Device and method for electrochemically processing a material

Abstract

The invention relates to a method and a device for electrochemically processing a material, which contains a hard phase and a binder phase. The method comprises preparing an aqueous, alkaline, complexing-agent-containing electrolyte and bringing the material to be processed into contact at least in part with the electrolyte and with a current source. In order to electrochemically oxidize the material, a pulsed electrical current is delivered to the material by means of the current source, the pulse sequence of the delivered electrical current being adjusted to the amount of the binder phase in the material to be processed. By means of the method and by means of the device, it is also possible to process materials having a high content of binder phase in such a way that matter can be removed from the material evenly (homogeneously), i.e. both from the hard phase and from the binder phase of the material.

Claims

1. A method for electrochemical machining of a material which comprises a hard phase and a binder phase, comprising the steps of: (a) providing an aqueous, alkaline electrolyte which includes an electrode which is connected to a first pole of a direct current source; (b) at least partially contacting the material to be machined with the aqueous, alkaline electrolyte, the material comprising the hard phase and the binder phase; (c) electrically contacting the material with a second pole of the direct current source; and (d) supplying a pulsed electrical current to the material via the direct current source for electrochemical oxidation of the material, the pulsed current having a specific pulse sequence; wherein the electrolyte comprises a complexing agent which is suitable for complexing at least one metal ion of the binder phase, wherein the pulse sequence of the pulsed current supplied by the direct current source is adjusted to the quantity of binder phase in the material to be machined in a manner that sufficient time is provided for the complexing agent in the electrolyte to complex the metal ions produced on the binder phase so that, during the material removal, no product film which could disturb the equilibrium between removal of the hard phase and removal of the binder phase can be formed and a uniform removal of both phases is made possible.

2. The method according to claim 1, wherein a material is machined, wherein (i) the hard phase has a Vickers hardness HV10 of at least 750, and/or (ii) the binder phase comprises a metal which is suitable for forming a metal hydroxide in aqueous, alkaline solution by electrochemical oxidation.

3. The method according to claim 2, wherein the hard phase comprises a hard metal.

4. The method according to claim 3, wherein the hard metal is a transition metal.

5. The method according to claim 2, wherein the hard phase is selected from the group consisting of tungsten carbide, titanium carbide, titanium nitride, tantalum carbide, niobium carbide, zirconium carbide, vanadium carbide and mixtures thereof.

6. The method according to claim 1, the aqueous alkaline electrolyte comprises: a base which is selected from the group consisting of hydroxide, carbonate, ammonia, alcoholate, alcohol amine, silicate and mixtures thereof; and/or an additive for increasing the viscosity of the aqueous, alkaline electrolyte; and/or no halogenide.

7. The method according to claim 6, wherein the hydroxide is selected from the group consisting of alkali metal hydroxide, alkaline earth metal hydroxide, and mixtures thereof and/or the carbonate is selected from the group consisting of alkali metal carbonate, alkaline earth metal carbonate, and mixtures thereof.

8. The method according to claim 1, wherein the aqueous, alkaline electrolyte is provided in a bath and the aqueous, alkaline electrolyte is circulated via at least one fluid inflow and at least one fluid outflow of the bath.

9. The method according to claim 1, wherein the electrode comprises a material selected from the group consisting of metal, metal alloy, carbon, electrically conductive plastic material and combinations thereof; and/or is mechanically oscillated.

10. The method according to claim 1, wherein the direct current source is configured such that it (i) supplies a voltage in the range of 0.1 to 50 V; and/or it supplies a current density in the range of maximally 400 A/cm.sup.2; (ii) supplies pulses with a pulse length in the range of max. 50 ms; and/or (iii) supplies pulses with a pulse pause between the pulses in the range of at least 0.1 ms.

11. The method according to claim 1, wherein the complexing agent comprises a material selected from the group consisting of alcohol amine, alkyl carbonate, carboxylic acid, ammonia, inorganic ammonium salt, primary amine, secondary amine, tertiary amine, and mixtures thereof.

