US20080178907A1

US20080178907A1 - Method for treating a thermally loaded component

Info

Publication number: US20080178907A1
Application number: US12/018,539
Authority: US
Inventors: Thomas Duda; Lars Engl
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2007-01-23
Filing date: 2008-01-23
Publication date: 2008-07-31
Also published as: GB0801245D0; DE102008004559A1; GB2446056A; GB2446056B; DE102008004559B4

Abstract

A method for treating a thermally loaded component having a metallic substrate and at least partially coated on an outer side with a protective coating, is provided. The method includes the step of predamaging the protective coating before removing the protective coating from the substrate using dry ice blasting. The predamaging is performed so as to lead to an increase of efficiency of the removal process.

Description

Priority is claimed to Swiss Patent Application No. CH 00103/07, filed Jan. 23, 2007, the entire disclosure of which is incorporated by reference.
The present invention relates generally to the field of thermal machine, and more particularly to a method for treating a thermally loaded component having a metallic substrate and a protective coating on at least a portion of an outer side.

BACKGROUND

Thermally loaded components of gas turbines, such as stator blades or rotor blades or elements of the associated combustion chamber, which on account of the high operating temperatures are produced from Ni-based alloys, are frequently coated on the surface with a ceramic thermal barrier coating (TBC). When applying these thermal barrier coatings, local defects can occur, which are not acceptable with regard to later application, or which have to be removed for other reasons. If such local defects occur, the whole thermal barrier coating, inclusive of the bonding layer which lies beneath it, currently has to be removed by means of a chemical and/or combined chemical/mechanical removal process. This chemical process is very burdensome and costly, and requires greater finish-machining. Also, it is not possible to selectively remove only areas of the protective coating in order to finish used components, for example, for renewed application.
It would be desirable to have available a less costly and locally applicable process by which only the protective coating or thermal barrier coating of such a component can be purposefully removed.
From the prior art, different methods for local removal of ceramic coatings are already known (see, for example, the publications US-A1-2005/0126001, US-A1-2004/0244910, WO-A1-02/103088, WO-A1-2005/083158, DE-A1-10 2004 009 757, US-A1-2004/0115447, US-A1-2004/0256504, US-A1-2003/0100242 and DE-B4-103 60 063). Other methods for local repair of coating systems are known from the publications US-A1-2002/0164417, DE-T2-601, 03 612, US-A1-2003/0101687, EP-A1-1 304 446, EP-A1-0 808 913 and US-B1-6, 235, 352.
The complete removal of thermal barrier coatings by means of chemical methods alone, or in combination with other methods, have been handled in a different way in the publications DE-A1-10 2004 049 825, US-A1-2001/0009246, US-A1-2001/0009247 and EP-B1-1 076 114.
Furthermore, it is known (Fr.-W. Bach et al., “Abtragen von thermisch gespritzten Schichten mit dem Trockeneis-Laserstrahl”, GTS-Strahl Vol. 14, September 2004; Fr.-W. Bach et al., “Dry ice blasting and water jet processes for the removal of thermal sprayed coatings”, Conf. Proc. ITSC 2005, Basle, p. 1542-1548 (2005)) to remove protective coatings, such as thermal barrier coatings, which are on components, by means of a dry ice blasting process.
With this type of coating removal, however, the complete removal of the coating on the one hand, and the least possible influencing of the metallic coatings (either the metal substrate as such, or the metallic bonding layer or adhesion mediating coating which lies above it) which lies beneath the coating which is to be removed on the other hand, is problematic. Previous processes suffer from low efficiency or very long treatment durations.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for removing such protective coatings or thermal barrier coatings, which utilize the advantages of the dry ice blasting process without being encumbered with its disadvantages.
The present invention provides a method for treating a thermally loaded component having a metallic substrate areally coated with a protective coating on a at least a portion of an outer side. The method includes removing the protective coating from the substrate using a dry ice blasting process, wherein, before the application of the dry ice blasting process the protective coating is first of all predamaged in a first step in such a way that the predamaging leads to an increase of the efficiency of the removal process.
The invention thus provides a two-stage treatment process, in which before the application of the dry ice blasting process the protective coating is first of all purposefully predamaged in a first step. By means of the purposeful predamaging of the coating which is to be removed, this can be more effectively, i.e. in shorter time, completely removed in the subsequent dry ice blasting process and without greater influence upon possible metal coatings which lie beneath it.
A development of the invention is characterized in that cracks are created in the protective coating for predamaging the protective coating, or the component is locally heated, or heated as a whole, to a temperature far above room temperature.
For this purpose, the component can be locally heated, or heated as a whole, by means of a burner, a plasma jet or a laser jet. Furthermore, heating of the whole component, for example in an oven, is possible. If the component has a substrate consisting of an Ni-based or Co-based alloy, it can be heated up as a whole to a temperature of up to 600° C.
Alternatively or additionally to heating up, the protective coating, however, can also be shot-peened for creating cracks or creating predamage, wherein steel balls, with a diameter of between 0.5 and 5 mm, are preferably used, and for blast formation the steel balls are introduced into a high-speed gas flow, especially consisting of compressed air. The use of ceramic balls is also conceivable.
During the subsequent dry ice blasting, dry ice grains consisting of carbon dioxide at a temperature of about −78° C. are preferably used. In particular, the dry ice grains consist of compressed carbon dioxide snow and have a diameter of between 1 and 3.5 mm and a length of between 2 and 10 mm.
It is especially favorable if the dry ice grains are accelerated to speeds of about 300 m/s in a compressed air flow, before striking the predamaged protective coating.
According to another development of the invention, there is a thermally grown oxide coating directly beneath the protective coating, and the thermally grown oxide coating is removed together with the protective coating.
A bonding layer can be provided between the substrate and protective coating; the protective coating can then be removed down to the bonding layer.
The component to be treated preferably has a substrate consisting of an Ni-based alloy, and the protective coating is a ceramic thermal barrier coating, especially consisting of yttrium-stabilized zirconium oxide.
The component can preferably be a stator blade or rotor blade of a gas turbine, or a protective segment of the combustion chamber of an internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is subsequently explained in more detail with reference to exemplary embodiments in conjunction with the drawings, in which:

