WO2003095696A2

WO2003095696A2 - Method and device for eliminating oxide(s) present on the surface of a metal material and for restoring an oxide layer of said surface

Info

Publication number: WO2003095696A2
Application number: PCT/FR2003/001424
Authority: WO
Inventors: Thierry Belmonte; Thierry Czerwiec; Jean-Marie Thiebaut; Henri Michel
Original assignee: Centre National De La Recherche Scientifique (C.N.R.S.)
Priority date: 2002-05-14
Filing date: 2003-05-07
Publication date: 2003-11-20
Also published as: AU2003251041A1; FR2839728A1; WO2003095696A3

Abstract

The invention concerns a method and device for eliminating oxides present on the surface of a metal material and for restoring an oxide layer on said surface which consists in: immersing said material in a gas medium which has a reducing effect on oxides initially present on the surface of said material during the cold plasma etching phase, and an oxidizing effect on the exposed surface of said material during a post-discharge phase; generating a cold plasma around said material to eliminate said oxides initially present on its surface; and generating a temporal or spatial post-discharge around said material to restore an oxide layer on its surface. The invention also concerns a device for implementing said method.

Description

Method and device for removing the oxide (s) present on the surface of a metallic material and for reconstituting a layer of oxides on said surface.

The invention relates to the field of cleaning and coating surfaces of metallic materials. More precisely, it relates to the elimination of the oxides originally present and the controlled reconstruction of a layer of oxides on the surface of these materials. The oxides present on the surface of metallic materials, whether they are formed naturally or during metallurgical treatments causing or facilitating the surface oxidation of the material (which will be called "native oxides"), must often be removed before a surface treatment. ulterior. The most commonly practiced method is pickling with acidic chemical solutions. It has the drawback of using solutions of strong acids which are dangerous to handle and which must then be reprocessed.

Another known possibility consists in carrying out the elimination of the oxides by a treatment by means of a cold plasma, ensuring a chemical cleaning of the surface of the metallic product. But generally, these methods are used under reduced pressure: their use therefore proves difficult on moving parts such as strips.

On the other hand, one of the treatments which one may wish to carry out following this elimination of native oxides consists in a controlled reoxidation of the surface. The aim is thus to form one or more native oxides on this same surface in a controlled manner. These oxides can serve as a diffusion barrier, subsequent bonding layers for ceramic, metallic or other paints or coatings, etc. But between the two operations, the product must not be exposed to the atmosphere, under penalty of reconstituting uncontrollably native oxides on its surface. The object of the invention is to propose a method making it possible to carry out, during the same operation, the two stages of elimination of the native oxides and of controlled reoxidation, under industrially practicable conditions.

To this end, the subject of the invention is a process for removing the oxide or oxides present on the surface of a metallic material and for reconstituting an oxide layer (s) on said surface, characterized in that:

said material is immersed in a gaseous medium of a composition such that it behaves in a reducing manner with respect to the oxide or oxides initially present on the surface of said material during the phase of establishment of a cold plasma, and oxidizingly with respect to the bare surface of said metallic material during a post-discharge phase, - A cold plasma is created around said material for a sufficient time to remove the said oxide (s) initially present on the surface of said material;

- And a temporal or spatial post-discharge is created around said material to reconstitute a layer of oxide (s) on its surface.

Said gaseous medium can be an Ar-H ₂ -H ₂ 0 mixture.

Said metallic material can be a moving strip; said post-discharge is then a spatial post-discharge, and said strip passes continuously from a zone in which the cold plasma is created to a zone of spatial post-discharge.

Prior to its passage in the zone where said cold plasma is created, the strip can pass through at least one zone where cold plasma is created in a reducing atmosphere, said atmosphere being non-oxidizing in post-discharge. Said cold plasmas can be created using resonant cavities.

The method according to the invention can be preceded by the immersion of said material in cold plasma under conditions of intermittent polarization or self-polarization, so as to obtain a removal by spraying of contaminants from the surface of said material.

The invention also relates to a device for removing the oxide or oxides present on the surface of a metallic material and for reconstituting an oxide layer (s) on said surface, characterized in that it comprises :

- an enclosure provided with means for controlling the composition of its atmosphere allowing the introduction into at least one of its zones of an oxidizing gaseous medium during the establishment of a cold and reducing plasma during a post-phase -dump ;

- Means for establishing a cold plasma around said material; - And means for establishing a spatial or temporal post-discharge around said material following the establishment of said cold plasma.

