Patent Application
20180245214
The invention relates to an apparatus for the cleaning and the coating of metal strip, wherein the cleaning is carried out by means of a plasma treatment process and the coating is applied by means of a vacuum vapour deposition process. Vacuum vapour deposition includes Physical Vapour Deposition (PVD) and Chemical vapour Deposition (CVD) and although the term PVD will be used to throughout the description it is meant to refer both to PVD and CVD as applicable.
Important issues with applying a coating on a metal substrate, such as a steel strip, by means of PVD on an industrial scale are to maintain the required vacuum for the PVD process and to keep the deposition chamber in which the coating is applied to the strip free from contaminations as much as possible. Contamination of the deposition chamber may occur by means of coating vapour that instead of on the metal strip deposits on the walls of the chamber. Another form of contamination is residues resulting from cleaning the metal strip.
A continuous PVD coating installation is provided with entry and exit sections provided with air locks to feed the metal strip in and out of the installation. To operate a PVD coating installation cost effectively the process should be operated in a continuous process as much as possible. A prerequisite for this is to keep the level of contamination to an absolute minimum and therewith the frequency of maintenance stops.
Known PVD coating technologies like electron beam or magnetron sputtering have a vapour yield between 60 and 95%, wherein the vapour yield is the ratio of deposited vapour on the metal strip divided by the total produced vapour. This means that with these technologies between 5-40% of the vapour will contaminate the deposition chamber requiring regular cleaning of the deposition chamber therewith interrupting the coating process frequently and for considerable time.
EP0909342 discloses a method to produce zinc coated steel strip with a vapour yield of almost 100%. This is achieved by combining a vapour distribution section, which alone is already capable of realising a vapour yield of ≥99%, with a heated channel and using an increased pressure within the deposition chamber in comparison with the pressure used in conventional PVD coating processes, which is of the order 10−4 mbar or lower. The temperature of the walls of the heated channel are related to the vapour pressure of the material and the pressure in the channel. The use of a heated channel results in that coating material will not deposit on the wall but is redirect onto the strip and given a second or third or fourth change to deposit on the metal strip.
Vapour distribution sections are disclosed in EP1902152 and in “Jet vapor deposition a novel vacuum coating technique with superior properties”, B. Schmitz, Revue de Metallurgie, Vol. 97, Nr. 7/8, p. 971-978, 2000.
The use of a heated channel also allows to operate the PVD coating process at an increased pressure, which is about three orders of magnitude higher than the conventional pressure. The advantage of such a higher pressure is that the number of conventional air lock sections at the entry and exit for the metal strip can be reduced by a number of sections. An air lock with multiple sections is disclosed in EP1627096.
A metal strip also needs to be cleaned and activated prior to the coating deposition process. This cleaning is most frequently done with a plasma sputtering technology, like for example with magnetron sputtering. Another cleaning method that could be applied involves the use of an electric arc discharge device as disclosed in WO2008074312. In WO2012091409 the cleaning and coating of advanced high strength steels is disclosed, wherein oxides present on the surface of the steel are removed by means of plasma, plasma arc and or laser cleaning before applying a coating on such a steel.
However, all these cleaning methods suffer from the major problem, that the residues coming from the sputtered layer will inevitably contaminate the deposition chamber. The oxide layer on the steel strip can have a thickness from 1 to 50 nanometres. In some cases and especially for Advanced and Ultra High Strength Steels the oxide layer can be from several tens to several hundreds of nanometres.
Such amounts of residues result in a contamination of the deposition chamber that completely eliminates all the advantages obtained by using a combination of a vapour distribution box and a heated channel.
It is an objective of the present invention to provide a method and an apparatus for the cleaning and the coating of metal strip, wherein contamination is prevented as much as possible.
It is another objective of the present invention to provide a method and an apparatus for the cleaning and the coating of metal strip, wherein it is prevented as much as possible that residues of the cleaning process enter the deposition chamber and as such influences the coating deposition or coating properties.
