EP4515024A1

EP4515024A1 - Method for producing an iron nitride coating on the surface of an iron or iron alloy substrate

Info

Publication number: EP4515024A1
Application number: EP23723089.1A
Authority: EP
Inventors: Federico BERTASI; Marco BANDIERA; Arianna PAVESI; Alessandro MANCINI; Andrea BONFANTI
Original assignee: Brembo SpA
Current assignee: Brembo SpA
Priority date: 2022-04-27
Filing date: 2023-04-26
Publication date: 2025-03-05
Also published as: KR20250004292A; IT202200008309A1; JP2025514240A; WO2023209585A1; US20250290219A1; CN119487240A

Abstract

The present invention is directed to a method for producing an iron nitride coating on the surface of an iron or iron alloy substrate, said method comprising the following steps: a) immersion of said iron or iron alloy substrate, in the presence of a counter electrode acting as a cathode, into an electrolytic bath comprising an ionic liquid comprising nitrogen cations and/or nitrogen anions, said substrate acting as an anode; b) electrochemical nitriding process of said substrate, comprising at least one of the following steps: - a galvanostatic step, in which an anodic electric current representative of a predetermined reference current density is applied between the substrate and the counter electrode until a predetermined threshold electric voltage is reached, at which an iron nitride coating is generated on the substrate having a predetermined thickness; - a potentiostatic step, in which an electric voltage having a value equal to a predetermined reference electric voltage is applied between the substrate and the counter electrode until a threshold anodic electric current representative of a predetermined threshold current density is reached, at which an iron nitride coating is generated on the substrate having a predetermined thickness.

Description

METHOD FOR PRODUCING AN IRON NITRIDE COATING ON THE SURFACE

OF AN IRON OR IRON ALLOY SUBSTRATE

Description

Field of the invention

The present invention is directed to a method for producing an iron nitride coating on the surface of an iron or iron alloy substrate, as well as an item comprising an iron or iron alloy substrate and a surface coating of said substrate obtainable by said method.

Background art

Nitriding is a known surface treatment which is applied to iron substrates or iron alloys, for example steel or cast iron, to increase the surface hardness thereof as well as corrosion resistance.

Currently, the nitriding of iron substrates or iron alloys is carried out using the following techniques: a) plasma-assisted deposition; b) ion beam deposition; c) laser melting; d) gas phase deposition; e) cyanide- containing bath.

However, these nitriding techniques require: high treatment temperatures, generally between 400 and 1000°C; long process times, up to hundreds of hours; complex systems (e.g., vacuum systems and high temperature chambers). Moreover, these nitriding techniques have significant environmental and safety problems, for example arising from the use of extremely toxic cyanide baths.

Therefore, conventional nitriding techniques are not suitable for use in most industrial applications. In particular, with these techniques it is substantially impossible to treat large parts or components, as well as thin parts or components, as they are easily subject to thermal deformation.

The constraint given by conventional nitriding techniques on process temperatures is particularly problematic. In fact, these techniques are not applicable to parts or components which cannot be heated without losing some of the geometric characteristics thereof (e.g., flatness or roughness).

Therefore, the problem underlying the present invention is to provide a method for producing an iron nitride coating on the surface of an iron or iron alloy substrate, such as steel or cast iron, which can be carried out at significantly lower temperatures as compared to those required by conventional techniques, in particular at temperatures not exceeding 250°C, and which is easily applicable on an industrial scale.

Summary of the invention

The above problem is solved by a method for producing an iron nitride coating on the surface of an iron or iron alloy substrate as well as by an item comprising a coating obtainable by said method, as outlined in the appended claims, the definitions of which form an integral part of the present description.

A first object of the invention is a method for producing an iron nitride coating on the surface of an iron or iron alloy substrate, said method comprising the following steps: a) immersion of said iron or iron alloy substrate, in the presence of a counter electrode acting as a cathode, into an electrolytic bath comprising an ionic liquid comprising nitrogen cations and/or nitrogen anions, said substrate acting as an anode; b) electrochemical nitriding process of said substrate, comprising at least one of the following steps:

- a galvanostatic step, in which an anodic electric current representative of a predetermined reference current density is applied between the substrate and the counter electrode until a predetermined threshold electric voltage is reached, at which an iron nitride coating is generated on the substrate having a predetermined thickness;

- a potentiostatic step, in which an electric voltage having a value equal to a predetermined reference electric voltage is applied between the substrate and the counter electrode until a threshold anodic electric current representative of a predetermined threshold current density is reached, at which an iron nitride coating is generated on the substrate having a predetermined thickness.

A second object of the present invention is an item comprising an iron or iron alloy substrate and an iron nitride surface coating on said substrate, in which said surface coating is obtainable by the aforesaid method.

