WO2013110963A1

WO2013110963A1 - Apparatus and method for coating a moving metal substrate

Info

Publication number: WO2013110963A1
Application number: PCT/IB2012/000099
Authority: WO
Inventors: Vincent Leclercq; Florin DUMINICA; Eric Silberberg
Original assignee: Arcelormittal Investigacion Y Desarrollo Sl
Priority date: 2012-01-24
Filing date: 2012-01-24
Publication date: 2013-08-01

Abstract

The invention primarily relates to a method for depositing a coating on a metal substrate (4) comprising the steps whereby: - the substrate (4) is moved at a predetermined speed according to an axis of movement (11); - discharges, and therefore plasma, are generated by applying a voltage between the substrate (1, 4) and an electrode (2) formed from a plurality of elongate conductors (2a, 2b, etc.) equidistant from the substrate, the projections of the longitudinal axes of the elongate conductors being inclined by 3 to 45 degrees in relation to the axis of movement of the substrate (11) and the elongate conductors being arranged such that each point of the surface of the substrate is exposed consecutively to the discharge of at least two elongate conductors, a dielectric barrier (3) being disposed between the substrate (1, 4) and the electrode (2) at a predetermined distance from the substrate; - a mixture of gas comprising at least a precursor of the coating is circulated at a speed greater than or equal to 5 m/s between the substrate (1, 4) and the dielectric barrier (3) and in the axis of movement of the substrate (11). The invention further relates to the apparatus used to implement said method.

Description

APPARATUS AND METHOD FOR COATING A METAL SUBSTRATE

IN PROGRESS

Object of the invention

The object of the method according to the invention is to deposit a thin layer of oxides on a moving substrate. A particular object is to deposit a thin and homogeneous layer of oxides on a metal substrate.

The technical field of the invention is that apparatus and methods in which a substrate is exposed to a plasma generated by a dielectric barrier (DBD) electric discharge. Precursors that contain the chemical elements that must be deposited are injected into the plasma as vapor. Plasma assists the chemistry of the deposition process. This type of process is called "plasma-assisted vapor deposition" (PACVD). The deposits obtained are inorganic or partially organic and include stoichiometric or sub-stoichiometric oxygen oxides, preferably metals. These deposits provide the surface of the substrate an improvement in its performance in terms of, for example, protection against corrosion, abrasion resistance, adhesion promotion, cleanability, absorbency or reflective waves electromagnetic or photo-catalytic activity.

The invention also relates to a device specially designed for the implementation of the method. State of the art and technical problem to be solved

The PACVD is often used at low pressure (between 0.001 mbar and 1 mbar absolute pressure). However, there are applications of PACVD at higher pressure (PACVD-HP), including the PACVD-DBD which can be implemented at atmospheric pressure.

Figure 1 schematically shows a device of PACVD-DBD. This consists of at least two electrodes 1,2 separated by at least one dielectric barrier 3, the assembly being contained in an enclosure delimiting the deposition zone. An electric current applied between the two electrodes generates an electric discharge, and therefore a plasma, between the dielectric barrier and one of the electrodes. The role of the dielectric barrier is to distribute the electric discharge over the entire surface of the electrode 2, thus preventing the electric discharge from occurring at a single point. It is known that the electrode 2 can be flat as shown in FIG. 1, or slender as shown in FIG. 2. It is also known that the dielectric barrier 3 can be flat as shown in FIGS. 1 and 2, or that it envelops the elongate electrode 2 as illustrated in FIG.

If the substrate to be treated is an electrical conductor, it is advantageous to substitute it for the electrode 1. If this substrate is an electrical insulator (dielectric), it may constitute an additional dielectric barrier to the dielectric barrier belonging to the device, or alternatively it can, on its own, constitute this dielectric barrier.

An example of application of PACVD-DBD is disclosed in patent application EP 0 577 447 A1 which describes a method for forming a deposit containing silicon on the surface of a metal substrate. The proper functioning of such equipment, and in particular the homogeneity of the plasma, is directly conditioned by the homogeneity of the distance between the dielectric barrier and the electrode 1, called "spacing". Indeed, if this spacing is locally lower, the electric discharge will be preferentially at this point. Similarly, if this spacing is overall too large, the plasma is heterogeneous. It is therefore important to accurately set the position of the entire surface of the dielectric barrier, while maintaining a small gap. For this purpose, the spacings are between 1 and 2mm.

