SK281999B6 - Process for the galvanic application of a surface coating - Google Patents

Abstract

In the galvanic process method, a structured coating surface is formed on a workpiece with an electrically conductive surface on which the coating is formed, there are cathodes in a galvanic bath. The operating current gradually increases during the nucleation phase (10, 11), in which a stepwise increase in current causes the formation of a deposit from individual or connected bodies on the surface of the object. The operating current is then kept constant during the dwell time (12), as a result of which the growth of previously formed nuclei and bodies is achieved. The process can be repeated cyclically

Description

The invention relates to a process for the electrodeposition of a surface layer according to German patent application DE 42 11 881.6-24.

BACKGROUND OF THE INVENTION

These surface structures are nowadays achieved in practice more or less successfully by chemical etching processes carried out after application of the coating layer or by mechanical machining methods such as grinding or sanding. A hard chrome coating is then applied to the surface structure thus formed. These diverse work steps required to create a structured surface are expensive and technologically demanding and require complicated machining techniques. The costs are essentially determined by mechanical or chemical machining processes.

In the field of structuring deposited metal coatings, equally costly and difficult to handle dispersion deposition processes are also employed, in which the specific surface structure is achieved by using organic or inorganic foreign substances, for example either incorporated into the chromium layer and / or blocking the chromium coating during the process deposition, thereby creating roughened surfaces. These foreign substances are added as dispersants to the electrolytes.

DE 33 07 748 relates to a method of galvanic surface treatment in which a pulsed current is used to form seed particles. When a current of suitable density is used, the resulting germs form a dendritic structure. In this way, rough, dendritically structured surfaces can be achieved in a single operation. The current density is the mean value of the current density at the cathode surface.

The invention is based on the need to provide a more elaborate method of galvanic deposition of structured metal coatings, which makes it possible to dispense with the subsequent additional mechanical and / or mechanical deposition. chemical processing, which would make it possible to form a variety of structured metal coatings and, moreover, to provide an apparatus for carrying out the process.

From the inventive point of view, this task has been solved in accordance with the main guiding requirements that meet the necessary patent requirements.

SUMMARY OF THE INVENTION

The structured coating is applied directly onto the object to be treated. The surface layer of the article must be electrically conductive and the surface should be substantially ground in order to obtain a smooth surface for applying the structured surface layer. Prior to the coating process, the article is cleaned and degreased according to established electroplating rules. It is immersed as a cathode into a galvanic bath in which an anode is already placed. The chosen distance between the anode and cathode is generally 1 to 40 cm. The electrolyte is preferably a chromium electrolyte, in particular a chromium sulfuric acid electrolyte, chromium electrolytes of mixed acids or electrolytes of alloys.

An operating voltage is generated between the anode and the cathode and the flowing current causes a metal coating to settle on the surface of the workpiece material used as the cathode. The invention proposes to involve positive current jumps to form the seed layer. The surface forming process consists of two phases: the seed coating phase and the growth phase thereof. During the priming phase, the current voltage and operating current are gradually increased, in several stages, according to respective pre-selectable changes in the current density setting from 1 to 6 mA / cm ² at each switching stage, from the initial value to the current structuring density. . The initial value is 0 mA / cm ² , but may be higher if the electroplating phase immediately prior to the priming stage and the bath current has not dropped to zero. The time between two current density increases is about 0.1 to 30 seconds, most often a 7 second interval is used to increase the current density. Each time the current is switched to a higher level, additional germs are generated. Unlike metallization with pulsed current, the operating current does not switch back to 0, on the contrary, it increases each time it is switched. In this way, more rounded and more regular shaped germs are formed. particles that are deposited on the surface of an object than has ever been achieved by an available pulsed current plating method. The individual stages of the flow are fed into the bath until a structured layer consisting of clusters of nearly spherical or dendritic bodies individually or superimposed on the surface of the workpiece is formed.

In particular, during the embryonic coating phase, it is desirable to achieve a structured surface thickness of between 4 µm and 10 µm. 10 to 240 degrees of current increase are usually required to achieve this effect, particularly good results are obtained at 50 to 60 degrees.

The current density obtained after reaching the last degree of current density is the structuring current density.

By reaching this current structuring density, the actual formation of the structure on the surface is roughly completed. Finishing of the resulting structure is dependent on several parameters, but mainly on the selected current structuring density, the number, height and time interval between the individual stages of the current, the bath temperature and the electrolyte to be used. The current density at each switching stage, as well as the time interval between the individual stages of increasing the current density, can be varied during the coating phase. Depending on the nature of the current conductivity, it is possible to create different surface structures, which is manifested mainly by different depth of surface roughening. Ideal parameters of this process can be determined simply empirically. As a rule, a higher current structuring density can be used at a higher bath temperature and a higher acid content in the electrolyte.