The method of the invention allows producing iron nitride coatings operating at considerably lower temperatures as compared to the known processes, even at room temperature, as well as obtaining homogeneous coatings without conditioning the substrate morphology and size. It results that the method of the invention is particularly suitable for making surface coatings of substrates which cannot be heated, otherwise geometric features such as flatness and roughness will be lost.

Moreover, the method of the invention allows producing iron nitride coatings in a short time, of the order of a few tens of minutes, thus being particularly suitable for industrial scale application.

In light of the aforementioned advantages, the method of the invention can thus find application in any technological field, from precision mechanics to the aerospace industry, from the medical field to dental implantology, up to the automotive field.

Further features and advantages of the invention will become apparent from the description of some embodiments, given below by way of non-limiting indication.

Brief description of the drawings

Figure 1 shows the Raman profile of the coating of a cast iron sample obtained with the method of the invention (Profile A) compared to the Raman profile of a coating of the same sample obtained with a conventional ferritic nitrocarburization process or FNC (Profile B).

Figure 2 shows the glow discharge optical emission spectroscopy (GD-OES) profiles obtained for a cast iron sample coated with the method of the invention and the same cast iron sample without a coating.

Detailed description of the invention

The present invention relates to a method for producing an iron nitride coating on the surface of an iron or iron alloy substrate, comprising a step a) in which said substrate is immersed in an electrolytic bath containing an ionic liquid comprising nitrogen ions, in the presence of a counter electrode, and a subsequent step b) in which an electrochemical nitriding process of said substrate comprising a galvanostatic step and/or a potentiostatic step is activated. It was surprisingly found that during the electrochemical nitriding process the nitrogen ions of the ionic liquid contained in the electrolytic bath decompose, producing nitrogen species which diffuse into the iron or iron alloy substrate, generating a surface coating of iron nitride. It has surprisingly been observed that in the method according to the invention there is a surface conversion of the substrate rather than a deposition of the coating, thus obviating adhesion problems between substrate and coating.

The term "ionic liquid" denotes a chemical compound consisting of ions (salt) which, under certain temperature and pressure conditions, is presented in the liquid state. An ionic liquid can be equated in all respects to a molten salt and, according to an empirical definition, ionic liquids have a melting temperature lower than 100°C, thus being in the liquid state at low temperatures or already at room temperature. In the latter case, they are referred to as room temperature ionic liquids (or RTIL).

In an embodiment of the invention, the ionic liquid contained in the electrolytic bath is a room temperature ionic liquid (RTIL).

The upper temperature limit at which said electrochemical process can be carried out is defined by the thermal stability of the ionic liquid. Therefore, said electrochemical nitriding process is advantageously carried out at temperatures not higher than the thermal degradation temperature of said ionic liquid.

Preferably, said electrochemical nitriding process is carried out at a temperature below 250°C, more preferably below 200°C. Even more preferably, said electrochemical nitriding process is carried out at room temperature. The term "room temperature" means a temperature between about 20 and 30°C, for example between about 20 and 25°C.

Advantageously, both step a) and step b) of the method of the invention are carried out at the aforesaid temperature.

Advantageously, the electrolytic bath in which the substrate is immersed during step a) is at a temperature below 250°C, preferably below 200°C, more preferably is at room temperature.

Advantageously, the electrochemical nitriding process of step b) is carried out at a temperature below 250°C, preferably below 200°C, more preferably it is carried out at room temperature.

In addition to simplifying the operational management of the process, the fact of employing temperatures below 250°C advantageously allows obtaining an iron nitride coating (preferably Fe₂-₃N, Fe₃N) with a lower degree of crystallinity as compared to the iron nitride obtained with conventional nitriding techniques, for example with the plasma technique. In accordance with an embodiment, the method of the invention allows obtaining a mixture of Fe₂-₃N (HCP structure, compact hexagonal) and Fe₃N (rhombohedral structure).

It was surprisingly observed that the iron nitride coating obtained by the method of the invention has a higher corrosion resistance as compared to the iron nitride coatings obtained by the conventional techniques. Without being bound by theory, it is believed that a lower degree of crystallinity of the iron nitride results in a reduced extension of the grain edges and thus a reduction of the reactive sites on which oxidation processes can occur.

In accordance with different embodiments, the electrochemical nitriding process comprises a galvanostatic step or a potentiostatic step or both of said steps.

Galvanostatic step

In accordance with a first embodiment, said electrochemical nitriding process comprises at least one galvanostatic step.