However, such spacings are incompatible with moving metal substrates such as steel strips.

As it is quite easy to impose the position of the entire surface of the substrate, when the substrate is a plastic film or aluminum foil, because these substrates are flexible and have excellent flatness, as it is It is generally more difficult to guarantee the precise position of the entire surface of the steel strips, when they are moving, because these substrates are less flexible and their flatness is incompatible with the precision required for the PACVD-DBD.

The variations of the flatness of the metal substrate, which then serves as an electrode 1, cause a high heterogeneity of the spacing, which in turn causes a high heterogeneity of the discharge and therefore of the coating deposited on the substrate. . In addition, the variations in the flatness of the metal substrate encourage the use of larger spacings so as to facilitate the movement of the substrate in the equipment. However, as mentioned above, this increase in The spacing also contributes to deteriorating the homogeneity of the plasma.

A solution proposed by the prior art, and illustrated in Figure 4, is to deflect the steel strip 4 on a support roll 5 and put it under tension. The combination of this deflection and this traction temporarily improves the flatness of the substrate in the area of the roll. But this is not enough to make the band perfectly flat.

Furthermore, a precursor is injected in gaseous form between the dielectric barrier and the substrate to be treated. This precursor is degraded by the electric discharge, it reacts with the oxygen present and forms oxide particles which are deposited on the substrate, thus forming the desired oxide layer. However, these particles are also deposited on the rest of the equipment, in particular on the dielectric barrier, on the walls and on the possible support roller. This fouling requires the regular cleaning of the equipment and therefore the shutdown of the production line, which generates very high maintenance costs.

A proposed solution, in particular by

EP 0 622 474 A1, consists in creating a low air flow between the electrodes and thus sucking the particles ready to be deposited on the walls. However, such a device does not prevent fouling of the dielectric barrier.

In summary, devices and methods, known from the prior art, according to which a substrate is exposed to a plasma generated by a dielectric barrier electric discharge, in order to deposit a layer of oxides on this substrate , have two important weaknesses. The first weakness is the heterogeneity of the plasma, and therefore the deposition, during the treatment of metal substrates in scrolling such as steel strips. The second weakness is the fouling of the deposition apparatus on internal organs, such as support rollers, dielectric barriers or electrodes. These two weaknesses have hitherto prevented the use of the dielectric barrier discharge process in the steel industry.

Aims of the invention

The present invention aims to provide a solution that overcomes the disadvantages of the state of the art.

More specifically, the object of the invention is to propose an apparatus and a method for deposition, by DBD, of a coating on a metal strip substrate, preferably on a steel strip, ensuring a homogeneous treatment of the substrate. in band over its entire width.

In addition, the operation of this device must significantly minimize the fouling of the internal organs of the device such as the support roller and the dielectric used in the DBD, if the dielectric is not the substrate to be coated. .

Main characteristic elements of the invention

A first aspect of the present invention relates to a method of depositing a coating on a metal substrate comprising the steps of:

said substrate is scrolled at a predetermined speed along a scroll axis;

generating discharges and therefore a plasma by applying a voltage between said substrate and an electrode consisting of a plurality of elongate conductors equidistant from said substrate, the projections longitudinal axes of said elongate conductors being inclined 3 to 45 degrees with respect to said running axis of the substrate and said elongated conductors being arranged so that each point of the surface of said substrate is exposed consecutively to the discharge of at least two elongate conductors, a dielectric barrier being disposed between said substrate and said electrode at a determined distance from said substrate;

a mixture of gases, comprising at least one precursor of said coating, is circulated at a speed greater than or equal to 5 m / s between said substrate and said dielectric barrier and in the axis of travel of said substrate.

According to preferred embodiments of the method of the invention, it further comprises one or a suitable combination of the following steps:

said projections of the longitudinal axes of the elongate conductors are inclined by 5 to 15 degrees with respect to said running axis of the substrate;

said distance between the dielectric barrier and the substrate is greater than or equal to 3 mm;

said distance between the dielectric barrier and the substrate is between 7 and 20 mm;

the gas mixture comprises air in proportion at least ten times greater than that of the precursor;

the speed of the gas mixture is at least five times greater than the running speed of the metal substrate;

a shield is interposed between said moving substrate and said dielectric barrier;

- said shield is in the form of a flexible strip scrolling; the process is carried out at a pressure of between 50 and 250 mbar;

the coating is inorganic or partially organic and comprises oxides of stoichiometric or sub-stoichiometric oxygen metals.

