METHOD FOR MANUFACTURING A POROUS ABRADABLE COATING MADE OF CERAMIC MATERIAL

Abstract

A process for manufacturing a porous abradable coating includes: filling a mold with hollow glass or thermosetting polymer beads and a slurry; and sintering heat treatment to obtain a ceramic layer with pores. A maximum sintering temperature of the green body of the ceramic part is either higher than the melting temperature of the hollow glass beads so that at the end of the sintering heat treatment the hollow glass beads are melted, or higher than the decomposition temperature of the hollow thermosetting polymer beads so that at the end of the sintering heat treatment the hollow thermosetting polymer beads are decomposed.

Claims

1. A manufacturing process for a porous abradable coating made of ceramic material comprising a layer of ceramic material having pores, the process comprising the following steps: filling at least partially of a mold with hollow glass or thermosetting polymer beads; filling of the mold with a slurry; filtration and discharge of a solvent from the slurry so that the mold contains a green body of the ceramic part comprising the hollow glass or thermosetting polymer beads, sintering heat treatment of the green body of the ceramic part to obtain the layer of ceramic material having pores, a maximum sintering temperature of the green body of the ceramic part being either higher than the melting temperature of the hollow glass beads so that at the end of the sintering heat treatment the hollow glass beads are melted, or higher than the decomposition temperature of the hollow thermosetting polymer beads so that at the end of the sintering heat treatment, the hollow thermosetting polymer beads are decomposed.

2. The process according to claim 1, wherein the heat treatment comprises at least two sintering stages, a first sintering stage at a temperature below a glass transition temperature ETO of the hollow glass beads or below the decomposition temperature of the hollow thermosetting polymer beads to form a partially consolidated ceramic part and a second sintering stage at a temperature higher than the glass transition temperature of the hollow glass beads to melt the hollow glass beads or higher than the decomposition temperature of the hollow thermosetting polymer beads to decompose the hollow thermosetting polymer beads.

3. The process according to claim 1, wherein, during the at least partial filling of the mold with the hollow glass beads, the hollow glass beads are disposed in a mesh having a mesh size for containing the hollow glass or thermosetting polymer beads and for allowing the slurry to pass through, the mesh having a decomposition temperature below a final sintering temperature.

4. The process according to claim 3, wherein the decomposition temperature of the mesh is higher than the temperature of the first sintering stage.

5. The process according to claim 3, wherein the mesh is made of a material comprising a nylon, a polyimide or a polyamide.

6. The process according to claim 1, wherein, during the at least partial filling of the mold with the hollow beads, the hollow glass beads are disposed in the mold with a solvent to agglomerate the hollow glass beads with each other by adsorption of the solvent on the surface of the hollow glass beads, the solvent then being discharged from the mold.

7. The process according to claim 1, wherein the mold includes at least one liquid discharge port.

8. The process according to claim 1, wherein the slurry comprises a pore-forming agent.

9. The process according to claim 1, wherein the ceramic layer having pores has a porosity greater than or equal to 40% by volume, preferably greater than or equal to 60% by volume, more preferably greater than or equal to 80% by volume.

10. The process according to claim 9, wherein after the sintering heat treatment of the green body of the ceramic part, a slurry layer is applied to the ceramic layer with pores and a further sintering heat treatment is performed to sinter the slurry layer and form an additional ceramic layer having a porosity smaller than a porosity of the ceramic layer with pores and a roughness of less than or equal to 5 μm, preferably less than or equal to 3 μm, more preferably less than or equal to 1 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Other features and advantages of the invention will emerge from the following description of embodiments of the invention, given by way of non-limiting examples, with reference to the appended figures, in which:

[0067] FIG. 1 is a view of a fracture face of an abradable coating according to the disclosure;

[0068] FIG. 2 is a schematic perspective view of a stack of hollow glass or thermosetting polymer beads;

[0069] FIG. 3 is a schematic cross-sectional view of an abradable coating according to one variant of the disclosure;

[0070] FIG. 4 is a flow chart representing the steps of a process for manufacturing the abradable coating of FIGS. 1 and 3;

[0071] FIGS. 5A and 5B are schematic cross-sectional views of a mold for implementing a process for manufacturing the abradable coating;

[0072] FIG. 6 is a graph showing temperature change versus time during the sintering heat treatment;

[0073] FIG. 7 is a perspective view of the result of an abrasion test carried out on an abradable coating according to the disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0074] FIG. 1 is a view of a fracture face of a porous abradable ceramic coating 10. The porous abradable coating 10 includes a ceramic layer 12 having pores 14. As shown schematically in the enlargement of FIG. 1, the pores 14 of the layer 12 include glass 16A.

