GB2557580A

GB2557580A - Tablet tool coating

Info

Publication number: GB2557580A
Application number: GB1618965.6A
Authority: GB
Inventors: John Blanchard Robert; Kenneth Southall Wayne
Original assignee: Teer Coatings Ltd; I Holland Ltd
Current assignee: Teer Coatings Ltd; I Holland Ltd
Priority date: 2016-11-10
Filing date: 2016-11-10
Publication date: 2018-06-27
Anticipated expiration: 2036-11-10
Also published as: GB2557580B

Abstract

A method for coating a steel tablet tool or punch comprises applying a chromium and nitrogen coating using closed field unbalanced magnetron sputtering at a maximum process temperature of 180oC. The coating preferably comprises at least three coating layers. An inner layer may be formed of chromium only. A second layer may have a nitrogen concentration that increases outwardly through the layer. An outer layer may have a uniform concentration of nitrogen throughout. Sputtering is preferably performed using four magnetrons 12 of which two have chromium targets. The tablet tool is preferably held in a jig 16 that may be rotated about two axes during coating. Sputtering may take place in a reduced pressure atmosphere and may take place in a gaseous atmosphere comprising argon 20. The coating formed by the method has a columnar structure without significant voids between the columns (figure 2). Also disclosed is a coated tablet tool that is preferably made of cold formable tool steel.

Description

(71) Applicant(s):

I Holland Limited (Incorporated in the United Kingdom)

Meadow Lane, Long Eaton, NOTTINGHAM,

NG10 2GD, United Kingdom

Teer Coatings Limited (Incorporated in the United Kingdom)

West Stone House, West Stone,

Berry Hill Industrial Estate, DROITWICH, WR9 9AS, United Kingdom (72) Inventor(s):

Robert John Blanchard Wayne Kenneth Southall (74) Agent and/or Address for Service:

Swindell & Pearson Ltd

Friar Gate, DERBY, DE1 1GY, United Kingdom (51) INT CL:

C23C 14/35 (2006.01) C23C 14/06 (2006.01) (56) Documents Cited:

CN 105624623 A

Surface and Coatings Technology, Vol. 309, October 2016, Kabir et al., Structure and mechanical properties of graded Cr/CrN/CrTiN coatings synthesized by close field unbalanced magnetron sputtering, pp. 779 - 789.

Wear, Vol. 290, April 2012, Alegria-Ortega et al., Erosion corrosion wear of Cr/CrN multi-layer coating deposited on AISI-304 stainless steel using the unbalanced magnetron (UBM) sputtering system, pp. 149-153.

(58) Field of Search:

INT CL C23C

Other: EPODOC, WPI, PATENT FULLTEXT, XPESP, INSPEC, XPSPRNG (54) Title of the Invention: Tablet tool coating

Abstract Title: Methods for coating tools by closed field unbalanced magnetron sputtering (57) A method for coating a steel tablet tool or punch comprises applying a chromium and nitrogen coating using closed field unbalanced magnetron sputtering at a maximum process temperature of 180°C. The coating preferably comprises at least three coating layers. An inner layer may be formed of chromium only. A second layer may have a nitrogen concentration that increases outwardly through the layer. An outer layer may have a uniform concentration of nitrogen throughout. Sputtering is preferably performed using four magnetrons 12 of which two have chromium targets. The tablet tool is preferably held in a jig 16 that may be rotated about two axes during coating. Sputtering may take place in a reduced pressure atmosphere and may take place in a gaseous atmosphere comprising argon 20. The coating formed by the method has a columnar structure without significant voids between the columns (figure 2). Also disclosed is a coated tablet tool that is preferably made of cold formable tool steel.

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

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Fig.l

Fig. 2

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Fig. 3

Tablet Tool Coating

This invention concerns a method of coating a steel tablet tool, and a tablet tool coated by such a method.

In tablet presses a number of tools or punches are provided which are used to press powder into a tablet of a required shape, with a required finish and perhaps also a marking formed by the tip of the tool. It is often required to provide a coating on the tool to increase the longevity of the tool, to reduce corrosion of the tool, and/or to avoid sticking of the powder to the tool.

One method of coating steel tablet tools is to provide hard Chromium by electro plating. Such a coating tends though to embrittle the steel. The embrittlement can reduce the tablet tools ability to withstand cyclic loading, and therefore reduces the maximum press force. A reduction in press force can cause problems in compression by restricting the hardness of any tablet formed. The waste product from Chromium electro plating is carcinogenic and can therefore involve difficulties and cost with disposal.

Such a Chromium coating can also suffer from micro cracking that can increase the coating porosity, thus restricting the coating’s ability to act as a corrosion barrier, particularly during use or cleaning of the tools.

Physical Vapour Deposition (PVD) is another method for applying such coatings. Typically such a process will be carried out at a relatively high process temperature within the range of 300°C to 500°C. Accordingly such a process cannot be used with some materials which cannot be coated at such temperatures such as for instance cold formable tool steel.

