 881.6-24.

Such surface structures are more or less well achieved by chemical etching processes following the coating or by mechanical processing methods such as grinding or sandblasting. A Ha rtch romschi cht is then applied to the surface structure thus created. These various work steps required for the creation are complex and require complicated process engineering. The costs are mainly determined by the mechanical or chemical processing steps to create the structure.

In the field of structuring metal layers, complex and difficult to control dispensing processes are also used, in which a specific surface structure is achieved through organic or inorganic foreign substances that are built into a chrome layer, for example, and / or growth block the chromium layer during the deposition process, so that smoky surfaces are created, the foreign substances are present as dispersants in the electrolyte.

DE 33 07 748 relates to a method for electrochemical coating, in which a pulse-shaped current is used for the seed formation. If an appropriate current density is used, the resulting nuclei form a dendritic structure. Rough, dendritically structured surfaces can be created in one step. The current density is understood to mean the mean current density at the cathode surface. The invention has for its object to provide an improved method for the electrochemical application of structural metal layers, which dispenses with mechanical or chemical aftertreatments and with which various structural metal layers can be produced, and an apparatus for carrying out this method.

According to the invention, this object is achieved in accordance with the features of the characteristics of the claims.

The structure layer is applied directly galvanically to the object to be coated. For this purpose, this must have an electrically conductive surface, which is usually ground to provide a smooth base for the structural layer. Before the coating process, the object is cleaned and degreased according to the usual galvanotechnical rules. It is immersed as a cathode in a galvanic bath, which also contains an anode. The distance between the anode and cathode is usually chosen in the range between 1 and 40 cm. Chrome electrolyte, in particular sulfuric acid, chrome electrolyte, mixed acid chromium electrolyte or fermentation electrolyte, are preferably used as the electrolyte.

A process voltage can be applied between the anode and the cathode and the flowing current causes an application of material to the object to be coated, which is used as the cathode. The invention proposes to apply positive current jumps for the formation of germs. The process of structure generation consists of a germination phase and a germ growth phase. First, in the germination phase, process voltage and process current are increased in several stages with a predeterminable change in current density of 1 to 6 mA / cm ² per stage from an initial value to a structural current density. The initial value is 0 mA / cm ² , but this can also be higher if the germination phase directly follows a previous galvanic process phase and the current in between is not reduced to zero. The time between two current density increases is about 0.1 to 30 seconds, most often step intervals of about 7 seconds are used. With each jump in current, new germs are formed. In contrast to pulse current coating, the process current does not go back to zero after every positive jump, but is increased further with every current jump. In this way, in particular round and more uniformly shaped germs or bodies can be deposited on the object than is possible with the known pulse current methods. The flow stages are applied to the bath in such a number until a structural layer consisting of a precipitate of individual or adjacent, approximately spherical or dendritic bodies is reached on the surface of the object.

A structural layer thickness of 4 μm to 10 μm is preferably aimed for with the germination phase. This usually requires between 10 and 240 current levels, particularly good results are achieved with 50 to 60 levels.

The current density achieved after completion of the last current stage is the structure current density. When this structure current density is reached, the nucleation phase, the actual formation of the structure, is largely completed. The structure of the resulting structure depends on many parameters, above all on the selected structure structure, on the number, the height and the time interval of the current stages, on the bath temperature and on the electrolyte used. The change in current density per stage as well as the time between two current density increases can be changed during the germination phase. Depending on the character of the current point, different surface structures can be created, the main ones are characterized by different roughness depths. The ideal process parameters can easily be determined empirically. As a rule, it can be said that with a higher bath temperature and a higher acid content of the electrolyte, a higher structure current density is also used.

This structure current density is usually two to three times the current density used in normal DC coating. In direct current coating, current densities in the range from 15 to 60 mA / cm ^{2 are used} . The value of the current density is dependent on the electrolyte and the bath temperature. Current densities in the range from 30 to 180 mA / cm ^{2 are} possible for structure coating.

This is followed by the germ growth phase. A process stream with a current density in the range from 80% to 120% of the current is used during a predeterminable ramp working time

Structural stories. An approximately even current flows during the ramp working time, which leads to the growth of the structure produced on the object. Depending on the duration of the ramp working time, this structural layer can be more or less pronounced. The growth takes place faster at the highest points of the structural layer than at the low points between the bodies applying in the germination phase. This initially results in a further increase in roughness during the germ growth phase. The ramp working time is usually in a range from 1 to 600 seconds, preferably around 30 seconds.

