EP0722515B1 - Process for the galvanic application of a surface coating - Google Patents

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

The invention relates to a method for electrochemical (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 created by chemical Etching processes after the coating or through mechanical processing methods such as grinding or Sandblasting more or less well achieved. To that The created surface structure then becomes a Hard chrome layer applied. These different, for Creation of necessary work steps are complex and require complicated process engineering. The costs are essentially mechanical or chemical Processing steps for structure generation determined.

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

DE 33 07 748 relates to a method for electrochemical Coating in which a pulsed current is used for nucleation is used. If an appropriate current density is used the resulting germs form a dendritic Structure. This allows rough, Generate dendritic structured surfaces. Under the Current density becomes the average current density at the Understand the cathode surface.

The invention has for its object an improved Process for the electrochemical application of Structural metal layers based on mechanical or chemical Post-treatments are dispensed with and with the varied Structural metal layers can be generated, as well as a To provide an apparatus for performing this method.

According to the invention, this object is achieved according to the features of Characteristic of the claims solved.

The structure layer is directly galvanically towards the coated object applied. This must be a have an electrically conductive surface, as a rule is sanded around a smooth base for the structural layer to offer. Before the coating process, the object is after usual galvanotechnical rules cleaned and degreased. It is immersed as a cathode in a galvanic bath which also has an anode. The distance between the anode and cathode is usually in the range between 1 and 40 cm chosen. Chromium electrolytes are preferred as electrolyte, especially sulfuric acid, chromium electrolytes, mixed acid Chromium electrolytes or alloy electrolytes are used.

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 applying positive current jumps to form germs. The structure creation process consists of a nucleation phase and a germ growth phase. First, in the nucleation phase, the process voltage and process current are increased in several stages, each with a predeterminable change in the current density from 1 to 6 mA / cm ² per stage, from an initial value to a structure current density. The initial value is 0 mA / cm ² , but this can also be higher if the nucleation phase immediately follows a previous galvanic process phase and the current in between is not reduced to zero. The time between two current density increases is 0.1 to 30 seconds. step spacings of about 7 seconds are most commonly 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 current stages are applied to the bath in such a number until a structural layer consisting of a precipitate of individual or stacked, approximately spherical or dendritic bodies is reached on the surface of the object.

Preferably, with the nucleation phase Structural layer thickness of 4 µm to 10 µm is aimed for. To do this usually between 10 and 240 current steps necessary, particularly good results are achieved with 50 to 60 levels reached.

The reached after completion of the last current stage Current density is the structure current density. With reaching this structure current density is the nucleation phase that actual formation of the structure, largely completed. The structure of the emerging structure is many Parameters, especially the selected structure current density, on the number, the amount and the time interval of the Current levels, the bath temperature and the used Depends on electrolytes. The change in current density per stage as can the time between two current density increases be changed during the nucleation phase. Depending on Character of the current function can be different Surface structures are generated, the main thing are characterized by different roughness depths. The ideal process parameters can simply be empirical be determined. As a rule, it can be said that at higher bath temperature and higher acidity of the Electrolytes also used a larger structure current density becomes.

This structure current density is usually two to three times the current density used in normal DC coating. With DC coating, current densities in the range from 15 to 60 mA / cm ^{2 are used} . The value of the current density depends 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. It is during a Predeterminable ramp working time with a process stream Current density in the range of 80% to 120% of the Structure current density created. During ramp work hours an approximately even current flows, which leads to growth the structure created on the object. Depending on the duration of the Ramp working time can do this structural layer more or be less pronounced. The growth takes place doing it faster at the highest points of the structure layer than at the lows between those in the nucleation phase bulky bodies. This initially results in one 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 a ramp-shaped lowering is also possible. 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 level and the time between two current levels is preferably selected in the range from 0.1 to 1 second.

Three process steps have been described above: The gradually increasing the process stream during the Nucleation phase until the structure current density is reached, keeping the process stream in the range of Structure current density during ramp working hours (Germ growth phase) and the subsequent lowering of the Process stream to a final value. These process steps represent a structure creation cycle. You can be repeated cyclically. This is especially true of Advantage if a stronger structuring of the surface is desired. The end value of each corresponds previous cycle the initial value of the following cycle. The number of repetitions is the desired number Surface structure and layer thickness dependent. Good results were done with repetitions between two and twenty times reached. The end values of the individual cycles can be different heights.

Advantageously, the object to be coated already becomes a few Time, preferably one minute before the start of the process in the bath immersed. This waiting time mainly serves the Temperature adjustment, that is, the base material takes about 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 usual for normal chrome plating. This basic pulse has a duration of approximately 600 seconds. In order to eliminate changes in concentration due to the upstream direct current treatment in the phase boundary layer before the structure is generated, it is advantageous to insert a current-free intermediate time of approximately 60 seconds after the basic pulse and before the structure is started.

