Process for coating a substrate, plant for implementing the process and feeder for feeding such a plant with metal

A coating process for coating at least one side of a running substrate, by vacuum evaporation, with a layer of a sublimable metal or metal alloy, in which said metal or metal alloy is positioned so as to face said side of the substrate in the form of at least two ingots placed in contact with one another, that surface of said ingots facing said side of the substrate being kept parallel to the substrate and at a constant distance from the latter during coating, and also to a coating plant for implementing the process and to a feeder for supply.

The present invention relates to a process for coating a running substrate, more particularly for coating steel strip with layers based on metallic elements such as, for example, magnesium, but without being in any way restricted thereto.

BACKGROUND

Various processes are known for depositing, on a running substrate, such as a steel strip, metallic coatings consisting of a layer of metal, or several successive layers of different metals, or else metal alloys. Among these processes, mention may be made of hot-dip galvanizing and electrodeposition, or indeed the various vacuum deposition (magnetron sputtering, Joule evaporation, electron bombardment and SIP (self-induced plasma)) processes.

Vacuum deposition processes have the advantage of being environmentally friendly and of enabling virtually all known elements, but also alloys, to be deposited. When it is desired for a metallic layer to be continuously deposited on a running substrate, the problem of how to feed the deposition chamber with the metal to be deposited then arises.

A first type of coating plant is known in which the metal to be deposited is kept in liquid form by being melted in a furnace before being conveyed to the deposition zone, for example by means of a pump or else by the barometer principle. However, this type of plant is not suitable for depositing metallic elements that sublime, i.e. those which pass directly from the solid phase to the vapour phase.

A second type of coating plant is known in which the metal to be deposited is in solid form, such as for example in the form of wires, which are introduced in a regular manner therein, or else in the form of granules, which are placed inside an inert graphite crucible. These plants are able to deposit metallic elements that sublime, but the deposition process is unstable, resulting in non-uniform coating thicknesses over the course of time, and is limited in terms of heating power, preventing sufficiently high deposition rates from being achieved.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for coating a running substrate with a sublimable metal or metal alloy that allows stable operation over time at a high deposition rate.

The present invention provides a coating process for coating at least one side of a running substrate, by vacuum evaporation, with a layer of a sublimable metal or metal alloy, in which said metal or metal alloy is positioned so as to face said side of the substrate in the form of at least two ingots placed in contact with one another, that surface of said ingots facing said side of the substrate being kept parallel to the substrate and at a constant distance from the latter during coating.

The process according to the invention may also include various features, taken individually or in combination:

the ingots of a sublimable metal or metal alloy may be moved simultaneously, either continuously or sequentially, so as to keep their surfaces facing the substrate parallel to and at a constant distance from the latter;

the ingots of a sublimable metal or metal alloy may be moved simultaneously by translation, the opposite surface of the ingots from that facing the substrate being in contact with an inclined plane;

the ingots may be moved simultaneously by translation in a plane parallel to the plane in which the substrate runs and in a direction perpendicular to the direction in which the substrate runs, the opposite surface of the ingots from that facing the substrate being in contact with an inclined plane;

the ingots may be moved simultaneously by translation in a plane parallel to the plane in which the substrate runs and in a direction parallel to the direction in which the substrate runs, the opposite surface of the ingots from that facing the substrate being in contact with an inclined plane;

the substrate may run vertically, the ingots being stacked one on top of another;

the substrate may be coated by vacuum evaporation on both its sides with a layer of a sublimable metal or metal alloy, the metal or metal alloy being positioned so as to face each side of the substrate in the form of at least two ingots placed in contact with one another, that surface of the ingots facing each side of the substrate being kept parallel to and at a constant distance from the side of the substrate during coating;

the metal or metal alloy may be the same or different for each side of the substrate;

the vacuum evaporation process may be deposition by plasma-enhanced evaporation, such as deposition by SIP (self-induced plasma) evaporation;

the metal or metal alloy may be chosen from zinc, magnesium, chromium, manganese, silicon and alloys thereof and is preferably magnesium or one of its alloys;

the deposition by vacuum evaporation may be carried out in a reactive atmosphere; and

the running substrate may be a steel strip, possibly already coated beforehand, and preferably a steel strip which has been coated beforehand with zinc or a zinc alloy and on which a layer of magnesium or magnesium alloy is deposited.

