Title of Invention	A METHOD OF SURFACE TREATING A MECHANICAL PART MADE OF HIGH-STRENGTH STEEL, AND A SEALING SYSTEM OBTAINED BY IMPLEMENTING SAID METHOD
Abstract	ABSTRACT the invention relates to a method of treating the surface of a mechanical part made of high-strength steel, the method seeking to confer on said part friction and lubrication properties that are needed for its use. In accordance with the invention, the method comprises the following successive steps: a) subjecting the part (P) to a primary finishing step organized to lower its surface roughness (Ra) to a value less than or equal to a first predetermined threshold (SI); ; b) then subjecting the part (P) to surface cleaning by means of a depressing solution; c) subjecting the part (P) as cleaned in this way to a tribo-finishin(0 step organized firstly to further lower its surface roughness (Ra) to a value less than or equal to a second predf^termined threshold (32) that is less than the first predetermined threshold (SI), and secondly to increase its wet ability by hydraulic fluids; and d) subjecting the part (P) to projection, at high speed and at ambient temperature, of tungsten bisulfide (WSj) powder in the form of platelets (£) that break, thereby creating a dense and self-lubricating deposit on the surface of said part.

Title of Invention

A METHOD OF SURFACE TREATING A MECHANICAL PART MADE OF HIGH-STRENGTH STEEL, AND A SEALING SYSTEM OBTAINED BY IMPLEMENTING SAID METHOD

Abstract

ABSTRACT the invention relates to a method of treating the surface of a mechanical part made of high-strength steel, the method seeking to confer on said part friction and lubrication properties that are needed for its use. In accordance with the invention, the method comprises the following successive steps: a) subjecting the part (P) to a primary finishing step organized to lower its surface roughness (Ra) to a value less than or equal to a first predetermined threshold (SI); ; b) then subjecting the part (P) to surface cleaning by means of a depressing solution; c) subjecting the part (P) as cleaned in this way to a tribo-finishin(0 step organized firstly to further lower its surface roughness (Ra) to a value less than or equal to a second predf^termined threshold (32) that is less than the first predetermined threshold (SI), and secondly to increase its wet ability by hydraulic fluids; and d) subjecting the part (P) to projection, at high speed and at ambient temperature, of tungsten bisulfide (WSj) powder in the form of platelets (£) that break, thereby creating a dense and self-lubricating deposit on the surface of said part.

