EP4263921A1

EP4263921A1 - Method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate

Info

Publication number: EP4263921A1
Application number: EP21841004.1A
Authority: EP
Inventors: Federico BERTASI; Alessandro MANCINI; Marco BANDIERA; Andrea BONFANTI
Original assignee: Brembo SpA
Current assignee: Brembo SpA
Priority date: 2020-12-21
Filing date: 2021-12-16
Publication date: 2023-10-25
Also published as: US20240044035A1; CN116783330A; WO2022137036A1; JP2024500811A

Abstract

The invention relates to a method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate, comprising the following steps: a) immersing the titanium or titanium alloy substrate as an electrode in a non-aqueous electrolyte comprising a room temperature ionic liquid comprising nitrogen ions in the presence of a counter electrode; and b) activating an electrochemical process of nitriding the substrate by applying an electric potential between the electrode, acting as an anode, and the counter electrode, acting as a cathode, so as to generate an anodic electric current at least sufficient to decompose the nitrogen ions by releasing the nitrogen contained therein. The liberated nitrogen penetrates by diffusion into the titanium or titanium alloy substrate until it leads to the conversion to titanium nitride of a surface layer of said substrate, thereby generating a nitrided diffusion surface layer that forms a nitrided surface coating for the substrate. The electric potential and/or the anodic electric current are modulated in time according to the thickness to be obtained for said nitrided surface coating.

Description

"METHOD FOR PRODUCING A TITANIUM NITRIDE COATING ON THE

SURFACE OF A TITANIUM OR TITANIUM ALLOY SUBSTRATE" DESCRIPTION

Field of application

[0001] The subj ect of this invention is a method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate .

[0002] The method according to the invention finds application in any technological field which needs to improve the tribological performance of components made of titanium or titanium alloys by means of titanium nitride surface coatings . The application may therefore range from precision mechanics to the aerospace industry, from the medical sector to dental implantology .

[0003] Advantageously, the method according to the invention enables titanium nitride coatings to be produced on substrates made of titanium or its alloys operating at low temperatures , and in particular at room temperature . It therefore finds particular application in producing said coatings on parts or components that cannot be heated without losing some of their geometric features ( flatness , roughness , etc . ) .

[0004] The method according to the invention may find particular application in the automotive industry, for example in the manufacture of protective surface coatings for brake system components , such as brake caliper pistons or brake disc parts made of titanium or its alloys .

Prior art

[0005] Tit anium and its alloys have some very attractive properties that allow them to be used in many industries . Some of the aforesaid properties are : excellent corrosion resistance and erosion resistance ; low density, which gives high speci fic strength-to-weight ratios , allowing for lighter and stronger structures ; high temperature resistance ; and, in some cases , cryogenic properties .

[0006] Tit anium and its alloys , however, are also characteri zed by modest tribological properties , such as poor resistance to abrasive wear, poor resistance to fatigue wear ( fretting) , and a high friction coef ficient . All of this has signi ficantly limited the use of titanium and its alloys in mechanical engineering applications .

[0007] The friction problem is related to the crystal structure and reactivity of titanium and may be largely overcome by appropriate thermochemical treatments that superficially modi fy the titanium substrate , making it harder .

[0008] One of the most common thermochemical treatments of titanium and its alloys is nitriding .

[0009] To date , the nitriding of titanium and its alloys may only be achieved using the following techniques: a) plasma-assisted deposition; b) ion-beam deposition; c) laser melting; d) gas-phase deposition; e) cyanide- containing baths .

[0010] These nitriding techniques all require:

[0011] - high temperatures, between 400 and 1000°C;

[0012] - long processing times (up to hundreds of hours) ;

[0013] - complex installations (vacuum systems, high temperature chambers, etc.) .

[0014] Not least, the aforesaid techniques generally present significant health and safety issues. For example, the use of cyanide baths, an extremely toxic compound, is cited.

[0015] As a result, traditional nitriding techniques are not suitable for use in most industrial applications. In particular, it is basically impossible to process large parts, as well as thin parts, as they are easily subject to thermal deformation.

[0016] Particularly problematic is the constraint given by conventional nitriding techniques on process temperatures. In fact, these techniques are not applicable on parts or components that may not be heated without losing some of their geometric characteristics.

[0017] Ph. Roquiny et al., "Colour control of titanium nitride coatings produced by reactive magnetron sputtering at temperature less than 100°C", Surface and Coatings Technology 116-119 (1999) 278-283 [1] and S.

Bellucci et al., "Synthesis of Titanium Nitride Film by RF Sputtering", Nanosci. Nanotechnol. Lett. 3 (2011) 1-9 [2] describe methods for the preparation of titanium nitride films at low temperature using plasma technology (reactive magnetron sputtering) in DC or AC mode, respectively. However, this technique has some limitations, namely:

[0018] - it requires expensive high-vacuum systems and magnetron technology for radio frequency control, which seem difficult to scale to a practical industrial level;

[0019] - it allows only for the treatment of small components that will fit inside a vacuum chamber;

[0020] - it may not be used to treat components with complex geometry (presence of shielded areas that cannot be reached by the plasma) ; and

[0021] - it suffers from a poor deposition rate with the need for long treatment times to obtain submillimetric coatings .