Another aspect of the present invention relates to an apparatus for depositing a coating on a moving substrate comprising:

an enclosure delimiting a deposition zone and allowing said substrate to scroll along a scroll surface and along a scroll axis;

an electrode consisting of a plurality of longitudinal conductors equidistant from said running surface, the projections on said running surface of the longitudinal axes of said elongated conductors being inclined at 3 to 45 degrees with respect to said running axis of the substrate and said elongated conductors being arranged to allow recovery of deposits generated by each of said elongate conductors;

a dielectric barrier disposed between said electrode and said running surface at a determined distance from said surface;

- Means for circulating at a speed greater than or equal to 5 m / s a mixture of gas and at least one precursor of said coating between the scroll surface and said dielectric barrier and in the scroll axis;

an electrical device for applying a voltage between said substrate and said electrode.

According to preferred embodiments of the apparatus according to the invention, the latter further comprises one or a suitable combination of the following characteristics: said dielectric barrier consists of a plurality of tubes enveloping each of said elongated conductors;

said elongated conductors are parallel to one another and approximately equidistant from one another;

the apparatus comprises a shield interposed between said dielectric barrier and said scrolling surface; said shield is a fiberglass fabric;

- The apparatus comprises means for unwinding at the input of said deposition area and re-winding at the output of said area a flexible band constituting said shield;

the apparatus comprises means making it possible to maintain said enclosure at a pressure of between 50 and 250 mbar.

It will thus be understood that the invention consists in implementing the means for maintaining homogeneity of discharge and deposition under conditions of unusual flatness of the substrate, while avoiding the rapid fouling of the equipment.

The invention will be better understood on reading the description which follows, given for explanatory but nonlimiting, with reference to the accompanying figures.

Brief description of the figures

Figures 1 to 3, already mentioned, schematically show PACVD-DBD devices according to the state of the art consist of at least two electrodes and a dielectric barrier, the first electrode being an electrical conductor plane and the second electrode being constituted respectively by a flat electrical conductor with a plane dielectric barrier

(fig.l) or by an elongated driver, the barrier dielectric being respectively flat (fig.2) and tubular so as to wrap the elongated electrode (fig.3).

Figure 4, already mentioned, schematically shows the traction of the substrate and the improvement of the flatness of the latter by means of a support roller, according to the state of the art.

Figure 5 schematically shows a solution for homogenizing the discharge, according to the present invention, wherein the space between the substrate and the dielectric is scanned by a gas flow.

Figure 6 shows schematically the planar projection of a homogenization solution of the discharge, according to the present invention, wherein the second electrode consists of a plurality of elongate conductors.

Figure 7 shows schematically the planar projection of different arrangements of elongated conductors according to the invention.

Figure 8 schematically shows a solution according to the present invention for unwinding / rewinding a shield in the form of a flexible strip scrolling.

FIG. 9 shows a discharge photograph respectively in the case of an immobile atmosphere (FIG A) and in the case of the circulation of the gas layer constituting the atmosphere at a speed of 20 m / s between the substrate and the dielectric barrier, in the axis of travel of the substrate (Figure B).

Description of preferred embodiments of the invention

The substrates particularly targeted by the invention are electrical conductors, including metal substrates. Without excluding the other categories of substrates, the category of substrate which is preferred is that of substrates of large dimensions, flat and scrolling during their manufacture. Among these substrates, those which are even more preferred are steel strips. Hot-dip galvanized and / or "prepainted", that is, coated with an organic coating, are included in the preferred substrates. Consequently, the deposition apparatus according to the invention can in particular be installed on a galvanizing line or on a painting line.

The steel strips are characterized primarily by a thickness generally between 0.10 mm and 3.00 mm and a width generally between 500 mm and 2500 mm. These strips have a running speed generally between 30 m / min and 700 m / min.

The substrates treated in the context of the invention may be deflected on a support roll 5 or alternately tensioned between two deflector rollers 6, 7 located on either side of the deposit zone, as shown in FIG. In the latter case, the substrate strand will be held in place by adjusting, on the one hand, the deflection of the substrate on the baffle rollers and, on the other hand, the traction of the band, these adjustments being well known to those skilled in the art. Preferably, the deflection rollers will be used which have the advantage of being away from the area where the deposit takes place and do not become fouled or dirty more slowly.

