EP1062380A1

EP1062380A1 - Method and installation for treating a metal part surface

Info

Publication number: EP1062380A1
Application number: EP99939206A
Authority: EP
Inventors: Bernard Drevillon; Nicolas Bertrand; Jean-Christophe Rostaing
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; LAir Liquide SA a Directoire et Conseil de Surveillance pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 1998-03-10
Filing date: 1999-03-05
Publication date: 2000-12-27
Also published as: US6561198B1; WO1999046428A1; FR2775986B1; JP2002506128A; FR2775986A1

Abstract

The invention concerns a method for treating a metal part (12) surface for deoxidising and/or degreasing it which consists in: filling a sealed chamber wherein is set the part to be treated, with a low pressure reducing gas; generating a static magnetic field in a zone of the chamber (16) separate from the zone wherein the part to be treated (12) is located and in exciting the gas mixture with an electromagnetic wave injected into the chamber (16) so as to generate in the gas a treating plasma, the intensity of the static magnetic field corresponding to cyclotron resonance, uniformly distributed in the chamber.

Description

METHOD AND INSTALLATION FOR SURFACE TREATMENT OF A WORKPIECE

METALLIC

The present invention relates to the surface treatment of large metal parts, with a view to their deoxidation and / or their degreasing, by addressing in particular the metal sheets as they appear at the outlet of a rolling mill, to modify its surface properties while retaining its volume properties. Such a part generally has surface irregularities of the order of 0.1 to 0.5 μm, lubrication residues and traces of oxidation, which make coatings applied to this surface not very adherent. In general, the preparation of the surface of a thin sheet for subsequent treatment, and in particular with a view to increasing the adhesion of coatings applied to this surface, comprises a necessary step of degreasing and deoxidation of prison. To do this, a widely used surface treatment technique consists of immersing the part to be treated in an acidic or basic chemical bath or in an organic solvent.

Although this type of technique makes it possible to treat parts of relatively large dimensions, and this with a relatively high speed, it is accompanied by the emission of effluents harmful to the environment.

It has also been proposed, for example in document US Pat. No. 5,376,223 or also in the article by Sakamoto et al published in Japanese Journal of Applied Physics in May 1980 (page 839), to use hydrogen plasma, in Electronic Cyclotronic Resonance conditions (RCE, ECR in the English language literature), to perform surface treatments, in particular of silicon. We know that RCE is a particular way of coupling the energy of an electromagnetic field with the electrons of a plasma to maintain the latter.

The two documents cited above are representative of the classic method used to 2 produce an RCE plasma reactor, which consists in injecting microwaves from a waveguide into a vacuum enclosure, through a dielectric window (for example enclosure 1, waveguide 5, and window 4 in FIG. 1 of the document US cited above).

The work carried out by the Applicant in this field has shown that such a configuration is not satisfactory for the application targeted according to the present invention, ie surface treatment making it possible to modify the surface properties of large metal parts. dimensions, in particular of metal sheets.

Indeed, in the relatively large volume region of the enclosure, a static magnetic field is created created by induction coils (referenced 2 in Figure 1 already cited). It then appears that the region where the plasma is created by transfer of energy from the microwave field to the electrons is of a diameter necessarily limited by the transverse dimensions of the waveguide, even if downstream of the coils the plasma diffuses towards the substrate by extending radially, knowing however that this diffusion is naturally accompanied by a radial gradient of plasma density.

It should also be emphasized that the magnetic field does not suddenly stop at the output of the coils (2 in the figure already cited in the document US), but has a gradient of magnetic field which extends beyond the inter-coil space towards the substrate, and which has the effect of accelerating the ions from the plasma to the substrate, and this all the more that one is at a significant distance from the axis, thus causing it to be conceived of a strong radial inhomogeneity.

