The present invention relates to a metal strip or sheet comprising a substrate made of stainless steel coated with at least one layer of chromium nitride based coating.

The PEM fuel cells, that is to say membrane proton exchange, include battery units each consist of an anode assembly / electrolyte / cathode, also called MEA ( "membrane electrode assembly" in English ), diffusion layers of gas, also called GDL ( "gas diffusion layer" in English), extending on either side of the MEA, and bipolar plates. The bipolar plates ensure the connection between them elements of the battery unit. They further define fluid flow channels, ensure distribution of the gas, coolant and discharge of the water generated in the stack. They also have the function of collecting the current generated at the electrodes.

Given the essential role played by the bipolar plates in the fuel cell, as well as the growing importance of such cells in many areas, it is desirable to develop bipolar plates compact, inexpensive to manufacture and presenting in Also a long life in operation in the fuel cell.

An object of the invention is to provide bipolar plates inexpensive to manufacture and further having a long life in operation in the fuel cell.

To this end, the invention relates to a metal strip or sheet as above, wherein the coating layer based on chromium nitride is textured.

According to particular features, the strip or sheet metal present in one or more of the following features, (s) singly or in any (s) combination (s) technically feasible (s):

- the coating layer has an epitaxial relationship with the substrate;

- the layer of chromium nitride based coating is obtained by physical deposition of vapor phase process, in particular by sputtering;

- the layer of chromium nitride based coating is formed directly on the stainless steel substrate without interposition of a passive layer;

- the substrate has a thickness of between 75 micrometers and 200 micrometers, and in particular a thickness less than or equal to 100 micrometers;

- grains of the substrate have a size strictly less than 50 micrometers, particularly between 10 micrometers and 30 micrometers;

- the coating layer has a columnar structure, the width of the columns being preferably between 10% and 20% of the thickness of the coating layer;

- the coating layer optionally comprises oxygen, said coating layer being obtained by physical vapor deposition (PVD) and having, on its surface, a surface zone comprising an oxygen atom content is strictly less than its atomic content nitrogen;

- the surface area has a height less than or equal to 15% of the total thickness of the coating layer;

- the coating layer comprises, at the interface with the substrate, an interface area comprising an oxygen atom content is strictly less than its atomic nitrogen content;

- the interface area has a height less than or equal to 15% of the total thickness of the coating layer;

- the metal strip or sheet is constituted, starting from the substrate and toward the surface of the coating layer, the interface area, of a heart area and the surface area, said areas being superposed in a direction normal to the mean plane of the substrate.

The invention also relates to a plate obtained by deforming a sheet or strip as defined above.

The invention also relates to a bipolar plate for a fuel cell comprising at least one plate as defined above.

The invention also relates to a method for producing a metal strip or sheet, which comprises the steps of:

- providing a stainless steel substrate comprising at its surface, a passive layer formed by natural oxidation of the stainless steel substrate;

- stripped entirely passive layer at least in certain areas of the substrate;

- depositing chromium nitride on the substrate regions in which the passive layer was etched so as to form a coating layer based on chromium nitride directly on the stainless steel substrate without interposition of a passive layer, the coating layer thus formed being textured.

According to particular characteristics of the process:

- the coating layer thus formed has an epitaxial relationship with the substrate;

- stripping is a physical etching;

- the layer of chromium nitride based coating is deposited by physical deposition of vapor phase process, in particular by sputtering;

- depositing chromium nitride on the substrate in a deposition apparatus comprising a deposition chamber and a chromium target arranged in the deposition chamber, the traveling substrate through the deposition chamber along a longitudinal direction, the deposition chamber having a deposition zone of length strictly less than the length of the deposition chamber, taken along the longitudinal direction and at least a first prohibited zone, adjacent to the deposition zone in the longitudinal direction, and on deposition , chromium nitride is deposited on the substrate only in the deposition zone and no chromium nitride is deposited on the substrate in the first forbidden zone.

The invention will be better understood from reading the following description given solely by way of example and with reference to the accompanying drawings, wherein:

- Figure 1 is a schematic view of a metal strip according to the invention;

- Figure 2 is a schematic representation of a manufacturing process of the strip of Figure 1;

- Figure 3 is a schematic representation of a deposition apparatus;

- Figure 4 is a schematic representation of a bipolar plate according to the invention;

- Figure 5 is an image obtained by processing an electronic transmission microscopy image of a blank from the strip of Figure 1;

- Figure 6 is a similar picture to that of Figure 5 taken for comparison on a tape which is not according to the invention;

- Figure 7 is an image in a scanning electron microscope of a plate obtained by punching a blank from the strip of Figure 1;

- Figure 8 is a similar picture to that of Figure 7 of a plate obtained by punching a blank from a strip which is not according to the invention;

- Figure 9 is an electronic transmission microscopy image of a plate obtained by punching a blank from the strip of Figure 1; and

- Figure 10 is a similar picture to that of Figure 9 obtained from a plate obtained by punching a blank from a strip which is not according to the invention;

- Figure 1 1 is a schematic view of a metal strip according to another embodiment of the invention;

- Figure 12 is a schematic representation of a portion of a web manufacturing process of Figure 1 1;

- Figure 13 is a schematic representation of a deposition apparatus according to a first embodiment;

- Figure 14 is a schematic representation of a deposition apparatus which is not according to the invention;

- Figure 15 is a schematic representation of a deposition apparatus according to a second embodiment; and

- Figure 16 is a schematic representation of the assembly for measuring the contact resistance.

Throughout the description, the expression "between a and b" is understood to include the terminals a and b.

The metal strip 1 illustrated in Figure 1 comprises a substrate 3 made of stainless steel and at least one coating layer 5.

More particularly, the substrate 3 is a strip made of stainless steel, and in particular ferritic stainless steel or austenitic. For example, the substrate 3 is made of stainless steel 1 .4404, .4306 1, 1 or 1 .4510 .4509.