FIG. 1 shows in a plurality of sub-FIGS. 1( a) to 1(f) different steps during the removal of a protective coating, which is directly applied to a substrate, according to an exemplary embodiment of the method according to the invention, wherein on the left, a (new) component without thermally grown oxide coating, and on the right, a (used) component with thermally grown oxide coating, is shown in each case; and

FIG. 2 shows in a plurality of sub-FIGS. 2( a) to 2(f) different steps during the removal of a protective coating, which is applied to a substrate via an adhesion mediating intermediate coating (bonding layer), according to another exemplary embodiment of the method according to the invention, wherein on the left, a (new) component without thermally grown oxide coating, and on the right, a (used or preoxidized) component with thermally grown oxide coating, is shown in each case.

DETAILED DESCRIPTION

The present invention refers to a method for removing a brittle coating system (protective coating 12 in FIG. 1( a) or FIG. 2( a)) from a metallic surface (a substrate 11 in FIG. 1( a) or 2(a)). The coating system which is to be removed is typically a ceramic system, i.e. a thermal barrier coating such as yttrium-stabilized zirconium oxide (ZrO₂7% Y₂O₃or the like) or comprises a coating consisting of the system La-oxide, or ZrO with additives from the range of rare earths (Er, Yb) and P and Gd. The thermal barrier coating can be applied by means of thermal spraying processes, which use powder or solutions/suspensions as a carrier of the coating material, and also can also be applied by means of other physical processes like EB-PVD.
In the case of used components, which were already subjected to a longer thermal loading in the presence of oxygen, a thermally grown oxide coating (13 in FIG. 1( a) or 2(a), right-hand column), may have been formed beneath the upper protective coating 12. Such an oxide coating (TGO) can also be established by means of a purposeful preoxidation. This thermally grown oxide coating 13 can also be removed within the scope of the removal process according to the invention. The removing of the coating 12, or coatings 12 and 13, is carried out from a metallic surface of a substrate 11 (FIG. 1) which lies beneath it, which, if the component is a stator blade or rotor blade of a gas turbine, customarily consists of an Ni-based or Co-based alloy, without the surface of the substrate 11 being damaged.
The coating system which is on the substrate 11, however, can also comprise at least one intermediate coating in the form of a bonding layer 22 (FIG. 2) beneath the protective coating 12. In this case, the upper protective coating 12 is removed by the method according to the invention without the bonding layer 22 which lies beneath it being damaged.
The advantage of the method according to the invention is that the brittle protective coating 12 can be completely removed without it happening to the metallic coating which lies beneath it, whether it be the substrate 11 itself (FIG. 1) or an interposed bonding layer 22 (FIG. 2). That means that after removing the protective coating 12, the metallic surface which lies beneath it is in an activated and cleaned state, wherein the surface roughness is comparable to that before removing the protective coating 12. In this way, the metallic surface can be recoated with a bonding adhesive which is comparable to the original bonding adhesive of the protective coating 12.
A two-stage process is essential for the method according to the invention: in a first step, the coating which is to be removed is predamaged by means of a suitable pretreatment. The predamaging achieves the effect of the coating then being able to be more easily and completely removed in a second step. The predamaging of the protective coating can be carried out either by means of a heat treatment of the component, or by means of a mechanical action on the coating by means of a shot-peening process, or by means of a combination of the two methods. For the actual removal of the (predamaged) coating, a dry ice blasting process is used, in which a jet of greatly accelerated dry ice grains is directed onto the coating surface, and there, acts on the one hand mechanically and on the other hand thermally (by means of shock cooling) on the coating.
The two-stage process is necessary because just by the dry ice blasting process, i.e. without predamaging the coating, residues of the coating remain on the component which then hinders finishing of the component. The predamaging of the coating, therefore, effectively prevents coating residues remaining on the component surface and increases the efficiency or reduces the process duration.
According to FIG. 1( a), the method starts from a new component 10 which is provided with a protective coating 12, especially in the form of a ceramic thermal barrier coating (TBC), on a metallic substrate 11 (left-hand side of FIG. 1( a)). However, it can also start from a component 10′ which in use has already been subjected to a thermal loading in the presence of oxygen, and, therefore, a thermally grown oxide coating (TGO) 13 has formed on the interface between substrate 11 and the protective coating 12 (right-hand side of FIG. 1( a)).
