PROCESS FOR MANUFACTURING A MICROSTRUCTURED COATING, AND WALL HAVING SUCH A COATING

Abstract

This relates to a process for manufacturing a microstructured coating on a substrate, which comprises a step of spraying molten particles having a diameter comprised between 0.05 .mu.m and 5 .mu.m onto the substrate, the spraying being carried out by at least one plasma torch, which is moved relative to the substrate, the movement parameters and/or the flowrate of particles sprayed by the plasma torches being variable as a function of the position of the plasma torches, ensuring a variable thickness for the coating formed by the particles, at various points of the substrate. The embodiment also relates to a wall having a coating obtained by this process.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to French Patent Application No. 1462492, filed Dec. 16, 2014, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] The embodiments described herein relate to a process for manufacturing a microstructured coating on a wall, and especially on an external wall of an aircraft. The embodiment also relates to a wall having such a microstructured coating.

BACKGROUND

[0003] To improve the performance of an aircraft, and especially to decrease its fuel consumption, it would be described to decrease its aerodynamic drag. To decrease this drag, it is known to form on some of the external walls of the aircraft, for example on portions of the wing or fuselage elements, a microstructure, usually referred to as a "sharkskin" or by the term "riblets".

[0004] A microstructure, in the context of the present application, consists of a set of reliefs repeated over an area, the depth of which is generally comprised between 10 and 60 microns, and the geometry of which is optimised to meet a set objective.

[0005] A sharkskin or riblet type microstructure contains ribs that lie substantially parallel to one another, the ridge lines of the ribs being oriented in the flow direction of a fluid over the surface, for example the direction of flow of the air over the external wall of an aircraft. These ribs are, in transverse cross section, sawtooth-shaped. Such a microstructure is for example known from documents EP 2 729 363 and EP 1 283 163.

[0006] A number of ways of producing such microstructures on a wall are known. It is especially known to adhesively bond, to the wall, a plastic film containing troughs of a few hundreds of millimetres depth, the ribs being formed between two successive troughs. According to another known embodiment, grooves of a few hundreds of millimetres are machined into a metal wall, so as to produce the microstructure.

[0007] Walls containing such microstructures have characteristics that allow them to decrease their aerodynamic drag, when they are subjected to an airflow flowing in the direction of the ribs. However, this decrease in aerodynamic drag is effective only if the ribs have precise shapes. It is especially important, to ensure this effectiveness, for the ridge lines of the ribs to be sufficiently sharp.

[0008] Known processes for manufacturing microstructures allow such ridge lines to be obtained. However, there is a need to improve these manufacturing processes, in order to obtain microstructures having a better resistance to erosion, especially when the microstructured walls are subjected to air friction, water friction and dust, in order to guarantee, for the duration, the performance of the microstructures and the corresponding decrease in the aerodynamic drag of the wall.

[0009] There is therefore a need for microstructured walls the microstructures of which have a higher resistance to erosion, and for a process allowing such walls to be manufactured. The present embodiment especially has the objective of providing a process for manufacturing microstructured walls meeting this need.

SUMMARY

[0010] These objectives, and others that will become more clearly apparent below, are achieved by way of a process for manufacturing a microstructured coating on a substrate, which comprises a step of spraying molten particles having a diameter of between 0.05 .mu.m and 5 .mu.m onto the substrate, the spraying being carried out by at least one plasma torch which is moved relative to the substrate, the movement parameters and/or the flowrate of particles sprayed by said plasma torch(es) being variable as a function of the position of the plasma torch(es), ensuring a variable thickness for the coating formed by said particles, at various points of the substrate.

[0011] It is thus possible to obtain a microstructure coating having a very precisely controlled geometry, which has a surface finish making it less sensitive to erosion.

[0012] According to one advantageous embodiment, the variable thickness is produced by spraying, onto at least one portion of the surface of the substrate, at least two successive layers of molten particles, the number of layers of particles sprayed being variable at various points of the substrate.

[0013] According to another advantageous embodiment, the variable thickness is produced by spraying at least one layer of molten particles the thickness of which is variable at various points of the substrate.