During the galvanostatic step, an anodic electric current representative of a predetermined reference current density is applied between the substrate (which acts as an anode) and the counter electrode (which acts as a cathode) until a predetermined threshold electric voltage is reached, at which an iron nitride coating having a predetermined thickness is obtained. The anodic electric current applied during the galvanostatic step can have a constant trend or a pulsed trend; when the anodic electric current has a pulsed trend, it is the average value of said anodic electric current which is representative of said predetermined reference current density.

In other words, during the galvanostatic step an anodic electric current is applied to which a current density having a constant or average constant value corresponds, until a predetermined threshold electric voltage and therefore a desired thickness of the substrate surface coating are reached. In other words, during the galvanostatic step an anodic electric current having a constant or averagely constant value (constant average value) is applied.

Said predetermined reference current density is such as to ensure the at least partial decomposition of the nitrogen cations and/or anions of the ionic liquid, generating nitrogen species available to form the nitride coating of the substrate.

Preferably, said predetermined reference current density has a value of at least 0.025 mA/cm², or at least 0.050 mA/cm², or at least 0.075 mA/cm², or at least 0.1 mA/cm², or at least 0.2 mA/cm², or at least 0.3 mA/cm², or at least 0.4 mA/cm², or at least 0.5 mA/cm², or at least 0.6 mA/cm², or at least 0.7 mA/cm², or at least 0.8 mA/cm², or at least 0.9 mA/cm², or at least 1 mA/cm² .

Preferably, said predetermined reference current density has a value not exceeding 50 mA/cm², or not exceeding 45 mA/cm², or not exceeding 40 mA/cm², or not exceeding 35 mA/cm², or not exceeding 30 mA/cm², or not exceeding 25 mA/cm², or not exceeding 20 mA/cm², or not exceeding 15 mA/cm², or not exceeding 10 mA/cm², or not exceeding 5 mA/cm² .

Preferably, said predetermined reference current density has a value between 0.5 and 50 mA/cm², for example between 0.5 and 20 mA/cm², or between 0.5 and 10 mA/cm² .

The values of said reference current density vary according to the electrical resistance of the substrate to be coated, in particular according to how much native oxide is present on the surface of the substrate. This means that the thicker the native oxide layer covering the substrate (up to several nanometers thick), thus the more resistive the substrate, the higher the reference current density value will be.

During said galvanostatic step, the applied anodic electric current can be both direct and alternating. Preferably, said predetermined threshold electric voltage is at least 1 V, or at least 2 V, or at least 3 V, or at least 4 V, or at least 5 V, or at least 6 V, or at least 7 V, or at least 8 V, or at least 9 V, or at least 10 V.

Preferably, said predetermined threshold electric voltage is not more than 20 V, or not more than 19 V, or not more than 18 V, or not more than 17 V, or not more than 16 V, or not more than 15 V, or not more than 14 V, or not more than 13 V, or not more than 12 V, or not more than 11 V.

Preferably, said predetermined threshold electric voltage is between 1 and 20 V, more preferably between 1 and 12 V, still more preferably between 1 and 10 V, for example 5 V.

Preferably, upon reaching said threshold electric voltage, an average coating thickness of at least 0.040 μm, preferably of at least 0.1 μm, more preferably of at least 1 μm, even more preferably between 1 and 5 μm is obtained.

The higher the threshold electric voltage, the faster the growth rate of iron nitride on the substrate, resulting in the formation of the nitride coating. However, as the electric voltage increases, the evolution of gaseous products from the electrolytic bath also increases. It was found that threshold electric voltage values between 1 and 20 V ensure a good compromise between coating growth rate and process efficiency.

The duration of the galvanostatic step depends on the anodic electric current applied and the time taken to reach the aforesaid predetermined threshold electric voltage.

As already mentioned, the thickness of the iron nitride coating obtained during the galvanostatic step depends on the threshold electric voltage reached, therefore also on the time required for the achievement thereof. In other words, the thickness of the iron nitride coating also depends on the duration of the galvanostatic step .

Preferably, the galvanostatic step has a duration of at least 5 minutes, or of at least 10 minutes, or of at least 15 minutes, or of at least 20 minutes, or of at least 25 minutes.

Preferably, the galvanostatic step has a duration not exceeding 180 minutes, or not exceeding 120 minutes, or not exceeding 60 minutes, or not exceeding 55 minutes, or not exceeding 50 minutes, or not exceeding 45 minutes, or not exceeding 40 minutes, or not exceeding 35 minutes, or not exceeding 30 minutes. Preferably, the galvanostatic step has a duration between 10 and 180 minutes, or between 15 and 60 minutes, for example about 60 minutes.

Potentiostatic step

According to a second embodiment, said electrochemical nitriding process comprises at least one potentiostatic step.