Full Text	A METHOD OF SURFACE TREATING A MECHANICAL FART M-.DE OF HIGH-STRENGTH ST'EEL, AND A SEALING SYSTEM OBTAIIIED BY IMPLEMENTING SAIID METHOD The present invention relates to a method of surface treating mechanical parts made of high-strength steel for the purpose of conferring on said parts friction and lubrication properties that are needed for their use, and it also relates to a sealing system obtained by implementing said method. BACKGROUND OF THfc INVENTION It is known to perform surface treatment on metal parts in order tp obtain properties in terms of friction and lubrication that are needed for their use, where the treatment is conK/entionally electrolytic chromium plating. An electrolytic chromium plating makes it possible to obtain a hard chromium coating and it is in very widespread use in various fields such as the field of aviation, because of its excellent properties in terms of friction, resistance to wear, and providing protection against corrosion. Electrolytic chromium plating is generally finished off by rectification so as to guarantee that tfie coating is of thickness that is uniform and presents a surface state corresponding to surface roughness (Ra) that is less than 0.2 micrometers (lim) . The success of the above technique can be explained by the: fact that the characteristics obtained after such treat\|nent steps include firstly excellent friction strength because of good resistance to wear associated with a perfect surface state, and secondly excellent lubrication in the presence of fluids due to the microcracking effect that is inherent to hard chromium and that provides a retention zone. Nevertheless, hard chromium plating is performed in an electrolytic cell in the presence of chromic acid based on hexavalent chromium (Cr^^) , which is harmful to the environment and to human beings. That substance is classified as being CMR (carcinogenic, muragenic, and harmful for reprpduction). In addition, like numerous electrolytic methods, that substance embrittles steels because of hydrogen diffusion, and it requires operating precautions to be taken in order to avoid burn marks in the underlying steel after rectification, where such burn marks give rise to irreversible degradation of the treated metal pa^t. OBJECT OF THE INVENTION An object of the invention is to devise a method of surface treatment that is capable of replacing electrolytic chrpmium plating, making it possible to obtain a high lel/el of friction strength and also very good wettability by hydraulic fluids, while conserving a level of surface roughness (Ra) that is less than or equal to 0.2 jam. Another object of the invention is to devise a treatment method^ that makes it possible to avoid the above-mentioned drawbacks of electrolytic methods, while being easy to ad^pt to the types of mechanical part in question. Another objfect of the invention is to devise a hydraulic sealim^ system that includes a sliding part with its surface! treated by the above-specified method. GENERAL DEFINITION OF THE INVENTION The above-mentioned technical problem is solved in accordance with the invention by a method of treating the surface of a mechanical part made of high-strength steel, the method seeking to confer on said part friction and lubrication properties that are needed for its use, which method comprises: the following successive steps: a) subjecting the part to a primary finishing step organized to lower its surface roughness (Ra) to a value less than or equ^l to a first predetermined threshold; b) then subjecting the part to surface cleaning by means of a degrelasing solution; c) subjecting the part as cleaned in this way to a tribo-finishing ^step organized firstly to further lower its surface rougjhness (Ra) to a value less than or equal to a second predetermined threshold that is less than the first predetermined threshold, and secondly to increase its wettability by hydraulic fluids; and d) subjecting the part to projection, at high speed and at ambient temperature, of tungsten bisulfide powder (WS2) in the foritn of platelets that break, thereby creating a densej and self-lubricating deposit on the surface of said part. It should be observed that the above treatment method, that implements a step of projecting tungsten bisulfide powder, differs radically from prior methods that also make u\|se of tungsten bisulfide powder projection and cif the kind specially developed for coating cutting tools that are harder than the part they are to cut. In ;this context, reference can be made to the documents wa-A-2004/031433 and WO-A-2004/092429. In particular, it qhould be observed that those documents implement a treatment method that does not provide for any prior degre^sing step, and the step of projecting tungsten bisulfijde powder makes use of a powder constituted by particles that are spherical, which particles become encrusted in corresponding recesses previously made by a sanding operation implemented using particles havincf the same dimensions as the powder particles. On the contrary, in the present invention, use is made of a tungsten bisulfide powder that is in the form of platelets th^t break up into microparticles of powder on being projected at high speed against the surface of the part for treatment (which surface has been prepared accordingly and is free from spherical depressions) the microparticles creating on the surface a deposit that is dense and self-lubricating. Thus, projecting platelets of very small thickness gives rise to a genuine explosion of the platelets ^into microparticles that densify the resulting coating, such that such a process is in no way comparable to thg prior processes of encrusting powder particles of spherical shape, which particles are received in recesses previously prepared for this purpose. AdvantageouEfly, the tribo-finishing step c) includes a first step cl):of deburring by continuously agitating parts for treatment together with an oxidizing first aqueous solution■containing abrasive agents until the desired surface iijroughness (Ra) is obtained, followed by a second step c2) (!f>f polishing by subjecting said parts to continuous agitation together with a non-oxidizing second aqueous solution:containing abrasive agents. In particular, the tribo-finishing step c) includes a third step c3) of surface cleaning, followed by inspection of the surface rougfiness (Ra) . In an advantageous implementation, provision is made for the first predetermined roughness threshold to be substantially equal to 0.2 um, and for the second predetermined roUghness threshold to be substantially equal to 0.1 pm. Also advantageously, the powder projected during step d) is const}ituted almost exclusively by pure WS2, and is in the form o\|f platelets that are substantially hexagonal in sha^De, with a main dimension lying in the range 0.8 um to 1.5 pm, and with a thickness of the order o f ' 0.1 pm. It can also; be advantageous to make provision for the method to include, after tribo-finishing step c), an additional step Ic' ) of micro-sanding, organized to activate the surlface of the part in order to increase the adhesion of the icoating subsequently deposited during step d) of projelcting WS2 powder. In which case, and advantageously, the m.icrc-sandi step c') is followed by a surface cleaning s::eD c"), an then by inspectioh of the surface roughness (Ra). Also preferably, the micro-sanding step c') is organized in such: a manner that the surface roughness (Ra), which is injcreased as a result of the micro-sanding, remains below the first predetermined roughnes threshold. In which casie, and advantageously, the micro-sandi step c') is implejmented using particles that are not oxides, and of a size lying in the range 5 jam to 15 pm. Finally, and preferably, the method includes, afte the WSo powder projection step d), a step d') of surfac cleaning followeci by inspection of surface roughness (Ra), of wettability, and of coefficient of friction. The inventidn also provides a hydraulic sealing system including^a slide rod slidable in a sealing assembly, in whii+h system the sealing assembly is constituted by a;guide bearing made of a first materia. and by a sealing:gasket made of a second material of hardness less th^n the hardness of the first material, and in which the\|slide rod has an outside surface that has been worked ^y implementing a method presenting at least one of the.above characteristics, such that said rod presents required lubrication properties relative the guide bearing and required friction properties relative to the pealing gasket. In particular, the first material constituting th guide bearing is a thermoplastic polymer, and the seco material constituting the sealing gasket is a rubber. Other charaicteristics and advantages of the invention appear;' more clearly in the light of the following descri\|ption and the accompanying drawings, relating to a pajrticular implementation. BRIEF DESCRIPTION OF THE DRAWINGS Reference is made to the figures of zr.e accompanying drawings, in which: ■ Figure 1 :is a diagram showing the various steps in a treatment method in accordance with the invention, here with optional intermediate steps of micro-sanding and an optional step of: cleaning; • Figure 2 is a micrograph obtained by an electron microscope showifig a small volume of the tungsten bisulfide powder: that is used for the high speed projection at ambient temperature that is performed in the method of th^ invention, which powder is constituted by platelets;- • Figure 3 jis a diagram showing an individual platelet of hexa(^onal shape constituting the tungsten bisulfide powder-in question; • Figure 4 fehows the improvement of performance in terms of wettability by presenting a comparative graph that plots a plutality of curves showing how liquid/solid contact angle vafies as a function of time; and • Figure 5 is an axial section view showing a hydraulic sealing system in accordance with the invention, obtaiified by implementing the above-specified treatment method\| DETAILED DESCRIPTION OF THE PREFERRED IMPLEMENTATION There follovi/s a description in greater detail of the successive steps^of the method of the invention of treating the surface of a mechanical part made of high-strength steel, which method seeks to impart on said part properties of fraction and of lubrication that are needed for its use. The mechanicfal part in question, referenced