WO2020058336A1

WO2020058336A1 - Method for removing a mechanically resistant coating

Info

Publication number: WO2020058336A1
Application number: PCT/EP2019/075008
Authority: WO
Inventors: Wolfgang Hansal; Rudolf Mann
Original assignee: Hirtenberger Engineered Surfaces Gmbh
Priority date: 2018-09-18
Filing date: 2019-09-18
Publication date: 2020-03-26
Also published as: ES2875760T3; EP3626864A1; DK3626864T3; EP3626864B1

Abstract

The invention relates to a method for removing a mechanically resistant coating from the substrate of a tool, comprising the steps of providing an electrolyte, which comprises an aqueous solution with 4 to 6 mol/l OH-, 0.1 to 0.5 mol/l S2- and 0.5 to 2 mol/l oxalate, placing the tool in the electrolyte and dissolving the mechanically resistant coating by contacting the tool with a DC voltage source.

Description

METHOD FOR REMOVING A PLASTIC COATING

The present invention relates to a method for removing a hard material coating which is applied to the substrate of a tool. The invention further relates to a mixture of substances, an aqueous solution and an electrolyte for use in this method.

Background of the Invention

Cutting tools used in the metalworking sector e.g. used as milling cutters are exposed to high mechanical and thermal loads. To improve the cutting performance and to extend the service life, these tools are coated with hard materials on the work surfaces after their mechanical production. These hard materials are usually compounds of refractory metals (elements of the 4th, 5th and 6th subgroup of the periodic table, i.e.Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W) with the elements B, C, N, O and Si.

Despite their great hardness and abrasion resistance, these hard material coatings wear out during use, so that at the end of the service life the layer no longer meets the requirements. In order to be able to continue using the tool, which is usually undamaged, it must be coated again. For the successful application of a layer using vacuum technology (PVD or CVD), it is necessary that the surface to be coated is free of disruptive impurities; this also includes remnants of a previous coating.

Classic methods for stripping a tool include mechanical stripping using blasting material (such as sandblasting the tool) until the hard material coating is removed. When mechanically removing a hard material coating, large forces generally act on the tool, which can lead to deformations and mechanical damage to the substrate.

As an alternative to mechanical, chemical stripping processes are also used, in which the hard material coating is dissolved. Due to the chemical resistance of such hard material coatings, however, very aggressive chemicals have to be used, some of which also dissolve the carrier material or uncoated parts of the tool. A known method for the chemical removal of AlTiN on hard metal substrates uses a mixture of concentrated sulfuric acid and potassium permanganate. This creates the unstable and potentially explosive intermediate manganese heptoxide (Mn ₂ 0 ₇ ), which acts as the actual oxidizing agent. However, this method proved to be inefficient and required increased security requirements, so that it is not used in practice.

WO 99/64646 A1 describes a method in which the hard material coating is applied to an intermediate layer applied to the substrate. This intermediate layer is dissolved by means of a solution which can pass through pores in the hard material coating, which subsequently also leads to the hard material coating being removed. The solution includes hydrogen peroxide, disodium oxalate and sodium hydroxide. In addition to the increased outlay due to the application of the intermediate layer, this method and the chemical method described above have the disadvantage that the uncoated substrate is chemically attacked.

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is to completely remove a hard material coating present on a tool without damaging the uncoated substrate.

This object is achieved by a method for removing a hard material coating from the substrate of a tool, comprising the provision of an electrolyte which contains an aqueous solution with 4 to 6 mol / l OH, 0.1 to 0.5 mol / l S ² and 0, 5 to 2 mol / l oxalate, placing the tool in the electrolyte and dissolving the hard material coating by contacting the tool with a DC voltage source. This electrochemical method for dissolving the hard material coating has proven to be advantageous, since in such an electrochemical method the substrate material forms a water-insoluble sulfide which serves as passivation, thereby protecting the underlying substrate. At the same time, the hard material coating is dissolved. This effect proves to be an advantage in particular on uncoated areas of the substrate or on those areas which no longer have any coating.