The structuring current density is typically two to three times the density used in conventional DC plating. DC plating works with current densities ranging from 15 to 60 mA / cm ² . The current density depends on the type of electrolyte and the bath temperature. A current density of 30 to 180 mA / cm ² can be used to form the structured surface.

After the seed formation phase, the seed growth phase occurs. In this case, an operating current with a current density in the range of 80% to 120% of the structuring current density is supplied for a short time during a preset holding time. During stamina, the overflow current is almost uniform, resulting in an enhancement of the structure on the metallized object formed during the first phase. Depending on the duration of the holding time, this initial structured layer may be more or less improved. At the highest points of the structured layer, the coating increases more rapidly than in the depressions formed during the seed layer. Therefore, the surface roughness is further increased in the coating growth phase. The residence time ranges from 1 to 600 seconds, preferably about 30 seconds.

At the end of the holding time, the operating current is reduced to a final value, usually to zero. This reduction of the operating current to the final value can be effected by a jump, but a gradual reduction is also possible. Here, too, good results were achieved with a stepwise change in the operating current. Preferably, to reduce the current density, values range from -1 to -8 mA / cm ² per degree, and a preferred time interval between two degrees of current is from 0.1 to 1 second.

Three steps of the process have been described above: stepwise increasing the operating current during the seed formation phase until the structuring current density is reached, maintaining the operating current within the structuring current value during the holding time (seed growth phase) and then reducing the operating current to the final value. These successive steps represent one cycle of structured surface formation. They can be repeated cyclically. Determining repetition of the process is particularly advantageous if a thicker coating is desirable on the surface of the object. In this case, the respective final value of the preceding cycle corresponds to the starting value of the immediately following cycle. The number of repetitions depends on the desired texture and thickness of the surface layer. Good results were obtained with repeats of two to twenty times. The final values of the individual cycles may be different.

It has many times led to a better result if the object we were intending to apply was placed in the bath about a minute before the start of the process. This waiting acclimatization time contributes primarily to temperature equalization - which means that the temperature of the base material of which the workpiece is made is equal to that of the bath.

Good results are obtained by applying a base chromium backing layer prior to the deposition of the structured coating under the conditions used in conventional direct current chrome plating. This is achieved in such a way that a base pulse (voltage or current pulse) is applied at the beginning of the coating. In this case, a current density of 15 to 60 mA / cm @ ^{2 is used} , which corresponds to the current values of normal chromium. This basic pulse lasts about 600 seconds. In order to eliminate changes in bath concentration in the phase boundary layer prior to the structured surface formation when such a direct current step is selected, it is advantageous to include a pause for about 60 seconds before starting the structured surface preparation process.

This method is used in many branches of technology, especially in machining such components, where special emphasis is placed on the properties of their surface. It is known that the application of surface layers to components is done by electroplating. However, very often the plated articles must have a particular surface structure of the coating to be deposited. For example, cylindrical sliding surfaces have a specified lubricant deposit (adhesion) value and the surface of medical or optical devices should have a reduced degree of reflection.

The specified reflection level is determined by the furniture industry and the manufacturers of medical equipment for its functional and decorative elements. In the printing industry, moistening rollers with a specially roughened surface are again required. In the mechanical metal processing industry, structured surface tools are needed to mechanically affect the surface texture of the workpiece. By way of example, the surface treatment of sheet metal in a rolling mill by means of rolling stands with chrome-plated rolls.

The apparatus for carrying out the described method consists of a galvanic bath containing an electrolytic solution with a concentrated content of metal compounds. As electrolyte, chromium sulfuric acid electrolytes, mixed acid chromium electrolytes or alloy electrolytes are preferred.

Electrolytes having a concentration of from 180 to 300 grams of CrO ₃ chromium per liter of bath were preferred. In addition, other additives, such as e.g. sulfuric acid H ₂ SO ₄ , hydrofluoric acid H ₂ F ₂ , hexafluorosilicic acid H ₂ SiF ₆ and mixtures thereof. The electrolyte preferably contains 1 to 3.5 grams of H ₂ SO ₄ per liter. The galvanic bath is usually heated, the optimum electrolyte temperature is preferably 30 to 55 degrees Celsius.

One anode and one cathode are immersed in the electrolytic electroplating solution, the cathode being the object to be metallized or at least at the end of the cathode. When using chromium electrolytes, anodized platinum or PbSn _{7 is} preferably used as the anode. The anode and cathode are connected to the operating current supply device. The current is fed in steps, each with a selectable change in current density from 1 to 6 mA / cm ² per degree from the initial value up to the structuring current density. The time intervals between the two degrees of increment are set to a value of 0.1 to 30 seconds. Once the current structuring density has been reached, an operating current having a current density in the range of 80% to 120% of the structuring current density is connectable for a preset holding time. In order to achieve a uniform coating, the device can be equipped with a rotary drive unit for uniform rotation of the object. The distance between the anode and the object to be coated is in the range of 1 cm to 40 cm, preferably 25 cm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by the following examples, which are illustrated in the accompanying drawings.