The deposits targeted by the invention are inorganic or partially organic, comprising stoichiometric or sub-stoichiometric oxygen oxides, preferably metals, themselves preferably comprising silicon or titanium. The role of Targeted deposition is to provide the substrate with one or more functions. Preferred functions include corrosion protection, abrasion resistance, adhesion promotion, improved cleanability, electromagnetic wave absorbency or reflectivity, photo-catalytic activity.

Throughout the text, an axis is defined as a line that shares an object, a body in two symmetrical parts in the sense of the largest dimension.

In addition, the flatness (or more precisely the difference in flatness) is defined as the maximum distance between the substrate and the horizontal surface on which it is placed.

In the DBD deposition apparatus disclosed in the present invention, the spacing is preferably set at 3 mm minimum, and this mainly for two reasons.

The first reason is related to the flatness of the substrate. The flatness of the preferred substrate is unusually large compared to plastic films and aluminum foils usually treated with DBD. It is therefore preferable to increase the gap to a value which is also unusually large in order to facilitate the scrolling of the substrate in the equipment. A gap of 3 mm is considered a minimum for the treatment of the preferred substrate.

The second reason is related to the configuration of industrial lines of continuous manufacture. On these, the substrates come in the form of coils which are loaded, unrolled, processed and re-wound. Just after the end of unwinding of the reel being processed, the upstream end of this reel is welded or stapled to the downstream end of the next reel. treat. The thickness of these welds or staples is much greater than that of the substrate. It must therefore be ensured that the spacing is sufficient to allow these welds or staples to pass. As such, it is usually recommended that the substrate is at least 7 mm away from the equipment walls so that the welds or staples do not touch them. We will therefore preferentially use a spacing of 7 mm and more.

For larger planes, it may be necessary to increase the spacing to values up to 20 mm.

The flatness of the substrate being unusually large, it is appropriate to modify the installation to maintain the homogeneity of the plasma and thus the homogeneity of the deposit.

It specifies below all the means that can be implemented to achieve this goal.

The first homogenization means is described with reference to FIG. 5. It consists in scanning the space between the substrate and the dielectric, said substrate-dielectric space, with a gas flow and this for the following reason : A DBD discharge usually consists of a multitude of ionization filaments that appear and disappear very quickly. The new filaments often reappear where previous filaments had already existed. This phenomenon is called the memory effect. It is related to the existence of electrical charges and residual chemical species present, some time after extinction of the filaments, on the insulating surfaces such as the dielectric barrier of the apparatus and in the layer of gas contained in the space dielectric-substrate. Because of this phenomenon, there are areas where new filaments are generated preferentially, with the macroscopic consequence heterogeneity of the discharge.

By ensuring an intense scanning of the substrate-dielectric space, ensures a stirring and rapid mixing of the gas layer, resulting in permanent homogenization thereof. Thanks to this homogenization, the electrical charges and residual chemical species are better distributed over the entire substrate-dielectric space, reducing the memory effect and making the distribution of the next filaments more homogeneous.

In the case of the present invention, in addition to the precursor gas and its possible carrier neutral gas, required in small quantities, the scanning will preferably be carried out mainly with air for reasons of simplicity of use of the device. Preferably, plasmagene gases will not be used for cost reasons.

In practice, one or more flow rates of precursor gas and a possible flow of carrier neutral gas are premixed with a large air flow, the proportion of air being preferably at least 10 times greater than the proportion of air. precursor. All of these gases constitute the injection mixture 8. This mixture is injected into the substrate-dielectric space by means of an injection device and forms the gas layer 9 which circulates in this space. After passing through the substrate-dielectric space, the residual gases are sucked by a pumping unit.

The scanning is approximately parallel to the surface of the substrate, in the direction of travel of the strip or alternately in the opposite direction, to obtain a homogeneous deposition in the bandwidth. Indeed, if the scan was done, at least partially, perpendicular to the direction of travel, the injection mixture would deplete precursor gas between one bank and the other of the steel strip, which would lead to a variable deposit from one bank to another.

The scanning is at a speed greater than 5 m / s. Below 5 m / s, it has indeed been found that homogenization of the plasma was not sufficient. Preferably, the scanning is carried out at a speed of between 5 m / s and 100 m / s, and more preferably between 15 m / s and 50 m / s. Preferably, the scanning speed is at least five times greater than the running speed of the substrate.

