U.S. patent application number 13/253454 was filed with the patent office on 2012-04-12 for method of coating a surface.
Invention is credited to Malko Gindrat, Richard K. Schmid, Sascha Usai, Konstantin von Niessen.
Application Number | 20120087802 13/253454 |
Document ID | / |
Family ID | 43431975 |
Filed Date | 2012-04-12 |
United States Patent
Application |
20120087802 |
Kind Code |
A1 |
von Niessen; Konstantin ; et
al. |
April 12, 2012 |
METHOD OF COATING A SURFACE
Abstract
A method is proposed of coating a surface of a body which has at
least one throughgoing bore having an inlet and having an outlet,
wherein the outlet is provided in the surface to be coated, in
which method the coating takes place by means of thermal spraying,
wherein, during the thermal spraying, the bore is flowed through by
a fluid which flows out through the outlet of the bore and
substantially prevents a constriction of the bore caused by the
coating.
Inventors: |
von Niessen; Konstantin;
(Buttwil, CH) ; Schmid; Richard K.; (Gerlikon,
CH) ; Gindrat; Malko; (Wohlen, CH) ; Usai;
Sascha; (Bremgarten, CH) |
Family ID: |
43431975 |
Appl. No.: |
13/253454 |
Filed: |
October 5, 2011 |
Current U.S.
Class: |
416/241R ;
427/446 |
Current CPC
Class: |
C23C 24/04 20130101;
Y02T 50/67 20130101; C23C 4/134 20160101; Y02T 50/60 20130101; F01D
5/288 20130101; F05D 2260/202 20130101; Y02T 50/676 20130101; C23C
4/12 20130101; F05D 2230/312 20130101; C23C 4/137 20160101; C23C
4/01 20160101 |
Class at
Publication: |
416/241.R ;
427/446 |
International
Class: |
F01D 5/28 20060101
F01D005/28; C23C 4/12 20060101 C23C004/12; F01D 5/14 20060101
F01D005/14 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 7, 2010 |
EP |
10186831.3 |
Claims
1. A method of coating a surface of a body which has at least one
throughgoing bore having an inlet and having an outlet, wherein the
outlet is provided in the surface to be coated, in which method the
coating takes place by means of thermal spraying, characterized in
that, during the thermal spraying, the bore is flowed through by a
fluid which flows out through the outlet of the bore and
substantially prevents a constriction of the bore caused by the
coating.
2. A method in accordance with claim 1, wherein the fluid is a
gas.
3. A method in accordance with claim 1, wherein the flow rate of
the fluid amounts at most to 5 SLPM.
4. A method in accordance with claim 1, wherein the thermal
spraying is a plasma spray process.
5. A method in accordance with claim 1, wherein the thermal
spraying is a plasma spray process which is carried out at a
process pressure which amounts to less than 10,000 Pa.
6. A method in accordance with claim 5, wherein a value between 50
and 2000 Pa is selected for the process pressure.
7. A method in accordance with claim 5, wherein a starting material
for the coating is sprayed onto the body in the form of a process
jet, wherein the starting material is injected into a plasma
defocusing the process jet and there at least one of partly and
completely changes into a liquid phase, and wherein the coating
takes place by deposition from the liquid phase.
8. A method in accordance with claim 4, wherein a starting material
for the coating is sprayed onto the body in the form of a process
jet, wherein the starting material is injected into a plasma
defocusing the process jet and there at least partly changes into a
gaseous phase, and wherein the coating takes place by deposition
from the gaseous phase.
9. A method in accordance with claim 1, wherein the body is a
turbine vane and the at least one bore is a cooling air bore.
10. A method in accordance with claim 9, wherein the turbine vane
has a foot which is connected to a source for the fluid during the
coating so that the fluid can flow from the foot of the turbine
vane through the at least one cooling air bore to the outlet of the
cooling air bore.