It is another objective of the present invention to provide a method and an apparatus for the cleaning and the coating of metal strip, wherein residues of the cleaning process are removed from the apparatus.
It is another objective of the present invention to provide a method and an apparatus for the cleaning and the coating of metal strip, wherein the pressure for the cleaning of the metal strip is higher than in the conventional PVD process.
It is another objective of the present invention to provide a method and an apparatus for the cleaning and the coating of metal strip, wherein the pressures of the cleaning process and the coating process are of the same order or do not differ more than one order of magnitude.
It is another objective of the present invention to provide a method and an apparatus for the cleaning and the coating of metal strip that allows operating the PVD coating process of a metal strip at reduced costs in comparison with the conventional PVD coating process.
It is still another objective of the present invention to provide a Zn-based coated steel strip product with superior adhesive bonding properties in comparison with a Zn-based coated steel strip product, produced at conventional deposition chamber pressure.
It is yet another objective of the present invention to provide a Zn—Mg coated steel strip product with superior adhesive bonding properties in comparison with a Zn—Mg based coated steel strip product, produced at conventional deposition chamber pressure.
According to a first aspect of the invention one or more of the objectives of the invention are realized by providing a method for cleaning and coating a metal strip comprising the steps of:
Providing that the cleaning of the metal strip and the coating of the metal strip is carried out in separate chambers contributes to keeping the contamination of the coating chamber to a minimum.
A further advantage of the method is obtained by using low to medium vacuum for the cleaning chamber and coating chamber, which is easier to provide and easier to maintain than the high vacuum of the order 10−4 mbar, typically applied for metallic coating using PVD. Moreover, the pressure in the cleaning chamber and deposition chamber can be of the same order which has the advantage that there is no, or only limited need for air locks for the passage from the cleaning chamber to the deposition chamber.
According to a further aspect of the method it is provided that the metal strip is cleaned by using a plasma cleaning technique. Such plasma cleaning could for instance be done by means of a magnetron sputtering device, an electric arc discharge device or an dielectric barrier discharge device. Preferably one of the latter two devices are used in the method since these can be used at higher pressures than magnetron sputtering. Cleaning by means of an electric arc discharge device is disclosed in WO2008074312. Although these devices could even be used at atmospheric pressures, it is preferred to use these in a vacuum to avoid the vapour productivity disadvantages typically associated with atmospheric pressure.
According the invention it is further provided that a gas stream is maintained through the cleaning chamber to remove residues resulting from the cleaning of the metal strip before the metal strip enters the deposition chamber. The gas stream is kept below a certain limit such that it does not negatively influence the plasma cleaning. While using a cleaning device that operates in medium to low vacuum it will be easy to maintain the vacuum and at the same time have a certain gas flow through the cleaning chamber to remove residue resulting from the cleaning operation. For this purpose the cleaning chamber is provided with a gas inlet connected to gas supply means, while vacuum pump means take care of the removal of the supplied gas. In this respect “gas” means for instance dry air, nitrogen or argon or any other suitable gas which does not affect the surface of the metal strip.
Preferably a gas bearing lock is used between the cleaning chamber and the deposition chamber. The advantage of providing a gas bearing lock is that there is no direct contact between the gas bearing lock and the strip. Furthermore, the required pressure of the gas bearing lock and its working principle will generate a gas stream that will improve the removal of residue resulting from the cleaning operation. Gas bearing locks are known in the art, see for instance WO2007/016688. Such a gas bearing lock further provides that if necessary a difference in vacuum pressure between the cleaning chamber and deposition chamber can easily be maintained.