During the potentiostatic step, an electric voltage having a value equal to a predetermined reference electric voltage is applied between the substrate (which acts as an anode) and the counter electrode (which acts as a cathode) until a threshold anodic electric current representative of a predetermined threshold current density is reached, at which an iron nitride coating having a predetermined thickness is obtained. The electric voltage applied during the potentiostatic step can have a constant or pulsed trend; when the electric voltage has pulsed trend, it is the average value thereof which is equal to said predetermined reference electric voltage.

In other words, during the potentiostatic step a constant, or averagely constant, electric voltage is applied until a threshold current density and therefore a desired thickness of the substrate surface coating are reached. Therefore, the electric voltage applied during the galvanostatic step has a constant or averagely constant value (constant average value).

Preferably, said threshold current density corresponds to a current density value below which the efficiency of the electrochemical process is lower, as the thickness of the coating remains substantially constant in view of a progressive increase in electricity consumption.

Said predetermined reference electric voltage is such as to create an anodic electric current capable of decomposing, at least partially, the nitrogen cations and/or anions of the ionic liquid, generating nitrogen species available to form the nitride coating of the substrate; said predetermined reference electric voltage is also such as to overcome the electrical resistance of the nitride coating being formed.

Preferably, said predetermined reference electric voltage is at least 1 V, or at least 2 V, or at least 3 V, or at least 4 V, or at least 5 V, or at least 6 V, or at least 7 V, or at least 8 V, or at least 9 V, or at least 10 V.

Preferably, said reference electric voltage is not more than 50 V, or not more than 45 V, or not more than 40 V, or not more than 35 V, or not more than 30 V, or not more than 25 V, or not more than 20 V, or not more than 15 V. Preferably, said predetermined reference electric voltage is between 1 and 20 V, or between 1 and 10 V, for example about 5 V.

The values of said reference electric voltage vary according to the electrical resistance of the substrate to be coated. This means that the more resistive the substrate, the greater the value of the reference electric voltage so that the electrical resistance of the nitride coating being formed can be overcome.

During said potentiostatic step, the applied electric voltage can be either continuous or alternating.

Preferably, said predetermined threshold current density has a value of at least 20 pA/cm², or at least 25 pA/cm², or at least 30 pA/cm², or at least 40 pA/cm², or at least 45 pA/cm², or at least 50 pA/cm².

Preferably, said predetermined threshold current density has a value not exceeding 80 pA/cm², or not exceeding 75 pA/cm², or not exceeding 70 pA/cm², or not exceeding 65 pA/cm², or not exceeding 60 pA/cm², or not exceeding 55 pA/cm².

Preferably, said predetermined threshold current density is between 20 and 80 pA/cm², more preferably between 30 and 70 pA/cm², even more preferably about 50 pA/cm² . Preferably, upon reaching said threshold current density an average coating thickness of at least 0.040 μm, preferably of at least 0.1 μm, more preferably of at least 1 μm, even more preferably between 1 and 5 μm is obtained.

The duration of the potentiostatic step depends on the applied electric voltage and the time required to reach the threshold anodic electric current .

As already mentioned, the thickness of the iron nitride coating obtained during the potentiostatic step depends on the threshold electrical density, therefore also on the time required for the achievement thereof. In other words, the thickness of the iron nitride coating also depends on the duration of the potentiostatic step.

Preferably, the potentiostatic step has a duration of at least 5 minutes, or of at least 10 minutes, or of at least 15 minutes, or of at least 20 minutes, or of at least 25 minutes.

Preferably, the potentiostatic step has a duration not exceeding 180 minutes, or not exceeding 120 minutes, or not exceeding 60 minutes, or not exceeding 55 minutes, or not exceeding 50 minutes, or not exceeding 45 minutes, or not exceeding 40 minutes, or not exceeding 35 minutes, or not exceeding 30 minutes. Preferably, the potentiostatic step has a duration between 15 and 180 minutes, or between 15 and 60 minutes, for example about 60 minutes.

Galvanostatic step and potentiostatic step

In accordance with a preferred embodiment, the electrochemical nitriding process comprises a galvanostatic step and a potentiostatic step in sequence, in which the potentiostatic step follows the galvanostatic step. The two steps are as described above.

Preferably, the electric voltage applied during the potentiostatic step corresponds to the threshold electric voltage reached in the galvanostatic step.

It has been experimentally found that sequentially carrying out first a galvanostatic step and then a potentiostatic step maximizes the efficiency of the electrochemical nitriding process by obtaining a coating having a desired average thickness, preferably between 0.040 and 5 μm, more preferably between 0.5 and 5 μm, even more preferably between 1 and 5 μm.

