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

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.

1 - 26 . (canceled) 
     
     
         27 . 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. 
   
     
     
         28 . The method according to  claim 27 , wherein said ionic liquid is at a temperature below 250° C. 
     
     
         29 . The method according to  claim 27 , wherein the electrochemical nitriding process according to said step b) is conducted at a temperature below 250° C. 
     
     
         30 . The method according to  claim 27 , wherein said step b) comprises said galvanostatic step and said potentiostatic step in sequence, said potentiostatic step being subsequent to said galvanostatic step. 
     
     
         31 . The method according to  claim 27 , wherein, during the galvanostatic step, said anodic electric current has a constant trend. 
     
     
         32 . A method according to  claim 27 , 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. 
     
     
         33 . The method according to  claim 27 , wherein, during the potentiostatic step, said electric voltage has a constant trend. 
     
     
         34 . The method according to  claim 27 , 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. 
     
     
         35 . The method according to  claim 27 , wherein said predetermined reference current density has a value between 0.5 and 50 mA/cm 2 . 
     
     
         36 . The method according to  claim 27 , wherein said predetermined threshold electric voltage is between 1 and 20 V. 
     
     
         37 . The method according to  claim 27 , wherein said predetermined reference electric voltage is between 1 and 50 V. 
     
     
         38 . The method according to  claim 27 , wherein said predetermined threshold current density is between 20 and 80 μA/cm 2 . 
     
     
         39 . The method according to  claim 27 , wherein the electrochemical nitriding process according to said step b) has a duration between 5 and 180 minutes. 
     
     
         40 . The method according to  claim 27 , producing an iron nitride coating having an average thickness between 0.040 and 5 μm. 
     
     
         41 . The method according to  claim 27 , wherein said ionic liquid is an ionic liquid at room temperature (RTIL). 
     
     
         42 . The method according to  claim 27 , wherein said ionic liquid comprises:
 nitrogen cations selected from the group consisting of pyrrolidinium, imidazolium, morpholinium, piperidinium and ammonium cations, and/or   nitrogen anions selected from the group consisting of dicyanamide, tricyanomethanide, bis(trifluoromethylsulfonyl)imide and bis(fluorosulfonyl)imide, phosphate, hexafluorophosphate and nitrate anions.   
     
     
         43 . The method according to  claim 27 , wherein said ionic liquid are selected from the group consisting of: 1-propyl-1-methylpyrrolidinium dicyanamide; 1-ethyl-1-methylpyrrolidinium dicyanamide; 1-propyl-1-methylimidazolium dicyanamide; 1-ethyl-1-methylimidazolium dicyanamide; 1-ethyl-3-methylmorpholinium dicyanamide; Tributylmethylammonium bis(trifluoromethylsulfonyl)imide; Butyltrimethylammonium bis(trifluoromethylsulfonyl)imide; Choline bis(trifluoromethylsulfonyl)imide; 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide; 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide; 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide; 1-ethyl-3-methylimidazolium nitrate. 
     
     
         44 . The method according to  claim 27 , wherein said ionic liquid is Tributylmethylphosphonium bis(trifluoromethylsulfonyl)imide, or Diethylmethylsulfonium bis(trifluoromethylsulfonyl)imide, or 1-methyl-1-propylpiperidinium tetrafluoroborate. 
     
     
         45 . The method according to  claim 27 , wherein during said step a), the substrate is immersed in the electrolytic bath by means of a support structure. 
     
     
         46 . The method according to  claim 27 , wherein said counter electrode consists of a body made of graphite, or stainless steel, or titanium, or aluminum. 
     
     
         47 . The method according to  claim 27 , wherein said counter electrode consists of a body immersed in said electrolytic bath or of the container of said electrolytic bath. 
     
     
         48 . The method according to  claim 27 , 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, 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, and then washed with water.   
     
     
         49 . The method according to  claim 27 , 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. 
     
     
         50 . 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  claim 27 . 
     
     
         51 . The item according to  claim 50 , wherein said surface coating contains a mixture of iron nitride Fe 2-3 N and iron nitride Fe 3 N. 
     
     
         52 . The item according to  claim 50 , 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. 
     
     
         53 . The method according to  claim 28 , wherein said ionic liquid is at room temperature. 
     
     
         54 . The method according to  claim 29 , wherein the electrochemical nitriding process according to said step b) is conducted at room temperature. 
     
     
         55 . The method according to  claim 35 , wherein said predetermined reference current density has a value between 0.5 and 10 mA/cm 2 . 
     
     
         56 . The method according to  claim 36 , wherein said predetermined threshold electric voltage is between 1 and 10 V. 
     
     
         57 . The method according to  claim 37 , wherein said predetermined reference electric voltage is between 1 and 10 V. 
     
     
         58 . The method according to  claim 38 , wherein said predetermined threshold current density is about 50 μA/cm 2 .