According to a further aspect of the invention an intermediate chamber is provided between the cleaning chamber and the deposition chamber with gas bearing locks connecting the intermediate chamber to the cleaning chamber and the deposition chamber. With the use of an intermediate chamber with gas bearing locks on both sides a separate chamber is obtained which is used to remove residues coming from the cleaning operation and which prevents that any of these residues enters the deposition chamber. Moreover the pressure in the intermediate chamber can be adjusted to any difference in pressure between cleaning chamber and coating chamber. The size of the intermediate chamber could be far less than the size of the cleaning chamber and/or the deposition chamber, therewith allowing to easily maintain a certain pressure in the intermediate chamber. Moreover, the removal of residues can easier be achieved with a relatively small intermediate chamber.
According to a further aspect of the invention the method comprises that the metal strip is mechanically cleaned before entering the deposition chamber. With mechanical cleaning is meant that the surface of the metal strip is treated with scraper means, brush means or any other like mechanical means. Mechanical cleaning is carried out after the plasma cleaning step and is aimed at removing residue that is still present on or attached to the surface of the metal strip.
According to a further aspect of the invention the method comprises that residues still remaining on the metal strip are removed by subjecting the metal strip to a stream of pressurised gas inside the intermediate chamber. Such a stream of pressurised gas in not only effective in removing residue still present on the metal strip but can also be used to effectively cool the metal strip. This is in particular necessary when thick layers of oxide have been removed from the strip as a consequence of which the strip temperature has increased considerably.
The mechanical cleaning is done either in the cleaning chamber or in the intermediate chamber. Preferably the metal strip is mechanically cleaned inside the intermediate chamber which allows for an easy removal of the residue making use of the vacuum pump system of the gas bearing lock.
The method further provides that the metal strip is activated before a coating is applied. Activation of the metal strip is in fact a low intensity cleaning of the metal strip. Such a treatment is necessary to provide a surface with sufficient bonding properties for some Zn-based coatings like Zn—Mg coatings. The activation of the surface of the metal strip can take place in the intermediate chamber or in the deposition chamber. The latter is allowed because the activation is a low intensity cleaning step with hardly any resulting residue. It is important that the activation is done close to the location where the coating is applied on the metal strip. Therefore it will be dependent on the dimensions of the intermediate chamber and the deposition chamber if the activation step is carried out in the intermediate chamber or in the deposition chamber.
The invention further provides an apparatus for cleaning and coating a metal strip provided with:
According to a further aspect of the invention the cleaning chamber is provided with means to provide a gas stream through the cleaning chamber and maintaining a pressure in the range of 0.01-100 mbar, preferably in the range of 0.1-50 mbar. By cleaning within this pressure range a gas stream can be maintained through the cleaning chamber to remove residues resulting from the cleaning of the metal strip before the metal strip enters the deposition chamber. At the same time the desired pressure can be maintained, especially within the higher part of the pressure range, rather easily without the need of pumps specifically designed for high-vacuum purposes. With this embodiment the cleaning chamber could be connected directly to the deposition chamber. If both chambers are operated at about the same pressure the connection could be a simple slit shaped opening. With a difference in pressure an air lock or a gas bearing lock could be used to prevent any pressure equalisation between chambers.
A further embodiment according to the invention provides that the connection between cleaning chamber and deposition chamber includes an intermediate chamber provided with air locks on opposite sides. With such an intermediate chamber any pressure difference between cleaning chamber and deposition chamber can be maintained without difficulty. Moreover, with the use of such an intermediate chamber it can be prevented that almost any contamination from the cleaning chamber, such as residue resulting from the cleaning operation, reaches the deposition chamber.
A further elaboration of the invention provides that at least one of the air locks is a gas bearing lock with at least one gas permeable bearing surface with gas supply means and one or more grooves connected to gas pumping means. The grooves and pumping means provide that gas coming from the bearing parts is removed and that a certain pressure is maintained.
In the set-up with an intermediate chamber at least the gas bearing lock at the side of the cleaning chamber is provided with one or more grooves preceding a gas permeable surface. With the grooves preceding the bearing part at least part of the residue coming from the cleaning operation can be removed before it could enter into the intermediate chamber.