[0022] CN108752006, CN108946733, and CN108557783 describe the possibility of using plasma-based techniques to obtain titanium nitride (TiN) nanopowders at room temperature. However, these documents only refer to obtaining TiN in powder form and do not teach how to make homogeneous surface coatings. Similarly to the techniques described in [1] and [2] , complex vacuum systems are required. In addition, vacuum powder handling systems are also required (e.g., CN108752006 envisages a ball milling system) , making these techniques unsuitable for large batch production.

[0023] Thus, there remains a great need for a method to produce a titanium nitride coating on the surface of a titanium or titanium alloy substrate that may be carried out at low temperatures and is readily applicable on an industrial scale.

[0024] In this context, "low temperatures" means temperatures not exceeding 250°C, i.e., temperatures that may be considered low when compared to the operational temperatures of the conventional methods mentioned above for the formation of homogeneous surface coatings, which are not lower than 400°C.

Disclosure of the invention

[0025] Therefore, it is a principal object of this invention to eliminate, or at least reduce, the aforementioned problems related to the prior art by providing a method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate that may be carried out at low temperatures and is readily applicable on an industrial scale. [0026] A further obj ect of this invention is to provide a method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate that may be carried out at room temperature and is readily applicable on an industrial scale .

[0027] A further obj ect of this invention is to provide a method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate that allows submillimetric coatings , in particular of at least one micron, to be obtained in a short time .

[0028] A further obj ect of this invention is to provide a method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate , which allows a very homogeneous coating to be obtained .

[0029] A further obj ect of this invention is to provide a method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate that allows a homogeneous coating to be obtained without conditioning by the morphology and dimensions of the substrate .

Description of the drawings

[0030] The technical features of the invention are clearly identi fiable in the content of the claims set out below and the advantages thereof will become more readily apparent in the detailed description that follows , made with reference to the accompanying drawings , which represent one or more embodiments provided purely by way of non-limiting examples , wherein :

[0031] - Fig . 1 shows a flow diagram of a method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate according to a preferred embodiment of the invention ;

[0032] - Fig . 2 shows in two superimposed graphs the time pattern of electric potential and current density during an electrochemical nitriding process according to an example application of the method according to the invention;

[0033] - Fig . 3A and 3B show di f fractograms , respectively, of a stoichiometric TiNx coating obtained with the method according to the invention and of the comparison thereof with a stoichiometric TiN coating obtained by a conventional plasma technique ; and

[0034] - Fig . 4 shows the polari zation curves of a sub- stoichiometric TiNx coating obtained with the method according to the invention and a non-nitrided T16A14V titanium alloy; and

[0035] - Fig . 5 shows in two superimposed graphs the time pattern of the electric potential and current density during an electrochemical nitriding process according to a further example of application of the method according to the invention .

Detailed description

[0036] This invention relates to a method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate .

[0037] This method comprises the following operational steps :

[0038] a ) immersing the titanium or titanium alloy substrate as an electrode in a non-aqueous electrolyte consisting of a Room Temperature Ionic Liquid (RTIL ) , including nitrogen ions , in the presence of a counter electrode ; and

[0039] b ) activating an electrochemical process for nitriding the substrate , by applying an electric potential between the electrode , acting as an anode , and the counter electrode , acting as a cathode , so as to generate an anodic electric current at least suf ficient to decompose the nitrogen ions by releasing the nitrogen contained therein .

[0040] Operationally, the counter electrode acts as a cathode in the sense that reduction reactions may occur on its surface .

[0041] During the aforesaid electrochemical nitriding process , nitrogen released from the decomposition of the nitrogen ions of the ionic liquid penetrates by di f fusion into the titanium or titanium alloy substrate , leading to the conversion of a surface layer of said substrate into titanium nitride . Thus , a nitrided surface di f fusion layer is generated on the substrate , forming a surface coating for the substrate .

[0042] Operationally, said electric potential and/or anodic current are modulated over time as a function of the thickness desired for the aforesaid nitrided surface coating .

[0043] Advantageously, the electrochemical nitriding process is carried out at low temperatures , i . e . , at temperatures below 250 ° C, preferably below 200 ° C .

[0044] The upper temperature limit at which the electrochemical process may be carried out is defined by the thermal stability of the ionic liquid . Once the thermal stability limit is exceeded, the ionic liquid degrades completely or partially, and the electrochemical nitriding process may not proceed .

[0045] The electrochemical nitriding process is thus carried out at temperatures not exceeding the temperature of maximum thermal stability of the ionic liquid .