One approach is therefore to carry out the process in a number of distinct phases such that once the process temperature reaches a set level, the process is halted and the apparatus and punches are allowed to cool down. This will however not result in a uniform coating and can lead to areas of potential weakness or separation.

Typically with low temperature physical vapour deposition, a columnar structure is formed, as shown diagramatically and in an SEM image in Fig. 1. Such a formation can tend to break up between the upper and lower parts of the columns. This formation will also increase the amount of water absorption and therefore sticking of the powder to the punch during operation. Wear resistance of the coating will also be lower than that for a denser structure.

A particular physical vapour deposition technique is closed field unbalanced magnetron sputtering, as described in for instance UK patent specification no. 2462890 and European patent specification no. 0521045. This technique can be used either for reactive or non reactive coatings. This technique has typically been used at relatively high process temperatures such as 450°C, and has not therefore been useable with materials such as cold formable tool steels.

According to a first aspect of the invention there is provided a method of coating a steel tablet tool, the method comprising applying a Chromium and Nitrogen coating using closed field unbalanced magnatron sputtering, with a maximum process temperature of not more than 180°C.

At least three coating layers may be provided on the tool.

A first inner coating layer may be formed just of Chromium. The first inner layer may have a thickness of between 0.1 pm and 0.75pm.

A second mid coating layer may be formed, with the amount of Nitrogen in the coating material increasing outwardly through the layer.

The second mid layer may have a thickness in the range of 0.1 pm to 0.75pm. The minimum concentration of Nitrogen in any part of the second mid layer may be 5%, and the maximum Nitrogen concentration in any part of the second mid layer may be 65%.

A third outer coating layer may be provided with a substantially uniform amount of Nitrogen throughout the layer.

The thickness of the third outer layer may be between 1,0pm and 6pm.

The Nitrogen concentration in the third outer layer may be between 40 and 65%.

The sputtering may be carried out with two of four magnetrons with Chromium targets.

The tablet tools are preferably held in a jig during coating, and the jig may hold the tablet tools about a central part thereof.

The jig may be rotated during coating, and may be rotated also at two axes.

A pulsed DC power is preferably applied to the bias, and the pulse voltage may vary between 60V and 75V.

The power applied to the targets may vary between 1.5kW and 2kW. The sputtering may take place in a reduced pressure atmosphere, and the reduced pressure may be between 0.7 χ 10'⁴ torr and 3.0 χ 10'³ torr, and more particularly in a range between 1.0 to 10'⁴ mbar and 1.0 χ 10'³ torr.

The sputtering may be carried out in a gaseous atmosphere, and the atmosphere may be made up of Argon.

An Ion clean of the tool may be carried out prior to providing a coating layer on the tool.

The coating may have a total thickness of between 1.5pm and 6pm.

According to a further aspect of the invention, there is provided a coated tablet tool, the tablet being coated by a method according to any of the preceding sixteen paragraphs.

The tablet tool may be made of cold formable tool steel.

An embodiment of the present invention will now be described by way of example and with reference to the accompanying drawings, in which:

Fig. 1 is a diagrammatic schematic view of a prior art coating and an SEM view of this coating;

Fig. 2 is a similar view to Fig. 1, but of a coating according to the invention; and

Fig. 3 is a diagrammatic plan view of physical vapour deposition apparatus useable with methods according to the invention.

A plurality of cold formable tool steel tablet punches may be coated with a Chromium and Nitrogen coating by the following method. A physical vapour deposition apparatus 10 using closed field unbalanced magnetron sputtering is used. Fig. 3 shows the apparatus 10 with four equispaced magnetrons 12 spaced around a rotatable centre part 14 with four sample holders/jigs 16. The closed magnetic field lines 18 are illustrated, and an argon source 20 is shown.

The jigs 16 are set up to hold the punches about a generally central point of the punches, and a number of test discs and temperature discs are also mounted on the jigs 16. The jigs 16 and contents are cleaned in a solvent cleaning line with ultrasonic cleaning, vapour drying and vacuum drying. The jigs 16 may be rotated as shown during cleaning and/or coating, and may be rotated about two axes.

The jigs 16 and tools are cleaned with an anti static Nitrogen gun, and the chamber had previously been cleaned with this gun. The jigs 16 are located the apparatus 10 with a vacuum chamber, and the pressure is reduced to less than 2.5 x 10'⁵ torr. A “rate of rise” check is made to check for the vacuum seal within the chamber. Prior to the bonding layer being applied an Ion Clean on the punches is carried out to ensure that they are in a clean state prior to the adhesion layer being applied. This is followed by inner bonding layer of pure Chromium which is applied to the tools with no Nitrogen present. This acts as an adhesion layer onto the steel. A first coating layer with a thickness of between 0.1 pm and ,75pm is then formed.

The following parameters are applied to the sputtering during formation of the first coating layer. The target power is between 1.5 kW and 2.0kW. The bias power is between 40V and 70V. The bias pulse settings are between 200kH and 300 kHz and 500 Ns to 1500 Ns. The coating is carried out in a reduced pressure Argon atmosphere, at a pressure of around 1.0 x 10'³ torr.

