Rotor-stator assembly for a gas turbine engine

Abstract

A rotor-stator assembly for a gas turbine engine, the assembly including a rotor having a layer of ceramic material forming an abrasive coating deposited on its tip, the layer being constituted mainly by zirconia and possessing a void ratio less than or equal to 15%, and a stator arranged around the rotor and provided facing the tip of the rotor with a layer of ceramic material forming an abradable coating, the layer being constituted mainly by zirconia possessing a void ratio lying in the range 20% to 50% with pores having size less than or equal to 50 m.

Claims

1. A rotor-stator assembly for a gas turbine engine, the assembly comprising: a rotor including a layer of ceramic material forming an abrasive coating deposited on its tip, said abrasive coating layer being constituted mainly by zirconia and possessing a void ratio less than or equal to 15%; and a stator arranged around the rotor and provided facing the tip of the rotor with a layer of ceramic material forming an abradable coating, said abradable coating layer being constituted mainly by zirconia possessing a void ratio lying in a range 20% to 50% with pores including size less than or equal to 50 m to control a porosity of the abradable coating layer so as to provide the abradable coating with an erosion resistance, and wherein the porosity of the abradable coating layer presents a distribution that is bimodal with fine pores of a size less than or equal to 5 m and medium pores of a size lying in a range 15 m to 50 m.

2. An assembly according to claim 1, wherein the porosity of the abradable coating layer is shared uniformly within said abradable coating layer.

3. An assembly according to claim 1, wherein the abradable coating layer is made up of at least two layer portions including different void ratios.

4. An assembly according to claim 1, wherein the abradable coating layer presents a modulus of elasticity lying in a range 4 GPa to 10 GPa and a hardness lying in the range 70 HR15Y to 95 HR15Y.

5. An assembly according to claim 1, wherein the abrasive coating layer presents a modulus of elasticity greater than or equal to 30 GPa and a hardness greater than 600 HV.

6. An assembly according to claim 1, wherein the zirconia mainly constituting the abradable coating layer and the abrasive coating layer is doped with a stabilizer of zirconia in a tetragonal or cubic phase.

7. An assembly according to claim 1, wherein the abradable coating layer is deposited on the stator by thermal spraying, by sintering, or by a sol-gel method.

8. An assembly according to claim 1, wherein the abrasive coating layer is deposited on the tip of the rotor by thermal spraying, by sintering, by an electrolytic method, by a vapor deposition method, or by a sol-gel method.

9. An assembly according to claim 1, wherein the rotor is a turbine blade and the stator is a turbine casing.

10. A gas turbine engine including at least one rotor-stator assembly according to claim 1.

11. An assembly according to claim 1, wherein the fine pores and the medium pores are each uniformly distributed in said abradable coating layer.

12. An assembly according to claim 6, wherein the stabilizer of zirconia in the tetragonal or cubic phase is yttrium, gadolinium, or dysprosium.

13. An assembly according to claim 1, wherein the rotor and/or the stator includes an underlayer under the respective abrasive coating layer and/or abradable coating layer, and the underlayer includes an alloy of MCrAlY where M is Ni, Co, or NiCo, or the underlayer includes a platinum aluminide coating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the present invention appear from the following description made with reference to the accompanying drawings, which show an embodiment having no limiting character. In the figures:

(2) FIG. 1 is a diagrammatic view of an example of a rotor-stator assembly in accordance with the invention;

(3) FIGS. 2A-2C, 3A-3C, and 4A-4B show various embodiments of materials suitable for use as abradable coatings for the stator of the FIG. 1 assembly; and

(4) FIGS. 5 and 6 are photographs showing the behavior of the abrasive and abradable coatings during a test, respectively for a prior art rotor-stator assembly and for a rotor-stator assembly in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) The invention applies to any rotor-stator assembly for a gas turbine engine in which the tip of the rotor comes into contact with the inside surface of the portion of the stator surrounding the rotor.

(6) FIG. 1 is a diagram of an example of an application of the invention in which the rotor is made up of rotary blades 2 of a high pressure turbine in a turbojet and the stator is a turbine ring 4 made as a single piece or made up of a plurality of touching ring sectors surrounding the blades.

(7) In such an assembly, the clearance between the inside surface of the turbine ring 4 and the tips of the blades 2 is zero or almost zero. On the inside, the turbine ring carries an abradable coating 6 into which an abrasive coating 8 deposited on the tips of the blades 2 can penetrate in part without suffering a large amount of wear.

