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 method for producing a metal strip or sheet as described above, comprising providing a substrate made of stainless steel and depositing a chromium nitride layer on the substrate by physical vapor deposition (PVD) 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, wherein the deposition chamber includes a deposition zone 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 wherein, during the deposition, chromium nitride is deposited on the substrate only in the deposition zone and no chromium nitride is deposited on the substrate in the Premi re forbidden zone.

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

- the first prohibited zone is located downstream of the target in the path of the substrate;

- the rate of deposition of chromium on the substrate is greater than or equal to a predetermined threshold in the deposition zone, downstream of the target;

- the deposition chamber comprises a downstream cache, impervious chromium atoms, said cover being disposed downstream in the enclosure so as to prevent the projection of chromium nitride on the substrate in the first closed area and to allow the projection chromium nitride on the substrate in the deposition zone;

- the downstream cache is interposed on the path of the chromium atoms thrown towards the first closed area so as to prevent their projection in the first restricted area;

- the downstream cover is disposed in the deposition chamber so as to prevent deposit on the chromium atoms of the substrate from the target which the deposition rate on the substrate would be strictly smaller than the predetermined threshold;

- the deposition chamber further comprises a second Zone in which no chromium nitride is deposited on the substrate during the deposition step, the second restricted area being adjacent to the deposition zone so that the first forbidden zone and the second forbidden zone surround the deposition zone in the longitudinal direction;

- the second restricted area is located upstream of the target in the path of the substrate;

- throughout the deposition zone, the deposition rate of the chromium atoms on the substrate during deposition is greater than or equal to the predetermined threshold;

- the deposition chamber further comprises an upstream cache, impervious chromium atoms, said cover being disposed upstream in the chamber so as to allow the projection of chromium nitride on the substrate in the deposition zone and prevent projection of chromium nitride on the substrate in the second closed area;

- upstream cache is interposed on the trajectory of chromium atoms thrown towards the second forbidden area from the target so as to prevent their projection into the second closed area;

- the method further comprises, prior to the deposition step, a step of determining the predetermined threshold for a given repository, by calibration, the predetermined threshold corresponding to the minimum deposition rate for which a layer is obtained coating having a desired contact resistance;

- during the providing step, there is provided a strip or metal sheet made of stainless steel and comprising, at its surface, a passive oxidation layer, said providing step further comprising a passive layer etching step for completely remove the passive layer at least in areas of the metal strip or sheet intended to be coated with the coating layer so that, in these areas, there remains no remaining passive layer at the beginning of the step of deposit.

The invention also relates to a metal strip or sheet comprising a substrate made of stainless steel and a base coating layer of chromium nitride, the coating layer optionally comprising oxygen, said coating layer being obtained by deposition physical vapor phase (PVD), the coating layer comprising, on its surface, a surface zone comprising an oxygen atom content is strictly less than its atomic nitrogen content. 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 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 coating layer is composed, starting from the substrate and toward the surface of the coating layer, the interface area, of a heart area and surface area, said zones being superimposed in a direction normal to the mean plane of the substrate;

- the coating layer has a contact resistance (ICR) of less than 10 mû.cm 2 to 100 N.cm "2 ;

- the coating layer is formed directly on the stainless steel substrate without interposition of a passive layer between the coating layer and the stainless steel substrate;

- the coating layer is textured, and in particular has an epitaxial relationship with the stainless steel substrate.

The invention also relates to a bipolar plate comprising at least a plate obtained by deforming a sheet or a blank cut from a strip as mentioned above.

The invention also relates to a method of making a bipolar plate comprising cutting the metal strip obtained by the method as defined above to obtain a plate and shaping the plate.

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 according to a first embodiment;

- Figure 4 is a schematic representation of a deposition apparatus which is not according to the invention;

- Figure 5 is a schematic representation of a deposition apparatus according to a second embodiment;

- Figure 6 is a schematic representation of the assembly for measuring the contact resistance;

- Figure 7 is a schematic representation of a bipolar plate obtained from a metal strip according to the invention;

- Figure 8 is a schematic view of a metal strip according to another embodiment;

- Figure 9 is a schematic representation of a manufacturing process of the strip of Figure 8;

- Figure 10 is an image obtained by processing an electronic transmission microscopy image of a blank from the strip of Figure 8;

- Figure 1 1 is a similar picture to that of Figure 10 taken for comparison on a comparative belt;

- Figure 12 is an image in a scanning electron microscope of a plate obtained by punching a blank from the strip of Figure 8;

- Figure 13 is a similar picture to that of Figure 12 of a plate obtained by punching a blank from a comparative belt;

- Figure 14 is an electron microscopy image transmission of a plate obtained by punching a blank from the strip of Figure 8; and

- Figure 15 is a similar picture to that of Figure 14 obtained from a plate obtained by punching a blank from a comparative tape.

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.

5 the coating layer is a layer based on type of chromium nitride

CrN.

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 CNN ^ This crystal structure is known to those skilled in the art.

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.

As illustrated in Figure 1, the coating layer 5 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 proceeding toward the surface of the coating layer 5, in the direction normal to the mean plane of the substrate 3, an interface area 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 y1 , y1 with 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 Cri Ni.

