The invention to which this application relates is to the provision of a coating which has particular benefits when applied to a surface of a plate to be used in a printing proces s, with the plate surface including etchings therein for use in the printing process . In particular, although not neces sarily exclusively, the plate is for use in the intaglio printing process in which the image to be printed is etched into the plate surface and the etched portions are filled with ink and any exces s ink is wiped from the plate surface prior to the printing of the etched image onto a sheet of material such as paper.

The use of printing proces ses such as the intaglio process is well known, as is the use of the intaglio proces s for the high security printing of sheet material such as the printing of banknotes and packaging for ant-counterfeiting purposes, and printing of high definition images.

Over time of use of the printing plates, which are typically formed of, for example, nickel alloys or copper plated stainless steels, the same can become worn and the surfaces of the same can collect residue from the ink and/or other substances which may be left on the same due to insufficient wiping clean and/or portions of the ink remaining on the plate following the printing process. Over time the build up of residue can cause imperfections in the printed sheets which are applied such that the printing plate may eventually have to be redefined in terms of the etched image and/or cleaned more thoroughly, or even replaced for refurbishment, all of which can cause a delay in the printing procedure and/or the plate may need to be discarded. Additionally, in multiple processes where individual colours are applied sequentially, such degradation may result in gradual changes to the colour of the resulting image. Mechanical damage to the plate, including scratches and the trans fer of micro particle inclusions from the ink, can also result in image degradation, both in terms of definition and colour.

It is known to apply a coating on at least the surface of the plate to which the ink is applied and in which the etched image is formed and the known coatings are formed of chromium and applied by electroplating. However, this coating, whilst effective in protecting the plate surface, is found to be environmentally unfriendly due to the pollutants which are created during the coating proces s and/or the subsequent discarding of the plates . Attempts at using other coatings have so far failed due to the specific problems relating to the plates and the coating of the same. These problems include the fact that the plates are required to be flexible during installation and use. Flexibility may also be required when mounting the plates in the coating apparatus if they require to be held in a curved shape on a holder and this can cause unacceptable stres s in a coating applied to the surface, resulting in cracking and ultimately delamination from the surface. A further problem is that a wiping action is required to be repeatedly performed across the surface in order to remove any exces s ink and this wiping action can have an abrasive effect on the coating material such that relatively quickly the coating and any benefits therefrom are removed. A yet further problem is to ensure that the coating applied is resistant to potential corrosive effects of the inks which are applied to the same whilst ensuring that the accuracy of the etched image is maintained. High pres sures during the printing proces s will cause repeated flexure of coated plate, which will induce fatigue failure. It is therefore known that while the coating applied must be relatively tough, the same must have good adhesion to the plate and cannot be too hard or brittle.

The aim of the present invention is therefore to provide a coating for application to a plate for use in a printing process and which coating allows the longevity of the usage of the plate to be maximised whilst ensuring that the clarity o f the image on the plate and the print which is achieved is at least maintained at an acceptable level during the desired service of the plate.

In a first aspect of the invention there is provided a plate for use in a printing procedure, said plate having a coating applied by a physical vapour deposition technique to at least one face thereof, said coating may include, but is not limited to, a metal nitride layer.

In an alternative embodiment the coating includes a metal nitride but also includes other materials, said materials applied to provide certain desired characteristics for the coating.

In one embodiment the coating includes a layer in contact with the surface of the plate which acts as an adhesion layer for the coating. In one embodiment this layer is of chromium or a chromium alloy such as so called nickel chrome.

In one embodiment the coating layer includes any or any combination of Cr ₂ N, CrN, Ni ₃ N, and a nitride of a nickel chromium alloy Ni _x Cr _y N ₂ . also referred to herein as NiCr(N) .

Preferably the metal nitride includes nickel and in a preferred embodiment the material is Nickel Chromium Nitride (NiCr(N)) .

In one embodiment the coating comprises an adhesion and corrosion resistance promoting Cr layer or a Cr/NiCr layer followed by a blended Cr-NiCr layer followed by a CrN/NiCrN layer and then a NiCr(N) layer which forms the external surface of the coated plate.

In one embodiment the surfaces to be coated are provided to come into contact with an ink and means to apply the ink and remove any exces s ink therefrom as part of a wiping action. Typically at least those surfaces of the plate which are to be wiped and/or have ink applied thereto are provided with the coating.

Typically the surface of the plate to which the coating is applied includes an etched image therein which is to be applied to a sheet material in a printing process .

In one embodiment the sheet material, once printed, is further proces sed to form one or more banknotes .