[0075] This glass 16A is present in the pores 14 of the ceramic layer 12 and results from the manufacturing process of the porous abradable coating 10. This glass 16A may be identified, for example, during analysis by X-ray fluorescence (XRF) spectroscopy. This technique makes it possible to identify elements present in the glass 16A that are not present in the ceramic material. The presence of the glass 16A may thus be identified.

[0076] This glass 16A comes from hollow glass beads 16B that are used to create porosity in the porous abradable coating 10. Hollow glass beads 16B that may be used in the manufacturing process of the porous abradable coating 10 are represented in FIG. 2.

[0077] The hollow glass beads 16B may be borosilicate glass, soda-lime glass, lead glass, also commonly referred to as crystal, silica glass or aluminosilicate glass.

[0078] The glass transition temperature is generally comprised between 550° C. and 600° C. for borosilicate glasses; between 450° C. and 480° C. for soda-lime glasses; between 400° C. and 420° C. for lead glasses; between 1300° C. and 1400° C. for aluminosilicate glasses and between 900 and 1300° C. for silica glass.

[0079] The hollow glass beads 16B of FIG. 2 may for example have a diameter of about 100 μm (micrometers) and have a wall thickness comprised between about a few hundred nanometers and a few micrometers.

[0080] These hollow glass beads 16B may for example be made of borosilicate glass and have a glass transition temperature T.sub.g of about 800° C.

[0081] The hollow glass beads 16B may be obtained with a data sheet that indicates their glass transition temperature T.sub.g. Alternatively, the glass transition temperature T.sub.g may be measured for example by differential scanning calorimetry (DSC).

[0082] In the embodiment of FIG. 1, the ceramic layer 12 with pores 14 has a total porosity of about 60% by volume.

[0083] The ceramic layer 12 may for example comprise alumina (Al.sub.2O.sub.3), zirconia (ZrO.sub.2), yttrium disilicate (Y.sub.2Si.sub.2O.sub.7) or silicon carbide (SiC), or a mixture of these compounds. This list is not limiting.

[0084] In the following, elements common to the various embodiments are identified by the same numerical references.

[0085] In FIG. 3, another embodiment of a porous abradable coating 10 has been represented. In the embodiment of FIG. 3, the porous abradable coating 10 also includes an additional ceramic layer 36 having a porosity less than a porosity of the ceramic layer 12. The additional ceramic layer 36 has a roughness R.sub.a less than or equal to 5 μm.

[0086] The additional ceramic layer 36 has a thickness of 100 μm.

[0087] It is understood that when hollow thermosetting polymer beads 16B are used, there is no glass 16A present in pores of the porous abradable coating 10.

[0088] The porous abradable coating 10 of the embodiments of FIGS. 1 and 3 is obtained by the manufacturing process 100 which will be described below and illustrated in FIG. 4.

[0089] The manufacturing process 100 for the porous abradable coating 10 of FIG. 4 includes a first step 102 in which a mold 20 is filled with hollow glass beads 16B (see FIGS. 5A and 5B). The mold 20 has two parts: a lower part 22 and an upper part 24. When assembled, the lower part 22 and the upper part 24 of the mold 20 define a cavity 26 which is intended to receive the hollow glass beads 16B and a slurry intended to form the ceramic material of the ceramic layer 12 after heat treatment.

[0090] As represented in FIGS. 5A and 5B, the lower part 22 of the mold 20 has a discharge port 28 and the upper part 24 of the mold has two discharge ports 30. Alternatively, the lower part 22 and/or the upper part 24 of the mold 20 could be made of a porous material that allows for filtration and discharge of liquids from the mold 20. For example, the discharge ports 28, 30 in the lower and upper parts 22, 24 of the mold 20 may allow the liquid from the slurry to be discharged from the mold 20. In particular, these discharge ports 28, 30 allow a vacuum pump to be used and accelerate the discharge of the liquid to form a green body of the ceramic part without handling of the part. The discharge ports 28, 30 in the lower and upper parts 22, 24 of the mold 20 may also allow material to be introduced into the mold 20. For example, the discharge port 28 of the lower part of the mold 20 may be used to inject the slurry into the cavity 26 of the mold 20.

[0091] The cavity 26 of the mold 20 may be partially filled, i.e., over a given height of the cavity 26, a stack 18 of hollow glass beads 16B is present and over a height complementary to the given height of the cavity 26, the cavity does not include a stack of hollow glass beads 16B.

