Self-lubricating ring for fluid bearings and method for producing such a ring

Description

FIELD OF THE INVENTION

[0001] This disclosure relates to the field of fluid bearings, and more particularly to a method of manufacturing a self-lubricating ring for a fluid bearing as well as a self-lubricating ring for a fluid bearing. TECHNOLOGICAL BACKGROUND

[0002] Fluid bearings are known as an alternative to ball bearings in many applications, to provide suspension of a rotating part, such as a shaft, relative to a stationary housing. Fluid bearings generally have a ring mounted coaxially around a shaft and separated from this shaft by a fluid film. Their advantages are numerous and include in particular low wear during use, thanks to the absence of contact between the ring and the shaft in stationary conditions.

[0003] However, in transient conditions, the ring and the shaft may come into contact, which requires appropriate lubrication to reduce their wear.

[0004] To this end, so-called self-lubricating rings have been proposed, comprising an impregnated porous coating, deposited on a substrate or "back". The substrate is conventionally made of stainless steel; document US 2017 / 138396 also describes a bearing element whose substrate can be made of iron alloy and the coating of polymer. However, in components subject to large temperature variations, such as cryogenic propellant rocket engine turbopumps, stainless steel exhibits significant differential expansion compared to the more complex alloys generally used in these components. This leads to a risk of de-shearing or tightening of the ring and, consequently, to its untimely disassembly or plasticization.

[0005] Known methods for producing this type of ring do not allow the substrate to be modified satisfactorily to solve this problem. There is therefore a need for a new type of method for manufacturing a self-lubricating ring for a fluid bearing, and for a new type of self-lubricating ring for a fluid bearing. PRESENTATION OF THE INVENTION

[0006] For this purpose, the present disclosure relates to a method of manufacturing a self-lubricating ring for a fluid bearing according to claim 1.

[0007] A self-lubricating ring is a ring comprising a self-lubricating coating. For the purposes of this disclosure, alloys include metals. Some alloys have the ability to self-protect by the natural creation, on their surface, of a thin, impermeable and compact layer of oxides. Such a layer is an example of a passive layer, or passivation layer. According to another example, a passive layer may result from a passivation treatment actively applied to the alloy. In all cases, the removal of the passive layer makes it possible to expose at least locally the core of the substrate, having a composition relatively poor in oxides.

[0008] Indeed, the inventors realized that unlike stainless steel, alloy substrates having a highly chemically resistant passive layer offer poor adhesion for the deposition of a self-lubricating coating. Thus, it was not possible to simply replace the stainless steel of the rings of the prior art with a passive alloy. The present method, thanks to the removal of the passive layer and the deposition of an anchoring layer, makes it possible to ensure sufficient adhesion between the alloy substrate and the self-lubricating coating. Furthermore, thanks to the fact that the substrate can be chosen from an alloy close to or identical to that used in the members concerned, the problems of differential expansion can be significantly reduced. It is understood that the deposition of an anchoring layer is an active step during which an anchoring layer is deliberately deposited on the depassivated surface.

[0009] In some embodiments, removing the passive layer includes sandblasting the substrate. Sandblasting not only destroys and removes the passive layer, such as naturally occurring oxides on the surface of the substrate, but also creates a roughness on the substrate that allows the anchoring layer to be mechanically anchored to the substrate. In addition, depending on the composition of the substrate, sandblasting may be more effective than chemical removal, which the oxides would resist.

[0010] In some embodiments, the deposition of the anchoring layer is carried out electrolytically. The electrolytic route is relatively inexpensive and promotes interdiffusion of the materials of the anchoring layer and the substrate, which also results in better anchoring.