11. A method in accordance with claim 10, wherein the turbine vane
has a plurality of cooling air bores which are flowed through by
the fluid.
12. A turbine vane coated in accordance with a method in accordance
claim 1.
13. A turbine vane in accordance with claim 9, wherein the cooling
air bore opens obliquely into the surface to be coated.
14. A turbine vane in accordance with claim 9, wherein the
thickness of the coating increases as the distance from the outlet
of the bore increases until said thickness has reached its maximum
value (d).
15. A method in accordance with claim 1, wherein the fluid is an
inert gas.
16. A method in accordance with claim 15, wherein the inert gas is
argon.
17. A method in accordance with claim 1, wherein the flow rate of
the fluid amounts at most to 1 SLPM.
18. A method in accordance with claim 5, wherein a value between
100 and 800 Pa is selected for the process pressure.
Description
[0001] The invention relates to a method of coating a surface of a
body in accordance with the preamble of the independent method
claim. The invention further relates to a turbine vane which is
coated in accordance with such a method.
[0002] It is a usual measure today with turbine vanes, in
particular with turbine vanes of gas turbines which are subject to
very high thermal or corrosive stress such as are used as
land-based industrial turbines or as aircraft engines, to provide
the turbine vane with cooling air bores which extend through the
vane and open into the outer surface of the turbine vane. During
the operation of the turbine, a cooling medium, usually air, is
then conducted through the cooling air bores and then exits at the
surface of the turbine vane and provides a cooling. In this
respect, a so-called film cooling is also frequently implemented,
i.e. the fluid exiting from all the cooling air bores forms a
protective gas film or vapor film on the surface of the turbine
vane which considerably reduces the thermal strain on the vane.
[0003] Problems in particular occur when such components already
provided with cooling air bores should be coated by means of a
thermal spray process--either on repair or maintenance or on new
manufacture. Such coatings are frequently applied to the turbine
vanes as thermal protective layers or as protection against (hot)
corrosion.
[0004] In thermal spraying, coating material may now be deposited
in the openings of the cooling air bores and considerably restrict
their flow cross-section or the opening may completely clog. This
naturally has a very disadvantageous effect on the cooling effect
achieved in operation. The cooling air bores therefore have to be
liberated from unwanted coating material in a further processing
step.
[0005] This problem is particularly pronounced with bores having a
very small diameter of, for example, less than one millimeter
diameter. There is in particular the tendency with the already
mentioned film cooling to provide more and smaller, i.e. narrower,
cooling air bores, for example to realize a fluid film on the vane
surface which is as homogeneous as possible. In this cooling
principle, it is very important that all cooling air bores actually
remain open.
[0006] It is also generally necessary on the thermal spraying of
surfaces into which such cooling air bores open that the bores do
not clog or that their flow cross-section is not reduced. A
problem-free cooling performance can only be achieved in this
way.
[0007] Different measures have already been proposed to solve this
problem. It is, for example, proposed in WO-A-2010/078994 to
introduce a masking material, preferably a polymer, into the
cooling hole or into the outlet opening of the cooling air bore
before the thermal coating. This masking material is hardened
before the coating, for example by treating with UV radiation.
After the coating procedure, the masking material then has to be
removed again in a further processing step, wherein it has to be
ensured that the masking material really is completely removed.
This is a complex and/or expensive procedure, in particular with
very thin bores.
[0008] Starting from this prior art, it is therefore an object of
the invention to propose a method with which a coating of a surface
is made possible, wherein the flow cross-section of bores which
open into the surface to be coated can be kept substantially
constant. Furthermore, a turbine vane coated in this manner should
be provided by the invention.
[0009] The subjects of the invention satisfying this object are
characterized by the independent claims of the respective
category.
[0010] In accordance with the invention, a method is therefore
proposed of coating a surface of a body which has at least one
throughgoing bore having an inlet and having an outlet, wherein the
outlet is provided in the surface to be coated, in which method the
coating takes place by means of thermal spraying, wherein, during
the thermal spraying, the bore is flowed through by a fluid which
flows out through the outlet of the bore and substantially prevents
a constriction of the bore caused by the coating.
