IE911897A1

IE911897A1 - Method for colouring a moving band of a metallic material by¹a low-temperature plasma

Info

Publication number: IE911897A1
Application number: IE189791A
Authority: IE
Original assignee: Ugine Aciers
Priority date: 1990-06-05
Filing date: 1991-06-04
Publication date: 1991-12-18
Also published as: PT97863A; FR2662708B1; FR2662708A1; CA2043793A1; EP0461011A1

Abstract

Process for colouring a metallic material by exposing the latter to a low-temperature plasma resulting in the formation of a thin oxide layer on the material, characterised in that the said material to be coloured is placed, in the form of a strip (6) packaged beforehand in the form a reel (7) on a unwinding roller (8), the said reel is unwound while the speed of travel of the strip is controlled, the said strip is passed over a conductive support (11) faced by at least one electrode (15), known as source electrode, defining a space (20) between it and the surface of the strip, the source electrode being made of a material whose oxides it is intended to deposit on the strip and being cooled so as to ensure a uniform colouring of the strip, the said support and the said source electrode being placed inside a leakproof enclosure (1) into which is introduced, at a controlled pressure lower than approximately 1000 Pa, an atmosphere comprising at least one gas whose molecule contains oxygen atoms, a low-temperature plasma is generated in the said space while the conductive support is maintained at a potential which is higher than that of the source electrode, and the strip is wound up, after passing over the conductive support, around a receiving roller (9) while being repackaged in reel form (10).

Description

METHOD FOR COLOURING A MOVING BAND OF A METALLIC MATERIAL BY A LOW-TEMPERATURE PLASMA The present invention relates to the surface colouring of a moving band of a metallic material by the exposure of the surface of said material to a low temperature plasma.

Methods are known for the surface treatment of metallic materials employing a plasma termed a low-temperature or cold plasma as opposed to thermonuclear fusion plasmas termed hot plasmas. The degree of ionization of a low-7 -? temperature plasma is low: (10 to 10 ). The electric energy is transferred to the electrons which are rather few in number but very energetic (1 to 10 eV). They consequently excite the gas to be heated, whose temperature may vary from about 20 to 700°C, and produce a large number of excited species. One of the methods for producing such a plasma consists in establishing an electroluminescent discharge between two electrodes (cathode and anode) which are in confronting relation within a rarefied atmosphere under a pressure lower than 1000 Pa. Other methods for producing plasma, such as the excitation of a rarefied atmosphere by microwaves or radiofrequencies are also known and applicable to the surface treatment of metallic materials.

The Applicant has filed French patent applications relating to methods for the surface treatment of metallic materials by cold plasmas. The patent application FR 88 05091 concerns a method for improving the resistance of a specimen to corrosion. For this purpose, said specimen is negatively polarized and exposed to a plasma comprising, at least in the state of traces, an oxidizing gas in an - 2 enclosure at a pressure of 1 to 1000 Pa. In one embodiment, the voltage between the anode and the cathode, the latter being formed by the specimen itself, is of the order of 100 to 5000 V and the treatment time varies from about 1 sec. to min.

The method described in application FR 89 13879 has for object to improve the adherence on the surface of a specimen of a coating which is subsequently applied on this surface. As in the preceding method, the specimen to be treated must be negatively polarized.

Lastly, the method described in the patent application FR 89 08085 permits colouring the surface of a specimen such as a stainless steel plate. The specimen is placed in a rarefied atmosphere comprising an oxidizing plasma. As opposed to the foregoing two methods, the specimen is positively polarized. Atoms are torn off by the plasma at an electrode-source (cathode) placed in confronting relation to the specimen. These atoms are oxidized by the plasma and the oxides thus formed are deposited on the specimen. This layer of oxides produces on the surface of the specimen a colouring due to light interference phenomena, the tint of which is a function of the thickness of the layer and therefore depends on the operational conditions and the duration of the experiment.

Known examples of the application of the surface treatment methods just referred to all concern the treatment of unit parts of generally relatively small dimensions. - 3 An object of the invention is to render possible the application of these methods to the treatment of metallic bands of great lengths.

