Parts comprising aluminium or an alloy thereof coated with a self-lubricating anodization layer, and corresponding anodizing method

Abstract

The present invention relates to a method for anodizing at least one portion of a part comprising aluminium or an aluminium alloy, said method comprising the following successive steps: a) immersing said portion in an aqueous bath comprising etidronic acid and polymeric particles comprising at least one fluoropolymer, b) applying to the portion immersed in the bath an electrical current and / or an electrical voltage, so as to obtain on said portion an anode layer incorporating said polymeric particles comprising at least one fluoropolymer, and / or residues of said particulate polymeric particles over its entire thickness. The invention also relates to a surface treatment method comprising said anodizing method, and also to the parts that can be obtained via this surface treatment method. The anode layer thus obtained imparts in particular improved tribological properties to the parts comprising aluminium or an aluminium alloy.

Description

[0001] TITLE: Parts comprising aluminum or one of its alloys coated with a self-lubricating anodizing layer, and corresponding anodizing process

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of surface treatment of aluminum or aluminum alloy parts, aimed at improving their tribological properties. It relates more specifically to an improved anodizing process, aimed in particular at competing with conventional hard anodizing processes, as well as a process for surface treatment of a part comprising aluminum or one of its alloys implementing said anodizing process, and the parts capable of being thus obtained.

[0004] STATE OF THE ART

[0005] Aluminum alloy parts intended for use in the aeronautics sector in particular generally undergo a surface treatment before being used to improve their performance.

[0006] One of the most widespread techniques for doing this is hard anodizing, also called hard anodic oxidation, which consists of forming a porous layer of aluminum oxides / hydroxides on the surface of the part, called the anodic layer, by applying a current to the part immersed in an electrolytic bath containing a strong acid electrolyte, the part constituting the anode of the electrolytic device.

[0007] The electrolytic baths currently used industrially for anodizing aluminum parts include a cold acid electrolyte (around 0°C) with or without additives to the anodizing bath. The acid that provides the best compromise between the desired performance, ease of use and compatibility with environmental standards is sulfuric acid. This is referred to as "hard sulfuric" anodizing. Hard sulfuric anodizing of aluminum is used for the functional properties of corrosion resistance, electrical and thermal resistance and abrasion resistance that it provides. An example of the implementation of hard sulfuric anodizing is given in document FR3077303A1, in which the electrolytic bath comprises sulfuric acid in a concentration of between 100 and 350 g / L, preferably with additional oxalic acid or glycolic acid in a concentration of between 10 and 45 g / L.Anodization is carried out under a pulsed current or voltage, advantageously at a temperature between -5 and 15°C. Anodization with etidronic acid should also be mentioned, which gives interesting results, but is little used industrially to date. Recent examples of implementation are described in particular by Huang et al. (Surface and Coating Technologies 374 (2019) 83-94), Kikuchi et al. (Surface and Coating Technologies 326 (2017) 72-78) and Iwai et al. (Electrochimica Acta 320 (2019)134606). To improve the friction resistance properties of the surface of the anodic layer, the "anodized" part can then be subjected to a so-called post-treatment step which consists of coating or impregnating the anodic layer formed by a fluoropolymer such as polytetrafluoroethylene (PTFE). We can then speak of “lubricating” post-treatment.Document CN101736385A thus presents an example of a protocol for ultrasonically impregnating an anodic layer with PTFE nanoparticles. Another example of a process integrating a post-treatment step is described in document DE4124730.

[0008] Alternatively, self-lubricating properties can be provided by adding additives to the anodizing bath as shown in document CN103981556A, which describes the use of boric acid and iron sulfate for this purpose. The friction behavior before post-treatment of parts anodized according to the process of CN103981556A is slightly improved. However, this process is not very reproducible and difficult to industrialize.

[0009] In prior art processes with post-treatment or addition of additives in the anodizing bath, the fluoropolymer that provides the lubricating properties is deposited essentially on the surface of the layer, and not in depth, even when the polymer is mixed with the anodizing bath. This surface-only deposition of the fluoropolymer is problematic when the parts have dimensions that are too large compared to the specifications - due to the significant thickness of the hard anodic layers - or when the roughness of the coating layer is too great. It is then necessary to machine the parts, which first affects the surface of the layer containing the fluoropolymer. In addition, during their use, the parts are subject to wear phenomena by abrasion of the outer surface of the post-treatment layer, which leads to a loss of the tribological properties provided by the fluoropolymer with the age of the part.