According to a further aspect it is provided that the grooves and gas permeable bearing surfaces are provided in opposite pairs of grooves and bearing surfaces. With the gas coming from the opposite bearing parts of such gas bearing lock the metal strip is kept at a distance from the surface of both bearing parts and the metal strip passes through the gas bearing lock without that either side of the strip comes into contact with the gas bearing lock.
Instead of a gas bearing with opposite pairs of grooves and gas permeable surfaces it is also possible to use a configuration with a roll at one side of the strip and a number of grooves or a number of grooves and gas permeable surfaces at the other side of the strip. Such a configuration could be appropriate between the intermediate chamber and the deposition chamber at which point any possible residue is already removed from the strip.
According to the invention a brush is provided at the location of the one or more pairs of opposite grooves in between successive pairs of opposite gas permeable surfaces in order to mechanically remove any residue still present on or adhering to the metal strip.
According to a further aspect of the invention a pressurised gas supply is provided and inside the intermediate chamber means to guide a stream of pressurised gas at the surface of the metal strip. Such a stream of pressurised gas in not only effective in removing residue still present on the metal strip but can also be used to effectively cool the metal strip. This is in particular necessary when thick layers of oxide have been removed from the strip as a consequence of which the strip temperature has increased considerably.
Next to the stream of pressurised gas supply at least one outlet channel is provided for the removal of supplied pressurised gas or supplied pressurised gas together with residue.
The gas bearing lock between the intermediate chamber and the deposition chamber is the transition from the pressure inside the intermediate chamber and the deposition chamber. At the same time if still any of the residue from the cleaning operation reaches this stage it will be removed before entering the deposition chamber. At this point also a standard air lock could be used, the more so if there is only a small pressure difference between the intermediate chamber and the deposition chamber.
This example describes the tests conducted to investigate the influence of air bearings on an activated and cleaned strip. To this end the strip was firstly cleaned in a roll to roll batch chamber at an operating pressure of 5·10−4 mbar using a magnetron sputtering unit operating at a power density of 200 kJ/m2. The magnetron sputter unit was used to clean/activate and to heat the strip. From a cleaning and activation perspective a cleaning density of >40 kJ/m2 is sufficient. After the cleaning, the steel strip is coiled and the pressure in the chamber is increased to a pressure in the range from 0.1 to 5 mbar with either, argon, nitrogen or dry air. This is done while the booster and the roots pump are still on to maximise the gas flow through the deposition chamber. Then the coil is unwound travelling for a certain period in this increased atmosphere which results in gas atoms bouncing onto the strip, the same will happen when an activated strip passes through a gas bearing lock. Finally, in the same atmosphere zinc and zinc-magnesium coatings of 3 micron are deposited onto the strip. The adhesion of these samples is tested using a crash adhesion test (BMW AA-M223). The results are compared to adhesion tests of samples produced after sputtering at 5·10−4 mbar and directly coating the cleaned/activated steel strip at this pressure. The results show that for magnesium containing coating the adhesion deteriorates and no acceptable adhesion can be achieved. For zinc containing coating the adhesion is okay for all the tests samples and the contact with the gas atoms do not change the adhesion behaviour of the zinc coating.
The invention is further explained on hand of the figures in which:
In
One advantage of the method according to the invention is the application of the vapour distribution box, with which a vapour yield of 99% could be realised and which has further been improved with the use of a heat channel. With such a channel the metal coating atoms that initially bounce off the strip are given a second chance to condense on the metal strip. In the description the term “heat channel”, is used interchangeably with the terms “hood”, “heated hood” and “heat box” and will all mean an envelope forming a space connecting the vapour distribution section and the metal strip which is to be coated. With such an envelope the vapour flow from the vapour distribution section is largely restricted to flow directions directly to the strip surface. Where the metal strip is coated on both sides the “hoods” on either side of the metal strip are preferable connected to form a box, wherein the box is provided with an entry and exit slit for the metal strip.