The invention therefore provides a method for the colouring of metallic materials by the exposure thereof to a low-temperature plasma resulting in the formation on the material of a thin layer of oxides, characterized in that it comprises placing said material to be coloured, in the form of a band previously put in the form of a roll, on an unwinding roller, unwinding said roll while controlling the speed of travel of the band, passing said band over a conductive support which is in confronting relation to at least one electrode, termed an electrode-source, defining a space therebetween and the surface of the band, the electrode-source being of a material the oxides of which are desired to be deposited on the band and being cooled so as to ensure a uniform colouring of the band, said support and said electrode-source being placed inside a sealed enclosure in which there is introduced, under a controlled pressure lower than about 1000 Pa, an atmosphere comprising at least one gas whose molecule comprises oxygen atoms, generating a low-temperature plasma in said space while maintaining the conductive support at a potential higher than the potential of the electrode-source, and winding the band, after a passage over the conductive support, around a receiving roller by putting it back in the form of a roll. Preferably, the conductive support is also cooled. The - 4 plasma may be generated by the establishment of an electroluminescent discharge between the electrode-source and the portion of the band in contact with the conductive support, the electrode-source being placed as a cathode and the conductive support as an anode in the electroluminescent discharge-producing circuit.

As will have been understood, the method according to the invention aims to treat in one go, not isolated metallic parts, but bands whose length and width are only limited by the size of the installation. These bands may be subseguently cropped so as to form sheets of desired length. Depending on the number and arrangement of the electrodeconductive support assemblies employed, it is possible to treat only one or both sides of the band.

A better understanding of the invention will be had from the following description with reference to the accompanying single sheet of drawings in which: Fig. 1 is a diagrammatic side elevational view of an installation for carrying out the method of the invention, the enclosure enclosing this installation being shown in section.

Fig. 2 is a diagrammatic view of a variant of this installation.

The installation is placed in a sealed enclosure 1, the 25 wall 2 of which is provided with an orifice 3 for introducing into the enclosure the plasma-producing gas or gaseous mixture contained in a container 21 . This gas or - 5 gaseous mixture must contain, at least in the state of traces, a gas whose molecule includes oxygen atoms, such as oxygen itself, ozone, air, nitrogen oxides, water vapour. Another orifice 4 permits connecting the interior of the enclosure to a suction device 5 which permits maintaining the pressure in the enclosure at a known and controlled value. This pressure must be such that, in combination with the other operational parameters of the installation, it permits the production of a cold plasma in the enclosure by IQ electroluminescent discharge. In practice, this pressure must in any case be lower than about 1000 Pa and typically 0.1 to 1000 Pa.

The band 6 one of the faces of which is to be coloured is introduced into the enclosure in the form of a roll 7 15 mounted on a roller 8, termed unwinding roller capable of freely rotating or rotating in a controlled manner about its axis so as to permit the roll to be unwound. In the course of its unwinding, the band is wound round another roller 9, termed receiving roller, whose axis is parallel to the axis of the unwinding roller 8 so as to form another roll 10. By means of a driving device, such as a couple of rollers 22, 22’ undergoing a controlled rotation, the band 6 may be made to travel at a known linear speed which may be maintained constant. The driving device may also act, not directly on the band, but on the receiving roller 9. It may also in the latter case be connected to another device for driving the unwinding roller 8, these two devices cooperating so as to - 6 ensure a substantially constant tension of the band including during periods during which the speed of travel of the band 6 is varied. This speed is measured by a device 23 acting by contact with the band 6 or in any other manner.

In the course of its travel between the two rolls 7 and 10, the band 6 passes round an electrically conductive roller 11 provided with internal cooling means such as a circulation of water or any other cooling fluid comprising a supply pipe 12 and a discharge pipe 12', and rotatable about an axis parallel to the axes of the unwinding roller 8 and receiving roller 9. The rotation of the conductive roller follows the travel of the band 6 and may be free or controlled by driving means (not shown) at the speed of travel of the band 6. In the latter case, it is possible to avoid any slip of the band 6 on the roller 11 and thereby avoid any deterioration of the surface of the band 6 which is in contact with the outer surface of the roller 11, and the regularity of the travel of the band 6 is improved. By means of an electric contact 13 of any type compatible with the rotation of the roller 11, such as a brush, the roller 11 is connected to the ©pole of an electric supply 14.

