EP2130944A1 - Method for treating the surface of a high-strength steel mechanical part, and sealing system obtained by implementing said method - Google Patents

Abstract

The process comprises subjecting a mechanical component to a primary finishing step to reduce its roughness on a surface at a value less than or equal to a predetermined first threshold, subjecting the component (P) to a surface cleaning using a degreasing solution, subjecting the cleaned component to a tribofinishing step organized for reducing its surface roughness (Ra) to a value less than or equal to a second predetermined threshold and increasing its wettability to hydraulic fluids, and subjecting the component to a projection of tungsten disulfide powder. The process comprises subjecting a mechanical component to a primary finishing step to reduce its roughness on the surface at a value less than or equal to a predetermined first threshold, subjecting the component (P) to a surface cleaning using a degreasing solution, subjecting the cleaned component to a tribofinishing step organized for reducing its surface roughness (Ra) to a value less than or equal to a second predetermined threshold, which is lower than the first predetermined threshold, and increasing its wettability to hydraulic fluids, and subjecting the component to a projection, at high speed and at an ambient temperature, of tungsten disulfide powder (WS 2) in the form of platelets (p), which is broken thus creating a dense and self-lubricating deposit in surface of the component. The tribofinishing step comprises chamfering (C1) by continuous stirring of component to be treated with a fist oxidizing aqueous solution containing abrasive agents for obtaining surface roughness, and polishing (C2) by continuous stirring of components with a second non-oxidizing aqueous solution containing abrasive agents followed by surface cleaning (C3), then checking the surface roughness. The first predetermined threshold of the roughness is 0.2 mu m and the second predetermined threshold of the roughness is 0.1 mu m. The powder is composed of pure WS 2and is in the form of hexagonal shape whose main dimension is 0.8-1.5 mu m and thickness is 0.1 mu m. The process comprises microblasting step to activate the surface of the component to increase the adhesion of the coating deposited in the projection step, and after the projection of WS 2powder, a step of surface cleaning, then checking both the surface roughness, wettability and coefficient of friction. The microblasting step is: organized such that the surface roughness, which is increased due to the microblasting, remains less than the first predetermined threshold of roughness; implemented with the particles that are not of oxides, and of which the size is 5-15 mu m. An independent claim is included for a hydraulic sealing system.

Description

The present invention relates to a method of surface treatment of high strength steel mechanical parts, aiming at imparting to said parts friction and lubrication properties necessary for their use, as well as a sealing system obtained by the implementation said method.

BACKGROUND OF THE INVENTION

It is known to provide a surface treatment to provide metal parts with friction and lubrication properties which are necessary for their use, the treatment usually being an electrolytic chromium plating. Electrolytic chromium plating provides a hard chromium coating, which is still very widely used in various fields such as the aeronautical field, because of its excellent properties in friction, wear resistance, and as a protection against corrosion. The electrolytic plating is generally completed by a grinding guaranteeing a homogeneous coating thickness and a surface condition corresponding to a surface roughness (Ra) of less than 0.2 μm. This success is explained by the fact that the characteristics obtained after these processing steps are on the one hand excellent frictional resistance due to good wear resistance coupled with a perfect surface condition, and other Excellent lubrication in the presence of fluids thanks to micro-tiling, inherent to hard chrome, acting as a retention zone.

However, hard chromium plating is carried out in an electrolytic cell in the presence of chromic acid based on hexavalent chromium (Cr ⁶⁺ ) which is harmful for the environment and for man. This is indeed a product that is classified as CMR (Carcinogenic, Mutagenic, and Harmful to Reproduction). In addition, like many electrolytic processes, this product is weakening for steels because of the diffusion of hydrogen, and requires operational precautions to avoid burns of the base steel after rectification, burns that would induce irreversible alteration of the treated metal part.

OBJECT OF THE INVENTION

The object of the invention is to design a surface treatment method capable of replacing electrolytic chromium plating, and making it possible to obtain both a high friction resistance level and also a very good wettability in the face of hydraulic fluids, while maintaining a level of surface roughness (Ra) that is less than or equal to 0.2 μm.