P, is constituted for Example by a stainless steel friction rod of the type used^for fitting to vehicle brake pistons. Naturally, the invention is not limited in any way to one particular type qf mechanical part. In Figure \\, there can be seen a first step of the treatment method; of the invention, shown diagra.T-.aticaily at a). The starting metal part is a part made of steel, preferably of stainless steel, presenting high strength, i.e. of hardness: that is not less than 30 HRC (i.e. on the Rockwell C scale). The part will generally already have been subjected to appropriate heat treatment enabling it to rpach hardness typically of the order of 34 HRC to 39 HRC^ or will have been treated with thermochemical tteatment of the cementation type at low temperature or o\|£ the nitriding type at low temperature enabling it to conserve its stainless properties. During step a), the part P is subjected to a primary finishing step tjiat is organized to lower its surface roughness Ra to k value that is less than or equal to a first predetermijied threshold SI, e.g. equal to 0.2 pm. The part P is thus finished with machining and treatment (of the thermochpmical or passivation type), and is present in its fjLnal shape and dimensions. The primary finishing treatment of conventional type may comprise steps of turning^ rectifying, etc. ..., and should ensure that its roughnefes Ra can be made to be less than 0.2 pm, for example, oncfe the part has been finished and is ready for the following treatment. It is recalled that the parameter Ra useifl herein for characterizing surface roughness is a pkrameter that is representative of the geometrical irregularities of a surface, and corresponds to the arithmetical mean deviation from the mean line of the roughness. During the following step b), the part P is subjected to surface cleaning by means of a degreasing solution. This operation is important since it enables the surface of tjie part P to be completely cleaned of all traces of possible dirt (grease, oil, shavings, dust, plastics residue^, felts, substances for providing temporary protection). The degreasing solution used is preferably of thfe alkaline type and it is used at a temperature in the range 35°C to 60°C. The duraricn of the degreasing step is typically 5 minutes. Naruraliy, when the level of^ dirtying is very great, and in order to reduce the time needed for the degreasing treatment, it is possible to perform pre-degreasing on the metal part. During the fjollowing step, referenced c) , the part P as cleaned in this way is subjected to a tribo-finishing step organized firstly to further reduce its surface roughness Ra to a^ value that is less than or equal to a second predetermined threshold S2 that is less than the first predetermined threshold SI, and secondly to increase its wet1\|ability by hydraulic fluids. The hydraulic fluids I in question are constituted in particular by flijiids based on hydrocarbons or on ester-phosphates, or oily fluids. Such a tribp-finishing operation is essential for preparing and op1}:imizing the surface state of the metal part prior to th$ treatment by projecting tungsten bisulfide powder► As shown di^grammatically in Figure 1, the tribo-finishing step c\ advantageously comprises a first step cl) of deburring^ a second step c2) of polishing, and a third step c3) ot surface cleaning, followed by inspecting surface roughness. Deburrinq step cl) consists in continuously agitating parts P for treatment, generally in a vibrating bowl, together with an oxidizing first aqueous solution containing abrasive agents so as to obtain the desired surface roughnesis Ra. During this step, an oxide film is created on the surfaces of the parts, which film is of hardness that is: less than the hardness of the underlying metal. The film is removed progressively by the mechanical actidn of the abrasive agents which are of hardness greater! than that of the film but less than that of the underlyirig metal, which abrasive agents strike against the surfaces of the part, thereby reducing the roughness of said surfaces. As an indication, this first step cl) of debarring should be implemented for a duration of noteless than 60 minutes. The second;step c2) of polishing preferably consists in continuouslyiagitating parts together with a non-oxidising second aqueous solution that contains abrasive agents. This second step of polishing serves to remove all of the oxidg film created during the first step cl) by the mechanicall action of the abrasive agents. As an indication, the ;duration of the treatment for this second step c2) of polilshing should not be less than 120 minutes . At the end pf these two steps cl and c2), the surface roughness Ra is reduced to a value that is less than or equal toi the second predetermined threshold S2, which is less th^n the first predetermined threshold SI, for example being about 0.1 jam. A step c3) is then advantageously pjrovided for cleaning the surface, followed by a st^p of inspecting the surface roughness Ra, which inspection can be very reliable due to the previously performed cleaning. The cleaning in question seeks to guarantiee that the result of the inspection is a measurement of sijirface roughness that is representative. The surface of the part then presents less dirtying than at the end of th^ primary finishing step a), so it is possible to use a solvent that is not very aggressive, of the acetone type.; At the end 6f this tribo-finishing step c), it is possible either tlo subject the part P directly to the following essentijal step referenced d) , constituted by a step of projecting tungsten bisulfide powder in the form of platelets at hjigh speed and at ambient temperature, or else in a variant^ to begin, prior to step d) , by implementing an ajdditional micro-sanding step, possibly followed by surfaice cleaning and inspecting surface roughness. Thesei additional steps are represented herein as a step c') thalt is a micro-sanding step organized to activate the surface of the part P so as to increase adhesion of the icoating deposited subsequently during step d) of projejcting WS2 powder, with said addi-ional step c') being f[ollowed by a step c") of surface cleaning and then by inspecting the surface roughness Ra. In Figure 1, nozzles 10 are shown diagrammatically to symbolize the mijcro-sanding, with particles being projected onto the part P, these particles, which are not oxides, generally having a size lying in the range 5 pm to 15 ]jm, and prjeferably of the order of 10 \|im. The projection of particles during step c') is performed at high speed, obtaiined by using a pressure of the order of 5 bars to 10 barjs, with the projection jets being inclined at an angle lying substantially in the range 45° to 135°. Naturally, :such a micro-sanding step has the effect of slightly increasing the surface roughness Ra. Nevertheless, th\|e micro-sanding step c') is organized so that the surface roughness Ra continues to remain below the first predetermined roughness threshold SI, e.g. 0.2 lam. In Figure l, step c") of inspecting surface roughness Ra is isymbolized by a simple arrow pointing to the part P. As lin above-described step c3), surface cleaning, e.g. by means of a relatively non-aggressive solvent of the ajcetone type, is performed prior to inspecting the siurface roughness so as to guarantee better representjativity for the result of the inspecting measurement. Whether or jnot the micro-sanding step should be implemented depejnds on the friction properties that it is desired to obtaijn on the metal parts, in addition to the above-mentioned iwettability properties. In this respect, when micro-sandiing is used, it is appropriate to implement step d\|) of projecting WSj powder very quickly, e.g. within a delay of not more than 120 minutes. At the end \|of the tribo-finishing step c) , and possibly after ir^icro-sanding step c') and after cleaning and inspection $tep c"), the part P is optiirially prepared for being subjected to the treatment of prcjectir.r tungsten bisulfide powder. The roughness associated with the finishing operations has been greatly diminished by the tribo-finisHing operation, while the micro-sanding, if any, has alscj activated the surface so as to increase the adhesion of 'the coating that is to be formed. During step d) , the part P is therefore subjected to projection of tungsten bisulfide powder (WS,) at high speed and at ambient temperature. In accordanjce with an essential characteristic of the invention, tjhe WSj powder used in the method of the invention is in ^the form of platelets £, as shown in Figures 2 and 3,; thereby producing a technical effect that is radically different from that which has been obtained in prioir art techniques that also make use of projecting WSj powder and that consist in projecting spherical powder particles that are encrusted in a cutter part previously prepared to present associated powder-receiving recesses. Furthermore, the teaching consisting in providing rec'^sses for receiving spherical particles de facto impliesi a limit for the amount of surface roughness reduction that can be obtained, insofar as too small a value for roughness would eliminate the powder-receiving recesses, and would prevent spherical particles of WSj powder bec^oming encrusted. Specifically, the process is quite' different when using a powder made up of platelets, i.e. Very thin plates that disintegrate into microparticles op coming into contact with the surface of the part for treatment. Preferably, I the platelets £ used are substantially hexagonal in shape, as shown in Figure 3, having a main dimension referenced D lying in the range 0.8 iJm to 1.5 pm, and a thickness, referenced E, of the order of 0.1 pm. When thi^se platelets p are projected by associated nozzl^, referenced 20 in Figure 1, they break up into microparticles on coming into contact with the surface, thereby: creating a deposit on the surface cf said part, which deposit is dense and self-lubricazir.g. By way of i;ndication, for operating conditions in which the WSj poWder is projected in the form of platelets, cold land at high speed, it is possible to use a pressure of the order of 5 bars to 10 bars with an angle of inclination for the projection jet lying in the range 45° to 135:° relative to the plane of the surface that is to be trieated, the distance between the outlet from the projection