Without being bound by theory, during the galvanostatic step the nitrogen species resulting from the decomposition of the nitrogen cations and/or anions of the ionic liquid diffuse into the substrate in a controlled manner, forming iron nitrides. The electrical resistance of the substrate increases with the formation of said nitrides. During the potentiostatic step, the electric voltage is kept on average equal to a predetermined value so that the diffusion of the nitrogen species into the substrate proceeds efficiently.

Preferably, the electrochemical process comprising a galvanostatic step and a potentiostatic step in sequence has a duration between 5 and 180 minutes, more preferably between 10 and 120 minutes, still more preferably between 30 and 60 minutes, for example about 60 minutes. Said duration is variable according to the temperature used for said electrochemical process.

The following description relates to the embodiment in which the electrochemical nitriding process comprises only one galvanostatic step, the embodiment in which the electrochemical nitriding process comprises only one potentiostatic step, and the embodiment in which the electrochemical process comprises one galvanostatic step and one potentiostatic step in sequence.

As already mentioned above, the galvanostatic step and/or the potentiostatic step are carried out with a constant trend or with a pulsed trend, respectively, of the anodic electric current and of the electric voltage.

In a preferred embodiment, the galvanostatic step and/or the potentiostatic step are carried out with a pulsed trend, respectively, of the anodic electric current and of the electric voltage. In accordance with this embodiment, preferably, the amplitude of the pulses of the current and/or electric voltage with respect to the average value is at least ±10%; for smaller amplitudes, the pulsed trend would lead to effects substantially equivalent to those of the constant trend. Preferably, each current and/or electric voltage pulse has a duration of at least 100ms (milliseconds).

Advantageously, carrying out the galvanostatic step and/or the potentiostatic step with pulsed trend allows limiting the possible formation of by-products on the surface of the substrate, reducing the evolution of gaseous by-products, and improving the local mixing of the ionic liquid, all this contributing to increasing the efficiency of the process.

In accordance with a preferred embodiment, the electrochemical nitriding process according to step b) of the method of the invention is carried out under an inert atmosphere, for example under nitrogen flow, allowing increasing the efficiency of the nitriding process and reducing the occurrence of collateral reactions of iron oxide (e.g., rust) formation before nitriding.

In accordance with a preferred embodiment, during the aforesaid step a), the substrate is immersed in the electrolytic bath by means of a support structure (so- called rack) which has the function of keeping the substrate immersed in the bath and, at the same time, applying the electric current and/or the electric voltage coming, respectively, from a current and voltage generator. In other words, said support structure advantageously ensures the electrical contact between the voltage/current generator and the substrate to be nitrided. Moreover, said support structure advantageously allows measuring the electric voltage and current flow applied to the substrate. Preferably, said support structure is made of metal, for example titanium or stainless steel.

In accordance with an embodiment of the invention, the non-aqueous electrolytic bath consists of said ionic liquid. In accordance with this embodiment, no solvents are added to the electrolytic bath and the ionic liquid acts as both a reagent and a solvent ("neat reaction").

In accordance with an alternative embodiment, the non-aqueous electrolytic bath comprises said ionic liquid and a polar solvent, preferably an aprotic polar solvent. Preferably, said polar solvent is selected from the group consisting of acetonitrile, dimethylformamide, dimethylsulfoxide, acetone. The addition of a polar solvent to the electrolytic bath allows modulating the viscosity of the ionic liquid, increasing the conductivity of the ionic liquid and increasing the coulombic efficiency of the electrochemical process. This allows obtaining nitride coatings having a greater thickness.

Preferably, during the electrochemical process, the electrolytic bath is subjected to forced mixing, for example by means of mixers and/or stirrers, with an increase in the efficiency of the process.

As already mentioned, said ionic liquid comprises nitrogen cations and/or nitrogen anions.

Preferably, said nitrogen cations are selected from the group consisting of pyrrolidinium, imidazolium, morpholinium, piperidinium and ammonium cations. Preferably, said nitrogen cations are functionalized with alkyl groups, preferably selected from methyl, ethyl, propyl and butyl.

Preferably, said nitrogen anions are selected from the group consisting of dicyanamide, tricyanomethanide, bis (trifluoromethylsulfonyl)imide, bis (fluorosulfonyl)imide, phosphate, hexafluorophosphate and nitrate anions.