The invention also aims to design a treatment process to avoid the aforementioned drawbacks of electrolytic processes, while being easy to adapt to the types of mechanical parts involved.

The invention also aims to design a hydraulic sealing system including a sliding part surface-treated by the aforementioned method.

GENERAL DEFINITION OF THE INVENTION

1. a) on soumet la pièce à une étape de finition primaire organisée pour abaisser sa rugosité en surface (Ra) à une valeur inférieure ou égale à un premier seuil prédéterminé;
2. b) on soumet ensuite la pièce à un nettoyage de surface au moyen d'une solution de dégraissage ;
3. c) on soumet la pièce ainsi nettoyée à une étape de tribofinition organisée pour d'une part abaisser encore sa rugosité en surface (Ra) à une valeur inférieure ou égale à un deuxième seuil prédéterminé qui est inférieur au premier seuil prédéterminé, et d'autre part augmenter sa mouillabilité aux fluides hydrauliques ; et
4. d) on soumet la pièce à une projection, à haute vitesse et à température ambiante, de poudre de bisulfure de tungstène (WS₂) se présentant sous la forme de platelets qui se brisent ainsi en créant en surface de ladite pièce un dépôt dense et autolubrifiant.

The aforementioned technical problem is solved according to the invention by a method of surface treatment of a high strength steel mechanical part, aimed at imparting to said part friction and lubrication properties necessary for its use, which method comprises the following successive steps:

1. a) subjecting the workpiece to an organized primary finishing step to lower its surface roughness (Ra) to a value less than or equal to a first predetermined threshold;
2. b) the part is then subjected to surface cleaning by means of a degreasing solution;
3. c) the part thus cleaned is subjected to a tribofinition step organized to firstly lower its surface roughness (Ra) to a value less than or equal to a second predetermined threshold which is lower than the first predetermined threshold, and on the other hand to increase its wettability to hydraulic fluids; and
4. d) the part is subjected to a projection, at high speed and at ambient temperature, of tungsten disulfide powder (WS ₂ ) in the form of platelets which thus break by creating on the surface of said part a dense deposit and self-lubricating.

It should be noted that the above-mentioned treatment method, which implements a step of spraying tungsten disulfide powder, is radically different from the previous methods also using a spraying of tungsten disulfide powder and specially developed for the coating. cutting tools harder than the piece to cut. As such, it may be referred to the documents WO-2004/031433 and WO-2004/092429 . It will be noted in particular that these documents use a treatment method that does not provide for any prior degreasing step, and the tungsten disulfide powder sputtering step uses a powder consisting of spherical particles, which are embedded in hollow counterparts previously performed by sandblasting operation implemented with particles of the same size as the powder particles.

On the contrary, in the context of the invention, a tungsten disulfide powder is used in the form of platelets which, when they are projected at high speed against the surface (which is prepared accordingly while being free of spherical bowls) of the piece to be treated, break into microparticles of powder, in order to create on the surface a dense and self-lubricating deposit. Thus, the projection of very thin platelets realizes a real explosion of microparticle plates densifying the coating made, so that such a process has nothing to do with the previous processes of incrustation of spherical powder particles which are received in hollows previously made for this purpose.

Advantageously, the tribofinishing step c) comprises a first step c1) deburring by continuously stirring the parts to be treated with a first oxidizing aqueous solution containing abrasive agents until the desired surface roughness (Ra) is obtained. followed by a second polishing step c2) by continuously stirring said parts with a second non-oxidizing aqueous solution containing abrasive agents. In particular, the tribofinishing step c) comprises a third step c3) of surface cleaning, followed by control of the surface roughness (Ra).

In an advantageous embodiment, it will be provided that the first predetermined roughness threshold is substantially equal to 0.2 μm, and that the second threshold predetermined roughness is substantially equal to 0.1 microns.

