Method and device for polishing the surface of a gas turbine blade

Abstract

The invention relates to a method and device for polishing the surface of a gas turbine blade. According to the invention, a metallic anticorrosive layer is polished by using a stream of dry ice whereby preventing the surface from becoming contaminated and obtaining cost advantages.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP02/12520, filed Nov. 8, 2002 and claims the benefit thereof. The International Application claims the benefits of German application No. 01128915.4 EP filed Dec. 5, 2001, both of the applications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention is in the field of gas turbine blades or vanes and involves the application or refurbishment of a protective layer or protective layer system on gas turbine blades or vanes.

BACKGROUND OF THE INVENTION

Protective layer systems on gas turbine blades or vanes are shown, for example, in U.S. Pat. No. 4,321,310, U.S. Pat. No. 4,676,994 or U.S. Pat. No. 5,238,752. Gas turbine blades and vanes are exposed to very high temperatures. To enable them to have the required resistance to high temperatures, therefore, they are made from materials which are able to withstand high temperatures. Nickel- or cobalt-based superalloys are particularly suitable for this purpose. In general, a protective layer or protective layer system to prevent oxidation, corrosion and/or for thermal insulation is also applied to the base body of a gas turbine blade or vane of this type. A metallic alloy of the type MCrAlY, where M represents an element selected from the group consisting of Fe, Co and Ni, Cr is chromium, Al is aluminum and Y represents yttrium or a rare earth element, is particularly suitable for this purpose. A ceramic thermal barrier coating is often applied to a corrosion-resistant layer of this type. This thermal barrier coating is highly thermally stable and is used to shield the metallic base body from direct contact with the hot gas. A typical material for a ceramic thermal barrier coating is yttrium-stabilized zirconium dioxide applied, for example, by atmospheric plasma spraying (APS).

A corrosion-resistant layer of this type is often applied by means of a plasma spraying process. Consequently, it has a relatively high roughness, which is undesirable. It is therefore necessary to smooth the metallic protective layer. This requires either a time-consuming grinding process, in which the gas turbine blade or vane is moved for a prolonged period of time in a trough containing abrasive bodies until the desired roughness has been set, or requires the use of a sand-blasting process, in which solid particles, in particular corundum grains, are deflected from a jet onto the surface and in the process smooth the surface. The grinding process is not only time-consuming but also entails the risk of prior uncontrolled removal of material at exposed edges. The known sand-blasting processes in particular have the drawback that solid particles of the jet may become included in the protective layer, thereby unacceptably reducing the surface quality.

U.S. Pat. No. 5,645,893 relates to a coated component having a base body made from a superalloy and having a bond coat and a thermal barrier coating. The bond coat includes a platinum aluminide and a thin oxide layer adjoining it. The thin oxide layer contains aluminum oxide. This oxide layer is adjoined by the thermal barrier coating, which is applied by means of the electron beam PVD process. Yttrium-stabilized zirconium dioxide is applied to the bond coat. Before the bond coat is applied, the surface of the base body is cleaned by means of a coarse sand-blasting process. Alumina sand is used for the material-removing processing of the base body.

A further measure aimed at making a gas turbine blade or vane suitable for use at very high temperatures is cooling by means of cooling air. This air is passed into an internal cavity in the gas turbine blade or vane, from where it takes up heat. This cooling air is generally at least partially guided out of cool-air bores onto the surface of the gas turbine blade or vane, where it forms a protective film. The cooling-air bores are electrochemically eroded or drilled by means of a laser beam in the region close to the surface, as demonstrated in U.S. Pat. No. 5,609,779. Drilling by means of a laser beam offers time and cost benefits but often leads to the formation of burrs and contaminations formed from fused-on or remelted particles on account of the action of heat. These burrs have to be removed if only on account of the need to maintain an accurate geometry of the film-cooling bores, which has hitherto required a complex, manually operated process.

U.S. Pat. No. 3,676,963 has described a process for removing undesirable regions of thermoplastic or elastic materials in particular in inner, relatively inaccessible regions by means of an ice jet. A corresponding application by means of dry ice, i.e. solid CO₂, is disclosed by U.S. Pat. No. 3,702,519. The undesired regions are removed by supercooling, resulting in embrittlement of the plastic regions, by cold CO₂ particles, and these regions are then removed by further particles.