This process is used in many areas of technology Components with special surface properties are required. It is known to use surface coatings on components to apply galvanic processes. Certain are often Requirements for the surface structure of the coated Workpiece. For example, cylinder liners defined lubricant depots to hold lubricants have and should medical or optical devices Have surfaces with low reflectance. Defined reflectances are also for functional and decorative applications in furniture and Sanitary fittings industry required. In the graphic Industry are using dampening cylinders for printing presses a special, "rough" surface. In the Metal forming technology can use chromium-plated tools create a structured surface around the workpiece give. For example, the surface of sheet metal through Structured rollers with chrome-plated rollers.

The device for carrying out the described method consists of a galvanic bath which contains an electrolytic bath solution containing a metal concentration. Chromium electrolytes are preferred as the electrolyte, in particular sulfuric acid chromium electrolytes, mixed acid chromium electrolytes or alloy electrolytes. A preferred electrolyte has a concentration of 180 to 300 grams of chromic acid CrO ₃ per liter. For this, third-party additives such. B. sulfuric acid H ₂ SO ₄ , hydrofluoric acid H ₂ F ₂ , silica hydrofluoric acid H ₂ SiF ₆ and mixtures thereof are added. A preferred electrolyte contains 1 to 3.5 grams of sulfuric acid H ₂ SO ₄ per liter. The galvanic bath is 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 being at least part of the cathode. 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.

Fig. 1

Schematische Darstellung einer Vorrichtung zum galvanischen Aufbringen von Strukturschichten,

Fig. 2

Grafische Darstellung eines zeitlichen Stromdichteverlaufs beim Erzeugen einer Strukturschicht,

Fig. 3

Fotografische Abbildung im Maßstab 200:1 der Oberflächenstruktur eines Objekts, beschichtet nach dem zu Fig. 2 beschriebenen Verfahrensverlauf,

Fig. 4

Fotografische Abbildung im Maßstab 500:1 der in Fig. 3 gezeigten Oberflächenstruktur und

Fig. 5

Grafische Darstellung eines zeitlichen Stromdichteverlaufs beim Erzeugen einer Strukturschicht,

Fig. 6

Grafische Darstellung eines zeitlichen Stromdichteverlaufs beim Erzeugen einer Strukturschicht und

Fig. 7

Grafische Darstellung eines zeitlichen Stromdichteverlaufs beim Erzeugen einer Strukturschicht.

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

Fig. 1

Schematic representation of a device for the galvanic application of structural layers,

Fig. 2

Graphical representation of a current density profile over time when generating a structure layer,

Fig. 3

Photographic image on a scale of 200: 1 of the surface structure of an object, coated according to the course of the method described for FIG.

Fig. 4

500: 1 scale photographic image of the surface structure and

Fig. 5

Graphical representation of a current density profile over time when generating a structure layer,

Fig. 6

Graphic representation of a temporal current density curve when generating a structure layer and

Fig. 7

Graphic representation of a temporal current density curve when generating a structure layer.

Fig. 1 shows the schematic representation of a device for the galvanic application of structural layers. A with electrolytic liquid 1 filled container that galvanic bath. In the galvanic bath is a too coating object 2 and an anode 3 immersed. That too Coating object forms the cathode 2. Anode and cathode are with a controlled electrical energy source 4th connected. The energy source can be an electricity or be a voltage source. There what the affects electrical influences, the current, respectively the Current density at the cathode is decisive for the coating is the process can be more accurate with a power source check. The use of a voltage source has in contrast, the advantage of a lower electrical Circuit effort. As long as other parameters, such as. B. the bath temperature and the concentration of the electrolyte are not The process can also be significantly changed with a Check the voltage source well.

The electrical energy source 4 is from a programmable control unit 5 controlled. With the Control unit can be any time voltage, or specify current curves, which are then automatically via Energy source 4 are placed on the electrodes.

Fig. 2 shows the graphical representation of the temporal Process current density curve when generating a Structural layer. 2 is the horizontal axis Time axis, on the vertical axis y is the current density shown. This is an exemplary embodiment for a possible procedure that follows is described in more detail. Figures 3 and 4 show photographic representations of using this process generated structure layer.

A sulfuric acid chromium electrolyte with 200 grams of chromic acid CrO ₃ and 2 grams of sulfuric acid H ₂ SO _{4 is} used as the galvanic bath. The workpiece to be coated is a rotationally symmetrical component, a dampening cylinder for the printing industry. In order to create a starting surface suitable for structural chrome 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, crack-free 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.