This work then seems to demonstrate that it is not realistic to treat substrates with a surface greater than 200 or 300 mm, using such RCE reactors. The object of the invention is to overcome these drawbacks and to provide a method of surface treatment of a metal part which can be carried out quickly, over large surfaces and near the places of subsequent treatment of the parts. 3

It therefore relates to a method of treating the surface of a metal part with a view to its deoxidation and / or its degreasing, characterized in that it comprises the following stages: - filling a sealed enclosure, in which the part to be treated, of a reducing gas mixture at low pressure,

- establish a static magnetic field in an area of the enclosure separate from the area in which the part to be treated is located, and excite the gas mixture by means of an electromagnetic wave injected into the enclosure so as to generate in the gas a processing plasma, the intensity of the static magnetic field corresponding to an electronic cyclotron resonance, established in the enclosure in a distributed manner.

It will be understood, the method according to the invention allows the obtaining, according to the initial state of the part, of deoxidation and degreasing treatments, this by the conjugation of a static magnetic field established in an area of l enclosure separate from that in which the part to be treated is located, and a plasma excitation of the gas mixture enabling it to be placed under conditions of “Distributed Electronic Cyclotronic Resonance” (RCE, ECR in the English language literature). We will refer to this literature of the 80s and

90 concerning such plasma excitations, including in particular the following two articles:

- the article by M. Pichot et al published in Review of Scientific Instruments in 1988, July, vol. 59, p 1072; and - the article by J. Pelletier et al published in Thin

Solid Films in 1994, vol. 241, p240.

As will be clear to a person skilled in the art, the process according to the invention adopts "low pressure" conditions, which should be understood as pressure conditions which effectively excite the plasma under resonance conditions RCE electronic cyclotron, while at higher pressures the only effect of the magnetic field would be the confinement of the plasma by the multipolar magnetic field (regime of 4

magnetron). The work carried out by the Applicant has shown that this magnetron regime, well known to those skilled in the art, gives largely insufficient results as regards the application of the targeted surface treatment. One will then advantageously be in a range going from 0.5 to 10 Torr, and even more preferably in a range going from 1 to 10 mTorr.

Similarly, the frequency of the incident electromagnetic wave adopted will take into account, on the one hand, the desired RCE regime, but also the nature of the gas to be excited, without forgetting the practical and technical contingencies of commercial availability and cost (frequencies for use domestic), as well as those related to international radio regulations. We will then preferentially take all these factors into account at a frequency of 2.45 GHz., Or even at a frequency of 5.85 GHz.

The notion of “distributed / distributed electronic cyclotron resonance” according to the invention should be understood to mean that the excitation means used comprise at least one field applicator of generally tubular shape connected to a microwave source, each applicator being equipped with means for creating a static magnetic field in the vicinity of the applicator, over a length corresponding substantially to the entire length of the applicator, at an intensity corresponding to the electronic cyclotron resonance.

The process according to the invention may also include one or more of the following characteristics, taken in isolation or in any technically possible combination:

- The electromagnetic wave is injected by means of at least one applicator of generally tubular shape connected to a microwave source and in which are placed permanent magnets;

- The applicator (s) have a length at least equal to one of the dimensions of the metal part to be treated;

- The gas mixture comprises hydrogen; 5

- The part to be treated consists of a sheet of steel running on a substrate holder; and

- The method further includes a step of applying a high frequency electric field on the substrate holder so as to obtain a polarization of the latter and of the part to be treated.

Other characteristics and advantages will emerge from the following description, given solely by way of example and made with reference to the appended drawings in which:

- Figure 1 is a schematic sectional view of a distributed electronic cyclotron resonance reactor, used for the creation of a treatment plasma according to the invention; - Figure 2 is an Auger profile of a metal sheet treated by conventional techniques; Figure 3 is an Auger profile of a metal sheet treated with a hydrogen plasma according to a method according to one invention; and FIG. 4 illustrates the adhesion of a coating deposited on a surface treated by means of a method according to

1 invention.

In FIG. 1, a reactor for processing metal parts has been shown, designated by the general reference 10.

It is intended to ensure the surface treatment of a part 12, constituted for example by a steel sheet placed on a substrate holder 14, with a view to its deoxidation and / or its degreasing. As can be seen in this FIG. 1, the enclosure 16 comprises a set of nozzles for admitting a reducing gas mixture, such as 18.

