WO2019109118A1

WO2019109118A1 - Coating for the surface of an article and process for forming the coating

Info

Publication number: WO2019109118A1
Application number: PCT/AT2018/060283
Authority: WO
Inventors: Katrin ZORN; Parnia Navabpour; Hailin SUN; Joanne HAMPSHIRE
Original assignee: High Tech Coatings Gmbh
Priority date: 2017-12-05
Filing date: 2018-12-04
Publication date: 2019-06-13
Also published as: GB201819818D0; GB2569237A; GB2569237B; GB201720225D0

Abstract

The invention relates to the provision of a coating, an article including the coating and a method for applying the coating which has a conductive layer which includes a noble metal and in one embodiment the said conductive layer has a further layer applied so as to allow the conductive characteristic of the said conductive layer to be exhibited at the external surface of the coating whilst, at the same time, the said further layer provides greater wear resistance for the conductive layer and hence allows the conductive characteristic of the coating and hence the article to which the same is applied to be maintained. In one embodiment the said further layer is applied so as to only partially cover the said conductive layer and thereby provide gaps from the external surface of the coating to the conductive layer.

Description

Coating for the surface of an Article and process for forming the coating

The invention to which this application relates is to the formation of a coating on a surface of an article, the coating and to the article itself once the coating has been applied thereto. The invention also relates to the method of application of the coating and in particular, to the for mation of a coating which, when formed, has conductivity characteristics to thereby allow the coating and the article to which the coating has been applied, be used for a predetermined pur pose.

The provision of articles which are required to have conductive characteristics in order to be able to perform a particular function is well-known and one form of article of this type is a fuel cell electrode or Bi-Polar plate (BP) which can be used as part of a power generation sys tem, for example, for a vehicle. While the core of the fuel electrode or BP can be formed of a conductive material, it is found that the core material if exposed at the outer surface of the ar ticle is subject to corrosion due to the fluids in which the plate is placed during operation. As a result, it is known to provide a coating on the outer surfaces of the article core so as to pro vide a protective effect as the coating which is provided has a greater resistance to corrosion than the material used to form the base core of the article. However, the range of materials which can be used to form this type of coating is limited by the need for the outer surface of the article, once coated, to still have conductive characteristics.

The use of conductive precious metals, such as gold, to form the outer layer of the coating on the article, is known but it is found that an outer layer formed of a precious metal is particu larly susceptible to wear during use of the article and it is believed that the material is gradu ally removed from the surface as a result of electrolysis which occurs during the use of the ar ticle. As such, as the precious metal layer typically has a thickness in the region of 20nm, the Interfacial Contact Resistance (ICR) can increase over time and therefore the effective life of an article coated with an outer surface of precious metal, while effective conductively for an initial period of time, is limited, due to the gradual removal of the precious metal from the coating over time and hence a reduction in the performance. A further problem with the use of the precious metal to form the coating is that the precious metal is expensive and hence the cost of the coating and article to which the same is applied is relatively expensive. An aim of the present invention is therefore to provide a coating, an article with the coating, and a method of applying the coating, which allows the conductivity of the outer surface of the coating, and hence the article to which the coating is applied, to be at or above a required level for a prolonged period of time so as to provide an effective lifetime for the article. This allows the article to be used for its intended purpose whilst providing protection against corro sion and, at the same time, extending the life of the coating and hence increasing the life of use of the article to which the coating is applied. Another aim is to allow for a reduced quan tity of precious metal to be used in the coating whilst at least maintaining, and preferably im proving, the performance of the coating.

In a first aspect of the invention there is provided an article, said article including a base and a coating applied thereto to cover at least part of the outer surface of the same, wherein said coating includes a conductive layer including a noble metal and/or an alloy of a noble metal and a layer of a metal or metal alloy.

In one embodiment the said layer of metal or metal alloy is formed on the surface of the arti cle and formed from any, or any combination, of Titanium, Niobium, Zirconium, Hafnium and/or Rutherfordium.

In one embodiment the noble metal used is any, or any combination, of Gold, Silver or Cop per.