[0014] Advantageously, the process implements a robot that controls the position, the orientation and the flowrate of each of the plasma torches spraying particles onto the substrate.

[0015] According to one advantageous embodiment, the spraying step comprises: spraying a first layer of particles, of substantially uniform thickness at every point on the substrate, then spraying, on top of said first layer, a plurality of other layers of particles, the number and/or thickness of which are different at various points of the substrate.

[0016] According to one advantageous embodiment, the spraying step comprises: spraying layers of particles, the number and/or thickness of which are different at various points of the substrate, then spraying on top of the preceding layers of particles at least one external layer of particles, of substantially uniform thickness at every point of the substrate.

[0017] The process comprises, prior to the spraying step, a step of measuring the geometry of the surface of the substrate and a step of calculating the thickness and/or the number of layers of particles to be sprayed at various points of said substrate.

[0018] Preferably, the particles sprayed in the spraying step comprise particles of metal oxide.

[0019] The process comprises, prior to the spraying step, a step of dispersing particles to be sprayed in a sol-gel type compound.

[0020] The particles sprayed in the spraying step comprise particles of zirconia.

[0021] The particles sprayed in the spraying step comprise particles of titanium dioxide.

[0022] The present embodiment also relates to a wall, comprising a substrate covered by a coating, this coating being a microstructured coating obtained by the process described above.

[0023] Advantageously, the coating has an average thickness comprised between 0.01 mm and 0.25 mm.

[0024] Advantageously, the coating has, on its surface, a microstructure forming ribs that are substantially parallel to one another.

[0025] Advantageously, the ribs have a height comprised between 10 .mu.m and 60 .mu.m.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The various embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and:

[0027] FIG. 1 is a schematic representation, in perspective, of a wall having "riblet" type microstructures;

[0028] FIG. 2 is a schematic cross-sectional view of a wall having a "riblet" type microstructured coating produced according to one embodiment;

[0029] FIG. 3 is a schematic perspective view of the wall in FIG. 2;

[0030] FIG. 4 is a schematic cross-sectional view showing the wall in FIG. 2 during manufacture of the microstructured coating;

[0031] FIG. 5 is a schematic cross-sectional view of a wall able to receive a microstructured coating, according to another embodiment;

[0032] FIG. 6 is a schematic cross-sectional view of the wall in FIG. 5 having a "riblet" type microstructured coating;

[0033] FIG. 7 is a schematic cross-sectional view of a detail of a wall having a "riblet" type microstructured coating produced according to another embodiment; and

[0034] FIG. 8 is a schematic representation of the device allowing coating layers to be sprayed onto a substrate, according to one embodiment.

DETAILED DESCRIPTION

[0035] The following detailed description is merely exemplary in nature and is not intended to limit the disclosed embodiments or the application and uses thereof. Furthermore, there is no intention to be bound by any theory presented in the preceding background detailed description.

[0036] FIG. 1 shows in perspective a portion of a wall 1 having a "riblet" type microstructure. This microstructure has a plurality of ribs 11, 12, 13, 14 and 15 extending substantially parallel to one another. Conventionally, each of these ribs has a height of about 50 .mu.m. In the example shown, the ridge lines 110, 120, 130, 140 and 150 of the ribs are shown as the intersections of the planes formed by the flanks of each of the ribs. These ridge lines are thus very sharp. As those skilled in the art know, a wall 1 having ribs 11, 12, 13, 14 and 15 with sufficiently sharp ridge lines 110, 120, 130, 140 and 150 generates a relatively low aerodynamic drag, when it is subjected to an air or fluid flow flowing in the direction of the ribs.

[0037] In fact, the ridge lines obtained by known processes do not have exactly this theoretical shape, but a shape approaching this theoretical shape. This shape may vary slightly over the lifetime of the wall 1, especially under the effect of erosion in air or in a fluid flow to which the wall 1 is subjected.