A second subject of the invention is a coating plant for continuously coating a substrate S on at least one of its sides with a layer of a sublimable metal or metal alloy, comprising a vacuum chamber containing:

a vacuum evaporation coater;

means for making said substrate S run through the coater; and

a feeder1,11,21for feeding the coater with a metal or metal alloy, said metal or metal alloy being in the form of ingots L1, . . . , Ln, L′1, . . . , L′n and comprising means for keeping said ingots L1, . . . , Ln, L′1, . . . , L′n in contact with one another and for keeping that surface of the ingots L1, . . . , Ln, L′1, . . . , L′n facing the side of the substrate S to be coated parallel to and at a constant distance from the substrate S.

The plant according to the invention may also comprise the following embodiments, taken individually or in combination:

the feeder1,11,21may comprise means for simultaneously moving the ingots L1, . . . , Ln, L′1, . . . , Ln sequentially or continuously so as to keep their surfaces facing the substrate S parallel to and at a constant distance from the latter;

the feeder11may comprise at least one inclined plane2,2′ with which the ingots L1, . . . , Ln, L′1, . . . , L′n are in contact via their side opposite that facing the substrate S, the means for moving the ingots L1, . . . , Ln, L′1, . . . , L′n enabling them to be moved by translation in a plane parallel to the plane in which the substrate S runs and in a direction perpendicular to the direction in which the substrate S runs, and the inclination of the inclined plane2,2′ increasing in the direction of movement of the ingots L1, . . . , Ln, L′1, . . . , L′n;

the means for moving the ingots L1, . . . , Ln, L′1, . . . , L′n may consist of at least one piston3,3′ acting on a first ingot L1, L′1 in contact with a first end of the inclined plane2,2′, and extending transversely relative to the substrate S, the first ingot L1, L′1 acting in turn on the ingots L2, . . . , Ln, L′2, . . . , L′n preceding it on said inclined plane2,2′ as far as its second end;

the plant may have a first inclined plane2provided with ingots L1, . . . , Ln and with a first piston3acting on the first ingot L1 in contact with the first end of the first inclined plane2and a second inclined plane2′ provided with ingots L′1, . . . , L′n and with a second piston3′ acting on the first ingot L′1 in contact with the first end of the second inclined plane3′, it being possible for said pistons3,3′ to act in the same direction or in opposed directions;

the plant may comprise two tanks4for recovering, by gravity, the spent ingots U, said tanks being placed beneath each second end of the inclined planes2,2′;

the feeder11,21may have at least one inclined plane12,22with which the ingots L1, . . . , Ln are in contact via their side opposite that facing the substrate S, the means for moving the ingots enabling them to be moved by translation in a plane parallel to the plane in which the substrate S runs and in a direction parallel to the direction in which the substrate S runs, and the inclination of the inclined plane12,22increasing in the direction of movement of the ingots L1, . . . , Ln;

the means for moving the ingots L1, . . . , Ln may consist of at least one piston13,23acting on a first ingot L1 in contact with a first end of the inclined plane12,22, and extending longitudinally relative to the substrate S, the first ingot L1 acting in turn on the ingots L2, . . . , Ln preceding it on the inclined plane12,22as far as its second end;

the plant may comprise a tank for recovering, by gravity, the spent ingots, said tank being placed beneath the second end of the inclined plane12,22;

the substrate S may run vertically, the feeder21comprising at least one vertical plate24on which the ingots L1, . . . , Ln are stacked, said plate being in contact with at least one vertical piston23;

the plant may comprise a feeder1,11,21facing each side of the substrate S, it being possible for each feeder1,11,21to contain an identical or different metal or metal alloy, and a vacuum evaporator facing each side of the substrate S, the assembly being placed within the same vacuum chamber;

the plant may include means for introducing a reactive atmosphere into the vacuum chamber;

the vacuum evaporator(s) may be plasma-enhanced evaporation deposition devices, such as a device for deposition by self-induced plasma (SIP) evaporation; and

A third subject of the invention is a feeder1,11,21for feeding a vacuum coating plant as defined above with a metal or metal alloy.

DETAILED DESCRIPTION

Within the context of the present invention, the term “evaporation process” is understood to mean any deposition process by evaporation at a pressure below atmospheric pressure. Preferably, the evaporation will be carried out by the SIP (self-induced plasma) process in which vacuum evaporation is combined with magnetron sputtering.

This process consists in creating a plasma between a substrate and a crucible containing the material to be deposited, in a gas for generating radicals and/or ions. Under normal operating conditions, these ions will be accelerated onto the surface of the material to be deposited on the substrate and tear off surface atoms, which are then deposited on the substrate. At the same time, the bombardment of the ions created in the plasma heats up the material to be deposited, enabling an evaporation process to be added to the magnetron sputtering. The reader may refer in particular to patent EP 780 486 for more details about the implementation of this process.