[0011] In some embodiments, the thickness of the anchor layer is between 2 and 20 microns, preferably between 5 and 15 microns, more preferably between 8 and 12 microns. The thickness of the anchor layer refers to the average thickness of the anchor layer, measured perpendicular to the depassivated surface. In these embodiments, the thickness ensures sufficient layer continuity to effectively protect the substrate before continuing with manufacturing. A lower thickness would risk leaving discontinuities leading to repassivation of the substrate, while a higher thickness would increase the cost of implementing the method.

[0012] In some embodiments, the deposition of the self-lubricating coating comprises sintering. The sintering, typically of a powder, creates a porous layer which can then be impregnated with a material providing the self-lubricating nature of the coating.

[0013] In some embodiments, the composition of the substrate is as follows, expressed in mass percentages: 20.0 to 23.0% of Cr; at most 5.0% of Fe; 8.0 to 10.0% of Mo; 3.15 to 4.15% of Nb; at most 1.0% of Co; at most 0.50% of Mn; at most 0.40% of Al; at most 0.40% of Ti; at most 0.50% of Si; at most 0.10% of C; at most 0.015% of S; at most 0.015% of P; the balance being Ni. Such an alloy is known under the trade name “Inconel 625” or according to the names N06625 (UNS standard), NI-PH3601 (AECMA standard) or NC22DNb (AFNOR standard). Such an alloy may include unavoidable impurities.

[0014] In some embodiments, the anchoring layer is made of copper. This means that the anchoring layer consists of the chemical element copper, excluding unavoidable impurities. At the very least, the anchoring layer may comprise, by mass, at least 70% copper, preferably at least 90% copper, preferably at least 95% copper. Copper in particular facilitates the deposition of a self-lubricating coating comprising bronze. Other materials may be used for the anchoring layer, for example copper-based alloys (i.e. bronze or copper-zinc) or nickel, in particular if the substrate is a nickel-based alloy. In any event, a person skilled in the art may choose a material capable of allowing interdiffusion between the anchoring layer and the self-lubricating coating to improve its anchoring.

[0015] In some embodiments, the method includes a step of shaping the ring. Thus, in some embodiments, the substrate is wound to form the ring. The substrate may be wound with the anchoring layer and the self-lubricating coating deposited thereon. The substrate is typically wound with the self-lubricating coating on the inside, but alternatively, it may be wound with the self-lubricating coating on the outside. The substrate may be wound by forming under a press.

[0016] In some embodiments, the substrate is made of a nickel-based alloy. A nickel-based alloy is an alloy comprising more nickel than any other chemical element, preferably at least 50% nickel. Nickel-based alloys are commonly used in components subject to large temperature variations and form a passive layer on their surface relatively quickly. The application of the method described above therefore finds a particularly interesting application in these embodiments.

[0017] The present disclosure also relates to a self-lubricating ring for a fluid bearing according to claim 13. The ring may be produced by the method previously described and have all or part of the characteristics resulting from the different embodiments of the method as previously described. In particular, the fact that the anchoring layer is anchored on the substrate may be made possible by removing the passive layer from the substrate.

[0018] Such a ring has better resistance when used in alloy components, particularly nickel-based alloys. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The invention and its advantages will be better understood upon reading the following detailed description of embodiments of the invention given as non-limiting examples. This description refers to the appended drawings, in which: THE figures 1A, 1B, 1C, 1D, 1E represent, in schematic section, steps of a method of manufacturing a self-lubricating ring for a fluid bearing according to one embodiment; the figure 2 represents, in schematic section, a self-lubricating ring for a fluid bearing according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0020] A method of manufacturing a self-lubricating ring for a fluid bearing, according to one embodiment, will be described with reference to figures 1A-1E And 2 .

[0021] As illustrated in the figure 1A , first of all, a substrate 12 is provided made of an alloy having a passive layer 14. In this case, the substrate 12 is made of a nickel-based alloy, more precisely Inconel 625, the composition of which has been given above. The passive layer 14 is here a surface layer, present at the periphery of the substrate 12. This passive layer 14 results in this case from the natural oxidation of the substrate 12.