[0092] It is understood that when the cavity 26 is completely filled with the hollow glass beads 16B, there are gaps between the hollow glass beads 16B, gaps which will be filled by the slurry.

[0093] In the embodiment shown in FIG. 5A, the hollow glass beads 16B have for example a diameter of about 100 μm and the hollow glass beads 16B are disposed in a mesh 32. The mesh 32 has a mesh size to contain the hollow glass beads 16B and allow the slurry to pass through.

[0094] The mesh size of the mesh 32 is such that the hollow glass beads 16B are unable to exit the mesh 32, i.e., the mesh size is smaller than the diameter of the hollow glass beads 16B. However, the mesh size of the mesh 32 allows the slurry to seep between the hollow glass beads 16B, and more particularly into the spaces formed between the hollow glass beads 16B.

[0095] For example, the mesh 32 is flexible which allows the mesh 32 filled with hollow glass beads 16B to conform to the shape of the cavity 26 of the mold 20.

[0096] The mesh 32 also helps contain the hollow glass beads 16B in the cavity 26 when the mold 20 has discharge ports.

[0097] In the embodiment of FIG. 5B, the mold 20 has porous membranes 34 disposed in the cavity 26 of the mold 20 and the hollow glass beads 16B are disposed between these porous membranes 34. The porous membranes allow the hollow glass beads 16B to be contained within the cavity 26 when the mold 20 has discharge ports.

[0098] In the embodiment of FIG. 5B, the hollow glass beads 16B are disposed in the cavity 26 of the mold 20 with a solvent to agglomerate the hollow glass beads 16B with each other by adsorption of the solvent on the surface of the hollow glass beads 16B. The solvent is then discharged from the mold 20, for example through one of the discharge ports 28, 30. However, the adsorbed solvent remains on the surface of the hollow glass beads 16B which allows the hollow glass beads 16B to be maintained in a dense stack, even during the filling of the mold 20 with the slurry.

[0099] The manufacturing process 100 then includes a step of filling 104 the cavity 26 of the mold 20 with the slurry, for example, through the discharge port 28 of the lower part 22 of the mold 20. When the spaces between the hollow glass beads 16B are filled with the slurry, the discharge port 28 is closed.

[0100] Next, the step of filtering and discharging the solvent 106 from the slurry in the mold 20 to form a green body of the ceramic part comprising the hollow glass beads 16B is performed. During this solvent filtration and discharge step 106, the solvent is extracted from the slurry, for example by using a vacuum pump connected to one of the discharge ports 28, 30. This solvent filtration and discharge step 106 may last for more than 24 h (hours).

[0101] When the green body of the ceramic part has reached a proper moisture level, the green body of the ceramic part including the hollow glass beads 16B is placed in a furnace and undergoes a sintering heat treatment (steps 108 and 110) to obtain the ceramic layer 12 having the pores 14.

[0102] The heat treatment includes a first sintering stage (step 108) at a temperature T1, which is lower than the glass transition temperature T.sub.g of the hollow glass beads 16B (see FIG. 6).

[0103] After the first sintering stage at temperature T1, the green body of the ceramic part before heat treatment is partially consolidated and forms a partially consolidated ceramic part, with the ceramic material forming a partially consolidated structure around the hollow glass beads 16B. The temperature T1 being lower than the glass transition temperature T.sub.g of the hollow glass beads 16B, the hollow glass beads 16B do not soften under the effect of the temperature T1. Thus, the partial consolidation of the ceramic material around the hollow glass beads 16B is achieved around the hollow glass beads 16B which are not or only slightly deformed under the effect of the partial consolidation of the ceramic material around the hollow glass beads 16B.

[0104] The mesh 32 of FIG. 5A is made of a material with a decomposition temperature higher than the temperature T1 of the first temperature stage. Thus, during the partial consolidation of the ceramic material, the hollow glass beads 16B are held in place by the mesh 32, which is still present after the first sintering stage at temperature T1.

[0105] The mesh 32 is for example made of a material comprising a nylon.

[0106] The partially consolidated ceramic part is then heat treated in a second sintering stage (step 110) at a temperature T2, which is higher than the glass transition temperature T.sub.g of the hollow glass beads 16B. The temperature T2 is therefore higher than the temperature T1.

[0107] As shown in FIG. 6, the temperature T2 of the second sintering stage may be the final sintering temperature (curve 40) or the sintering heat treatment may comprise other sintering stages, at least one of which is at a temperature T3 higher than the temperature T2 of the second sintering stage (curve 42). The final sintering temperature is the maximum temperature imposed on the ceramic material in order to obtain the porous abradable ceramic coating 10.