[0011] By the measure of allowing a fluid to flow through the bore
during the thermal spraying, the coating material is prevented from
penetrating into the bore or being deposited in the bore by the
fluid exiting through the outlet. It is prevented in this manner
that the flow cross-section of the bore is reduced during the
thermal spraying or that the bore is even fully clogged. The
particular advantage of this process lies in the fact that, after
the thermal spraying, no additional processing step is required to
open the bore or to widen it to the desired diameter again or to
liberate it from hardened masking material.
[0012] The fluid is preferably a gas because the fluid flow can be
controlled better by it; but generally a liquid can also be used as
the fluid, for example also a liquid which then exits the outlet as
a vapor. With respect to the avoidance of unwanted chemical
reactions, the fluid is in particular preferably an inert gas,
especially argon, because this is frequently used as a process gas
in thermal spray processes.
[0013] The flow rate at which the fluid flows through the bore or
through the totality of all bores naturally depends on the
respective process parameters and can be optimized to the
application. It has in particular proved itself in practice with
processes in the underpressure area if the flow rate of the fluid
is at most 5 SLPM (standard liters per minute) and preferably
amounts to at most 1 SLPM.
[0014] In a preferred application, the thermal spraying is a plasma
spraying process.
[0015] The method in accordance with the invention is particularly
well suited when the thermal spraying is a plasma spraying process
which is carried out at a process pressure which is less than
10,000 Pa. LPPS processes (LPPS=low pressure plasma spraying) are
especially suitable, in particular those in which a value is
selected for the process pressure between 50 and 2000 Pa,
preferably between 100 and 800 Pa.
[0016] In a special embodiment of the LPPS process relevant to
practice, a starting material for the coating is sprayed onto the
body in the form of a process jet, wherein the starting material is
injected into a plasma defocusing the process jet and there partly
or completely changes into a liquid phase, and wherein the coating
takes place by deposition from the liquid phase. This process is
also called a LPPS-TF process, where TF stands for thin film.
[0017] A further embodiment very relevant to practice is an LPPS
process with deposition from the gas phase. In this respect, a
starting material for the coating is sprayed onto the body in the
form of a process jet, wherein the starting material is injected
into a plasma defocusing the process jet and there at least partly
changes into a gaseous state, and wherein the coating takes place
by deposition from the gaseous phase. The designation LPPS-TF is
sometimes also used for this process; however, since it is an--at
least partial--deposition from the gaseous phase or vapor phase,
the designation PS PVD (plasma spray physical vapor deposition) is
also used for this process.
[0018] The method in accordance with the invention is in particular
suitable for the coating of turbine vanes which have one or more
cooling air bores. The body is then a turbine vane and the at least
one bore is a cooling air bore.
[0019] The turbine vane preferably has a foot which is connected to
a source for the fluid during the coating so that the fluid can
flow from the foot of the turbine vane through the at least one
cooling air bore to the outlet of the cooling air bore.
[0020] The method is particularly suitable when the turbine vane
has a plurality of cooling air bores which are flowed through by
the fluid.
[0021] A turbine vane is further proposed by the invention which is
coated in accordance with the method in accordance with the
invention.
[0022] In an embodiment variant, the cooling air bore opens
obliquely into the surface to be coated. It is naturally also
possible that the cooling air bore opens substantially
perpendicular into the surface to be coated. Combinations of these
two variants are also possible.
[0023] In a preferred embodiment of the turbine vane, the thickness
of the coating increases as the distance from the outlet of the
bore increases until said thickness has reached its maximum value.
