Functional mechanical part and method for surface treatment thereof

Abstract

The present invention relates to a method for treating a mechanical part (2, 3) comprising a functional surface (8) that is made of NiP and is intended to come into frictional contact with another functional surface, the method comprising a step of oxidising and / or phosphating the functional surface so as to artificially form an oxide layer (9) and / or a phosphate layer (10), respectively, on the functional surface (8), the phosphate layer (10) being a layer of Ni or Zn phosphates.

Description

FUNCTIONAL MECHANICAL PART AND ITS SURFACE TREATMENT METHOD Technical field of the invention

[0001] The present invention relates to a mechanical part comprising a functional surface intended in use to come into frictional contact with another functional surface. It also relates to the method of surface treatment of said part. Technological background

[0002] The watch movement was built from the start with lubrication to function. This allows it to reduce friction losses and therefore the energy required for its proper functioning. Lubrication therefore allows it to maintain its good chronometric performance and limit wear. However, oil or grease is an element that ages due to its load of wear particles, its oxidation, or even its migration, or even its evaporation. The environment also plays a role, with temperature changing the viscosity properties and hindering the chronometric operation of the movement. Despite all these inconveniences, movements are still lubricated with oils and greases, which are nevertheless increasingly efficient.

[0003] Throughout the movement's kinematic chain, the crucial point is the escapement. Although it has been mastered for centuries with the Swiss lever design, which is the most widespread today, this escapement requires special attention for its lubrication. Between the epilame that is applied to the lifts to force the oil to remain in contact or even the application of lubricant to the escape wheels, everything is done to guarantee long-term lubrication and therefore chronometric performance.

[0004] Numerous lubricant developments have been carried out with different types of oils or even aimed at applying a solid coating to this Swiss lever escapement.

[0005] For a long time, the holy grail has been the elimination of lubrication, particularly in the exhaust, which would make it possible to overcome problems linked to lubrication.

[0006] One method is to limit the friction phases on the escapement. The best known is the coaxial escapement, which, due to its architecture, minimizes friction by replacing it with impacts, allowing it to operate without oil. Another method is to change the material of the escapement. Thus, other types of escapement made with silicon or diamond have been developed to minimize friction. Pairs of materials in a functional assembly have also been developed, such as the ceramic pair facing the diamond, which allows the formation of a third lubricating body.

[0007] Currently, no mass-produced movement runs dry and only movements that could be called "prototypes" have been marketed in very limited quantities.

[0008] A new solution is therefore always sought. Summary of the invention

[0009] To this end, an analysis of the condition of functional surfaces of mechanical parts subjected to friction for several years was carried out. The tests were carried out on parts coated with NiP or made in bulk from this material, NiP being known to improve shock resistance. NiP or Nickel-Phosphorus is a Nickel alloy containing 12% Phosphorus. It is classified among the high phosphorus alloys. It is non-magnetic and resists corrosion well. It is hard (350 to 450 HV at the bath outlet) and can also be hardened by heat treatment up to 900 HV by precipitation of NisP at the grain boundaries.

[0010] Watch wheels made of electroformed NiP and steel coated with a NiP deposit were subjected to a long-term "movement test". The "movement test" is a test in real conditions that allows the suitability of the escapement wheel / lever system to be assessed over a long period. Some NiP wheels tested without any lubrication gave very satisfactory performance over a period of 6 years. After stopping the test and dismantling the components, the wheels were analyzed to assess the condition of the functional surfaces. SEM (Scanning Electron Microscopy) analyses were performed on an ultra-precise microsection obtained by FIB (Focused Ion Beam) section. They revealed a "black layer" of unknown nature, related to a third body that would have formed spontaneously during these years of operation.This "black layer" was subsequently characterized by an EDX (Energy-dispersive X-ray spectroscopy) analysis in order to identify the elements; then by a TOF SIMS (Time-of-F light Secondary Ion Mass Spectrometry) analysis from the extreme surface to a depth of 150 nm to try to determine the chemical bonds present. The results of these analyses show that: - the elements Ni, P, O, C were detected, - PCs'; Ni2PO4; Ch-; PO Ni2Os' bonds were identified.

[0011] These elements and bonds constituting this "black layer" show that the nickel and phosphorus initially present have drastically oxidized and have even formed a new molecule like Ni2PO4. This intense oxidation on the one hand, and the birth of a new nickel phosphate molecule on the other hand, seem responsible for the good tribological behavior of the escape wheel / lever assembly.

