FIELD OF THE INVENTION
This invention relates to a robotic air plasma spray process for making abradable
coating on Gas turbine shroud segment with a patterned design.
BACKGROUND OF THE INVENTION
Gas turbines operate at very high temperatures and the turbine entry temperature (TET)
for the stage I nozzles are around 1100 C. The flow of this hot gas path is directed by
these nozzles onto the first stage blades which also experience temperatures greater
than 1000 C. The rotating blades and the stationary shroud are maintained at a
minimum clearance to prevent rub off due to linear thermal expansion of the blade
during service. This clearance results in a loss in efficiency of the gas turbine due to
leakage of the combustion products. Abradable coatings on flow path surfaces above
the moving metal blade tips in a turbine engine can reduce over tip leakage and
improve efficiencies. A rotating part can erode a portion of a fixed abradable
coating applied on an adjacent stationary part to form a seal having a very close
tolerance. An abradable seal can be used to minimize the clearance between blade tips
and an inner wall of an opposed shroud, which can reduce leakage or guide leakage
flow of a working fluid, such as steam or air, across the blade tips, and enhance turbine
efficiency. Patterned abradable architectures have been used to reduce blade tip wear
and are effective in reducing the leakage without increasing shroud weight significantly.
United States Patent, 20160236994906, by Vetters Daniel Kent, et al. August 18, 2016
entitled “PATTERNED ABRADABLE COATINGS AND METHODS FOR THE
MANUFACTURE THEREOF” relates to the development an abradable ceramic coating
with an arrangement of features selected such that the coating has a porosity of about
5% to about 90% wherein the abradable coating is directly coated on a Ceramic matrix
composite (CMC) substrate. The disclosure is directed to a method, including: forming
an abradable ceramic coating on a CMC component; and machining a pattern of
features into a surface of the abradable ceramic coating, wherein the features include
an array of hemi spherically-shaped depressions. The ceramic abradable coating can
include aluminum nitride, aluminum diboride, boron carbide, aluminum oxide, mullite,
zirconium oxide, carbon, silicon carbide, silicon nitride, transition metal nitrides,
transition metal borides, rare earth (RE) oxides, and mixtures and combinations thereof.
The array includes a regular pattern of pocket-like depressions that have shapes, sizes
and patterns selected to control the porosity of the coating, the flow of a fluid over the
surface, or both, while minimizing thermal stresses and stress concentrations in the
coating. The array includes linear, parallel rows, and linear, parallel columns of
depressions. In the rows, the depressions are a distance r apart, have a diameter x, and
the depressions of an adjacent row are offset a distance r/2 relative to the depressions
in the row. In columns, the depressions are a distance r apart, and the depressions of
an adjacent column are offset a distance r/2 relative to the depressions in the
column. This is shown in figure 1.
This patent involves creating a pattern of depression on a thick coating on CMC
substrate.
United States Patent, 20150354392 by Lipkin D. M., et al. December 10, 2015 entitled
“Abradable coatings” relates to abradable shroud coatings, and methods of making
abradable shroud coatings. A substantially smooth abradable surface of a shroud
maintains flowpath solidity but can result in severe blade tip wear. Patterned abradable
shroud surfaces result in significantly reduced blade tip wear as compared to
unpatterned or substantially smooth-flowpath shrouds, but allow leakage across the
blade tip that leads to decreased turbine efficiency. The present disclosure provides
shroud coatings, coated shrouds and methods of coating shrouds that include a hybrid
architecture that balances the apparently contradictory requirements of high flowpath
solidity, low blade tip wear and high durability. The pattern is shown in figure 2.
The abradable coating comprising patterned ridge architectures such as parallelograms,
may include first regions and second regions. In some embodiments, the second
regions may be more intrinsically abradable than the first regions. The first regions may
be a patterned structure or scaffold of relatively dense ridges or relative "high" portions
that provide mechanical integrity while supporting blade tip incursion without undue
blade wear. The second regions may include a highly friable microstructure that readily
abrades in response to blade incursion while having relatively poor mechanical integrity
as a stand-alone structure as compared to the first regions or scaffold. The highly friable
microstructure of the second regions can be achieved using a relatively porous and/or
micro cracked microstructure as compared to the first regions. The second regions may
be corralled by the relatively dense scaffold or first regions so as to facilitate blade
incursion while remaining substantially intact during typical turbine operation, including
operation under typical erosive, gas loading and dynamic conditions.