In principle, it is sufficient if the substrate has at least one of the metals W, Co or Fe or another metal which can form sulfides which are insoluble in water. Preferred substrate materials therefore comprise W, Co and / or Fe, since these form passivating sulfide layers. In some tools with a hard material coating, a base layer can be introduced between the hard material coating and the substrate. For example, a TiN base layer is often provided for an AlTiN hard material coating. This base layer can be easily removed in a next step after removing the hard material coating. For this purpose, it can preferably be provided that after removing the hard material coating, the tool is treated with a mixture comprising NH ₃ and H ₂ O ₂ to remove a base layer arranged between the hard material coating and the substrate. The mixture preferably comprises a freshly prepared solution of an aqueous NH3 solution and an aqueous H ₂ 0 ₂ solution.

One embodiment variant provides that the base layer is TiN.

The method according to the invention can be used for any hard material coating if the hard material coating contains at least one of the above-mentioned refractory metals (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo or W). The best results were achieved in the first tests with hard material coating made of AlCrN or AlTiN.

Furthermore, it has proven to be advantageous if the tool is galvanically provided with a metal layer, for example a cobalt layer, on those parts of the substrate which do not have a hard material coating before being placed in the electrolyte.

For the electrochemical process, a mixture of substances consisting of

70 to 87 mol% of a metal hydroxide,

1 to 6 mol% of a water-soluble sulfide,

• 11 to 24 mol% of oxalic acid or a salt of oxalic acid

• maximum 2 mol% of other ingredients

proven to be advantageous, which is why it forms an aspect of the invention.

An aqueous solution can be provided from the mixture of substances itself or also by means of the individual constituents, so that an aqueous solution also comprises

• 4 to 6 mol / l OH

0.1 to 0.5 mol / l S ²

• 0.5 to 2 mol / l oxalate

forms an aspect of the invention.

The aqueous solution is preferably characterized by

• 4.8 to 5.2 mol / l OH, preferably KOH or NaOH

0.24 to 0.27 6 mol / l S ² , preferably K ₂ S or Na ₂ S

0.8 to 1.2 mol / l oxalate, preferably disodium oxalate or dipotassium oxalate. The aqueous solution preferably comprises further constituents in a total amount of not more than 2 mol / l. The aqueous solution particularly preferably comprises no further constituents.

Another aspect relates to an electrolyte with such an aqueous solution.

Further details and advantages are explained below using examples and figures. Show it:

Fig. 1: SEM image of a hard metal substrate (5 OOOx)

FIG. 2: EDX spectrum of the hard metal substrate from FIG. 1

Fig. 3: SEM image of a high-speed steel substrate (5000x)

FIG. 4: EDX spectrum of the high-speed steel substrate from FIG. 3

5: SEM image of a TiN / AlTiN layer (100x)

FIG. 6: EDX spectrum of the TiN / AlTiN layer from FIG. 5

Fig. 7: SEM image of an AlCrN layer (100x)

8: EDX spectrum of the AlCrN layer from FIG. 7

Fig. 9: Photographs of contacted and painted samples

10: Assignment of the SEM images to the respective zones of the samples according to

Fig. 9

Fig. 11: System hard metal TiN / AlTiN: originally uncoated

Tungsten carbide substrate after electrochemical treatment, lOOOx

Fig. 12: Carbide-TiN / AlTiN system: Carbide electrochemically decoated from TiN / AlTiN

Fig. 13: Carbide-TiN / AlTiN system: transition from originally uncoated

Carbide substrate (left) to untreated carbide (right).

Fig. 14: Carbide-TiN / AlTiN system: transition from originally uncoated

Carbide substrate (top) to decoated carbide (bottom)

Fig. 15: System carbide-AlCrN: Originally uncoated carbide substrate after electrochemical treatment, 100x

Fig. 16: Carbide-AlCrN system: Electrochemically decoated from AlCrN

hard metal

Fig. 17: Carbide-AlCrN system: transition from originally uncoated

Carbide substrate (left) to untreated carbide (right).

Fig. 18: Carbide-AlCrN system: transition from originally uncoated

Carbide substrate (top) to decoated carbide (bottom) Fig. 19: High-speed steel TiN / AlTiN system: originally uncoated

High speed steel substrate after electrochemical treatment, lOOOx

Fig. 20: High-speed steel system - TiN / AlTiN: High-speed steel electrochemically decoated from TiN / AlTiN

Fig. 21: System high-speed steel-TiN / AlTiN: transition from originally uncoated high-speed steel substrate (left) to untreated

High-speed steel (right).