For predamaging the protective coating 12, the components 10, 10′ are now subjected to a thermal treatment at temperatures T far above room temperature T_R(FIG. 1( b)), and/or are mechanically machined by means of a shot-peening process (FIG. 1( c)).
The thermal treatment (for the component 10, 10′ as a whole) can be undertaken in an oven, or by means of local heating with a burner, a plasma jet or a laser jet. If the substrate 11 is produced from an Ni-based alloy, the thermal treatment can be carried out at temperatures of up to 600° C. On account of the different thermal expansion coefficients of the substrate material and of the protective coating material, according to FIG. 1( d) the formation of cracks 17 in the protective coating 12, and therefore an effective predamaging of the protective coating 12, occurs. The heating up leads to an increase of the temperature difference between component and dry ice, which leads to an increased removal rate.
For implementing the shot-peening process, according to FIG. 1( c) steel balls (or ceramic balls) 16, with a diameter of between 0.5 and 5 mm, are introduced into a high-speed gas jet (for example compressed air) and are shot through the nozzle 15 of a corresponding shot-peening device 14 onto the surface of the protective coating 12 which is to be removed. The impact of the balls 16 on the surface creates cracks 17 and partially even creates a delamination of the protective coating 12, and so leads to an effective predamaging.
After the predamaging of the protective coating 12 which is created in this way in the first step, the predamaged protective coating 12 is completely removed in the second step (FIG. 1( e)) by using a dry ice blasting device 18. The dry ice blasting process uses dry ice grains 21 consisting of solid carbon dioxide at temperatures of about −78° C. The dry ice grains consist of compressed carbon dioxide snow and have a diameter of between 1 and 3.5 mm and a length of between 2 and 10 mm. The dry ice grains 21 are accelerated in a compressed air flow to speeds of about 300 m/s and are then shot through a nozzle 19 onto the surface of the predamaged protective coating 12. Typical pressures of the compressed air which is used are between 0.1 and 1.6 MPa in this case. Depending upon application, round or flat nozzles 19 can be used in this case.
The dry ice blasting process acts upon the protective coating 12 by means of a combination of mechanical and thermal mechanisms: the kinetic energy of the blast medium, on account of shock waves on the interface between protective coating 12 and substrate 11 creates further cracks and removes particles of the protective coating 12 which are already blasted off, or which have only poor adhesion to the coating.
During impact of the dry ice grains 21, some of the kinetic and thermal energy is converted into sublimation energy. The volume of the solid carbon dioxide grows by a factor of up to 700. Large pressure gradients on the surface of the protective coating 12 result from it. The gaseous carbon dioxide can be drawn off together with the blasted off particles of the coating and, after filtering, can be let out into the environment without producing further waste.
A very thin surface layer of the protective coating 12 is cooled down at high cooling speed to −50° C., wherein thermal stresses are created, which, in the case of large differences in the thermal expansion coefficients between substrate 11 and protective coating 12, are particularly large. These thermal stresses also lead to cracks 17 in the protective coating 12, weaken these further, and promote delamination. Since dry ice, comparable with gypsum, is rather soft, there is practically no abrasive effect. Consequently, the protective coating 12 can be removed without the metallic coating which lies beneath it being damaged, even if it consists of mild steel or high-grade steel, nickel alloys or even aluminum alloys.
By means of the dry ice blasting process, the protective coating is completely removed in the case of a new component 10, and the thermally grown oxide coating 13 (FIG. 1( f) which lies beneath it is additionally also removed in the case of an already used component 10′.
With thermally loaded components, a metallic bonding layer 22 (for example MCrAlY), or an aluminum layer or PtAl layer (FIG. 2( a)), in most cases is arranged between the protective coating 12 and the substrate 11 for better bonding adhesion of the protective coating 12. If it concerns a new component 20, the protective coating 12 and the bonding layer directly adjoin each other (left-hand section of FIG. 2( a)). If the component 20′ is used, a thermally grown oxide coating 13 (right-hand side of FIG. 2( a)) has been formed in turn on the interface between the two coatings.
With this type of coating, exactly the same procedure is also followed as with components without a bonding layer. FIGS. 2( a) to 2(f), therefore, show the same steps and process parameters similar to FIGS. 1( a) to 1(f). In the end (FIG. 2( f)), the protective coating 12 and a possible thermally grown oxide coating 13 are completely removed from the component 20, 20′ so that the bonding layer 22 which lies beneath it is extensively revealed.

