Abradable coating made of a material having a low surface roughness

Abstract

The present invention relates to an abradable coating for a turbine engine part, characterized by the fact that said coating includes a layer made of an abradable material, the surface asperities of which are filled with thermally bonded ceramic grains, forming a smooth, free surface having a low roughness.

Claims

1. A method of manufacturing a stator part of a turbine engine, the stator part configured to be outside a rotor part or facing a rotor part, the stator part comprising an internal face that comprises a layer of a MCrAlY abradable coating wherein M is selected from the group consisting of Ni, Co, NiCo, and CoNi, the layer of the MCrAlY alloy being intended to form a seal between the stator part and the rotor part, the method comprising: filling surface irregularities of the layer of the abradable coating with ceramic grains, the ceramic grains being alumina, and heating the layer of the abradable coating to bind the ceramic grains by partial sintering, a free surface of the layer of the abradable coating comprising portions of the MCrAlY alloy and the alumina.

2. The method according to claim 1, wherein a ceramic grain size is between 0.1 and 15 μm.

3. The method according to claim 1, wherein a ceramic grain size is less than 1 μm.

4. The method according to claim 1, wherein the ceramic grains penetrate the layer of the abradable coating over a depth of between 50 and 1000 μm.

5. The method according to claim 1, wherein the surface irregularities are filled by a slip comprising the ceramic grains in suspension in water, the method further comprising removing the excess of slip by scraping the free surface of the layer of the abradable coating.

6. The method according to claim 5, wherein the slip further comprises a dispersant and a binder.

7. The method according to claim 1, wherein the layer of the abradable coating is heated at a temperature of less than 1200° C.

8. The method according to claim 1, wherein the layer of the abradable coating is heated at a temperature of 600° C. for 4 hours.

9. The method according to claim 1, wherein the layer of the abradable coating is heated by applying a laser beam or other localized heating source.

Description

PRESENTATION OF THE FIGURES

(1) Other features and advantages will emerge upon reading the detailed description of a non-limitative embodiment of the invention, with reference to the drawings, in which:

(2) FIG. 1 shows a photographic view in cross section of a coating of the invention with a first magnification;

(3) FIG. 2 shows a detail of the view of FIG. 1 with greater magnification.

DETAILED DESCRIPTION OF THE INVENTION

(4) The following example relates to a high-pressure radial compressor of a gas turbine engine; the cover of the wheel, made of steel such as that known by the designation INCO 909, is covered on its internal face with an abradable coating over a thickness of 1 mm. The coating was produced by the thermal spraying of a CoNiCrAlY alloy associated with a porogenic agent such as an aromatic polyester, sold under the name METCO 2043. The production of the abradable material comprises a step of pyrolysis of the porogenic agent during which the pores are created within the material.

(5) On the surface of the material, the pores form irregularities that have to be filled in. In accordance with the invention, a powder of a ceramic material is deposited on the surface of the abradable material so as to fill in the surface porosity.

(6) Here, the material is alumina, which has the advantage of being stable at the operating temperature of the turbine engine and of being an oxide and therefore not sensitive to oxidation phenomena.

(7) The alumina powder is preferably provided in the form of an aqueous slip, the viscosity parameters and load factor of which, in particular, are determined so as to make it possible to cover the material optimally and to control the penetration depth, between 50 and 1000 μm.

(8) The granulometry of the powder is selected according to the size of the pores to be filled in in the abradable material. The granulometry of the powder is between 0.1 and 15 μm and is for example less than 1 μm.

(9) A ceramic grain size of less than 1 μm makes it possible to effect a grain binding treatment at a temperature compatible with that which can be withstood by the abradable material substrate.

(10) If, however, the maximum temperature acceptable to the substrate is less than that acceptable to the abradable material, then local cooling of the substrate is advantageously established, for example by means of cooling coils.

(11) The procedure for smoothing via the ceramic material is as follows:

(12) Preparation of a slip by mixing the alumina powder at the appropriate granulometry, for example 0.3 μm, with water in the presence of a dispersant such as nitric acid and optionally an organic binder, such as PVA (polyvinyl alcohol);

(13) Deposition of the slip on the surface of the abradable material;

(14) Removal of the excess by scraping the surface; If the porosity is closed then only the surface porosity is filled in, If the porosity is open, the infiltration depth is governed by the number of passes; a penetration depth greater than the maximum touch depth is then aimed for, so as to preserve a low roughness even in the case of touching;

(15) Drying in the open air or in an oven;

(16) Heat treatment at 600° C. for 4 hours so as to eliminate the PVA if necessary and to bind the ceramic grains together. Partial sintering is carried out so as to bind the grains in a microporous alumina phase, which is both neither too fragile so as not to lose its cohesion during operation of the turbine engine before any contact, nor too strong so as to prevent behaviour that is no longer abradable but abrasive.

(17) The solution of the invention made it possible to obtain a coating with a smooth surface of Ra<1 μm. A gain in specific consumption was estimated at 0.4% in relation to such roughness compared with a roughness of 12 μm of the prior art.

(18) In FIGS. 1 and 2, depicting in cross section the coating of the invention, the abradable material CoNiCrAlY can be seen in light print with the filled pores P being in dark print, and the alumina Al.sub.2O.sub.3 also visible on the surface, smoothing the irregularities formed by the surface pores. FIG. 2, produced with greater magnification compared with FIG. 1, shows the cohesion between the alumina grains.

(19) The present invention is not limited to the formation of a smooth abradable coating on a compressor casing but applies to any part when this presents an advantage or interest.

(20) The alumina can be replaced by any other ceramic material in so far as it can be in the form of a sufficiently reactive powder to be able to carry out partial sintering at relatively low temperatures, according to the temperature resistance of the substrate. Oxides are preferred.

(21) The abradable material cited in the example is Metco 2043: the invention is not limited to this. Ni graphite 75/25, Metco 2460, Metco 310 or Metco 314 can also be cited. It should be noted that the materials Ni graphite 75/25, Metco 310 and Metco 314 do not contain any polyester to be pyrolised in order to obtain porosities. The porosities come directly from the process of depositing the abradable material.