nozzles and the part P typically lying in the range 20 millimeters (mm) to 100 mm. These operating conditjions enable platelets of WSj powder to be projected at high speed so that they break up into microparticles oh striking the surface of the part to be treated. Tests undertaken by the Applicant have shown that it is then easy to obtain a coating of thickness lying in the range 0.4 pmi to 0.6 \|im with the liquid/solid contact angle at the surjface of the WS, coating varying in a manner that is perfectly reproducible (which is not true for the prior art techniques mentioned above). The treated parts arje then of a bluish gray color that is entirely charactpristic of a deposit of uniform thickness. Visujal inspection of the color of the part thus makes it possible to guarantee that the treatment has taken place properly and that the desired characteristics have indeed been achieved. Furthermore,!, as shown in Figure 1, it is also possible to provjide for the method to include, after step d) of subjecting! WSj powder, a step d') of cleaning its surface, followejd by inspection. As for the preceding step c3) and c"):, the surface cleaning may be performed by means of a soilvent that is not very aggressive, of the acetone type, thereby guaranteeing better representativityl for the results of the inspection measurements. Performing $uch a final step prior to using the treated parts is; of great advantage, and it; serves ir. particular to pe):form three inspections, represented by three arrows in the figure, relating respectively to surface roughnesfe, to wettability by hydraulic fluids, in particular fluid^ based on hydrocarbons or on ester-phosphates, or o.JLly fluids, and to the coefficient of friction (staticland/or dynamic). This ensures that a treated part is obtained presenting surfafce roughness with a value Ra of less than 0.2 pm, with a d\|/namic friction coefficient (WSj against WSj and plane onlplane) of less than 0.03, and a static friction coefficjient (WSo against WS, and plane on plane) of less than 0.0\|7. The wettability that is obtained is also extremely discriminating ifisofar as it is very good for hydraulic fluid, in particular for fluids based on hydrocarbons or on ester-phosphates, or oily fluids, while being very bad for aqueous fluids. Figure 4 shdws the improvement obtained in performance in terms of wettability for the WS2 coating when made in accbrdance with the invention. Curves CI, \|C2, and C3 in the graph of Figure 4 correspond to vajriation in the liquid/solid contact angle (in degrees) as la function of time (in seconds) . Curve CI corresponds to a treatment method of traditional type, while curves C2 land C3 correspond to treatment in accordance with Ithe invention, respectively with and without final cleaning. A coating is thus obtained with a coefficient of friction that is\| very low, and that is self-lubricating because of the continuous film created on the surface of the part, with this taking place over a very wide temperature range, the coating furthermore being lipophilic and hydrophobic. This represents considerable progress compared with the above-mentioned prior techniques corresponding to electrolytic processes. With referelnce to Figure 5, there follows a description of a\| hydraulic sealing systerr. in acccriance with the invenT:iion obtained by implementing the above-described surfade treatment method. In Figure 5j, there can thus been seen a hydraulic sealing system referenced 100 comprising a slide rod 101 of axis X that i\|s made of high-strength stainless steel, and that slides iin a sealing assembly 102. The sealing assembly 102 is Ireceived in a housing 106 formed in a support element dOS, being disposed between shoulders 107 and 108. The sealing! assembly 102 is constituted by a guide bearing 103 made of a first material and by a sealing gasket 104 made lof a second material of hardness lower than that of thei first material. By way of example, the first material constituting the guide bearing 103 is a thermoplastic polymer, and the second material constituting the! sealing gasket 104 is a rubber. When the rod 101 movejs from the right to the left in the figure, the guidb bearing 103 that is capable of sliding on the rod 101 cb-operates with the sealing gasket 104 by compressing it, thereby reinforcing sealing. The outside: surface 110 of the rod 101 has been treated by imple\|nenting a method as described above, such that said rod presents required properties both concerning lubripation relative to the guide bearing 103, i.e. at the inte[rface between the outside surface 110 of the rod 101 and the inside surface 103.1 of the guide bearing 103, and in terms of friction relative to the sealing gasket 104, i.e. at the interface between the outside surface 110 of the rod 101 and the inside surface 104.1 of the sealing gasket 104, in order to avoid abrasion. The dual function of the WSj coating lining the sliding rod 101 optimizes co-operation with both of the components 103, LL04 constituting the sealing assembly 102. Such a hydi-aulic sealing system is particularly advantageous for fitting to vehicle brake pisrons. This can apply in particular to a friction rod for arranging in a piston in a hydraulic ring in an aircraft brake. The role of such a friction rod is to guide the piston when applying braking force to the disk(s) of the brake, the rod being fitted with a sealing system constituting a cjuide bearing made of polytetrafluoroethylene and a sealing gasket made of elastomer of the ethylene propylene type. Such a rod/gasket system can then satisfy numerous requirements, in particular iti can present excellent friction behavior serving to limit gasket wear and damage to the rods, and it can present good sealing for the piston against the hydraulic fluid. The invention is not limited to the implementations described above,: but on the contrary covers any variant using equivalent means to reproduce the essential characteristics ispecified above. CLAIMS 1. A method of treating the surface made of high-strength steel, the method seeking to confer on said part friction and lubrication properties that are needed for its usfe, the method being characterized in that it comprises: the following successive steps: a) subjectinig the part (P) to a primary finishing step organized to\| lower its surface roughness (Ra) to a value less than ot egual to a first predetermined threshold (SI); b) then subjbcting the part (P) to surface cleaning by means of a deglreasing solution; c) subjecting the part (P) as cleaned in this way to a tribo-finishingi step organized firstly to further lower its surface roughness (Ra) to a value less than or equal to a second predetermined threshold (S2) that is less than the first predetermined threshold (SI), and secondly to increase its v\|ettability by hydraulic fluids; and d) subjectinjg the part (P) to projection, at high speed and at ambijent temperature, of tungsten bisulfide (WSo) powder in t^e form of platelets (p) that break, thereby creating ia dense and self-lubricating deposit on the surface of s4id part. 2. A method accoifding to claim 1, characterized in that the tribo-finishing step c) includes a first step cl) of deburring by coniinuously agitating parts (P) for treatment together with an oxidizing first aqueous solution containing abrasive agents until the desired surface roughnes$ (Ra) is obtained, followed by a second step c2) of polishing by subjecting said parts to continuous agitation together with a non-oxidizing second aqueous solution;containing abrasive agents. 3. A method according to claim 2, characterized in that the tribo-finishiLng step c) includes a third step c3) of surface cleaning, followed by inspection of the surface roughness (Ra). 4. A method according to any one of claims 1 to 3, characterized in that the first predetermined roughness threshold (SI) isi substantially equal to 0.2 ^m, and the second predetermiined roughness threshold (S2) is substantially equial to 0.1 ]am. 5. A method according to any one of claims 1 to 4, characterized in Ithat the powder projected during step d) is constituted almost exclusively by pure WSj, and is in the form of platelets (p) that are substantially hexagonal in shape, with a main dimension (D) lying in the range 0.8 pm Ito 1.5 pm, and with a thickness (E) of the order of 0.1 iiim. 6. A method accorlding to claim 1, characterized in that it includes, afteir tribo-finishing step c) , an additional step c') of micrd-sanding, organized to activate the surface of the palrt (P) in order to increase the adhesion of the coating subsequently deposited during step d) of projecting WS2 polwder. 7. A method according to claim 6, characterized in that the micro-sandinc^ step c') is followed by a surface cleaning step c"], and then by inspection of the surface roughness (Ra). 8. A method according to claim 6 or claim 7, characterized in;that the micro-sanding step c') is organized in sucl\|i a manner that the surface roughness (Ra) , which is iijicreased as a result of the micro-sanding, remainsi below the first predetermined roughness threshold (SI). i 9. A method accotding to any one of claims 6 to 3, characterized in\|that the micro-sanding step z'] is implemented usin-:^ particles that are not oxides, and of a size lying in thfe range 5 \im to 15 jam. 10. A method according to claim 1, characterized in that it includes, aftfer the WSj powder projection step d), a step d') of surf surface roughness (Ra), of wettability, and of coefficient of fjriction. 11. A hydraulic pealing system including a slide rod (101) slidable ih a sealing assembly (102), the system being characterised in that the sealing assembly (102) is constituted by a; guide bearing (103) made of a first material and by h sealing gasket (104) made of a second material of hardhess less than the hardness of the first material, and in: that the sliding rod (101) has an outside surface :(110) that has been worked by implementing a mfethod according to any one of claims 1 to 10, such that said rod presents required lubrication properties relative to the guide bearing (103) and required frictioh properties relative to the sealing gasket (104) . 12. A system according to claim 11, characterized in that the first material constituting the guide bearing (103) is a thermoplastic polymer, and the second material constituting thei sealing gasket (104) is a rubber.