[0010] Prior art documents also propose combining an anodizing process with another process, the latter allowing the inclusion of polymeric particles in the anodic layer. Thus, we can notably cite document CH499057, which describes a mixed process combining (sulfuric) anodization with an electrophoresis process, and document RU2483144 which discloses a process for anodizing a metal by plasma electrolytic oxidation, with a specific electrolyte comprising PTFE particles. However, these processes are complex and expensive to implement.

[0011] The present invention therefore aims to overcome the drawbacks of the prior art, by proposing parts coated with an anodizing layer having improved tribological properties, more constant over time and / or with reduced losses of performance over time. The invention also aims to obtain parts with thin coatings, requiring little or no subsequent rework due in particular to their low roughness. Preferably, the method for achieving this does not use any harmful substance, in particular based on hexavalent chromium, and makes it possible to obtain parts with performances at least equivalent to the parts obtained with the methods of the prior art, in particular in terms of fatigue reduction of the part. The method is also easy to implement, in particular in existing installations, because it does not require substantial modifications to the latter.

[0012] BRIEF STATEMENT OF THE INVENTION

[0013] To this end, the invention proposes a method for anodizing at least one part of a part comprising aluminum or an aluminum alloy, said method comprising the following successive steps: a) immersion of said part in an aqueous bath comprising etidronic acid and polymeric particles comprising at least one fluoropolymer, b) application to the part immersed in the bath of an electric current and / or an electric voltage, so as to obtain on said part an anodizing layer (also called "anodic layer") incorporating said polymeric particles comprising at least one fluoropolymer, and / or residues of said polymeric particles over its entire thickness.

[0014] The anodizing process of the invention makes it possible to obtain thinner anodic layers for improved, or at least identical, tribological and mechanical performance compared to “hard” anodic layers ( / .e. obtained by conventional hard anodizing processes) even after reworking or lapping. They have a gray-white appearance on aluminum and aluminum alloys.

[0015] According to another aspect, the invention relates to a method of surface treatment of at least a portion of a part comprising aluminum or an aluminum alloy, said method comprising the anodizing method of the invention.

[0016] According to another aspect, the invention also relates to parts comprising aluminum or an aluminum alloy coated with an anodizing layer on at least part of its surface, said anodizing layer incorporating polymeric particles comprising at least one fluoropolymer, and / or residues of said polymeric particles over its entire thickness. The presence of organic particles over the entire thickness of the anodic layer can be demonstrated for example with glow discharge optical emission spectroscopy or Energy Dispersive Spectrometry: in the anodic layer, a substantially constant level of oxygen and carbon (or fluorine) is detected, which is indicative of carbon compounds throughout the oxide layer formed by the anodizing process.However, the analytical techniques used do not allow us to state with certainty that the polymer particles are not modified during anodization: this is why we speak not only of polymer particles, but also of their residues.

[0017] Thus, the parts of the invention exhibit improved tribological (and mechanical) performance, which decreases little - if at all - over time.

[0018] DEFINITIONS

[0019] For the purposes of the present invention, the term "fluoropolymer" or "fluoropolymer" means a polymer comprising multiple carbon-fluorine bonds, generally obtained by polymerization of monomers comprising at least one Carbon-Fluorine bond, in particular by polymerization of olefin comprising at least one Carbon-Fluorine bond. An example of a fluoropolymer is polytetrafluoroethylene (PTFE). Fluoropolymers are characterized by high chemical stability, which gives them high resistance to solvents, acids and bases in particular.

[0020] By "residues of polymeric particles comprising at least one fluoropolymer" is meant in particular polymeric fragments, in particular fluorinated polymeric fragments, which may result from degradation of the polymers during step b), or "melted" particles.

[0021] For the purposes of the present invention, the term "tribological properties" means, in particular, resistance to friction, wear, and lubrication properties. Tribological properties are generally measured using pin-plane type tribometers, for example, or standardized functional tests such as the Taber abrasion test of standard ASTM D 4060-14.

[0022] By "etidronic acid" is meant the molecule with CAS number 2809-21-4, also called 1-hydroxyethylidenediphosphonic acid, of formula [Chem.

[0023] Anodic layers can be characterized by glow-discharge optical emission spectroscopy (GDOES). By successive abrasion using argon atoms (plasma), GDOES can determine the atomic composition of a material, as well as the relative abundance of its constituent elements. More precisely, metal samples are used as a cathode in a plasma. From the surface, the sample is analyzed in successive layers, by spraying argon ions. The atoms removed pass into the plasma by diffusion. Photons related to the de-excitation of atoms in the plasma are emitted: they have characteristic wavelengths that are recorded by a downstream spectrometer and then quantified. The composition of each of the layers can then be determined. This technique can be used to identify oxygen, aluminum, and carbon, among other elements.However, the use of argon plasma does not allow the detection of fluorine atoms. Examples of SDL implementation are described for example by Moutarlier et al. (Ultrasonics Sonochemistry 64 (2020) 104879).