DE-A 205 87 66 shows a process for cleaning metallic, radioactively contaminated surfaces by means of an ice jet. The use of dry ice is also proposed for readily soluble deposits on the surface.

U.S. Pat. No. 4,038,786 and the corresponding DE-A 254 30 19 disclose a device which can be used to generate a dry ice jet with a favorable particle size and shape and without aggregation of the particles.

DE-C 196 36 305 shows a process for eliminating coatings and coverings from a sensitive substrate. These coverings include soot, moss, pollutant deposits and high-viscosity non-impact-resistant coatings on substrates such as wood, plastic foams or sandstone.

Gentle removal of the coverings or coatings from the sensitive substrates is possible by means of a dry ice jet.

Comparable applications for dry ice blasting, for example for removal of silicone seals or paints from, for example, plastic moldings or other shape-critical base bodies are described in the following articles:

“Trockeneis-Strahlreinigen” [Dry ice blast cleaning], A. Buinger, Kunststoffe 86 (1996) 1, p.58; “CO2 blast cleaning”, Ken Lay, Rubber Technology International '96, pp. 268-270; “Reinigen mit Trockeneisstrahlen in der Austauschmotorenfertigung” [Cleaning by dry ice blasting in reconditioned engine manufacturing], Eckart Uhlmann, Bernhard Axmann, Felix Elbing, VDI-Z 140 (1998) 9, pp.70-72; “Dry-ice blasting for cleaning: process, optimization and application”, G. Spur, E. Uhlmann, F. Elbing, Wear 233-235 (1999) pp. 402-411; “Stoβkraftmessung beim Strahlen mit CO₂-Pellets” [Impact force measurement during blasting with CO₂ pellets], Eckart Uhlmann, Bernhard Axmann, Felix Elbing, ZWF 93 (1998) 6 pp.240-243.

SUMMARY OF THE INVENTION

The invention provides a process for removing ceramic material from the surface of a gas turbine blade or vane in accordance with patent claim 1, the gas turbine blade or vane having a metallic high-temperature corrosion-resistant layer, to which a ceramic thermal barrier coating has been applied. In the process, a dry ice jet of dry ice particles is passed over the surface, so that material is removed from the ceramic thermal barrier coating by the action of the impinging dry ice particles.

The invention is based on the surprising discovery that the dry ice blasting is suitable for removing a ceramic thermal barrier coating.

Previous applications of dry ice blasting have been based to a considerable extent on a thermal-mechanical action on a relatively soft coating. A covering or coating of this type is removed in pieces by flaking off as a result of cold embrittlement and subsequent kinetic action. By contrast, a ceramic thermal barrier coating consists of a hard, robust material. Furthermore, a ceramic thermal barrier coating is specifically designed to withstand fluctuating temperatures and thermal stresses. For this purpose, it is customary to build up a columnar structure which allows thermal transverse stresses to be compensated for. The ceramic thermal barrier coating should therefore in actual fact be insensitive in particular to attempts to remove it by thermo-mechanical means.

Removal by means of dry ice avoids contamination by foreign substances. Moreover, the metallic base body of the gas turbine blade or vane is not adversely affected, and moreover no material whatsoever is removed from the high-temperature corrosion-resistant layer.

Therefore, the surface of the gas turbine blade or vane can be smoothed or deburred easily and with a high quality, or may also have the entire ceramic thermal barrier coating removed from it.

Smoothing by means of a dry ice jet on the one hand provides the time and cost benefit of a sand-blasting process over, for example, a grinding process. On the other hand, however, the main drawback of a conventional sand-blasting process, namely contamination of the blade or vane surface with particles of the sand jet, is avoided. The dry ice particles sublime immediately and without leaving residues on contact, with the result that there is no accumulation or chemical reaction of any type. Furthermore, no blasting residues remain to be disposed of.

The ceramic thermal barrier coating preferably includes zirconium dioxide, and even more preferably it is formed entirely from yttrium-stabilized zirconium dioxide.

The process according to the invention may in particular also be employed for refurbishment of a layer system of a gas turbine blade or vane. For this purpose, the entire old ceramic thermal barrier coating is removed.