12. A device for electrochemical machining of a material which comprises a hard phase and a binder phase, comprising (a) an aqueous, alkaline electrolyte, the aqueous, alkaline electrolyte contacting an electrode of a direct current source and being contactable at least in regions with the material which comprises the hard phase and the binder phase; and (b) the direct current source with a first pole which is connected electrically to the electrode and a second pole which can be connected electrically to the material, the direct current source being adjusted to supply a pulsed electrical current with a specific pulse sequence to the material for electrochemical oxidation of the material; wherein the electrolyte comprises a complexing agent which is suitable for complexing at least one metal ion of the binder phase, wherein the direct current source is configured to supply the pulsed current in a pulse sequence which is adjusted to the quantity of binder phase in the material to be machined in a manner that sufficient time is provided for the complexing agent in the electrolyte to complex the metal ions produced on the binder phase so that, during the material removal, no product film which could disturb the equilibrium between removal of the hard phase and removal of the binder phase can be formed and a uniform removal of both phases is made possible.

13. The device according to claim 12, wherein (i) the hard phase has a Vickers hardness HV10 of at least 750; and/or (ii) the binder phase comprises a metal suitable for forming a metal hydroxide in aqueous, alkaline solution by electrochemical oxidation.

14. The device according to claim 12, wherein the aqueous, alkaline electrolyte (i) comprises a base selected from the group consisting of hydroxide, carbonate, ammonia, alcoholate, alcohol amine, silicate and mixtures thereof; and/or (ii) comprises an additive for increasing the viscosity of the aqueous, alkaline electrolyte; and/or (iii) comprises no halogenide.

15. The device according to claim 13, wherein the hard phase comprises a hard metal.

16. The device according to claim 13, wherein the hard phase is selected from the group consisting of tungsten carbide, titanium carbide, titanium nitride, tantalum carbide, niobium carbide, zirconium carbide, vanadium carbide, and mixtures thereof.

17. The device according to claim 12, wherein the aqueous, alkaline electrolyte is contained in a bath which has at least one fluid inflow and at least one fluid outflow and is configured to circulate the aqueous, alkaline electrolyte.

18. The device according to claim 12, wherein the electrode (i) comprises a material selected from the group consisting of metal, metal alloy, carbon, electrically conductive plastic material and combinations thereof; and/or (ii) is mechanically oscillatable.

19. The device according to claim 12, wherein the direct current source (i) is configured such that it supplies a voltage in the range of 0.1 to 50 V; and/or (ii) is configured such that it supplies a current density in the range of max. 400 A/cm.sup.2; and/or (iii) is configured such that it supplies pulses with a pulse length in the range of max. 50 ms; and/or (iv) is configured such that it supplies pulses with a pulse pause between the pulses in the range of at least 0.1 ms.

20. The device according to claim 12, wherein the complexing agent comprises a material selected from the group consisting of alcohol amine, alkyl carbonate, carboxylic acid, ammonia, inorganic ammonium salt, primary amine, secondary amine, tertiary amine and mixtures thereof.

Description

(1) On the basis of the subsequent Figures and of the subsequent example, the subject according to the invention is intended to be explained in more detail without wishing to restrict said subject to the specific embodiments illustrated here.

(2) FIG. 1 shows four FESEM photographs (BSE) of material surfaces which were achieved by a different combination of pulse modulation and electrolyte composition. The machined material concerns WC30Co, i.e. a mixture of 70% by weight of tungsten carbide as hard phase and 30% by weight of cobalt as binder phase. A direct voltage of 11 volts was used. FIGS. 1A and 1B show the machining in an alkaline electrolyte without complexing agent for complexing a metal ion of the binder phase. In FIG. 1A, the result of a machining of WC30Co with unpulsed direct current is illustrated, whereas FIG. 1B illustrates the result of a machining of WC30Co with pulsed direct current. In the case of the result illustrated in FIG. 1C, in fact an electrolyte with complexing agent was used, but only unpulsed direct current in the machining. FIG. 1D alone shows the result of a machining according to the invention of WC30Co since the WC30Co was subjected here to a complexing agent-containing electrolyte and was machined with pulsed direct current.

