Coating for porous substrate

Abstract

The present disclosure relates to a method for manufacturing a coating for a porous substrate, and to a mechanical part equipped with such a coating, the method comprising the following steps: providing a substrate to be protected (31), the substrate (31) containing pores (34); providing a liquid suspension (15) containing at least a first powder (21) and a second powder (22), the first powder (21) possessing a D50 strictly smaller than that of the second powder (22) and an electrophoretic mobility strictly higher than that of the second powder (22); providing a DC electric generator (12); placing the substrate (31) in the suspension (15), as a first electrode, and connecting the substrate (31) to a first terminal of the electric generator (12); placing a second electrode (14) in the suspension (15) and connecting the second electrode (14) to a second terminal of the electric generator (12); and applying a pulsed or continuous voltage of at least 10 V across the two electrodes (31, 14) for at least 1 minute.

Description

Description Title of the invention: COATING FOR POROUS SUBSTRATE Technical Field

[0001] This disclosure relates to a method for manufacturing a coating for a porous substrate, as well as a mechanical part provided with such a coating. This method is applicable to any type of porous substrate. However, it is particularly suitable for parts made of C / C (carbon / carbon) composite material, in particular those subject to high heating, such as brakes, and exposed to particularly oxidizing environments, particularly in the aeronautical field. Prior art

[0002] Thanks to their excellent tribological properties and their lightness, C / C composite materials have now established themselves as the reference material for aeronautical braking.

[0003] However, due to the very significant heating experienced by C / C brakes, which can exceed 1400°C and sometimes even reach 2000°C, the carbon constituting these brakes is subject to oxidation phenomena by the ambient air, which considerably reduces the service life of these C / C brakes.

[0004] In order to combat these oxidation phenomena, anti-oxidation protective coatings are conventionally applied to the non-friction parts of C / C brakes, i.e. to the parts exposed to the oxygen in the air.

[0005] However, due to the great variability of oxidizing conditions that such C / C brakes can encounter, depending on temperature levels, operating times, or even chemical elements in the environment, in particular the presence of water, it is generally necessary to combine two protective mechanisms. The first aims to trap carbon oxidation catalysts, such as potassium acetate from particularly de-icing products used on runways. The second aims to reduce the diffusion rate of oxygen to the carbon substrate.

[0006] Traditionally, these two protective mechanisms are the subject of separate layers. Indeed, anti-catalytic protection is generally obtained by applying a precursor to the surfaces to be protected, using a gun, which must then undergo a transformation requiring a temperature below 1000 °C. Conversely, anti-diffusion protection is generally obtained by high-temperature deposition processes, above 1000 °C.

[0007] However, as shown in Figure 1, the conventional deposition process for anti-catalytic protection leads to the formation of discontinuous islands of active principle 91, here alumino-phosphate AI(PO3)3, leaving exposed areas of the substrate 92, the precursor AI(H2PO4)3 in fact comprising a portion of water which evaporates during its transformation: the layer thus obtained, discontinuous, cannot therefore act as a barrier against the diffusion of oxygen towards the substrate.

[0008] Therefore, it is generally necessary to multiply the deposition and stabilization steps in order to obtain multi-layer anti-oxidation protection that presents the desired level of effectiveness. This classic process therefore presented numerous constraints.

[0009] Consequently, a new deposition process using the electrophoresis technique was proposed in application FR 21 / 1 1927. This new process has shown very good results for forming a single-layer protective coating: in fact, as shown in Figure 2, the coating 93 obtained is dense and continuous, which makes it possible to effectively protect the substrate 94, at least as long as the coating is intact.

[0010] However, as can be seen in Figure 2, this method only makes it possible to form an external layer on the surface of the substrate: in particular, it is observed that the coating 93 does not penetrate at all into the porosities 95 of the substrate 94. Consequently, the substrate 94 is vulnerable in the event of scratching or flaking of the coating 93: in fact, in such a case, the oxidizing or corrosive agents of the environment can penetrate deep into the substrate 94. through the defect in the coating 93 and then through the network of pores 95, which may cause corrosion of the substrate 94 under the coating 93.