(8) According to the invention, the layer forming the abrasive coating 8 is made of a ceramic material mainly constituted by zirconia and having a void ratio less than or equal to 15%, while the layer forming the abradable coating 6 is made of a ceramic constituted mainly by zirconia and possessing a void ratio lying in the range 20% to 50%, with pores of size less than or equal to 50 m.

(9) The term mainly is used herein to mean that the zirconia of the abradable coating and of the abrasive coating represents at least 80% of the composition of those layers.

(10) The abradable coating 6 and the abrasive coating 8 of the rotor-stator assembly in accordance with the invention are thus made using the same ceramic, namely zirconia, but with microstructures that are different. The zirconia layer constituting the abrasive coating possesses relatively low porosity so as to guarantee the best possible mechanical properties (hardness and resistance to thermal shocks), while the zirconia layer constituting the abradable coating presents a microstructure that is fine (i.e. pores of a size that is less than or equal to 50 m), and porosity that is controlled so as to enhance the erosion resistance of the coating and so as to guarantee high-performance abradability properties.

(11) For both coatings, the zirconia that is used is advantageously doped (with yttrium, gadolinium, dysprosium, or any other stabilizer of zirconia in tetragonal or cubic phase). Nevertheless, it is not necessary for the composition of the zirconia power forming these coatings to be strictly identical for both coatings. Thus, by way of example, it is possible to add stabilizer agents to one or the other of the powders.

(12) The abrasive coating 8 deposited on the tips of the blades 2 preferably also presents a modulus of elasticity that is greater than or equal to 30 GPa and hardness greater than 600 HV.

(13) The abrasive coating layer may be deposited on the tips of the blades by methods that are known, such as thermal spraying, sintering, the electrolytic method, vapor deposition, or the sol-gel method.

(14) Recourse may also be had to an underlayer 10 in order to facilitate depositing the abrasive coating layer on the tips of blades that are made of metal. For example, the bonding underlayer 10 may be constituted by MCrAlY (where M may be Ni, Co, or NiCo), or by a coating of platinum aluminide.

(15) With reference to FIGS. 2A-2C, 3A-3C, and 4A-4B, there follows a description of various embodiments of microstructures for layers of material forming the abradable coating of the rotor-stator assembly of the invention.

(16) FIGS. 2A and 2B show a first embodiment respectively in the form of a photograph (scale: 1 m) and in the form of a diagram, for such a layer 12 constituted mainly by zirconia and deposited on a substrate 14. In this embodiment, the porosity (where pores are represented by black dots 16) is said to be fine, i.e. it is constituted by pores of size less than or equal to 5 m, and uniform in its distribution across the thickness of the layer. In this example, the distribution of the porosity in the layer is monomodal.

(17) It should be observed that this layer 12 may be deposited on the substrate via a bonding underlayer 18, e.g. formed by depositing an alloy of the MCrAlY type (where M is Ni, Co, or NiCo), or depositing a platinum aluminide coating, etc.

(18) Furthermore, in a variant of this first embodiment shown in FIG. 2C, the layer 12 of zirconia also comprises particles 17 of solid lubricant, such as for example particles of hexagonal boron nitride. Such particles make it easier for the blades to move during rotor-stator contact and they limit forces between the rotor and the stator.

(19) FIGS. 3A to 3C show a second embodiment of a layer 20 deposited on a substrate 14 and suitable for use in constituting the abradable coating of the rotor-stator assembly.

(20) In this second embodiment, the layer 20 is constituted by two layers 20a and 20b of zirconia having different microstructures. Thus, the layer 20a deposited beside the abrasion zone presents porosity that is high (i.e. having a void ratio lying in the range 20% to 50%), while the layer 20b deposited beside the substrate 14 presents lower porosity (of the order of 10% to 20%).

(21) A layer is thus obtained that presents high porosity at the surface in order to act as an abradable coating and a denser underlayer beside the substrate in order to act as a thermal barrier. Such variation in porosity across the depth of the abradable coating serves to improve the resistance of the coating to thermal shocks, to improve its resistance to thermal cycles, and to increase the insulating power of the coating (it provides a thermal barrier that is more effective).

(22) It should be observed that this layer 20 may be deposited on the substrate via an intermediate bonding underlayer 18.

(23) It should also be observed that in each of the two layers 20a and 20b, the pores are preferably distributed in uniform manner throughout the thickness of the layers.