It is also provided with a pipe 20 for connecting the enclosure 16 to a pumping station (not shown) allowing the extraction of the gas delivered by the nozzles 18 and the maintenance of the pressure of this gas at a desired value , for example between 1 and 10 mTorr.

It can also be seen that the enclosure 16 is equipped with a device 22, in accordance with the invention, ensuring excitation 6 of a plasma in the treatment gas, with electronic cyclotron resonance.

This device 22 is constituted by several field applicators, such as 24, each having a tubular shape and connected by one of their ends, by any suitable means, such as a coaxial cable, to an energy source in the field of microwave (not shown), for example at a frequency equal to 2.45 GHz.

More particularly, the applicators 24 are constituted by applicators known under the name "Distributed Electronic Cyclotronic Resonance" (DECR).

Furthermore, each applicator 24 is equipped with means for creating a static magnetic field in the vicinity of the applicator, at an intensity corresponding to the electronic cyclotron resonance, that is to say a static magnetic field whose intensity B is related to the excitation frequency f of an electron placed in this static magnetic field by the following relation:

B = π x m y f e

in which m and e are respectively the mass and the charge of the electron.

Thus, for example, for incident microwave radiation having a frequency of 2.45 GHz, the intensity of the static magnetic field created in the vicinity of each applicator is chosen to be 875 Gauss. In the embodiment shown in FIG. 1, the means for creating the static magnetic field consist of a magnet 26 of longitudinal shape and arranged here inside each applicator 24.

This construction makes it possible to obtain a static magnetic field whose value decreases relatively quickly and, thus, to obtain a weak or even zero static magnetic field in the area in which the part 12 to be treated is placed. In addition, due to the intense absorption of 7

microwave field in the vicinity of the applicators, the excitation of the plasma does not take place in the immediate vicinity of the substrate, thus limiting the risk of alteration of the latter.

Finally, it can be seen in FIG. 1 that the reactor 10 is completed by a set of metal bars, such as 25, extending transversely and parallel to the applicators 24. These bars 25 are connected to earth, that is to say to the wall of the enclosure 16, to constitute a mass reference along each applicator in order to structure the microwave field in this zone and facilitate the propagation of the incident radiation.

Finally, we see that the reactor 10 is, for the embodiment shown, supplemented by an electric field source 30, for example as here at a frequency equal to 13.56 MHz, making it possible to ensure polarization of the substrate holder, as will be described later.

The cleaning and deoxidation of the part 12 by means of the reactor 10 as described above is carried out as follows. In the following description, it will be considered that the part 12 is constituted by a sheet of steel running on the substrate holder 14 under the action of conventional motor means, not shown.

First of all, after having positioned the sheet 12 on the substrate holder 14, the enclosure 16 is filled by means of the nozzles 18 with a reducing gas mixture at low pressure, that is to say preferably comprised between 1 and 10 mTorr.

For example, the reducing gas mixture consists of hydrogen, possibly associated with argon or another neutral gas.

It will be understood that if according to the invention hydrogen is a preferred gas, other reducing gases are of course conceivable.

Microwave power is then injected by means of each applicator 24, in the vicinity of each inlet nozzle 18 and, simultaneously or successively, is created in this zone, that is to say in a zone of the enclosure 16 distinct from the area in which the sheet 12 to be treated is placed, a static magnetic field whose intensity 8

corresponds, as previously mentioned, to electronic cyclotron resonance.

Note that the static magnetic field lines thus created loop between two neighboring magnets. A multipolar magnetic field structure is thus defined between the applicators.

In this zone, the electrons are subjected to the action of microwaves and the effect of electronic cyclotron resonance, and can thus reach a high energy. The most energetic electrons undergo little action from the self-coherent electromagnetic field of the plasma, responsible for the diffusion of the latter towards the volume of the enclosure, and therefore remain trapped on the lines of the multipolar magnetic field. These very energetic electrons will ensure, by inelastic ionizing collisions, the dissociation of neutral species from the gas mixture found in this region to form secondary electrons and ionized species, in particular H ⁺ ions (even H ⁺ and Ar ⁺ ) . The latter can diffuse out of the trapping zone under the effect of the self-coherent electric field of the plasma. Radical species can then form within this plasma. A plasma is thus formed which substantially fills the entire internal volume of the enclosure 16. This gives a highly dissociated hydrogen plasma, at low pressure and very active, allowing the treatment of the sheet 12.