Said metallic material may be a strip, and the device comprises means for passing said strip by making it pass through a zone provided with means for establishing a cold plasma and for introducing said gaseous medium.

The device may comprise, on the path of said strip, at least one zone provided with means for creating a cold plasma in a reducing atmosphere, and not oxidizing in post-discharge.

The device may comprise, on the path of said strip, before said zone or zones provided with means for establishing a cold plasma, a zone provided with means for creating an intermittent cold plasma and for polarization or self-polarization of said strip. The metallic material may be a fixed part, and said device may include means for establishing an intermittent cold plasma around said material and means for polarizing or self-polarizing said material.

As will have been understood, the invention consists in immersing the part to be treated in an atmosphere containing both a gas which can behave as a reducing agent and a gas which can behave as an oxidant, and an area is created around the part. plasma under conditions such that after being exposed to plasma, the part is exposed to a post-discharge. The respective proportions of the gases are chosen so that during the plasma establishment phase, the reducing effect is predominant, and that during the post-discharge phase, the oxidizing effect is predominant. Thus, during the first phase, the native oxides are eliminated, and during the second phase, a layer of oxides of controlled composition is reconstructed on the surface of the part. The aim is to obtain on the surface of the part only the oxide or oxides satisfying the desired properties, therefore constituting for example a good diffusion barrier and / or a primary adhesion layer for subsequent coatings.

To this end, it is possible to allow the passage from a reducing medium to an oxidizing medium in the same enclosure and using the same atmosphere in the following manner. An H ₂ -H ₂ O mixture at x% of H ₂ 0 is used as the atmosphere, the whole being able to be diluted in an inert gas such as argon. In the absence of plasma, this gaseous medium is oxidizing for values of x greater than a value denoted x ₀ which depends on the nature of the metal to be treated, operating conditions, etc. However, in the presence of plasma, for values of x lying in the interval [Xo, x _. ] where x * ι> Xo, this medium is reducing, since the dissociation of H ₂ leads to the formation of atomic hydrogen which is more reducing than molecular hydrogen. Xi is defined as being the upper limit beyond which this reducing effect on exposure to plasma is no longer observed. This reasoning applies to any other reducing species whose dissociation in a plasma would lead to the formation of more reducing subspecies than it. Thus, in the post-discharge period of the plasma, whether temporal or spatial, the medium is oxidizing while it is reducing in plasma.

Consequently, by precisely controlling the value of x between the limits x ₀ and xi previously defined, it is possible in the same process to have successive times and / or at different places in the enclosure of the oxidizing and reducer.

It is therefore necessary that the variation of the partial pressure of the oxidizing agent is well controlled and that it does not depend or little on external elements. Indeed, if plasmas are used in contact with walls, such as those of the reactor enclosure, contamination occurs with elements coming from the surface layers of the walls, such as oxygen which changes the partial pressure d oxygen in the enclosure. Indeed, if we are able to remove oxides such as alumina (when treating an aluminum part), we will also remove any oxide from the constituent walls of the reactor. It is then necessary to resort to plasmas without direct influence of the walls, like resonant cavities. It is also necessary to avoid the backscattering of the air species when working at atmospheric pressure, for example. But if this condition is respected, work at atmospheric pressure is possible.

As mentioned, post-discharge can be temporal or spatial. In other words, essentially two solutions are possible for implementing the method according to the invention:

- create a reducing cold plasma around the part while keeping it fixed in the treatment enclosure: the elimination of native oxides takes place during the plasma creation phase, and the reformation of oxides takes place during the post-discharge phase which follows, during which the atmosphere surrounding the part becomes oxidizing;

- Or create a cold reducing plasma in a given zone of the enclosure and allow the part to remain there for a sufficient time to allow the elimination of native oxides, then move the part out of the zone where the reducing plasma prevails until the neighboring area where the post-discharge oxidizing atmosphere prevails; this last way of proceeding is particularly suitable for the case of processing of moving metal strips. The invention will be better understood on reading the description which follows, given with reference to the appended figure, which schematically shows an example of installation allowing the implementation of the method according to the invention, applied to the processing of tapes scrolling. This installation comprises an enclosure 1 which, in the case which will be described, can operate under atmospheric pressure but could also, if necessary, be provided with means for maintaining a reduced pressure therein. A pipe 2 allows the evacuation of the gases present.