(24) Furthermore, in a variant of this second embodiment, as shown in FIG. 3C, the layer 20a deposited beside the abrasion zone may present an array of vertical cracks 22 (at least two cracks every millimeter), so as to improve the thermomechanical behavior of the coating.

(25) FIGS. 4A-4B show a third embodiment of a layer 24 deposited on a substrate 14 and suitable for use in constituting the abradable coating of the rotor-stator assembly.

(26) In this third embodiment, the layer 24 is formed by a layer of zirconia with high porosity (i.e. having a void ratio lying in the range 20% to 50%).

(27) Compared with the first embodiment, the distribution of porosity in the layer in this example is bimodal, i.e. the pores are shared between fine pores (i.e. having size less than or equal to 5 m), and medium pores (i.e. having a size lying in the range 15 m to 50 m).

(28) In the photograph of FIG. 4A, the pores of fine size are thus represented by black zones 26, while the pores of medium size are represented by black zones 28. It should be observed that regardless of pore size, the way the pores are distributed in the layer 24 is uniform.

(29) Other characteristics common to the various embodiments of the layer forming the abradable coating of the rotor-stator assembly are described below.

(30) This abradable coating layer preferably presents a modulus of elasticity lying in the range 4 GPa to 10 GPa. Such values give the coating better resistance to thermal shocks.

(31) Also preferably, the abradable coating layer presents hardness lying in the range 70 to 95 on the surface Rockwell hardness scale (HR15Y), thus making it possible to guarantee sufficient resistance to erosion while presenting good abradable character.

(32) Furthermore, the abradable coating layer may be deposited by thermal spraying of an agglomerated powder preferably containing a pore-generating agent and a solid lubricant. It is also possible to have recourse to thermal spraying of a suspension of nanometer powder which facilitates obtaining finer microstructure. Likewise, it is possible to modify the spraying parameters so as to obtain a microstructure with vertical cracks and a void ratio greater than 20%.

(33) Recourse may also be had to deposition by sintering of powders having a submicrometer microstructure with or without a pore-generating agent and a solid lubricant. Sintering provides increased control over the microstructure by shaping the deposit at a temperature lower than the melting temperature of the ceramic. In particular, the microstructure may be conserved as being fine and the porosity may be controlled both in terms of uniformity and in terms of size and distribution.

(34) Recourse may also be had to a sol-gel method for depositing the abradable coating layer.

(35) With reference to FIGS. 5 and 6, there follows a description of the results of testing abradability and wear for a prior art abrasive/abradable coating pair (FIG. 5) and for an abrasive/abradable coating pair in accordance with the invention (FIG. 6).

(36) These tests were performed using a test bench of the kind made available by the supplier Sulzer Innotec. In known manner, such a test bench comprises both a rotor carrying a blade that is provided at its free end with a layer abrasive coating, and a plate carrying a sample of abradable coating, it being possible to move the plate towards the rotor so as to enable incursion of the abrasive material into the abradable material. The test bench also has a high-speed flame generator for heating the abradable coating during testing.

(37) Tests were performed under the same conditions with the rotor rotating at a speed of 410 meters per second (m/s), with heating of the abradable coating to a temperature of about 1100 C., and with an incursion speed of the abrasive coating into the abradable coating of about 50 micrometers per second (m/s).

(38) FIG. 5 shows the surface state after testing a prior art abradable coating 30 constituted by dense zirconia (void ratio of about 10%) and a plan view of the tip of a blade (without coating) 32 after contacting the coating.

(39) In this figure, a high degree of wear can be seen at the tip of the blade, with poor contacting in the abradable coating deposited on the stator, together with the presence of material being transferred from the blade to the stator.

(40) FIG. 6 shows the surface state after testing an abradable coating 34 in accordance with the invention and a section view of an abrasive coating 36 in accordance with the invention. In particular, in this test, the abradable coating 34 that was used possessed a void ratio lying in the range 25% to 28% with a microstructure that was fine, being constituted by fine pores and medium pores (bimodal distribution) that were distributed uniformly as described with reference to FIGS. 3A and 3B. Similarly, the abrasive coating 36 made of zirconia partially stabilized with yttrium possessed a void ratio of about 10%.

(41) Compared with FIG. 5, it can be seen in this figure that wear is low in the abrasive coating deposited on the rotor and contact is good in the abradable coating deposited on the stator. This leads to good behavior in terms of abradability and of wear for the abrasive/abradable coating pair of the rotor-stator assembly in accordance with the invention.