[0011] There is therefore a real need for a method of manufacturing a coating for a porous substrate, as well as a mechanical part provided with such a coating, which are free, at least in part, from the drawbacks inherent in the aforementioned known methods. Statement of the invention

[0012] The present disclosure relates to a method of manufacturing a coating for a porous substrate, comprising the following steps: - provision of a substrate to be protected, the substrate comprising porosities, - supply of a liquid suspension comprising at least a first powder and a second powder, the first powder having a D50 strictly lower than that of the second powder and an electrophoretic mobility strictly greater than that of the second powder, - supply of a direct current electric generator, - placing the substrate in the suspension, as a first electrode, and connecting the substrate to a first terminal of the electrical generator, - placing a second electrode in the suspension and connecting the second electrode to a second terminal of the electric generator, and - application of a direct or pulsed voltage of at least 10 V between the two electrodes for at least 1 minute.

[0013] By using a liquid suspension of two powders with different sizes and electrophoretic mobilities, it is possible, using such an electrophoresis method, to form a double-layer coating in which the particles of the finer powder penetrate deep into the porosities of the substrate, forming an inner layer, while the particles of the coarser powder are deposited on the surface of the substrate, forming an outer layer.

[0014] Due to higher electrophoretic mobility, the first powder reaches the substrate faster than the second powder, allowing it to penetrate the substrate's porosities before the latter are covered on the surface by the particles of the second powder.

[0015] This method thus makes it possible to obtain in a single step a double-layer coating fulfilling at least two functions. It is thus possible to form, on the one hand, a continuous external layer over the entire surface of the substrate immersed in the suspension, with a sufficient density to form an effective barrier, for example against the diffusion of oxygen, and, on the other hand, an internal layer filling at least in part the porosities of the substrate so as to form a second barrier preventing, in the event of rupture of the external layer, the penetration of elements of the environment within the network of porosities.

[0016] Thanks to this process, it is possible, if necessary, to include several different active ingredients in the suspension in order to provide the coating with additional functions without multiplying the layers and therefore without multiplying the manufacturing costs and without being limited by differential expansion issues. In particular, it is possible to choose one or more different active ingredients for each powder in order to provide the outer layer and the inner layer with different properties. That said, the two powders can also have the same chemical composition.

[0017] Furthermore, the use of electrophoresis allows this coating to be formed from a liquid suspension directly comprising the active ingredient(s), and not a precursor as was the case in some previous methods. Similarly, since the active ingredient can be deposited directly and uniformly on the substrate, it is possible to form a layer as thick as desired in a single electrophoresis step, without resorting to intermediate stabilization steps. This also reduces cycle time.

[0018] In this presentation, the characteristic diameters of the powders, including D10, D50 and D90, are determined by laser diffraction.

[0019] In some embodiments, the D90 of the first powder is strictly less than the D10 of the second powder. This ensures a significant size difference between the two powders, which reduces the risk of particles of the first powder remaining blocked on the surface of the substrate or, conversely, that particles of the second powder do not penetrate into the porosities of the substrate.

[0020] In certain embodiments, the D50 of the first powder is strictly less than the average diameter of the porosities of the substrate. Preferably, the D50 of the first powder is less than or equal to 50%, more preferably 10% or 5%, of the average diameter of the porosities of the substrate. This allows good penetration of the particles of the first powder into the porosities of the substrate.

[0021] In some embodiments, the D50 of the first powder is less than or equal to 0.5 pm, preferably less than or equal to 0.2 pm.

[0022] In some embodiments, the D90 of the first powder is less than or equal to 1 pm, preferably less than or equal to 0.5 pm.

[0023] In some embodiments, the D50 of the second powder is strictly greater than the average diameter of the pores of the substrate. Preferably, the D50 of the second powder is greater than or equal to 150%, more preferably 200%, of the average diameter of the pores of the substrate. This reduces the risk of particles of the second powder penetrating into the pores of the substrate.

[0024] In some embodiments, the D50 of the second powder is greater than or equal to 0.5 pm, preferably greater than or equal to 2 pm.