To optimize the characteristics of the discharge, the scanning speed is adjusted to a value that depends in particular on the spacing. For example, if this spacing is 7 mm, the preferred speed at the entrance of the substrate-dielectric space is approximately 15 m / s. If this spacing is 15 mm, the preferred speed is approximately 30 m / s. Under these conditions, the gas layer 9 in the substrate-dielectric space undergoes a permanent renewal which makes it possible to obtain a discharge whose homogeneity is satisfactory in the direction of the sweep.

However, it has been found in the case of a planar electrode 2 that the oxide deposits made with a sweep as described above still have variations in thickness in the cross direction of the moving metal strip. It seems that these variations in thickness result from a less uniform mixing in the direction perpendicular to the scan than in the direction of the scan. Without wishing to be bound by any theory, the inventors believe that the heating of the gas layer in contact with the plasma and the temperature difference between the substrate and the dielectric barrier generate a heterogeneous gaseous flow, in the form of convection cells distributed in the bandwidth (Rayleigh-Bénard type thermo-aerodynamic instabilities). At the junction between two successive convection cells, the mixing of the gas layer is insufficient, which allows the preferential localization of the filaments of the discharge.

In order to avoid this problem, use is made of a second homogenization means. This means is described with reference to FIG. 6. A first electrode 1 is formed by the metal strip 4 running along a running surface and along a scroll axis 11. The second electrode 2 is formed of a plurality of elongated conductors 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, ... of length, width and thickness approximately identical and shown in FIG. 6 by their projection on the substrate 4. The reason for Such a projection representation resides in the fact that these conductors can be just as straight as curvilinear, or polygonal, or else formed of a set of line or curve segments, intersections between conductors being possible, that depending on the configuration of the substrate in the equipment and the desired homogeneity of deposit. Advantageously, their length is much greater than one spacing and preferably between 200 mm and 1 m. Their width and thickness are preferably approximately equal to the spacing. Their section is preferably circular, although other sections are possible. Preferably, each of the conductors is surrounded by a tubular electrical barrier, although other forms of dielectric barrier are possible, such as for example a single dielectric barrier disposed between the set of elongate conductors and the substrate. All these elongate conductors are equidistant from the substrate. In the example of Figure 6, they are also parallel to each other and approximately equidistant from each other.

The projections, on the surface of the substrate, of the longitudinal axes of these elongate conductors, or in the absence of segments thereof, are inclined with respect to the axis of travel of the substrate at an angle 12.

All elongated conductors are brought to the same electrical potential, the substrate being grounded. Preferably, the longiline conductors are provided with a high alternating voltage at a high frequency of between 1 kHz and 500 kHz, and preferably between 10 kHz and 100 kHz. Electrical discharges are produced between these elongated conductors and the substrate, but not between adjacent straight conductors.

The precursor contained in the gas layer is degraded by the discharges which are located along the elongated conductors. It is in these places that the deposit is made on the moving substrate.

Those skilled in the art will adjust the dimensions of the elongate conductors, their number, their arrangement and their inclination so as to allow recovery of the deposits generated by each of the conductors, each point of the surface of the substrate being thus exposed to the discharge several of these consecutive straight conductors during scrolling and thus obtain the desired homogeneity.

For this purpose, we will use inclinations of long line conductors between 3 and 45 degrees. Below 3 degrees, the overlap between the elongate conductors is not sufficient to obtain sufficient homogeneity of the plasma. Above 45 degrees, the gas flow in the axis of the substrate does not allow not sufficient homogenization of the discharges along each rectilinear conductor. According to a preferred embodiment, inclinations of between 5 and 15 degrees will be used. This allows a better compromise between the homogenization of the plasma according to the length of the substrate obtained by the scanning of the gas layer and the homogenization according to the width of the substrate obtained by the inclination of the elongated conductors. More preferably, it will use a tilt of 10 degrees which represents the best compromise.

In order to obtain the desired homogeneity, it will be possible in particular to use provisions of elongated conductors such as those shown in Figure 7 in planar projection.

For simplicity, preference will be given to the arrangement of FIG. 6. In this, the upstream ends of all the elongated conductors are connected by an imaginary line segment 13, referred to as "upstream edge". and the downstream ends of all the elongated conductors are connected by an imaginary line segment 14, referred to as "downstream edge" such that the upstream edge and the downstream edge are perpendicular to the axis of the substrate.

