ABRADABLE COATING

Abstract

The invention relates to an abradable coating for a turbomachine, comprising a mineral compound having a Mohs hardness of 6 or less and a melting temperature of more than 450° C. or even 800° C., and a polymeric compound with a content of between 40% and 70% by volume.

Claims

1. An abradable coating for a turbomachine, comprising a mineral compound, the Mohs hardness of which is less than or equal to 6 and the melting temperature of which is greater than 450° C. or even 800° C., and a polymeric compound, with a content of between 40% and 70% by volume, characterized in that the mineral compound is selected from: calcium fluoride, hydroxyapatite, lanthanum phosphate, diatomite, muscovite or barium sulfate.

2. The abradable coating according to claim 1, wherein the polymeric compound is a polyester or a projectable polymer.

3. The abradable coating according to claim 1, wherein the porosity is less than 5%.

4. The abradable coating according to claim 1, wherein the mineral compound is hydroxyapatite and the polymeric compound is polyester.

5. The abradable coating according to claim 1, wherein the coating is insoluble in water and acetone.

6. A turbomachine module, comprising: a rotor provided with a plurality of moving vanes, a stator, and at least one abradable coating according to claim 1 provided at the interface between a portion of the rotor and a portion of the stator.

7. The module according to claim 6, comprising at least one underlayer with a roughness Ra between 9 μm and 40 μm on the portion of the rotor and on the portion of the stator, under the abradable coating.

8. The module according to claim 7, wherein the roughness underlayer is located between the portion of the stator and the abradable coating.

9. The module according to claim 7, wherein the roughness underlayer is composed of a Nickel-Aluminum alloy or a Nickel-Chromium-Aluminum alloy.

10. An aircraft turbomachine comprising a module according to claim 6.

11. A method of making an abradable coating according to claim 1, comprising the steps of: mixing a powder of a mineral compound, the Mohs hardness of which is less than or equal to 6 and the melting temperature of which is greater than 450° C. and a powder of a polymeric compound, forming an abradable coating from said powder mixture, the polymeric compound having a content between 40% and 70% by volume of the abradable coating, applying the abradable coating to the interface between a portion of a rotor having a plurality of moving vanes and a portion of a stator of a turbomachine module.

12. The method of claim 11, wherein the formation of the abradable coating is performed by single or double injection thermal spraying, extrusion, or hot molding.

13. The method of claim 11, wherein the application of the abradable coating is performed by bonding with an adhesive film or by direct injection between the portion of the rotor and the portion of the stator.

14. The method according to claim 11, wherein a roughness underlayer is deposited on the stator portion by plasma spraying in the area, this underlayer has a roughness Ra of 10 μm and said underlayer is composed of a Nickel-Aluminum alloy or a Nickel-Chromium-Aluminum alloy.

15. The method of claim 11, wherein a roughness underlayer is deposited on the stator portion by arc-wire spraying, said underlayer has a roughness Ra of 40 μm and said underlayer is composed of a Nickel-Aluminum alloy or a Nickel-Chromium-Aluminum alloy.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0054] The present invention will be better understood and further details, features and advantages of the present invention will become clearer upon reading the following non-limiting example description, with reference to the appended drawings in which:

[0055] FIG. 1, already described, represents a microstructure of abradable coating composed of Al—Si and polyester, after cyclic corrosion tests,

[0056] FIG. 2, already described, represents a wear map of an abradable coating composed of Al—Si and polymer, obtained by rubbing a vane,

[0057] FIG. 3, already described, represents a trace left by a vane on an abradable coating composed of Al—Si and h-BN, after abradability tests,

[0058] FIG. 4 shows an axial sectional view of a turbomachine according to the invention,

[0059] FIG. 5 represents a perspective view of a turbomachine according to the invention,

[0060] FIG. 6 represents a front view of a turbomachinery module according to the invention,

[0061] FIG. 7 is a graph representing the erosion and furrowing resistance of an abradable coating according to the invention as a function of the ratio between the mineral compound and the polymeric compound,

[0062] FIG. 8 shows a double injection thermal spraying device,

[0063] FIGS. 9 and 10 represent a microstructure of an abradable coating according to the invention composed of Hydroxyapatite and Polyester, and

[0064] FIG. 11 represents a trace left by a vane on an abradable coating according to the invention composed of Hydroxyapatite and Polyester, after abradability tests.