Due to the fluid flow exiting during the coating, it is more
difficult for the coating material to be deposited in the direct
vicinity of the respect outlet than somewhat further away where the
flow is already weaker. The transition from the outlet of the bore
to the surface of the body is no longer so abrupt or sharply edged
due to this increase in the layer thickness, but is rather somewhat
sloped or rounded. This is advantageous for the operation of the
turbine vane viewed from the aspect of the flow behavior of the
cooling air. In particular with film cooling processes, an even
more homogeneous cooling film can hereby be realized at the surface
of the turbine vane.
[0024] Further advantageous measures and preferred embodiments of
the invention result from the dependent claims.
[0025] The invention will be explained in more detail in the
following with reference to embodiments and to the drawing. There
are shown in the schematic drawing, partly in section:
[0026] FIG. 1 a sectional representation of a fluid-cooled turbine
vane;
[0027] FIG. 2 a cross-sectional representation of a section of the
wall of a gas-cooled turbine vane, which is coated in accordance
with an embodiment of the method in accordance with the invention,
with a bore serving as a cooling air bore; and
[0028] FIG. 3 as FIG. 2, but for a different bore.
[0029] The method in accordance with the invention serves for the
coating of a surface of a body which has at least one throughgoing
bore which opens into the surface to be coated. In the following,
reference is made with exemplary character to the application
particularly important for practice that the body is a turbine vane
1 (see FIG. 1) which is gas cooled in operation.
[0030] FIG. 1 shows, in a longitudinal sectional representation, an
embodiment of a gas-cooled turbine vane 1 such as is known from the
prior art. The turbine vane 1 includes a turbine blade 2 and a foot
3 with which the turbine vane 1 is installed on the rotor or on the
hub. A plurality of cooling air passages 4 are typically provided
in the interior of the turbine vane 1 and are flowed through by a
gas--usually air--in the operation of the turbine to cool the
turbine vane 1. The flow development of the cooling air is
illustrated in FIG. 1 by the arrows without reference symbols. A
plurality of cooling air bores 5 extend from the cooling air
passages 4 through the wall of the turbine vane 1 so that the
cooling air can exit at the surface 10 of the turbine vane 1. Each
cooling air bore 5 has an inlet 51 (see FIG. 2) through which the
cooling air enters into the bore 5 and an outlet 52 which is
disposed in the outer surface 10 of the turbine vane 1 and through
which the cooling air flows out of the bore 5.
[0031] As already mentioned, the method in accordance with the
invention is not restricted to the coating of turbine vanes, but is
rather very generally suitable for the coating of a surface 10 of a
body 1 which has at least one throughgoing bore 5, i.e. which can
be flowed through by a fluid, having an inlet 51 and an outlet 52,
wherein the outlet 52 is provided in the surface 10 to be coated.
Such a body can, for example, also be a tubular body whose outer
surface is to be coated and whose wall is provided with a plurality
of bores 5.
[0032] In the method in accordance with the invention, the surface
10 of the body 1, here the surface 10 of the turbine vane 1, is
coated by means of thermal spraying. In accordance with the
invention, the bore 5 is flowed through by a fluid during the
thermal spraying, said fluid flowing out through the outlet 52 of
the bore 5 and thus substantially preventing a narrowing of the
outlet 52 or of the bore 5 caused by the coating. It is at least
largely prevented by the fluid exiting during the thermal spray
process that coating material is deposited in the outlet 52 or in
the bore 5. The flow cross-section of the bore 5 is thus
maintained; even after the coating, the bore 5 still has the same
flow cross-section as before the coating.
[0033] The same cooling air passage 4 or cooling air bores 5 are
therefore used for the throughflowing fluid during the thermal
spraying as are used in the normal operation of the turbine for the
cooling medium.
[0034] The method in accordance with the invention is suitable for
all thermal spray processes such as flame spraying, high-velocity
flame spraying (HVOF), arc spraying, e.g. wire arc spraying, laser
spraying, cold gas spraying and all plasma spraying processes, e.g.
plasma spraying at normal pressure (APS), plasma spraying at
underpressure (VPS or under a protective gas atmosphere.