However, in the case of an electrical barrier surrounding each of the elongate conductors, the succession of the latter and in particular the spacing between two successive conductors tends to disturb the scanning of the gas layer, schematically represented by the vector In addition, the dielectric barrier becomes clogged during deposition.

In order to avoid these problems, it may be necessary to use an additional means. This means is described with reference to FIG. 8. This shows the geometry of Figure 5, to which has been added a shield 16, located between the substrate and the dielectric barrier.

The shield 16 divides the gas layer which is located between the substrate and the dielectric barrier in two sub-layers approximately parallel to the substrate. The first sub-layer is that which is located between the substrate and the shield 16 and which corresponds to the swept gas layer 9. This first sub-layer contains the injection mixture, in order to perform the deposition on the substrate. The second sublayer 17 is the one located between the shield and the dielectric barrier. This second underlayer is free of precursor gas, in order to prevent fouling of the dielectric barrier. The role of this shield is twofold: it homogenizes the plasma by guiding the layer of swept gas and prevents the deposition of the oxide layer on the dielectric barrier and thus its fouling by preventing any movement of gas from the first underlayer to the second sublayer.

To avoid degrading the homogeneity of the discharge to which the substrate is exposed, this shield must be made of an electrically insulating material (or dielectric). However, it is not necessary for this shield itself to constitute a dielectric barrier proper. In other words, it does not have to have a minimum capacitive impedance. As this shield does not constitute a real dielectric barrier, the discharge is little modified by the presence of this shield.

This shield clogs instead of the surface of the dielectric barrier. When the thickness of the deposit which constitutes the fouling on this shield reaches the maximum acceptable value, the shield is replaced. This shield must be considered as a consumable. The replacement of this shield can be done in a discontinuous or continuous manner, the shield being for example in the form of a flexible strip unwound at the entrance of the deposition area and re-wound at the output. In the preferred version of the industrial depot, this shield scrolls continuously. The optimal speed of scrolling of this shield is reached when the maximum acceptable thickness for the fouling on this shield is reached at the moment when this shield leaves the deposition zone. For this reason, the running speed is preferably between 0.01 mm / s and 10 mm / s, and more preferably between 0.1 mm / s and 1 mm / s.

Preferably, the shield will be formed of a glass fabric. Without excluding other types of glass fabric, the preferred type of glass fabric is a fabric usually used to improve the strength of various products, such as electronic appliance housings. It has the advantage of being a cheap and available consumable.

The glass has all the expected qualities. It is resistant to thermal stresses in the deposition area subjected to scanning. It is chemically inert. Its thermal conductivity is quite good. Its dielectric constant is pretty good too.

The glass fabrics are easy to use because they can be loaded in the form of rolls, be unwound in the apparatus, especially through the deposition area and be re-wound easily, as shown in the diagram. Figure 8.

The glass fabrics comprise woven glass fibers. As these fibers are very brittle, when they touch each other, glass fabric producers almost always add an organic matrix that prevents the different fibers from touching each other. and allows easy manipulation and wrapping of the fabric.

Unlike the fibers, this organic matrix is however not able to withstand the thermal and chemical stresses present in the discharge. This organic matrix is modified or burned at least partially by the discharge.

However, since the speed of travel of the glass fabric is much slower than that of the substrate, the fumes emitted by the combustion of this organic matrix are negligible and they have no effect on the quality of the layers. deposited on the substrate.

In the case where the glass fabric used is porous, it is envisaged several solutions preventing the diffusion of the precursor gas through its shield.

The first solution is to maintain the pressure of the gas in the second sub-layer greater than the pressure of the gas in the first sub-layer. This pressure difference is sufficient to ensure, wherever this shield is porous, a gaseous flow that passes through the shield, from the second underlayer to the first sub-layer. This gas flow is fast enough to prevent any diffusion of the precursor gas from the first sub-layer to the second sub-layer.

Alternatively the shield is previously rendered non-porous by means of a layer of oxides deposited on its surface. In a preferred version, this oxide deposit is a silica deposit, which can be applied by one of the deposition techniques known in the prior art. This deposit can be applied before loading this shield into the deposition apparatus or it can be applied in the device itself. In the latter case, it may be before it passes through the zone where the coating is carried out on the substrate or during its passage in this zone.