[0065] The elements having the same functions in the different implementations have the same references in the figures.

DETAILED DESCRIPTION THE EMBODIMENTS

[0066] FIGS. 4 and 5 show a dual flow turbojet engine 1 extending along a main axis A, constituting an example of a turbomachine according to the invention. The turbojet engine 1 comprises, from upstream to downstream in the direction of air flow represented by arrow F, a fan 2, a low-pressure compressor 3, a high-pressure compressor 4, a combustion chamber 5, a high-pressure turbine 6 and a low-pressure turbine 7. The low-pressure compressor 3 and high-pressure compressor 4 have several compression stages.

[0067] The rotor of each compression stage comprises a plurality of moving vanes, mounted on a disc coupled to a high-pressure shaft of the turbojet engine 1. A shroud connects the disc to the disc of the previous stage.

[0068] The stator of each stage comprises a shroud, provided opposite the moving vanes of the rotor, and a plurality of fixed vanes provided opposite the rotor shroud. The shroud of the stator has abradable tracks against which the outer ends of the moving vanes of the rotor rub.

[0069] The contact areas 8 between the moving vanes 9 of the rotor 13 and the abradable tracks 14 of the stator 15 are in particular visible on FIGS. 5 and 6. The abradable tracks 14 join the tops of the vanes 9 of the rotor 13. The contact areas 8 correspond to the clearance between the tops of the vanes 9 of the rotor 13 and the abradable tracks 14.

[0070] Examples of abradable coatings for forming these abradable tracks will now be described.

[0071] The abradable coating includes a mineral compound and a polymeric compound. More specifically, the abradable coating may consist of a mineral compound and a polymeric compound. With the exception of any impurities, such abradable coating does not include any other component. In particular, this abradable coating does not include a metal compound.

[0072] The mineral compound has a Mohs hardness of less than or equal to 6, and a melting temperature greater than 450° C., or even 800° C. In other words, the mineral compound is stable at least up to 450° C. or 800° C.

[0073] The mineral compound can be selected from: Calcium Fluoride (CaF2), Hydroxyapatite (Ca10(PO4)6(OH)2), Lanthanum Phosphate (LaPO4), Diatomite (SiO2), Muscovite (KAl2(AlSi3O10)(OH,F)2) or Barium Sulfate (BaO4S).

[0074] The Hydroxyapatite has a hexagonal crystal system and a 6/m space group. This mineral compound is stable up to at least 900° C. and has a hardness of 5 on the Mohs scale. In addition, this mineral compound is insoluble in water, acetone and alcohol.

[0075] The polymeric compound may be a polyester, for example liquid crystal, or a sprayable polymer, such as a thermal spraying polymer.

[0076] The abradable coating has a volume average polymeric compound content of between 40% and 70%.

[0077] The proportions of polymeric compound in the abradable coating depend on the function of the engine part. For example, the abradable coating is low in polyester (around 40% by volume of the abradable coating) for the high pressure compressor, and is higher in polyester (around 70% by volume of the abradable coating) for the low pressure compressor.

[0078] FIG. 7 shows the erosion resistance (RE) (i.e., anti-corrosion properties) and the furrowing resistance (RS) (i.e., anti-furrowing properties) of the abradable coating as a function of the ratio (RP) between the mineral compound and the polymeric compound.

[0079] The furrowing resistance RS (represented by curve a) is stable for an abradable coating comprising between 0% and 80% of polymeric compound, and between 20% and 100% of mineral compound, while the erosion resistance RE (represented by curve b) increases when the percentage of polymeric compound increases in the abradable coating (from 0% to 80% of polymeric compound by volume of the abradable coating), and thus when the percentage of mineral compound decreases in the abradable coating (from 100% to 20% of mineral compound by volume of the abradable coating).

[0080] In FIG. 7, the area Z1 corresponds to the optimal percentages of polymeric compound in the abradable coating. This area Z1 ranges from 40% to 70% polymeric compound by volume in the abradable coating. An abradable coating comprising such a percentage of polymeric compound by volume exhibits the optimal anti-corrosion and anti-furrowing properties.