Advantageously, the powder sprayed during step d) is almost exclusively composed of pure WS ₂ , and is in the form of platelets of substantially hexagonal shape, the main dimension of which is between 0.8 μm and 1.5 μm. μm, and whose thickness is of the order of 0.1 microns.

It may be furthermore advantageous to provide that the method comprises, after step c) of tribofinishing, a complementary step c ') micro-sandblasting, organized to activate the surface of the piece in order to increase the adherence of the coating deposited subsequently in the step d) powder projection of WS ₂ .

Advantageously then, step c ') of micro-sandblasting is followed by a step c' ') of surface cleaning, then control of the surface roughness (Ra).

More preferably, the micro-sanding step c ') is organized in such a way that the surface roughness (Ra), which is increased because of the micro-sanding, remains below the first predetermined roughness threshold.

Advantageously, the micro-sanding step c ') is carried out with particles which are not oxides and whose size is between 5 μm and 15 μm.

Preferably, finally, the method comprises, after step d) of spraying WS ₂ powder, a step d) of) surface cleaning, then control of both the surface roughness (Ra), the wettability , and the coefficient of friction.

The invention also relates to a hydraulic sealing system, comprising a sliding rod in a sealing assembly, wherein the sealing assembly is constituted by a guide bearing formed of a first material, and by a seal. sealant formed by a second material of lesser hardness than the first material, and wherein the sliding rod has an outer surface which has been treated using a method having at least one of the above-mentioned features, so that said rod has properties required in lubrication vis-à-vis the guide bearing and friction vis-à-vis the seal.

In particular, the first constituent material of the guide bearing is a thermoplastic polymer, and the second material constituting the seal is a rubber.

Other features and advantages of the invention will emerge more clearly in the light of the following description and the accompanying drawings, relating to a particular embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 illustre schématiquement les différentes étapes d'un procédé de traitement conforme à l'invention, avec ici des étapes intermédiaires optionnelles de micro-sablage et une étape finale optionnelle de nettoyage ;
• la figure 2 est un cliché réalisé par microscopie électronique illustrant un petit volume de la poudre de bisulfure de tungstène qui est utilisée pour la projection, à haute vitesse et à température ambiante, prévue dans le procédé de l'invention, laquelle poudre est constituée de platelets ;
• la figure 3 illustre schématiquement un platelet individuel de forme hexagonale constituant la poudre de bisulfure de tungstène concernée ;
• la figure 4 illustre l'amélioration des performances en matière de mouillabilité grâce à un diagramme comparatif donnant diverses courbes de variations de l'angle de contact liquide/solide en fonction du temps ; et
• la figure 5 est une vue en coupe axiale illustrant un système d'étanchéité hydraulique conforme à l'invention, obtenu en mettant en oeuvre le procédé de traitement précité.

Reference will be made to the figures of the accompanying drawings, in which:

• the figure 1 schematically illustrates the different steps of a treatment process according to the invention, with here optional intermediate micro-sandblasting steps and an optional final cleaning step;
• the figure 2 is an electron microscope picture illustrating a small volume of tungsten disulfide powder that is used for high-speed, high-temperature projection ambient, provided in the process of the invention, which powder consists of platelets;
• the figure 3 schematically illustrates an individual plate of hexagonal form constituting the tungsten disulfide powder concerned;
• the figure 4 illustrates the improvement in wettability performance by means of a comparative diagram giving various curves of variations of the liquid / solid contact angle as a function of time; and
• the figure 5 is an axial sectional view illustrating a hydraulic sealing system according to the invention, obtained by implementing the aforementioned treatment method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The successive steps of the surface treatment method of a high-strength steel mechanical part according to the invention will now be described in greater detail, said method aiming at imparting to said part friction and lubrication properties necessary for its operation. use.

The mechanical part concerned, noted P, will for example be a stainless steel friction rod of the type used to equip brake pistons of vehicles. Of course, the invention is in no way limited to a particular type of mechanical part.