It is preferable for the smoothing to set a predetermined maximum roughness of the surface. The maximum roughness of a roughness average is preferably less than 30 μm, more preferably less than 15 μm.

As has been stated above, cooling-air bores often open out at the surface. For production reasons, burrs often remain in the region where these bores open out, these burrs in particular comprising remelted material if the bores have been drilled by means of a laser beam. It is preferable for these burrs to be removed by means of the dry ice jet.

It is preferable for the smoothing to be carried out in fully automated fashion. A multiaxis holder for the gas turbine blade or vane or multiaxis guidance of the dry ice jet makes it possible to reach any region of the surface which is to be smoothed.

It is preferable for the dry ice jet to leave a nozzle at a pressure of from 10 bar to 30 bar.

The embodiments described under sections A) to F) may also be combined with one another.

The invention also provides an apparatus for removing ceramic material of a ceramic thermal barrier coating on the surface of a gas turbine blade or vane, having a holder for the gas turbine blade or vane and having a nozzle for emitting a dry ice jet, and having a multi-axis manipulation device for moving the dry ice jet relative to the gas turbine blade or vane in such a manner that completely automated removal of the ceramic material of the gas turbine blade or vane becomes possible.

It is preferable for the dry ice jet also to be used to clean the gas turbine blade or vane. Cleaning of this type is to be carried out in particular prior to coating of a gas turbine blade or vane. Any impurity may have an adverse effect on the bonding of the coating to be applied. With conventional cleaning processes, there is a risk of foreign material being included in the surface to be cleaned as a result of the cleaning agents used, for example sand in the case of sand-blasting. This risk is avoided by the use of a dry ice jet, since the dry ice sublimes without leaving any residues.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with reference to an exemplary embodiment, in which, in some cases diagrammatically and not to scale:

FIG. 1 shows an apparatus for removing ceramic material from a gas turbine blade or vane,

FIG. 2 shows a longitudinal section through a surface region of a gas turbine blade or vane having a protective layer system, and

FIG. 3 shows a process for removing ceramic material from a gas turbine blade or vane.

DETAILED DESCRIPTION OF INVENTION

Identical reference numerals have the same meaning throughout the figures.

FIG. 1 shows an apparatus 15 for removing ceramic material from the surface of a gas turbine blade or vane 20. To produce a compressed-air flow, a screw compressor 1, a compensation vessel 2, an adsorption dryer 3, a cooler 4 and a measurement system 5 are connected in series. Air is highly compressed, in particular to a pressure of from 3 to 12 bar, in the screw compressor 1. The compensation vessel 2 is used to stabilize a constant mass flow. In the adsorption dryer 3, the air is dried, and then cooled in the cooler 4. A measurement system 5 is used to record the compressed-air parameters.

The compressed-air stream is then fed to a pellet supply device 12, in which dry ice pellets 6 are stored. The dry ice pellets 6 are fed to the compressed-air stream by means of a screw conveyor 7 via a star feeder 8 and are fed with the compressed-air stream to a Laval nozzle 10 which can be moved by means of a robot 9. They emerge from the nozzle as a dry ice jet 14 at virtually the speed of sound and strike a gas turbine blade or vane 20. The gas turbine blade or vane 20 is mounted in a multi-axis holder 22, so that the dry ice jet 14 can be guided onto any point on the surface of the gas turbine blade or vane 20.

FIG. 2 shows a longitudinal section through a surface region of a gas turbine blade or vane 20. An MCrAlY corrosion-resistant layer 32 has been applied to a base body 30 made from a nickel- or cobalt-base superalloy. A thin bond coat 34 has been formed on the corrosion-resistant layer 32. The bond coat 34 improves the bonding of a ceramic thermal barrier coating 36 of yttrium-stabilized zirconium dioxide applied to the corrosion-resistant layer 32.

A cooling passage 38 leads from an internal cavity (not shown) in the gas turbine blade or vane 20 to the surface 24 of the gas turbine blade or vane 20. A trapezoidal opening region 40 is formed in the vicinity of the surface. The opening region 40 is drilled by means of a laser beam, which leads to partial melting of material. As a result, a burr 42 is formed and has to be removed. This is done by means of the dry ice jet 14.