The substrate 3 has a thickness of between 75 micrometers and 200 micrometers, and in particular a thickness less than or equal to 100 micrometers.

The substrate 3 strip-shaped is obtained by any suitable conventional method, for example by hot rolling in one or more passes, followed by a cold rolling in one or more passes, a slab made of the desired alloy , the method may comprise one or more thermal treatment, including annealing.

The stainless steel of the substrate 3 is polycrystalline. It therefore consists of a plurality of grains. The steel grains forming the substrate 3 have a size strictly less than 50 micrometers, particularly between 10 micrometers and 30 micrometers.

The coating layer 5 is a layer made of CrN-type chromium nitride.

As will be seen subsequently, the chromium nitride layer optionally comprises oxygen within limits which will be specified further.

For example, the coating layer 5 made of chromium nitride is formed of CrN x O y with x between 0.6 and 2 and y strictly less than 1, 4 and incidental impurities, in particular of impurities resulting from manufacture.

The sum of x and y is such that the coating layer 5 made of chromium nitride has the cubic crystal structure of face-centered Cri Ni. This crystal structure is known to the skilled person.

The coating layer 5 has for example a thickness of between 3 nm and 150 nm. It is more particularly greater than or equal to 50 nm, for example greater than or equal to 50 nm and less than or equal to 100 nm.

The coating layer 5 is obtained by a physical vapor deposition process.

The use of the physical deposition method for vapor deposition of the coating layer 5 made of chromium nitride results in a distinctive microstructure of this coating layer. In particular, the coating layer 5 has a columnar structure with grain size substantially the same as or comparable to those of steel constituting the substrate 3.

The coating layer 5 has a normal growth direction to the substrate 3.

The use of the vapor deposition process resulting in a columnar structure in the coating layer 5, consisting of columns of width between 10% and 20% of the thickness of the coating layer 5. By way of example , the column width is of the order of 10 nanometers.

The columns of the coating layer 5 grows in the direction of growth of the coating layer 5. In this context, the term column length, the size of a column in a direction normal to the substrate 3 and the width dimension a column in a plane parallel to the mean plane of the substrate 3.

The coating layer 5 of the metal strip 1 is textured to a crystallographic point of view. This texturing result of the implementation of the process which will be described later.

By "textured" means that the crystallographic directions of growth of the columns constituting the coating layer 5 are not random, but strongly oriented along one of the crystallographic axes defining the unit cell CrN. In this case, the relative orientation of the unit cells of the coating layer 5 and the substrate 3 used to align the crystallographic planes of the two phases having similar interplanar spacings and ensure the continuity of these crystallographic planes perpendicular to the interface between the substrate 3 and the coating layer 5.

Preferably, the coating layer 5 has an epitaxial relationship with the substrate 3. "epitaxy" means, in conventional manner, the three crystallographic axes of the columns of the coating layer 5 are aligned with those of the grains 3 adjacent the substrate to these columns.

As will be seen subsequently, a coating layer 5 having an epitaxial relationship with the substrate 3 is particularly advantageous.

The metal strip 1 is advantageously constituted of the substrate 3 and one or more coating layers 5 made of chromium nitride. In particular, the coating layer 5 made of chromium nitride is formed directly on the stainless steel substrate 3, without the interposition of an intermediate layer, such as a passive or oxide layer, resulting from oxidation natural stainless steel constituting the substrate 3. by natural oxidation, refers to the oxidation in air, for example when using or storing the substrate 3 made of stainless steel.

In addition, preferably, the metal strip 1 does not include layers formed over the coating layer 5 furthest from the substrate 3.

A method for obtaining a metal strip 1 according to the invention will now be explained with reference to FIG 2.

In a first step there is provided a metal strip 9, comprising a substrate 3 made of stainless steel covered at its surface with a passive layer 10 made of chromium oxide formed by natural oxidation of the stainless steel constituting the substrate 3. such passive chromium oxide layer forms on the surface of stainless steel when the latter comes into contact with air. It is the basis of the stainless character of stainless steel. The passive layer 10 may typically comprise, in addition chromium oxide, and to a minor extent, oxides of other chemical elements present in the steel constituting the substrate 3.

In a second step, completely scours the passive layer 10 in certain areas at least of the metal strip 9 so that, in these areas, there remains no remaining passive layer 10.

Preferably, during the second stage, the passive layer 10 is etched over the entire surface of the substrate 3 to be coated with the coating layer 5.

Pickling is performed by a physical etching process. Preferably, the etching is ion etching, conducted by bombarding the initial metal strip by a neutral gas. The inert gas used is for example argon. Alternatively, it may be any other suitable inert gas.

Such ion etching process is known as such and will not be described in more detail below.

After this stripping, one obtains the substrate 3 having the metal of the stainless steel atoms on its surface in the areas in which etching was performed.

During a third step, by physical deposition is deposited in vapor phase (also referred to as "PVD" or "physical vapor deposition" in English) by means of a physical deposition installation vapor 14, chromium nitride on the substrate 3 in the areas in which the passive layer 10 was etched.

Conventionally, the physical repository vapor 14, shown in Figure 3, includes a deposition chamber 20, adapted to be evacuated and a target 22. The target 22 consists of chromium.

The substrate 3 travels through the enclosure 20 in a direction of travel, called longitudinal direction hereinafter. Throughout the description, the terms upstream and downstream are used with reference to the travel of the substrate 3 through the enclosure 20.

The enclosure 20 includes, at each of its longitudinal ends, a passage opening 25 of the substrate 3. The openings 25 are advantageously sealed.

In the example shown, the enclosure 20 includes a source of inert gas 24. The inert gas is for example argon.

The PVD method is advantageously used a sputtering method. In this case, the target 22 is known as "sputtering target". The enclosure 20 includes means for applying a potential difference between the target 22 and the substrate 3, so that the target 22 as the cathode of the installation of deposit 14 and the substrate 3 forms the anode the repository 14.