On the figure 1 , there is a first step of the treatment method according to the invention, schematized in a). The starting metal part is a mechanical part made of steel, preferably a stainless steel, which is of high strength, that is to say whose hardness is at least 30 HRC. This part will generally have already undergone a heat treatment adapted to achieve a hardness typically of the order of 34 to 39 HRC, or will have been treated by a low-temperature cementation type thermochemical treatment or nitriding at low temperature allowing it to maintain its stainless properties.

During step a), the piece P is subjected to a primary finishing step which is organized to lower its surface roughness Ra to a value less than or equal to a first predetermined threshold S1, which is, for example, 0.2 μm . The piece P is thus complete machining and treatment (thermochemical type or passivation), and is in its form and final dimensions. This primary finishing treatment, of conventional type, can comprise stages of turning, grinding, etc ..., which must therefore make it possible to achieve a roughness Ra, for example less than 0.2 μm, once the part is finished and ready for use. next treatment. It is recalled that the parameter Ra used here to characterize the surface roughness is a parameter representative of the geometrical irregularities of a surface, which corresponds to the average arithmetic deviation with respect to the average roughness line.

In the next step b), the piece P is subjected to a surface cleaning by means of a degreasing solution. This operation is important because it removes the entire surface of the part P from all possible traces of dirt (grease, oil, chips, dust, plastic residues, felts, protective products temporary). The degreasing solution used will preferably be of the alkaline type at a temperature of between 35 ° C and 60 ° C. The duration of the degreasing step will typically be, for example, 5 minutes. Of course, in the case where the level of soiling is very important, and in order to reduce the degreasing treatment time, it will be possible to perform a pre-degreasing of the metal parts.

In the next step, denoted c), the piece P thus cleaned is subjected to a tribofinition step organized to first lower its surface roughness Ra to a value less than or equal to a second predetermined threshold S2 which is lower than the first predetermined threshold S1, and secondly increase its wettability to hydraulic fluids. The hydraulic fluids concerned are in particular fluids based on hydrocarbons or ester-phosphates, or oily fluids.

Such a tribofinishing operation is essential to prepare and optimize the surface state of the metal part before the tungsten disulfide powder spraying treatment.

As is schematized on the figure 1 the tribofinishing step c) advantageously comprises a first deburring step c1), a second polishing step c2), and a third surface cleaning step c3), followed by a surface roughness check.

The debinding step c1) consists in a continuous stirring, generally in a vibratory bowl, of the pieces P to be treated with a first oxidizing aqueous solution containing abrasive agents until the desired surface roughness Ra is obtained. At During this stage, an oxide film is created on the surface of the parts, with a hardness lower than that of the base metal. This film is removed gradually by the mechanical action of the abrasive agents which are of hardness greater than that of the film, but lower than that of the base metal, which abrasive agents come to clash on the surface of the pieces, and thus reduce the roughness of said surface. As an indication, this first step c1) deburring will be carried out for a period of at least 60 minutes.

The second polishing step c2) preferably consists in a continuous stirring of the pieces with a second non-oxidizing aqueous solution containing abrasive agents. This second polishing step makes it possible to completely remove the oxide film created during the first step c1) by the mechanical action of the abrasive agents. As an indication, the treatment time for this second polishing step c2) will be at least 120 minutes.

At the end of these two steps c1) and c2), the surface roughness Ra is lowered to a value less than or equal to a second predetermined threshold S2, which is less than the first predetermined threshold S1, and which will be, for example order of 0.1 μm. Advantageously, a surface cleaning step c3) will be provided, followed by control of the surface roughness Ra, which control is very reliable thanks to the cleaning carried out beforehand. The cleaning concerned in this case aims to guarantee the representativeness of the result of the surface roughness control measure. The surface of the part then has less dirt than at the end of the finishing step a) primary, so that one can use a solvent slightly aggressive acetone type.