[0022] The passive layer 14 is removed, at least in part. In the present embodiment, this removal is carried out by sandblasting, possibly preceded by degreasing the substrate 12. The sandblasting parameters can be chosen by those skilled in the art depending on the thickness and nature of the oxide layers formed on the surface. As illustrated in the figure 1B , the passive layer 14 is removed on one face of the substrate 12, here the upper face. This makes it possible to expose a depassivated surface 14a of the substrate 12. In addition, thanks to the sandblasting, the depassivated surface 14a acquires a roughness which may be greater than or equal to the roughness of the surface before removal of the passive layer.

[0023] After removing the passive layer 14, an anchoring layer 16 is deposited on the depassivated surface 14a. This deposition can be carried out directly after removing the passive layer 14, without waiting, so as not to allow a passive layer to reform naturally. This step is shown in the figure 1C . In this embodiment, the anchoring layer 16 is made of copper. Furthermore, the anchoring layer 16 is here deposited electrolytically, and, in this example, over the entire depassivated surface 14a. Thus, for this embodiment, we speak of electrolytic copper plating. Alternatively, the anchoring layer could also be deposited by other means such as physical vapor deposition. Furthermore, the anchoring layer 16 could only be deposited on certain areas of the depassivated surface 14a, typically via masking.

[0024] Electrolytic deposition is known per se to those skilled in the art. The deposition parameters are determined by those skilled in the art according to their knowledge to obtain, for example, an average thickness of the anchoring layer 16 between 8 and 12 microns. In order to guarantee good adhesion of the copper to the depassivated surface 14a, matt copper is preferably used, without additives in order to avoid any degradation at high temperature, in particular without organic brightener type additives which, by decomposing at high temperature, would form blisters between the substrate and the self-lubricating coating. Pull-off tests carried out by the inventors according to the ISO 2409 standard have shown satisfactory adhesion of the anchoring layer 16 to the depassivated surface 14a, the anchoring layer 16 being truly anchored to the depassivated surface 14a.

[0025] After the deposition of the anchoring layer 16, a self-lubricating coating is deposited on the anchoring layer 16. To do this, according to the invention, a porous layer 18 is first deposited on the anchoring layer 16.

[0026] In this case, the porous layer 18 is obtained by sintering a powder, and more particularly a bronze powder. The bronze powder, here of chemical formula CuSn 12 , can be sintered by spreading in a belt sintering furnace. The speed of movement of the belt can be between 8 and 12 meters per hour. The temperature of the furnace can be between 700°C and 900°C. A sintered porous layer 18 is thus obtained having a porosity rate of between 15% and 30%, as illustrated in the figure 1D The thickness of the porous layer 18 may be between 0.2 and 2 millimeters, preferably between 0.5 and 1 millimeter, depending on the application.

[0027] The sintering of the CuSn bronze 12 on the copper anchoring layer 16 is a homogeneous sintering, which consequently ensures good adhesion and optimal interface quality between the anchoring layer 16 and the porous layer 18. As a result, the porous bronze layer 18 has metallurgical adhesion with the substrate 12 via the copper anchoring layer 16, which eliminates any risk of detachment.

[0028] The porous layer 18 is then impregnated so as to form a self-lubricating coating, for example by dipping. In this embodiment, the integral assembly formed by the substrate 12, the anchoring layer 16 and the porous layer 18 is dipped in a bath. The bath may comprise an aqueous solution of polytetrafluoroethylene (PTFE). Homogeneous and total penetration of the solution throughout the thickness of the porous layer 18 may be obtained, according to one example, by performing the dipping in a vacuum chamber and alternating the pressure in said chamber between vacuum and atmospheric pressure, in order to expel the air present in the porosities of the porous layer 18. At the end of this impregnation, the impregnated porous layer 18 forms a self-lubricating coating 20, as illustrated in the figure 1E .