[0038] In the embodiments that are presented below, substrates are covered with microstructured coatings, forming a wall having microstructures possibly of the "riblet" type. The substrate may for example consist of a panel such as a metal sheet or a panel made of a composite material. The microstructured coatings, the thickness of which is preferably comprised between 0.01 mm and 0.25 mm, are obtained by spraying onto the substrate, by a plasma torch, fine particles, of diameter comprised between 0.05 .mu.m and 5 .mu.m. Preferably, these particles are sprayed in the form of a plurality of successive layers of particles, forming together the coating.

[0039] The use of fine particles to form the coating makes very precise adjustment of the thickness of this coating possible. Specifically, since the particles have a size very much smaller than the thickness of the coating, it is necessary to spray a great many thereof to obtain the final coating. The coating is thus obtained gradually, thereby enabling a very good control of its thickness, and therefore of the geometry of the microstructures.

[0040] Moreover, the use of fine particles allows a surface finish to be obtained for the coating that is very uniform, thereby limiting the risk of erosion and preventing dirt or water from adhering to the wall.

[0041] According to one preferred embodiment schematically shown in FIG. 8, the particles are sprayed onto the substrate by an atmospheric plasma beam, made up of ionised gas in which the particles to be sprayed are included.

[0042] As shown in FIG. 8, the plasma torch 81 forms a plasma beam 810. Particles 83 are introduced into this plasma beam 810 by a nozzle 82. In the plasma beam 810, these particles 83 are heated until molten and are sprayed towards the substrate 80. In certain cases, these particles may also undergo chemical transformations in this plasma beam 810, especially due to the heat. On contact with the surface of the substrate 80, the particles 83 solidify thereon so as to form a coating layer 830. By way of example, in one embodiment of the embodiment, the temperature of the plasma is about 200 to 14000 K, the velocity of the plasma jet is comprised between 50 and 2500 m/s, and the particles move in the plasma at a velocity comprised between 20 and 400 m/s. The distance between the plasma torch and the substrate to be coated is about 25 cm.

[0043] The particles sprayed by the plasma onto the substrate are heated to very high temperatures, of about one thousand degrees, for a very short time, of about one thousandth of a second. This increase in temperature may affect the cohesion of the sprayed particles, or generate chemical reactions in these particles. The small size of these particles prevents unacceptable heating of the substrate on impact of the hot particles.

[0044] According to one preferred embodiment, the particles implemented to manufacture the coating are dispersed beforehand in a liquid in order to form a sol-gel type compound that is itself inserted into the beam of the plasma torch. The use of such a sol-gel process allows a good particle-size distribution to be obtained for the particles to be sprayed.

[0045] As FIG. 8 shows, a sol-gel type compound 820, contained in a container 821, may be fed to the nozzle 82 via a duct 822, so as to be introduced into the plasma beam 810.

[0046] Advantageously, the nozzle introduces a precise flowrate of sol-gel type compound, in order to control at each instant the amount of particles deposited by the plasma beam. A plurality of nozzles may be provided, for a given plasma beam, so as to introduce simultaneously or in succession a plurality of types of particles into the plasma, in desired proportions.

[0047] The formation of such a sol-gel type compound containing fine particles, and the introduction of this sol-gel type compound into a plasma beam allowing the particles to be sprayed onto a substrate in order to form thereon a thin coating layer, are especially described by the documents WO2006/043006 and WO2007/122256.

[0048] Preferably, the particles sprayed by the plasma beam in order to form the coating layers are particles of metal oxides. The use of such particles allows a resistive layer that has a good resistance to erosion to be manufactured, thereby allowing the microstructures to durably provide a good aerodynamic performance.

[0049] More precisely, the particles used may be chosen from a plurality of types of particles, depending on the properties sought for the coating. Each layer may be composed of a single type of particle or of a plurality of types of mixed particles. Moreover, certain of the layers of particles may be porous so as to allow the particles forming successive layers to interpenetrate. Such an interpenetration may allow the sought-after final properties to be optimized.

[0050] Thus, according to one preferred embodiment, the first layers deposited on the substrate may be tie sublayers made of corrosion-blocking polymers. According to one variant, the plasma torches may generate on a metal substrate metal oxides allowing subsequent layers to adhere.