Fig. 22: High-speed steel-TiN / AlTiN system: transition from originally uncoated high-speed steel substrate (top) to decoated

High-speed steel (bottom)

Fig. 23: High-speed steel AlCrN system: originally uncoated

High speed steel substrate after electrochemical treatment, lOOOx

Fig. 24: High-speed steel AlCrN system: Electrochemically decoated from AlCrN

High-speed steel

Fig. 25: High-speed steel-AlCrN system: transition from originally uncoated high-speed steel substrate (left) to untreated

High-speed steel (right)

Fig. 26: System high-speed steel-AlCrN: transition from originally uncoated high-speed steel substrate (top) to decoated

High-speed steel (bottom)

Fig. 27: Co layer on hard metal after the stripping process

Fig. 28: Hard metal decoated from TiN / AlTiN after coating the exposed hard metal with cobalt

DETAILED DESCRIPTION OF THE INVENTION

The samples examined (according to the substrate) consisted of high-speed steel and hard metal. The samples were each coated with TiN / AlTiN or AlCrN. The hard metal coating was incomplete because it was important to determine whether the task was completely solved to have uncoated parts on the substrate and to show that these were not attacked by the electrochemical treatment. The task was solved by developing an alkaline electrolyte containing oxalate and sulfide ions. The sulfide ions form an insoluble phase with the substrate, which protects against attack by the stripping process by passivation.

Another option to protect areas that were uncoated from the start was to selectively apply a thin layer to them. In the case described, this was a co-layer. Experimental implementation:

At the beginning of the stripping tests, the initial state of the samples was recorded by SEM:

The blanks provided were wrapped with wire to make contact and all sides except the half front surfaces were coated with a resist. This means that the following tests, e.g. in a scanning electron microscope, the direct comparability with the initial state given (Fig. 9).

De-coating attempts in detail

Electrochemical stripping with alkaline electrolyte

In order to protect the substrate, in particular the co-phase of the hard metal, during the stripping process, sulfide-containing electrolytes were tested. Sulfide is an ideal candidate because all substrate materials (W, Co, Fe) form water-insoluble sulfides in alkaline, which protect the material from further attack by passivation, while the elements in the PVD layer (Al, Ti, Cr) do not Form phases and therefore remain electrochemically active.

With a solution (5M KOH + 20g / l Na ₂ S + 1M K2C2O4) was layered as follows:

Hard material coating AlCrN: 60 ° C, 200 mV vs Hg / HgO, 25 min

Hard material coating TiN / AlTiN: 60 ° C, 600 mV vs Hg / HgO, 45 min

This electrolyte turned out to be the best among the tested candidates, it gives good results in all four sample types in terms of the stripping performance and the area removal.

11 to 26 are the representation of four representative locations of four stripped samples with different substrate / layer combinations, ie FIGS. 11 to 14, 15 to 18, 19 to 22 and 23 to 26 form respectively a sample. The first of the respectively associated figures (that is, FIGS. 11, 15, 19 and 23) represents the originally uncoated substrate after the electrochemical treatment, the second figure (that is, FIGS. 12, 16, 20 and 24) that stripped substrate. The respective third FIG. (Thus FIGS. 13, 17, 21 and 25) represents the transition from the treated, originally uncoated substrate to the original, uncoated substrate protected during the treatment with lacquer and serves to represent the substrate change during the treatment. The fourth figure in each case (that is to say FIGS. 14, 18, 22 and 26) represents the transition from the stripped substrate to the treated but originally uncoated substrate. The decoating of hard metal / TiN / AlTiN functioned satisfactorily. 20 shows the exposed hard metal substrate after stripping, an attack can be seen, which is, however, significantly weaker than in the systems tested so far. At the stripped locations, the substrate is only attacked at the locations where pores are present in the TiN / AlTiN layer (FIG. 21). At the transitions (FIGS. 22 and 23) it can be seen that the attack has only a minor effect on the dimensions of the substrate.