The deposition of the coating layer 5 on the substrate 3 is formed by bombardment of the target 22 by means of the inert gas from the inert gas source 24 also in an atmosphere comprising nitrogen.

More specifically, and as illustrated in Figure 3, during the deposition step, the substrate 3 strip-shaped scrolls through the enclosure 20.

A suitable potential difference is applied between the target 22 and the substrate 3. The projected inert gas in plasma form on the target 22, extract chromium atoms thereof, which then condense on the substrate 3 so as to form the coating layer 5 made of chromium nitride, chromium combining with the nitrogen present in the chamber 20.

The deposit is made on a substrate 3 at room temperature, for example of the order of 20 ° C.

During this deposition step, the nitrogen flow injected into the chamber 20 is adjusted to achieve the desired stoichiometry of nitrogen in the coating layer 5 made of chromium nitride. The constancy of the deposit stoichiometry is provided by the analysis of the spectrum of light emitted by the plasma during the deposition of the PVD coating. Indeed, the analysis of this spectrum can be deduced the relative concentrations of chromium and nitrogen present in the plasma.

The specific nitrogen flow rates to use based on the stoichiometry vary depending on the deposit facility PVD used. However, the skilled person is capable, through its general knowledge and through a limited number of calibration tests connecting the light spectrum emitted by the plasma and stoichiometry measurements of the coating layers obtained for various nitrogen flow, to determine the nitrogen flow rate to use for a particular installation, depending on the desired stoichiometry in the coating layer 5.

This forms the coating layer 5 made of chromium nitride directly on the substrate 3 made of stainless steel, without the interposition of a passive layer. The coating layer 5 obtained after this step is textured. More particularly, it has an epitaxial relationship with the substrate 3.

After the third step, thereby obtaining the metal strip 1 according to the invention.

The invention also relates to a bipolar plate 1 1 obtained from the metal strip 1. Such bipolar plate 1 1 is shown in Figure 4.

As shown in Figure 4, the bipolar plate 1 1 comprises two plates 13 joined together, particularly by welding. More particularly, the plates 13 define, after joining, distribution channels and fluid evacuation.

Each plate 13 is obtained from a metal strip 1 according to the invention. More particularly, it is obtained by deformation, in particular cold-cut a blank from the metal strip 1.

Each plate 13 thus comprises a substrate 3 made of stainless steel, coated with at least one coating layer 5 made of chromium nitride, as previously described in relation to the metal strip 1.

The final shape of plates 13 is advantageously obtained by cold drawing of a blank cut from the metal strip 1.

The invention also relates to a method for manufacturing a bipolar plate 1 1 from a metal strip 1 comprising:

- providing a plate 13 comprising a substrate 3 made of stainless steel, coated with at least one coating layer 5 made of chromium nitride, as described above; and

- the securing of the plate 13 with another plate, and preferably to another similar plate 13, so as to form the bipolar plate 1 1.

The step of providing the plate 13 comprises:

- cutting the metal strip 1 to form at least one blank; and

- the deformation of the blank, in particular by stamping, to form a plate

13.

As explained above, the deformation is preferably a cold deformation, in particular a cold-stamping.

The joining together of the two plates to form the bipolar plate 1 1 is performed by any suitable method, particularly by welding.

The inventors of the present invention carried out the following experiments. They implemented on a metal strip 9 as described above, the method for manufacturing a metal strip 1 as shown in Figure 1, and thus obtained a metal strip 1.

They then cut blanks from the metal strip 1. They imaged such blanks by transmission electron microscopy (or TEM) and analyzed the images obtained by techniques known image analysis to check if there was an epitaxial relationship between the substrate 3 and the coating layer 5.

More particularly, they calculated the Fourier transforms of the images obtained by TEM, Fourier transforms them to explore the frequency distributions of the images obtained by TEM. On images Fourier transforms, they selected frequencies corresponding to the crystallographic planes of the substrate 3 and the coating layer 5 to which an adjacent orientation is observed in the frequency space. They then final images obtained by inverse Fourier transform of filtered from the selected frequencies.

An example of image obtained after analysis is shown in Figure 5. In this figure, the diagonal lines represent the atomic planes. The continuity of these features the passage of the interface between the substrate 3 and the coating layer 5 shows that the coating layer 5 has an epitaxial relationship with the substrate 3.

The inventors have carried out such analyzes for metal strips 1 obtained with different flow rates of nitrogen during the step of depositing the coating layer 5. They observed that the epitaxial relationship between the substrate 3 and the layer of coating 5 is whatever the flow rate of nitrogen used.

For comparison, the inventors conducted experiments with the metal strips obtained by a method differing from the process according to the invention only by the fact that during the second stage, the passive layer 10 was not entirely etched in areas of the substrate 3 coated with the coating layer 5. Thus, this passive layer 10 remains in areas then coated with the coating layer 5 made of chromium nitride, and is interposed between the coating layer 5 and the substrate 3.

6 illustrates an example of an image obtained after analysis of a TEM image of such band, which is not according to the invention, according to the analytical method explained above. In this case, we see that there is no continuity of atomic planes at the interface between the substrate 3 and the coating layer 5. In this image, the white area between the substrate 3 and the coating layer 5 corresponds to the passive layer 10.

Thus, the inventors have observed that the epitaxial relationship between the coating layer 5 and the substrate 3 is not obtained when the passive layer 10 was not etched, or has not been completely etched before coating in areas of the substrate 3 are intended to be covered with the coating layer 5.

The inventors then deformed by stamping blanks obtained by cutting a metal strip 1 according to the invention for the plates 13.

Figure 7 is an image in a scanning electron microscope of a plate 13 thus obtained. In this image is observed no peeling between the coating layer 5 and the substrate 3 resulting from stamping. Thus, the coating layer 5 has a satisfactory adhesion to the substrate 3. However, a good adhesion of the coating layer 5 is particularly advantageous during use of the band 1 in a bipolar plate. Indeed, it ensures the bipolar plate good electrical properties, in particular good electrical conductivity and prevents poisoning of the electrolyte.