In one embodiment the said conductive layer includes an alloy of at least one of the noble metal and at least one of the metals. In one embodiment the conductive layer includes Tita nium and/or a titanium alloy and gold and/or a gold alloy

In one embodiment, the said conductive layer applied forms the external surface of the coat ing.

In another embodiment a further layer is applied to partially cover the conductive layer, leav ing at least part of the conductive layer exposed so as to be exposed at the external surface of the coating. Typically said portions are spaced apart. In one embodiment the further layer is formed of a metal or metal alloy which is applied onto the said conductive layer and the further layer forms at least part of the external surface of the coating.

In one embodiment the said further layer is formed of Titanium or a Titanium alloy.

Typically, the further layer is applied with a thickness and/or pattern which is such as to allow the conductive characteristic of the said conductive layer to still be effective to a predesig nated conductivity level at the external surface of the coating.

In one embodiment the said further layer is applied to a thickness in the region of lOnm or less and/or with a coverage of the surface of the conductive layer of more than 25% and more preferably 30-80%.

This in contrast to conventional coatings of this type in which the conductive material layer of precious metal would be applied to a greater thickness to prolong the effective life of the coat ing. In contrast, in accordance with the invention, the effective life of the conductive layer is prolonged by applying a different form of conductive layer which may in fact include less precious metal and the precious metal is retained in position by the other materials, such as further metals, which are included in the layer.

In one embodiment the conductivity is in the range of 10 milli-Ohms per square metre.

Typically the said conductive layer is formed as a matrix of the metal materials.

In one embodiment, the materials used to form the conductive layer, are deposited using sput tering techniques. In one embodiment all of the layers of the coating are deposited using sput tering apparatus.

In one embodiment the article is an electrode or Bi-Polar plate. In another aspect of the invention there is provided a fuel cell apparatus including an article with a coating as herein described.

In another aspect of the invention there is provided a coating as herein described.

In a further aspect of the invention there is provided a method for forming a coating on an ar ticle, said method including the steps of applying a first layer of a metal or metal alloy to a surface of said article and applying a conductive layer including sufficient noble metal to pro vide the said layer with conductive characteristic and providing the said conductive layer in a position in the coating so as to provide a conductive characteristic at the external surface of the coating.

In one embodiment said method includes the steps of placing the article into a coating cham ber on a holder, providing a plurality of sputter magnetron assemblies, providing at least one target of a metal and at least one target of the noble metal, said targets provided with respec tive sputter magnetron assemblies to allow the selected sputter deposition of material from the targets onto the article surface to form the coating and wherein, during the formation of at least part of the said conductive layer, said first and second materials are sputter deposited simultaneously.

In one embodiment the method includes the step of controlling the time and/or power supply of operation of the magnetron assemblies for the co-deposition so as to control the composi tion and/or thickness of said conductive layer.

In one embodiment the first material is Titanium and/or an alloy including the same and the second material is Gold and/or an alloy containing the same.

In one embodiment the method includes the step of further operating the sputter assembly with the target of the first material to form a further coating of the same on top of the said conductive layer which, is discontinuous so as to partially overlay the said conductive layer and allow the conductive characteristic to be exhibited at the external surface of the coating or is of a thickness selected so as allow the conductive characteristic of the conductive layer to be exhibited at the external surface of the coating. In one embodiment, the material used to form the further layer is sputtered for a period of time so as to form an irregular layer in that there are gaps in the same which allow the under lying conductive layer to be partially exposed.

Typically, the size of the gaps are such as to allow the conductive characteristic to be exhib ited at the external surface whilst, at the same time, reducing the opportunity for the material of the conductive layer to leave the said coating during use of the article.

In one embodiment, the articles are located on a holder and there is relative movement of the holder and the magnetron assemblies during the coating process.

In one embodiment the relative movement is caused by rotation of the holder or, in another embodiment, the relative movement is caused by linear movement of the holder.

In one embodiment a vacuum is created in the chamber and an inert gas, such as Argon, is in troduced into the chamber.

In one embodiment, when a particular material target is not in use, a mask is provided to pre vent material sputtered from other targets contaminating said target.