[0035] A gas, and specifically an inert gas, is preferably used as
the fluid for the throughflow of the bore 5 during the coating
process. It can be avoided by the use of an inert gas that unwanted
chemical reactions which can be caused by the fluid flowing out of
the bores 5 occur during the thermal spraying. A particularly
preferred gas is argon because this inert gas is used as a process
gas in a number of spray processes.
[0036] The flow rate at which the fluid flows out of the bore 5 or
out of the bores 5 during the thermal coating can be adapted in
dependence on the application and on the thermal spraying process.
The flow rate should, on the one hand, be selected so high that a
deposition of the coating material in the bore is prevented. On the
other hand, the flow rate should be selected so high that it does
not result in a considerable degrading of the coating process.
[0037] In practice, flow rates of at most 5 SLMP (standard liters
per minute) and especially of at most 1 SPLM have specifically
proved themselves for plasma spray processes in the underpressure
range on the use of argon.
[0038] A special embodiment of the method in accordance with the
invention will now be explained with reference to FIGS. 2 and 3 in
which an LPPS process (LPPS: low pressure plasma spraying) is used
for the thermal spraying. This process is described generically in
WO-A-03/087422. The process described there is one which is there
also described as an LPPS thin-film process (LPPS-TF=LPPS thin
film). A conventional LPPS plasma spray method is modified in a
technical process manner by the LPPS thin film process, with a
space through which a plasma flows ("plasma flame" or "plasma jet")
being widened due to the changes and being expanded to a length of
up to 2.5 m. The geometrical extent of the plasma results in a
uniform expansion--a "defocusing"--of a powder jet which is
injected into the plasma using a propellant gas. The material of
the powder jet which is dispersed to form a cloud in the plasma and
is melted partly or completely there moves in a uniform
distribution over a widely expanded surface of the body to be
coated.
[0039] It is disclosed in WO-A-03/087422 that a plasma is generated
with sufficiently high specific enthalpy so that a substantial
portion of the coating material changes into the vapor phase and is
deposited from the vapor phase on the surface to be coated.
Reference is made here to WO-A-03/087422 with respect to the plasma
parameters and the details of the process.
[0040] A major aspect of the process disclosed in WO.-A-03/087422
is that a substantial portion of the coating material is deposited
from the vapor phase on the body to be coated. Since this process
is not necessarily directed to the generation of thin films it is
today frequently also called a PS PVD process (PS PVD: plasma spray
physical vapor deposition) instead of an LPPS-TF process, whereas
the term LPPS-TF process is used for such processes in which thin
films of less than 50 micrometers, for example, are generated,
wherein the deposition usually takes place from the liquid phase,
i.e. the coating material moves onto the surface to be coated in
the form of small droplets.
[0041] In the embodiment of the process in accordance with the
invention explained with reference to FIGS. 2 and 3, a PS PVD
process is used as thermal spraying such as is disclosed, for
example, in the already cited WO-A-.03/087422 (but called LPPS-TF
there), i.e. the deposition of the coating material on the surface
10 to be coated takes place at least to a substantial amount of at
least 50% of the coating deposited on the surface 10 from the gas
phase.
[0042] FIG. 2 shows a cross-sectional representation of a section
of the wall of a gas-cooled turbine vane 1 which is coated in
accordance with such an embodiment of the method in accordance with
the invention having the bore 5 serving as a cooling air bore. The
outlet 52 of the bore 5 opens substantially perpendicular into the
surface 10 to be coated of the turbine vane 1. FIG. 3 shows, in a
representation analog to FIG. 2, a bore 5 which opens
obliquely--that is not at a right angle--into the surface 10 to be
coated. Both FIG. 2 and FIG. 3 show the turbine vane 1 with a
coating 11 already deposited on the surface 10. Since it is
sufficient for understanding, only one bore 5 is shown in each case
in FIGS. 2 and 3.
