Abradable coating having variable densities

Abstract

A method of fabricating an abradable coating of varying density, and such an abradable coating of varying density. According to the invention, the method comprises the following steps: providing a substrate (32) having a first portion with its surface situated at a first level (A), and a second portion with its surface situated at a second level (B) different from the first level; depositing a precursor material on the first and second portions of the substrate (32); compressing the precursor material between the substrate and a bearing surface; and sintering the precursor material as compressed in this way in order to obtain an abradable coating (36) having a first portion (36a) on the first portion of the substrate, and possessing a first density, and a second portion (36b) on the second portion of the substrate, and possessing a second density distinct from the first.

Claims

1. A fabrication method for an abradable coating of varying density, the method comprising the following steps: providing a substrate having a first portion with its surface situated at a first level, and a second portion with its surface situated at a second level different from the first level; depositing a precursor material on the first and second portions of the substrate; compressing the precursor material between the substrate and a bearing surface; and sintering the precursor material as compressed in this way in order to obtain an abradable coating having a first portion on the first portion of the substrate, and possessing a first density, and a second portion on the second portion of the substrate, and possessing a second density distinct from the first.

2. A method according to claim 1, wherein the second portion of the substrate is obtained by machining at least one groove in a blank of the substrate.

3. A method according to claim 1, wherein the bearing surface is continuous and rectilinear at least in a direction extending transversely relative to the first and second portions of the substrate.

4. A method according to claim 1, wherein the first portion of the abradable coating possesses final porosity of less than 15%.

5. A method according to claim 1, wherein the second portion of the abradable coating possesses final porosity greater than 20%.

6. A method according to claim 1, further comprising, before the step of depositing the precursor material on the first and second portions of the substrate, a step of forming by sintering a backing layer on the second portion of the substrate, the final porosity of the backing layer being less than 15%.

7. A method according to claim 1, further comprising, after the step of sintering the precursor material, a step of forming by sintering a surface layer on at least a portion of the abradable coating, the surface layer having final porosity of less than 15%.

8. A method according to claim 1, wherein the substrate is a ring sector.

9. A method according to claim 1, wherein the substrate possesses a longitudinal channel between two longitudinal shoulders, the shoulders forming part of the first portion of the substrate, and the bottom of the channel forming part of the second portion of the substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings are diagrammatic and seek above all to illustrate the principles of the invention.

(2) In the drawings, from one figure to another, elements (or portions of an element) that are identical are identified by the same reference signs. In addition, elements (or portions of an element) belonging to different examples but having functions that are analogous are identified in the figures by numerical references incremented by 100, 200, etc.

(3) FIG. 1 is a section view of a turbine engine of the invention.

(4) FIG. 2 is a fragmentary perspective view of an example of a stator ring of the invention.

(5) FIGS. 3A to 3E show various successive steps in an example method of the invention.

(6) FIG. 4 is a section view of a second example of an abradable track.

(7) FIG. 5 is a section view of a third example of an abradable track.

DETAILED DESCRIPTION OF EXAMPLE(S)

(8) In order to make the invention more concrete, examples of methods and abradable tracks are described below in detail with reference to the accompanying drawings. It should be recalled that the invention is not limited to these examples.

(9) FIG. 1 is a section view of a bypass turbojet 1 of the invention, the section being on a vertical plane containing the main axis A of the turbojet. Going from upstream to downstream in the air stream flow direction, the turbojet comprises: 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) The high pressure turbine 6 has a plurality of blades 6a rotating with the rotor and a plurality of guide vanes 6b mounted on the stator. The stator of the turbine 6 comprises a plurality of stator rings 10 arranged facing the movable blades 6a of the turbine 6. As can be seen in FIG. 2, each stator ring 10 is subdivided into a plurality of sectors 11, each provided with an abradable track 20 against which the movable blades 6a rub in the event of a radial excursion of the rotor.