In
In
In the cleaning chamber 2 plasma cleaning means 9, 10 are provided such as electric arc discharge means or dielectric barrier discharge means. Cleaning by magnetron sputtering is also possible, but the other cleaning techniques are preferably used since these can be operated at much higher vacuum pressures than magnetron sputtering. These techniques can be used at pressures in the range of 0.01-100 mbar and at even higher pressures.
The cleaning chamber 2 is provided with a gas inlet 15 to supply gas into the cleaning chamber 2 and with pumping means to remove the gas from the cleaning chamber 2, by means of which a sufficient gas flow through the cleaning chamber 2 is realized to remove residue resulting from the cleaning operation from the cleaning chamber 2. The supply of gas is limited such that a certain pressure is maintained inside cleaning chamber 2. The gas used could be dry air, nitrogen or argon. In the set-up of
In the deposition chamber 4 on both sides of the metal strip 11, for instance a steel strip, a vapour distribution section 12 is provided. The vapour distribution section 12 is provided with nozzles and/or slits as to cover the total width of the metal strip 11. A hood 13 is connected to the vapour distribution section 12 and heating means 14 are provided to heat the hood. If the metal strip 11 is to be coated on both sides the hoods 13 on both sides are preferably connected to each other to form a box, wherein the box is provided with entry and exit slits for the metal strip.
A vapour chamber (not shown in the drawing) wherein a metal is heated to generate a coating vapour is connected to the vapour distribution section 12. The vapour distribution section 12 is operated such that the vapour leaves the vapour distribution section 12 through the nozzles at sonic speed. The hood or heat box is heated to reduce the deposition of the vapour on the heat box. To assure that no contamination is occurring the hood or heat box is heated to a temperature that is equal to or larger than the saturation vapour temperature of the deposited material that corresponds to the pressure of the vapour in the head box or hood. For a Zn-coating the hood or heat box is heated in the temperature interval between 330 and 580° C., this is roughly the Zn temperature range that coincides with a vapour pressure between 0.01 and 10 mbar. The exact temperature of the heat box will also be determined by the maximum allowable strip temperature.
In
With a conventional air lock the strip 11 has to pass one or more sections with rolls and if there is still residue on the strip or adhering to the strip, the residue will be rolled into the strip which may give rise to certain surface defects in the final product. With the set-up according to
In
Inside the intermediate chamber a pressurised gas supply 19 is provided which is used to remove any residue still on the surface of strip 11. The gas supply may comprise a long slit extending over the width of the strip or a number of nozzles. This manner to remove residue from the strip brings a lot of gas into the intermediate chamber and in order to remove that gas outlet channels 18, 18′ are provided at both sides of the pressurised gas supply 19. As far as necessary pumping means may be provided to improve the removal of used supply of pressurised gas. Together with the used gas the residue that is blown of the strip is removed. Through these outlet channels 18,18′ also part of the gas coming through the gas bearing parts 17,20 is removed. The amount of residue removed through outlet channel 18 may be larger than the amount that is removed through outlet channel 18′ because of the location with respect to the cleaning chamber 2.
The grooves 16, 21, outlet channels 18,18′ and gas bearing parts 17,20 continue across the width of the metal strip 11.
The gas supplied through the pressurised gas supply 19 can also be used to cool the strip. Cooling of the strip will be necessary if the temperature of the strip as a result of the cleaning operation has risen above a value that is still acceptable to apply a coating on the strip or where the temperature has risen such that the properties of the strip are altered.
In
Instead of providing that the complete deposition chamber is formed as a heat box, it is also possible to provide that only part of the deposition chamber facing one side of the metal strip is formed as a heated hood. This would be a suitable embodiment for metal strip that is to be coated on one side only.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15020138.2 | Aug 2015 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/069505 | 8/17/2016 | WO | 00 |