After the ramp working time has expired, the process current is reduced to a final value, often to zero. This lowering of the process stream to the final value can occur suddenly, but it is also possible to lower the ramp. Here too, good results were achieved with a gradual change in the process stream. The current levels are preferably in a range from -1 to -8 mA / cm ² per stage and the time between two current stages is preferably selected in the range from 0.1 to 1 second.

Three process steps have been described above: gradually increasing the process current during the germination phase until the structure current density is reached, keeping the process current in the region of the structure current density during the ramp working time (germ growth phase) and then reducing the process current to a final value. These process steps represent a structure generation cycle. They can be repeated cyclically. This is particularly advantageous if a stronger structuring of the surface is desired. The end value of the previous cycle corresponds to the start value of the following cycle. The number of repetitions depends on the desired surface structure and layer thickness. Good results were achieved with repetitions between two and twenty times. The final values of the individual cycles can be of different heights.

With Vortei l, the object to be coated is immersed in the bath for some time, preferably one minute before the start of the process. This waiting time is primarily used to adjust the temperature, i.e. the base material assumes the temperature of the electrolyte.

Good results are achieved if a DC base layer is applied before the application of the structural layer under the conditions customary for normal chrome plating. This is achieved by applying a basic pulse (voltage or current pulse) at the beginning of the coating. A current density of 15 to 60 mA / cm ^{2 is} used, which corresponds to the current values customary in normal chrome plating. This basic pulse has a duration of approximately 600 seconds. Concentration changes due to the upstream DC treatment in the

Eliminating the phase boundary layer before the structure generation, it is advantageous if a current-free intermediate time of about 60 seconds is inserted after the basic pulse and before the start of the structure generation.

This process is required in many areas of technology for components with special surface properties. It is known to apply surface coatings to components by means of galvanic processes. Certain requirements are often placed on the surface structure of the coated workpiece. For example, cylindrical surfaces should have defined lubricant depots for holding lubricants, and medical or optical devices should have surfaces with a low degree of re ﬂ ection. Defined ref lex onsg are also for functional and decorative applications in furniture and

Sanitary fittings industry required. In the graphics industry, wet scrubbers with a special, "rough" surface are required for printing machines. Chromium-plated tools can be used in forming technology to give the workpiece a structured surface. For example, the surface of sheet metal can be structured by rolling with chrome-plated rollers.

The device for performing the described method consists of a galvanic bath, which is a

Meta 11 concentrate containing e le ct ro lyt i see bath solution contains. Chrome electrolyte are preferred as electrolyte, in particular sulfuric acid chrome electrolyte, mixed acid

Ch rome le kt ro lyte or alloying lect ro lyte. A preferred one

Electrolyte has a concentration of 180 to 300 grams

Chromic acid CrO per liter. For this, third-party additives such. B. sulfuric acid H SO, hydrofluoric acid H F,

2 4 2 2

Silicic hydrofluoric acid H SiF and mixtures thereof

2 6 be added. A preferred electrolyte contains 1 to 3.5

Grams of sulfuric acid H SO per liter. The galvanic bath is

2 4 usually heated, the temperature of the electrolyte is preferably 30 to 55 degrees Celsius.

An anode and a cathode are immersed in the electrolytic bath solution, the object to be coated forming the cathode or at least being the cathode's egg. If a chrome electrolyte is used, platinum-plated platinum or PbSn7 are preferably used as the anode material. Anode and cathode are connected to a device for feeding a process current. The process current can be increased from the initial value to the structure current density in several stages, each with a predeterminable change in the current density of 1 to 6 mA / cm ² per stage. The time intervals between two current increases can be set between 0.1 and 30 seconds. After reaching the structure current density, a process current with a current density in the range of 80% to 120% of the structure current density can be applied for a predeterminable ramp working time. In order to achieve a uniform coating, the device can be equipped with a rotary drive for continuously rotating the object. The distance between the anode and the object to be coated is selected in the range from 1 to 40 cm, preferably at 25 cm.

The invention is explained in more detail in the following exemplary embodiments with reference to drawings. Show it:

1 shows a schematic representation of a device for the galvanic application of structural layers,

2 shows a graphical representation of a temporal current density profile when generating a structure layer, Fig. 3 Photographic illustration in 200: 1 scale

Surface structure of an object, coated in accordance with the course of the method described for FIG. 2,

FIG. 4 photographic illustration on a scale of 500: 1 of the surface structure shown in FIG. 3 and

5 shows a graphical representation of a current density profile during the generation of a structure layer,

6 shows a graphical representation of a current density profile during the generation of a structure layer and

7 shows a graphical representation of a temporal current density profile when generating a structure layer.

