Method for Delamination of Ceramic Hard Material Layers from Steel and Cemented Carbide Substrates

Abstract

In order to improve a method for decoating of ceramic hard material layers from steel and cemented carbide substrates having a ceramic hard material layer on part of the surface thereof and to make it amenable to further applications, it is proposed that the workpieces (10) to be decoated be insertedpreferably with a part thereof without a ceramic hard material layerinto guard elements, preferably protective plugs, which fit in diameter and height, and be pressed into a holder (50), the holder with the workpieces (10) to be decoated be contacted with the plus pole of the current pulse driver, an either acidic or basic electrolytic bath be selected, the contacted holder be placed into the selected electrolytic bath (30), at least one electrode (20) be positioned at a predetermined distance from the holder and the latter be contacted with the negative pole of the power pulse generator (40), the decoating is performed by means of the current pulse driver, with endpoint detection being performed continuously or a control for decoating being conducted at time intervals.

Claims

1. A method for decoating of ceramic hard material layers from at least one workpiece having a ceramic hard material layer on a part of the surface thereof, wherein at least one electrode is arranged as a cathode in an electrolytic liquid, wherein the workpiece or the workpieces acting as anode are also arranged at least partially in said electrolyte liquid, wherein a pulse driver means for generating voltage pulses is arranged between the cathode or the cathodes and the anode or the anodes, and wherein guard elements are provided, comprising the steps that the workpieces to be decoated are inserted into the guard elements that are matching in diameter and height and pressed into a holder, that the holder with the workpieces to be decoated is contacted with the plus pole of the pulse driver means, that an acidic electrolytic bath is selected, that the contacted holder is placed into the selected electrolytic bath, at least one electrode is placed at a predetermined distance from the holder and is contacted with the negative pole of the pulse driver means, and that the decoating is performed by means of the pulse driver means, wherein a continuous end point detection or a control for decoating at time intervals is carried out, wherein the end point detection comprises measuring or determining the voltage which is required to establish a specific current, the endpoint being reached when, after observing a drop of the voltage, the voltage again reaches its original value, characterized in that a 2 to 50% mineral acid with a pH value of 0.5 to 1.1 is used as electrolyte.

2. The method according to claim 1, characterized in that the workpieces are inserted into a holder, thereby contacting them and simultaneously protecting the uncoated material surfaces from attack, and to subsequently decoat them.

3. The method according to claim 1, characterized in that the power supply is designed in such manner that it supplies a current of 10 A to 50 A at a voltage (U.sub.0Max) of 20 V to 60 V, which is current-controlled pulsed with a frequency of 1 Hz to 40 Hz and a sampling rate greater than 25%.

4. The method according to claim 1, wherein a holder is used for workpieces, particularly hobs, having uncoated surfaces in several regions thereof, the holder having a base plate in which an isolating mounting protects the workpiece to be received therein from chemical attack, an electrical contact for the current supply acting as anode, a conductive cylinder provided as a cathode and which can be contacted via electrical contacts, preferably a current rail, and an isolating plug which protects the workpiece from chemical attacks at other locations.

5. The method according to claim 4, characterized in that a holder is used in which the cylinder, the isolating mounting and the isolating plug are configured exchangeable in order to cover and to contact workpieces with various sizes and shapes.

6. The method according to claim 2, characterized in that the power supply is designed in such manner that it supplies a current of 10 A to 50 A at a voltage (U.sub.0Max) of 20 V to 60 V, which is current-controlled pulsed with a frequency of 1 Hz to 40 Hz and a sampling rate greater than 25%.

7. The method according to claim 2, wherein a holder is used for workpieces, particularly hobs, having uncoated surfaces in several regions thereof, the holder having a base plate in which an isolating mounting protects the workpiece to be received therein from chemical attack, an electrical contact for the current supply acting as anode, a conductive cylinder provided as a cathode and which can be contacted via electrical contacts, preferably a current rail, and an isolating plug which protects the workpiece from chemical attacks at other locations.

8. The method according to claim 3, wherein a holder is used for workpieces, particularly hobs, having uncoated surfaces in several regions thereof, the holder having a base plate in which an isolating mounting protects the workpiece to be received therein from chemical attack, an electrical contact for the current supply acting as anode, a conductive cylinder provided as a cathode and which can be contacted via electrical contacts, preferably a current rail, and an isolating plug which protects the workpiece from chemical attacks at other locations.