At the end of this step c) of tribofinishing, it is possible either to directly submit the piece P to the essential next step denoted d), which is a step of projection, at high speed and at ambient temperature, of disulfide powder. tungsten in the form of platelets, or alternatively, first implement, prior to step d), a micro-sanding complementary step, optionally followed by a surface cleaning and a control of roughness on the surface. These complementary steps have been illustrated here in the form of a step c ') which is a micro-sandblasting step, organized to activate the surface of the piece P in order to increase the adhesion of the subsequently deposited coating during the period of time. WS ₂ powder projection step d), this complementary step c ') being followed by a surface cleaning step c'') and then controlling the surface roughness Ra. On the figure 1 schematically shown nozzles 10 symbolizing the micro-sanding, with a projection of particles on the part P, these particles, which are not oxides, generally having a size between 5 microns and 15 microns, and preferably of the order of 10 microns. The projection of particles during step c ') is carried out at high speed, obtained by a pressure of the order of 5 to 10 bars, and with an inclination of the spray jets substantially between 45 ° and 135 °.

Of course, such a micro-sandblasting step has the effect of slightly increasing the surface roughness Ra. However, micro-sandblasting step c ') will be organized in such a way that the surface roughness Ra is still lower than the first predetermined threshold of roughness S1, for example 0.2 μm.

On the figure 1 , for step c ''), the surface roughness Ra is symbolized by a simple arrow directed on the part P. As for the step c3) previously described, a surface cleaning, for example by means of a slightly aggressive solvent of the acetone type is carried out before the control of the surface roughness, which guarantees a better representativeness of the result of the control measurement.

The choice of whether or not to use the micro-sandblasting step will depend on the friction properties that are desired on the metal parts, in addition to the aforementioned wettability properties. As such, in the case of micro-sanding, it will be necessary to implement fairly quickly step d) powder projection of WS ₂ , for example within a period not exceeding 120 minutes.

At the end of step c) of tribofinishing, and possibly steps c ') micro-sanding and c' ') cleaning and control, the piece P is now optimally prepared to undergo the projection treatment of tungsten disulfide powder. The roughness related to the finishing operations has in fact been greatly reduced by the operation of tribofinishing, and the eventual micro-sandblasting has further activated the surface to increase the adhesion of the coating that will be formed.

During step d), the piece P is subjected to a projection, at high speed and at ambient temperature, of tungsten disulfide powder (WS ₂ ).

According to an essential characteristic of the invention, the WS ₂ powder used in the context of The method of the invention is in the form of platelets p, as illustrated in FIGS. Figures 2 and 3 , which provides a technical effect radically different from that which was obtained with the prior art also implementing a projection of WS ₂ powder, and of projecting spherical particles of powder which are embedded in a cutting piece previously prepared for have associated receiving valleys. Moreover, the teaching of providing receiving cavities for the spherical particles implies de facto a limitation for the reduction of surface roughness that can be obtained, since a too low value of the roughness would eliminate the reception hollows, and prevent the encrustation of spherical particles of WS ₂ powder. In this case, it is quite different because of the use of a powder consisting of platelets, that is to say, very thin platelets which disintegrate in microparticles in contact with the surface of the piece to be treated .

Preferably, platelets p of substantially hexagonal shape, as illustrated in FIG. figure 3 , whose main dimension denoted D is between 0.8 μm and 1.5 μm, and whose thickness denoted E is of the order of 0.1 μm. These platelets p, when projected by associated nozzles, noted 20 on the figure 1 , break in microparticles in contact with the surface, creating on the surface of said part a dense and self-lubricating deposit.

As an indication, for the operating conditions of the projection of WS ₂ powder in the form of platelets, cold and at high speed, it will be possible to use a pressure of the order of 5 to 10 bar, with an angle of inclination of the projection jets from 45 ° to 135 ° with respect to the plane of the surface to be treated, the distance between the projection nozzle outlets and the workpiece P typically ranging from 20 to 100 mm. These operating conditions make it possible to project WS ₂ powder platelets at high speed, so that they break into microparticles by crashing onto the surface of the workpiece.