[0081] The abradable coating according to the invention does not comprise more than 70% polymeric compound by volume. Indeed, beyond 70% of polymeric compound by volume in the abradable coating, the latter would have insufficient adhesion to the substrate, particularly in the case of an abradable coating obtained by thermal spraying. In FIG. 7, the area Z2 corresponds to the percentages of polymeric compound in the abradable coating for which the abradable coating would have insufficient adhesion to the substrate.

[0082] The abradable coating according to the invention does not comprise less than 40% polymeric compound by volume. Indeed, below 40% polymeric compound by volume in the abradable coating, the latter would have insufficient corrosion resistance. In FIG. 7, the area Z3 corresponds to the percentages of polymeric compound in the abradable coating for which the abradable coating would have insufficient erosion resistance.

[0083] Preferably, the porosity of the abradable coating is less than 5%. This porosity can be assessed by microscopic observation at a magnification of ×200.

[0084] The abradable coating is insoluble in water and acetone, sparingly soluble in alcohol, and may be soluble in engine washes. In particular, the mineral compound dissolves significantly at pH 3 and below (pH below that of acid rain).

[0085] The abradable coating can be deposited by thermal spraying on the substrate to be coated, using powders. In the case of an abradable coating composed of Hydroxyapatite and Polyester, the powder particle size before spraying of the mineral compound varies from −130 μm to +45 μm, and that of the polyester varies from −150 μm or −145 μm to +45 μm.

[0086] In the case of single injection thermal spraying, the powders of the mineral compound and the polymeric compound are mixed beforehand and then the powder mixture is sprayed onto the substrate to be coated by means of a plasma torch.

[0087] In the case of a double injection thermal spraying, the powders of the mineral compound and the polymeric compound are mixed during the spraying on the substrate to be coated. FIG. 8 shows a double injection thermal spraying device. This device 20 includes a mineral compound injector 21, a polymeric compound injector 22 and a plasma torch 23. The mineral compound and the polymeric compound are injected into the plasma torch beam, which projects them onto the substrate to be coated.

[0088] The abradable coating can be machined by any known technique, including turning, milling, or grinding. After surface machining, an abradable coating is obtained as shown in FIG. 9, whose microstructure is visible in FIG. 10. The porosity rate of this abradable coating is less than or equal to 5%.

[0089] The abradability of this abradable coating (evaluated according to DMC 0420) is in the range of 100%-130%. With such an abradable coating, there is no vane wear.

[0090] The DMC 0420 test evaluates the performance of an abradable coating using the A/O (Abradability to Overpenetration) ratio, which is measured using a measuring device not shown in the drawings: three simulated vanes are arranged protruding from the perimeter of a rotating wheel. An abradable sample to be tested is placed below the rotating wheel. The rotating wheel advances at constant speed towards the abradable sample and penetrates it to a set depth. The actual depth dug into the abradable is then measured and the ratio of the set depth to the dug depth is calculated. This ratio is called the A/O ratio and is expressed as a percentage.

[0091] The abradable coating composed of Hydroxyapatite and Polyester can be deposited on the substrate to be coated, preferably a low pressure compressor, according to another technique. The Hydroxyapatite and Polyester powders are mixed and then extruded or heat molded to form a ring. This ring is an abradable track blank, which is then bonded to the substrate to be coated or directly injected onto the substrate, for example the casing of the low pressure compressor. The abradable track blank can be bonded by means of an adhesive film. Alternatively, in order to minimize the thickness of the adhesive seal and to ensure better adhesion of the abradable coating to the substrate, the ring can be inserted cold. The bonding is then carried out by baking in a furnace, for example at a temperature of about 175° C. The abradable track is then machined to the desired dimensions.

[0092] An adhesive underlayer can be added between the substrate to be coated and the abradable coating. The abradable coating is more adherent the rougher the underlayer. Preferably, the underlayer has a roughness Ra between 9 μm and 40 μm. For example, the underlayer can be composed of a Nickel-Aluminum (NiAl) alloy or a Nickel-Chromium-Aluminum (NiCrAl) alloy deposited on the substrate by plasma spraying in air. Such an underlayer has a roughness Ra of about 10 μm. The underlayer can be composed of NiAl or NiCrAl deposited by arc-wire spraying. Such an underlayer has a roughness Ra of about 40 μm.