Figure 8 is an image in a scanning electron microscope of a plate obtained by drawing of blanks obtained by cutting the strip not according to the invention described above. In this image, there is a detachment of the coating layer 5. Thus, the adhesion of the coating layer 5 is, in this case, sufficient to resist deformation by stamping the blank.

Note that, in the context of the invention, the deformation by stamping the blank obtained from this metal strip 1 to form a plate 13 may result in a plate 13 having a layer of non-continuous coating 5.

Indeed, due to the different mechanical properties of the substrate 3 and the coating layer 5, stamping results in a much higher relative elongation of the substrate 3 with respect to the coating layer 5. Due to the good adhesion between the substrate 3 and the coating layer 5 according to the invention, this differential elongation results in the formation of micro-cracks 26 in the coating layer 5 of the plate 13, and thus the formation of a layer of discontinuous coating on the plate 13 . these microcracks 26 are formed in particular between two adjacent columns of the coating layer 5. the passive layer reforms naturally in these microcracks 26 by natural oxidation of the stainless steel substrate 3, which is exposed in these areas.

The inventors have observed that, even in the presence of microcracks 26, there occurs no loss of coating during drawing blanks obtained from tape 1 according to the invention.

Moreover, thanks to the good adhesion between the coating layer 5 and the substrate 3, the coating layer 5 does not come off the substrate 3 during the drawing and is observed in the plate 13, a very good adhesion between the discontinuous coating layer 5 and the substrate 3.

Figure 9, which is an electron microscopy image of a transmission plate 13 according to the invention, illustrates these observations.

The inventors measured the electrical conductivity of such a discontinuous layer and found that the electrical performance remained satisfactory, despite the presence of microcracks 26. The passive layer is reuniting after drawing between adjacent columns of the coating layer 5 is not harmful not for the electrical conductivity and protects the stainless steel of the substrate 3 in the areas where it is flush.

Figure 10 is an electron microscopy image transmission of a plate obtained by drawing from the strip which is not according to the invention described above. In this case, it is observed that the coating layer 5 is peeled away from the substrate 3 and slid in one piece in response to the elongation of the substrate 3 under the effect of stamping. In the sheet thus obtained, the coating layer 5 thus adheres more to the substrate 3. However, it is sought to avoid such separation, which adversely affects the electrical conductivity of the bipolar plate and may lead to poisoning of the electrolyte of the fuel cell.

The metal strip 1 according to the invention is particularly suitable for producing bipolar plates having a long lifetime for reduced manufacturing costs.

In fact, stainless steel is an inexpensive material and which also has very advantageous properties for use as a bipolar plate. In particular, it has excellent mechanical properties. It is further deep-drawable, weldable, gas impermeable, and has a high electrical conductivity in its thickness, as well as good thermal conductivity.

However, when a bare stainless steel substrate is used in bipolar plates of fuel cells, these bipolar plates have insufficient electrical properties. The textured coating layer based on chromium nitride according to the invention colleague good electrical conduction properties to the bipolar plate, in particular resulting from the excellent adhesion of the coating layer to the substrate 3.

In one embodiment, the invention relates to a metal foil or blank obtained by cutting the metal strip 1.

The metal sheet according to the invention could also be obtained by a method analogous to that described above for obtaining the strip 1, but starting from a metal sheet during the first step of the process, rather than a gang.

Such a film has similar properties to those of the band 1.

A metal strip 1 'according to another embodiment of the invention will now be described with reference to Figure 1 1. This band 1 'has all the characteristics of the web 1, and in particular its texturing properties, but also has the specific features described below.

As illustrated in Figure 1 1, the coating layer 5 'of the web 1' is constituted by three superimposed zones in the direction of growth of the coating layer 5 ', that is to say in the direction normal to the mean plane of the substrate 3 '. Each zone extends over the entire surface of the coating layer 5 ', taken parallel to the mean plane of the substrate 3'. Preferably, each zone has a substantially constant thickness.

More particularly, the coating layer 5 is constituted, starting from the substrate 3 'and moving towards the surface of the coating layer 5' in the direction normal to the mean plane of the substrate 3 ', zone d interface 6, a heart region 7 and a surface zone 8.

The surface region 8 of the coating layer 5 is located at the surface of the coating layer 5 '.

The surface zone 8 has an oxygen atom content is strictly less than its atomic nitrogen content. It is considered that the difference between the ratio of the atomic oxygen content on the atomic content of chromium and the ratio of the nitrogen content on the atomic atomic chromium content in the surface zone 8 is preferably greater than or equal to 0 1.

The surface zone 8 has a composition of the type: CrN x1 O y i, y1 strictly less than x1, the balance being incidental impurities, in particular impurities resulting from manufacture. These impurities do not include oxygen. The difference between x1 and y1 is preferably at least 0.1.

X1 coefficient corresponds to the ratio of the atomic nitrogen content on the atomic content of chromium in the surface region 8. The coefficient y1 corresponds to the ratio of the atomic oxygen content on the atomic chromium content in the surface zone 8.

The sum of the coefficients x1 and y1 is such that the surface zone 8 has the cubic crystal structure of face-centered CNN ^

x1 is advantageously between 0.6 and 2.

Regardless of the value of x1, y1 is advantageously less than or equal to

1, 4, while being strictly less than x1, and being such that the surface region 8 retains the crystallographic structure of Cri Ni.

In particular, the surface region 8 extends over at least 5% of the thickness of the coating layer 5 '. It extends at most about 15% of the thickness of the coating layer 5 '.

No oxidation layer, which would result from the oxidation of the coating layer 5 'is formed on the surface region 8.