[0046] According to a preferred embodiment of the method, the electrochemical nitriding process is carried out at room temperature . This is particularly advantageous as it simpli fies the operational management of said process . [0047] From an operational point of view, however, carrying out the electrochemical nitriding process at low temperatures (not exceeding 250°C) limits the mobility of nitrogen in the crystalline matrix of the titanium or titanium alloy, with consequences on the chemical features of the titanium nitride obtained, as illustrated hereinafter .

[0048] Advantageously, said nitrided surface coating that is obtained by the method according to the invention is composed of sub-stoichiometric titanium nitride TiNx, wherein 0<x<0.3. Such sub-stoichiometric titanium nitride has a lower degree of crystallinity than the degree of crystallinity of stoichiometric titanium nitride TiN.

[0049] The titanium nitride obtainable with the conventional plasma technique is a stoichiometric (Ti:N equal to 1:1) and crystalline TiN. In contrast, the titanium nitride obtainable with the method according to the invention is a sub-stoichiometric TiN (Ti:N equal to 1:0.3 max) and has a low degree of crystallinity.

[0050] The low crystallinity of the sub-stoichiometric TiNx makes this titanium nitride much more corrosion-resistant than stoichiometric titanium nitride. The low degree of crystallinity implies, in fact, a more limited extension of grain boundaries and therefore a reduction of reactive sites on which oxidation processes may occur. [0051] The maximum ratio Ti:N of 0.3 is related to the low temperatures at which the electrochemical process is carried out. Low process temperatures limit the mobility of nitrogen in the crystalline matrix of the titanium or titanium alloy substrate. The lower the temperature, the more difficult it will be to insert nitrogen into the titanium crystal lattice. The use of a room temperature ionic liquid as a nitrogen source, instead of a high T gas (as in the conventional plasma technique) thus affects the features of the titanium nitride obtained. In other words, the sub-stoichiometry (and thus the superior corrosion resistance) is a direct result of operating at low temperatures.

[0052] Advantageously, the nitrided surface coating obtainable by the method according to the invention has an average thickness between 0.040 pm and 5 pm. These thicknesses may be obtained by prolonging the aforementioned step b) of activating an electrochemical nitriding process for a period of time between 5 and 45 minutes, variable according to the selected temperature and the way in which the electrochemical process is carried out.

[0053] According to a preferred embodiment of the invention, as illustrated in the flow diagram of Fig. 1, the aforesaid electrochemical process comprises two consecutive steps :

[0054] - a first galvanostatic step, wherein an electric potential is applied, modulated in time so as to generate an anodic electric current having a value on average equal to a predefined base current density, until a predefined threshold electric potential is reached; and [0055] - a second potentiostatic step, wherein an electric potential equal to a predefined base electric potential is applied and maintained until at least one anodic current with a predefined threshold current density is reached .

[0056] It has been experimentally veri fied that performing in sequence first a galvanostatic step and then a potentiostatic step maximi zes the ef ficiency of the electrochemical nitriding process in terms of the coating thickness obtained and the amount of nitrogen inserted into the crystalline matrix of the titanium or titanium alloy substrate .

[0057] More speci fically, at the beginning of the electrochemical nitriding process ( zero coating thickness ) , it is preferable that the di f fusion of nitrogen into the substrate matrix occurs in a slow and controlled manner . A galvanostatic step is performed for this purpose . Once a certain amount of TiNx is formed, the electrical resistance of the coating begins to increase , and in order for nitrogen di f fusion in the substrate matrix to proceed ef ficiently, it is necessary to maintain an electric potential on average equal to a predefined value . A potentiostatic step is carried out for this purpose .

[0058] The electrochemical nitriding process may also be carried out by performing only a galvanostatic step or only a potentiostatic step . However, being the conditions equal , with respect to the execution of the electrochemical process in the aforesaid two consecutive steps , a nitrided coating with a lower thickness and associated lower nitrogen insertion i s obtained .

[0059] As described above , during the first galvanostatic step, a time-modulated electric potential is applied so as to generate an anodic electric current on average equal to a predefined base current density until a predefined threshold electric potential is reached .

[0060] Preferably, said predefined base current density is between 0 . 025 and 0 . 5 mA/ cm2 .

[0061] The current density values to be used depend on the electrical resistance of the substrate being treated and how much native oxide is present on the surface . I f the substrate is not very resistive (with a thickness of a few nanometers of native oxide ) , a base current density value close to 0 . 025 mA/ cm2 may be used; conversely, i f the substrate is very resistive (with many nanometers of native oxide) , a base current density value in the vicinity of 0.5 mA/ cm2 may be used.