[0012] The inventors therefore sought to artificially reproduce, in a short time, what had happened naturally over several years. The idea was to create intense oxidation of the NiP surface using several technologies and also to reproduce in a similar way simple as possible a molecule close to Ni2PO4, namely nickel orthophosphate with the formula Ni3(PO4)2. The two approaches, namely oxidation and phosphating, were examined individually, then combined.

[0013] More specifically, the present invention relates to a method for treating a mechanical part comprising a functional NiP surface intended to come into frictional contact with another functional surface, said method comprising a step of oxidation and / or phosphating said functional surface so as to artificially form a layer of oxides and / or a layer of phosphates respectively on said functional surface. According to the invention, the layer of phosphates is a layer of Ni phosphates or alternatively a layer of Zn phosphates which could also improve the tribological properties.

[0014] To carry out this oxidation and / or phosphating treatment, several possibilities have been successfully evaluated on a tribometer: - Dry oxidation with O2 plasma or with a sweep of an O2 / O3 mixture, - Germination of Nis(PO4)2 by chemical reaction (hydrolysis), - Dry oxidation + Germination of Nis(PO4)2, - Oxidation by electrolysis in an aqueous medium.

[0015] The presence of oxides and phosphates on a surface composed of NiP allows a significant dry tribological gain with stabilization and a reduction in the coefficient of friction compared to raw NiP compared to, for example, ruby.

[0016] The present invention also relates to a mechanical part comprising a functional surface intended to come into frictional contact with another functional surface, said NiP functional surface having been subjected to the above treatment method and comprising a layer of oxides and / or a layer of Ni or Zn phosphates.

[0017] Another aspect of the invention relates to a functional assembly comprising the mechanical part described above and another mechanical part comprising the other functional surface intended to be in frictional contact with the functional surface of said mechanical part, the functional assembly being characterized in that the frictional contact is dry.

[0018] Other characteristics and advantages of the invention will appear on reading the detailed description which follows, with reference to the attached drawings. Brief description of the figures

[0019] Figure 1 partially represents a functional assembly comprising two parts, namely an escape wheel and an anchor pallet with contact surfaces treated according to the method of the invention.

[0020] Figure 2 is a schematic sectional representation of the functional part treated with the method according to the invention.

[0021] Figure 3 represents an electron microscopy image of the distribution of Ni3(PO4)2 seeds on the surface of a sample.

[0022] Figure 4 represents an electron microscopy image of a Ni3(PO4)2 seed.

[0023] Figure 5 represents the curves of the dynamic friction coefficient as a function of the distance traveled for the NiP / ruby pair with the NiP treated by dry oxidation according to the invention for two samples and the NiP / ruby pair without NiP treatment for comparison.

[0024] Figure 6 shows the curves of the dynamic friction coefficient as a function of the distance traveled for the NiP / ruby couple with NiP treated by germination of Nis(PO4)2 according to the invention for two samples and the NiP / ruby couple without NiP treatment for comparison.

[0025] Figure 7 represents the curves of the dynamic friction coefficient as a function of the distance traveled for the NiP / ruby pair with the NiP treated by dry oxidation and germination of Nis(PO4)2 according to the invention for two samples and the NiP / ruby pair without NiP treatment for comparison.

[0026] Figure 8 represents the curves of the dynamic friction coefficient as a function of the distance traveled for the NiP / ruby pair with the NiP treated by oxidation in an aqueous medium according to the invention for a sample and the NiP / ruby pair without NiP treatment for comparison. Detailed description of the invention

[0027] The present invention relates to a mechanical part subjected on one or more of its so-called functional or contact surfaces to friction with one or more functional surfaces of another part or of the same part. The mechanical part can be used in any system where friction is a concern. This can be applications for automotive parts, electronic parts, etc. More specifically, it can be a part in the watchmaking field and in particular a part of the movement. Examples of parts include a pallet, an escape wheel, an axis of a mobile, a bearing, a barrel spring or even gear wheel teeth. Said part can be in contact with another part. For example, in the watchmaking field, the functional assembly 1 visible in FIG. 1 can comprise a first part 2 which is a pallet 4 of an anchor 5 and a second part 3 which is an escape wheel 6.More specifically, the pallet 4 has a rest plane A and an impulse plane B which cooperate with the rest planes C and impulse planes D of the tooth 7 of the escape wheel 6. These planes A, B, C, D are functional surfaces which are highly stressed and subject to high levels of friction and / or. contact requiring the use of special materials to reduce friction. Alternatively, one functional surface of a part may be in contact with another functional surface of the same part. For example, this may be a barrel spring formed from a blade with one face of the spring intended to be in contact with another face of the spring.