The interface area 6 of the coating layer 5 is located at the interface with the substrate 3 '. It is in direct contact with the steel constituting the substrate 3. It forms part of the coating layer 5 'closest to the substrate 3'.

Advantageously, the interface area 6 has an oxygen atom content is strictly less than its atomic nitrogen content. The difference between the ratio of the atomic oxygen content on the atomic content of chromium and the ratio of the nitrogen content on the atomic atomic chromium content in the interface area 6 is preferably greater than or equal to 0.1 .

The interface area 6 has a composition of the type: CrN x2 0y 2 , with y2 strictly less than x2, the balance being incidental impurities, in particular impurities resulting from manufacture. These impurities do not include oxygen. The difference between x2 and y2 is preferably at least 0.1.

X2 ratio is the ratio of the nitrogen content on the atomic atomic chromium content in the interface area 6. The coefficient y2 corresponds to

ratio of the atomic oxygen content on the atomic content of chromium in the interface area 6.

The sum of x2 and y2 coefficients is such that the interface area 6 has the cubic crystal structure of face-centered Cri Ni.

x2 is advantageously between 0.6 and 2.

Regardless of the value of x2, y2 is preferably less than or equal to 1, 4, while being strictly less than x1, and being such that the interface area 6 retains the crystallographic structure of CNN ^

In particular, the interface area 6 extends over at least 1% of the thickness of the coating layer 5 '. It extends at most about 15% of the thickness of the coating layer 5 '. More particularly, it extends over at most 10% of the thickness of the coating layer 5 '.

The heart zone 7 of the coating layer 5 'forms the heart of the coating layer 5'. It extends, in the direction normal to the mean plane of the substrate 3 ', the interface between zone 6 and the surface region 8. It constitutes the major part of the thickness of the coating layer 5'. Preferably, it extends over at least 70% of the thickness of the coating layer 5 '.

In particular, the heart 7 zone has an oxygen content atomic strictly less than one third of its atomic nitrogen content. In other words, it has a composition as x3: CrN x3 O y3 , y3 with strictly less A- else

being comprised of impurities, including impurities resulting from manufacture. These impurities do not include oxygen.

X3 ratio is the ratio of the nitrogen atomic content on the atomic content of chromium in the heart area 7. y3 ratio is the ratio of the atomic oxygen content on the atomic content of chromium in the heart area 7.

The sum of x3 and y3 coefficients is such that the heart zone 7 has the cubic crystal structure of face-centered Cri Ni.

x3 is advantageously between 0.6 and 2.

Regardless of the value of x3, y3 is advantageously less than or equal to x3

1, 4, while being strictly less than -, and being such that the heart area 7

retains the crystallographic structure of CNN ^

A coating layer 5 'having a surface zone 8 as defined above, wherein the atomic oxygen content is strictly less than the atomic nitrogen content, is particularly advantageous for use as a bipolar plate in fuel cells. Indeed, the inventors have found that when the atomic oxygen content in the surface zone 8 is strictly less than the atomic nitrogen content, the contact resistance ( "Interfacial Contact Resistance" or ICR English) measured between a sheet cut from such a strip and a reference gas diffusion layer SGL 34BC Group, is less than 10 mû.cm 2 to 100 N.cm "2 .

On the contrary, the inventors have found that when the atomic oxygen content of the surface zone 8 is greater than or equal to its atomic nitrogen content is measured contact resistances much greater, at least equal to 100 mû.cm 2 to 100 N.cm "2 , which is not entirely satisfactory for use as a bipolar plate in a fuel cell.

In addition, the inventors have found that when the coating layer 5 'comprises in its interface region 6, an oxygen atom content is strictly less than its atomic nitrogen content, the coating layer 5' has a better adhesion to substrate 3 'a coating layer in which this condition is not respected. This property is particularly advantageous when the belt 1 is used to manufacture bipolar plates of fuel cells. Indeed, insufficient adhesion of the coating layer 5 'on the substrate 3' increases the risk of its detachment during shaping, in particular by stamping, such detachment may contaminate the electrical conduction properties of the bipolar plate .

A portion of a method for manufacturing a metal strip 1 'comprising a coating layer 5' is illustrated schematically in Figure 12.

The first two steps of the method (supplying the metal strip comprising the substrate 3 'and a passive layer and etching the passive layer) are identical to those described above with reference to the metal strip 1. In Figure 12, these steps have not been represented, only the third step being illustrated.

The third step differs from the third step of the process previously described by the features mentioned below.

According to this embodiment, and as illustrated in Figure 13, the deposition chamber 20 includes a deposition zone 30 and a first zone called "forbidden" 32, adjacent the deposition zone 30 in the longitudinal direction. A closed area is defined as an enclosure area 20 on the substrate of the route 3 'wherein it is not desired to occur chromium based coating.

The length of the deposition zone 30 is strictly less than the length of the deposition chamber 20. More particularly, the deposition zone 30 has a length strictly less than the length of the zone of the chamber 20 wherein the chromium would be deposited on the substrate 3 'in the absence of forbidden zone 32.

The deposition zone 30 and the first area prohibited 32 are thus defined so that, during deposition, chromium nitride is deposited on the substrate 3 'only in the deposition zone 30 and prevents the deposition of chromium nitride on the substrate 3 'in the first closed area 32.

The first closed area 32 is located downstream of the target 22 in the path of substrate 3 '.

The deposition zone 30 and the first closed area 32 are configured such that, in the entire area of ​​the deposition zone 30 located downstream of the target 22 in the path of substrate 3 ', the deposition rate of carbon chromium on the substrate 3 'is greater than or equal to a predetermined threshold speed. The first closed area 32 corresponds to a zone in which the chromium atoms from the target 22 would be deposited on the substrate 3 'at a speed strictly less than the threshold speed if they were free to settle according to their natural path.

Indeed, the inventors have found that the oxygen content of the coating is locally strictly greater than the previously set desired content in the areas of the coating formed by depositing chromium atoms in a strictly lower speed than the predetermined threshold speed.