Full Text

A METHOD OF SURFACE TREATING A MECHANICAL FART M-.DE OF HIGH-STRENGTH ST'EEL, AND A SEALING SYSTEM OBTAIIIED BY IMPLEMENTING SAIID METHOD
The present invention relates to a method of surface treating mechanical parts made of high-strength steel for the purpose of conferring on said parts friction and lubrication properties that are needed for their use, and it also relates to a sealing system obtained by implementing said method.
BACKGROUND OF THfc INVENTION
It is known to perform surface treatment on metal parts in order tp obtain properties in terms of friction and lubrication that are needed for their use, where the treatment is conK/entionally electrolytic chromium plating. An electrolytic chromium plating makes it possible to obtain a hard chromium coating and it is in very widespread use in various fields such as the field of aviation, because of its excellent properties in terms of friction, resistance to wear, and providing protection against corrosion. Electrolytic chromium plating is generally finished off by rectification so as to guarantee that tfie coating is of thickness that is uniform and presents a surface state corresponding to surface roughness (Ra) that is less than 0.2 micrometers (lim) . The success of the above technique can be explained by the: fact that the characteristics obtained after such treat|nent steps include firstly excellent friction strength because of good resistance to wear associated with a perfect surface state, and secondly excellent lubrication in the presence of fluids due to the microcracking effect that is inherent to hard chromium and that provides a retention zone.
Nevertheless, hard chromium plating is performed in an electrolytic cell in the presence of chromic acid based on hexavalent chromium (Cr^^) , which is harmful to the environment and to human beings. That substance is

classified as being CMR (carcinogenic, muragenic, and harmful for reprpduction). In addition, like numerous electrolytic methods, that substance embrittles steels because of hydrogen diffusion, and it requires operating precautions to be taken in order to avoid burn marks in the underlying steel after rectification, where such burn marks give rise to irreversible degradation of the treated metal pa^t.
OBJECT OF THE INVENTION
An object of the invention is to devise a method of surface treatment that is capable of replacing electrolytic chrpmium plating, making it possible to obtain a high lel/el of friction strength and also very good wettability by hydraulic fluids, while conserving a level of surface roughness (Ra) that is less than or equal to 0.2 jam.
Another object of the invention is to devise a treatment method^ that makes it possible to avoid the above-mentioned drawbacks of electrolytic methods, while being easy to ad^pt to the types of mechanical part in question.
Another objfect of the invention is to devise a hydraulic sealim^ system that includes a sliding part with its surface! treated by the above-specified method.
GENERAL DEFINITION OF THE INVENTION
The above-mentioned technical problem is solved in accordance with the invention by a method of treating the surface of a mechanical part made of high-strength steel, the method seeking to confer on said part friction and lubrication properties that are needed for its use, which method comprises: the following successive steps:
a) subjecting the part to a primary finishing step organized to lower its surface roughness (Ra) to a value less than or equ^l to a first predetermined threshold;

b) then subjecting the part to surface cleaning by means of a degrelasing solution;
c) subjecting the part as cleaned in this way to a tribo-finishing ^step organized firstly to further lower its surface rougjhness (Ra) to a value less than or equal to a second predetermined threshold that is less than the first predetermined threshold, and secondly to increase its wettability by hydraulic fluids; and
d) subjecting the part to projection, at high speed and at ambient temperature, of tungsten bisulfide powder
(WS2) in the foritn of platelets that break, thereby creating a densej and self-lubricating deposit on the surface of said part.
It should be observed that the above treatment method, that implements a step of projecting tungsten bisulfide powder, differs radically from prior methods that also make u|se of tungsten bisulfide powder projection and cif the kind specially developed for coating cutting tools that are harder than the part they are to cut. In ;this context, reference can be made to the documents wa-A-2004/031433 and WO-A-2004/092429. In particular, it qhould be observed that those documents implement a treatment method that does not provide for any prior degre^sing step, and the step of projecting tungsten bisulfijde powder makes use of a powder constituted by particles that are spherical, which particles become encrusted in corresponding recesses previously made by a sanding operation implemented using particles havincf the same dimensions as the powder particles.
On the contrary, in the present invention, use is made of a tungsten bisulfide powder that is in the form of platelets th^t break up into microparticles of powder on being projected at high speed against the surface of the part for treatment (which surface has been prepared

accordingly and

is free from spherical depressions) the

microparticles creating on the surface a deposit that is

dense and self-lubricating. Thus, projecting platelets of very small thickness gives rise to a genuine explosion of the platelets ^into microparticles that densify the resulting coating, such that such a process is in no way comparable to thg prior processes of encrusting powder particles of spherical shape, which particles are received in recesses previously prepared for this purpose.
AdvantageouEfly, the tribo-finishing step c) includes a first step cl):of deburring by continuously agitating parts for treatment together with an oxidizing first aqueous solution■containing abrasive agents until the desired surface iijroughness (Ra) is obtained, followed by a second step c2) (!f>f polishing by subjecting said parts to continuous agitation together with a non-oxidizing second aqueous solution:containing abrasive agents. In particular, the tribo-finishing step c) includes a third step c3) of surface cleaning, followed by inspection of the surface rougfiness (Ra) .
In an advantageous implementation, provision is made for the first predetermined roughness threshold to be substantially equal to 0.2 um, and for the second predetermined roUghness threshold to be substantially equal to 0.1 pm.
Also advantageously, the powder projected during step d) is const}ituted almost exclusively by pure WS2, and is in the form o|f platelets that are substantially hexagonal in sha^De, with a main dimension lying in the range 0.8 um to 1.5 pm, and with a thickness of the order o f ' 0.1 pm.
It can also; be advantageous to make provision for the method to include, after tribo-finishing step c), an additional step Ic' ) of micro-sanding, organized to activate the surlface of the part in order to increase the adhesion of the icoating subsequently deposited during step d) of projelcting WS2 powder.