The preferred embodiment is obtained when the shield is rendered non-porous while it passes into the deposition zone. In this case, it is the fouling naturally generated by the deposition apparatus that renders the shield non-porous. As this shield scrolls very slowly, fouling makes the shield non-porous long before the shield exits the deposition zone. In other words, only a small proportion of the surface of the dielectric barrier is covered by a porous shield. Thus, the pressures of the gases must be controlled only over a zone of reduced dimensions.

Moreover, under the conditions of the invention, it can be difficult to obtain, at atmospheric pressure, a discharge whose homogeneity is satisfactory. Preferably, therefore, the pressure of the chamber will be slightly reduced, by means of a pumping unit, to reach a light vacuum. A pressure of between 50 mbar and 250 mbar absolute, and more preferably between 100 mbar and 150 mbar absolute, will preferably be chosen. However, because the targeted vacuum is light, only one pumping stage is required. This makes it possible to limit the amount of the investment and the degree of complexity of the equipment.

Tests have been carried out and will now be described with reference to FIG. 9. The photo (A) in FIG. 9 shows a very heterogeneous discharge, obtained under one of the conditions described below, but with a still atmosphere. Photo (B) in FIG. 9 is obtained under identical conditions except that the gas layer constituting the atmosphere moves at a speed of 20 m / s. These photos are representative of the visual impression obtained on site. In the tests corresponding to FIG. 9, the test conditions were as follows:

the first electrode 1 is the substrate 4: the substrate is a 500 mm wide steel strip whose thickness is varied between 0.4 mm and 0.7 mm. It is tensioned between two deflector rollers 6, 7;

- two types of steel are tested: stainless steel and pre-painted galvanized steel. The latter is therefore a galvanized steel already covered with a layer of paint;

- The second electrode 2 consists of a plurality of elongate conductors 2a, 2b, 2c, ... These are identical to each other, cylindrical, outer diameter equal to 13 mm, length equal to 500 mm, copper. They are 20 in number, spaced from each other by a regular distance equal to 25 mm and equidistant from the substrate;

each of these elongated conductors is surrounded by a dielectric tube. All of these dielectric tubes form the dielectric barrier. They are identical to each other, elongated, cylindrical, of internal diameter equal to 13 mm, outside diameter equal to 17 mm, length greater than 500 mm, extruded alumina, sintered and non-porous gas;

- the spacing is varied between 7 mm and 15 mm;

- Longiline conductors are inclined between 3 ° and 15 ° relative to the axis 11 of travel of the substrate;

the shield 16 is a "resistance" type glass fabric whose surface mass is 50 g / m ² . This fabric consists of glass fibers embedded in an organic matrix. The shield is previously made non-porous by covering it with an organic silica deposit (SiCxHyOz) by one of the known means of the prior art. This shield is arranged parallel to the substrate 4, between the latter and the dielectric barrier 3. The distance between this shield and the outer surface of this dielectric barrier is 1 mm;

the injection mixture 8 consists of air and precursor gas. Air is the main constituent of this mixture. The air consists of filtered ambient air. The precursor used is HMDSO (hexamethyledisiloxane). The air flow is varied between 0.4 Nm ³ / min and 0.8 Nm ³ / min. The precursor flow rate is varied between 50 g / h and 500 g / h. As a result, the velocities of the gas layer vary between 10 m / s and 35 m / s. These speeds are at least 10 times greater than the speed of travel of the substrate;

the pressure of the chamber is varied between 100 mbar and 150 mbar absolute;

- The substrate is electrically grounded via pads or brushes applied to the two deflector rollers 6, 7. A second alternating voltage is applied to the second electrode, which varies between 1000 V _rms and 1500 V _rms , at a frequency of 40 kHz. Under these conditions, the elongate lead injected current constituting the second electrode varies between 0.3 A _rms and 1.1 A _rms .

Visual observations, FTIR analyzes and adhesion tests carried out on all the substrates coated during the tests confirmed the homogeneity and the quality of the deposits obtained.

Although the invention has been described in the case of a metal substrate, it is easy to apply it to the case of a non-conductive substrate which would have a flatness incompatible with the devices of PACVD-DBD of the prior art.

Similarly, although the invention has been described in the case of a horizontal substrate and a deposit made on the upper face thereof, it is conceivable to apply it to the case of a substrate. vertical and / or deposit made on the underside. Such configurations may make it possible to limit the pollution of the deposit by the dusting which tends to form in the plasma, and this in addition to the already existing suction.