United States Patent, 20130017073 by Ali Sulfiker, et al. January 17, 2013 entitled
“PATTERN-ABRADABLE/ABRASIVE COATINGS FOR STEAM TURBINE
STATIONARY COMPONENT SURFACES” relates to a pattern-abradable seal
assembly provided for a stationary steam turbine component. The seal assembly, in
use, is oriented in opposition to at least one seal tooth on a rotatable turbine component
so as to inhibit leakage flow across the seal assembly in one direction, the seal
assembly may include an annular seal carrier having at least one axially-oriented
surface; a pattern-abradable/abrasive seal coating or insert at least partially covering
the at least one axially-oriented surface, the pattern-abradable/abrasive seal coating
having a pattern formed thereon adapted to face and be at least partially penetrated by
the at least one seal tooth. A plurality of anti-swirl elements project radially beyond
the pattern and are arranged to provide at least an axial component of flow across the
abradable seal assembly. The coating or insert may also be used on other stationary
turbine component surfaces to direct flow in a predetermined direction. This is shown in
figure 3.
United States Patent, 20150031272 by Fulton B.A., et al. January 29, 2015 entitled
“Machining tool and method for abradable coating pattern” relates to a method for
forming a pattern in an abradable coating includes the step of machining a groove in
the abradable coating with a machining tool. The machining tool is configured to
machine a top surface, a side surface and a bottom surface of the groove
simultaneously. A repeating step repeats the machining step until a desired number of
grooves is obtained in the abradable coating.
The machining tool is mounted in a three-axis rotary mill and the machining tool is
rotated at about 1,000 to about 30,000 revolutions per minute (rpm) during the
machining step. The machining tool contains an abrasive surface comprising at least
one of: diamond, cubic boron nitride (CBN), ceramic or silicon carbide. The abradable
coating is comprised of a ceramic material and the turbo machine is a gas turbine. This
is shown in figure 4.
United States Patent, 20060110248 by Nelson W.A. et al. May 25th 2006 entitled
“pattern for the surface of a turbine shroud” relates to a pattern for improving
aerodynamic performance of a turbine includes a material disposed in a pattern at a
base surface of a turbine shroud such that the material is capable of abradable contact
with a tip portion of a turbine bucket. The pattern includes a first plurality of ridges
disposed at the base surface such that a first portion of the first plurality of ridges
corresponding to a back portion of the turbine bucket is oriented at a first angle with
respect to an axis of rotation of the turbine bucket. Each ridge of the first plurality of
ridges has a first sidewall and a second sidewall having a first end and a second end.
The first ends of the first and second sidewalls extend from the base surface. The first
and second sidewalls slope toward each other with substantially equal but opposite
slopes until meeting at the second ends of respective first and second sidewalls defining
a centreline and a top portion of the ridge. The pattern is shown in figure 5.
United States Patent, 20100003894 by Miller M.O. et al. Jan 7th 2010 entitled “Method
and apparatus for selectively removing portions of an abradable coating using a water
jet ” relates to a method and apparatus for forming raised ridges on the surface of a
turbine component having an abradable coating formed on an outer surface thereof
which includes a mask having a predetermined pattern of openings therein adjacent
the abradable coating on a surface of the turbine component; and a high pressure water
jet that has movement relative to the mask so that the high pressure water jet passes
along the extent of the openings in the mask and passes through the openings in the
mask to remove portions of the abradable coating on the turbine component located
beneath the openings in the mask. The abradable coating is a TBC coating that
includes an AlSi-polyester and nickel graphite filler. The process of abrasive water jet
cutting using mask is shown in figure 6.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose a robotic air plasma spray process
for making abradable coating on Gas turbine shroud segment with a patterned design.
SUMMARY OF THE INVENTION
Accordingly, there is provided a robotic air plasma spray process for making abradable
coating on Gas turbine shroud segment with a patterned design. The process
comprising fabricating a metal mask made of Inconel of adequate thickness to fit
completely on the shroud segment taking into account the curvature of the shroud front
face; designing a metal mask such that it can be secured on the shroud segment in a
repeatable manner for the purpose of robot programming; carrying-out EDM/wire cutting
of the pattern design on the front face of the mask so as maintain structural stability;
coating the metal mask with a low viscosity high temperature paint to ensure that very
little coating sticks onto the mask during the plasma spray process; commencing the
coating process with partial masking of the shroud with grit blast resistant masking tape
in region “A” to ensure only remaining area gets grit blasted; depositing after the grit
blasting is completed, around 100 micron of NiCrAlY based bond coat powder, viz.,
AMDRY 9621 robotically by plasma spray process on the entire surface except on
region “A” covered with masking tape; further depositing around 350 micron of YSZ
based top coat abradable powder, viz., Metco 2395 robotically by plasma spray process
on the entire surface except on region “A” covered with masking tape; fitting the metal
mask onto the shroud after around 450-500 micron of coating is deposited on the
shroud;and pre-setting the robot program in such a way that the process follow the
pattern in the desired number of steps (22 nos.) so that only the unmasked region of the
shroud is exposed to plasma spraying. Plasma spray using the above robot sequence
ensures that the coating deposits only beneath the openings in the mask thereby
resulting in less powder consumption, reducing coating time and also reducing heat
input to the mask thereby extending its life. After 20 passes of plasma operation, the
spraying is stopped to permit for cooling of the mask and also to determine the accuracy
of powder deposition through the openings in the mask. Subsequently, another 20 -30
passes are carried out to ensure a final abradable coating thickness of around 1.5 mm.