[0051] On the exterior layers of the coating, layers based on zirconia particles may advantageously be implemented. This material has a high resistance to wear, and allows a coating having a good wear resistance to be obtained. The degree of porosity of these layers may be chosen to allow particles conferring particular properties on the coating to penetrate. Thus, a zirconia-based layer, providing good erosion and shock resistance mechanical performances, may also have properties resulting from the presence of other particles.

[0052] Among these particles, particles of titanium dioxides may provide the coating with a self-cleaning function. Specifically, the photo-catalytic properties of this material, in combination with an ultra-violet source, allows organic elements, such as debris associated with the impacts of mosquitoes or any other organic element, to be continuously converted. The ultraviolet light may be of natural origin or projected onto the wall with a suitable light source.

[0053] According to another possible embodiment, a self-cleaning function may be provided by spraying of piezoelectric ceramic particles. Under the effect of a current, these ceramics vibrate, generating surface waves and enabling detachment and then the walls to be cleaned, guaranteeing an improvement in the appearance and durability of the aerodynamic performance of the wall.

[0054] According to yet another possible embodiment, the spraying of organometallic particles allows the coating to be given a super-hydrophobic function, especially decreasing the risk that ice will form on the wall.

[0055] The implementation of such particles or combinations of particles, to obtain a coating having particular properties, is especially described by documents WO2006/043006 and WO2007/122256.

[0056] The movements of the torch, or of the torches spraying the coating layers, are preferably controlled by a robot that moves the torch(es) so as to perform a certain number of particle sprays at each point of the substrate. This robot, by moving the torch relative to the substrate, allows the distance between the torch and the substrate, which is generally smaller than 25 cm, to be controlled. The robot may also spray, with one or more torches, a plurality of successive layers onto the wall, while varying, if necessary, the orientation of the torches. The robot may especially ensure that the torch is positioned at a suitable distance relative to the substrate, and with a suitable angle, to ensure uniform spraying of the coating layers onto a substrate possibly of complex nonplanar shape.

[0057] This robot may, for example, control a bank of torches each having different flowrates and orientations.

[0058] The robot may also actuate, cut or modulate the supply of particles to the nozzles introducing the particles into the plasma beams of the torches. Thus, this robot makes it possible for the torch, during its passage over each point of the wall of the substrate, to deposit a layer of particles of desired thickness, or for no particles to be deposited, when the supply of particles is interrupted. This embodiment allows a layer of particles to be deposited on certain zones of the wall, and not on certain other zones. It also makes it possible to vary the thickness of a given layer of particles, as a function of the zones of the wall. This embodiment thus allows the geometry of the coating to be effectively controlled.

[0059] The robot may be combined with a means, such as a camera, for monitoring coating deposition conditions in real-time.

[0060] FIGS. 2 and 3 schematically show a wall 2 that is covered, according to a first embodiment, with a microstructured coating forming a plurality of ribs 21, 22, 23 and 24. In this embodiment, the wall 2 is formed on the base of a substrate 20 that is a planar panel. This substrate may for example be a metal plate, for example made of aluminium, or a plate made of a composite material. It is advantageously entirely covered with a first coating layer 201, which may for example be a tie layer ensuring good adhesion of subsequent layers.

[0061] In this embodiment, each of the ribs 21, 22, 23 and 24 is formed by a plurality of coating layers stacked one on top of the other, each new layer having a width smaller than the preceding layer so that the stack of layers has a pyramidal shape. All the coating layers thus form the microstructured coating, comprising the ribs 21, 22, 23 and 24.

[0062] FIG. 4 schematically shows the spraying of the particles forming the microstructured coating onto the substrate 20, according to one possible embodiment, in order to form the wall 2 shown in FIGS. 2 and 3.

[0063] According to this embodiment, the coating layers are sprayed onto the substrate 20 by a plurality of torches 51 and 52, each spraying a directional jet, 510 and 520, respectively, of particles onto the substrate 20. In this embodiment, the two plasma torches 51 and 52 move from left to right, while each sprays particles intended to form a coating layer. Each of these torches thus deposits on the wall a separate layer of particles. It may be seen in FIG. 4 that the first layer 201 and the two following layers of particles forming the base of the ribs 21, 22, 23 and 24 have been deposited before the passage of the plasma torches 51 and 52. Thus, second layers, 211, 221, 231 and 241, respectively, have been respectively deposited on the first layer 201, to form the base of the ribs 21, 22, 23 and 24, respectively. Third layers, 212, 222, 232 and 242, respectively, have been respectively deposited on the second layers 211, 221, 231 and 241, respectively.