The installation further comprises an electrode-source 15 mounted on a support 16 connected to the θ pole of the electric supply 14, and which therefore performs the function of a cathode whereas the conductive roller 11 performs the function of an anode. A regulating device 24 controls the difference of potential between the conductive - Ί support 11 and the electrode-source 15. The electrode 15 is internally cooled, for example by means of a circuit circulating water or any other cooling fluid and comprising a supply pipe 17 and a discharge pipe 17'. The surface 18 of the electrode-source 15 facing toward the band 6 is generally parallel to the portion of the surface 19 of the band 6 which is in facing relation thereto. The surfaces 18 and 19 thus define a space 20 of constant width 1. Created inside this space by an electroluminescent discharge is the cold plasma which effects the colouring treatment of the surface of the band 6 facing toward the surface 18 of the electrode 15. As explained in the aforementioned document FR 89 08085, this colouring is achieved by the deposition on the surface 19 of the band 6 of a thin layer (a few hundreds of A ) of metal oxides the metal atoms of which have been torn away from the surface 18 of the electrode-source 15. The nature of these oxides depends on that of the material of the electrode-source. An electrode of ferritic stainless steel forms a deposit based on iron and chromium oxides. An electrode of austenitic stainless steel forms a deposit based on iron, chromium and nickel oxides, etc. For the purpose of obtaining a deposit which is particularly resistant to corrosion, there may be employed an electrode source of a pure noble metal (titanium, chromium or aluminium for example) which forms a deposit consisting of only the oxides of this metal. The other operational conditions required for carrying out such - 8 a treatment are: difference of potential between the electrode-source 15 and the assembly formed by the conductive roller 11 and the band 6: about 200 to 5000 V; current density in the band 6: 1 to 100 mA/sq.cm.; the time during which a given portion of the band 6 faces the electrode-source 15: up to 60 minutes; distance 1 between the band 6 and electrode-source 15: mm to several centimetres, preferably 1 to 50 mm. 1o For given experimental conditions, the colouring obtained depends on the treatment time and it is easy to determine experimentally which colour corresponds to which treatment time, i.e. to the time during which a portion of the surface faces the electrode-source. The thickness of 15 the oxide layer increases if the treatment time increases, if the density of the current in the band 6 increases, if the difference of potential between the band 6 and the electrode-source 15 increases, if the width 1 of the space decreases.

In order to ensure that the plasma is not formed from the whole of the surface of the electrode-source 15, which would result in spurious phenomena harmful to a correct regularity of operation of the apparatus and would result in an exaggerated wear of the electrode-source, the electrode25 source 15 may be sheathed with an insulating material, such as Sirilite (Registered Trademark), while leaving bare the active surface 18 which faces the surface 19 of the band 6.

It 911897 - 9 This sheath (not shown in the drawing) may be replaced by a guard ring, i.e. a positively polarized electrode which has a shape matching that of the surfaces of the electrodesource 15 other than the active surface 18 and is placed at a short distance from said surfaces, namely on the order of 0.5 mm to a few centimetres. This distance is so arranged as to render impossible the formation of a plasma between the confronting surfaces of these two electrodes bearing in mind the other experimental conditions. It is in any case less than the distance between the active surface 18 of the electrode source and the surface 19 of the band 6.

Arranged as just described, the installation permits colouring a single one of the surfaces of the band 6. If it is desired to colour both surfaces, it is then necessary to add to the installation a second assembly comprising a conductive roller and an electrode-source similar to the first assembly and exposing the surface of the band 6 which is not yet coloured to a plasma which is established therebetween and the second electrode-source, the surface already coloured being put in contact with the second conductive roller.

It is indispensable to cool the electrode-source 15 if a uniform colouring of the surface of the band 6 is to be obtained. The cooling of the conductive roller 11 is also useful for this purpose.

The conductive roller 11 may be replaced by any conductive support which is preferably capable of - 10 accompanying the moving band. But such a roller is particularly advantageous owing to the facility with which it is employed and the quality and constancy of the contact with the band that this roller permits.

Optionally, the support 16 of the electrode-source may be shifted in such manner as to vary the width 1 of the space 20. However, if it is desired to maintain an identical value of 1 throughout the space 20, it is necessary to provide for each width of the space 20 a model of the electrode-source 15 whose active surface 18 has a radius of curvature adapted for this purpose. If r is the radius of curvature of the conductive roller 11, the active surface 18 must have a radius of curvature equal to r + 1, and its centre of curvature must coincide with the centre of the conductive roller 11. However, such a constancy of 1 is not in fact indispensable to the good operation of the apparatus and in particular to the reproducibility and the homogeneity of the colourings obtained. Indeed, if the configuration of the zone in which the plasma is established remains the same from the beginning to the end of the travel of the band from the roll, which occurs at constant speed, all the parts of the roll will have undergone an identical treatment and it is sufficient to know by experience, for a given configuration of the zone of the electrodes, which other operational conditions result in such and such a colouring for this to be reproducible.

Means of known type (not shown) may also be provided for IE 9118®7 - 11 interposing between the coils of the roll 10 a sheet of paper or plastics material whose function is to avoid deterioration of the surfaces of the band by rubbing between the coils.