It may also be considered to preheat the substrate and / or the injection mixture. This can improve the quality of the deposit. The preheating temperature depends mainly on the composition of

The atmosphere and the desired deposit.

Reference symbols

1 first electrode

2 second electrode

2a, 2j individual slender conductors

3 dielectric barrier

4 substrate

5 roll support

6.7 baffle rollers

8 injection mix

9 layer of gas

10 residual gases

11 axis of travel of the substrate

12 angle of inclination of the second electrode

13 upstream edge

14 downstream edge

15 vector displacement of the gas layer

16 shield

17 second gas underlayer

Claims

A method of depositing a coating on a metal substrate (4) comprising the steps of:

said substrate (4) is scrolled at a predetermined speed along a scroll axis (11);

- discharges and therefore a plasma are generated by applying a voltage between said substrate (1, 4) and an electrode (2) consisting of a plurality of elongate conductors (2a, 2b, ...) equidistant from said substrate, the projections of the longitudinal axes of said elongate conductors (2a, 2b, ...) being inclined by 3 to 45 degrees relative to said running axis of the substrate (11) and said elongate conductors being arranged so that each point the surface of said substrate is exposed consecutively to the discharge of at least two elongate conductors, a dielectric barrier (3) being disposed between said substrate (1, 4) and said electrode (2) at a determined distance from said substrate;

a mixture of gases, comprising at least one precursor of said coating, is circulated at a speed greater than or equal to 5 m / s between said substrate (1, 4) and said dielectric barrier (3) and in the scrolling axis; said substrate (11).

2. Method according to claim 1, wherein said projections of the longitudinal axes of the elongated conductors (2a, 2b, ...) are inclined by 5 to 15 degrees relative to said running axis of the substrate (11).

3. Method according to claim 1 or 2, wherein said distance between the dielectric barrier (3) and the substrate (1, 4) is greater than or equal to 3 mm.

4. Method according to claim 3, wherein said distance between the dielectric barrier (3) and the substrate (1, 4) is between 7 and 20mm.

5. A process according to any one of the preceding claims, wherein the gas mixture comprises air in a proportion at least ten times greater than that of the precursor.

6. A method according to any one of the preceding claims, wherein the speed of the gas mixture is at least five times greater than the running speed of the metal substrate.

7. Method according to any one of the preceding claims, for which there is interposed a shield (16) between said moving substrate and said dielectric barrier.

8. The method of claim 7, wherein said shield (16) is in the form of a flexible strip scrolling.

9. Process according to any one of the preceding claims, implemented at a pressure of between 50 and 250 mbar.

10. A process according to any one of the preceding claims, wherein the coating is inorganic or partially organic and comprises stoichiometric or sub-stoichiometric oxides of oxygen.

Apparatus for depositing a coating on a moving substrate (4) comprising:

an enclosure delimiting a deposition zone and allowing said substrate (4) to scroll along a scroll surface and along a scroll axis (11);

an electrode (2) consisting of a plurality of elongate conductors (2a, 2b, ...) equidistant from said running surface, the projections on said scrolling surface of the longitudinal axes of said elongated conductors being inclined at 3 to 45 degrees relative to said running axis of the substrate and said elongate conductors being arranged to allow recovery of the deposits generated by each of said elongate conductors;

a dielectric barrier (3) disposed between said electrode and said running surface at a determined distance from said surface;

an electrical device for applying a voltage between said substrate (1, 4) and said electrode (2).

Apparatus according to claim 11, wherein said dielectric barrier (3) is comprised of a plurality of tubes enclosing each of said elongate conductors.

Apparatus according to any one of claims 11 or 12, wherein said elongated conductors are parallel to each other and approximately equidistant from each other.

14. Apparatus according to any one of claims 11 to 13, comprising a shield (16) interposed between said dielectric barrier and said scroll surface.

Apparatus according to claim 14, wherein said shield (16) is a glass fiber fabric.

16. Apparatus according to any one of claims 14 or 15, comprising means for unfolding at the entrance of said deposition and repositioning zone. winding at the output of said zone a flexible band constituting said shield.

17. Apparatus according to any one of claims 11 to 16, comprising means for maintaining said enclosure at a pressure of between 50 and 250 mbar.