[0064] The torches 51 and 52 respectively deposit the fourth and fifth layers. More precisely, in FIG. 4, the torch 51 has deposited the fourth layer 213 on the layer 212, the fourth layer 223 on the layer 222, and is in the process of depositing the fourth layer 233 on the layer 232. The torch 52 has deposited the fifth layer 214 on the layer 213, and is in the process of depositing the fifth layer 224 on the layer 223. The succession of layers deposited one on top of the other by the torches thus allows the controlled geometry of the ribs shown in FIGS. 2 and 3 to be formed.

[0065] Of course, the embodiment may be implemented with a larger number of torches than that shown in FIG. 4. It may also be implemented with a single torch making a plurality of passes so as to produce the various layers of the microstructure.

[0066] Therefore, the microstructure of the wall 2 is composed of a large number of successive layers, preferably more than ten layers. Since the quantity of particles deposited in each layer can be determined with precision, it is possible to produce layers having a very precise thickness. It is thus possible to control the geometry of the microstructure, by adapting the number of layers.

[0067] FIGS. 5 and 6 show the implementation of another possible embodiment. FIG. 5 shows (solid line) a substrate 30 on which a microstructured coating must be deposited, so as to produce the wall 3 including ribs the desired shape of which is shown by the dotted line. In this embodiment, the substrate 30 is not flat, but has reliefs 310, 320, 330 and 340 intended to form seeds of the ribs. These reliefs may be obtained, for example, with one of the known processes for forming "riblet" type microstructures.

[0068] The process according to this embodiment comprises a preliminary step consisting in precisely measuring the surface of the substrate 30, in order to measure the precise dimensions of the reliefs 310, 320, 330 and 340. This measurement may for example be carried out by systematically scanning the surface with the beam of a laser telemeter. Another step consists in calculating, depending on these measurements, using a suitable software package implemented by computer, the number and/or thickness of the coating layers to be deposited at each point of the substrate 30 in order to produce a coating having the desired microstructure, which is shown by the dotted line in FIG. 5. Lastly, in the following steps, these coating layers are deposited on the substrate 30 to form the microstructured coating that is shown in FIG. 6. The various coating layers are stacked one on top of the other, each layer matching the shape of the layer on which it is deposited, until the ribs, 31, 32, 33 and 34, respectively, are formed on the reliefs, 310, 320, 330 and 340, respectively, of the substrate 30.

[0069] In the embodiment shown, the coating layers comprise a first layer 301, which is continuous over the substrate, which may for example be a tie layer ensuring good adhesion of subsequent layers. The coating layers also comprise a last layer 302, which is also continuous and covers all the other layers. This last layer 302 is advantageously a layer having characteristics that make it apt to ensure the surface finish of the coating, and especially its erosion resistance.

[0070] FIG. 7 shows the implementation of another possible embodiment. In this embodiment, a plurality of coating layers, referenced 411 to 419, have been deposited on a substrate 40 to form a microstructured coating. Each of the coating layers 411 to 419 has a thickness that varies depending on the zone that it is in. Thus, in the zones 401 and 402 of the coating, each of these layers has a minimal thickness, such that the coating formed from these layers has a minimal thickness.

[0071] In the zone comprised between these zones 401 and 402, the thickness of the layers of the coating gradually increases to a maximal thickness value, then gradually decreases to a minimal thickness value. This variation in the thickness of a coating layer may be obtained by varying the quantity and/or the size of the particles introduced into the plasma beam, and/or by varying the speed of movement of the plasma beam over the substrate. In the embodiment shown, the thickness of all the layers is identical, at a given point of the substrate. Therefore, the stack of coating layers having a large thickness forms a relief 41 in the coating, thereby allowing a microstructure to be formed in the coating.