The base layer of TiN of the TiN / AlTiN coating can be removed with a mixture of NH3 (25%) and H2O2 (30%) in a ratio of 1 + 3 at room temperature within 4-5 minutes without attacking the substrates. This mixture should always be freshly prepared in small quantities (<50ml), since H2O2 quickly breaks down in an alkaline environment. Dissolved heavy metals (Co, Fe) catalyze the decomposition and lead to strong heating and foaming. Alternatively, the solution can be applied to the substrates by means of a spray process, the solution being produced with a static mixer only immediately before the atomizing nozzle.

With this system, stripping is also possible without severely attacking the substrate. Since the stripping process is more gentle with AlCrN than with TiN / AlTiN, the substrate attack is less.

As expected, the electrolyte system also works with high-speed steel substrates. The stripping is complete, the attack on the substrate minimal.

AlCrN can also be removed from high-speed steel substrates with this system. It can also be observed here that the attack on the substrate is less due to the milder conditions when removing AlCrN than with the high-speed steel-TiN / AlTiN system.

Galvanic co-layer as additional substrate protection:

The attack on the substrate was further restricted by a selective, thin galvanic coating of the hard metal substrate. In the specific case, cobalt was chosen because it is the matrix material of the hard metal and its use therefore does not introduce any undesirable impurities into the system.

In principle, any galvanically separable metal or alloy is suitable as protection, which is insoluble in the stripping electrolyte and can then be removed again without attacking the substrate (for example iron, zinc, manganese, copper, etc.). When coating, it surprisingly turned out that the TiN / AlTiN layer can be coated with cobalt in an adhesive manner at high currents despite its lower electrical conductivity and the ceramic structure. Trying to sample this anyway decoating (based on the consideration that the cobalt layer adhesion to the TiN / AlTiN coating is not as good as that of the hard metal) failed because the cobalt layer proved to be adhesive and insoluble, which was the original concept that a cobalt layer underneath Material protects, confirmed in principle. During the subsequent removal of the cobalt layer, the exposed substrate material was naturally severely attacked. Coating with cobalt with a greatly reduced current density left the TiN / AlTiN layer largely free, so that the subsequent stripping could be carried out without any problems at the free areas (see FIGS. 27 and 28). The method of reducing the current density in the galvanic coating is a quick and simple solution which uses the lower electrical conductivity of the coating in comparison to the substrate in order to prevent galvanic coating of the free substrate.

Claims

EXPECTATIONS

1. A method of removing a hard material coating from the substrate of a tool, comprising

The provision of an electrolyte which comprises an aqueous solution with 4 to 6 mol / l OH, 0.1 to 0.5 mol / l S ² and 0.5 to 2 mol / l oxalate,

• Placing the tool in the electrolyte and

• Dissolving the hard material coating by contacting the tool with a DC voltage source.

2. The method according to claim 1, characterized in that the tool is then treated with a mixture comprising NH ₃ and H ₂ 0 ₂ to remove a base layer arranged between the hard material coating and the substrate.

3. The method according to claim 1 or claim 2, characterized in that the base layer is TiN.

4. The method according to any one of claims 1 to 3, characterized in that the hard material coating consists of AlCrN or AlTiN.

5. The method according to any one of claims 1 to 4, characterized in that the tool is electroplated before placing in the electrolyte, a metal layer, preferably cobalt layer, on the substrate, which has no hard material coating.

Mixture consisting of

70 to 87 mol% of a metal hydroxide,

I to 6 mol% of a water-soluble sulfide,

I I to 24 mol% of oxalic acid or a salt of oxalic acid

maximum 2 mol% of other ingredients.

7. Aqueous solution, comprehensive

• 4 to 6 mol / l OH

0.1 to 0.5 mol / l S ²

• 0.5 to 2 mol / l oxalate.

8. Aqueous solution according to claim 7, characterized by

• 4.8 to 5.2 mol / l OH, preferably KOH or NaOH

0.24 to 0.27 6 mol / l S ² , preferably K ₂ S or Na ₂ S 0.8 to 1.2 mol / l oxalate, preferably disodium oxalate or dipotassium oxalate.

9. electrolyte comprising a mixture of substances according to claim 6 or an aqueous solution according to claim 7 or claim 8.

10. Galvanic cell comprising an electrolyte according to claim 9.