[0028] The mechanical part is at least partly made of NiP. Thus, at least the functional surface(s) are made of NiP. The part may be made in bulk of NiP or include a NiP coating at least at the functional surfaces. The other mechanical part comprising the other functional surface intended to be in frictional contact with the functional surface of said mechanical part may be made of a material chosen from ruby, steel and NiP treated or not according to the method of the invention.

[0029] According to the invention, at least the functional surfaces comprise oxides and / or phosphates. In Figure 2, the functional surface 8 can be schematically visualized with a layer of oxides 9 and a layer of phosphates 10. To do this, the functional surfaces have been subjected to an oxidation and / or phosphating treatment with, for the example of Figure 2, an oxidation and phosphating treatment.

[0030] The oxidation treatment can be carried out by dry oxidation or by electrolysis. Dry oxidation can be obtained by atmospheric plasma or vacuum plasma or thermally in an oxygen-flushed furnace. For example, the samples can be oxidized in a vacuum reactor under oxygen plasma or under a flush of an O2-O3 mixture when the equipment is equipped with an ozone generator (O3). The oxidation artificially created by the process appears in the form of a thin and very homogeneous layer of a darker color than the initial substrate. It may be thought that the conversion layer produced is isotropic, although this has not been proven. Advantageously, the oxide layer has a thickness of between 7 and 13 nm, preferably between 8 and 12 nm. The thickness measurement can be carried out by an ellipsometric analysis with for example the SEMILAB Spectroscopic Ellipsometer SE 2000 equipment. Advantageously, the oxide layer has in the CIELAB color space (compliant with standards CIE n°15, ISO 7724 / 1, DIN 5033 Teil 7, ASTM E-1164), a value of a* between 2.2 and 3 and a value of b* between 8 and 12, preferably between 9 and 11.

[0031] Phosphating treatment can be carried out by germinating a phosphate on the NiP substrate. This treatment consists of creating seeds that are favorable to good tribology. Preferably, it is a nickel orthophosphate Ni3(PO4)2 which is relatively easy to germinate. It is also possible to germinate a zinc phosphate (Zn3(PO4)2).

[0032] Germination involves hydrolysis, i.e. the breaking of a covalent bond in an aqueous medium. Its principle consists of introducing a nickel supply in the form of nickel chloride hexahydrate (NiCl2.6H2O) and a phosphate supply in the form of potassium dihydrogen orthophosphate (KH2PO4). These two molecules in the presence of each other do not cause any reaction. On the other hand, if a hydrolyser such as urea (NH2CONH2) is added; the whole thing is brought to a certain temperature which is typically between 70°C and 100°C, the two molecules break to form a third which is nickel orthophosphate, according to the following chain of reactions: 3[Ni(NH3)] 2+ + 2H2PO4- + 8H2O Ni3(PO4)2. 8H2O + 3NH3+ 2H2

[0033] Typically, nickel chloride hexahydrate is in an aqueous solution with a molar concentration between 0.01 and 0.06 M, potassium dihydrogen orthophosphate is in an aqueous solution with a molar concentration between 0.02 and 0.09 M and urea in an aqueous solution with a molar concentration between 0.01 and 0.15 M.

[0034] Nickel orthophosphate germinates on NiP. The germination has an average density of 35 germs per 100 micron square by 100 microns. A representation is given in Figure 3. This is equivalent to saying that there are 0.0035 germs per square micron. The germs have a leaflet-like appearance that forms a flower petal structure. A germ measures approximately 5 to 6 microns (see Figure 4) and is very adherent to the NiP surface.

[0035] There are alternative methods for generating nickel orthophosphate (Nis(PO4)2). The previous hydrolysis reaction requires an energy input to occur. In this case, it was heat that enabled this reaction to occur. However, it is possible to imagine achieving this energy input using a cold plasma (under reduced or atmospheric pressure) or even ultrasonic energy.

[0036] Furthermore, other reactions lead to the formation of nickel orthophosphate in aqueous or solid media, we can cite: - Saifon Kullyakool et al in “Determination of kinetic triplet of the synthesized Ni3(PO4)2_8H2O by non-isothermal and isothermal kinetic methods” (journal of thermal analysis and calorimetry feb 2014): v' NiSCM and Na2HPÛ4 at 90°C for 1 to 5 days NiSCM (0.5M) and NasPCM (0.5M) at 70°C for 1 day.