The predetermined threshold rate is equal to a percentage of the maximum rate of deposition of the chromium atoms on the substrate 3 '. The deposition rate is maximum compared to the target 22. It decreases moving away from the target 22 in the longitudinal direction, in the path of substrate 3 '.

The value of the predetermined threshold speed is obtained experimentally by the skilled artisan, for a given deposition facility by a limited number of experiments. It corresponds to the minimum deposition rate of the chromium atoms on the substrate 3 'downstream of the target 22 which is measured a contact resistance of the obtained coating layer of less than 10 mu. cm 2 to 100 N.cm "2 .

For example, in the case of the apparatus used by the inventors of the present invention, this threshold rate is equal to about 10% of the maximum deposition rate.

To deposit chromium nitride on the substrate 3 'only in the deposition zone 30, the second step comprises the implementation of at least one downstream cover 28 in the deposition chamber 20, said cover 28 being intended downstream to prevent the projection of atoms of chromium on the substrate 3 'from the target 22 outside the deposition zone 30.

The cover 28 prevents the projection of atoms of chromium on the substrate 3 'in the first closed area 32, wherein the deposition rate of the chromium atoms on the substrate 3' is strictly less than the given threshold. The downstream cover 28 is disposed on the chromium atoms of the trajectory, in the absence of downstream cover 28, would be deposited on the substrate 3 ', downstream of the target 22 according to the path of the substrate 3', with a speed of deposit strictly less than the predetermined threshold speed, and thus prevents the deposit of such carbon chromium on the substrate 3 '.

The cache 28 is disposed between the target 22 and the substrate 3 '. The cache 28 is disposed at a distance from the downstream wall 21 of the enclosure 20. The first prohibited zone 32 extends downstream of the cover 28.

For example, the cache 28 is formed by an impermeable plate to chromium atoms from the target 22. It is substantially normally disposed substrate 3 'scrolling through the enclosure 20.

The inventors have discovered that the delimitation in the deposition chamber 20 of such a deposition zone 30, associated with a first closed area 32, can be formed on the substrate 3 a coating layer 5 'having a surface area 8 as defined above, having an oxygen atom content is strictly less than its atomic nitrogen content.

optionally it is noted that the oxygen present in the coating layer 5 'resulting unavoidable enclosure sealing imperfections 20 and the desorption from the walls of the chamber 20 or even of the substrate 3'.

According to an alternative embodiment, the downstream boundary of the deposition zone 30 is determined by measuring the atomic content profile oxygen in a coating layer based on chromium nitride formed in the deposition chamber in the absence of bounding prohibited area, and deducting the area of ​​the enclosure 20, wherein the surface area of ​​the coating layer has an oxygen atom content is strictly less than its atomic nitrogen content.

Thus, a coating layer 5 'having a surface zone 8 as defined above, wherein the atomic oxygen content is strictly less than the atomic nitrogen content.

The location of the deposition area obtained according to this variant is substantially identical to the location determined from the deposition rates.

Figure 14 shows an enclosure that is not within the invention, due to the absence of forbidden zone, but which illustrates the need for such areas. In this figure, there is shown a curve 31, obtained by the inventors, showing the evolution of the atomic oxygen content in the base coating layer of chromium nitride obtained in such a chamber. On this curve, it is observed that the atomic oxygen content in said coating layer deposited under conditions identical to the conditions of the invention, but in the absence of forbidden zones, is minimum in the portions of the coating layer 5 'deposited facing the target 22. This atomic oxygen content increases in the portions of the coating layer 5' filed in the direction of the upstream and downstream ends of the chamber 20 according to the travel direction of the substrate 3 ', being maximum in the vicinity of these ends.

Optionally, the second step of the method further comprises, prior to depositing the coating layer 5 ', minimizing the outgassing rate of the enclosure 20 to minimize the amount of residual gas in the chamber 20 as much as possible . This minimization is particularly carried out by pumping the residual gases from the enclosure 20.

Determining the minimum and maximum presence of oxygen in the atmosphere in the chamber 20 may be effected by experience depending on local conditions of implementation of the invention.

By "outgassing" means the capacities of the gas desorbed from all enclosure surfaces 20 and in addition to the nitrogen controlled rate. This degassing acts as a disruption or chemical pollution vis-à-vis the deposition by PVD carried out in the deposition chamber 20.

The coating layer 5 'is advantageously performed in a single pass of coating, that is to say, by a single pass through the deposition chamber 20.

At the end of the deposition step, a metal strip is obtained 1 'comprising a coating layer 5' having a surface zone 8 as defined above.

The manufacturing method according to a second embodiment differs from the method which has just been described in that, as shown in Figure 15, the deposition zone 30 is limited, not only downstream of the target 22 as described above, but also upstream of the target 22 in the path of substrate 3 '. In this embodiment, the deposition zone 30 'is defined such that the deposition rate of the chromium atoms on the substrate 3' is greater than or equal to the threshold speed previously described throughout the deposition zone 30 '.

In this embodiment, the chamber 20 then comprises a second forbidden zone 33, adjacent the deposition zone 30 'upstream of the target 22 on the path of the substrate 3. The first closed area 32 and second closed area 33 surround the deposition zone 30 'in the longitudinal direction. The first prohibited zone 32 and the second closed area 33 are areas in which the atoms of chromium would be deposited on the substrate 3 'in a strictly less than the predetermined threshold speed speed if they were free to settle according to their natural path.

To deposit chromium nitride on the substrate 3 'only in the deposition zone 30', the second step comprises the introduction, into the deposition chamber 20, a cache 28 downstream as defined above, and an upstream cover 29.