[0108] The temperature T2 of the second sintering stage being higher than the glass transition temperature T.sub.g of the hollow glass beads 16B, the consolidation of the partially consolidated ceramic part continues and the hollow glass beads 16B soften.

[0109] As the hollow glass beads 16B soften, they leave cavities in the ceramic material. These cavities will form the pores 14 of the porous abradable coating 10. Thus, when the hollow glass beads 16B melt, the ceramic material is already sufficiently consolidated and the cavities left by the hollow glass beads 16B are not filled by the ceramic material.

[0110] When the sintering heat treatment is complete, a porous abradable ceramic coating 10 whose pores 14 include a small amount of glass 16A is obtained. The beads being hollow glass beads 16B, the wall of the hollow glass bead 16B being relatively thin, the amount of glass 16A remaining in the pores 14 is relatively small. The glass 16A present in the pores 14 of the ceramic layer 12 does not adversely affect the abradability of the porous abradable coating 10.

[0111] The mesh 32 material has the advantage of having a decomposition temperature that is lower than the final sintering temperature. Thus, at the end of the sintering process, the mesh 32 is decomposed and at most traces of carbon remain in the porous abradable ceramic coating 10 obtained at the end of the sintering heat treatment.

[0112] To obtain the porous abradable coating 10 of the embodiment of FIG. 3, the manufacturing process 10 comprises, after the sintering heat treatment of the green body of the ceramic part, an additional step 112 during which a layer of slurry is applied to the ceramic layer 12 comprising the pores 14.

[0113] The assembly formed by the ceramic layer 12 with the pores 14 and the slurry layer is then subjected to an additional sintering heat treatment 114 to sinter the slurry layer and form the additional ceramic layer 36.

[0114] In combination with the hollow glass beads 16B, the slurry may include a pore-forming agent that makes it possible, during the sintering heat treatment, to create, in the ceramic material, additional porosity to the porosity generated from the hollow glass beads 16B. Additional porosity may thus be created during the sintering heat treatment and thus increase the total porosity of the ceramic layer 12 comprising the pores 14. It is then understood that the glass 16A will not be present in all of the pores 14 of the ceramic layer 12.

[0115] FIG. 7 presents the results of an abrasion test of a porous abradable coating 10 by a titanium-based alloy metal blade performed under standard test conditions. The porous abradable coating 10 was obtained by using an alumina slurry comprising 25% by volume alumina. The alumina slurry comprises water (solvent) and polyvinyl acetate. The hollow glass beads are made of borosilicate glass and have a diameter of about 100 μm. After filtration and discharge of the solvent, the green body of the ceramic part is heat treated with an intermediate stage at 80° C. for at least 2 h to dry the green body of the ceramic part. Then, the heat treatment includes a first sintering stage at a temperature T1 equal to 500° C. for 2 h, which is lower than the glass transition temperature T.sub.g of the hollow glass beads 16B which is about 800° C. The temperature rise to 500° C. is carried out at 15° C./min (degrees Celsius/minute). The heat treatment includes a second sintering stage at a temperature T2 equal to 1050° C. for 8 h. The temperature rise from 500° C. to 1050° C. is carried out at 10° C./min. The ceramic part is then cooled freely. The porosity obtained is about 60% by volume.

[0116] The standard test conditions are as follows: three TA6V blades with a thickness of 0.7 mm were rotated at a circumferential speed of 200 m/s (meters/second) with a penetration speed into the porous abradable coating 10 of 0.15 mm/s (millimeters/second) until a penetration depth into the porous abradable coating 10 that is equal to 1 mm is reached. The measured blade wear is less than 0.01 mm.

[0117] When using hollow beads 16B made of thermoplastic polymer, the manufacturing process described above in that the glass transition temperature T.sub.g of the hollow beads 16B made of glass is replaced by the decomposition temperature T4 of the hollow beads 16B made of thermoplastic polymer.

[0118] Although the present disclosure has been described with reference to a specific example embodiment, it is evident that various modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. Furthermore, individual features of the various embodiments discussed may be combined in additional embodiments. Consequently, the description and drawings should be considered in an illustrative rather than restrictive sense. It will be noted that the sintering heat treatment may include additional temperature stages at temperatures intermediate to temperatures T1, T2 and T3. It may also include temperature stages when cooling the porous abradable coating from the final sintering temperature to room temperature.