[0037] In the latter two cases, a precipitate of nickel orthophosphate is obtained.

[0038] Two other methods are described in: - Ismaël Saaddoune et al in “Synthesis characterization, Electrochemistry and in situ XRD investigation of Nis(PO4)2 as negative electrode material for lithium ion batteries” (ChemElectroChem 10.1002 / celc.202001065): Synthesis of Ni3(PO4)2 by solid-state route using NiO and ammonium phosphate (NH4)2HPO4. These ingredients are mixed (powders) and then calcined in steps of 200 to 900°C in a furnace without gas protection. - Lowie Henderick et al in “Plasma enhanced atomic layer deposition of nickel and cobalt phosphate for lithium ion batteries” (2022,51,2059 Dalton Transactions) suggest the precursors Nicp2 (cyclopentadienyl-nickel) as well as TMP (trimethylphosphate) in an O2 plasma at 300°C.

[0039] It is also possible to deposit a nickel orthophosphate film using ALD (Atomic Layer Deposition) technology. In this case, we are dealing with a film and not germs.

[0040] Samples were prepared with the oxidation and / or phosphating treatment according to the invention and tribological tests were carried out on these samples.

[0041] Oxidation treatments were carried out with dry vacuum oxidation. The latter was obtained by vacuum plasma treatment. The equipment used is the “PVD / PECVD Denton Discovery” machine. The samples are placed in a vacuum chamber. They are preheated between 100 and 200°C. The highly plasmagenic Ar gas is introduced into the chamber, the substrate holder of which is subjected to a negative potential that can vary between 500 and 1000 V; which generates a power varying between 90 W and 380 W in the Denton equipment used. The pressure is set at 15 pbar. Typically, it can be between 10 and 30 pbar. An Ar plasma is ignited. The first step consists of cleaning the surface by bombarding it with relatively heavy Ar ions for a few minutes. Once the surface is decontaminated, the Ar is gradually replaced by O2 until a pure oxygen plasma is obtained, which then takes on a yellow tint. It is the oxygen plasma that will generate the expected oxide layer. This very energetic plasma has an electronic temperature of around 100,000 °K (one hundred thousand degrees Kelvin). This temperature, which is not physical, simply reflects the agitation and intense reactivity of the atoms confined in this plasma.

[0042] About ten tests were carried out under different conditions where a certain number of parameters are fixed: the pressure in the vacuum chamber as well as the flow of Ar and O2 while other parameters are variable: the applied voltage, the temperature of the chamber and the residence time of the sample in the chamber.

[0043] Furthermore, 2 additional tests were carried out in ALD Encapsulix equipment which has the particularity of having an ozone generator. This Os gas is particularly reactive but also very unstable and ephemeral. This generator provides an O2 / O3 gas mixture which can be used with or without plasma and which sweeps the samples in a chamber in which a vacuum has been previously created as in the previous equipment. One test is carried out with a plasma at the risk of destroying the O3 molecule; while the other is carried out in the form of a simple chemical sweep without plasma.

[0044] 10 tests were carried out on the 2 pieces of equipment described above. The tribological results given below are presented for one sample of each. One, numbered 36, was carried out in the ALD machine with chemical sweeping (without plasma) of an O2 / O3 mixture for 4 hours. Another, numbered 8, was carried out in the PVD machine, with 700 V (172 W) for 15 minutes at 150°C (figure 5).

[0045] Oxidation treatments have also been carried out in aqueous media. Oxidation is carried out during simple electrolysis of water, by connection to the positive pole (+) of the electrodes. There, a release of O2 occurs, well known to those skilled in the art. The variable parameters are: - The type of solution o Electrolytic degreasing bath (alkaline), o 1 M KOH solution (alkaline). - Anodic oxidation in H3PO4 medium at 0.1 M (acid), - A stay in immersion (therefore chemical) in alkaline, neutral and acidic environments. The result for the tribological test is shown below for the sample with number 35 (figure 8).

[0046] For the phosphating treatment, the germination of Nis(PO4)2 was carried out following the following protocol: - Source of nickel: nickel chloride hexahydrate (NiCL, 6H2O) dissolved at a rate of 1570 mg per 200cc of water; i.e. a 0.033 M solution, - Phosphate source: potassium dihydrogen orthophosphate (KH2PO4) dissolved at a rate of 1794 mg per 200 cc of water; i.e. a 0.066 M solution, - Hydrolyzer source: urea (NH2CONH2) dissolved in different proportions: o 300 mg per 200 cc of water (0.025 M solution), o 600 mg per 200 cc of water (0.05 M solution), o 900 mg per 200 cc of water (0.075 M solution), o 1200 mg per 200 cc of water (0.1 M solution).