In one embodiment the first material is sputter deposited to form an underlayer on the article and onto which underlayer the said conductive layer is applied. In one embodiment the said first material is sputter deposited substantially continuously during the formation of the under layer and the said conductive layer. Typically the continuous deposition of the first material allows a smooth transition between the said underlayer and the conductive layer.

Specific embodiments of the invention are now disclosed with reference to the accompanying drawings; wherein

Figure 1 illustrates a cross section through an article and coating in accordance with one em bodiment of the invention; Figure 2 illustrates a cross section through an article and coating in accordance with a second embodiment of the invention;

Figure 3 illustrates a cross section through an article and coating in accordance with a further embodiment of the invention;

Figure 4 graphically illustrates an accelerated corrosion test on coatings in accordance with the invention;

Figures 5a and b graphically illustrate the Effect of Corrosion Testing on Gold Content and Electrical Conductivity of Coatings formed in accordance with the invention; and

Figures 6a and b illustrate two examples of apparatus which can be used to apply the coating using a method in accordance with the invention.

Referring now to Figures 1-3 there is illustrated cross sectional views through part of an arti cle 2, such as an electrode or bi-polar plate which has a surface 4, onto which a coating 6 is applied and which is show in cross section. In these examples the materials applied are Tita nium (Ti) and Gold (Au) but it should appreciated that other materials previously mentioned could be substituted. In each of the Figures 1-3 the coating 6 is formed in accordance with dif ferent embodiments of the invention and, in each embodiment there is provided a conductive layer 10 which is applied onto a previously applied underlayer 8 of, in these embodiments, Ti tanium. The Titanium underlayer 8 may be applied directly onto the article surface 4 but in other embodiments one or more additional layers may be deposited between the conductive layer 8 and the surface 4 in order to provide improved adhesion of the coating 6 to the surface 4 and/or further characteristics of the coating to suit particular requirements.

The conductive layer 10 in these embodiments is formed by the co- sputtering of Titanium and Gold and in many cases the sputter deposition of the titanium material will continue dur ing the application of the underlayer 8 and the sputtering of the Gold is commenced so as to form the conductive layer 10. The continuous sputtering deposition of the Titanium material allows a relatively smooth transition between the layers 8 and 10 to be achieved. It is found that by sputtering a sufficient quantity of gold to provide a required percentage of the same in the conductive layer 10, so the required conductive characteristics of the layer can be achieved, whilst the Titanium material acts to retain the gold in the layer as an integral part of the same and so allows the Gold to be retained as part of the conductive layer for a signifi cantly greater time than if the layer was formed only of Gold. This therefore means that the conductive layer has a significantly reduced wear rate in comparison to conventional coatings and so the coating has a significantly improved life and resists any increase in Interfacial Con tact Resistance.

Figure 1 illustrates a coating in accordance with one embodiment of the invention in which the layer 10 forms the external surface of the coating. In order to further protect the Gold ma terial from wear it is found that the application of a further, outer, layer 12 of a material such as Titanium is of benefit, as is illustrated in Figures 2 and 3. In Figure 2 the application of the further layer 12 is controlled to be sufficiently thin such that the conductive characteristics of the layer 10 are still exhibited to sufficient extent at the external surface 14 of the coating and indeed the further coating may be discontinuous when applied so that there are holes or pock ets through to the conductive layer 10. The formation of the discontinuities may be due to the relative thinness of the further layer 12 which is applied or the further layer may be applied in a specific pattern.

In Figure 3 it is illustrated how the further layer 12 is applied as a mesh pattern so as to delib erately leave passages 16 at irregular or irregular intervals from the external surface 14 of the coating to the conductive layer and thereby allow the conductive characteristic to be exhibited at the external surface 14 of the coating.

Thus, in accordance with each of the embodiments there is provided the deposition of an ini tial underlayer layer 8 to promote adhesion and corrosion resistance, followed by co-deposi tion of two materials, typically Ti and Au, so that the Au is embedded in the Ti to form the conductive layer 10, and, in the embodiments of Figures 2 and 3, a relatively thin Ti cap outer layer 12 is applied to further restrict and retain the Au within the conductive coating 10.