In accordance with a preferred embodiment, said ionic liquid comprises nitrogen cations and nitrogen anions. Preferably, said ionic liquid is selected from the group consisting of: 1-propyl-l-methylpyrrolidinium dicyanamide; 1-ethyl-l-methylpyrrolidinium dicyanamide; 1-propyl-l-methylimidazolium dicyanamide; 1-ethyl-l- methylimidazolium dicyanamide; l-ethyl-3- methylmorpholinium dicyanamide; Tributylmethylammonium bis (trifluoromethylsulfonyl)imide; Butyltrimethylammonium bis (trifluoromethylsulfonyl)imide; Choline bis (trifluoromethylsulfonyl)imide; l-ethyl-3- methylimidazolium bis(fluorosulfonyl)imide; l-ethyl-3- methylimidazolium bis (trifluoromethylsulfonyl)imide; 1- methyl-l-propylpiperidinium bis (trifluoromethylsulfonyl)imide; 1-butyl-l- methylpyrrolidinium bis(trifluoromethylsulfonyl)imide; 1- ethyl-3-methylimidazolium nitrate. In accordance with another embodiment, said ionic liquid comprises nitrogen cations or nitrogen anions. Preferably, said ionic liquid is selected from the group consisting of: Tributylmethylphosphonium bis (trifluoromethylsulfonyl)imide, Diethylmethylsulfonium bis (trifluoromethylsulfonyl)imide, 1-methyl-l- propylpiperidinium tetrafluoroborate. As already highlighted above, the iron or iron alloy substrate, for example cast iron or steel, is immersed as an electrode in the ionic liquid, in the presence of a counter electrode. Said substrate acts as an anode, while said counter electrode acts as a cathode. Preferably, said counter electrode consists of a body made of graphite, or stainless steel, or titanium, or aluminum. Advantageously, said materials have a high electrical conductivity and, at the same time, do not degrade in the ionic liquid. More preferably, said counter electrode is graphite.

In accordance with different embodiments, said counter electrode consists of a body immersed in the electrolytic bath, or of the same container of the electrolytic bath in which the substrate is immersed.

In accordance with a preferred embodiment, the method of the invention comprises a step of pre-treating the substrate. Said pre-treatment step is carried out upstream of step a) of immersing the substrate in the electrolytic bath.

Preferably, said pre-treatment step comprises a "degreasing" step in which any traces of grease and/or lubricant-coolant liquid are removed from the surface of the substrate. Preferably, during said "degreasing" step, the substrate is immersed in a polar solvent for a predetermined period of time, preferably at least 30 seconds, optionally with ultrasound application. The polar solvent is water, ethanol, or a mixture thereof, for example; optionally, the water contains a surfactant. The substrate can then be washed with water and air dried. Preferably, said pre-treatment step comprises a "surface preparation" step in which any traces of oxides are removed from the substrate surface. Preferably, during said "surface preparation" step, the substrate is immersed in an acidic solution for a predetermined period of time, preferably at least 1 minute, and then washed with water. Said acidic solution is an aqueous solution of 5% wt. hydrochloric acid, for example.

Preferably, said pre-treatment step comprises both said "degreasing" step and said "surface preparation" step .

Following said pre-treatment step, the substrate to be subjected to steps a) and b), of immersion in the electrolytic bath and electrochemical nitriding process, respectively, advantageously exhibits an electrically conductive surface. Said pre-treatment step is particularly preferred; in fact, if the surface of the substrate were contaminated by processing residues (e.g., lubricant-coolant liquid), islands with lower electrical conductivity could be formed, on which the nitride coating would grow unevenly.

In accordance with a preferred embodiment, the method of the invention comprises a step of post-treating the substrate. Said post-treatment step is carried out downstream of step b), i.e., downstream of the electrochemical nitriding process.

Preferably, said post-treatment step comprises a "rinsing" step in which any ionic liquid residues are removed from the nitrided surface of the substrate. Preferably, during said "rinsing" step, the coated substrate is immersed in a polar solvent for a predetermined period of time, preferably at least 30 seconds. The polar solvent is water, ethanol, or a mixture thereof, for example. The substrate can then be washed with distilled water. Preferably, the substrate is then air dried.

Preferably, said post-treatment step further comprises a subsequent "heat treatment" step such as to increase the corrosion resistance of the substrate. During said "heat treatment" step, the substrate is heated to a temperature between 50 and 200°C, for example between 50 and 150°C, for a predetermined period of time, preferably at least 30 seconds.

As already mentioned above, the object of the present invention is also an item comprising an iron or iron alloy (e.g., cast iron or steel) substrate and an iron nitride surface coating on said substrate, in which said surface coating is obtainable by the method described above. Preferably, said surface coating contains a mixture of iron nitride Fe₂-₃N and iron nitride Fe₃N.

Since the method of the invention allows treating any complex geometry which can be immersed in an electrolytic bath, the item of the invention is not limited to particular geometries or morphologies.

Preferably, said item is a component of automobiles or motorcycles, or a part of said component.