[0024] Anodic layers can also be characterized by energy-dispersive X-ray spectroscopy (EDS), which is used for the elemental analysis of a sample. It also allows the relative abundance of the detected chemical elements to be analyzed. It is based on the interaction between an X-ray excitation source and the sample to be analyzed. The sample is subjected to an electron beam powerful enough to excite the low-energy electrons in the sample's atoms. These are ejected and then replaced by higher-energy electrons. During this process, energy is also released in the form of X-rays, corresponding to the energy difference between the two electrons. The number and energy of the emitted X-rays are characteristic of the emitting element, which makes it possible to determine the composition of the sample.This technique can identify oxygen, aluminum, carbon, and fluorine, among other things. Examples of EDS implementation are described, for example, by Torkar et al. Engineering Failure Analysis 16(3) (2009).

[0025] DETAILED PRESENTATION

[0026] Anodizing process

[0027] The invention relates firstly to a method for anodizing at least one part of a part comprising aluminum or an aluminum alloy, said method comprising the following successive steps: a) immersion of said part in an aqueous bath comprising etidronic acid and polymeric particles comprising at least one fluoropolymer, b) application to the part immersed in the bath of an electric current and / or an electric voltage, so as to obtain on at least one part of the surface an anodic layer incorporating said particles over its entire thickness.

[0028] Part

[0029] The part is metallic, and comprises aluminum and / or an aluminum alloy. According to a first embodiment, the part consists of aluminum and / or an aluminum alloy. The part or piece may further comprise one or more additive elements, and in particular (%m meaning mass percentage relative to the total weight of the part):

[0030] • copper, for example between 0 and 6%m, and / or

[0031] • zinc for example between 0 and 7%m, and / or

[0032] • silicon, for example between 0 and 13%m, and / or • lithium, for example between 0 and 5%m lithium.

[0033] The part may comprise or be a 2000, 6000 and / or 7000 series alloy.

[0034] The process can be applied to an entire part, this part being intended to be entirely immersed during the process.

[0035] The method is applied to at least one part of at least one part. The method can also be applied to several parts of the same part, for example simultaneously.

[0036] According to one embodiment, the method can be applied simultaneously to one or more parts of several parts, the parts being intended to undergo the steps of the method simultaneously.

[0037] When the process is applied to only part of the part, the process is not applied to a so-called spared (or to be spared) part of the part. In this case, the part can be completely immersed during the process, the part to which the process is to be applied being then immersed and the spared part being either inert with respect to anodizing, or masked for protection. Masking can be achieved by applying a protective film, varnish, masking tape or a cap to the spared part.

[0038] In the remainder of the description, and unless otherwise stated, the term "part" refers to the at least one part to which the process is applied (as opposed to the spared part), or to the entire part if the process is applied to it entirely.

[0039] The part is, for example, a simple part, meaning that it has a shape that can be easily modeled by a two-dimensional object. A simple part is, for example, a substantially flat object such as a plate, or an object with two smooth faces. A rod is another example of a simple part. Alternatively, the part may be a complex part, meaning a part requiring three-dimensional modeling, for example because it includes one or more hollow bodies.

[0040] Bath

[0041] The anodizing process is typically carried out in a tank comprising a cathode and at least part of the part to be anodized, which acts as an anode. According to this embodiment, the tank contains the bath. The tank is preferably provided with temperature regulation and heating means, and preferably with stirring means and / or circulation means intended to keep the aqueous bath mixed.

[0042] The concentration of etidronic acid in the aqueous bath is advantageously between 5 and 300 g / L, preferably between 10 and 150 g / L, for example 100 or 120 g / L.

[0043] According to one embodiment, the aqueous bath comprises one or more additives.

[0044] The aqueous bath may further comprise a silicate or aluminate salt, preferably at a concentration of between 0.1 and 50 g / L, advantageously between 10 and 30 g / L, to increase the compactness of the layer, and therefore its resistance to corrosion. The silicates or aluminates may have sodium or potassium as counterions.

[0045] The aqueous bath may further comprise one or more salts intended to increase the conductivity of the bath. They are typically inorganic, and advantageously chosen from the group consisting of sulfate, nitrate or halide salts. Advantageously, these salts are at a concentration of between 1 and 50 g / L in the aqueous bath.