Fig. 1 is a schematic representation of a galvanic coating device having a structured coating.

Fig. 2 is a graphical representation of the current density over time of a structured coating on a surface.

Fig. 3 is a photographic image of the surface structure of an object with a 200: 1 magnification ratio, with the coating of the process shown in FIG. Second

Fig. 4 is a photograph of the structure shown in FIG. 3, at a magnification ratio of 500: 1.

Fig. 5 is a graphical representation of the current density over time in a structured surface coating process.

Fig. 6 is a graphical representation of the current density over time in a structured surface coating process.

Fig. 7 is a graphical representation of the current density over time in a structured surface coating process.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 schematically shows an apparatus for the galvanic deposition of structured coatings. The vessel filled with the electrolyte solution 1 forms a galvanic bath. The object 2 to which the coating and the anode 3 are to be dipped is immersed in the galvanic bath. The object to be coated forms the cathode 2. The anode and cathode are supplied from a controlled power source 4. The power source may be either a current source or a voltage source. As it is in terms of the effects of electricity, current, respectively. the cathode current density is decisive for the whole process, the process can be more easily controlled by the current source. The use of a voltage source, on the other hand, has the advantage of lower switching requirements. Unless other parameters, such as e.g. bath temperature and electrolyte concentration, the process can be controlled equally well by a voltage source.

The electric power source 4 is controlled by a programmable control unit 5. The control unit can be used to set any voltage waveforms, resp. current, which is then automatically fed through the energy source 4 to the electrodes.

Figure 2 is a graphical representation of the time current operating density for forming a structured coating surface layer. The horizontal axis of FIG. 2 is the timeline, the vertical Y-axis shows the current density. This is an example of the possible course of the processing method, which is described in more detail below. Figures 3 and 4 are photographic images of a structured layer prepared by this plating method.

A chromium electrolyte with sulfuric acid in a ratio of 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 thus treated is a rotationally symmetrical part, a humidifying drive roller for the printing industry. In order to provide a suitable surface for structured chromium plating, a cylinder made of steel St52 is first finely ground with a roughness depth Rz < 3 [mu] m. Subsequently, about 30 µm thick nickel layer and 10 µm continuous chromium coating are applied to the object under normal conditions in electroplating conditions. The workpiece thus prepared is rotated in a galvanic bath in order to achieve a uniform distribution of the coating layer. The workpiece is a cathode, the material used for the anode is clad titanium or PbSn ₇ . The anode / cathode spacing was adjusted to 25 cm.

At the beginning of the first phase 7 of the process, the current remains switched off. This phase serves to acclimatize the workpiece to a galvanic bath. The workpiece then reaches the electrolyte temperature. After about one minute, the anode and cathode are connected to direct current. This remains on for the entire phase 8, which lasts about 600 seconds. During this time, a base chrome coating is applied to the workpiece by direct current. The current density used is also common in traditional chrome, 20 mA / cm ² . After deposition of the chromium base layer with a direct current, the second phase 9 is free of current.

Then the actual structure creation begins. During phases 10 and 11, the current density gradually increases to the structuring current density 14. The characteristic data of the individual stages (step height or time interval between the two current changes) varies during the increase. In the first phase 10, the current is increased in 16 degrees to 40 mA / cm ² . This corresponds to a change in current density of 2.5 mA / cm ² per stage. The time interval 28 between the two stages is 5 seconds. Then, the current density during phase 11 is increased in an additional 62 steps to a structuring current density of 100 mA / cm ² , the time between the two switching stages is 6 seconds (the current density shown in the graph in Fig. 2 is not quite dimensionally true. 5 and 6).

After the structuring current density is reached, the current density is maintained at the same level throughout the holding time. During this time, the direct current flow causes an increase in the thickness of the structured coating formed in phases 10 and 11. The residence time lasts 60 seconds. Then, the current density is again reduced to a final value of 0 mA / cm ² through the intermediate steps in 22 steps. The time between two current changes lasts 4 seconds. For reasons of technical use, in the case of a humidifying roller, in addition to the structured chromium layer formed according to the process of the invention, a 4-8 mm chromium layer with micro-cracks is applied. This procedure is carried out under the conditions common to galvanic processes and is not explained in detail herein.

Figures 3 and 4 are photographic shots of a structured chrome surface - a microscope view, obtained according to the procedure described in Figs. 2. The structured layer consists predominantly of approximately spherical, sporadically or partially superimposed particles. The structured layer shown is defined by a rough surface Rz = 8 µm, with a carrier fraction of 25%. According to DIN 4762, the "load factor" can also be defined as "material load".