The downstream cover 28 and the upstream cover 29 are configured to prevent the chromium atoms projection on the substrate 3 'from the target 22 outside the deposition zone 30', that is to say, in the enclosure zones 20 in which the deposition rate of the chromium atoms on the substrate 3 'is strictly less than the given threshold. They are arranged in the path of atoms of chromium that would, in the absence of caches 28, 29, deposited on the substrate 3 'respectively in the first zone 32 or prohibited in the second closed area 33, and prevent the deposit of chromium atoms in these areas 32, 33.

The downstream cover 28 and the upstream cover 29 are disposed on either side of the target 22, according to the substrate travel direction 3 '.

In the example shown, the covers 28, 29 are equidistant from the target 22 and the deposition zone 30 'is centered on the target 22. In this example, the deposition zone 30' extends under the target 22, in a central region of the chamber 20.

For example, the upstream cover 29 is formed by a plate impermeable to chromium atoms from the target 22. It is substantially normally disposed substrate 3 'scrolling through the enclosure 20.

The upstream cover 29 is spaced apart from the upstream wall 23 of the enclosure 20.

For example, in the system used by the inventors, the covers 28, 29 were spaced apart by 90 cm.

The inventors of the present invention have found that the boundary of the deposition zone 30 'upstream of the target 22, including through the upstream cover 29, provides a coating layer 5' comprising, in its area of interface 6, an oxygen atom content is strictly less than its atomic nitrogen content. As explained previously, such a coating layer 5 'has a better adhesion to the substrate 3' a layer in which this condition is not respected.

Thus when the deposition zone 30 is delimited upstream and downstream of the target 22 in the manner specified above, one obtains, at the end of the deposition step a coating layer 5 'comprising, on its surface, the surface region 8 as defined above and at the interface between the substrate 3 'and the coating layer 5', the interface area 6 as defined above.

Moreover, this coating layer 5 'comprises, between the interface region 6 and the surface zone 8, a heart region 7 as defined above.

According to a variant of the second embodiment, the upstream and downstream limits of the deposition zone 30 'are determined by measuring the atomic content profile oxygen in a coating layer based on chromium nitride formed on a substrate under conditions identical to the conditions of the invention, but without bounding prohibited areas in the enclosure, and deducting the area of ​​the enclosure 20 wherein the surface zone and the interface zone of such coating layer have an oxygen atom content is strictly less than its atomic nitrogen content.

The location of the deposition zone 30 'obtained according to this variant is substantially identical to the location determined from the deposition rates.

At the end of the deposition step, a metal strip is obtained 1 'according to the invention.

Plates 13, and a bipolar plate having the coating 1 1 5 'may be obtained by a method identical to the method described above for the metal strip 1, but applied to the metal strip 1'.

As explained above, the metal strip 1 'according to the invention is particularly suitable for the production of such bipolar plates 1 1.

The inventors have in particular measured for three different samples of metal strip on a base coating layer of chromium nitride, the contact resistance (ICR) between the sample and a gas diffusion layer of a fuel cell using the arrangement shown in Figure 16.

Samples 1 and 2 are samples of strips 1 ', obtained according to the second embodiment of the method described above by sputtering in a deposition chamber 20 having an upstream and a downstream cover 29 cover 28. Sample 3 comes from a band that is not according to the invention.

As illustrated in Figure 16, the contact resistance is measured by sandwiching an assembly formed by a reference 32 gas diffusion layer and a sheet obtained by cutting from the strip to be tested between two copper plates 34, and then applying a known current intensity I in this set. The gas diffusion layer 32 used by the inventors is a reference layer 34BC marketed by SGL Group.

One then measures the potential difference between the diffusion layer 32 and the surface of the sheet, and is deduced in the contact resistance between the sheet and the diffusion layer 32.

This method of measuring the contact resistance of a bipolar plate of fuel cell is conventional and well known in the art.

The inventors have also experimentally determined the composition of the surface region 8 of the coating layer.

Table 1 below shows the results of these experiments.

Table 1: Measured contact resistance for samples tested

In this table, there is obtained lower contact resistance values at 10 MU. cm 2 to 100 N.cm "2 for coatings having surface areas in which the atomic oxygen content is, according to the invention is strictly less than the nitrogen atom content in the surface region (samples 1 and 2) . on the contrary, in the case of the sample 3, wherein the atomic oxygen content is greater than or equal to the nitrogen atom content in the surface of the coating zone and that is not a sample according to the invention , the contact resistance exceeds 100 mû.cm 2 100 N.cm "2 . She is very driven above 10. cm 2 to 100 N.cm "2 .

The inventors have also measured for samples 1 to 3, the composition of the interface area 6 and the heart area 7. The results of these measurements are shown in Table 2 below.

coating layers

As previously explained, the inventors of the present invention have also found that the particular composition of the interface area 6 of the covering layer 5 'according to the invention is advantageous. Indeed, the inventors have observed that the adhesion of the coating layer 5 'is particularly good when the atomic oxygen content of the interface area 6 is strictly less than its atomic nitrogen content. Thus, the risk of detachment of the coating layer 5 'of substrate 3' during the deformation of the blank obtained from the web 1 'to form the plates 13, for example by pressing, is minimized. We try to avoid such separation, as it may compromise the integrity of the bipolar plate 1 1 or denuire to its electrical conductivity and can result in poisoning of the electrolyte.

Obtaining the preferred atomic oxygen content of the surface zone 8 due to the presence of the first closed area 22, that is to say of limiting the deposition of chromium nitride downstream of the target 22 in the manufacturing process of the metal strip 1 'according to the first and second embodiments.

Obtaining the preferred atomic oxygen content of the interface area 6 due to the presence of the second closed area 23, that is to say of limiting the chromium nitride deposit upstream of the target 22 in the manufacturing process of the metal strip 1 'according to the second embodiment.