[0025] In some embodiments, the D50 of the second powder is less than or equal to 5 pm, preferably less than or equal to 2 pm. This reduces the risk of sedimentation of the second powder in the liquid suspension. In particular, the D50 of the second powder may be between 0.5 and 2 pm.

[0026] In some embodiments, the D10 of the second powder is greater than or equal to 0.5 pm, preferably greater than or equal to 1 pm.

[0027] In some embodiments, the D50 of the second powder is less than or equal to 5 pm.

[0028] In some embodiments, the electrophoretic mobility of the first powder is at least 25% higher, preferably at least 50% higher than that of the second powder. Such a difference reduces the risk that too many particles of the second powder reach the substrate before the particles of the first powder.

[0029] In some embodiments, the electrophoretic mobility of the first powder is greater than or equal to 0.5 m 2 / Vs, preferably greater than or equal to 0.7 m 2 / Vs

[0030] In some embodiments, the electrophoretic mobility of the first powder is less than or equal to 2 m 2 / Vs

[0031] In some embodiments, the electrophoretic mobility of the second powder is less than or equal to 0.5 m 2 / Vs, preferably less than or equal to 0.3 m 2 / Vs

[0032] In some embodiments, the electrophoretic mobility of the second powder is greater than or equal to 0.3 m 2 / Vs

[0033] In some embodiments, the method comprises, before the step of providing the liquid suspension, a step of treating the first powder resulting in the ionization of the surface of the particles of the first powder. Such ionization makes it possible to artificially increase the electrophoretic mobility of the first powder.

[0034] In some embodiments, the concentration, by mass, of the first powder is higher than that of the second powder. Indeed, since the particle size of the first powder is smaller than the second, it is preferable to compensate for this difference in size by a higher concentration, for example approximately twice as high.

[0035] In some embodiments, the particles of the first powder penetrate into the porosities of the substrate at least 1 mm, preferably at least 2 mm, forming an internal coating layer.

[0036] In some embodiments, the particles of the first powder penetrate into the porosities of the substrate at most 5 mm, preferably at most 3 mm.

[0037] In some embodiments, the method further comprises, after the step of applying the voltage, a heat treatment step resulting in crystallization of the particles of the first powder. Such crystallization causes an expansion of the material of the first powder, which makes it possible to more effectively seal the porosities of the substrate within the inner layer of the coating.

[0038] In certain embodiments, the particles of the second powder are deposited on the surface of the substrate, without penetrating into the porosities of the substrate, forming an external coating layer having a thickness greater than or equal to 10 μm, preferably greater than or equal to 50 μm.

[0039] In some embodiments, the thickness of the outer coating layer is between 10 and 300 μm, preferably between 50 and 150 μm. Such thickness ranges effectively restrict the diffusion of oxygen within the coating.

[0040] In some embodiments, the resulting coating has a surface density of between 10 and 50 mg / cm 2 , preferably between 20 and 40 mg / cm 2 Such density ranges effectively restrict the diffusion of oxygen within the coating.

[0041] In some embodiments, the substrate is made of a C / C composite material.

[0042] In some embodiments, at least the first powder or the second powder comprises an active ingredient from the phosphate family. Phosphates in fact make it possible to trap carbon oxidation catalysts. It is thus possible to obtain dual anti-oxidation protection, having both an anti-catalytic function and a diffusion barrier function.

[0043] In some embodiments, the first powder comprises a monoaluminum phosphate precursor, silica, alumina or aluminophosphate. In particular, aluminophosphate of formula AI(PO3)3 offers very good anti-catalytic properties.

[0044] In some embodiments, the second powder comprises borosilicates with fluxing agents such as boron. This allows during heat treatment to form a protective glass layer. This second powder may also contain a precursor of monoaluminum phosphate, silica, alumina or aluminophosphate, in particular, aluminophosphate of formula AI(PO3)3.

[0045] In some embodiments, the liquid phase of the suspension comprises 1-propanol and / or 2-propanol. In particular, both of these species may constitute at least 90% of the liquid phase, or even 100% of the liquid phase. In particular, 1-propanol and 2-propanol may be present in equal parts in the liquid phase.