Thus, there is no longer any requirement to use conventional, relatively thick, high percentage gold coatings and instead an Au containing conductive coating 10 of, for example, 2-4 nm can be as effective in reducing ICR by retaining the Au within the coating and so the conductive performance of the coating 10 will last at least as long as the conventional coatings, as is illus trated in Figures 4 and 5a and b.

In Figure 4 the performance of different coatings in terms of reduction of the Au component present in the conductive coating over time is shown, with graph lines 18 and 20, showing performance of the coatings of Figures 1 and 2 respectively. It is clear that significantly less Au is lost in comparison to a conventional coating of a layer of Titanium and a layer of Au only, which is illustrated is shown by graph line 22. Similarly, the improved performance of the coatings of Figures 1 and 2 in comparison to the same conventional coating indicated by reference numeral 22 is shown in Figures 5a and b.

The coating can be applied using sputtering apparatus 24, two forms of which are shown in Figures 6a and b, a rotary form of apparatus is shown in plan in Figure 6a, and an in-line form of the apparatus is shown in elevation in Figure 6b. In both embodiments, the article 2 to be coated (only one of which is shown in each embodiment for ease of illustration) is mounted on a holder 26. The holder 26 is provided for rotational movement in Figure 6a and for linear movement in Figure 6b as indicated by arrows 28, 30 respectively. The articles and holder are held in a chamber 32 in which a vacuum is created and an inert gas, such as Argon, can be selectively introduced during the coating process. A series of magnetrons are provided, each with a target of material and, in these embodiments, the magnetrons 34 are provided with tar gets of Titanium material and magnetrons 36 are provided with targets of Gold material. The magnetrons 34 are initially operated to allow the sputter deposition of Titanium from the Tita nium targets only to form coating underlayer 8 and this sputtering is continued with the com mencement of operation of the magnetrons 36 to sputter Gold so that the co sputtering of both Gold and Titanium material forms the coating layer 10. In order to form the coatings shown in Figures 2 and 3 the operation of the magnetrons 36 is stopped and the sputtering of magne trons 34 is continued so as to form the outer layer 12 of the coating from Titanium.

A typical example of specific steps for the method of forming the coating is now provided; wherein: In one embodiment a titanium layer is applied to a surface of a base plate and the targets of the magnetrons include one titanium target and one gold target.

The base plates which are to be coated are retained on a holder and are held at an average bias voltage value between -350V and -50V which varies depending on the specific process step. In one embodiment the negative substrate bias can be generated preferably by a pulsed DC power supply to the apparatus used for the coating method.

The first step of the process is an ion cleaning step with a relatively low current of typically 0.45 mAcm ² provided on the magnetron Ti target

A Ti adhesion layer is then deposited with, typically, a current of 6 mAcm^-2 supplied to oper ate the magnetron with the Ti target.

A layer of titanium plus gold is then deposited to form the conductive layer of the coating by causing the simultaneous operation of the magnetrons with the Ti target and the Gold target to sputter deposit material from the same onto the previously applied TI layer of the coating on the base plates.

In on embodiment the method is then completed and the conductive layer forms the external surface of the coating and hence the base plate.

In another embodiment a further layer is deposited from the Ti target operating at a current of 6 mAcm^-2. In order to define the outer layer of the coating and which in one embodiment has gaps therein so as to allow the conductive characteristic of the conductive layer to be exhib ited at the external surface of the coating.

Thus, there is no longer any requirement to use conventional, relatively thick, high percent age, and expensive exclusively noble metal containing coating layers as the conductive per formance of the coatings formed in accordance with the invention will last at least as long as the conventional coatings with significantly less noble metal content. The potential uses of the coating and method in accordance wit the invention are extensive and includes electrolys- ers, fuel cells or any application requiring corrosion resistance and conductivity characteris tics to be provided.