In a specific embodiment, said item is the brake caliper of the braking system of a car or motorcycle, or a part of said brake caliper.

In another embodiment, said item is the brake disc of the braking system of a car or motorcycle, or a part of said brake disc, for example the braking bands.

Experimental section

A cast iron sample was first immersed in an ultrasound ethanol bath for more than 30 seconds and then in an acidic solution for more than 1 minute, then washed with water.

The sample thus treated was mounted on a support structure (also referred to as a rack, i.e., a metal rod on which the sample was hooked), then immersed in an electrolytic bath comprising tributylmethylammonium bis(trifluoromethylsulfonyl )imide. A graphite counter electrode was also immersed in the electrolytic bath. The sample was then subjected to an electrochemical process at room temperature, comprising a first galvanostatic step and a second potentiostatic step.

More specifically, an anodic electric current having an average current density of 10 mA/cm² was initially applied between the sample and the counter electrode until an electric voltage of 5 V was reached; the first galvanostatic step lasted about 10 minutes. An average electric voltage of 5 V was then applied between the sample and the counter electrode until a current density of 50 pA/cm² was measured; the second galvanostatic step lasted about 50 minutes.

After the electrochemical nitriding process was completed, the sample was immersed in an ethanol bath for more than 30 seconds. After immersion, the sample was rinsed with distilled water and allowed to air dry.

The sample thus obtained was coated with a homogeneous layer of iron nitride with a thickness between about 0.5 and 1.0 μm. Said layer also showed a low degree of crystallinity and a corrosion potential 78 mV higher than the uncoated sample, demonstrating that the sample coated using the method of the invention has a higher corrosion resistance as compared to the uncoated sample.

Figure 1 shows the Raman profile of the coating obtained with the method described above (Profile A) compared to the Raman profile of a coating obtained with a conventional ferritic nitrocarburization process, or FNC (Profile B). From a comparison of the two profiles, it can be seen that the coating obtained with the method of the invention has a lower degree of crystallinity and a different concentration of oxidation by-products (e.g., Fe₂O₃, FeOOH). In the first approximation, the degree of crystallinity can be evaluated by looking at the width of the peaks at mid-height; the peaks of the coating obtained with the method of the invention show a greater width at mid-height as compared to the coating obtained with the FNC technique.

Figure 2 shows the glow discharge optical emission spectroscopy (GD-OES) profiles obtained for the cast iron sample coated with the method of the invention and for the same cast iron sample without a coating. As compared to the uncoated sample, the nitrided sample shows: 1) a nitrogen concentration peak just below the surface of the sample, and 2) a non-negligible nitrogen concentration moving away from the surface of the sample. This confirms that the coating obtained by the method of the invention is integrated in the sample matrix.

It is apparent that only one particular embodiment of the present invention was described. Those skilled in the art will be able to make all modifications required to the method for producing an iron nitride coating on the surface of an iron or iron alloy substrate for the adaptation thereof to particular conditions, without however departing from the scope of protection as defined in the appended claims.

Claims

1. A method for producing an iron nitride coating on the surface of an iron or iron alloy substrate, said method comprising the following steps: a) immersion of said iron or iron alloy substrate, in the presence of a counter electrode acting as a cathode, into an electrolytic bath comprising an ionic liquid comprising nitrogen cations and/or nitrogen anions, said substrate acting as an anode; b) electrochemical nitriding process of said substrate, comprising at least one of the following steps:

2. The method according to claim 1, wherein said ionic liquid is at a temperature below 250°C, preferably below 200°C, more preferably is at room temperature.

3. The method according to claim 1 or 2, wherein the electrochemical nitriding process according to said step b) is conducted at a temperature below 250°C, preferably below 200°C, more preferably is conducted at room temperature.

4. The method according to any one of the preceding claims, wherein said step b) comprises said galvanostatic step and said potentiostatic step in sequence, said potentiostatic step being subsequent to said galvanostatic step .

5. The method according to any one of the preceding claims, wherein, during the galvanostatic step, said anodic electric current has a constant trend.

6. A method according to any one of claims 1 to 4, wherein, during the galvanostatic step, said anodic electric current has a pulsed trend, the average value of said anodic electric current being representative of said predetermined reference current density.

7. The method according to any one of the preceding claims, wherein, during the potentiostatic step, said electric voltage has a constant trend.

8. The method according to any one of the preceding claims 1 to 7, wherein, during the potentiostatic step, said electric voltage has a pulsed trend, the average value of said electric voltage being equal to said predetermined reference electric voltage.

9. The method according to any one of the preceding claims, wherein said predetermined reference current density has a value between 0.5 and 50 mA/cm², preferably between 0.5 and 20 mA/cm², or between 0.5 and 10 mA/cm² .