[0046] In some embodiments, the liquid phase of the suspension is a 50 / 50 mixture of 1-propanol and / or 2-propanol.

[0047] In some embodiments, the suspension comprises a stabilizer. This stabilizer makes it possible to reduce the sedimentation rate of the suspended particles, by steric or electrostatic repulsion effect between the particles.

[0048] In some embodiments, the stabilizer comprises phosphoric acid. Preferably, the stabilizer is exclusively phosphoric acid.

[0049] In certain embodiments, the concentration of stabilizer in the suspension is between 1 and 10 g / L, preferably between 2 and 8 g / L, more preferably between 3 and 5 g / L.

[0050] In some embodiments, the distance separating the electrodes is between 5 and 50 mm.

[0051] In some embodiments, the voltage applied between the electrodes is between 10 and 200 V, preferably between 30 and 80 V, more preferably between 40 and 60 V.

[0052] In some embodiments, the voltage applied between the electrodes is a direct voltage.

[0053] In other embodiments, the voltage applied between the electrodes is a pulsed voltage. The inventors have discovered that a pulsed voltage allows better infiltration of the suspension for the same overall duration. The pulse frequency is for example between 1 Hz and 10 kHz, preferably between 1 Hz and 100 Hz.

[0054] In some embodiments, the phase during which the voltage is applied (high state and low state for a pulsed voltage) has a duration of between 1 and 30 minutes, preferably between 1 and 10 minutes.

[0055] The present disclosure also relates to a mechanical part, comprising a substrate, comprising porosities and having a surface to be protected, and a continuous and waterproof coating, including - an internal coating layer, extending under the surface to be protected and sealing the porosities of the substrate by at least 2 mm, and - an external coating layer, extending over the surface to be protected by at least 10 pm.

[0056] This mechanical part therefore benefits from a double-layer coating capable of protecting the part on the surface and in depth. This coating can in particular be obtained using a manufacturing process according to any of the preceding embodiments. This therefore results in all the advantages mentioned above regarding the process. In particular, all the optional characteristics of the process, and all the advantages associated with them, can be transposed directly to the mechanical part.

[0057] In some embodiments, the substrate is made of C / C composite material and the coating is an anti-oxidation protection comprising an active ingredient from the phosphate family. The part thus benefits from anti-oxidation protection having both the anti-catalytic function and the diffusion barrier function.

[0058] The present disclosure also relates to an aircraft brake, comprising a mechanical part according to any of the preceding embodiments. The brake can then have both a relatively low mass and good resistance to oxidation, including at high temperatures.

[0059] The above-mentioned and other features and advantages will become apparent from the following detailed description of examples of the proposed method and mechanical part. This detailed description refers to the attached drawings. Brief description of the drawings

[0060] The attached drawings are schematic and are intended primarily to illustrate the principles of the presentation.

[0061] In these drawings, from one figure to another, identical elements (or parts of elements) are identified by the same reference signs.

[0062] [Fig. 1] Figure 1 is a micrograph of an anti-corrosion coating according to a first prior art.

[0063] [Fig. 2] Figure 2 is a micrograph of an anti-corrosion coating according to a second prior art.

[0064] [Fig. 3] Figure 3 is a schematic diagram of the electrophoresis step.

[0065] [Fig. 4] Figure 4 represents a liquid suspension in the context of an exemplary embodiment.

[0066] [Fig. 5] Figure 5 represents an electrophoresis installation according to the framework of the exemplary embodiment.

[0067] [Fig. 6] Figure 6 shows the same electrophoresis setup after the electrophoresis step. Description of the embodiments

[0068] In order to make the disclosure more concrete, an example of a method for manufacturing a coating is described in detail below, with reference to the accompanying drawings. It is recalled that the invention is not limited to this example.