In one embodiment the printing proces s is an intaglio printing proces s.

Typically the coating is applied using a Physical Vapour Deposition (PVD) coating process and more preferably a reactive Closed Field Unbalanced Magnetron Sputter Ion Plating proces s.

In a further aspect of the invention there is provided a coating applied to a surface of a plate for use in a printing proces s to form an external surface thereof, said coating including an outer surface of a metal nitride, said metal nitride having a structure with a highly oriented basal plane arrangement.

In one embodiment the metal nitride has a highly oriented configuration in that the maj ority of the basal planes are substantially parallel with the surface to which the material is applied. Preferably the whole coating is so oriented.

In one embodiment the material used to form at least the outer surface of the coating is Nickel Chromium nitride.

In a further aspect of the invention, there is provided a coating applied using Closed Field Unbalanced Magnetron Sputter Ion Plating apparatus and the coating has at least one layer which is of high orientation of the basal planes to lie parallel with the surface of the surface of the printing plate to which the coating is applied.

In one embodiment the coating is multi layered and at least the outer layer of the coating has high orientation of the basal planes.

In one embodiment the at least outer layer includes or comprises Cr ₂ N.

In a further aspect of the invention there is provided a method for the application of a coating to a surface of a printing plate on which an image is etched, said coating including a metal nitride, said method comprising the steps of sputtering nickel or a nickel alloy in a nitrogen atmosphere to produce a metal or alloy nitride structure on the surface to be coated, said material sputtered from a NiCr target in a nitrogen atmosphere, and characterised in that the coating is applied in an oriented manner such that more than 50% of the basal planes of the structure are substantially parallel to the surface of the printing plate to which the coating is applied.

Typically, any of the coatings, Cr ₂ N, Ni ₃ N, NiCr(N), deposited using reactive CFUBMSIP in accordance with the invention, are smooth and have dense structures, and are tough with requisite hardness and corrosion resistance.

In one embodiment the coating application sputtering method produces smooth external surface with a dense form and with a minimum areal density of defects. Typically the Closed Field arrangement of Unbalanced magnetrons in the apparatus gives very high ion current densities during in situ bombardment cleaning and during the coating proces s hence producing the very adherent, dense coatings. In one embodiment the coating is applied to a thicknes s in the range of 0.5-5 microns.

In one embodiment the coating is applied at a temperature in the range of 100 to 400 degrees Celsius.

A specific embodiment of the invention is now described with reference to the accompanying drawings; wherein

Figure 1 illustrates a printing plate of the type in relation to which the current invention may be utilised;

Figure 2 illustrates a cros s section along line AA of the printing plate of Figure 1 in accordance with one embodiment of the invention; and

Figure 3 illustrates apparatus suitable for applying the coating in accordance with the invention.

Figures 1 and 2 illustrate a plate for use in a printing proces s in accordance with the invention. The plate 25 comprises a first surface 26 on which is formed an etched image 28 which is filled with ink during the process and which allows the image to be printed onto sheet material, in this case using the known intaglio printing procedure. In this example, the image shown is that of a face of a banknote. It is this surface 26 which is required to be coated in order to allow the life of the plate to be increased relative to a non-coated surface or a surface which has an inferior coating applied thereto.

The coating is shown in more detail in Figure 2 and in a larger scale than in use, for ease of illustration, in which there is shown the surface 26 on which the coating is applied. The coating comprises a first layer 30 of Cr alone or Cr/NiCr followed by a blended layer 32 o f Cr-NiCr followed in turn by a layer 34 of CrN/NiCr(N) and then a final external layer 36 of NiCr(N) which forms the external face 38 of the coating.

Figure 3 illustrates apparatus referred to as a closed field unbalanced magnetron sputter ion plating apparatus which is subject to protection in the applicant's patent GB2258343 and reference is hereby made to the contents of that patent.