[0062] Tit anium and titanium alloys have an extremely negative redox potential (-1.63V vs. NHE, normal hydrogen electrode) . This means that titanium is always covered with a thin layer of native oxide and the natural oxidation of its surfaces is virtually instantaneous. Moreover, the native titanium oxide is particularly dielectric, i.e., it has a dielectric constant higher than many other transition metal oxides; therefore, even very small thicknesses, for example less than 100 nm, are sufficient for the electric field to be very attenuated therein. As already pointed out, this behavior forces the use of current densities that may be different depending on the initial conditions of the substrate to be treated. [0063] The fact that many titanium compounds (including nitride) are very dielectric explains the reason why very thick coatings cannot be achieved and the titanium nitride coating has excellent corrosion resistance.

[0064] Preferably, the aforesaid predefined electrical threshold potential is between 2 and 12V, and even more preferably between 4 and 10V, and most preferably equal to 5V.

[0065] The minimum potential value to be applied is the one necessary to make the nitrogen ions in the ionic liquid decompose and thus make the nitrogen comprised therein available .

[0066] Operationally, the more the electric potential grows and approaches 12V, the faster the growth of TiNx occurs . However, for values of applied electric potential greater than 10V, the rate of parasitic reactions ( causing gas evolution from the ionic liquid bath) becomes non- negligible and therefore the ef ficiency of the process is reduced . In other words , the goal in the galvanostatic step is to use the current to degrade the nitrogen ions in the ionic liquid and not to evolve gaseous products . Electric potential values not exceeding 10V therefore ensure a good compromise between the speed of growth of the coating and the columbic ef ficiency of the process .

[0067] As described above , during the second potentiostatic step, an electric potential on average equal to a predefined base electric potential is applied and maintained until at least one anodic current having a predefined threshold current density is reached .

[0068] Preferably, the aforesaid predefined base electric potential is between 8 and 50V, and even more preferably equal to 10V .

[0069] The goal of the potentiostatic step is to support the nitrogen insertion reaction in the substrate matrix by overcoming the electrical resistance of the nitrate coating that is being formed . The electric potential may therefore exceed the value of 10V to reach values even close to 50V, although it is preferable to maintain values close to 10V to contain the evolution of gaseous products from the ionic liquid .

[0070] Preferably, the aforesaid predefined threshold current density is between 20 and 80 pA/cm2 , and even more preferably 50 pA/ cm2 .

[0071] It has been experimentally veri fied that below this threshold value the electrochemical process becomes completely inef ficient as there is almost no growth in coating thickness against a progressive increase in energy consumption .

[0072] Preferably, the aforesaid second potentiostatic step has a duration of at least 5 minutes , regardless of the value of the threshold current density .

[0073] During the aforesaid first galvanostatic phase , the anodic electric current may be maintained on average equal to the predefined base current density in a constant manner ( as illustrated for example in Fig . 2 ) or in a pulsed pattern ( as illustrated for example in Fig . 5 ) .

[0074] Preferably, in the case of a pulsed pattern, the amplitude of the current density pulses with respect to the mean value is at least ± 10% . For smaller amplitudes the pulsed pattern leads to ef fects substantially equivalent to those of the constant pattern .

[0075] Preferably, each current density pulse has a duration of at least 100 ms (milliseconds ) .

[0076] During the aforesaid second potentiostatic step, the electric potential may be maintained on average equal to the predefined base electric potential in a constant manner ( as illustrated for example in Fig . 2 ) or in a pulsed pattern .

[0077] Preferably, in the case of a pulsed pattern, the amplitude of the electric potential pulses with respect to the mean value is at least ± 10% . For smaller amplitudes the pulsed pattern leads to ef fects substantially equivalent to those of the constant pattern .

[0078] Preferably, each electric potential pulse has a duration of at least 100 ms (milliseconds ) .

[0079] It was possible to veri fy that carrying out the galvanostatic step and/or the potentiostatic step with a pulsed pattern allows a series of advantages to be obtained with respect to the case of a constant pattern : [0080] - Limiting the possible formation of undesirable byproducts on the surface of the nitrided workpiece ; it is hypothesi zed that these by-products could be due to parasitic reactions leading to degradation of the ionic liquid without an associated nitride formation;

[0081] - Reducing the evolution of gaseous by-products .

[0082] - Improving local mixing of the ionic liquid by allowing the surface of the treated substrate to be replenished with new ions ; this helps to increase the ef ficiency of the process .

[0083] Preferably, the galvanostatic step and/or the potentiostatic step are carried out with pulsed patterns of the electric current density and the electric potential , respectively .

[0084] Advantageously, both direct current and alternating current may be applied in the electrochemical process .

[0085] Preferably, the substrate is immersed in the ionic liquid bath via a support structure, which has the function of keeping the substrate immersed and simultaneously supplying current from a current/voltage generator . Through this support structure it is also possible to measure the electric potential and the current flow applied to the substrate in order to manage the electrochemical nitriding process .

[0086] The use of a Room Temperature Ionic Liquid (RTIL ) as a non-aqueous electrolytic bath is an essential feature of the method . No solvents are added to the ionic liquid .