[0046] The aqueous bath may further comprise a surfactant, in particular at concentrations of between 0.001 and 10 g / L. The surfactants have the effect of stabilizing the suspension of the polymer particles in the aqueous bath. The surfactant may comprise an ionic or non-ionic surfactant or a mixture of ionic and non-ionic surfactants. Examples of ionic surfactants include organic compounds comprising at least one sulfate or sulfonate function, typically in the form of sodium salts, such as sodium dodecyl sulfate and sodium dodecyl benzene sulfonate. Examples of non-ionic surfactants are polyethylene glycol derivatives (PEGs of different molecular weights) and fluorinated surfactants (in particular per- and polyfluoroalkyls).

[0047] The bath is advantageously maintained at a temperature between 15 and 60°C, for example between 20°C and 50°C. Preferably, the bath is maintained at a temperature between 25°C and 40°C, or at room temperature. Typically, the temperature is constant during the implementation of the method, i.e. during steps a) of immersion and b) of application of an electric current and / or voltage.

[0048] Polymeric particles comprising at least one fluoropolymer

[0049] The polymeric particles comprising at least one fluoropolymer are advantageously of a size between 0.001 and 10 pm, preferably between 0.01 and 1 pm.

[0050] The polymeric particles are suspended in the aqueous bath, and are at a concentration between 0.1 and 500 g / L, preferably between 10 and 200 g / L.

[0051] Preferably, the fluoropolymer comprises or consists of PTFE.

[0052] The particles may further comprise particles selected from graphite particles, molybdenum disulfide particles, ceramic particles, polymeric particles or mixtures thereof.

[0053] According to a particular embodiment, the polymeric particles consist of fluoropolymer particles, preferably PTFE.

[0054] Application of an electric current and / or an electric voltage (step b)

[0055] The method comprises a step of applying to the part immersed in the aqueous bath an electric current and / or an electric voltage, carried out after the immersion step has started and while the part is immersed. Advantageously, the electric current and / or the electric voltage is at a current density of between 0.1 and 3 A / dm 2 , in direct current or in pulsed current. The current density can be constant or variable during step b).

[0056] Step b) is typically carried out under direct current with a constant or variable current density over time, between 0.1 and 3 A / dm 2 .

[0057] Alternatively, step b) can be implemented in so-called "pulsed current" mode or in "pulsed voltage" mode. Thus, the electric current and / or the electric voltage can be applied in pulsed mode with at least one positive pulse, and preferably at least one pause time. The average current density in pulsed current is advantageously between 0.1 and 5 A / dm 2. By "pulsed current", we mean for example a signal whose intensity is periodic, the period or pulsation being made up of one or more pulses during which the current is non-zero, and one or more rest periods, also called pause times, during which the current is zero. By "pulsed voltage", we mean for example a signal whose voltage is periodic, the period or pulsation being made up of one or more pulses during which the voltage is non-zero, and one or more rest periods during which the voltage is zero. It is possible to introduce periods where the current or voltage are reversed, we then speak of "reverse pulsed current" or "reverse pulsed voltage". The electric current and / or the electric voltage is then applied in pulsed mode with at least one positive pulse and at least one negative pulse.

[0058] The pulsed current is for example such that the pulse duration of the pulsed current is between 0.1 and 60 ms, for example between 1 and 5 ms, for example equal to 2 ms and / or the duration of the rest time of the pulsed current is between 0.1 and 60 ms, for example between 4 and 30 ms, for example equal to 8 ms. Alternatively or in addition, the pulsed voltage is for example such that the pulse duration of the pulsed voltage is between 0.1 and 60 ms, for example between 1 and 5 ms, for example equal to 2 ms and / or the duration of the rest time of the pulsed voltage is between 0.1 and 60 ms, for example between 4 and 30 ms, for example equal to 8 ms. The pulse duration and / or the duration of the rest time is for example fixed.

[0059] The frequency of the pulsed current and / or the pulsed voltage may be between 5 and 1000 Hz, i.e. a period between 1 and 200 ms, for example between 40 and 200 Hz, for example equal to 100 Hz.

[0060] The reverse pulsed current is for example such that the pulse duration of the anodic pulsed current is between 0.1 and 60 ms, for example between 1 and 5 ms, for example equal to 2 ms and / or the duration of the cathodic reverse current time is between 0.1 and 60 ms, for example between 4 and 30 ms, for example equal to 8 ms. Alternatively or in addition, the reverse pulsed voltage is for example such that the pulse duration of the pulsed voltage is between 0.1 and 60 ms, for example between 1 and 5 ms, for example equal to 2 ms and / or the duration of the reverse pulsed voltage time is between 0.1 and 60 ms, for example between 4 and 30 ms, for example equal to 8 ms. The pulse duration and / or the rest time duration is for example fixed. A rest time of variable duration can be introduced between the anodic and cathodic sequences.

