Abradable coating

Abstract

An abradable coating for a turbomachine, as well as a turbomachine module and a turbomachine including such an abradable coating, the abradable coating including, with a content of greater than 50% by volume, an inorganic compound whose Mohs hardness is less than 6 and whose melting temperature is greater than 900 C.

Claims

1. An abradable track suitable for a turbomachine, the abradable track comprising: a substrate; and abradable coating, provided on the substrate; wherein the abradable coating comprises, in a content of greater than 95 vol. %, Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, LaPO.sub.4, or diatomaceous earth, and wherein the abradable coating has a surface roughness Ra of less than 3 m.

2. The abradable track of claim 1, wherein the abradable coating has a porosity of less than 15%.

3. The abradable track of claim 1, wherein the abradable coating further comprises a metal compound.

4. The abradable track of claim 3, wherein the metal compound is based on nickel, cobalt, or iron.

5. The abradable track of claim 3, wherein the metal compound is NiAl.

6. The abradable track of claim 3, wherein the Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 and the metal compound together constitute at least 99 vol. % of total abradable coating material.

7. A turbomachine module, comprising: a rotor, provided with a plurality of moving blades; a stator; and the abradable track of claim 1, provided at the interface between a portion of the rotor and a portion of the stator.

8. A turbomachine, comprising: a module of claim 7.

9. A process for manufacturing the abradable track of claim 1, the process comprising: depositing the Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, LaPO.sub.4, or diatomaceous earth by thermal spraying from a powder having a particle size in a range of from 45 to 90 m; and surface machining the abradable coating.

10. The abradable track of claim 1, wherein the abradable coating further comprises NiCrAl.

11. The abradable track of claim 1, wherein the abradable coating further comprises CoNiCrAlY.

12. The abradable track of claim 1, wherein the abradable coating further comprises FeCrAlY.

13. The abradable track of claim 1, wherein the abradable coating comprises the LaPO.sub.4.

14. The abradable track of claim 1, wherein the abradable coating comprises the diatomaceous earth.

15. The abradable track of claim 1, wherein the Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 is at least 99 vol. % of the abradable coating.

16. The abradable track of claim 1, wherein the abradable coating consists of the Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 and possible impurities.

17. The abradable track of claim 1, wherein the LaPO.sub.4 is at least 99 vol. % of the abradable coating.

18. The abradable track of claim 1, wherein the abradable coating consists of the LaPO.sub.4 and possible impurities.

19. The abradable track of claim 1, wherein the diatomaceous earth is at least 99 vol. % of the abradable coating.

20. The abradable track of claim 1, wherein the abradable coating consists of the diatomaceous earth and possible impurities.

21. The abradable track of claim 1, wherein the abradable coating has a porosity of less than 15%, and wherein the Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 is greater than 95 vol. % of the abradable coating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The appended drawings are schematic and are intended primarily to illustrate the principles of the disclosure.

(2) In these drawings, from one figure to another, identical elements (or parts of elements) are marked with the same reference signs.

(3) FIG. 1 is an axial cross-sectional view of a turbomachine according to the disclosure.

(4) FIG. 2 is a cross-sectional view of a module according to the disclosure.

(5) FIG. 3 is a photograph illustrating the microstructure of a first example coating according to the disclosure.

(6) FIG. 4 is a graph showing the aerodynamic losses within a module as a function of the roughness of the abradable coating.

(7) FIG. 5 shows a schematic illustration of an abradability test.

DESCRIPTION OF THE EMBODIMENTS

(8) To make the disclosure more concrete, an example of an abradable coating is described in detail below, with reference to the appended drawings. It should be recalled that the invention is not limited to this example.

(9) FIG. 1 shows, in cross-section along a vertical plane passing through its main axis A, a turbofan engine 1, constituting an example of a turbomachine according to the disclosure. It comprises, from upstream to downstream according to the air flow, 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.