[0069] Figure 3 schematically represents an electrophoresis device 10 involved in the process which will be described below. It comprises a tank 11, a direct current electric generator 12, a first electrode 13, connected to the positive terminal of the electric generator 12, and a second electrode 14, connected to the negative terminal of the electric generator 12. The tank 11 is filled with a suspension 15 comprising particles 16 suspended in a liquid phase. The two electrodes 13 and 14 are immersed in the suspension 15. They preferably face each other, parallel to each other. A distance D separates the two electrodes 13 and 14.

[0070] When the electric generator 12 applies a voltage U between the two electrodes 13, 14, an electric field E crosses the suspension 15 and causes the migration of particles 16 towards one or other of the electrodes 13, 14 depending on their electrical charge. Thus, particles 16 carrying a negative electrical charge here migrate towards the positive electrode 13 and are deposited there. The migration speed of particles 16 depends on the intensity of the electric field E and the electrophoretic mobility p of particles 16: for a given electric field E, the higher the electrophoretic mobility p of particle 16, the faster particle 16 will migrate towards the oppositely charged electrode, according to the following formula: v = px E.

[0071] The quantity of material deposited on electrode 13 can then be expressed using Hamaker's law: Am = A / D xpx E x C x At, with A, the surface of the electrode 13, D, the distance between the electrodes 13, 14, p, the electrophoretic mobility of the particles 16, E, the imposed electric field, C, the concentration of particles 16 in suspension 15, At, the duration of the electrophoresis step.

[0072] An example of a coating manufacturing process will now be described in more detail. In this example, the suspension 15, shown in Figure 4, consists of: - a 50 / 50 mixture of 1-propanol and 2-propanol, constituting the liquid phase of the suspension, - 65% by mass of AI(PO3)3, in the form of a first powder 21 having a D50 equal to 0.2 pm, suspended in the liquid phase, - 35% by mass of borosilicate and boron (90 / 10), in the form of a second powder 22 having a D50 equal to 2 pm, suspended in the liquid phase, and - 4 g / L of phosphoric acid, acting as a stabilizer, in solution in the liquid phase.

[0073] The electrophoretic mobility of the first powder is equal to 0.6 m 2 / Vs while the electrophoretic mobility of the second powder is equal to 0.3 m 2 / Vs

[0074] In the present example, the substrate to be protected 31 is a C / C (carbon / carbon) composite substrate comprising porosities whose average diameter is of the order of 10 μm. In Figure 5, the substrate 31 is schematically represented with its peripheral surface 32, zones of material 33 and its porosities 34 (white zones forming the interstices between the zones of material 33).

[0075] The substrate 31 is thus immersed in the liquid suspension 15 and connected to the positive terminal of the generator 12: the substrate 31 being made of C / C composite material, it is conductive and can therefore constitute such an electrode. A counter-electrode 14 is also immersed in the liquid suspension 15 and connected to the negative terminal of the generator 12.

[0076] The substrate 31 and the counter-electrode 14 are then separated by a distance D of between 5 and 50 mm, this distance being variable due to the geometry of the part to be coated, and the electric generator 12 applies a voltage U of 50 V between its terminals.

[0077] This voltage U is maintained for 5 minutes. During this electrophoresis step, as can be seen in Figure 6, the particles of the first powder 21 migrate more quickly towards the substrate 31 due to their higher electrophoretic mobility and, thanks to their diameter being smaller than the average diameter of the porosities 34 of the substrate 31, penetrate into the porosities 34 at which they are deposited, gradually forming an internal layer 41. The particles of the second powder 22, which are slower, reach the substrate with a lower flow rate and, being too large to penetrate into the porosities 34, are deposited on the surface 32 of the substrate 31, gradually forming an external layer 42.

[0078] The speed of formation of the external layer 42 being lower than that of the internal layer 41, the particles of the first powder 21 have time to fill substantially all the porosities 34 located under the surface 32 of the substrate 31, to a depth of between 2 and 3 mm, before the external layer 42 comes to block the porosities 34 on the surface.