[0061] The frequency of the pulsed current and / or the pulsed voltage may have a frequency between 5 and 1000 Hz, or a period between 1 and 200 ms, for example between 40 and 200 Hz, for example equal to 100 Hz.

[0062] The anodic current density during the pulse duration is for example between 0.5 A / dm 2 and 20 A / dm 2 , for example between 5 A / dm 2 and 12 A / dm 2 , for example 10 A / dm 2 . The cathode current density during the reverse pulse duration is for example between 0.5 A / dm 2 and 20 A / dm 2 , for example between 5 A / dm 2 and 12 A / dm 2 , for example 10 A / dm 2 .

[0063] Anodic layer

[0064] The thickness of the anodic layer obtained is advantageously between 1 and 40 pm, typically between 2 and 15 pm. Such a thickness is particularly advantageous when low roughness is required: in particular, it makes it possible to avoid re-machining.

[0065] The anodic layer typically has an arithmetic roughness (Ra) between 0.2 pm and 2 pm measured with a mechanical profilometer.

[0066] The process described here makes it possible to greatly improve the tribological properties (in particular the sliding or rolling friction behavior) of the part comprising an aluminum alloy due to the anodic layer thus formed, while ensuring hardness properties similar to those obtained by a hard anodizing process, even when the anodic layer is said to be "thin", i.e. less than 10 μm thick.

[0067] The anodic layer has a characteristic white color, quite different from the color of conventional anodic layers, which gives it a favorable (finished) appearance.

[0068] Surface treatment process

[0069] The invention also relates to a method for surface treatment of at least a portion of a part comprising aluminum or an aluminum alloy, said method comprising the anodizing method according to the invention.

[0070] The part is as described above in connection with the anodizing process.

[0071] Advantageously, the surface treatment method further comprises a surface preparation step, prior to immersion, which may comprise a degreasing step and / or a stripping step. Degreasing allows the removal of fatty substances. Degreasing may be electrolytic or chemical, and is preferably carried out without borate. The degreasing step is advantageously followed by a rinsing step.

[0072] Pickling allows deoxidation of the surface to be treated before any subsequent surface treatment. If a degreasing step is implemented, the pickling step is preferably carried out after the degreasing step, more preferably after the rinsing step associated with the degreasing step.

[0073] The pickling step may comprise alkaline pickling (particularly sodium pickling) and / or acid pickling. The pickling step is for example followed by a rinsing step, for example with demineralized water, advantageously at room temperature. This rinsing step is then prior to the immersion step. When the pickling comprises only alkaline pickling, an acid rinse will be preferred to neutralize the surface. When the pickling is acidic, a rinse with a basic solution will be preferred, also to neutralize the surface.

[0074] The surface treatment method may further comprise a first anodic oxidation, for example a sulfuric, phosphoric or sulfo-tartaric anodization, in an electrolyte making it possible to obtain a first anodic layer of 5 to 50 pm, preferably 5 to 15 pm. The electrolyte typically comprises sulfuric acid, and / or phosphoric acid, and / or tartaric acid.

[0075] This anodic layer can be obtained by sulfuric anodization for example, in potentiostatic or galvanostatic mode, typically at temperatures between -5°C and 25°C. The conditions for implementing this step are well known to those skilled in the art, and are not particularly limited.

[0076] The part is then coated with two anodic layers: the first anodic layer is the sulfuric anodizing layer, while the second anodic layer, superimposed on the first, is the anodic layer obtained by the anodizing process of the invention. The thickness of the superposition of the two anodic layers is then typically between 6 and 50 μm.

[0077] According to a particular embodiment, the surface treatment method comprises the following successive steps:

[0078] (1) degreasing which can be done with an organic solvent (methyl ethyl ketone or acetone for example) or an aqueous degreasing bath, in order to eliminate surface contaminations from the parts, as usually carried out in the preparation of aluminum parts, then

[0079] (2) pickling, preferably alkaline, as usually carried out in the preparation of aluminum parts, possibly followed by an acid neutralization step to remove the hydroxides resulting from the alkaline pickling step, (4) optionally anodization, for example in an acid electrolyte, in order to obtain an oxide layer with a thickness of between 5 and 50 μm, preferably between 5 and 15 μm. This anodization can be carried out in an electrolyte composed of sulfuric acid, phosphoric acid, oxalic acid, tartaric acid or a mixture of these acids, or their mixtures. The bath can also include additives.