The substrate to be coated moves through a vacuum chamber facing the crucible containing the material to be deposited. The crucible is negatively biased relative to the substrate, the latter preferably being earthed. An array of magnets positioned behind the crucible confines the plasma created therein. In order for the substrate to be coated to be positioned very precisely with respect to the crucible, the substrate is generally placed on a support roll, which may be rotated about its axis. However, this type of roll is unnecessary when substrates in rigid plate form, such as thick metal plates or glass plates, are treated.

The process according to the invention therefore consists in depositing a layer of a metal or metal alloy on a running substrate.

This process applies more particularly, but not solely, to the treatment of steel strip, preferably strip coated with zinc or a zinc alloy. The term “zinc alloy” is understood to mean any compound comprising at least 50% zinc and able to contain, for example, aluminum, iron, silicon, etc.

This prior coating may be obtained by any known galvanizing process, whether for example hot-dip galvanizing, electrodeposition or deposition by vacuum evaporation. However, vacuum deposition processes are preferred.

The coating deposited within the context of the present invention is a metallic coating based on elements that can sublime during their deposition by vacuum evaporation. In particular, this covers elements such as zinc, magnesium, chromium, manganese and silicon, for example. It is particularly preferable to deposit magnesium on a strip precoated with zinc after which a diffusion treatment may be carried out by any suitable means, so as in the end to obtain a Zn—Mg coating having, in particular, a high performance in terms of corrosion resistance.

The present inventors firstly conducted evaporation trials on a static bulk magnesium ingot facing a steel strip running through a SIP vacuum deposition plant.

After a certain operating time, it was observed that the coating deposited did not have a uniform thickness and that the deposition rate was relatively low. The deposition therefore had to be interrupted, making it possible to observe the state of the ingot, a representation of which is reproduced inFIG. 1.

It may be seen that the wear of the ingot is very irregular and to particularly accentuated in line with the high-magnetic-field zone. Without wishing to be tied to one particular theory, the present inventors consider that the instability of the deposition process is directly due to the irregular wear of the ingot.

To stabilize the process, the inventors therefore split the bulk ingot into at least two ingots and kept the top sides of these ingots parallel to and at a constant distance from the substrate to be coated, continuously or sequentially compensating for the wear due to evaporation.

For this purpose, a first coating plant according to the invention using the SIP process was developed, this being partly reproduced inFIG. 2in which a feeder1may be seen.

The feeder is placed beneath the substrate to be coated, shown schematically as100, which runs horizontally, for example on a support roll102, above the system of magnets A for confining the magnetic field, these being placed in a water box. A counterelectrode (not shown) is placed beneath the feeder1and a plasma is created by discharge between the counterelectrode/magnets assembly and the steel strip. To make the system easier to understand, the preferential erosion zone has been shown by means of a circuit T, which represents the zone where the magnetic field is intense.

The feeder1comprises a first inclined plane2, the inclination of which increases from left to right. This inclined plane may be made of any appropriate material, provided that it is not liable to be sputtered during the operation, which would contaminate the coating obtained. This first inclined plane2could for example be made of tungsten.

A series of n ingots L1 to Ln is placed on this inclined plane2, these being in contact with one another and their heights decreasing from left to right. The inclination of the inclined plane2is adapted so as to compensate for the wear of the ingots L1 to Ln, in such a way that the top sides of the ingots L1 to Ln remain mutually parallel and parallel to the substrate to be coated that is running above the device1, perpendicular to the inclined plane2. A constant distance is thus maintained between the magnets and the top side of the ingots, and also a constant distance between the top side of the ingots and that side of the substrate to be coated. The reason for this is that, to obtain the most homogeneous plasma possible, it is important for the distance between electrodes (consisting here of the ingots on the one hand and the substrate on the other) to be as constant as possible. It is also just as important for the distance between the magnets of the magnetron and the top side of the ingots to remain constant. If these conditions are not met, owing to consumption of the ingots, at any point on their top side, the plasma will be locally intensified or diminished, generating instabilities in this plasma, and thus instabilities in the evaporation.

The use of an inclined plane2thus makes it possible to obtain a very stable plasma at every point in the evaporation zone.

Provided on either side of the ingots L1 to Ln are two lateral tungsten guides6that keep these ingots L1 to Ln properly aligned.

Placed to the left of the first ingot L1 is a piston3that acts on this first ingot L1 so as to move it translationally to the right, while still being guided by the inclined plane2and the lateral guide6. The movement of the ingot L1 shifts, in a cascade fashion, all the ingots placed on the inclined plane2until the end ingot Ln drops under gravity into a recovery tank4for collecting the spent ingots, which may be melted and reused.