In which case, and advantageously, the m.icrc-sandi step c') is followed by a surface cleaning s::eD c"), an then by inspectioh of the surface roughness (Ra).
Also preferably, the micro-sanding step c') is organized in such: a manner that the surface roughness (Ra), which is injcreased as a result of the micro-sanding, remains below the first predetermined roughnes threshold.
In which casie, and advantageously, the micro-sandi step c') is implejmented using particles that are not oxides, and of a size lying in the range 5 jam to 15 pm.
Finally, and preferably, the method includes, afte the WSo powder projection step d), a step d') of surfac cleaning followeci by inspection of surface roughness (Ra), of wettability, and of coefficient of friction.
The inventidn also provides a hydraulic sealing system including^a slide rod slidable in a sealing assembly, in whii+h system the sealing assembly is constituted by a;guide bearing made of a first materia. and by a sealing:gasket made of a second material of hardness less th^n the hardness of the first material, and in which the|slide rod has an outside surface that has been worked ^y implementing a method presenting at least one of the.above characteristics, such that said rod presents required lubrication properties relative the guide bearing and required friction properties relative to the pealing gasket.
In particular, the first material constituting th guide bearing is a thermoplastic polymer, and the seco material constituting the sealing gasket is a rubber.
Other charaicteristics and advantages of the invention appear;' more clearly in the light of the following descri|ption and the accompanying drawings, relating to a pajrticular implementation.

BRIEF DESCRIPTION OF THE DRAWINGS
Reference is made to the figures of zr.e accompanying drawings, in which:
■ Figure 1 :is a diagram showing the various steps in a treatment method in accordance with the invention, here with optional intermediate steps of micro-sanding and an optional step of: cleaning;
• Figure 2 is a micrograph obtained by an electron microscope showifig a small volume of the tungsten bisulfide powder: that is used for the high speed projection at ambient temperature that is performed in the method of th^ invention, which powder is constituted by platelets;-
• Figure 3 jis a diagram showing an individual platelet of hexa(^onal shape constituting the tungsten bisulfide powder-in question;
• Figure 4 fehows the improvement of performance in terms of wettability by presenting a comparative graph that plots a plutality of curves showing how liquid/solid contact angle vafies as a function of time; and
• Figure 5 is an axial section view showing a hydraulic sealing system in accordance with the invention, obtaiified by implementing the above-specified treatment method|
DETAILED DESCRIPTION OF THE PREFERRED IMPLEMENTATION
There follovi/s a description in greater detail of the successive steps^of the method of the invention of treating the surface of a mechanical part made of high-strength steel, which method seeks to impart on said part properties of fraction and of lubrication that are needed for its use.
The mechanicfal part in question, referenced P, is constituted for Example by a stainless steel friction rod of the type used^for fitting to vehicle brake pistons. Naturally, the invention is not limited in any way to one particular type qf mechanical part.

In Figure \\, there can be seen a first step of the treatment method; of the invention, shown diagra.T-.aticaily at a). The starting metal part is a part made of steel, preferably of stainless steel, presenting high strength, i.e. of hardness: that is not less than 30 HRC (i.e. on the Rockwell C scale). The part will generally already have been subjected to appropriate heat treatment enabling it to rpach hardness typically of the order of 34 HRC to 39 HRC^ or will have been treated with thermochemical tteatment of the cementation type at low temperature or o|£ the nitriding type at low temperature enabling it to conserve its stainless properties.
During step a), the part P is subjected to a primary finishing step tjiat is organized to lower its surface roughness Ra to k value that is less than or equal to a first predetermijied threshold SI, e.g. equal to 0.2 pm. The part P is thus finished with machining and treatment (of the thermochpmical or passivation type), and is present in its fjLnal shape and dimensions. The primary finishing treatment of conventional type may comprise steps of turning^ rectifying, etc. ..., and should ensure that its roughnefes Ra can be made to be less than 0.2 pm, for example, oncfe the part has been finished and is ready for the following treatment. It is recalled that the parameter Ra useifl herein for characterizing surface roughness is a pkrameter that is representative of the geometrical irregularities of a surface, and corresponds to the arithmetical mean deviation from the mean line of the roughness.
During the following step b), the part P is subjected to surface cleaning by means of a degreasing solution. This operation is important since it enables the surface of tjie part P to be completely cleaned of all traces of possible dirt (grease, oil, shavings, dust, plastics residue^, felts, substances for providing temporary protection). The degreasing solution used is preferably of thfe alkaline type and it is used at a

temperature in the range 35°C to 60°C. The duraricn of the degreasing step is typically 5 minutes. Naruraliy, when the level of^ dirtying is very great, and in order to reduce the time needed for the degreasing treatment, it is possible to perform pre-degreasing on the metal part.
During the fjollowing step, referenced c) , the part P as cleaned in this way is subjected to a tribo-finishing step organized firstly to further reduce its surface roughness Ra to a^ value that is less than or equal to a second predetermined threshold S2 that is less than the first predetermined threshold SI, and secondly to increase its wet1|ability by hydraulic fluids. The hydraulic fluids I in question are constituted in particular by flijiids based on hydrocarbons or on ester-phosphates, or oily fluids.
Such a tribp-finishing operation is essential for preparing and op1}:imizing the surface state of the metal part prior to th$ treatment by projecting tungsten bisulfide powder►
As shown di^grammatically in Figure 1, the tribo-finishing step c\ advantageously comprises a first step cl) of deburring^ a second step c2) of polishing, and a third step c3) ot surface cleaning, followed by inspecting surface roughness.
Deburrinq step cl) consists in continuously agitating parts P for treatment, generally in a vibrating bowl, together with an oxidizing first aqueous solution containing abrasive agents so as to obtain the desired surface roughnesis Ra. During this step, an oxide film is created on the surfaces of the parts, which film is of hardness that is: less than the hardness of the underlying metal. The film is removed progressively by the mechanical actidn of the abrasive agents which are of hardness greater! than that of the film but less than that of the underlyirig metal, which abrasive agents strike against the surfaces of the part, thereby reducing the roughness of said surfaces. As an indication, this first

step cl) of debarring should be implemented for a duration of noteless than 60 minutes.
The second;step c2) of polishing preferably consists in continuouslyiagitating parts together with a non-oxidising second aqueous solution that contains abrasive agents. This second step of polishing serves to remove all of the oxidg film created during the first step cl) by the mechanicall action of the abrasive agents. As an indication, the ;duration of the treatment for this second step c2) of polilshing should not be less than 120 minutes .
At the end pf these two steps cl and c2), the surface roughness Ra is reduced to a value that is less than or equal toi the second predetermined threshold S2, which is less th^n the first predetermined threshold SI, for example being about 0.1 jam. A step c3) is then advantageously pjrovided for cleaning the surface, followed by a st^p of inspecting the surface roughness Ra, which inspection can be very reliable due to the previously performed cleaning. The cleaning in question seeks to guarantiee that the result of the inspection is a measurement of sijirface roughness that is representative. The surface of the part then presents less dirtying than at the end of th^ primary finishing step a), so it is possible to use a solvent that is not very aggressive, of the acetone type.;
At the end 6f this tribo-finishing step c), it is possible either tlo subject the part P directly to the following essentijal step referenced d) , constituted by a step of projecting tungsten bisulfide powder in the form of platelets at hjigh speed and at ambient temperature, or else in a variant^ to begin, prior to step d) , by implementing an ajdditional micro-sanding step, possibly followed by surfaice cleaning and inspecting surface roughness. Thesei additional steps are represented herein as a step c') thalt is a micro-sanding step organized to activate the surface of the part P so as to increase