[0080] According to another embodiment, the method comprises a step c) of post-treatment after the step of implementing the anodizing method.

[0081] Advantageously, post-treatment step c) comprises a step c1) of sealing with hot water, with or without corrosion inhibitor. The corrosion inhibitors may for example be chromium, nickel, cobalt, molybdate or silicate salts. The sealing is typically a sealing with deionized water at a temperature greater than or equal to 75°C.

[0082] According to one embodiment, post-treatment step c) comprises a step c2) of immersion in a sol-gel capable of improving the corrosion resistance of the surface of the part. A surface is then obtained with an anodic layer coated with a sol-gel layer, which makes it possible to improve the corrosion resistance or tribological properties. To do this, the bath may, for example, comprise metasilicates. Step c2) makes it possible to improve the corrosion resistance of the part.

[0083] According to another particular embodiment, the post-treatment step c) comprises an impregnation step c3) capable of improving the tribological properties (and in particular reducing the coefficient of friction) of the surface of the part, for example an impregnation of PTFE. According to another particular embodiment, the post-treatment step c) comprises a step c4) of immersion in a coloring bath. To do this, the coloring bath may for example comprise pigments of a desired color, at a concentration of between 5 and 30 g / L.

[0084] According to a particular embodiment, the surface treatment method comprises a post-treatment step comprising step c1) and / or c2) and / or c3) and / or c4).

[0085] Parts comprising aluminum or an aluminum alloy coated with an anodic layer

[0086] The invention also relates to a part comprising aluminum or an aluminum alloy coated with an anodizing layer on at least part of its surface, said anodizing layer incorporating polymeric particles comprising at least one fluoropolymer, and / or residues of said polymeric particles over its entire thickness. The part is capable of being obtained by the surface treatment method of the invention. The presence of polymeric particles comprising at least one fluoropolymer, and / or residues of said polymeric particles over the entire thickness in the anodic layer is characterized by glow discharge optical emission spectroscopy (GDOES), or by energy dispersive X-ray spectroscopy (EDS).

[0087] According to a first aspect, the anodic layer comprises crystallized alumina. In other words, the anodic layer has at least in part a crystalline organization detectable by X-ray diffraction (analysis of the interaction of X-rays with a crystalline material) and by Raman spectrometry (analysis of the interaction of a monochromatic laser with the material). The presence of crystallized alumina is entirely characteristic of the anodic layers of the invention, and is not found in anodic layers obtained according to conventional methods, which lead to completely amorphous structures. They are similar to the layers obtained by micro-arc oxidation for current densities at least three times higher on average, and therefore much lower energy requirements.

[0088] According to a second aspect, the anodic layer has a non-zero concentration of oxygen - indicative of the presence of oxide, in particular aluminum oxide - as well as a non-zero concentration of fluorine - indicative of the presence of polymeric particles comprising at least one fluoropolymer, and / or residues of said polymeric particles.

[0089] - over the entire thickness of the anodic layer, by EDS analysis. The fluorine concentration is lower than the oxygen concentration. As it approaches the metal, the amount of fluorine decreases, reaching a value that is substantially zero in the metal of the coated part. When the anodic layer is itself coated - for example with a layer of PTFE

[0090] - the thickness of this second coating is generally very low (less than 900 nm), so that the detected elements cannot be considered significant. Indeed, they are confused with any atmospheric contamination which adsorbs on the surface, and disrupts the measurements, particularly with regard to carbon.

[0091] Thus, the part is characterized in that: the anodic layer has over its entire thickness a non-zero concentration of oxygen, as well as a non-zero concentration of fluorine, the fluorine concentration being lower than the oxygen concentration, the oxygen and fluorine concentrations being measured by energy dispersive X-ray spectroscopy, and / or the anodic layer comprises crystallized alumina.

[0092] For routine measurements, GDOES analysis of carbon can also be used. Thus, according to a third aspect, the anodic layer has a non-zero oxygen concentration - indicative of the presence of oxide, particularly aluminum oxide - as well as a non-zero carbon concentration - indicative of the presence of polymer residues - throughout the thickness of the anodic layer, by GDOES or EDS analysis. The carbon concentration is lower than the oxygen concentration. But this analysis must be combined with EDS mapping of fluorine to confirm the presence of fluoropolymer particles.