Figure 5 is a representation of a time course of current density of another of the phases of the structured surface forming process. Phases 7, 8 and 9 have already been explained in the comments to figure 2. In phase 15, immediately following, the current density increases in 110 equal steps up to a structuring current density value of 100 mA / cm ² . The time interval between the two current switches was 10 seconds. After a holding time of 16 to 60 seconds, the current density, but now in 22 equal steps, decreases to a final value of 0 mA / cm ² . The time between the two current switches was 4 seconds. Subsequently, the process cycle consisting of phases 15, 16 and 17 is repeated after a short time without current.

Figure 6 shows the current density of the next phase of the deposition process. The waiting time 7 for acclimatizing the workpiece to the galvanic bath is followed by a direct current pulse 18 corresponding in nature to the direct current 8 in FIG. 2. This is followed directly by the seed layer stage 19, during which the current density increases stepwise up to the value of the structuring current density 24. The current density is then maintained at the structuring current density for a holding time of 20 and then reduced to a final value of 26 during phase 21. After a short waiting time of 22, a further seed formation phase 23 follows, gradually increasing the current density to a new current density of 25. structuring. In this case, the initial current density during the crystal formation phase 23 is equal to the final value 26 to which the current density has decreased at the end of the previous structured coating cycle. The current value is then maintained at the structuring current density 25 during the dwell time 27 and is consequently reduced to a new final value of 0 mA / cm ² .

Figure 7 shows the current density over time of another variant of the deposition process. Technological course

Stage 7 to 9 was discussed already in Figure 2. The operating current is then increased in stages to the current structuring density 30 in stages 29 and then 80% of the structuring current density 30 is added over a holding time of 32. Meanwhile, a current-free rest period of 31 is inserted. After the end-of-life period of 32, the operating current during phase 33 is reduced to a final value. This final value serves as an initial value for the next structured coating forming cycle, starting with the first stepwise current rise in phase 35. After the new structuring current density 36 has been reached, an operating current of 120% of the structuring current density 36 is applied during the dwell time 38. In the meantime, however, a rest period 37 without current is again inserted.

PATENT CLAIMS

Claims (11)

A method of galvanic coating of machine components by a direct current coating process, in particular for forming structured cylindrical surfaces as depots for receiving lubricants, for setting defined levels of reflection from surfaces for optical, medical tools and for functional and decorative use in furniture and medical applications industry, to create surfaces with a specified roughness in printing industry products, and to provide structured tool surfaces where at least one initial pulse of electrical voltage and / or electrical current is deposited on the coated surface with embedded material layers and where subsequently by at least one subsequent pulse to the growth of the embryonic secreted material by depositing additional secreted material, characterized in that during the germ layer formation phase the electrical voltage and / or electric current increases in several stages, the time between increments being 0.1 to 30 seconds, the variations in current density in stages being from 1 to 6 mA / cm ² . Method according to claim 1, characterized in that the steps are each increased by a preset change of current density from 1 to 6 mA / cm ² per degree from the initial value to the structuring current density (14,24,25), wherein the time (28) between the two current density increases is about 0.1 to 30 seconds and the next stages are fed into the bath until a continuous structured coating consisting of individually or superimposed, approximately spherical or dendritic particles on the surface of the object is achieved and then during the phase of growth of the coating particles, an operating current with a current density in the range of 80% to 120% of the structuring current density is fed during a preselected holding time (12, 16). 3. A process according to claim 1 or 2, wherein each step of the process takes 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 degrees. Method according to at least one of Claims 1 to 4, characterized in that the structuring current density (14, 24, 25) ranges from 30 mA / cm ² to 180 mA / cm ² . Method according to at least one of Claims 1 to 5, characterized in that the residence time (12, 16) ranges from 1 to 600 seconds, preferably about 30 seconds. Method according to at least one of Claims 1 to 6, characterized in that the operating current is reduced to a final value (26) after the holding time has elapsed. Method according to claim 7, characterized in that the operating current decreases by a predetermined varying value from -1 to -8 mA / cm ² per stage up to the final value after each time of holding time. The method of applying a coating to an electrically conductive surface of an object according to any one of claims 7 or 6, characterized in that the process is repeated cyclically two to twenty times, wherein each respective final value of the previous cycle corresponds to the initial value of the next cycle. Method according to claim 9, characterized in that the final values (26) have different sizes. Method according to at least one of claims 1 to 10, characterized in that, before the structured layer is formed, a DC pulse (8, 18) with a current density of 15 mA / cm ² to 60 mA / cm ² is supplied to the bath. forming a direct current primer coating.