Claims

1. A method for treating a thermally loaded component having a metallic substrate and at least partially coated on an outer side with a protective coating, the method comprising:

predamaging the protective coating; and

removing the protective coating from the substrate using dry ice blasting, wherein the predamaging is performed so as to lead to an increase of efficiency of the removal process.

2. The method as recited in claim 1, wherein the predamaging includes creating cracks in the protective coating.

3. The method as recited in claim 1, further comprising heating the component at least locally to a raised temperature substantially greater than room temperature.

4. The method as recited in claim 3, wherein the heating is performed using at least one of a burner, a plasma jet and a laser jet.

5. The method as recited in claim 3, wherein the substrate includes at least one of an Ni-based and a Co-based alloy, and wherein the raised temperature is less than or equal to 600° C.

6. The method as recited in claim 2, wherein the predamaging includes shot-peening the protective coating so as to create the cracks.

7. The method as recited in claim 6, wherein the shot-peening is performed using steel balls having a diameter between 0.5 and 5 mm.

8. The method as recited in claim 7, wherein the shot-peening includes introducing the steel balls into a high-speed flow of a gas for blast formation.

9. The method as recited in claim 8, wherein the gas includes compressed air.

10. The method as recited in claim 1, wherein the dry ice blasting is performed using dry ice grains including carbon dioxide having a temperature of about −78° C.

11. The method as recited in claim 10, wherein the dry ice grains include compressed carbon dioxide snow and have a diameter of between 1 and 3.5 mm and a length of between 2 and 10 mm.

12. The method as recited in claim 11, wherein the dry ice blasting includes accelerating the dry ice grains in a compressed air flow to speeds of about 300 m/s before striking the predamaged protective coating.

13. The method as recited in claim 1, wherein the component has a thermally grown oxide coating directly beneath the protective coating, and wherein the removing includes removing the thermally grown oxide coating together with the protective coating.

14. The method as recited in claim 1, wherein the component has a bonding layer disposed between the substrate and the protective coating, and wherein the removing includes removing the protective coating down to the bonding layer.

15. The method as recited in claim 1, wherein the includes at least one of a Ni-based and a Co-based alloy, and the protective coating is a ceramic thermal barrier coating.

16. The method as recited in claim 15, wherein the thermal barrier coating includes yttrium-stabilized zirconium oxide.

17. The method as recited in claim 1, wherein the component is at least one of a stator blade and a rotor blade of a gas turbine.

18. The method as recited in claim 1, wherein the component is a burner component.

19. The method as recited in claim 18, wherein the burner component includes a protective segment of a combustion chamber of an internal combustion engine.