[0093] The thickness of the anodic layer of the invention coating the part is advantageously between 1 and 40 μm, typically between 2 and 15 μm. Such a thickness is particularly advantageous, especially when it is accompanied by low roughness. Thus, advantageously, the part has an arithmetic roughness (Ra) measured with a mechanical profilometer of between 0.2 μm and 2 μm. This makes it possible in particular to avoid re-machining. Such a thickness is also advantageous when the treatment is followed by impregnation with polytetrafluoroethylene to improve tribological performance.

[0094] When the part is coated with two anodic layers of different nature, in particular a sulfuric anodic layer then an anodic layer of the invention, the thickness of the superposition of the two anodic layers is then typically between 6 and 50 μm. The method described here makes it possible to greatly improve the tribological properties (in particular the behavior in sliding or rolling friction) of the part comprising an aluminum alloy by the anodic layer thus formed, while ensuring hardness properties similar to those obtained by a hard anodizing process, even when the anodic layer is said to be "thin", that is to say of a thickness less than 10 μm.

[0095] The part can be a functional part such as a slide rail, a tube, a pulley, a gear.

[0096] The parts of the invention are useful in particular as construction materials, particularly maritime, aeronautical or even aerospace.

[0097] FIGURES

[0098] Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which:

[0099] [Fig. 1]: section of the part obtained in example 1, used to measure the thickness of the anodic layer obtained by the method of the invention.

[0100] [Fig. 2]: graph showing the measurement of the coefficient of friction after 600 cycles under 1 N for two parts. In solid lines are presented the measurements obtained for a part having undergone hard anodization followed by PTFE impregnation (anodic layer coated with a layer of PTFE obtained by a dip-coating (the part is immersed then removed more or less quickly depending on the desired thickness for the PTFE coating)), and in dotted lines are presented the measurements obtained for a part of the invention, having undergone a surface treatment process of the invention according to example 1. [Fig. 3]: graph showing the measurement of the coefficient of friction after 600 cycles under 1 N for two parts.In solid lines are presented the measurements obtained for a part having undergone hard anodization followed by PTFE impregnation (anodic layer coated with a layer of PTFE obtained by “dip-coating”), and in dotted lines are presented the measurements obtained for a part of the invention, having undergone a surface treatment process of the invention according to example 2.

[0101] [Fig. 4]: graph showing the measurement of the coefficient of after 600 cycles under 1 N for two parts. In solid lines are presented the measurements obtained for a part having undergone hard anodization followed by PTFE impregnation (anodic layer coated with a layer of PTFE obtained by "dip-coating"), and in dotted lines are presented the measurements obtained for a part of the invention, having undergone a surface treatment process of the invention according to example 3.

[0102] [Fig. 5]: EDS maps of the part obtained in example 3. For the analysis area, we see four images:

[0103] A: SEM (scanning electron microscopy) image.

[0104] B: carbon mapping (C) obtained by EDS.

[0105] C: Oxygen (O) mapping obtained by EDS.

[0106] D: Fluorine (F) mapping obtained by EDS.

[0107] EXAMPLES

[0108] Example 1

[0109] The composition of the bath and the process parameters are given in Table 1 below. The tests were carried out at variable current density during handling, on a 2024 grade aluminum substrate.

[0110] [TABLE 1]

[0111] Under these conditions, the anodic layer obtained after treatment has a gray-white appearance. The thickness of the anodic layer is 4.2 pm measured in section, presented in Figure 1. The roughness measured with the Veeco Dektak 150 mechanical profilometer is 0.3 pm. The coefficient of friction, presented in Figure 2, is approximately 0.35 measured after 600 cycles under 1 N, with a rotation speed of 2.5 cm / s, and an alumina friction ball.

[0112] Example 2 The composition of the bath and the process parameters are given in Table 2. The tests were carried out at direct current density during handling on a 7175 grade aluminum substrate.

[0113] [TABLE 2]

[0114] The resulting anodic layer is gray-white in color. The thickness measured by eddy current on a Fischer Fischerscope MMS PC2 device is 3 pm and the roughness measured with the Veeco Dektak 150 mechanical profilometer is 0.5 pm. The coefficient of friction, shown in Figure 3, is approximately 0.2 after 600 cycles under 1 N, with a rotation speed of 2.5 cm / s, and an alumina friction ball.

[0115] Example 3

[0116] The composition of the bath and the process parameters are given in Table 3. The tests were carried out at variable current density during handling on a 2024 grade aluminum substrate which had already undergone anodization in an acid electrolyte and had an anodic layer with a thickness of between 9 pm and 11 pm.