In fact , the ionic liquid acts as both a solvent and a reagent. Such a reaction is referred to in the jargon as a "neat reaction." In general, room temperature ionic liquids are non-volatile, non-toxic compounds with high thermal stability and ionic conductivity.

[0087] Advantageously, during the electrochemical process the ionic liquid may be subjected to a forced stirring (e.g., by mixers) . Stirring allows the surface of the treated substrate to be replenished with new ions, helping to increase the efficiency of the process.

[0088] Preferably, the aforesaid room temperature ionic liquid comprises nitrogen anions. In fact, it was possible to verify that by using ionic liquids with nitrogen cations the electrochemical process is triggered with great difficulty.

[0089] According to a preferred embodiment of the method, the aforesaid room temperature ionic liquid comprises: [0090] - pyrrolidinium, imidazolium and/or morpholinium cations and

[0091] - dicyanamide and/or tricyanomethanide anions.

[0092] Preferably, the pyrrolidinium, imidazolium, and morpholinium cations are functionalized with radical groups chosen from the group consisting of: methyl, ethyl, propyl, and butyl, preferably methyl, ethyl, and propyl .

[0093] Particularly preferred are anionic liquids having dicyanamide as the nitrogen anion . In fact , the dicyanamide anion consists of 64 % nitrogen . There are no liquid compounds with nitrogen anions having suf ficient ionic conductivity and a similar nitrogen concentration as dicyanamide .

[0094] Advantageously, i f ionic liquids with anions other than dicyanamide are used, in order to compensate for the lower amount of available nitrogen, for example, the viscosity, melting temperature , and solvent power of the ionic liquid may be modulated in order to increase the ionic conductivity of the ionic liquid and thus increase the columbium ef ficiency of the electrochemical proces s . The " solvent power" of the ionic liquid refers to its ability to behave like a solvent and thus , for example , to dissolve other materials . However, for the purposes of the electrochemical nitriding process , what is important is for there to be the highest possible concentration of nitrogen, preferably of the anionic type .

[0095] According to a wholly preferred embodiment of the method, the aforesaid room temperature ionic liquid is chosen from the group comprised of : 1-propyl- l- methylpyrrolidinium dicyanamide , 1-ethyl- l- methylpyrrolidinium dicyanamide , 1-propyl- l- methylimidazolium dicyanamide , 1-ethyl- l- methylimidazolium dicyanamide , and l-ethyl-3- methylmorpholinium dicyanamide.

[0096] According to an alternative embodiment of the method, the aforesaid room temperature ionic liquid comprises bis (trifluoromethylsulfonyl) imide or bis ( fluorosulfonyl ) imide anions.

[0097] In particular, the ionic liquid may be chosen from the group consisting of: Tributylmethylmethylammonium bis (trifluoromethylsulfonyl) immide;

Butyl trimethylammonium bis (trifluoromethylsulfonyl) immide; Choline bis (trifluoromethylsulfonyl) immide; l-Ethyl-3- methylimidazolium bis ( fluorosulf onyl ) immide ; l-Ethyl-3- methylimidazolium bis (trifluoromethylsulfonyl) immide; 1- Methyl-1 -propylpiperidinium bis (trifluoromethylsulfonyl) immide; 1-Butyl-l- methylpyrrolidinium bis (trifluoromethylsulfonyl) immide; Tributylmethylmethylphosphonium bis (trifluoromethylsulfonyl) immide;

Di ethylmethyl sulf onium bis (trifluoromethylsulfonyl) immide.

[0098] According to a further alternative, the ionic liquid may be chosen from the group consisting of: l-Ethyl-3- methylimidazolium nitrate; and 1-Methyl-l- propylpiperidinium tetrafluoroborate .

[0099] As pointed out above, the titanium or titanium alloy substrate is immersed as an electrode in the room temperature ionic liquid in the presence of a counter electrode .

[00100] Preferably, said counter electrode consists of a body made of graphite , stainless steel , titanium, or aluminum . Even more preferably the counter electrode is made of graphite .

[00101] In fact , these materials have a high electrical conductivity and at the same time do not degrade in the ionic liquid when an electric potential is appl ied .

[00102] The aforesaid counter electrode ( cathode ) may consist of :

[00103] a body immersed in the non-aqueous electrolyte ( ionic liquid) similarly to the substrate ( electrode/anode ) ; or

[00104] - the same container of the aforesaid nonaqueous electrolyte ( ionic liquid) within which the substrate ( electrode/anode ) is immersed .

[00105] Advantageously, as illustrated in the diagram in Fig . 1 , the method may comprise a pre-treatment step c ) of the substrate , to be carried out prior to the aforesaid steps a ) and b ) . Pre-treatment consists of removing any traces of grease and/or coolant from the surface of the substrate .

[00106] Preferably, the pre-treatment for removing any traces of grease and/or lubricant-coolant is carried out by immersing the substrate in a polar solvent for a predetermined period of time , preferably with the additional application of ultrasound . In particular for example water and/or ethanol may be used as a polar solvent . I f water is used, a surfactant may be added . Immersion in the polar solvent may be followed by washing with distilled water and subsequent air drying .