The process stream remains switched off during a first process phase 7. 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-G DC base layer is applied to the workpiece. The current density used is also common for normal chrome plating, here 20 mA / cm ² . After the application of the direct current 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 in FIG 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 technical reasons, in the case of Wet friction cylinder then on the after Processes produced according to the invention Chromium structure layer still a 4 to 8 µm thick micro-cracked Chrome layer applied. This happens among those in the Electroplating common DC conditions and will be here not explained in more detail.

3 and 4 show microscopic images of the Chromium structural layer according to that described in FIG. 2 Process was generated. The structure layer exists predominantly of spherical, individual and partial also bodies lying against each other. The one shown The structural layer has a surface roughness of Rz = 8 µm a share of 25%. The "load share" is also as "Material proportion" defined in accordance with DIN 4762.

5 shows the current density profile over time of a further process sequence for the coating of structures. 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 increased in 110 identical steps to the structure current density of 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 O 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 temporal current density profile of a further method profile. After the waiting phase 7 for acclimatizing the workpiece to the galvanic bath, there follows a direct current pulse 18, which corresponds in its nature to the direct current pulse 8 in FIG. 2. This is immediately followed by a nucleation 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 nucleation phase 23 with a gradual increase in the current density up to the new structure current density 25. The initial current density of the nucleation 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 of another Variant of the procedure. Process sections 7 to 9 have already been discussed for FIG. 2. The process stream will then be phased in during phase 29 Structure current density 30 increased. After that, during the Ramp working time 32 a process stream with one Current density value of 80% of the structure current density 30 is applied. In between there is a current-free rest period 31. After the Ramp working time 32 becomes the process stream during phase 33 lowered to a final value. This final value serves as Initial value for a second structure generation cycle, starting with the gradual increase in electricity in phase 35. After the new structure current density 36 will be reached during the Ramp working time 38 is a process stream with one Current density value of 120% of the structure current density 36 is applied. In between there is again a current-free rest 37.

Claims (12)

1. Process for the electrochemical (galvanic) application of a surface coating to a machine component, by means of a direct current application process, the formation of nuclei of the deposition material on the area to be coated being achieved by means of at least one starting pulse of the electrical voltage and/or the electrical current, and subsequently a growth of the deposition material nuclei being brought about by means of at least one following pulse, as a result of the addition of further deposition material, characterized in that, during the nucleus formation phase, the increase in the electrical voltage and/or the electrical current is carried out in a plurality of stages, the time between the increases lying between 0.1 and 30 seconds, current density changes being carried out in stages of 1 to 6 mA/cm².
2. Process according to Claim 1, characterized in that the stages are increased with in each case a predefinable change of the current density of 1 to 6 mA/cm² per stage, from a starting value to a structuring current density (14, 24, 25), the increases are increases in current density, and the stages are applied to the bath in a sufficient number until a structure layer, consisting of a deposit of individually superimposed, approximately spherical or dendritic bodies, is achieved on the surface of the object, and thereafter in a nucleus growth phase, during a predeterminable ramp working time (12, 16), a process current having a current density in the range from 80% to 120% of the structuring current density is applied.
3. Process according to Claim 1 or 2, characterized in that the duration of the individual stages is about 7 seconds.
4. Process according to at least one of Claims 1 to 3, characterized in that the current density is increased in 10 to 240 stages.
5. Process according to at least one of Claims 1 to 4, characterized in that the structuring current density (14, 24, 25) lies in the range from 30 mA/cm² to 180 mA/cm².
6. Process according to at least one of Claims 1 to 5, characterized in that the ramp working time (12, 16) is 1 to 600 seconds, preferably about 30 seconds.
7. Process according to at least one of Claims 1 to 6, characterized in that the process current is reduced to a final value (26) after the ramp working time has elapsed.
8. Process according to Claim 7, characterized in that the process current, after the ramp working time has elapsed, is lowered to the final value in stages with in each case a predefined change of -1 to -8 mA/cm² per stage.
9. Process for the application of'a surface coating to the electrically conductive surface of an object, characterized in that the process according to one of Claims 7 or 8 is repeated cyclically two to twenty times, in each case the final value of the preceding cycle corresponding to the starting value of the following cycle.
10. Process according to Claim 9, characterized in that the final values (26) have different levels.
11. Process according to at least one of Claims 1 to 10, characterized in that, before the generation of the structure, a direct current pulse (8, 18) having a current density of 15 to 60 mA/cm² is applied in order to build up a direct current base layer.
12. Use of the process according to at least one of Claims 1 to 11, characterized in that it is applied

    in order to generate structure on cylinder surfaces for lubricant stores for the accommodation of lubricants,

    in order to set defined degrees of reflectance of surfaces in optical and medical instruments and for functional and decorative applications in the furniture and sanitary fittings industry,

    in order to generate surfaces with defined roughness in products in the graphics industry,

    in order to generate structured surfaces on tools.

Images