9. The method according to claim 7, characterized in that a holder is used in which the cylinder, the isolating mounting and the isolating plug are configured exchangeable in order to cover and to contact workpieces with various sizes and shapes.

10. The method according to claim 8, characterized in that a holder is used in which the cylinder, the isolating mounting and the isolating plug are configured exchangeable in order to cover and to contact workpieces with various sizes and shapes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Further details, advantages and features of the object of the present invention will become apparent from the following description and the corresponding drawings, in which methods for decocting of ceramic hard material layers according to the present invention are illustrated by way of example. In the drawings there are shown in:

[0027] FIG. 1 a schematic view of the arrangement for carrying out the method according to a first exemplary embodiment of the present invention with a holder for a plurality of workpieces;

[0028] FIG. 2 a perspective view of a holder for mounting of a plurality of workpieces, in this case of shaft tools for positioning in an electrolyte;

[0029] FIG. 3 a detailed view of the functional elements according to FIG. 2;

[0030] FIG. 4 a view from the side onto the holder according to FIGS. 2 and 3;

[0031] FIG. 5 a schematic view of the arrangement for carrying out the method according to a second exemplary embodiment of the present invention;

[0032] FIG. 6 a perspective view of an alternative holder for mounting of a workpiece, here of a hob, in which the surface to be decocted is located between two uncoated regions thereof, according to the arrangement of FIG. 5;

[0033] FIG. 7 a detailed view of the functional elements according to FIG. 6;

[0034] FIG. 8 a perspective view, namely a photograph of the holder according to FIGS. 2 to 4, into which shaft tools are inserted;

[0035] FIG. 9 a view, namely a photograph of the holder according to FIGS. 2 to 4, into which shaft tools are inserted;

[0036] FIG. 10 a view, namely a photograph of the shaft tools according to FIGS. 8 and 9, after decoating;

[0037] FIG. 11 a perspective view, namely a photograph of a workpiece for insertion into the holder according to FIGS. 5 to 7; and

[0038] FIG. 12 a view of the voltage curve which can be used for the end point detection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] The hard material layers of the first group and the third group can have a layer structure comprising a TiN adhesion promoting layer with a layer thickness of <0.5 m between the tool and the actual hard material layer. This forms a transition phase to the actual functional hard material layer.

[0040] It has been found that these hard material layers of the first and the third group can be selectively decoated from the surface down to the adhesion layer made of TiN within a very short time using a suitable wet-chemical approach and applying electrical pulses.

[0041] Moreover, it was apparent from the experiments that in case the hard material layers do not have a TiN adhesion layer between the hard metal tool and the hard material layer the decoating can be carried out by means of the same wet-chemical approach and by means of electrical pulses in an equally fast manner. This is especially true for the hard material layers of the second group. However, in this case the hard metal tool is attacked at the surface thereof and has to be post-treated.

[0042] Moreover, it was found in experiments that the hard material layers of the second and the third group can be selectively decoated within a very short time from the surface either down to the adhesion layer made of TiN or, in the absence of such an adhesion layer made of TiN, down to the surface of the high speed steel tool in a suitable wet-chemical approach by means of electrical pulses.

[0043] Hard material layers of the first group on high speed steel tools cannot be de-coated with this method because the wet-chemical approach used here destroys the high speed steel substrate.

[0044] In the case of pulsed decoating, the coated tool serves as a positive pole (electrical anode), whereas steel shields or steel rings or other metal objects serve as negative pole (electrical cathode). The electrolyte used depends on the ceramic components in the hard material layer.

[0045] Hence, for the hard material layers classified as above, two different electrolyte media are employed, namely for hard material layers of the first group, that is Ti, Al based layers, an acidic electrolyte, which in the exemplary embodiment described here consists of 10 to 15% nitric acid (c=1.67 to 2.58 mol/l) and a pH value of 0.23 pH to 0.41 pH, and for hard material layers of the second and the third group, that is Cr and CrTi based layers, a basic electrolyte, which in the exemplary embodiment described here consists of 1 L water with 50 mL KOH 50% (c=0.67 mol/l) and 20.6 g potassium permanganate (c=0.13 mol/l) and a pH value of the solution of 13.5. In the exemplary embodiment described here, both electrolytes are operated at room temperature. Now a uniformly positive current-pulsed signal is induced by means of a pulse generator until the decoating has started occurring. The decoating time starting with a 2 m thickness hard material layer is between 10 secs and 5 min, depending on the hard material layer, the electrolyte used and the tool material used.