[00107] The pre-treatment step c ) , while optional , is nevertheless preferred . In fact , it is preferable that before proceeding to steps a ) and b ) the substrate to be coated exposes an electrically conductive surface . I f the surface of the substrate is contaminated with processing residues ( e . g . , lubricant-coolant ) , it is possible for islands of lower electrical conductivity to be formed on which the nitrided coating grows unevenly .

[00108] Advantageously, as illustrated in the diagram in Fig . 1 , the method may comprise a post-treatment step d) of the substrate to be carried out after the aforesaid steps a ) and b ) . The post-treatment involves removing any residual ionic liquid from the nitrided surface of the substrate .

[00109] Preferably, the post-treatment for removing any residual ionic liquid is carried out by immersing the nitrided substrate in a polar solvent for a predefined period of time. In particular for example water and/or ethanol may be used as a polar solvent. Immersion may be followed by washing with distilled water and subsequent air drying.

Example of application

[00110] A sample of titanium alloy T16A14V (Al 6% by weight; V 4%) , having an area of about 8 cm2, was used as the substrate.

[00111] The sample was immersed in an ultrasonic bath in ethanol for more than 30 seconds. After immersion, the sample was rinsed with distilled water and allowed to air dry. The purpose of this pre-treatment is to remove any grease and/or traces of lubricant-coolant liquid.

[00112] In this case, the method for producing a titanium nitride coating on the surface of the substrate according to the invention was carried out at room temperature (25°C) .

[00113] A bath consisting of 1-propyl-l- methylpyrrolidinium dicyanamide was used as the electrolytic bath (room temperature ionic liquid; CAS No. : 327022-60-6; C13H20N4, the structural formula of which is provided below) .

[00114] After undergoing pre-treatment , the sample was mounted on a support structure (rack) configured to measure both the electric potential and current flow of the sample during the electrochemical nitriding process (step (b) ) , which will be described hereinafter. Thus mounted, the sample was immersed in the electrolytic bath and used as the working electrode (anode) . A graphite counter electrode (cathode) was also immersed in the bath .

[00115] After being immersed in the electrolytic bath (step a) ) , the sample was subjected to an electrochemical nitriding process (step b) ) .

[00116] The electrochemical nitriding process consists of two consecutive electrochemical steps: the first is a galvanostatic step, while the second is a potentiostatic step .

[00117] As shown in the graph in Fig. 2, during the galvanostatic step, an anodic current having a density of

0.025 mA/ cm2 was applied to the T16A14V sample until an electric potential of 5V was reached . This first galvanostatic step lasted approximately 25 minutes . Subsequently, during the potentiostatic step , the electric potential of the rack ( and thus of the sample ) was kept constant at 10V until a current of 50 pA/ cm2 was measured . This second galvanostatic step lasted approximately 15 minutes .

[00118] 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 .

[00119] The sample is found to be coated with a homogeneous layer of sub-stoichiometric titanium nitride TiNx with x=0 . 3 . The coating had an average thickness of about 1 pm and had a typical bright golden color .

[00120] Fig 3A shows the di f f ractogram of the sub- stoichiometric TiNx coating obtained on the T16A14V titanium alloy sample . Fig . 3B shows the measurement of a stoichiometric TiN sample for comparison . It may be observed that the coating obtained by the method according to the invention comprises a TiNx layer with x=0 . 3 . With respect to a crystalline TiN (which may be obtained using conventional methods such as plasma techniques ) , the pattern of the coating obtained appears clearly di f ferent , therefore indicating a lower degree of crystallinity and a sub-stoichiometric composition .

[00121] As a first approximation, the degree of crystallinity may be assessed by looking at the width of the di f fraction peaks . The peaks of the coating obtained according to the invention TiNo. a show a larger mid-height width with respect to the crystalline TiN .

[00122] The value of x may be determined by comparing the measured di f f ractogram with that of materials comprised in appropriate databases .

[00123] The TiNx coating thus obtained was subj ected to line scan voltammetry measurements to calculate the corrosion potential and corrosion current . The results are shown in Fig . 4 where the polari zation curves of the TiNx ( top curve ) and the non-nitrided T16A14V alloy sample (bottom curve ) are shown . The comparison of the two curves demonstrates the excellent corrosion protection capability of TiNx with 0<x<0 . 3 . In fact , for the TiNO . 3 coating, a corrosion potential of + 104 mV vs SCE ( Saturated Calomel Electrode ) and a corrosion current of 8 nA cm-2 were detected; the non-nitrided T16A14V alloy sample , on the other hand, had a corrosion potential of -297 mV vs SCE and a corrosion current of 39 nA cm- 2 .