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 CNN ^

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 nearest 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 atomic nitrogen content on the atomic content of chromium in the interface area 6. y2 ratio is the 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 CNN ^

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 Cri Ni.

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 between the interface region 6 and the surface region 8 . it constitutes the major part of the thickness of the coating layer 5. Preferably, it covers 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 0y 3 , with y3 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 the coefficients x3 and y3 is such that the heart area has 7 cubic crystal structure of face-centered CNN ^

x3 is advantageously between 0.6 and 2.

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

1, 4, while being strictly less than x3, and being such that the heart zone 7 retains the crystallographic structure of Cri Ni.

The strip 1 according to the invention is particularly advantageous for use as a bipolar plate in a fuel cell proton exchange membrane.

Indeed, the use of stainless steel as a substrate for the production of bipolar plates is particularly advantageous. 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. He is also thermoformable, sealable, gas impermeable, and this

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 coating layer 5 made of chromium nitride is electrically conductive, and has a function to improve the surface electrical properties of the bipolar plates made by means of a metal strip 1.

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 piles fuel. 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 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 better adhesion to the 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 detachment during its shaping, in particular by stamping, such detachment may contaminate the electrical conduction properties of the bipolar plate.

A method of obtaining a metal strip 1 according to a first embodiment of the invention will now be explained with reference to Figures 2 and 3.

During a first step of this method there is provided a metal substrate 3 in strip form. The substrate 3 is made of stainless steel.

During a second step, by physical deposition is deposited in vapor phase (also referred to as "PVD" or "physical vapor deposition" in English) of chromium nitride on the substrate 3 in a physical repository vapor 14, as shown in Figure 3.

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 is scrolled through the enclosure 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, an opening 25 for passage 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 coating

PVD. 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 be employed for a given disposal facility, depending on the desired stoichiometry in the coating layer 5.

According to the invention, and as illustrated in Figure 3, 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 area of ​​the chamber 20 located on the path of the substrate 3 in which 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 on the path of the 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 on the path of the substrate 3, the deposition rate of the chromium atoms 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 maximum deposition rate corresponding to the rate of deposition of the chromium atoms on the substrate 3 facing the target 22.

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 downstream cover 28 being intended 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 would be strictly smaller 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 deposition rate 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 cover 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 in traveling substrate 3 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 comprising a surface zone 8 such as defined above, having an oxygen atom content is strictly less than its atomic nitrogen content.

Note that the oxygen possibly present in the coating layer 5 resulting inevitable imperfections of the sealing enclosure 20 and 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 comprising 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 4 illustrates 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 increases in the portions of the coating layer 5 deposited 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 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.

5 the coating layer is advantageously carried out 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, there is obtained a metal strip 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, only in that, as shown in Figure 5, the deposition zone 30 is limited, not only downstream of the target 22 as described above, but also upstream of the target 22 on the path of the 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 at 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 projection of atoms of chromium on the substrate 3 from the target 22 outside the deposition zone 30 ', that is to say in areas of the enclosure 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 deposition of carbon chromium 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 facing 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 in traveling substrate 3 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 interface region 6, an oxygen atom content is strictly less than its atomic nitrogen content. As explained previously, such a coating layer 5 has better adhesion to the substrate 3 as 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, 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.

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 7.

As shown in Figure 7, 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.

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 6.

Samples 1 and 2 are samples of webs 1 according to the invention, 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. L sample 3 is derived from a band which is not according to the invention.

As illustrated in Figure 6, 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.

Sample x2 y2 x3 y3

1 0,9 0,4 1 ,3 0,4

2 1 ,1 0,3 1 ,0 0,3

3 OJJ OJJ 0.77 0.44

Table 2: Compositions measured heart of el areas of interface areas

coating layers

CLAIMS

1. - A method for manufacturing a metal strip or sheet (1; 1 '), comprising:

- providing a substrate (3; 3 ') made of stainless steel; and

- depositing a chromium nitride layer on the substrate (3; 3 ') by physical vapor deposition (PVD) in a deposition device (14) comprising a deposition chamber (20) and a target chromium (22) disposed in the deposition chamber (20), the substrate (3; 3 ') running through the deposition chamber (20) in a longitudinal direction,

wherein the deposition chamber (20) includes a deposition zone (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 to the deposition zone (30; 30 ') in the longitudinal direction, and

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

2. - A method of manufacturing according to claim 1, wherein the first restricted area (32) is located downstream of the target (22) on the path of the substrate (3).

3. - A method of manufacturing according to claim 2, wherein the rate of deposition of chromium on the substrate (3) is greater than or equal to a predetermined threshold in the deposition zone (30; 30 ') downstream of the target (22).

4. - A method of manufacturing according to any preceding claim, wherein the deposition chamber (20) comprises a downstream cover (28), impermeable to chromium atoms, said downstream cover (28) being disposed in the chamber (20) so as to prevent the projection of chromium nitride on the substrate (3; 3 ') in the first closed area (32) and to allow the projection of chromium nitride on the substrate (3) in the region of deposit (30; 30 ').