The moving strip 3 to be treated is unwound from a reel 4, at a speed chosen by the operator and which can be, for example, around 150 m / min and it is wound on a mandrel to form another reel 5, these two coils being driven in rotation by conventional means not shown. The strip 3 is made of a metallic material, such as steel, aluminum, titanium or one of their alloys, etc. When it is introduced into enclosure 1, it has on its surface a layer of native oxides which it is desired to eliminate and replace with a layer of oxide (s) of controlled composition, structure and thickness.

In the example shown, a microwave plasma is created in four zones 6, 7, 8, 9 crossed successively by the strip 3 in movement, these four zones being delimited by resonant cavities 10, 11, 12, 13, according to a known technology. We can thus isolate the effects of these different zones. Another advantage of limiting the formation of the plasma to well-defined zones 6, 7, 8, 9 is that the walls of the enclosure 1 are not affected by the plasmas: this prevents their degradation and the introduction into the plasmas. parasitic elements which could negatively influence the results of the operation. The microwave plasma is generated by a microwave generator 14 connected to the resonant cavities 10, 11, 12, 13 by an electronics known in itself making it possible to adjust the characteristics of the plasma and which may include, in particular, a power distributor , a circulator, an impedance adapter, etc. The installation is equipped with means 15 for introducing a purely reducing plasma gas (for example an Ar-H ₂ mixture) into the resonant cavities 10, 11, 12, 13, and means 16 for adding into the last resonant cavity 13 a gas which may behave in an oxidative manner (for example water vapor) in the post-discharge zone 17 of the plasma 9 generated in said last resonant cavity 13.

In the first three resonant cavities 10, 11, 12, the conditions are therefore created so that the plasma behaves in a reducing manner and eliminates the layer of native oxides from the surface of the strip 3. It would be possible to use only for this purpose only one resonant cavity, active on a longer strip length than each of the resonant cavities 10, 11, 12. But spread the operation of removing the oxides over several zones of limited length rather than over a single large area and / or high power injected avoids bringing, under the effect of plasma, the band 3 to a temperature which could be excessive. Thus when processing a strip 3 of aluminum alloy, it is preferable not to bring the strip to more than 150 ° C. The number of resonant cavities, their spacing and the operating conditions prevailing in each of them must be fixed according to needs, in particular the nature of the native oxides to be eliminated, the thickness of their layer, etc.

In the last resonant cavity 13, the preceding Ar-H ₂ mixture is added as plasma gas, in the example shown, to which a proportion x of water vapor is added, calculated such that:

- In the resonant cavity 13, the plasma 9 behaves in a reducing manner, so as, if necessary, to remove the last traces of native oxides still present on the surface of the strip 3;

- And in the zone immediately following the resonant cavity 13 and where the plasma 17 is in a post-discharge state, said plasma 17 behaves in an oxidative manner according to the mechanism which has been previously described so as to reconstitute on the surface of the strip 3 an oxide layer (s) having a composition and a thickness suitable for the future uses of strip 3.

The value of x can vary as a function of the gas flows injected, the speed of travel of the strip 3, and various other factors, for a material constituting the given strip 3.

A heating system 18 acting on the strip 3 when it is in the post-discharge zone 17 can make it possible to control the temperature in a manner which promotes the constitution of the oxide layer (s) targeted. We have detailed here the case of the establishment of a microwave plasma in the resonant cavities 10, 11, 12, 13. But the use of other types of cold plasmas is possible, for example radio-frequency plasmas .

Depending on the requirements of the treatment, one can work at atmospheric pressure or under reduced pressure. The oxide layer obtained at the end of the treatment can then see its thickness increased by means of other more conventional treatments, selectively. If the strip is made of Inconel, a layer of chromium oxides alone can be formed and then grown. In the example described and shown, we have only considered the case where one cleans and covers only one side of the strip 3. But one can have symmetrically with respect to the strip 3 an apparatus capable of acting in the same way way on the other side of the strip 3 if the treatment must be carried out on both sides.

Advantageously, one can devote the first 10 of the resonant cavities (or an additional resonant cavity placed before the others) not purely for the elimination of native oxides, but for the elimination of various other compounds which may be present on the surface of the band 3, such as oils, in particular silicone oils which are difficult to remove by conventional cleaning processes. For this purpose, an appropriate atmosphere will be used which may be identical to that used for the removal of oxides. If the process is carried out under vacuum, this step can be carried out under conditions such that chemical elimination and spraying of the layer of impurities are alternately created. This makes it possible to get rid, before the use of the process of elimination and reconstitution of the oxides according to the invention, of contaminants which would risk making this elimination more difficult, or even impossible if the contaminants are present in too large quantities. A start of elimination of native oxides can also be obtained during this spray cleaning phase. The use of an intermittent polarization (or self-polarization, in the case where a radiofrequency plasma is used, which makes it possible to work at atmospheric pressure) makes it possible to avoid bringing the band 3 to excessive temperatures.