[0047] These 4 concentrations have an influence on the dimensions of the sheets (length, width and thickness) during the crystallization of the seed. Then, 1 cc of surfactant (sodium lauryl sulfonate) is added. The samples are previously cleaned and activated by degreasing cathodic electrolytic at 5 A / dm 2 . Then they are immersed vertically in the hydrolysis solution. It is heated to 90°C and the samples remain there for 45 minutes once the solution has reached the right temperature. After this time the samples are removed, rinsed and dried. The parameters set are as follows: - Reagent concentrations (NiCL 0.033 M & KH2PO4 0.066 M), - The hydrolysis temperature (90°C). The variable parameters are as follows: - The hydrolyzer concentration, - The time the platelets are kept in the beaker, - The pH of the solution. In addition, some samples are abraded with an abrasive disc (P4000 or 5 pm) in the hope of creating more germination sites.

[0048] 24 samples were treated. The tribological results are presented below for a sample, numbered 15, treated with the maximum urea concentration of 0.1 M for a time of 45 minutes and a pH of 4.08 and for the sample, numbered 14, treated under the same conditions but with a time of 30 minutes (figure 6).

[0049] Samples were also prepared by combining both treatments, preferably first the oxidation treatment, followed by the phosphating treatment. Samples were subjected to the dry oxidation treatment, in accordance with sample 8 above, followed by germination carried out with 750 mg of hydrolyser (urea). It should be noted that in order to preserve the surface previously oxidised by plasma, electrolytic degreasing before germination is carried out in anodic and not cathodic degreasing.

[0050] Tribological tests were conducted in alternating linear mode against a 02mm ruby ​​ball. The test conditions are as follows: - Normal effort: 1 mN, - Maximum sinusoidal speed: 10mm / s, - Amplitude: 4mm, - Distance covered: 25m, - Condition: dry.

[0051] The reference consisting of a raw NiP disc following the LIGA (Lithography Galvanoformung Abformung) manufacturing process was tested with a tribological result which shows a strong evolution of the friction coefficient: - A start beyond 0.5, - A running-in phase at 0.5, - A fall to 0.25, - Stabilization at 0.25 with many peaks.

[0052] All tests were carried out in the same configuration and compared to this reference.

[0053] In Figure 5, it can be seen that dry oxidation stabilizes the CoF but also limits the NiP running-in phase. A significant advantage in terms of CoF gain is visible for oxidation via O2 / O3 scavenging (sample 36).

[0054] In Figure 6, we see that the germination of Nis(PO4)2 makes it possible to limit the CoF peaks but also to reduce it to reach 0.15 dry. The drop in CoF is very rapid at start-up and therefore limits the running-in phase.

[0055] With oxidation in aqueous medium, we observe in Figure 8 a decrease and stabilization of the CoF at 0.15 dry with an extremely short running-in phase.

[0056] Samples combining both treatments (Figure 7) show the decrease and stabilization of the CoF at 0.15 dry. Depending on the degree of germination, the most notable impact is the running-in phase which is more or less long but the CoF at start-up is significantly lower than for the reference.

[0057] Colorimetric measurements were also carried out on ALD-oxidized samples and comparative samples that had not been subjected to accelerated oxidation treatment but to natural oxidation that can last for several years. The L*a*b* colorimetric values ​​were measured on the polished samples with a KONICA MINOLTA CM-3610A spectrophotometer under the following conditions: SCI (specular reflection included) and SCE (specular reflection excluded) measurements, 8° tilt, SAV measurement area of ​​4 mm in diameter. The results are given in the table below.

[0058] The color difference between the two pellets is determined on the basis of delta E with the latter calculated on the basis of the values ​​L*a*b* as follows with Lj, a] and b^ referring to the values ​​of the reference pellet and L2, a2 ​​and b2 to the values ​​of the pellet oxidized with the treatment according to the invention.