A second mid coating layer with a thickness of between 0.15 pm and 0.7 pm is formed, in which the amount of Nitrogen is increased outwardly from around 5% to 65%. The conditions in which the second layer is applied by sputtering is with a target power of between 1.5kW and 2kW. There is a biased power of between 40V and 75V. The bias pulse settings are between 200kHz and 300kHz and 500 Ns to 1500 Ns. The Argon atmosphere is again at a pressure of around 1.0 x 10'³ torr.

A third outer coating layer is formed with a thickness of between 1.0 pm and 6.0pm. The third outer coating is produced by sputtering with the following conditions. A target power of between 1.5kW and 2kW, a bias power of between 40V and 70V. The bias pulse settings are between 200 kHz and 300 kHz and 500 Ns to 1500 Ns. The Argon pressure is again around 1.0 x 10'³ torr. Initially the Nitrogen level is set at 90% but is reduced to between 60% and 80% in around 5 to 15 minutes. The coating takes place overall for between 6 and 9 hours, and with these power levels the process temperature within the chamber does not exceed 180°C.

The coating thus formed is shown diagammatically and in an SEM in Fig. 2. This shows that the coating still has a columnar structure but there are no significant voids between the columns, and therefore a denser columnar structure is produced than the prior art lower temperature arrangement shown in Fig. 1. The cold formable tool steel of the tablet tools has a tempering temperature of 200°C, and therefore the steel is not weakened by the temperature of the sputtering technique.

This method has been found to be useable to provide a coating of between 1.5 and 6 microns, with the wear rate of the coating being of a required level of around 1 x 10'¹⁷ m³ N'¹m'¹. The method therefore allows a strong coating to be applied to for instance cold formable tool steel at low temperatures, whilst providing a durable coating. This coating and method therefore allows substrates such as cold formable tool steels which are very fracture resistant and tough, to be provided with a durable dense coating as previously can only be attained at higher temperatures by using for instance electroplating and the resultant problems this can entail.

It is to be realised that a wide range of variations may be made without departing from the scope of the invention. For instance different materials and/or conditions may be applied.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

5 1. A method of coating a steel tablet tool, the method comprising applying a Chromium and Nitrogen coating using closed field unbalanced magnatron sputtering, with a maximum process temperature of not more than 180°C.
2. A method according to claim 1, in which at least three coating layers

10 are provided on the tool.
3. A method according to claim 2, in which a first inner coating layer is formed just of Chromium.

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4. A method according to claim 3, in which the first inner layer has a thickness of between 0.1pm and 0.75pm.
5. A method according to any of claims 2 to 4, in which a second mid coating layer is formed, with the amount of Nitrogen in the coating material

20 increasing outwardly through the layer.
6. A method according to claim 5, in which the second mid layer has a thickness in the range of 0.1pm to 0.75pm.

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7. A method according to claims 5 or 6, in which the minimum concentration of Nitrogen in any part of the second mid layer is 5%.
8. A method according to any of claims 5 to 7, in which the maximum Nitrogen concentration in any part of the second mid layer is 65%.
9. A method according to any of claims 2 to 8, in which a third outer coating layer is provided with a substantially uniform amount of Nitrogen throughout the layer.
10. A method according to claim 9, in which the thickness of the third outer layer is between 1 .Opm and 6pm.
11. A method according to claims 9 or 10, in which the Nitrogen 5 concentration in the third outer layer is between 40 and 65%.
12. A method according to any of the preceding claims, in which the sputtering is carried out with two of four magnetrons with Chromium targets.

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13. A method according to any of the preceding claims, in which the tablet tools are held in a jig during coating.

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14. A method according to claim 13, in which the jig holds the tablet tools about a central part thereof.
15. A method according to claims 13 or 14, in which the jig is rotated.
16. A method according to claim 15, in which the jig is rotated about two axes.
17. A method according to any of the preceding claims, in which a pulsed DC power is applied to the bias.
18. A method according to claim 17, in which the pulse voltage varies 25 between 60V and 75V.
19. A method according to any of the preceding claims, in which the power applied to the targets varies between 1,5kW and 2kW.

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20. A method according to any of the preceding claims, in which the sputtering takes place in a reduced pressure atmosphere.
21. A method according to claim 20, in which the reduced pressure is between 0.7 x 10-4 torr and 3.0 x 10-3 torr.
22. A method according to claim 21, in which the reduced pressure is

5 between 1.0 to 10’4 torr and 1.0 x 10’3 torr.
23. A method according to any of the preceding claims, in which the sputtering is carried out in a gaseous atmosphere.

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24. A method according to claim 23, in which the atmosphere is made up of Argon.

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25. A method according to any of the preceding claims, in which an Ion clean of the tool is carried out prior to providing a coating layer on the tool.
26. A method according to claim 23, in which the coating has a total thickness of between 1.5pm and 6pm.
27. A coated tablet tool, the tablet being coated by a method according to 20 any of the preceding claims.
28. A tool according to claim 27, in which the tablet tool is made of cold formable tool steel.

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29. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.

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Application No: GB1618965.6