[0072] Those skilled in the art will obviously be able to imagine other particle-spraying solutions allowing, via variation of the thickness and/or position of each layer, a microstructure to be produced in the coating.

[0073] Thus, the process according to the embodiment allows a wall, for example a metal or composite wall of an aircraft exterior, to be obtained that is covered with a microstructured coating possibly including troughs or reliefs of a height of 10 to 60 microns. These reliefs may be formed very precisely, with a tolerance of 0.8 to 3 microns, and have a very smooth surface having a good erosion resistance, guaranteeing the durability over time of the microstructure. This coating may also have characteristics that make it possible to limit the effects of contamination (for example by mosquitoes, or dust, resistance to chemical attack and mechanical stresses) and provide an aesthetic effect such as a uniform and durable colouring.

[0074] Such a structure thus makes it possible to satisfy the various constraints to which a wall element of an aircraft is subjected, among which resistance to environmental conditions, and especially to atmospheric conditions; compatibility with various non-planar shapes, the coating possibly being deposited in varied directions; coating aerodynamic performance, the coating possibly having microstructures decreasing the drag generated by a fluid flow; and aesthetic conditions on the exterior coatings of aircraft.

[0075] In the embodiments shown, the microstructured coatings obtained form ribs of substantially triangular cross section. Of course, those skilled in the art will be able, without difficulty, to implement this process to manufacture microstructures of different type.

[0076] While at least one exemplary embodiment of the present embodiment(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms "comprise" or "comprising" do not exclude other elements or steps, the terms "a" or "one" do not exclude a plural number, and the term "or" means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.

Claims

1. A process for manufacturing a microstructured coating on a substrate comprising: spraying molten particles having a diameter between 0.05 .mu.m and 5 .mu.m onto said substrate, the spraying being carried out by at least one plasma torch, which is moved relative to the substrate, the movement parameters and/or the flowrate of particles sprayed by the plasma torch(es) being variable as a function of the position of the plasma torch(es), and ensuring a variable thickness for the coating formed by the particles, at various points of the substrate.

2. The process of claim 1, wherein the variable thickness is produced by spraying, onto at least one portion of the surface of said substrate, at least two successive layers of molten particles, the number of layers of particles sprayed being variable at various points of the substrate.

3. The process of claim 2, wherein variable thickness is produced by spraying at least one layer of molten particles the thickness of which is variable at various points of the substrate.

4. The process of claim 3, further comprising a robot that controls the position, the orientation and the flowrate of each of the plasma torches spraying particles onto the substrate.

5. The process of claim 4, the spraying step comprises: spraying a first layer of particles, of substantially uniform thickness at every point on the substrate; and spraying, on top of the first layer, a plurality of other layers of particles, the number and/or thickness of which are different at various points of the substrate.

6. The process of claim 5, wherein the spraying step comprises: spraying layers of particles, the number and/or thickness of which are different at various points of the substrate (20, 30, 40, 80); and spraying on top of the preceding layers of particles at least one external layer of particles, of substantially uniform thickness at every point of the substrate.

7. The process of claim 6, wherein prior to the spraying step, a step of measuring the geometry of the surface of the substrate and a step of calculating the thickness and/or the number of layers of particles to be sprayed at various points of the substrate (20, 30, 40, 80).

8. The process of claim 7, the particles sprayed in the spraying step comprise particles of metal oxide.

9. The process of claim 8, wherein, prior to the spraying step, a step of dispersing particles to be sprayed in a sol-gel type compound is performed.

10. The process of claim 9, wherein the particles sprayed in the spraying step comprise particles of zirconia.

11. The process of claim 9 wherein the particles sprayed in said spraying step comprise particles of titanium dioxide.

12. A wall, comprising: a substrate; and a coating covering the substrate, wherein the coating is a microstructured coating.

13. The wall of claim 12, wherein the coating has an average thickness between 0.01 mm and 0.25 mm.

14. The wall of claim 12, wherein the coating has, on its surface, a microstructure forming ribs that are substantially parallel to one another.

15. The wall of claim 14, wherein the ribs have a height of between 10 .mu.m and 60 .mu.m.