[00124] These two values attest to the impressive corrosion resistance of the TiNx coating, which is higher than that of the T16A14V alloy, one of the most commonly used titanium alloys also due to its excellent corrosion resistance .

[00125] Some physicochemical properties of the TiNx coating have been measured and are shown in Table 1 below :

Table 1

[00126] It is also an obj ect of this invention to provide an item, comprising at least one titanium or titanium alloy portion, said portion having a nitrided surface coating consisting of sub-stoichiometric titanium nitride TiNx, wherein 0<x<0 . 3 . Sub-stoichiometric titanium nitride has a lower degree of crystallinity than stoichiometric titanium nitride TiN . The aforesaid surface coating is integrated into the crystalline matrix of the titanium or titanium alloy portion .

[00127] Preferably, said nitrided surface coating has an average thickness between 0 . 040 and 5 pm .

[00128] Advantageously, the nitrided surface of the aforesaid article is obtained by subj ecting the titanium or titanium alloy portion to nitriding by applying the method according to the invention, and in particular as described above .

[00129] The invention allows numerous advantages to be obtained which have been explained throughout the description .

[00130] The method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate according to the invention may be carried out at low temperatures and is readily applicable on an industrial scale .

[00131] In particular, the method according to the invention may be carried out at room temperature in a manner that is easily applicable on an industrial scale .

[00132] The method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate according to the invention allows for sub- stoichiometric titanium nitride coatings of at least one micron thickness to be obtained in a short time ( on the order of a few tens of minutes ) . [00133] The method according to the invention also provides a homogeneous titanium nitride coating .

[00134] The method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate according to the invention enables a homogeneous titanium nitride coating to be obtained without conditioning by the morphology and dimensions of the substrate to be coated .

[00135] With respect to the conventional techniques for generating titanium nitride coatings , the method according to the invention :

[00136] - is based on an electrochemical nitriding process that involves the simple immersion of the substrate to be treated in an electrolytic bath, with economic and easy-to-implement modes ;

[00137] allows for large components to be treated without the constraint of mounting a vacuum chamber ;

[00138] - allows for treating any complex geometry that may be immersed in an electrolytic bath;

[00139] - requires short process times , allowed by the adoption of an electrochemical process in contrast to the long times of the traditional plasma technique ;

[00140] is based on a surface conversion of the substrate rather than a coating deposition, thus avoiding adhesion problems between the substrate and the surface layer;

[00141] - avoids the use of very toxic reagents, such as cyanide baths .

[00142] The invention thus conceived therefore achieves its intended purposes.

[00143] Of course, in its practical embodiment it may also assume different forms and configurations from the one illustrated above, without thereby departing from the present scope of protection. [00144] Furthermore, all details may be replaced with technically equivalent elements, and dimensions, shapes, and materials used may be any according to the needs.

Claims

1 . A method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate , comprising the following steps : a ) immersing the titanium or titanium alloy substrate as an electrode in a non-aqueous electrolyte consisting of an ionic liquid at room temperature , comprising nitrogen ions , in the presence of a counter electrode ; and b ) activating an electrochemical nitriding process of the substrate , by applying an electric potential between an electrode , acting as an anode , and the counter electrode , acting as a cathode , so as to generate an anodic electric current at least suf ficient to decompose the nitrogen ions releasing the nitrogen contained therein, the released nitrogen penetrating by di f fusion into the titanium or titanium alloy substrate until leading to the conversion of a surface layer of said substrate into titanium nitride , thereby generating a nitrided di f fusion surface layer constituting a nitrided surface coating for the substrate , wherein said electric potential and/or said anodic electric current are modulated in time as a function of the thickness to be obtained for said nitrided surface coating .

2 . The method according to claim 1 , wherein said nonaqueous electrolyte is at a temperature of less than 250 ° C, preferably less than 200 ° C, and even more preferably at room temperature .

3. Method according to claim 1 or 2 , wherein said nitrided surface coating consists of sub-stoichiometric TiNx titanium nitride , where 0<x<0 . 3 , said sub- stoichiometric titanium nitride having a degree of crystallinity lower than the degree of crystallinity of stoichiometric TiN titanium nitride .

4 . The method according to claim 1 , 2 or 3 , wherein said nitrided surface coating has an average thickness between 0 . 040 and 5pm obtainable by prolonging said step b ) of activating an electrochemical nitriding process for a period of time between 5 and 45 minutes .

5 . The method according to any one of the preceding claims , wherein said electrochemical process comprises two consecutive steps : a first galvanostatic step, in which an electric potential is applied which is modulated in time so as to generate an anodic electric current having a value on average equal to a predefined base current density, until a predefined threshold electric potential is reached; and a second potentiostatic step, in which an electric potential is applied on average equal to a predefined base electric potential , maintaining it until at least reaching an anodic current having a predefined threshold current density .

6. The method according to claim 5 , wherein during said first galvanostatic step the anodic electric current is maintained on average equal to said predefined base current density in a constant manner or in a pulsed pattern .