[0059] A delta E of 5.4 is obtained. We can conclude that the treated sample has darkened compared to the reference sample. An oxidation layer has therefore developed on the surface of the sample. A more detailed characterization of this oxidation layer was carried out by an ellipsometric analysis. It appears that the thickness of the layer is between 9 and 10 nm. On the other hand, its index located between 1.8 and 2 shows that the composition of this layer is not preferentially composed of NiO, whose index of 2.1818 does not correspond, but rather of phosphorus oxides, which does not prohibit the minority presence of NiO. Indeed, phosphorus trioxides (P2O3) or phosphorus tetroxides (P2O4) or phosphorus pentoxides (P2O5) have an index close to 1.82; which is within the identified range. This result is logical because the reaction kinetics of NiO is lower than that of phosphates, which take over and form more easily and more quickly than NiO.

[0060] In conclusion, the presence of oxides and phosphates on a surface composed of NiP allows a significant dry tribological gain.

Claims

CLAIMS 1. Functional assembly (1) comprising a first mechanical part (2.3) comprising a first functional surface (8) made of NiP and a second mechanical part comprising a second functional surface intended to be in frictional contact with the first functional surface (8) made of NiP of said first mechanical part (2.3), wherein the first NiP functional surface comprises an oxide layer (9) and / or a phosphate layer (10) respectively, the phosphate layer (10) being a Ni or Zn phosphate layer, the frictional contact between the first and second mechanical parts being dry.

2. Functional assembly (1) according to the preceding claim, characterized in that at least the second functional surface of the second mechanical part is made of a material chosen from ruby, steel and NiP.

3. Functional assembly (1) according to the preceding claim, characterized in that the second functional surface in NiP comprises a layer of oxides (9) and / or a layer of phosphates (10) respectively, the layer of phosphates (10) being a layer of Ni or Zn phosphates.

4. Functional assembly according to the preceding claim, characterized in that the layer of Ni or Zn phosphates (10) comprises seeds.

5. Functional assembly according to the preceding claim, characterized in that it is a Nis(PO4)2 seed.

6. Functional assembly according to one of the preceding claims, characterized in that the oxide layer (9) has a thickness of between 7 and 13 nm, preferably between 8 and 12 nm.

7. Functional assembly according to the preceding claim, characterized in that the oxide layer (8) mainly comprises phosphorus oxides.

8. Functional assembly according to one of the preceding claims, characterized in that the oxide layer (9) has in the colorimetric space L*,a*,b*, a value of a* between 2.2 and 3 and a value of b* between 8 and 12, preferably between 9 and 11.

9. Functional assembly according to one of the preceding claims, characterized in that it is a part of a watch movement.

10. Functional assembly according to one of the preceding claims, characterized in that it is a mechanical part (2, 3) chosen from a pallet (4), an escape wheel (6), an axis of a mobile, a bearing, a barrel spring and gear wheel teeth.

11. Method for treating a mechanical part (2, 3) of a functional assembly according to any one of claims 1 to 10, said method comprising a step of oxidation and / or phosphating said functional surface (8) so as to artificially form a layer of oxides (9) and / or a layer of phosphates (10) respectively on said functional surface (8), the layer of phosphates (10) being a layer of Ni or Zn phosphates.

12. Treatment method according to claim 11, characterized in that the oxidation step is carried out by dry oxidation by atmospheric plasma, by vacuum plasma, by sweeping an O2-O3 mixture with or without plasma, or thermally in a furnace swept by oxygen or by oxidation by electrolysis in an aqueous medium.

13. Treatment method according to one of claims 11 to 12, characterized in that the phosphating step is carried out by ALD deposition to form a phosphate film or by chemical reaction to form phosphate seeds.

14. Treatment method according to one of claims 11 to 13, characterized in that it comprises an oxidation step followed by a phosphating step.

15. Treatment method according to one of claims 11 to 14, characterized in that the oxidation step is a dry oxidation with the following sub-steps: - Heating the mechanical part (2,3) between 100 and 200°C and placing said mechanical part (2,3) on a substrate holder in a vacuum chamber, - Introduction of Ar into the vacuum chamber with the substrate holder subjected to a negative potential between 500 and 1000 V.

16. Treatment method according to one of claims 11 to 15, characterized in that the phosphating step is carried out by chemical reaction between nickel chloride hexahydrate (NiCl2.6H2O), potassium dihydrogen orthophosphate (KH2PO4) and urea (NH2CONH2).

17. Treatment method according to claim 16, characterized in that the nickel chloride hexahydrate is in an aqueous solution with a molar concentration of between 0.01 and 0.06 M, the potassium dihydrogen orthophosphate is in an aqueous solution with a molar concentration of between 0.02 and 0.09 M and the urea is in an aqueous solution with a molar concentration of between 0.01 and 0.15 M.

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