It will be noted that, as mentioned previously, the substrate holder 14 is possibly subjected to the influence of a high frequency electric field, which can make it possible to polarize the latter.

Indeed, when the substrate holder 14 is subjected to the influence of a positive alternation of the electric field delivered by the source 30, the electrons are attracted towards the sheet 12, while when the substrate holder 14 is subjected to the influence of a negative alternation, the positive ions, in this case the H ⁺ ions, are attracted towards the sheet 12. 9

It is understood that the electrons being more mobile than the ions, a polarization of the substrate holder is obtained, which polarization is adjustable under the control of the source 30. It is thus possible to control the energy of the ions coming to attack the surface of the sheet 12 to be treated.

In FIG. 2, the Auger profile is shown, obtained by an Auger electron spectroscopy analysis technique, also known under the name “AES”, as a function of the depth P, of a steel sheet. treated by conventional techniques. It can therefore be considered that this sheet is at present deemed clean with the user site considered, using such conventional techniques for cleaning sheets in the liquid phase.

FIG. 3 shows the Auger profile, as a function of the depth, of a substrate treated with a hydrogen plasma according to the invention, as described above, in the absence of polarization of the holder. substrate.

It will be noted that, in order to avoid reoxidation of the samples between their unloading from the treatment enclosure 16 and their introduction into an Auger spectrometer, an encapsulation layer of hydrogenated amorphous silicon, d, is deposited in situ, immediately after treatment. '' a thickness of a few tens of nanometers, typically 30 to 50 nanometers. During the Auger analysis of the composition profile as a function of depth, this layer is easily distinguished from the rest of the sample. Referring first to Figure 2, it is noted that oxygen and carbon are present in significant quantities in the transition zone between the steel substrate and the silicon encapsulation layer. Oxygen is naturally associated with iron oxide, while carbon reveals residual contamination by hydrocarbons not removed by solvent degreasing.

Referring now to FIG. 3, it can be seen that, on the surface of the steel, that is to say in the transition zone between the encapsulation layer and the surface of 10

the sheet metal, the carbon and oxygen concentrations are considerably reduced.

Thus, the hydrogen plasma treatment according to the invention is not only capable of reducing the native iron oxides, but also of removing traces of hydrocarbons.

The surface treatment process which has just been described, allowing the removal of residual hydrocarbons from the surface as well as the deoxidation of the latter, also makes it possible to considerably improve the adhesion of subsequent coatings.

FIG. 4 shows the result of tests making it possible to assess the adhesion of a thin layer silica coating deposited on surfaces thus treated, consisting in applying an increasing force to a hard tip pressing on the coating, while the sample to be analyzed is moved in translation. The critical charge L _c corresponding to the start of the flaking of the film is noted. For a surface treated using conventional techniques, that is to say by solvent, it can be seen in FIG. 4 that the adhesion, characterized in this way, is relatively poor, since the critical load L _c is 0.35 Newton. On the contrary, for a surface treated with hydrogen plasma, the value of the critical charge L _{c is} established at approximately 1.20 Newton, which corresponds to an increase in a ratio of approximately 4.

Thus, the surface treatment method which has just been described is extremely effective in ensuring satisfactory adhesion of the subsequent functional coatings, in particular of layers of paint or of anticorrosion layer, such as a layer of silica, which could be applied subsequently. . It also makes it possible to considerably reduce the treatment time of the surfaces of metal parts.

Although the present invention has been described in relation to particular embodiments, it is not thereby limited, but is on the contrary 11 susceptible of modifications and variants which will appear to those skilled in the art within the scope of the claims below.

Thus for example, as indicated above, if the concept of "low pressure" has been particularly described and exemplified in the above as being in the range from 0.5 to 10 mTorr, we will have understood this condition must above all be understood as effectively allowing the excitation of the plasma under RCE electronic cyclotron resonance conditions. We can then leave some of this range while remaining within the scope of the present invention, insofar as an NCE regime is effectively respected.