However, it is not advisable to seek to eliminate all of the native oxides from band 3 during this spraying step. Some oxides can be sprayed faster than others. There is then a risk of discovering the metallic surface of the strip 3 in certain zones which will in turn undergo spraying, and will be given a roughness different from their initial roughness. It will be noted that the method according to the invention, which, unlike spraying which is a physical effect, is based on a chemical effect, ceases to be effective when the metal surface of the strip 3 is reached, and therefore does not modify the morphology of this surface.

The use of the invention has been described in the case of treatment of a moving strip 3. However, those skilled in the art will obviously be able to adapt it to the case where he wants to process isolated parts discontinuously. In this case, there is only one phase of elimination of the native oxides by the plasma, and the reconstitution of a layer of controlled oxide (s) takes place in the post-discharge phase which follows, unless this phase is preceded by one or more phases in which a gas having no oxidizing properties is used in post-discharge, unlike the gas used for the process according to the invention. Again, it is conceivable to precede the complete elimination of the native oxides by a spraying phase of the contaminants carried out by means of the establishment of a polarization or self-polarization drawn from the part to be treated.

Claims

1. A method of removing the oxide (s) present on the surface of a metallic material and of reconstituting an oxide layer (s) on said surface, characterized in that:

said material is immersed in a gaseous medium of a composition such that it behaves in a reducing manner with respect to the oxide or oxides initially present on the surface of said material during the phase of establishment of a cold plasma, and oxidizingly with respect to the bare surface of said metallic material during a post-discharge phase,

- A cold plasma is created around said material for a sufficient time to remove the said oxide (s) initially present on the surface of said material;

2. Method according to claim 1, characterized in that said gaseous medium is a mixture of Ar-H ₂ -H ₂ O.

3. Method according to claim 1 or 2, characterized in that said metallic material is a moving strip, in that said post-discharge is a spatial post-discharge, and in that said strip runs continuously passing from an area in which the cold plasma is created to a spatial afterload area.

4. Method according to claim 3, characterized in that, prior to its passage in the zone where said cold plasma is created, the strip passes through at least one zone where cold plasma is created in a reducing atmosphere, said atmosphere being non oxidizing in post-discharge.

5. Method according to one of claims 1 to 4, characterized in that said cold plasmas are created using resonant cavities.

6. Method according to one of claims 1 to 5, characterized in that it is preceded by the immersion of said material in a cold plasma under conditions of intermittent polarization or self-polarization, so as to obtain elimination by spraying of contaminants from the surface of said material. ιυ

7. Device for removing the oxide (s) present on the surface of a metallic material (3) and for reconstituting an oxide layer (s) on said surface, characterized in that it comprises:

- An enclosure (1) provided with means (15, 16) for controlling the composition of its atmosphere allowing the introduction into at least one of its zones (9) of an oxidizing gaseous medium during the establishment of a cold and reducing plasma during a post-discharge phase;

- means (10, 11, 12, 13) for establishing a cold plasma around said material; - And means for establishing a spatial or temporal post-discharge (17) around said material following the establishment of said cold plasma.

8. Device according to Claim 7, characterized in that the said metallic material (3) is a strip, in that the said device comprises means (4, 5) for passing the said strip (3) through it through an area provided means (13, 15, 16) for the establishment of a cold plasma and for the introduction of said gaseous medium.

9. Device according to claim 8, characterized in that it comprises, on the path of said strip (3), at least one zone (6, 7, 8) provided with means (16, 10, 11, 12) of creation of a cold plasma in a reducing, non-oxidizing atmosphere in post-discharge.

10. Device according to claim 8 or 9, characterized in that it comprises on the path of said strip (3), before the said zone or zones provided with means (10, 11, 12, 13) for the establishment of a cold plasma, an area provided with means for creating an intermittent cold plasma and for polarizing or self-polarizing said band.

11. Device according to claim 7, characterized in that the metallic material is a fixed part, and in that said device comprises means for the establishment of an intermittent cold plasma around said material and means for polarization or self-polarization of said material.