10. The method according to any one of the preceding claims, wherein said predetermined threshold electric voltage is between 1 and 20 V, preferably between 1 and 12 V, more preferably between 1 and 10 V.

11. The method according to any one of the preceding claims, wherein said predetermined reference electric voltage is between 1 and 50 V, preferably between 1 and 20 V, more preferably between 1 and 10 V.

12. The method according to any one of the preceding claims, wherein said predetermined threshold current density is between 20 and 80 pA/cm², preferably between 30 and 70 pA/cm², more preferably of about 50 pA/cm² .

13. The method according to any one of the preceding claims, wherein the electrochemical nitriding process according to said step b) has a duration between 5 and 180 minutes, preferably between 10 and 120 minutes, more preferably between 30 and 60 minutes.

14. The method according to any one of the preceding claims, producing an iron nitride coating having an average thickness between 0.040 and 5 μm, preferably between 0.5 and 5 μm, more preferably between 1 and 5 μm.

15. The method according to any one of the preceding claims, wherein said ionic liquid is an ionic liquid at room temperature (RTIL).

16. The method according to any one of the preceding claims, wherein said ionic liquid comprises: nitrogen cations selected from the group consisting of pyrrolidinium, imidazolium, morpholinium, piperidinium and ammonium cations, preferably said cations being functionalized with alkyl groups preferably selected from methyl, ethyl, propyl and butyl, and/or nitrogen anions selected from the group consisting of dicyanamide, tricyanomethanide, bis (trifluoromethylsulfonyl)imide and bis (fluorosulfonyl)imide, phosphate, hexafluorophosphate and nitrate anions.

17. The method according to any one of the preceding claims, wherein said ionic liquid comprises nitrogen cations and nitrogen anions, preferably said ionic liquid being selected from the group consisting of: 1-propyl-l- methylpyrrolidinium dicyanamide; 1-ethyl-l- methylpyrrolidinium dicyanamide; 1-propyl-l- methylimidazolium dicyanamide; 1-ethyl-l- methylimidazolium dicyanamide; l-ethyl-3- methylmorpholinium dicyanamide; Tributylmethylammonium bis (trifluoromethylsulfonyl)imide; Butyltrimethylammonium bis (trifluoromethylsulfonyl)imide; Choline bis (trifluoromethylsulfonyl)imide; l-ethyl-3- methylimidazolium bis(fluorosulfonyl)imide; l-ethyl-3- methylimidazolium bis (trifluoromethylsulfonyl)imide; 1- methyl-l-propylpiperidinium bis (trifluoromethylsulfonyl)imide; 1-butyl-l- methylpyrrolidinium bis(trifluoromethylsulfonyl)imide; 1- ethyl-3-methylimidazolium nitrate.

18. The method according to any one of claims 1 to 16, wherein said ionic liquid is Tributylmethylphosphonium bis (trifluoromethylsulfonyl)imide, or Diethylmethylsulfonium bis(trifluoromethylsulfonyl)imide, or 1-methyl-l-propylpiperidinium tetrafluoroborate.

19. The method according to any one of the preceding claims, wherein during said step a), the substrate is immersed in the electrolytic bath by means of a support structure (so-called rack).

20. The method according to any one of the preceding claims, wherein said counter electrode consists of a body made of graphite, or stainless steel, or titanium, or aluminum .

21. The method according to any one of the preceding claims, wherein said counter electrode consists of a body immersed in said electrolytic bath or of the container of said electrolytic bath.

22. The method according to any one of the preceding claims, comprising a step of pre-treating said substrate upstream of said step a), said pre-treatment step in turn comprising: a "degreasing" step, in which the substrate is immersed in a polar solvent for a predetermined period of time, preferably at least 30 seconds, optionally with ultrasound application, and/or a "surface preparation" step, in which the substrate is immersed in an acid solution for a predetermined period of time, preferably at least 1 minute, and then washed with water.

23. The method according to any one of the preceding claims, comprising a step of post-treating said substrate downstream of said step b), in which said substrate is immersed in a polar solvent for a predetermined period of time, preferably at least 30 seconds, and optionally heated to a temperature between 50 and 200°C for a predetermined period of time, preferably at least 30 seconds.

24. An item comprising an iron or iron alloy substrate and an iron nitride surface coating on said substrate, wherein said surface coating is obtainable by the method according to any one of the preceding claims.

25. The item according to claim 24, wherein said surface coating contains a mixture of iron nitride Fe₂-₃N and iron nitride Fe₃N.

26. The item according to claim 24 or 25, being the brake caliper or the brake disc of the braking system of a car or a motorcycle, or a part of said brake caliper or said brake disc.