When the end of the piston3which is the first end of the inclined plane2it is at the end of its travel, it is then actuated in the opposite direction. Secondly, a vertical piston5acting on a support plate, supporting a series of p fresh ingots R1 to Rp, is actuated upwards so as to present a new ingot level with the first end of the inclined plane2. The piston3is then brought into contact with the lateral side of the ingot R1, which is pressed against the first ingot L1 in contact with the inclined plane2.

Thus, it may be seen that the feeder may be supplied with ingots continuously, without interrupting the process, and keeping the surface of the ingots in use both flat and parallel to the substrate to be coated thanks to the inclined plane2and to the regular movement imposed by the piston3.

To cover the majority of the intense magnetic field zone T and therefore to optimize the deposition rate, a second feeder1′ in every way identical to the feeder1is placed facing the second half of the zone T. Here, the piston3′ moves the second series of n ingots L′1 to L′n from right to left, the inclined plane2′ being of the opposite orientation to the inclined plane2, but it would be quite possible to provide two feeders which are identical in all points for each half of the zone T.

It may be seen that, in this embodiment, it is possible to optimize the size of the ingots by determining their width depending on the width of the zone T.

A second embodiment, as shown inFIG. 3, shows a feeder11comprising, in a similar way to the first embodiment, an inclined plane12supporting a series of n ingots L1 to Ln extending transversely relative to the running substrate in a horizontal plane (not shown). The environment of the feeder11is identical to that described in the case ofFIG. 2.

The ingot L1 is moved through the action of the pistons13, which advance the ingots L1 to Ln on the inclined plane12until they drop, under gravity, into a recovery tank (not shown) placed beneath the second end of the inclined plane12.

The system is recharged with ingots by means of a plate14moved by two pistons15, which plate supports a stack of p ingots R1 to Rp and operates in the same way as the device corresponding toFIG. 2. Provided on both sides of the ingots L1 to Ln are two lateral tungsten guides16which keep these ingots L1 to Ln properly aligned.

It may also be seen that this embodiment of the plant according to the invention allows easy adaptation to various widths of substrates to be coated since all that is required is to modify the width of the ingots and the spacing of the parts16in order to obtain a feeder having precisely the width necessary for the coating in progress.

A third embodiment, as shown inFIG. 4, shows a feeder21comprising, in a similar manner to the first embodiment, two inclined planes22supporting a series of n ingots L1 to Ln extending transversely relative to the substrate S running in a vertical plane. These inclined planes22also have lateral cheek plates which ensure correct alignment of the ingots.

The ingot L1 is moved through the action of two vertical pistons23on a vertical plate24, thereby advancing the ingots L1 to Ln along the inclined planes22.

When the top ingot Ln arrives beyond the end of the inclined planes22, a horizontal piston25is actuated laterally against the side of this ingot, which can thus be removed sideways.

This embodiment allows the substrate to be easily coated in a vertical position. This position also makes it possible for a two-side coating to be very easily applied by placing a feeder according to the invention on each side of the substrate, for example a feeder121with a vacuum evaporation123opposite feeder21shown schematically, while still being placed within the same reduced-pressure chamber.

Trials

Trials were carried out using an arrangement in accordance with the first embodiment and an SIP evaporation device. Pure zinc ingots or pure magnesium ingots 4 cm in thickness and 10 cm in width were used to coat steel strips varying in width between 50 and 200 cm. Each steel strip ran over a support roll at 100 m/min. The ingot evaporation rate was also varied.

The distance between the top side of the ingots and the steel strips to be coated was kept at 5 cm and the chamber opening, corresponding to the deposition zone in the direction in which the strip runs, was set to 40 cm. In all the trials carried out, the ingot run speed necessary for obtaining a coating thickness of around 1.5 μm was determined.

The results are given in the following three tables:

These show that the ingot run speed essentially depends on the width of the strip to be coated and on the rate of evaporation of the coating material.

Similar results were obtained using arrangements in accordance with the second and third embodiments.

More generally, it was found that an ingot run speed of between 1 and 15 cm/min allowed the desired coating thickness targets to be achieved. If it is desired to achieve greater coating thicknesses, all that is then required is to pass the substrate through a series of coaters. Thus, to obtain a zinc thickness of around 7.5 μm under the evaporation conditions indicated in the above tables, five coaters would be required.

Whatever the embodiment adopted, the invention also has the advantage of not requiring the use of graphite crucibles, thereby making it possible for deposition to take place in a reactive atmosphere. This makes it possible in particular to deposit, for example, metal oxides, nitrides, sulphides and fluorides, with high deposition rates.

As was seen over the course of the description of a few preferred embodiments of the invention, the coating may especially be carried out when the substrate runs either horizontally or vertically. It goes without saying that the coating may also be carried out in any position of the substrate that would be intermediate between horizontal and vertical.