(3) FIG. 2 illustrates the difference of the material removal on the hard metal as a function of the use of a complexing agent in the electrolyte and as a function of the type of direct current (“pulse-modulated” stands for pulsed direct current and “direct current/voltage” stands for unpulsed direct current).

EXAMPLE—METHOD FOR ELECTROCHEMICAL MACHINING OF A HARD METAL MADE OF TUNGSTEN CARBIDE AND COBALT

(4) In the following, the chemical reactions during the anodic dissolving of a hard metal material, which comprises tungsten carbide (WC) as hard phase and cobalt as binder phase, in an aqueous, alkaline electrolyte are described.

(5) It should hereby be assumed that the hard phase is oxidised via reaction (1) to form tungstenate and carbon dioxide (Schubert et al. (2014) Int. Journal of Refractory Metals and Hard Materials, issue 47, p. 54-60).
WC+7H.sub.2O.fwdarw.WO.sub.4.sup.2−+CO.sub.3.sup.2−+14H.sup.++10e.sup.−  (1)

(6) As a result of the transpassive dissolving of WC, a fourteen-fold material quantity of protons per WC unit are generated from the decomposition of the water.

(7) Furthermore, it should be assumed that also the dissolving of the binder phase is effected transpassively, the process in this case being two-stage. In the alkaline medium, a thin, electron-conducting cobalt(II)hydroxide layer is firstly formed on the binder phase via reaction (2):
Co+2H.sub.2O.fwdarw.Co(OH).sub.2+2H.sup.++2e.sup.−  (2)

(8) This passive layer is insoluble in the alkaline medium so that, on this, water would be converted into oxygen readily via reaction (3):
2H.sub.2O.fwdarw.O.sub.2+4H.sup.++4e.sup.−  (3)

(9) A local acidification, i.e. a local increase in the H.sup.+ concentration, could dissolve the passive layer according to reaction (4):
Co(OH).sub.2+2H.sup.+.fwdarw.Co.sup.2++2H.sub.2O  (4)

(10) In fact, the oxidation of hard- and binder phase leads to the electrolyte solution becoming acidified locally on the interface to the material (see reactions (1) and (2) above). In the practical implementation of the method, such a local acidification is however prevented by the electrolyte convection which is necessary for the removal process and hence unavoidable: by means of the electrolyte convection, the interface to the material is permanently alkalised and therefore does not reach the necessary H.sup.+ concentration for dissolving the passive layer.

(11) Therefore, in the method according to the invention and in the device according to the invention, an electrolyte is used, which electrolyte complexes and hence dissolves the metal ion of the passive layer (i.e. of the oxidised binder phase), for example via reaction (5), (complexing agent here is NH.sub.3):
Co(OH).sub.2+6NH.sub.3.fwdarw.[Co(NH.sub.3).sub.6].sup.2++2OH.sup.−  (5)

(12) The complexing of the metal ions from the hydroxide layer is however subjected to a specific speed.

(13) In order to adjust the removal speeds of hard- and binder phase with each other, a temporal interruption in the anodic dissolving process is therefore necessary. This is achieved according to the invention by the current source supplying a pulsed electrical current to the material and the pulse sequence, supplied by the current source, of the pulsed current being adjusted to the quantity of binder phase in the material to be machined.

(14) By means of such a pulse-modulated polarisation of the material with a suitable adjustment of the pulse-polarisation-specific parameters, consisting of pulse duration <t.sub.on> and pulse pause <t.sub.off> and also the voltage amplitudes <U.sub.on> during the pulse or current amplitudes <I.sub.on> during the pulse and the voltage pauses <U.sub.off> or current pauses <I.sub.off>, the removal process can be adapted to the respective material properties (e.g. binder phase content, particle size of the hard phase). A bipolar modulation of voltage or current (i.e. alternating current) is not required because of the complexing additive.