adhesion of the icoating deposited subsequently during step d) of projejcting WS2 powder, with said addi-ional step c') being f[ollowed by a step c") of surface cleaning and then by inspecting the surface roughness Ra. In Figure 1, nozzles 10 are shown diagrammatically to symbolize the mijcro-sanding, with particles being projected onto the part P, these particles, which are not oxides, generally having a size lying in the range 5 pm to 15 ]jm, and prjeferably of the order of 10 |im. The projection of particles during step c') is performed at high speed, obtaiined by using a pressure of the order of 5 bars to 10 barjs, with the projection jets being inclined at an angle lying substantially in the range 45° to 135°.
Naturally, :such a micro-sanding step has the effect of slightly increasing the surface roughness Ra. Nevertheless, th|e micro-sanding step c') is organized so that the surface roughness Ra continues to remain below the first predetermined roughness threshold SI, e.g. 0.2 lam.
In Figure l, step c") of inspecting surface roughness Ra is isymbolized by a simple arrow pointing to the part P. As lin above-described step c3), surface cleaning, e.g. by means of a relatively non-aggressive solvent of the ajcetone type, is performed prior to inspecting the siurface roughness so as to guarantee better representjativity for the result of the inspecting measurement.
Whether or jnot the micro-sanding step should be implemented depejnds on the friction properties that it is desired to obtaijn on the metal parts, in addition to the above-mentioned iwettability properties. In this respect, when micro-sandiing is used, it is appropriate to implement step d|) of projecting WSj powder very quickly, e.g. within a delay of not more than 120 minutes.
At the end |of the tribo-finishing step c) , and possibly after ir^icro-sanding step c') and after cleaning

and inspection $tep c"), the part P is optiirially prepared for being subjected to the treatment of prcjectir.r tungsten bisulfide powder. The roughness associated with the finishing operations has been greatly diminished by the tribo-finisHing operation, while the micro-sanding, if any, has alscj activated the surface so as to increase the adhesion of 'the coating that is to be formed.
During step d) , the part P is therefore subjected to projection of tungsten bisulfide powder (WS,) at high speed and at ambient temperature.
In accordanjce with an essential characteristic of the invention, tjhe WSj powder used in the method of the invention is in ^the form of platelets £, as shown in Figures 2 and 3,; thereby producing a technical effect that is radically different from that which has been obtained in prioir art techniques that also make use of projecting WSj powder and that consist in projecting spherical powder particles that are encrusted in a cutter part previously prepared to present associated powder-receiving recesses. Furthermore, the teaching consisting in providing rec'^sses for receiving spherical particles de facto impliesi a limit for the amount of surface roughness reduction that can be obtained, insofar as too small a value for roughness would eliminate the powder-receiving recesses, and would prevent spherical particles of WSj powder bec^oming encrusted. Specifically, the process is quite' different when using a powder made up of platelets, i.e. Very thin plates that disintegrate into microparticles op coming into contact with the surface of the part for treatment.
Preferably, I the platelets £ used are substantially hexagonal in shape, as shown in Figure 3, having a main dimension referenced D lying in the range 0.8 iJm to 1.5 pm, and a thickness, referenced E, of the order of 0.1 pm. When thi^se platelets p are projected by associated nozzl^, referenced 20 in Figure 1, they break up into microparticles on coming into contact with the

surface, thereby: creating a deposit on the surface cf said part, which deposit is dense and self-lubricazir.g.
By way of i;ndication, for operating conditions in which the WSj poWder is projected in the form of platelets, cold land at high speed, it is possible to use a pressure of the order of 5 bars to 10 bars with an angle of inclination for the projection jet lying in the range 45° to 135:° relative to the plane of the surface that is to be trieated, the distance between the outlet from the projection nozzles and the part P typically lying in the range 20 millimeters (mm) to 100 mm. These operating conditjions enable platelets of WSj powder to be projected at high speed so that they break up into microparticles oh striking the surface of the part to be treated.
Tests undertaken by the Applicant have shown that it is then easy to obtain a coating of thickness lying in the range 0.4 pmi to 0.6 |im with the liquid/solid contact angle at the surjface of the WS, coating varying in a manner that is perfectly reproducible (which is not true for the prior art techniques mentioned above). The treated parts arje then of a bluish gray color that is entirely charactpristic of a deposit of uniform thickness. Visujal inspection of the color of the part thus makes it possible to guarantee that the treatment has taken place properly and that the desired characteristics have indeed been achieved.
Furthermore,!, as shown in Figure 1, it is also possible to provjide for the method to include, after step d) of subjecting! WSj powder, a step d') of cleaning its surface, followejd by inspection. As for the preceding step c3) and c"):, the surface cleaning may be performed by means of a soilvent that is not very aggressive, of the acetone type, thereby guaranteeing better representativityl for the results of the inspection measurements.

Performing $uch a final step prior to using the treated parts is; of great advantage, and it; serves ir. particular to pe):form three inspections, represented by three arrows in the figure, relating respectively to surface roughnesfe, to wettability by hydraulic fluids, in particular fluid^ based on hydrocarbons or on ester-phosphates, or o.JLly fluids, and to the coefficient of friction (staticland/or dynamic).
This ensures that a treated part is obtained presenting surfafce roughness with a value Ra of less than 0.2 pm, with a d|/namic friction coefficient (WSj against WSj and plane onlplane) of less than 0.03, and a static friction coefficjient (WSo against WS, and plane on plane) of less than 0.0|7.
The wettability that is obtained is also extremely discriminating ifisofar as it is very good for hydraulic fluid, in particular for fluids based on hydrocarbons or on ester-phosphates, or oily fluids, while being very bad for aqueous fluids.
Figure 4 shdws the improvement obtained in performance in terms of wettability for the WS2 coating when made in accbrdance with the invention.
Curves CI, |C2, and C3 in the graph of Figure 4 correspond to vajriation in the liquid/solid contact angle (in degrees) as la function of time (in seconds) . Curve CI corresponds to a treatment method of traditional type, while curves C2 land C3 correspond to treatment in accordance with Ithe invention, respectively with and without final cleaning.
A coating is thus obtained with a coefficient of friction that is| very low, and that is self-lubricating because of the continuous film created on the surface of the part, with this taking place over a very wide temperature range, the coating furthermore being lipophilic and hydrophobic. This represents considerable progress compared with the above-mentioned prior techniques corresponding to electrolytic processes.