Claims

C l a i m s

1. An article, said article including a base and a coating applied thereto to cover at least part of the outer surface of the same, wherein said coating includes a conductive layer includ ing a noble metal and/or an alloy of a noble metal and a layer of a metal or metal alloy.

2. An article according to claim 1 wherein the said conductive layer is an intermediate layer of the coating or forms the external layer of the coating.

3. An article according to claim 2 wherein the conductive layer is formed by a mixture of said noble metal and another metal or metal alloy.

4. An article according to claim 4 wherein the said other metal or metal alloy is any of titanium, niobium, or zirconium.

5. An article according to any of the preceding claims wherein the said conductive layer is applied onto the layer of metal applied to the article surface and the metal is any of tita nium, niobium and/or zirconium.

6. An article according to claim 1 wherein the total thickness of the coating is 20-100 nm.

7. An article according to any of the preceding claims wherein the said conductive layer is 20-80 % based on the noble metal.

8. An article according to any of the preceding claims wherein the coating includes a further layer which forms an external surface of the coating.

9. An article according to claim 8 wherein the said further layer is formed of a metal or a metal alloy and includes any or any combination of titanium, niobium or zirconium.

10. An article according to claim 9 wherein the said further layer also includes a noble metal.

11. An article according to any of claims 8-10 wherein the noble metal in the said con ductive layer is protected from wear by the inclusion of the said further layer.

12. An article according to any of claims 8-11 wherein the said further layer has a thick ness of 3-10 nm.

13. An article according to any of claims 8-12 wherein the said further layer covers 55- 80%of the surface of the said conductive layer.

14. An article according to any of claims 8-13 wherein the said further layer includes substantially 20% noble metal.

15. An article according to any of claims 8-14 wherein the said further layer has a pattern so as to leave sufficient gaps therein to allow the conductive characteristic of the said underly ing conductive layer to be exhibited at the external surface of the coating.

16. An article according to claim 15 wherein the said pattern is a mesh pattern with regu larly spaced gaps.

17. An article according to any of the preceding claims wherein the article is a bi-polar plate for use as part of a fuel cell.

18. An article according to any of the preceding claims wherein the said coating has an interfacial contact resistance (ICR) of less than 25 mQ centimetre squared.

19. An article according to claim 12 wherein the said coating has an interfacial contact resistance (ICR) of less than 15 mQ centimeter squared.

20. An article according to any of the preceding claims wherein the conductive charac teristic of the said conductive layer is greater than that of the said further layer.

21. A coating for an article wherein said coating comprises a layer applied to the article and a conductive layer applied thereto which includes a noble metal and a further metal.

22. A coating according to claim 21 wherein a further layer is applied to the conductive layer which has gaps therein to allow the conductive characteristic of the conductive layer to be exhibited at the external surface of the coating.

23. A method for forming a coating on an article, said method including the steps of ap plying a first layer of a metal or metal alloy to a surface of said article and applying a conduc tive layer including sufficient noble metal to provide the said layer with conductive character istic and providing the said conductive layer in a position in the coating so as to provide a conductive characteristic at the external surface of the coating.

24. A method according to claim 23 wherein said conductive layer is formed as a noble metal alloy.

25. A method according to claim 24 wherein any of titanium, niobium, or zirconium is simultaneously applied with the noble metal to form the said conductive layer.

26. A method according to any of the claims 23-25 wherein a further layer is applied onto said conductive layer to at least partially cover said conductive layer and said further layer is formed of a metal or metal alloy and includes any of titanium, niobium, or zirconium.

27. A method according to claim 26 wherein the said further layer also includes a noble metal.

28. A method according to any of claims 22 to claim 26 wherein the said further layer includes gaps to allow the conductive characteristic of the conductive layer to be exhibited at the external surface of the coating.

29. A method according to claim 27 wherein the said further layer is applied so as to pro vide a substantially regular spaced arrangement of said gaps in the coating so as to provide a substantially uniform conductive characteristic at the external surface of the coating.

30. A method according to any of the claims 23-29 wherein the material to form the lay ers of the coating is selectively sputter deposited from at least a first magnetron with a metal target and a second magnetron with a noble metal target.