U.S. patent application number 12/018539 was filed with the patent office on 2008-07-31 for method for treating a thermally loaded component.
This patent application is currently assigned to ALSTOM Technology Ltd.. Invention is credited to Thomas Duda, Lars Engl.
Application Number | 20080178907 12/018539 |
Document ID | / |
Family ID | 37847035 |
Filed Date | 2008-07-31 |
United States Patent
Application |
20080178907 |
Kind Code |
A1 |
Duda; Thomas ; et
al. |
July 31, 2008 |
METHOD FOR TREATING A THERMALLY LOADED COMPONENT
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.
Inventors: |
Duda; Thomas; (Wettingen,
CH) ; Engl; Lars; (Baden, CH) |
Correspondence
Address: |
DARBY & DARBY P.C.
P.O. BOX 770, Church Street Station
New York
NY
10008-0770
US
|
Assignee: |
ALSTOM Technology Ltd.
Baden
CH
|
Family ID: |
37847035 |
Appl. No.: |
12/018539 |
Filed: |
January 23, 2008 |
Current U.S.
Class: |
134/1.1 ;
134/2 |
Current CPC
Class: |
B24C 1/003 20130101;
F01D 5/286 20130101; F01D 5/005 20130101; B24C 1/086 20130101; B24C
1/10 20130101; F01D 5/288 20130101 |
Class at
Publication: |
134/1.1 ;
134/2 |
International
Class: |
C23G 1/00 20060101
C23G001/00; C25F 5/00 20060101 C25F005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 23, 2007 |
CH |
00103/07 |
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.degree. 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.degree. 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.
Description
[0001] Priority is claimed to Swiss Patent Application No. CH
00103/07, filed Jan. 23, 2007, the entire disclosure of which is
incorporated by reference.
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.degree. C.
[0014] 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.
[0015] During the subsequent dry ice blasting, dry ice grains
consisting of carbon dioxide at a temperature of about -78.degree.
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.
[0016] 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.
[0017] 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.
[0018] A bonding layer can be provided between the substrate and
protective coating; the protective coating can then be removed down
to the bonding layer.
[0019] 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.
[0020] 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
[0021] The invention is subsequently explained in more detail with
reference to exemplary embodiments in conjunction with the
drawings, in which:
[0022] 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
[0023] 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
[0024] 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 (ZrO.sub.27% Y.sub.2O.sub.3 or
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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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)).
[0031] 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.sub.R (FIG. 1(b)), and/or are
mechanically machined by means of a shot-peening process (FIG.
1(c)).
[0032] 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.degree. 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.
[0033] 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.
[0034] 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.degree. 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.
[0035] 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.
[0036] 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.
[0037] A very thin surface layer of the protective coating 12 is
cooled down at high cooling speed to -50.degree. 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.
[0038] 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'.
[0039] 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.
[0040] 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.
* * * * *