(10) FIG. 2 shows, schematically, a stage of the high-pressure compressor 4, the high-pressure compressor 4 comprising a succession of such stages.

(11) The rotor 10 of each stage comprises a plurality of moving blades 11, mounted on a disk 12 coupled to the high pressure shaft of the turbomachine 1. In addition, a shroud 13 connects the disk 12 to the disk 12 of the previous stage. The stator 20 of each stage comprises a shroud 21, provided opposite the moving blades 11, and a plurality of fixed blades 22 provided opposite the shroud 13 of the rotor 10.

(12) The stator shroud 21 carries abradable tracks 31 against which the external ends of the moving blades 11 rub. Furthermore, another abradable track 32 is provided on the inner end of each fixed blade 22; knife edges 15 provided on the rotor shroud 13 then rub against this abradable track 32.

(13) Examples of abradable coatings to form these abradable tracks 31 and 32 will now be described.

(14) In a first example, the abradable coating is made of hydroxyapatite, an inorganic compound of the formula Ca.sub.10(PO.sub.4).sub.6(OH).sub.2. Except for possible impurities, this abradable coating does not comprise any other component.

(15) This inorganic compound has a hexagonal crystal system and a 6/m space group. It is stable up to at least 900 C. and has a hardness of 5 on the Mohs scale. Furthermore, it is insoluble in water, acetone and alcohol.

(16) It is deposited by thermal spraying on the substrate to be coated, in this case the shroud 21 and the ring 32, from a powder with a particle size between 45 and 90 m. In this example, a thickness of 1.5 mm is desired for the coating.

(17) After surface machining, a coating whose microstructure is visible on FIG. 3 is obtained: its porosity is lower than 15% and its roughness Ra lower than 3 m.

(18) In this respect, FIG. 5 represents the aerodynamic losses suffered by the air stream circulating in a high-pressure compressor equipped with abradable tracks, as a function of the roughness of the coating forming these abradable tracks. This curve 50 was drawn by comparing several materials on a test bench. Points 51 and 52 correspond to the cases of two abradable coatings currently preferred for a high-pressure compressor: a raw Metco 2043 coating for point 51 and a Metco 2043 coating with an alumina slurry for point 52.

(19) The point 53 corresponds to the case of this coating made of hydroxyapatite: it can be seen that this coating has a roughness about three times lower than that of the known Metco 2043 coatings and therefore causes almost half the aerodynamic losses of these coatings of the state of the art.

(20) Furthermore, the performance of this abradable coating was evaluated using the A/O ratio (abradability to overpenetration) which is measured using a measuring device 90 shown in FIG. 6: three simulated vanes 91 are arranged protruding from the perimeter of a rotating wheel 92. An abradable sample 93 to be tested is placed below the rotating wheel 92. The rotating wheel 92 advances at a constant speed towards the abradable sample 93 and penetrates it to a set depth. The actual depth dug into the abradable is then measured and the ratio of set depth to dug depth is calculated. This ratio is called the NO ratio and is expressed as a percentage. The test parameters are as follows. The rotation speed at the end of the simulacrum blades 91 is 210 m/s, the feed speed of the rotating wheel 92 towards the sample 93 is 150 m/s and the set depth is 0.5 mm.

(21) Therefore, this hydroxyapatite coating showed during these tests an NO ratio comprised between 110% and 120%, without any wear on the blades.

(22) In addition, an erosion test according to standard ASTM G76 was performed on this abradable coating. An erosion of 1.7 mm.sup.3/g for an angle of 90 was then measured for this hydroxyapatite coating.

(23) Although the present invention has been described with reference to specific example embodiments, it is apparent 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. Consequently, the description and drawings should be considered in an illustrative rather than restrictive sense.

(24) It is also obvious that all the features described with reference to a process are transposable, alone or in combination, to a device, and conversely, all the features described with reference to a device are transposable, alone or in combination, to a process.