1 shows the schematic representation of a device for the galvanic application of structural layers. A container filled with electrolytic liquid 1 forms the galvanic bath. An object 2 to be coated and an anode 3 are immersed in the galvanic bath. The object to be coated forms the cathode 2. The anode and cathode are connected to a controlled electrical energy source 4. The energy source can be a current or a voltage source. Since the current or the current density at the cathode is decisive for the coating as far as the electrical influences are concerned, the process can be controlled more precisely with a current source. In contrast, the use of a voltage source has the advantage of less electrical circuit complexity. As long as other parameters, such as. B. not the bath temperature and the concentration of the electrolyte change significantly, the process can also be easily controlled with a voltage source.

The electrical energy source 4 is controlled by a programmable control unit 5. The control unit can be used to specify any desired voltage or current profiles, which are then automatically applied to the electrodes via energy source 4.

FIG. 2 shows the graphic representation of the time course of the process current density when generating a structure layer. The 'horizontal axis of Fig. 2 is the time axis, on the vertical axis y the current density is shown. This is an exemplary embodiment of a possible process sequence, which is described in more detail below. FIGS. 3 and 4 show photographic representations of the structural layer produced using this method.

A sulfuric acid chromium electrolyte with 200 grams of chromic acid CrO and 2 grams of sulfuric acid H SO is used as the galvanic bath

3 2 'used. The workpiece to be coated is a rotationally symmetrical component, a dampening device for the printing industry. In order to create a starting surface suitable for structural plating, the cylinder consisting of St52 is first finely ground, with a roughness depth of Rz <3 μm. A 30 μm thick nickel layer and then a 10 μm thick, low-crack chrome layer are then applied according to the conditions customary in electroplating. The workpiece prepared in this way is rotated for structural chrome plating in the galvanic bath in order to achieve a coating that is as uniform as possible. The workpiece forms the cathode, platinum-plated titanium or PbSn7 is used as the anode. The anode / cathode electrode spacing is set to 25 cm. During a first process phase 7, the process stream switches off. This phase serves to acclimatize the workpiece to the galvanic bath. The workpiece takes on the temperature of the electrolyte. After about a minute, a direct current between the anode and cathode is switched on. This remains switched on during phase 8, which lasts about 600 seconds. A chrome DC base layer is applied to the workpiece. The current density used is also common for standard chrome, here 20 mA / cm ² . After the application of the DC base layer, a second phase 9 follows without current.

Then the actual creation of the structure begins. During phases 10 and 11, the current density is increased in steps to the structure current density 14. The characteristic data of the stages (height of the current stages and time interval between two current steps) are varied during the ascent. In the first phase 10, the current is increased in 16 steps to 40 mA / cm ² . This corresponds to a change in the current density of 2.5 mA / cm ² per stage. The time 28 between two current stages is 5 seconds. The current density during phase 11 is then increased in 62 further steps to the structure current density of 100 mA / cm ² , the time between two current stages is 6 seconds (the current density curve shown in the graph in FIG. 2 is not to scale, the same applies to that 5 and 6 graphs).

After the structure current density is reached, this current density is maintained during the ramp working time 12. The direct current flowing thereby leads to the growth of the structural layer produced in phases 10 and 11. The ramp work time is 60 seconds. Then the current density is again gradually reduced in 22 steps to the final value of 0 mA / cm ² . The time between two current steps is 4 seconds. For reasons of application technology, in the case of the moist egg or gum, the liquid is subsequently produced by the process according to the invention

Chrome structure layer applied a 4 to 8 μm thick micro-cracked chrome layer. This is done under the direct current conditions common in electroplating and is not explained in more detail here.

3 and 4 show microscopic images of the chromium structure layer which was produced by the method described for FIG. 2. The structural layer consists predominantly of spherical, individual and partly also adjacent bodies. The one shown

The structural layer has a surface roughness of Rz = 8 μm with a supporting part of 25%. The "Tragantei l" is also defined as "Materialantei l" according to DIN 4762.