In particular, measurements have shown that a chromium nitride deposit produced on the stainless steel substrate in a deposition chamber 20 and placed under vacuum which has been minimized, before deposition, the outgassing rate, but allowing nitride chromium deposit according to its natural path without limiting the deposition zone for example by covers, resulting in a coating layer having atomic oxygen levels far outside the ranges described above, particularly in the interface area and the surface area of the coating layer. In this case, the inventors have obtained a coating layer having a contact resistance (ICR) greater than 100 mû.cm 2 100 N.cm "2 and further having a lower adhesion than when the deposition zone is limited upstream of the deposition cathode.

Thus, a simple minimization of the presence of oxygen in the deposition chamber 20 by conventional techniques do not provide the coating layer 5 'as described above.

In one embodiment, the invention relates to a metal foil or blank obtained by cutting the metal strip 1 '.

The metal sheet according to the invention could also be obtained by a method analogous to that described above for obtaining the strip 1 ', but starting from a substrate 3' in sheet form, rather than a substrate 3 'in strip form. Such a film has similar properties to those of the web 1 '.

The invention has been described for a coating layer 5, 5 'formed on a face of the substrate 3, 3'. Alternatively, the metal strip 1, 1 ', as well as the plates 13 and bipolar plates 1 1 manufactured from this strip 1, 1' may comprise a coating layer 5, 5 'of the type previously defined on each of their faces.

Such a coating layer on both sides of the substrate 3; 3 'can be obtained in a single pass or in several passes, for example two passageways, in the deposition chamber 20.

CLAIMS

1. - strip (1; 1 ') or metallic sheet comprising a substrate (3; 3') made of stainless steel coated with at least one coating layer (5; 5 ') made from chromium nitride, characterized in that that the coating layer (5; 5 ') based on chromium nitride is textured.

2. - strip (1; 1 ') or sheet according to claim 1, wherein the coating layer (5; 5') has an epitaxial relationship with the substrate (3; 3 ').

3. - strip (1; 1 ') or sheet according to one of claims 1 and 2, wherein the coating layer (5; 5') based on chromium nitride is obtained by a physical deposition process stage steam, in particular by sputtering.

4. - strip (1; 1 ') or sheet according to any preceding claim, wherein the coating layer (5; 5') based on chromium nitride is formed directly on the substrate (3; 3 ' ) of stainless steel, without the interposition of a passive layer (10).

5.- strip (1; 1 ') or sheet according to any preceding claim, wherein the substrate (3; 3') has a thickness of between 75 micrometers and 200 micrometers, in particular less than or equal 100 micrometers.

6. - strip (1; 1 ') or sheet according to any preceding claim, wherein the grains of the substrate (3; 3') have a size strictly less than 50 micrometers, particularly between 10 micrometers and 30 micrometers.

7. - strip (1; 1 ') or sheet according to any preceding claim, wherein the coating layer (5; 5') has a columnar structure, the width of the columns being preferably between 10% and 20% of the thickness of the coating layer (5; 5 ').

8. A strip (1 ') or sheet according to any preceding claim, wherein the coating layer (5') optionally comprises oxygen, said coating layer (5 ') being obtained by physical deposition vapor deposition (PVD), characterized in that the coating layer (5 ') has, on its surface, a surface region (8) comprising an oxygen atom content is strictly less than its atomic nitrogen content.

9. - band or metal sheet (1 ') according to claim 8, wherein the surface region (8) has a height less than or equal to 15% of the total thickness of the coating layer (5').

10. - strip (1 ') or metal foil according to one of claims 8 or 9, wherein the coating layer (5') comprises, at the interface with the substrate (3), an interface area (6) comprising an oxygen atom content is strictly less than its atomic nitrogen content.

January 1. - strip (1 ') or foil according to claim 10, wherein the interface region (6) has a height less than or equal to 15% of the total thickness of the coating layer (5; 5').

12. - strip (1 ') or metallic sheet according to any one of claims 8 to 1 1, taken in combination with Claim 10, which is constituted, starting from the substrate (3') and towards the surface the coating layer (5 ') of the interface region (6) of a heart area (7) and the surface region (8), said areas (6,7,8) being superimposed following a direction normal to the mean plane of the substrate (3 ').

13. - plate (13) obtained by deformation of a strip (1; 1 ') or sheet according to any preceding claim.

14. - bipolar plate (1 1) for a fuel cell comprising at least one plate (13) according to claim 13.

15.- A method of making a strip (1; 1 ') or metal foil, comprising the steps of:

- providing a substrate (3; 3 ') of stainless steel comprising, on its surface, a passive layer (10) formed by natural oxidation of the stainless steel substrate (3; 3')

- stripping entirely passive layer (10) at least in some areas of the substrate (3; 3 ');

- depositing chromium nitride on the substrate areas (3; 3 ') in which the passive layer (10) has been etched so as to form a coating layer (5; 5') based on chromium nitride directly on the substrate (3; 3 ') of stainless steel, without the interposition of a passive layer (10), the coating layer (5; 5') thus formed being textured.

16. - The method of claim 15, wherein the coating layer (5; 5 ') thus formed has an epitaxial relationship with the substrate (3; 3').

17. - Method according to any one of claims 15 and 16, wherein the etching is a physical etching.

18. - Method according to one of claims 15 to 17, wherein the coating layer (5; 5 ') based on chromium nitride is deposited by physical deposition of vapor phase process, in particular by sputtering.

19. - The method of claim 18, wherein the chromium nitride is deposited on the substrate (3 ') in a deposition apparatus comprising a chamber

deposit (20) and a chromium target (22) disposed in the deposition chamber (20), the substrate (3 ') passing through the deposition chamber (20) in a longitudinal direction,

the deposition chamber (20) comprising a deposit area (30; 30 ') length strictly less than the length of the deposition chamber (20) taken along the longitudinal direction and at least a first prohibited zone (32 ) adjacent the deposition zone (30; 30 ') in the longitudinal direction, and

during the deposition, chromium nitride is deposited on the substrate (3 ') only in the deposition region (30; 30') and no chromium nitride is deposited on the substrate (3 ') in the first restricted area (32).