In Figure 3 there is illustrated the apparatus in plan and in a schematic manner in a form which can be used for a batch coating proces s. However, in-line coating proces ses may equally be used to apply the coating in accordance with the invention. In this apparatus there is provided a coating chamber 2 in which there is disposed two sets of opposing unbalanced magnetrons 4,6; 8,10. Each of the unbalanced magnetrons include a target 12 of suitable material to allow the coating to be formed. The magnetrons are arranged to operate in a closed field configuration in that the magnetrons are arranged around a carrier 9 in the chamber 2 which is centrally spaced relative to the magnetrons . The magnetrons are arranged so that adj acent magnetrons have outer magnetic assemblies 14 of opposite polarity, to those adj acent thereto hence allowing magnetic field lines to link adj acent magnetrons so as to produce a substantially closed ring of magnetic flux which substantially traps all electrons generated in the system and increases the level of ionisation surrounding the carrier. It should be appreciated that some of the unbalanced magnetrons may be replaced by arrays of magnets with suitable magnetic configuration to retain the closed filed of magnetic flux in the coating chamber. In another embodiment, as long as at least one magnetron is of opposite polarity to the remaining magnetrons and/or magnetic assemblies in the coating chamber then benefits of the closed field configuration still apply, in accordance with the teaching in the applicant's patent GB2258343. The carrier 9 is provided in the form of a drum driven to rotate about axis 13 and on the external surface 1 1 of which can be mounted the devices or items to be coated. The devices are mounted such that the surface of the same which is to be coated, faces towards the unbalanced magnetrons and magnetic assemblies, if present.

Inlet means are provided to allow the selective introduction of working gases into the chamber and pumps are provided in order to allow the pres sure in the chamber to be controlled.

In operation, power is applied to the magnetrons under controlled conditions via power supplies 20, 21 , 22, 23 rotating carrier receive coating flux material from the targets as they pass the same and as a result the layer of the material is built up to the required thicknes s of coating. The carrier is insulated from the body of the machine to allow it to float electrically (or for a bias voltage to be applied) .

In accordance with the invention, at least one, but typically two Cr targets are utilised 12' and an NiCr material target or targets 12" is/are utilised in the closed field arrangement. By the selective operation of the deposition means a multilayer coating is applied to the surface or surfaces of the item to be coated and which item is positioned within the coating apparatus to be exposed to the deposited material. A suitable reactive gas to be introduced into the coating chamber in this case would be nitrogen in addition to the inert gas, Ar. In this example a coating structure of CrN/NiCr(N) CrN/NiCr(N) is deposited by the sequential operation of the deposition means to cause the sputtering of material from either set of targets 12' or 12" to form the inner and outer layers respectively and co-sputtering of targets 12', 12" of required to form the middle layer. Thus in accordance with the invention there is provided a CrN containing coating which is applicable to printing plates. In an alternative embodiment a Ni ₃ N coating can be applied or yet further a NiCr(N) coating.

As described above by the selective operation of the coating apparatus coatings comprising a series of layers can be applied to form the coating, such as those comprising CrN and Ni ₃ N layers or CrN and NiCr(N) layers, or multilayer coatings comprising CrN, Ni ₃ N and NiCr(N) layers.

It should therefore be appreciated that in accordance with the invention, any hard metal nitride, carbo nitride, oxide, or oxynitride can be utilised to advantage in accordance with the invention.

A specific example of the invention is now described in a non limiting manner.

The plates to be coated are mounted, and typically "wrapped" around a cylindrical drum in a coating chamber in which there are provided a series of unbalanced magnetrons arranged in a closed field configuration. The magnetrons include targets of Cr and NiCr. A plasma cleaning operation is performed initially to clean the surface which is to be coated with Ar gas being introduced into the chamber.

Material is deposited firstly from the Cr target or from a combination of Cr and NiCr to apply the initial layer onto the surface of the plate which is to be coated and this is found to improve adhesion of the coating to the surface and reduce stres ses in the coating which is formed. This is then followed by the combined operation of the apparatus to provide a blended coating to be formed from the material from both targets and to allow a graded coating to be formed. The pressure in the coating chamber is in the region of 1.0E-3 torr, with the thicknes s of the coating being in the range of 0.5- 5 microns.

Typically a bias voltage is applied to the plates during the coating application and a temperature in the region o f 350degrees Celsius is maintained during the coating application. The power to operate the targets is preferably a switched-mode DC supply. The bias power to the plate is preferably a pulsed switched-mode DC supply which is found to provide improved coating enhancement.

During the coating procedure the introduction of nitrogen gas into the coating chamber is controlled using a feedback loop control.

The coating which is applied has a hardnes s in the range of 1000- 1 800Hv, measured using a ultra-micro hardness dynamic hardnes s tester, such as a Fischerscope EM300 instrument operated with a peak load of 50mN.

Typically the multi layered coating tends to exhibit enhanced in comparison with single layer coatings . As the wiping action on the coating, the corrosive nature of the ink applied thereto and/or the flexing of the plate can all cause damage to the coating, the combination of hardnes s, wear resistance properties and ability to withstand stres ses of the coating applied in accordance with the invention are important advantages.