[0046] The applied current for a given tool depends on the coated surface, and accordingly also on the diameter and geometry of the tool, on the type of the ceramic coating, and thus also on the electrolyte, and can be specifically determined in experiments. The applied current for a cemented carbide end mill (=8 mm. coated length 40 mm) with a coating of the layer type of the second group with a layer thickness of 3 m which is decoated in the basic electrolyte, is about 10 to 11 A. The applied current for the same cemented carbide end mill tool as described above, but coated with a layer type of the first group, which is decoated with an acidic electrolyte, is 3 A. If several tools are clamped into the holder, then the tools act as resistors in a parallel circuit.

[0047] In the case of high speed steel tools, the same dependences were found as in the case of cemented carbide tools. The applied current for a high speed steel tool with a diameter between 6 mm to 12 mm which is decoated in the basic electrolyte is 10 to 11 A. In the acidic electrolyte a corresponding decoating is not possible because the tool would be destroyed.

[0048] The frequency of the pulse and its function shape are also critical parameters for this type of the decoating. A current-controlled pulse mode, preferably with a uniform geometry and most preferably with a rectangular bipolar pulse shape, is used. The frequency of the pulse in the case of the basic electrolyte is 5 Hz to 40 Hz, preferably 10 Hz to 35 Hz and most preferably 20 Hz to 30 Hz, and a sampling rate (duty cycle) smaller than 50%, preferably smaller than 35% and most preferably smaller than 25% is used. In the case of the acidic electrolyte, the frequency is 1 Hz to 40 Hz, preferably 2 Hz to 20 Hz and most preferably 3 Hz to 8 Hz and a sampling rate (duty cycle) greater than 50%, preferably greater than 70% and most preferably greater than 85% are used.

[0049] The TiN adhesion layer remaining on the tools is subsequently decoated by means of a wet-chemical method suitable for the base material, that is high speed steel or cemented carbide. By using e.g. hydrogen peroxide solutions, where the cemented carbide tool is protected by applying a protection voltage, the TiN adhesion layer can be removed within 5 to 10 min. The cemented carbide is not attacked in such short time.

[0050] If hard material layer systems which do not comprise a TiN adhesion layer are decoated with the pulsed method, then the cemented carbide is attacked in the acidic as well as in the basic electrolyte. Then, a post-treatment by means of re-grinding or microblasting or burnishing is necessary. Also, a slight attack on high speed steel tools can occur by applying the basic electrolyte. However, this attack is only minimal and causes a slight optical dulling of the surface.

[0051] Non-coated surfaces such as, for example, shafts of end mill tools are attacked by the pulsed method in acidic and in basic electrolyte and therefore have to be covered by a suitable holder with protection plugs. For shaft tools a holder with protection plugs was specifically developed for the pulsed decoating method. However, the holder can also used e. g. for other chemical decoating methods in which attacks on the cemented carbide can occur. The holder serves the purpose of receiving shaft tools with different diameters, thereby contacting them and simultaneously protecting the uncoated shaft surface from attack and for subsequently decoating it with the pulsed method.

[0052] The holder 50 for shaft tools comprises a conductive base housing 52 with electrical contacts and at least one current supply element, which in the present exemplary embodiment are current supply rails 56, a cover 55 with bore openings and seals for different plugs 54, which in turn are preferably provided with bores with different diameters. The base housing 52, cover 55 and current supply rails 56 are coated with an isolator wherein the isolating material has to be resistant against chemicals and may not be applied at the contacting surfaces. The plugs 54 which are provided with bores with different diameters in order to receive different diameters of shaft tools are made of non-conductive materials that are chemically resistant. The height of the plugs varies in order to cover non-coated shaft lengths with different heights. The plugs 54 are provided with o-rings in order to prevent chemicals from penetrating between the shaft and the plug 54. Moreover, in FIG. 3 there is shown a contact rail 57 on which rests the tool 10, and a two-side contact coil 58, wherein the contact rail 57 serves as a clamping device for the contact coil.

[0053] A characteristic feature in the use of the holder in combination with the guiding plugs is the fact that after pulsed decoating and subsequent removal of the TiN adhesion layer a small ring of non-decocted or slightly attacked surface remains on the shaft tool since a small overlap between the plugs and the coated shaft surface and/or a small overlap between the free shaft surface and the electrolyte is present.

[0054] A special embodiment of a holder servers the purpose of receiving e.g. hobs with different diameters, thereby contacting them and simultaneously protecting the uncoated material surface from attack, and of subsequently decoating them with the pulsed method.