(11) An example of such an abradable track 20 is described with reference to FIGS. 3A to 3E. In FIG. 3A, a blank 30 is initially provided. Specifically, it comprises a ring sector obtained using a conventional method. Its surface 30s is regular, rectilinear in the axial section plane of FIG. 3A, and circularly arcuate in a radial section plane.

(12) As shown in FIG. 3B, a groove 31 is then machined longitudinally, i.e. circumferentially, in the surface of the blank 30 so as to form a channel: this produces a substrate 32 possessing two shoulders 33 on either side of the groove 31, respectively upstream and downstream.

(13) Together, these two shoulders form a first substrate portion 33 having its surface 33s extending at a first level A corresponding to the initial level of the blank 30. The portion of the substrate 32 situated at the bottom of the groove 31 forms a second substrate portion 34 having its surface 34s extending at a second level B that is lower than, i.e. deeper than, the first level A and that corresponds to the bottom of the groove 31. In the present example, the groove 31 is 12 mm deep, in other words the difference between the levels A and B is 12 mm.

(14) As shown in FIG. 3C, the substrate 32 as formed in this way is then placed in the cavity of a shaping mold 40 of axial dimensions that correspond to the dimensions of the substrate 32.

(15) A precursor material 35, specifically a metal powder, is then deposited in uniform manner over all of the substrate 32. The powder 35 thus completely fills the channel 31 and forms a continuous layer of constant thickness over level A on the shoulders 33 of the substrate 32. The powder 35 is thus added until it reaches a third level C: in the present example, this level C is located 20 mm above the level A of the shoulders 23 of the substrate. In the present example, the powder is a nickel powder of grain size centered around 100 m; its initial porosity is about 70%.

(16) Naturally, this initial porosity may vary depending on the type of powder used and on the desired final porosities: for example, for nickel powders having a grain size of about 4 m to 7 m, the initial porosity may be about 23% to 33%. Preferably, powders having a high initial porosity are used for the abradable zone of low density. In addition, it is possible to obtain a greater initial porosity by adding a pore-generating agent to such powders, which is subsequently eliminated during the method, e.g. during a pyrolysis step.

(17) In contrast, powders that are finer with a lower initial porosity can be used for the higher-density zones such as the low walls, the backing layer, or the surface layer of low roughness, as described below.

(18) Thus, the powder 35 forms a 20 mm thick layer over the shoulders 33, and a 32 mm thick layer over the central channel 31.

(19) As shown in FIG. 3D, the mold 40 is then closed. A bearing face 42 of its cover 41, which is rectilinear in the axial plane of FIG. 3D and circularly arcuate in a radial plane, then bears against the surface 35s of the powder layer 35.

(20) Stress is then exerted on the cover 41 of the mold 40 in order to press against the powder layer 35 and compress it between the substrate 32 and the bearing face 42 of the cover 41 of the mold 40. The powder layer 35 is thus compressed to a fourth level D that lies specifically 4.2 mm above the shoulders 33 of the substrate, i.e. 4.2 mm above the level A.

(21) During this compression step, the particles of powder 35 are compacted against one another, thereby filling in some of the voids initially present between the particles, with the air that is expelled in this way being discharged from the mold 40. The porosity of the powder therefore decreases during this compression step, and the density of the powder increases.

(22) Nevertheless, this densification depends on the position of the volume of powder under consideration within the powder layer 35. Specifically, ignoring phenomena of the powder migrating naturally, the volumes of powder 35a situated between the shoulders 33 and the bearing surface 42 are subjected to an available volume reduction and thus to compression that is greater than the volume of powder 35b situated in and over the channel 31.

(23) Specifically, over the shoulders 33, the initially available thickness corresponds to the difference in level between the levels A and C, i.e. 20 mm in this example, while the thickness available after compression corresponds to the difference of level between the levels A and D, i.e. 4.2 mm: the powder volume 35a is thus subjected to a 79% reduction in volume.

(24) In contrast, over the second portion 34 of the substrate 32, the initially available thickness corresponds to the level difference between the levels B and C, i.e. 32 mm in this example, while the thickness available after compression corresponds to the level difference between the level B and D, i.e. 16.2 mm: the powder volume 35a is thus subjected to a 49% reduction in volume.