5 shows the current density profile over time of a further process sequence for structure coating. The process phases 7, 8 and 9 have already been discussed in the explanations for FIG. 2. In the subsequent phase 15, the current density is reduced to the same in 110 equal steps

Structure current density increased by 100 mA / cm ² . The time between two current steps is 10 seconds. After the ramp working time 16 of 60 seconds, the current density is reduced, this time in 22 identical steps, to the final value of 0 mA / cm ² . The time between two current steps is 4 seconds. Following this, after a brief current-free moment, the process cycle consisting of phases 15, 16 and 17 is repeated.

6 shows the current density profile over time of a further procedure. After the waiting phase 7 for acclimatizing the workpiece to the galvanic bath, a direct current pulse 18 follows, which in its nature is the direct current pulse 8 in Fig. 2 corresponds. This is immediately followed by a seed phase 19 in which the current density is gradually increased to the structure current density 24. The current density is then kept at the structure current density during the ramp working time 20 and is then ramped down to an end value 26 during the phase 21. After a short waiting time 22 there follows again a germination phase 23 with a gradual increase in the current density up to the new structure current density 25. The initial current density of the germination phase 23 is equal to the end value 26 to which the current density was reduced at the end of the previous structure generation cycle. The current density is then kept at the structure current density 25 during the ramp working time 27 and subsequently abruptly reduced to the new final value of 0 mA / cm ² .

7 shows the current density profile over time of a further variant of the method sequence. Process sections 7 to 9 have already been discussed in relation to FIG. 2. The process stream is then gradually increased to the structure current density 30 during phase 29. A process current with a current density value of 80% of the structure current density 30 is then applied during the ramp working time 32. In between there is a current-free rest time 31. After the ramp working time 32 has expired, the process current is reduced to a final value during phase 33. This final value serves as an initial value for a second structure generation cycle, starting with the gradual current increase in phase 35. After reaching the new structure stems 36, a process current with a current density value of 120% of the structure stomp density 36 is applied during the ramp working time 38. In between there is again a current-free rest 37.

Claims

PATENT CLAIMS

1. A method for electrochemical (galvanic) application of a surface according to the German application with the file number P 42 11 881.6-24, characterized in that during the germination phase the electrical voltage and / or the electrical current was acquired in several stages Igt.

2. The method according to claim 1, characterized in that the steps with a predetermined change in the current density of 1 to 6 mA / cm ² per step from an initial value to a st strömst romdi chte (14, 24, 25) are increased, wherein the time (28) between two increases in current density is approximately 0.1 to 30 seconds and the steps are placed in the bath in such a number until a structural layer consisting of a precipitate composed of individual or stacked, approximately spherical or dendritic bodies is reached on the surface of the object and then a process current with a current density in the range from 80% to 120% of the structure current density is applied in a seed growth phase during a predeterminable ramp working time (12, 16).

3. The method of claim 1 or 2, d a d u r c h g e k e n n z e i c h n e t that the duration of the individual stages is about 7 seconds.

Method according to at least one of claims 1 to 3, characterized in that the current density is increased in 10 to 240 steps.

5. The method according to at least one of claims 1 to 4, characterized in that the structure current density (14, 24, 25) is in the range from 30 mA / cm ² to 180 mA / cm ² .

6. The method according to at least one of claims 1 to 5, d a d u r c h g e k e n n z e i c h n e that the ramp working time (12, 16) 1 to 600 seconds, preferably about. Is 30 seconds.

7. The method according to at least one of claims 1 to 6, that the process stream is lowered to a final value (26) after the ramp working time has elapsed.

8. The method according to claim 7, characterized in that the process current is gradually reduced to the final value after the ramp working time with a predetermined change of -1 to -8 mA / cm ² per level.

9. A method for applying a surface coating to the electrically conductive surface of an object, characterized in that the method according to one of claims 7 or 8 is repeated cyclically two to twenty times, the end value of the previous cycle corresponding to the initial value of the following cycle .

10. The method according to claim 9, characterized in that the final values (26) are of different heights.

11. The method according to at least one of claims 1 to 10, characterized in that before the structure generation, a direct current pulse (8, 18) with a current density of 15 to 60 mA / cm ^{2 is applied} to build up a direct current base layer.

12. Use of the method according to at least one of the

Claims 1 to 11, characterized in that it is used for the structure generation of cylinder surfaces for Schmi erstoffdepot s for receiving lubricants, for setting defined degrees of reflection of surfaces in optical, medical devices and for functional and decorative applications in furniture and Sani tä ri ndust ri e, for the production of surfaces with defined roughness in products of the graphics industry, for the production of structured surfaces for tools.