[0055] The holder comprises a base plate 75 in which an isolating mounting 74 is incorporated and which protects the workpiece 10 to be received therein from chemical attack and holds the workpiece 10 preferably in a standing manner. An electrical contact 76 for the workpiece serves as anode, and there is a conductive cylinder 72 which is provided as a cathode and which can be contacted via an electrical contact, and an isolating plug 60 which protects the workpiece 10 from chemical attacks at other locations. The cylinder 72, the isolating mounting 74 and the isolating plug 60 can be exchanged in order to cover and to contact the different sizes and shapes of workpieces 10.

[0056] The method for decoating of shaft tools is carried out in the exemplary embodiment described hereshown in FIG. 1as follows: [0057] 1. The shaft tools 10 to be decoated are inserted into protection plugs that are matching in diameter and height and pressed into the holder 50. [0058] 2. The holder with the shaft tools 10 to be decoated is contacted with the plus pole of a current pulse driver 40. [0059] 3. It must be decided which electrolytic bath 30 shall be used, namely an acidic electrolyte for layers of the first group and a basic electrolyte for layers of the second and the third group. [0060] 4. The contacted holder 50 is placed into the selected electrolytic bath. [0061] 5. Two electrodes 20 made of steel are placed on both sides of the holder and the latter is contacted with the negative pole of the current pulse driver. The distance of the electrodes made of steel to the shaft tool is 0.5 cm to maximally 2.5 cm. [0062] 6. At the pulse generator 40 the conditions are adjusted for shaft tools (with a diameter of 6 mm to 12 mm). Thereby, nine shaft tools per decoating are assumed. The holders in the exemplary embodiment described here are designed for nine tools.

TABLE-US-00001 layers of the 2nd and 3rd layers of the 1st group: group: 1st Example: 1st Example: Number of shaft tools 9 Number of shaft tools 9 with with diameter of 12 mm diameter of 12 mm current: 15 A current: 100 A voltage (U.sub.0Max): 40 V voltage (U.sub.0Max): 50 V current-controlled, current-controlled, pulse shape rectangular pulse shape rectangular frequency 5 Hz frequency 25 Hz symmetry/sampling rate: symmetry/sampling rate: 98% 20% 2nd Example: 2nd Example: Number of shaft tools 9 Number of shaft tools 9 with with diameter of 6 mm diameter of 6 mm current: 15 A current: 100 A voltage (U.sub.0Max): 40 V voltage (U.sub.0Max): 50 V current-controlled, current-controlled, pulse shape rectangular pulse shape rectangular frequency 5 Hz frequency 25 Hz symmetry/sampling rate: symmetry/sampling rate: 98% 20% [0063] 7. Decoating begins immediately. [0064] 8. In the case of shaft tools 10 of the first group an end point detection is used. [0065] In the case of tools of the first group, an effect was surprisingly detected which may serve as end point detection. The electrical power supply provides a function of the current during the decoating time whereby a constant, exactly stable current is generated. Due to the fact that during the decoating process the surface of the tools and therefore, also the resistance is changed, a drop of the voltage is found. When the titanium nitride layer is reached, the resistance increases until the voltage reaches its original value. Hereby, the voltage curve is in the range of about 2 to 10 V and a voltage difference of about 2 to 4 V is to be expected. [0066] In the case of tools of the second and third group the current supply is stopped every 20 to 30 seconds, and the holder with the shaft tools is controlled with respect to decoating. [0067] 9. In the case of a layer with a thickness of 2 m decoating down to the tool or the TiN adhesion layer is completed, depending on the composition of the hard material layer, within 10 seconds to 30 minutes.

[0068] Subsequently the TiN adhesion layer is completely decoated with a conventional wet-chemical approach. Decoating without a TiN adhesion layer requires the same pulsed decoating time. A further chemical decoating is not necessary, but a mechanical post-treatment is carried out due to the attacks of the substrate.

[0069] A slightly different process is provided in an exemplary embodiment for decoating hobs, as shown in FIG. 5: [0070] 1. The hob 10 to be decoated is contacted with the plus pole of the current pulse driver 30 and placed into the holder according to FIGS. 6 and 7 and provided with a protection plug 60. [0071] 2. It must be decided which electrolytic bath 30 shall be used, namely an acidic electrolyte for layers of the first group and a basic electrolyte for layers of the second and third group. [0072] 3. The contacted hob 10 is placed into the selected electrolytic bath 30. A steel ring electrode made of stainless steel that was gold coated is centrally placed at a distance of 0.5 cm to maximally 2.5 cm around the hob. This steel electrode is connected with the negative pole of the pulse generators 30. [0073] 4. At the pulse generator 30 the conditions are adjusted for the hob 10.