[0029] The ring is also shaped. For example, as indicated above, the substrate 12 can be cut to the desired dimensions, according to the target diameter and width, and wound so that its opposite ends are in contact. The winding can be carried out by press forming. To ensure the continuity of the ring, said ends can then be welded together, or simply placed end to end by shrink fitting in a support whose internal diameter is sized so as to keep said ends in contact, without play. A self-lubricating ring 10 is thus obtained, as shown in the figure 2 .

[0030] In addition to the steps previously described, it is possible to apply a heat treatment to the substrate 12 covered with the anchoring layer 16 and the porous layer 18. Such a heat treatment can improve the interdiffusion between the layers and, in fine,the adhesion of the self-lubricating coating 20 to the substrate 12. However, if the selected temperature is too high, such treatment may degrade the mechanical properties of the substrate 12.

[0031] Although the present invention has been described with reference to specific exemplary embodiments, modifications may be made to these examples without departing from the general scope of the invention as defined by the claims. Furthermore, individual features of the various illustrated / mentioned embodiments may be combined in additional embodiments, while remaining within the scope of the claims. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

Claims

1. A method of manufacturing a self-lubricating ring (10) for a fluid bearing, comprising the following steps: - providing an alloy substrate (12) having a passive layer (14); - at least partially removing the passive layer (14) of said substrate, so as to obtain a depassivated surface (14a); - depositing an anchoring layer (16) on the depassivated surface (14a); - depositing a self-lubricating coating (20) on the anchoring layer (16), the coating comprising a porous layer (18) having a metallurgical adherence with the substrate (12) via the anchoring layer.

2. The manufacturing method according to claim 1, wherein the substrate (12) is made from a nickel-based alloy, preferably wherein the composition of the substrate (12) is as follows, expressed in percentages by weight: 20.0 to 23.0% Cr; not more than 5.0% Fe; 8.0 to 10.0% Mo; 3.15 to 4.15% Nb; not more than 1.0% Co; not more than 0.50% Mn; not more than 0.40% Al; not more than 0.40% Ti; not more than 0.50% Si; not more than 0.10% C; not more than 0.015% S; not more than 0.015% P; the balance being Ni.

3. The manufacturing method according to claim 1 or 2, wherein the removal of the passive layer (14) comprises sanding the substrate (12).

4. The manufacturing method according to any one of claims 1 to 3, wherein the deposition of the anchoring layer (16) is performed electrolytically.

5. The manufacturing method according to any one of claims 1 to 4, wherein the thickness of the anchoring layer (16) is between 2 and 20 microns, preferably between 5 and 15 microns, more preferably between 8 and 12 microns.

6. The manufacturing method according to any one of claims 1 to 5, wherein depositing the self-lubricating coating (20) comprises sintering.

7. The manufacturing method according to any one of claims 1 to 6, wherein the anchoring layer (16) is made of copper.

8. The manufacturing method according to any one of claims 1 to 7, wherein the substrate (12) is wound to form the ring.

9. The manufacturing method according to any one of claims 1 to 8, wherein the substrate (12) forms a back for the ring (10).

10. The manufacturing method according to any one of claims 1 to 9, wherein the substrate (12) is wound with the anchoring layer (16) and the self-lubricating coating (20) deposited on top.

11. The manufacturing method according to any one of claims 1 to 10, wherein the anchoring layer (16) is made from a copper-based alloy or from nickel.

12. The manufacturing method according to any one of claims 1 to 11, wherein the anchoring layer (16) is made from copper and the porous layer is made from bronze.

13. A self-lubricating ring (10) for a fluid bearing, comprising an alloy substrate (12) configured to have a passive layer, the ring further comprising an anchoring layer (16) anchored to said substrate (12) whereof the passive layer has been removed, and a self-lubricating coating (20) deposited on the anchoring layer (16), the coating (20) comprising a porous layer (18) having a metallurgical adherence with the substrate (12) via the anchoring layer (16).