With referelnce to Figure 5, there follows a description of a| hydraulic sealing systerr. in acccriance with the invenT:iion obtained by implementing the above-described surfade treatment method.
In Figure 5j, there can thus been seen a hydraulic sealing system referenced 100 comprising a slide rod 101 of axis X that i|s made of high-strength stainless steel, and that slides iin a sealing assembly 102. The sealing assembly 102 is Ireceived in a housing 106 formed in a support element dOS, being disposed between shoulders 107 and 108.
The sealing! assembly 102 is constituted by a guide bearing 103 made of a first material and by a sealing gasket 104 made lof a second material of hardness lower than that of thei first material. By way of example, the first material constituting the guide bearing 103 is a thermoplastic polymer, and the second material constituting the! sealing gasket 104 is a rubber. When the rod 101 movejs from the right to the left in the figure, the guidb bearing 103 that is capable of sliding on the rod 101 cb-operates with the sealing gasket 104 by compressing it, thereby reinforcing sealing.
The outside: surface 110 of the rod 101 has been treated by imple|nenting a method as described above, such that said rod presents required properties both concerning lubripation relative to the guide bearing 103, i.e. at the inte[rface between the outside surface 110 of the rod 101 and the inside surface 103.1 of the guide bearing 103, and in terms of friction relative to the sealing gasket 104, i.e. at the interface between the outside surface 110 of the rod 101 and the inside surface 104.1 of the sealing gasket 104, in order to avoid abrasion.
The dual function of the WSj coating lining the sliding rod 101 optimizes co-operation with both of the components 103, LL04 constituting the sealing assembly 102.

Such a hydi-aulic sealing system is particularly advantageous for fitting to vehicle brake pisrons.
This can apply in particular to a friction rod for arranging in a piston in a hydraulic ring in an aircraft brake. The role of such a friction rod is to guide the piston when applying braking force to the disk(s) of the brake, the rod being fitted with a sealing system constituting a cjuide bearing made of
polytetrafluoroethylene and a sealing gasket made of elastomer of the ethylene propylene type. Such a rod/gasket system can then satisfy numerous requirements, in particular iti can present excellent friction behavior serving to limit gasket wear and damage to the rods, and it can present good sealing for the piston against the hydraulic fluid.
The invention is not limited to the implementations described above,: but on the contrary covers any variant using equivalent means to reproduce the essential characteristics ispecified above.

CLAIMS
1. A method of treating the surface
made of high-strength steel, the method seeking to confer
on said part friction and lubrication properties that are
needed for its usfe, the method being characterized in
that it comprises: the following successive steps:
a) subjectinig the part (P) to a primary finishing step organized to| lower its surface roughness (Ra) to a value less than ot egual to a first predetermined threshold (SI);
b) then subjbcting the part (P) to surface cleaning by means of a deglreasing solution;
c) subjecting the part (P) as cleaned in this way to a tribo-finishingi step organized firstly to further lower its surface roughness (Ra) to a value less than or equal to a second predetermined threshold (S2) that is less than the first predetermined threshold (SI), and secondly to increase its v|ettability by hydraulic fluids; and
d) subjectinjg the part (P) to projection, at high speed and at ambijent temperature, of tungsten bisulfide (WSo) powder in t^e form of platelets (p) that break, thereby creating ia dense and self-lubricating deposit on the surface of s4id part.
2. A method accoifding to claim 1, characterized in that the tribo-finishing step c) includes a first step cl) of deburring by coniinuously agitating parts (P) for treatment together with an oxidizing first aqueous solution containing abrasive agents until the desired surface roughnes$ (Ra) is obtained, followed by a second step c2) of polishing by subjecting said parts to continuous agitation together with a non-oxidizing second aqueous solution;containing abrasive agents.
3. A method according to claim 2, characterized in that the tribo-finishiLng step c) includes a third step c3) of

surface cleaning, followed by inspection of the surface roughness (Ra).
4. A method according to any one of claims 1 to 3, characterized in that the first predetermined roughness threshold (SI) isi substantially equal to 0.2 ^m, and the second predetermiined roughness threshold (S2) is substantially equial to 0.1 ]am.
5. A method according to any one of claims 1 to 4, characterized in Ithat the powder projected during step d) is constituted almost exclusively by pure WSj, and is in the form of platelets (p) that are substantially hexagonal in shape, with a main dimension (D) lying in the range 0.8 pm Ito 1.5 pm, and with a thickness (E) of the order of 0.1 iiim.
6. A method accorlding to claim 1, characterized in that it includes, afteir tribo-finishing step c) , an additional step c') of micrd-sanding, organized to activate the surface of the palrt (P) in order to increase the adhesion of the coating subsequently deposited during step d) of projecting WS2 polwder.
7. A method according to claim 6, characterized in that the micro-sandinc^ step c') is followed by a surface cleaning step c"], and then by inspection of the surface roughness (Ra).
8. A method according to claim 6 or claim 7, characterized in;that the micro-sanding step c') is organized in sucl|i a manner that the surface roughness (Ra) , which is iijicreased as a result of the micro-sanding, remainsi below the first predetermined roughness threshold (SI). i

9. A method accotding to any one of claims 6 to 3,
characterized in|that the micro-sanding step z'] is
implemented usin-:^ particles that are not oxides, and of a
size lying in thfe range 5 \im to 15 jam.
10. A method according to claim 1, characterized in that
it includes, aftfer the WSj powder projection step d), a
step d') of surf surface roughness (Ra), of wettability, and of
coefficient of fjriction.
11. A hydraulic pealing system including a slide rod
(101) slidable ih a sealing assembly (102), the system
being characterised in that the sealing assembly (102) is constituted by a; guide bearing (103) made of a first material and by h sealing gasket (104) made of a second material of hardhess less than the hardness of the first material, and in: that the sliding rod (101) has an outside surface :(110) that has been worked by implementing a mfethod according to any one of claims 1 to 10, such that said rod presents required lubrication properties relative to the guide bearing (103) and required frictioh properties relative to the sealing gasket (104) .
12. A system according to claim 11, characterized in that
the first material constituting the guide bearing (103)
is a thermoplastic polymer, and the second material
constituting thei sealing gasket (104) is a rubber.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=AxYUbb47dp+aWPxkbAdzgQ==&loc=egcICQiyoj82NGgGrC5ChA==

« Previous Patent

Next Patent »

Patent Number

272217

Indian Patent Application Number

1271/CHE/2009

PG Journal Number

14/2016

Publication Date

01-Apr-2016

Grant Date

22-Mar-2016

Date of Filing

02-Jun-2009

Name of Patentee

MESSIER-BUGATTI-DOWTY

Applicant Address

INOVEL PARC SUD 78140 VELIZY VILLACOUBLAY

Inventors:

#	Inventor's Name	Inventor's Address
1	GARIN, AUDE,	10 RUE DE LA POUDRIERE, 67120 MOLSHEIM,
2	VIOLA, ALAIN,	3 RUE DU CIMETIERE, 67800 HOENHEIM,

PCT International Classification Number

C21C1/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	08 3093	2008-06-04	France