TABLE-US-00002 Layers of the 2nd and 3rd Layers of the 1st group: group: 1st Example: 1st Example: hob with a diameter of 47 hob with a diameter of 47 mm; height 1510 mm mm; height 1510 mm current: 30 A current: 30 A voltage (U.sub.0Max): 40 V voltage (U.sub.0Max): 50 V current-controlled, current-controlled, pulse shape rectangular pulse shape rectangular frequency 5 Hz frequency 25 Hz symmetry/sampling rate: symmetry/sampling rate: 98% 20% 2nd Example: 2nd Example: hob with a diameter of 33 hob with a diameter of 33 mm; height 110 mm mm; height 110 mm current: 30 A current: 30 A voltage (U.sub.0Max): 40 V voltage (U.sub.0Max): 50 V current-controlled, current-controlled, pulse shape rectangular pulse shape rectangular frequency 5 Hz frequency 25 Hz symmetry/sampling rate: symmetry/sampling rate: 98% 20% [0074] 5. Switch on the pulse generator 30. Decoating begins immediately. [0075] 6. The current supply is stopped every 20 to 30 seconds, and the holder 50 with the hob 10 is controlled with respect to decoating. [0076] 7. In the case of a layer with a thickness of 2 m decoating down to the TiN adhesion layer is completed, depending on the composition of the hard material layer, within 1 minute to 10 minutes.

[0077] Subsequently the TiN adhesion layer is completely decoated with a conventional wet-chemical approach. Decoating without a TiN adhesion layer requires the same pulsed decoating time. A further chemical decoating is not necessary, but a mechanical post-treatment is carried out due to the attacks of the substrate.

EXAMPLES FOR DECOATING

Example 1

[0078] Nine cemented carbide shaft tools (spiral drills d=12 mm, K type) with an AlTiN layer (layer type table: layer #6) with a thickness of 3.4 m and a TiN adhesion promoting layer were inserted into the specifically developed holder with protection plugs and immersed into a 10% nitric acid solution acting as electrolyte, and decoated down to the TiN adhesion layer with a pulsed current I.sub.Function of 15 A with a frequency of 5 Hz and a sampling rate of 98%. The steel electrodes had a distance to the cemented carbide tool of 1 to 2 cm. The decoating time was 2 min and was terminated by the end point detection.

[0079] In a further process step according to the state of the art, the TiN adhesion layer is completely decoated in a peroxidic decoating bath under the application of a protection voltage on the shaft tools. Here, the decoating time is about 5 to 10 min. After decoating, no attacks on the tools were found in the scanning electron microscope.

Example 2

[0080] A cemented carbide hob (d=470 mm) with an AlTiN layer (layer type table: layer #6) with a thickness of 7.2 m, a coloring cover layer consisting of Al, Ti, N and a TiN adhesion promoting layer was immersed into a 12% nitric acid solution acting as electrolyte, and decoated down to the TiN adhesion layer with a pulsed current I.sub.Function of 30 A with a frequency of 5 Hz and a sampling rate of 98%. The ring steel electrode had a distance to the cemented carbide tool of 1.5 cm. The decoating time was 3 min.

Example 3

[0081] Nine cemented carbide rods (d=6 mm, K type) each with a TiAlN/SiN layer (layer type table: layer #7) with a thickness of 3.7 m and a TiN adhesion promoting layer were inserted into the specifically developed holder with protection plugs and immersed into a 12% nitric acid solution acting as electrolyte, and decoated down to the TiN adhesion layer with a pulsed current I.sub.Function of 15 A with a frequency of 5 Hz and a sampling rate of 98%. The steel electrodes had a distance to the cemented carbide tool of 1 to 2 cm. The decoating time was 2 min and was terminated by the end point detection.