(25) Under such circumstances, insofar as the mass of powder remains constant in each powder volume 35a and 35b, it is possible to calculate the densification of the material using the following formula, in which e.sub.i is the initial thickness of the material and e.sub.f is its final thickness:

(26) $\mathrm{densification}=\frac{d_f-d_i}{d_i}=\frac{m\text{/}{V}_f-m\text{/}{V}_i}{m\text{/}{V}_i}=\frac{V_i-V_f}{V_f}=\frac{e_i-e_f}{e_f}$

(27) It can thus be deduced that the first powder volume 35a is subjected to densification, i.e. to an increase in its density, of 376%, while the second powder volume 35b is subjected to densification of 98%.

(28) Once such a compressed state has been obtained, the powder layer 35 as differently compressed in this way is sintered using a conventional method.

(29) At the end of the sintering step, the abradable track 20 of FIG. 3E is obtained in which the substrate 32 is covered in a coating 36 having a first portion 36a overlying the shoulders 33 with a thickness of 4.2 mm and a final porosity of 14.7%, together with a second portion 36b overlying the channel 31 and possessing thickness of 16.2 mm and final porosity of 35.7%.

(30) In this respect, the final porosity P.sub.f can be calculated generally as a function of the initial porosity P.sub.i and of the compression ratio Tc, i.e. the reduction in volume, of the portion of material under consideration:
P.sub.f=P.sub.i(1Tc)

(31) Naturally, the depth of the groove 31, the initial thickness of powder 35, and the amplitude of compression may be freely adjusted in order to achieve the desired densities and thickness for the coating.

(32) Furthermore, in this example, the channel 31 of the substrate extending at a level B lower than the level of the shoulders 33 is a groove obtained by machining the blank 30. Nevertheless, in a variant of the first implementation, an analogous channel could be obtained by adding low walls onto the blank 30 so as to form shoulders 33 on either side of the channel 31: under such circumstances, the initial level of the blank defines the level B of the bottom of the channel 31, while the top of the walls define the level A.

(33) In a second example, shown in FIG. 4, the method has an additional step that takes place immediately after providing a substrate 132 having two levels A and B. A backing layer 137 of high density, e.g. having final porosity in the range 0 to 15% is put into place on the bottom of the channel 131 by sintering a powder. Thereafter the method remains unchanged compared with the first example, the layer of precursor material being deposited onto the shoulders 133 and onto the backing layer 137.

(34) At the end of the method, an abradable track 120 as shown in FIG. 4 is thus obtained in which the second coating portion 136 of lower density covers the backing layer 137, which backing layer protects the substrate in the event of a radial offset of the body traveling past the coating that is greater than the maximum intended offset, e.g. in the event of a large unbalance of the moving body.

(35) In a third example, compatible with the first and second examples and shown in FIG. 5, the method includes an additional step that takes place immediately after the steps of compressing and sintering the powder, the beginning of the method remaining unchanged compared with the first or the second example. A surface layer 238 of high density, i.e. possessing final porosity in the range 0 to 15%, and of small thickness, e.g. lying in the range 0.05 mm to 0.10 mm, is put into place by sintering a powder at the surface of the coating 236 of varying density. At the end of the method, the abradable track 220 of FIG. 5 is thus obtained in which the surface layer 238, covers all of the coating 236b, which surface layer possesses surface roughness that is less than that of the lower density second portion of the coating 236b, thereby improving aerodynamic friction.

(36) The examples described in the present disclosure are given by way of non-limiting illustration, and a person skilled in the art can easily, in the light of this disclosure, modify these examples or envisage others, while remaining within the scope of the invention.

(37) Furthermore, the various characteristics of these embodiment or implementation examples may be used singly or combined with one another. When they are combined, the characteristics may be combined as described above or in other ways, the invention not being limited to the specific combinations described in the present disclosure. In particular, unless specified to the contrary, any characteristic described with reference to any one embodiment or implementation may be applied in analogous manner to any other embodiment or implementation.