5. - A method of manufacturing according to claim 4, wherein the downstream cover (28) is interposed on the trajectory of the chromium atoms thrown towards the first closed area (32) so as to prevent their projection in this first prohibited zone (32).

6. - A method of manufacturing according to one of Claims 4 or 5 taken in combination with Claim 3, wherein the downstream cover (28) is arranged in the deposition chamber (20) so as to prevent deposition on the substrate (3; 3 ') chromium atoms from the target (22), the deposition rate on the substrate (3; 3') is strictly less than the predetermined threshold.

7. - A method of manufacturing according to any preceding claim, wherein the deposition chamber (20) further comprises a second closed area (33) in which no chromium nitride is deposited on the substrate (3 ; 3 ') during the deposition step, the second closed area (33) being adjacent to the deposition zone (30') so that the first closed area (32) and the second closed area (33) flank the deposition zone (30 ') in the longitudinal direction.

8. - Manufacturing process according to claim 7, wherein the second closed area (33) is located upstream of the target (22) on the path of the substrate (3; 3 ').

9. - Manufacturing process according to Claim 8, taken in combination with Claim 3, wherein in all the deposition zone (30 '), the deposition rate of the chromium atoms on the substrate (3; 3') for deposition is greater than or equal to the predetermined threshold.

10. - A method of manufacturing according to one of claims 7 to 9, wherein the deposition chamber (20) further comprises an upstream cover (29), impermeable to chromium atoms, said upstream cover (29) being arranged in the enclosure (20) so as to allow the projection of chromium nitride on the substrate (3; 3 ') into the deposition zone (30') and to prevent the projection of chromium nitride on the substrate (3; 3 ') in the second closed area (33).

January 1. - Fabrication process according to claim 10, wherein the upstream cover (29) is interposed on the path of the chromium atoms thrown towards the second closed area (33) from the target (22) so as to prevent their projection in the second closed area (33).

12. - A method according to any preceding claim, taken in combination with Claim 3, comprising, before the deposition step, a step of determining the predetermined threshold for a given disposal facility, for calibration, predetermined threshold corresponding to the minimum deposition rate is obtained for which a coating layer having the desired contact resistance.

13. - A method of manufacturing according to any preceding claim, wherein during the providing step, there is provided a strip or metal sheet made of stainless steel and comprising, at its surface, a passive oxidation layer , said step of providing further comprises a step of etching the passive layer (10) to completely remove the passive layer (10) at least in areas of the metal strip (1 ') or of the sheet intended to be coated with the coating layer (5 ') so that, in these areas, there remains no remaining passive layer (10) at the beginning of the deposition step.

14. - band or metal sheet (1; 1 ') comprising a substrate (3; 3') made of stainless steel and a coating layer (5; 5 ') based on chromium nitride, the coating layer (5 ; 5 ') optionally comprising oxygen, said coating layer (5; 5') being obtained by physical vapor deposition (PVD), characterized in that the coating layer (5; 5 ') has, at its surface, a surface region (8) comprising an oxygen atom content is strictly less than its atomic nitrogen content.

15. - band or metal sheet (1; 1 ') according to claim 14, wherein the surface region (8) has a height less than or equal to 15% of the total thickness of the coating layer (5; 5 ').

16. - strip (1; 1 ') or metal foil according to one of claims 14 or 15, wherein the coating layer (5; 5') comprises, at the interface with the substrate (3; 3 ') , an interface region (6) comprising an oxygen atom content is strictly less than its atomic nitrogen content.

17.- strip (1; 1 ') or foil according to claim 16, wherein the interface region (6) has a height less than or equal to 15% of the total thickness of the coating layer (5; 5 ').

18. - strip (1; 1 ') or metallic sheet according to any one of claims 14 to 17, taken in combination with claim 16, which is constituted, starting from the substrate (3; 3') and towards the surface of the coating layer (5; 5 ') of the interface region (6) of a heart area (7) and the surface region (8), said zones (6,7, 8) being superimposed in a direction normal to the mean plane of the substrate (3; 3 ').

19. - strip (1; 1 ') or metallic sheet according to any one of claims 14 to 18, which has a contact resistance (ICR) of less than 10 mû.cm 2 to 100 N.cm "2 .

20. - strip (1 ') or metallic sheet according to any one of claims 14 to 19, wherein the coating layer (5; 5') is formed directly on the substrate (3 ') of stainless steel, without interposition a passive layer (10) between the coating layer (5 ') and stainless steel substrate (3').

21 .- strip (1 ') or metallic sheet according to any preceding claim, wherein the coating layer (5') is textured, and has in particular an epitaxial relationship with the stainless steel substrate (3 ').

22. A bipolar plate (1 1) comprising at least one plate (13) obtained by deformation of a sheet (1; 1 ') according to any one of claims 14 to 21 or a cut blank from of a strip (1; 1 ') according to any one of claims 14 to 21.

23.- A method for manufacturing a bipolar plate (1 1) comprises cutting the metal strip (1; 1 ') obtained by the method according to any one of claims 1 to 13 for a plate (13) and the shaping of this plate (13).