Example 4

[0082] Nine cemented carbide shaft tools (d=12 mm, K type) with an AlTiCrN layer (layer type table: layer #23) with a thickness of 3.1 m and a TiN adhesion promoting layer were inserted into the specifically developed holder with protection plugs and immersed into a basic solution of potassium permanganate with the following composition: 1L H.sub.2O; 50 ml KOH (50%); 20.6 g KMnO.sub.4 and decoated down to the TiN adhesion layer with a pulsed current I.sub.Function of 100 A with a frequency of 25 Hz and a sampling rate of 20%. The steel electrodes had a distance to the cemented carbide tool of 1 to 2 cm. The decoating time was 2 min. In a further process step according to the state of the art, the TiN adhesion layer is completely decoated in a peroxidic decoating bath under the influence of a protection voltage on the shaft tools. Here, the decoating time was about 5 to 10 min. After decoating, no attacks on the tools were found in the scanning electron microscope.

Example 5

[0083] A cemented carbide hob (d=470 mm) with an AlTiCrN layer (layer type table: layer #23) with a thickness of 5.7 m and a TiN adhesion promoting layer was immersed into a basic solution of potassium permanganate with the following composition: 1L H.sub.2O; 50 ml KOH (50%), 20.6 g KMnO.sub.4 acting as electrolyte, and decoated down to the TiN adhesion layer with a pulsed current I.sub.Function of 30 A with a frequency of 25 Hz and a sampling rate of 20%. The ring steel electrode had a distance to the cemented carbide tool of 1 to 2 cm.

Example 6

[0084] Nine cemented carbide rods (d=10 mm, K type) with an AlTiCrN layer (layer type table: layer #22) each with a thickness of 3.4 m without a TiN adhesion promoting layer were inserted into the specifically developed holder with protection plugs and immersed into a basic solution of potassium permanganate with the following composition: 1L H.sub.2O; 50 ml KOH (50%); 20.6 g KMnO.sub.4 acting as electrolyte, and decoated down to the TiN adhesion layer with a pulsed current I.sub.Function of 100 A with a frequency of 25 Hz and a sampling rate of 20%. The steel electrodes had a distance to the cemented carbide tool of 1 to 2 cm. The decoating time was 2 min. The substrate was attacked. Thereafter, the attacked surface was wet-blasted at 1.5 bar. The surface was examined by REM. A roughening of the surface can be recognized in this case.

[0085] In a comparative milling test, on the one hand with a cemented carbide tool which was decoated without a TiN adhesion layer and then recoated, and on the other hand with a new tool which was only coated, conducting the following working steps [0086] coating with AlTiCrN without a TiN adhesion layer [0087] decoating with the pulsed method/KMnO.sub.4 basic [0088] wet-blasting with F400A at 1.2 bar [0089] wet-sharpening of the front face (decoated tools and one new tool) [0090] Edge treatment in the Otec (KV1:2/25 rpm/5 min) [0091] coating with AlCrN [0092] Otec: Polish walnut (Topping) [0093] quality control: Alicona, SEM [0094] Fehlmann: milling test!
gave the following result: After a once only reprocessing of cemented carbide end mills a considerable tool life of approximately 80% as compared to a new tool is possible.

Example 7

[0095] Eight high speed steel tools (d=6 mm, standard) each with an AlCrTiN layer (layer type table: layer #25) with a thickness of 2.8 m with a TiN adhesion promoting layer were inserted into the specifically developed holder with protection plugs and immersed into a basic solution of potassium permanganate with the following composition: 1L H.sub.2O; 50 ml KOH (50%); 20.6 g KMnO.sub.4acting as electrolyte, and decoated down to the TiN adhesion layer with a pulsed current I.sub.Function of 100 A with a frequency of 25 Hz and a sampling rate of 20%. The steel electrodes had a distance to the cemented carbide tool of 1 to 2 cm. The decoating time was 2 min. In a further process step according to the state of the art, the TiN adhesion layer is completely decoated in a peroxidic decoating bath under the influence of a protection voltage on the shaft tools. Here, the decoating time was about 10 to 15 min.

Example 8

[0096] A high speed steel hob (d=700 mm) with an AlTiCrN layer (layer type table: layer #22) with a thickness of 2.6 m without a TiN adhesion promoting layer was immersed into a basic solution of potassium permanganate with the following composition: 1L H.sub.2O; 50 ml KOH (50%); 20.6 g KMnO.sub.4acting as electrolyte, and decoated down to the TiN adhesion layer with a pulsed current I.sub.Function of 30 A with a frequency of 25 Hz and a sampling rate of 20%. The steel electrodes had is a distance to the high speed steel hob of 1.0 cm. The decoating time was 11 min. In a further process step according to the state of the art, the brownish discoloration which was formed by the pulsed decoating is removed in a peroxidic decoating bath at increased temperature. Here, the length of stay in the bath was about 5 min.