In the case of refurbishment of a gas turbine blade or vane 20 which has already undergone prolonged use, the ceramic thermal barrier coating 36 is replaced. For this purpose, the ceramic thermal barrier coating 36 or residues thereof is completely removed by means of the dry ice jet 14.

However, depending on the setting of the dry ice jet hardness, partial removal is also possible, so that the surface 24 is smoothed to a predetermined roughness by the dry ice jet 14.

Claims

1-10. (canceled)

11. A process for removing ceramic material from the surface of a gas turbine component, comprising:

compressing air;

drying the compressed air;

cooling the compressed air;

feeding the dried cooled compressed air to a dry ice supply device;

feeding dry ice to the dry ice supply device;

releasing a mixture of the compressed air and the dry ice towards the surface to remove from the ceramic material from the surface by the impinging dry ice particles.

12. The process as claimed in claim 11, wherein the dry ice particles are fed to the compressed air stream by a star feeder.

13. The process as claimed in claim 11, wherein the air is compressed to a pressure level of 3 to 12 bar.

14. The process as claimed in claim 11, wherein a compensation vessel is used to stabilize a constant mass flow.

15. The process as claimed in claim 11, wherein the ceramic material comprises zirconium dioxide.

16. The process as claimed in claim 11, wherein the surface is smoothed by the removal of the ceramic material.

17. The process as claimed in claim 16, wherein a predetermined maximum roughness of the surface is set by the removal of the ceramic material.

18. The process as claimed in claim 11, wherein a cooling passage that extends through the surface having a production-related burr formed at least in part in an opening region of the cooling passage is removed by the dry ice particles.

19. The process as claimed in claim 18, wherein the cooling passage is formed by a laser beam.

20. The process as claimed in claim 17, wherein a multi-axis manipulation device for moving the dry ice jet relative to the gas turbine blade or vane is used to remove the ceramic material and achieve the predetermined maximum roughness.

21. The process as claimed in claim 11, wherein the dry ice jet exits a nozzle at a pressure of from 10 bar to 30 bar.

22. The process as claimed in claim 11, wherein the ceramic material is completely removed by the dry ice particles.

23. The process as claimed in claim 11, wherein the dry ice is provided in pellet form.

24. The process as claimed in claim 11, wherein the mixture of compressed air and dry ice pellets is released towards the surface at near the speed of sound.

25. The process as claimed in claim 11, wherein the mixture of compressed air and dry ice is passed over the surface a plurality of times.

26. An apparatus for removing ceramic material from the surface of a gas turbine component, comprising:

a gas turbine component holder;

a compressor for compressing air;

a dryer for drying the compressed air;

a cooler for cooling the compressed air;

a dry ice supply device to receive the dried cooled compressed air;

a dry ice emitter for emitting a stream of dry ice within the compressed air; and

a multi-axis manipulation device for moving the dry ice jet relative to the gas turbine component.

27. The apparatus as claimed in claim 26, wherein the gas turbine component is a blade or vane.

28. The apparatus as claimed in claim 26, wherein the air is compressed to a pressure level from 3 to 12 bar.

29. The apparatus as claimed in claim 26, wherein the surface is smoothed by the removal of the ceramic material.

30. The apparatus as claimed in claim 29, wherein a predetermined maximum roughness of the surface is set by the removal of the ceramic material.

31. The apparatus as claimed in claim 26, wherein a cooling passage that extends through the surface having a production-related burr is formed at least in part in an opening region of the cooling passage is removed by the dry ice jet.

32. The apparatus as claimed in claim 31, wherein the cooling passage is formed by a laser beam.

33. The apparatus as claimed in claim 30, wherein the multi-axis manipulation device removes the ceramic material from the surface and achieves the predetermined maximum roughness.

34. The apparatus as claimed in claim 26, wherein the dry ice jet exits the supply device with an initial pressure of from 10 bar to 30 bar.

35. The apparatus as claimed in claim 26, wherein the ceramic material is completely removed by the dry ice jet.

36. The apparatus as claimed in claim 26, wherein the dry ice is provided in pellet supply form.

37. The apparatus as claimed in claim 26, wherein the mixture of compressed air and dry ice pellets is released towards the surface at near the speed of sound.

38. The apparatus as claimed in claim 26, wherein the mixture of compressed air and dry ice pellets is passed over the surface a plurality of times.