As a variant, diagrammatically represented in Fig. 2, the unwinding roller 8 and receiving roller 9 may be placed outside the sealed enclosure 1. After having left the unwinding roller 8, the band 6 enters the enclosure 1 through an opening 25 provided with means for ensuring a IQ sealed contact between the wall 2 and the band 6. A similar opening 26 enables the band to travel out of the enclosure 1 toward the receiving roller 9. Outside the enclosure 1, the band 6 passes around capstans 27, 28 which orient the direction of travel so that the band 6 enters and leaves the enclosure in a constant direction which is independent of the diameters of the rolls 7, 10 wound onto the rollers 8 and 9. Other capstans 29, 30, placed inside the enclosure, complete the guiding of the band 6 and guarantee in particular a constant arc of contact of the band 6 on the conductive roller 11. Fig. 2 also shows the electrodesource 15 and its support 16, the gas supply 21 and the device for regulating the pressure in the enclosure 5. The other elements common to those of the installation of Fig. 1 are not shown.

In the configurations just described, the metallic material is included as the anode in the electric circuit which produces the electroluminescent discharge causing the - 12 formation of the cold plasma. But this condition is not essential to obtain the colouring of the specimen. The essential feature is that the installation comprise means for maintaining the specimen at a positive potential relative to that of the electrode-source from which are torn away the atoms whose oxides are deposited on the specimen. Likewise, the plasma is not obligatorily produced by an electroluminescent discharge between the electrode-source and the specimen, but may be produced by excitation of the atmosphere by microwaves or radiofrequency.

The dimensions of the treated rolls are only limited by the size of the installation.

The surface colouring treatment mainly applies to stainless steel rolls, since the metal does not have to subsequently undergo a surface treatment (for example against corrosion) which would conceal the effects of the treatment. However, it is possible to apply the treatment to any metallic material put in the form of rolls, whether it is bare or provided with a metallic covering (for example steels covered with zinc by electrodeposition or galvanization). The expression metallic material is intended to mean both a solid metallic material and a composite material in which metallic components and nonmetallic components are intimately mixed, or a metallic layer in one piece with the surface of a non-metallic substrate, as in the case of sheets termed sandwich sheets. •Ε911897 - 13 It must be understood that it is possible to substitute, without departing from the spirit and scope of the invention, for the elements of the installation just described their technical equivalents and that improvements may be made in the installation which do not adversely affect the architecture and the operation thereof. Thus, any arrangement for improving the ionization efficiency in the plasma may be added to the installation. Such an arrangement may comprise in the known manner means for creating a magnetic field between the electrode and the conductive support, such as a permanent magnet or an electromagnet. The magnetic field has for effect to increase the path of the ions and electrons and thereby results in the desired improvement in the ionization efficiency in the plasma. It is thus possible also to operate at pressures lower than 0.1 Pa.

Claims

1. Method for colouring a metallic material by exposing the material to a low-temperature plasma resulting in the formation of a thin layer of oxides on the material, 5 characterized in that it comprises placing said material to be coloured, in the form of a band previously put in the form of a roll, on an unwinding roller, unwinding said roll by controlling the speed of travel of the band, passing said band round a conductive support which is in confronting Ιθ relation to at least one electrode, termed electrode-source, defining a space therebetween and the surface of the band, the electrode-source being of a material the oxides of which are desired to be deposited on the band and being cooled so as to ensure a uniform colouring of the band, said support 15 and said electrode-source being placed inside a sealed enclosure in which there is introduced, under a controlled pressure lower than about 1000 Pa, an atmosphere comprising at least one gas whose molecule comprises oxygen atoms, generating a low-temperature plasma in said space while 20 maintaining the conductive support at a potential higher than the potential of the electrode-source, and winding the band, after its passage over the conductive support, around a receiving roller by putting it back in the form of a roll.

2. Method according to claim 1, characterized in that 25 it comprises also cooling the conductive support for the band.

3. Method according to claim 1 or 2, characterized in that it comprises generating the plasma by the establishment of an electroluminescent discharge between the electrodesource and the portion of the band in contact with the conductive support, the electrode-source being placed as a cathode and the conductive support as an anode in the electroluminescent discharge-producing circuit.

4. Method according to any one of the claims 1 to 3, characterized in that it comprises creating a magnetic field between the electrode-source and the conductive support.

5. Method according to any one of the claims 1 to 4, characterized in that the metallic material to be coloured is a stainless steel band.

6. Method according to any one of the claims 1 to 4, characterized in that the metallic material to be coloured is a steel band covered with a metallic layer.

7. A method according to claim 1 for colouring a metallic material by exposing the material to a low-temperature plasma, substantially as hereinbefore described.

8. A coloured metallic material whenever obtained by a method claimed in a preceding claim.