The tests carried out by the Applicant have shown that it is then easy to obtain a coating with a thickness of between 0.4 μm and 0.6 μm, with a change in the contact angle (liquid / solid) in surface of the WS ₂ coating which is perfectly reproducible (which was not the case with the earlier techniques mentioned above). The treated parts then have a bluish gray color, which is quite characteristic of a deposit of uniform thickness. A visual check of the color of the part thus makes it possible to guarantee that the treatment has proceeded correctly, and that the desired characteristics are well achieved.

Moreover, as illustrated on the figure 1 it may also be provided that the method comprises, after step d) of spraying WS ₂ powder, a step d) of) surface cleaning, and then control. As for the previous steps c3) and c ''), the surface cleaning can be performed using a mild solvent acetone type, which ensures a better representativeness of the results of the control measures.

Such a final step is very interesting before the use of the treated parts, and it allows in particular to perform a triple control, illustrated here by three arrows in the figure, relating both the surface roughness, the wettability to hydraulic fluids, in particular hydrocarbon-based fluids or ester-phosphates, or oily fluids, and the coefficient of friction ( static and / or dynamic).

It is thus ensured to obtain a treated part having a surface roughness with a Ra value of less than 0.2 μm, with a dynamic coefficient of friction (WS ₂ against WS ₂ and plane plane) of less than 0.03, and a static coefficient of friction (WS ₂ against WS ₂ and planar plane) less than 0.07.

The wettability obtained is also extremely discriminating, insofar as it is very good for hydraulic fluids, in particular fluids based on hydrocarbons or ester-phosphates, or oily fluids, while being very bad with regard to aqueous fluids.

The figure 4 allows to appreciate the improved performance in terms of wettability for the WS ₂ coating produced in accordance with the invention.

The curves C1, C2 and C3 represented on the diagram of the figure 4 correspond to the variations of the liquid / solid contact angle (in degrees) as a function of time (in seconds). Curve C1 corresponds to a traditional type of treatment process, while curves C2 and C3 correspond to a treatment according to the invention, respectively without and with final cleaning.

It has thus been possible to achieve a coating with a very low coefficient of friction, which is self-lubricating thanks to the continuous film created on the surface of the part, and this over a very wide range of temperatures, this coating being more is lipophilic and hydrophobic. This represents a considerable advance compared to previous techniques mentioned above corresponding to electrolytic processes.

We will now describe, with reference to the figure 5 , a hydraulic sealing system according to the invention, obtained by implementing the surface treatment method which has just been described.

On the figure 5 there is thus distinguished a hydraulic sealing system rated 100 having a high-strength stainless steel X axis rod 101 which slides in a sealing assembly 102. The sealing assembly 102 is received in a housing 106 formed in a support member 105, being disposed between two shoulders 107, 108.

The sealing assembly 102 is constituted by a guide bearing 103 formed of a first material and by a seal 104 formed of a second material of lower hardness than that of the first material. By way of example, the first constituent material of the guide bearing 103 is a thermoplastic polymer, and the second material constituting the seal 104 is a rubber. When the rod 101 moves from right to left in the figure, the guide bearing 103, which can slide on the rod 101, cooperates with the seal 104 by compressing it to enhance the seal.

The outer surface 110 of the rod 101 has been treated by implementing a method as described above, so that said rod has properties required both in lubrication vis-à-vis the guide bearing 103, it is to say at the level the interface between the outer surface 110 of the rod 101 and the inner surface 103.1 of the guide bearing 103, and in friction with respect to the seal 104, that is to say at the level of the interface between the outer surface 110 of the rod 101 and the inner surface 104.1 of the seal 104, to avoid abrasion.

The dual function of the WS ₂ coating on the sliding rod 101 allows optimum cooperation with each of the two components 103, 104 constituting the sealing assembly 102.

Such a hydraulic sealing system is particularly advantageous for fitting brake pistons of vehicles.