Claims

12 RESENDTCATTONS

1. A method of surface treatment of a metal part (12) with a view to its deoxidation and / or its degreasing, characterized in that it comprises the following steps:

filling a sealed enclosure (16), in which the part (12) to be treated is placed, with a reducing gas mixture at low pressure,

- establish a static magnetic field in an area of the enclosure separate from the area in which the part to be treated is located and,

- excite the gas mixture by means of an electromagnetic wave injected into the enclosure (16) so as to generate in the gas a treatment plasma, the intensity of the static magnetic field corresponding to the electronic cyclotron resonance, established in the pregnant in a distributed manner.

2. Method according to claim 1, characterized in that said establishment of a magnetic field and said excitation of the gas mixture take place simultaneously.

3. Method according to claim 1, characterized in that said establishment of a magnetic field and said excitation of the gas mixture take place successively.

4. Method according to one of claims l to 3, characterized in that the electromagnetic wave is injected by means of at least one applicator (24) of generally tubular shape connected to a microwave source and in which are placed permanent magnets (26).

5. Method according to claim 4, characterized in that the applicator (s) have a length at least equal to one of the dimensions of the metal part to be treated.

6. Method according to one of claims 1 to 5, characterized in that the gas mixture comprises one hydrogen.

7. Method according to any one of the preceding claims, characterized in that the pressure of the gas mixture is between 0.5 and 10 mTorr. 13

8. Method according to any one of the preceding claims, characterized in that the frequency of the incident electromagnetic wave is approximately equal to 2.45 GHz.

9. Method according to any one of the claims

1 to 7, characterized in that the frequency of the incident electromagnetic wave is approximately equal to 5.85 GHz.

10. Method according to any one of the preceding claims, characterized in that the part

(12) to be treated is constituted by a sheet of steel running on a substrate holder (14).

11. The method of claim 10, characterized in that it further comprises a step of applying a high frequency electric field on the substrate holder (14) so as to obtain a polarization of the latter.

12. Method according to one of claims 1 to 9, characterized in that the part (12) to be treated rests on a substrate holder (14), the method further comprising a step of applying a high frequency electric field on the substrate holder (14) so as to obtain a polarization of the latter.

13. Installation for surface treatment of a metal part (12) with a view to its deoxidation and / or its degreasing, suitable for implementing the method according to one of the preceding claims, comprising:

- a sealed enclosure (16), in which the part (12) to be treated can be placed, - a source of a reducing gaseous mixture, making it possible to fill the enclosure using the gaseous mixture at low pressure,

means (26) making it possible to establish a static magnetic field in an area of the enclosure distinct from the area in which the part to be treated is placed, and

- Means (24) for exciting the gas mixture by means of an electromagnetic wave injected into the enclosure

(16), capable of generating a treatment plasma in the gas, 14 characterized in that the intensity of the static magnetic field corresponds to the electronic cyclotron resonance, established in the enclosure in a distributed manner as follows: said excitation means comprise at least one field-shaped applicator (24) tubular connected to a microwave source, each applicator being equipped with means for creating a static magnetic field in the vicinity of one applicator, at an intensity corresponding to the electronic cyclotronic resonance.

14. Installation according to claim 13, characterized in that the applicator (s) have a length at least equal to one of the dimensions of the metal part to be treated.

15.. Installation according to claim 13 or 14, characterized in that said means for creating a static magnetic field in the vicinity of one applicator comprise a permanent magnet (26) placed inside each applicator.

16. Installation according to one of claims 13 to

15, characterized in that said source of reducing gas mixture comprises hydrogen.

17. Installation according to one of claims 13 to

16, characterized in that it comprises a substrate holder capable of receiving the part to be treated.

18. Installation according to claim 17, characterized in that it comprises means for scrolling the part (12) to be treated on the substrate holder (14).

19. Installation according to claim 17 or 18, characterized in that it further comprises means

(30) making it possible to apply a high frequency electric field to the substrate holder (14) so as to obtain a polarization of the latter.