[0079] After this electrophoresis step, the substrate 21, now carrying an internal layer 41 of AI(PO3)3 particles and an external layer 42 of borosilicate particles (the boron serving as a fluxing agent having volatilized), is removed from the tank 11 and undergoes a stabilization step carried out at 700°C for 1 h to 5 h in a nitrogen atmosphere. It makes it possible to consolidate the protective layers 41, 42 thus obtained, thus forming a protective coating 40. In particular, this step makes it possible to crystallize the particles 21 of the internal layer 41 and, thus, to fill the porosities 34 of the internal layer 41.

[0080] Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various illustrated / mentioned embodiments may be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

[0081] It is also obvious that all the characteristics described with reference to a method are transposable, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device are transposable, alone or in combination, to a method.

Claims

Demands

1. A method for manufacturing a coating for a porous substrate, comprising the following steps: - provision of a substrate to be protected (31), the substrate (31) comprising porosities (34), - supply of a liquid suspension (15) comprising at least a first powder (21) and a second powder (22), the first powder (21) having a D50 strictly lower than that of the second powder (22) and an electrophoretic mobility at least 25% higher than that of the second powder (22), - supply of a direct current electric generator (12), - placement of the substrate (31) in the suspension (15), as the first electrode (13), and connection of the substrate (31) to a first terminal of the electrical generator (12), - placement of a second electrode (14) in the suspension (15) and connection of the second electrode (14) to a second terminal of the electrical generator (12), and - application of a continuous or pulsed voltage (U) of at least 10 V between the two electrodes (13, 14) for at least 1 minute.

2. A method according to claim 1, wherein the D90 of the first powder (21) is strictly less than the D10 of the second powder (22).

3. A method according to claim 1 or 2, wherein the D50 of the first powder (21) is less than or equal to 0.5 pm, preferably less than or equal to 0.2 pm, and wherein the D50 of the second powder (22) is greater than or equal to 0.5 pm, preferably greater than or equal to 2 pm.

4. A method according to any one of claims 1 to 3, wherein the electrophoretic mobility of the first powder (21) is at least 50% higher than that of the second powder (22).

5. A method according to any one of claims 1 to 4, wherein the electrophoretic mobility of the first powder (21) is greater than or equal to 0.5 m 2 / Vs, preferably greater than or equal to 0.7 m 2 / Vs, and in which the electrophoretic mobility of the second powder (22) is less than or equal to 0.5 m 2 / Vs, preferably less than or equal to 0.3 m 2 / Vs

6. A method according to any one of claims 1 to 5, wherein the mass concentration of the first powder (21) is greater than that of the second powder (22).

7. A method according to any one of claims 1 to 6, wherein the particles of the first powder (21) penetrate the pores (34) of the substrate (31) to a minimum of 1 mm, preferably to a minimum of 2 mm, forming an internal coating layer (41), and wherein the particles of the second powder (22) are deposited on the surface (32) of the substrate (31), without penetrating the pores (34) of the substrate (31), forming an external coating layer (42) having a thickness greater than or equal to 10 pm, preferably greater than or equal to 50 pm.

8. A method according to claim 7, wherein the particles of the first powder (21) penetrate the pores (34) of the substrate (31) to a maximum of 5 mm, preferably to a maximum of 3 mm.

9. A method according to any one of claims 1 to 8, wherein the substrate (31) is a C / C composite substrate.

10. A process according to any one of claims 1 to 9, wherein the first powder (21) comprises a precursor of monoaluminium phosphate, silica, alumina or aluminophosphate, and wherein the second powder (22) comprises borosilicates with fluxing agents such as boron.

11. Mechanical part, comprising a substrate (31), comprising porosities (34) and having a surface to be protected (32), and a continuous and waterproof coating (40), including - an internal coating layer (41), formed by particles of a first powder, extending under the surface to be protected (32) and sealing the porosities (34) of the substrate (31) over at least 2 mm, and - an external coating layer (42), formed by particles of a second powder, extending over the surface to be protected (32) over at least 10 pm, the first powder having a D50 strictly lower than that of the second powder.

12. Mechanical part according to claim 11, wherein the coating (40) is an anti-oxidation protection comprising an active principle from the phosphate family.

13. Aircraft brake, comprising a mechanical part according to claim 11 or 12.

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