It may be in particular a friction rod intended to be arranged in a piston of a hydraulic crown of an aircraft brake. The role of such a friction rod is to guide the piston during the application of the braking force on the brake disc or discs, the rod being equipped with a sealing assembly consisting of a bearing of polytetrafluoroethylene guidance and an ethylene propylene elastomer seal. Such a rod / seal system is then capable of meeting multiple requirements, in particular having an excellent frictional behavior making it possible to limit the wear of the joints and the damage of the rods, and to have an optimum seal of the piston against the hydraulic fluid.

The invention is not limited to the embodiments which have just been described, but on the contrary covers any variant taking again, with means equivalent, the essential characteristics set out above.

Claims (12)

A method of surface treatment of a high-strength steel mechanical part, aimed at imparting to said part friction and lubrication properties necessary for its use, characterized in that it comprises the following successive steps: a) subjecting the workpiece (P) to an organized primary finishing step to lower its surface roughness (Ra) to a value less than or equal to a first predetermined threshold (S1); b) the part (P) is then subjected to surface cleaning by means of a degreasing solution; c) the piece (P) thus cleaned is subjected to an organized tribofinishing step in order firstly to lower its surface roughness (Ra) to a value less than or equal to a second predetermined threshold (S2) which is lower than the first predetermined threshold (S1), and secondly increase its wettability to hydraulic fluids; and d) the part (P) is subjected to a projection, at high speed and at ambient temperature, of tungsten bisulphide powder (WS ₂ ) in the form of platelets (p) which thus break, creating on the surface of said part a dense deposit and self-lubricating. Process according to Claim 1, characterized in that the tribofinishing step c) comprises a first step c1) of deburring by continuous stirring of the pieces (P) to be treated with a first oxidizing aqueous solution containing abrasive agents until obtaining surface roughness (Ra) desired, followed by a second polishing step c2) by continuously stirring said parts with a second non-oxidizing aqueous solution containing abrasive agents. Process according to Claim 2, characterized in that the tribofinishing step c) comprises a third surface cleaning step (3) and then controlling the surface roughness (Ra). Method according to one of claims 1 to 3, characterized in that the first predetermined threshold (S1) of roughness is substantially equal to 0.2 microns, and the second predetermined threshold (S2) of roughness is substantially equal to 0.1 .mu.m. Method according to one of claims 1 to 4, characterized in that the powder sprayed during step d) is almost exclusively composed of pure WS ₂ , and is in the form of platelets (p) of substantially hexagonal shape, whose main dimension (D) is between 0.8 microns and 1.5 microns, and whose thickness (E) is of the order of 0.1 microns. Process according to claim 1, characterized in that it comprises, after step c) of tribofinishing, a complementary step c ') micro-sandblasting, organized to activate the surface of the piece (P) in order to increase the adhesion of the coating subsequently deposited during the step d) of spraying WS ₂ powder. Process according to Claim 6, characterized in that the micro-sandblasting step c ') is followed by a surface cleaning step c''), followed by control of the surface roughness (Ra). Process according to Claim 6 or Claim 7, characterized in that step c ') of micro-sanding is organized in such a way that the surface roughness (Ra), which is increased due to the micro-sanding, remains below the first predetermined roughness threshold (S1). Process according to one of Claims 6 to 8, characterized in that the micro-sandblasting step c ') is carried out with particles which are not oxides and whose size is between 5 μm and 15 μm. .mu.m. Process according to claim 1, characterized in that it comprises, after the step d) of spraying WS ₂ powder, a step d) of surface cleaning and then controlling both the surface roughness ( Ra), wettability, and coefficient of friction. Hydraulic sealing system, comprising a rod (101) sliding in a sealing assembly (102), characterized in that the sealing assembly (102) is constituted by a guide bearing (103) formed of a first material and by a seal (104) formed of a second material of lower hardness than the first material, and in that the sliding rod (101) has an outer surface (110) which has been treated with a method according to one of claims 1 to 10, so that said rod has properties required in lubrication vis-à-vis the guide bearing (103) and friction vis-à-vis the seal (104). The system of claim 11, characterized in that the first material constituting the guide bearing (103) is a thermoplastic polymer, and the second material constituting the seal (104) is a rubber.

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