7 . The method according to claim 5 or 6 , wherein during said second potentiostatic step the electric potential is maintained on average equal to said predefined base electric potential in a constant manner or in a pulsed pattern, preferably in a pulsed pattern .

8 . The method according to claim 5 , 6 or 7 , wherein said predefined base current density is between 0 . 025 to 0 . 5 mA/ cm2 .

9. The method according to any one of the claims from 5 to 8 , wherein said predefined threshold electric potential is between 2 and 12V, and preferably between 4 and 10V, even more preferably equal to 5V .

10 . The method according to any of the claims from 5 to 9 , wherein said predefined base electric potential is between 8 and 50V, and is preferably 10V .

11 . The method according to any of the claims from 5 to

11 , wherein said predefined threshold current density is between 20 and 80 pA/cm2 , and preferably 50 pA/ cm2 .

12 . The method according to any of the claims from 5 to 11 , wherein said second potentiostatic step has a duration of at least 5 minutes , regardless of the threshold current density value .

13. The method according to any of the preceding claims , wherein said ionic liquid at room temperature comprises nitrogen anions .

14 . The method according to any of the preceding claims , wherein said ionic liquid at room temperature consists of pyrrolidinium, imidazolium and/or morpholinium cations and dicyanamide and/or tricyanomethanide anions , wherein preferably the pyrrolidinium, imidazolium and morpholinium cations are functionali zed with radical groups chosen from the group consisting of : methyl , ethyl , propyl and butyl , preferably methyl , ethyl and propyl .

15 . The method according to claim 14 , wherein said ionic liquid at room temperature 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 and l-Ethyl-3- methylmorpholinium dicyanamide .

16. The method according to any of the claims from 1 to 13, wherein said ionic liquid at room temperature comprises bis (trifluoromethylsulfonyl) imide or bis ( fluorosulfonyl ) imide anions.

17. The method according to claim 16, wherein said ionic liquid at room temperature is selected from the group consisting of:

Tributylmethyl ammonium bis (trifluoromethylsulfonyl) immide;

Butyl trimethylammonium bis (trifluoromethylsulfonyl) immide; Choline bis (trifluoromethylsulfonyl) immide; l-Ethyl-3- methylimidazolium bis ( fluorosulf onyl ) immide ; l-Ethyl-3- methylimidazolium bis (trifluoromethylsulfonyl) immide; 1- Methyl-1 -propylpiperidinium bis (trifluoromethylsulfonyl) immide; 1-Butyl-l- methylpyrrolidinium bis (trifluoromethylsulfonyl) immide; Tributylmethylphosphonium bis (trifluoromethylsulfonyl) immide;

Di ethylmethyl sulf onium bis (trifluoromethylsulfonyl) immide.

18. The method according to any of the claims from 1 to

13, wherein said ionic liquid at room temperature is selected from the group consisting of: l-Ethyl-3-methylimidazolium nitrate; and 1-Methyl-l- propylpiperidinium tetrafluoroborate . 37

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

20 . The method according to any of the preceding claims , wherein said counter electrode consists of a body immersed in said non-aqueous electrolyte or the container of said non-aqueous electrolyte .

21 . The method according to any of the preceding claims , comprising a pre-treatment step c ) of the substrate , to be carried out before said steps a ) and b ) , wherein said pre-treatment consists of removing any traces of grease and/or lubricant-coolant liquid from the surface of the substrate .

22 . The method according to claim 21 , wherein said pretreatment of removing any traces of grease and/or lubricating coolant liquid is carried out by immersing the substrate in a polar solvent for a predefined period of time , preferably with the additional application of ultrasound, preferably said immersion being followed by washing with distilled water and subsequent air drying .

23. The method according to any of the preceding claims , comprising a post-treatment step d) of the substrate , to be carried out after said steps a ) and b ) , wherein said post-treatment consists of removing any residues of ionic liquid from the nitrided surface of the substrate .

24 . The method according to claim 23 , wherein said posttreatment of removing any residues of ionic liquid is carried out by immersing the substrate in a polar solvent for a predefined period of time , preferably said immersion being followed by washing with distilled water and subsequent air drying .

25 . Article , comprising at least a portion in titanium or titanium alloy, said portion having a nitrided surface coating consisting of sub-stoichiometric TiNx titanium nitride , where 0<x<0 , 3 , said sub-stoichiometric titanium nitride having a degree of crystallinity less than the degree of crystallinity of stoichiometric TiN titanium nitride , wherein said surface coating is integrated into the crystalline matrix of said portion in titanium or titanium alloy .

26. The article according to claim 25, wherein said nitrided surface coating has an average thickness of between 0 . 040 and 5 pm .

27 . The article according to claim 25 or 26 , wherein said nitrided surface is obtained by subj ecting said portion in titanium or titanium alloy to nitriding by applying the method according to any of the claims from 1 to 24 .