[0043] To prepare for the coating, the turbine vane 1 is first
fastened to a holder, not shown, with which it can be introduced
into the coating chamber. It is in this respect in particular
preferred with plasma processes in the underpressure range if the
coating chamber is situated before an antechamber from where the
turbine vane 1 to be coated can be introduced by means of the
holder through a sluice wall into the coating chamber. Such a plant
having an antechamber is disclosed, for example, in EP-A-1 006
211.
[0044] The holder or the foot 3 of the turbine vane 1 is now
connected to a source in which an inert gas, preferably, argon, is
stored. In this respect, the connection takes place such that the
argon can flow from the source through the foot 3 of the turbine
vane 1 through the at least one cooling air bore 5 in a
correspondingly same manner as the cooling medium flows through the
turbine vane 1 during the operation of the turbine. A plurality of
cooling air bores 5 are naturally preferably provided which are
then all flowed through by the argon during the coating, as the
arrows without reference symbols in FIGS. 2 and 3 indicate.
[0045] The pressure in the coating chamber is brought to the
desired process pressure of less than 10,000 Pa, preferably to a
value between 50 Pa and 2000 Pa, and especially to a value between
100 and 800 Pa.
[0046] After the turbine vane 1 has been brought into the coating
chamber at the desired process pressure, the argon flow through the
cooling air bore 5 is started as the arrows without reference
symbols in FIGS. 2 and 3 indicate. A starting material for the
coating in the form of a process jet is now sprayed onto the
turbine vane 1 by means of a plasma spray apparatus, wherein the
starting material is injected into a plasma defocusing the process
jet and changes there at least partly into the gaseous phase. The
coating of the turbine vane 1 now takes place by deposition from
the gaseous phase while the argon gas flows through the bore 5.
[0047] The exiting argon can in this respect additionally effect a
positive cooling effect on the surface 11. The argon is pumped out
of the coating chamber together with the process gas.
[0048] In an alternative process management, the LPPS-TF process is
carried out so that the starting material is partly or completely
plasticized in the plasma or changes into the liquid phase and the
coating then takes place by deposition from the liquid phase.
[0049] During the coating process, the argon gas flowing through
the bore 5 prevents the coating material from being deposited in
the bore 5 or in the outlet 52. It is thus ensured that each bore 5
still has the same flow cross-section as before the thermal
spraying after the end of the thermal spraying.
[0050] A particular advantage of the method explained with
reference to FIG. 2 and FIG. 3 is that a more homogeneous, that is
slower, transition takes place from the edge of the bore 5 to the
coating 11 due to the fluid flowing out of the bore 5 during the
thermal spraying. As FIG. 2 and FIG. 3 show, the thickness of the
coating 11 increases continuously in the region marked by B as the
distance from the outlet 52 of the bore 5 increases until said
thickness has reached its maximum value d. This slow increase of
the coating is due to the fact that the outflowing fluid makes the
deposition of the coating material more difficult in the direct
proximity of the outlet 52 during the coating, with this difficulty
decreasing as the distance from the outlet 52 increases.
[0051] It can be seen in FIG. 2 that with a perpendicular opening
of the bore 5 this continuous increase of the thickness of the
coating takes place symmetrically, that is, in accordance with the
illustration, on both sides of the outlet 52 of the bore 5.
[0052] On an oblique opening of the bore 5, as is shown in FIG. 3,
the inclined increase of the coating 11 occurs essentially only on
one side, namely on the left side of the outlet 52 in accordance
with the illustration.
[0053] This slow increase in the thickness of the coating 11 seen
in the direction of flow also has advantages for the operation of
the turbine because the cooling air exiting from the cooling air
bore 5 is hereby better conducted over the surface of the turbine
vane 1 in the operating sate and turbulence at the outlet 52 of the
bore 5 can be at least reduced.
* * * * *