[0117] [TABLE 3]

[0118] The anodic layer obtained after treatment has a gray-white appearance with a thickness of 7.6 pm, measured by eddy current on a Fischer Fischerscope MMS PC2 device. The roughness measured with the Veeco Dektak 150 mechanical profilometer is 1.1 pm. The coefficient of friction, shown in Figure 4, is approximately 0.3 measured after 600 cycles under 1 N, with a rotation speed of 2.5 cm / s, and an alumina friction ball. The hardness measured in section on a Shimadzu HMV-G microdurometer is 624 Hv. It is therefore much higher than the hardness of the layers obtained by conventional hard anodizing, which is generally between 350 and 400 Hv. The distribution of the elements is given in Figure 5. Comparative Example 1

[0119] The composition of the bath and the process parameters are given in Table 4 below. The tests were carried out at variable current density during handling, on a 2024 grade aluminum substrate.

[0120] [TABLE 4]

[0121] Analysis of the anodic layer by EDS measurements does not allow the presence of the fluorine element to be detected in the anodic (i.e. oxidized) layer.

[0122] Furthermore, no significant improvements in the coefficient of friction measured after 600 cycles under 1 N were observed compared to a control without PTFE particles (approximately 0.8 for each of the tests and the control).

[0123] The addition of PTFE particles in a conventional hard anodizing or “soft” sulfuric anodizing process has no impact on the growth or nature of the anodic layer, nor on the coefficient of friction obtained after anodizing.

[0124] It is concluded that PTFE particles were not incorporated into the anodic layer under any of the experimental conditions tested.

Claims

CLAIMS 1. Method for anodizing at least one part of a part comprising aluminum or an aluminum alloy, said method comprising the following successive steps: a) immersing said part in an aqueous bath comprising etidronic acid and polymeric particles comprising at least one fluoropolymer, b) applying to the part immersed in the bath an electric current and / or an electric voltage, so as to obtain on said part an anodic layer incorporating said polymeric particles comprising at least one fluoropolymer, and / or residues of said polymeric particles over its entire thickness.

2. Method according to claim 1, characterized in that the polymeric particles of step a) are suspended in the aqueous bath at a concentration of between 0.1 and 500 g / L, preferably between 10 and 200 g / L.

3. Method according to any one of the preceding claims, characterized in that the concentration of etidronic acid in the aqueous bath is between 5 and 300 g / L, preferably between 10 and 150 g / L.

4. Method according to any one of the preceding claims, characterized in that the aqueous bath further comprises a surfactant, in particular at concentrations of between 0.001 and 10 g / L.

5. Method according to any one of the preceding claims, characterized in that the aqueous bath further comprises: • a silicate or aluminate salt, preferably at a concentration of between 0.1 and 50 g / L, advantageously between 10 and 30 g / L, and / or • one or more salts, in particular inorganic salts such as sulfate, nitrate or halide salts, advantageously at a concentration of between 1 and 50 g / L.

6. Method according to any one of the preceding claims, characterized in that the bath is maintained at a temperature between 15°C and 60°C.

7. Method according to any one of the preceding claims, characterized in that the electric current and / or the electric voltage is at a constant current density or variable during treatment, advantageously between 0.1 and 3 A / dm 2 , in direct current, and 0.1 and 5 A / dm 2 of average current density in pulsed current.

8. Method according to any one of the preceding claims, characterized in that the electric current and / or electric voltage is applied in pulsed mode with at least one positive pulse, and preferably at least one pause time.

9. Method according to any one of the preceding claims, characterized in that the electric current and / or electric voltage is applied in pulsed mode with at least one positive pulse and at least one negative pulse.

10. A method of surface treatment of at least a portion of a part comprising aluminum or an aluminum alloy, said method comprising the anodizing method according to any one of claims 1 to 9.

11. Surface treatment method according to claim 10, characterized in that it further comprises a surface preparation step which may comprise a degreasing step and / or an alkaline pickling step.

12. Surface treatment method according to claim 10 or 11, characterized in that it further comprises a first anodization in an electrolyte typically comprising sulfuric acid, phosphoric acid, and / or tartaric acid, making it possible to obtain a first anodic layer of 5 to 50 μm, preferably 5 to 15 μm thick.

13. Surface treatment method according to any one of claims 10 to 12, characterized in that it is followed by a post-treatment step c) comprising: • a step c1) of sealing with hot water, with or without corrosion inhibitor, and / or • a step c2) of immersion in a sol-gel capable of improving the corrosion resistance of the surface of the part, and / or • an impregnation step c3) to form a coating capable of improving the tribological properties of the surface of the part.

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