The patterned abradable coating on the GT shroud using the mask is shown in figure
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 showing abradable coating in the pattern of depressions.
Figure 2 showing abradable coating comprising patterned ridge architectures such as
parallelograms,
Figure 3 showing pattern-abradable seal assembly provided for a stationary steam
turbine component.
Figure 4 showing machining tool for making abradable coating in the pattern.
Figure 5 shows the pattern which includes a first plurality of ridges disposed at the base
surface.
Figure 6 showing process of water jet cutting of abradable coating to form a pattern
using mask.
Figure 7 showing the design drawing for developing patterned abradable coating.
Figure 8 showing one as-coated patterned abradable GT shroud.
DETAIL DESCRIPTION OF THE INVENTION
Gas turbines generate power from the energy delivered by the flow of hot combustion
gases directed onto the first stage blades. These rotating blades are maintained at a
minimum clearance with the stationary shroud to prevent rub off due to linear thermal
expansion of the blade during service. This clearance results in a loss in efficiency of
the gas turbine due to leakage of the combustion products. Abradable coatings on flow
path surfaces in a turbine engine can reduce over tip leakage and improve efficiencies.
A rotating part can erode a portion of a fixed abradable coating applied on an adjacent
stationary part to form a seal having a very close tolerance. An abradable seal can be
used to minimize the clearance between blade tips and an inner wall of an opposing
shroud, which can reduce leakage or guide leakage flow of a working fluid, across the
blade tips and enhance turbine efficiency. Where large areas are involved, patterned
abradable architectures are used to reduce blade tip wear and are effective in reducing
the leakage without increasing shroud weight significantly. The details of the desired
coating architecture as provided by the collaborator is given in figure 7.
The inventive step involves the development of the process to deposit the abradable
coating powders in a pattern formation using a metal mask on the shroud segment of
the Gas turbine by robotic plasma spray. This coating includes a bond coat and a top
coat.
The inventive process allows a patterned abradable coating of thickness around 1.5 mm
suitable for gas turbine shrouds, which ensures that the patterned coating meets the
abradable properties required for gas turbine shroud including hardness, bond strength
and porosity/microstructure.
According to the invention, a metal mask is used for the above said coating for buildingup
the desired architecture of the pattern on the shroud. The robotic process enables
coating build-up beneath the openings in the mask to the desired architecture of the
pattern on the shroud. The mask is made of Inconel and can be used repetitively for
more than one shroud coatings with minimum distortion due to continuous plasma spray
heating. The height of the patterned ridgesformed by using the mask are in the range
900-1000 micron on the shroud top surface. Application of plasma spray resistant
masking liquid on the metal mask ensures that the plasma sprayed powder does not
adhere to the metal mask while the coating operation is in progress. An expensive
coating top coat powder, viz., Metco 2395 is used optimally by adopting the robotic
operation and masking process for coating build-up. The process enables a significant
saving of powder and elimination of a post machining step leading to higher throughput.
WE CLAIM
1. A robotic air plasma spray process for making abradable coating on Gas turbine
shroud segment with a patterned design, the process comprising the steps of:-
(i) fabricating a metal mask made of Inconel of adequate thickness to fit
completely on the shroud segment taking into account the curvature of the
shroud front face;
(ii) designing a metal mask such that it can be secured on the shroud segment in
a repeatable manner for the purpose of robot programming;
(iii) carrying-out EDM/wire cutting of the pattern design on the front face of the
mask so as maintain structural stability;
(iv) coating the metal mask with a low viscosity high temperature paint to ensure
that very little coating sticks onto the mask during the plasma spray process;
(v) commencing the coating process with partial masking of the shroud with grit
blast resistant masking tape in region “A” to ensure only remaining area gets
grit blasted;
(vi) depositing after the grit blasting is completed, around 100 micron of NiCrAlY
based bond coat powder, viz., AMDRY 9621 robotically by plasma spray
process on the entire surface except on region “A” covered with masking
tape;
(vii) further depositing around 350 micron of YSZ based top coat abradable
powder, viz., Metco 2395 robotically by plasma spray process on the entire
surface except on region “A” covered with masking tape;
(viii) fitting the metal mask onto the shroud after around 450-500 micron of coating
is deposited on the shroud; and
(ix) Pre-setting the robot program in such a way that the plasma gun follows the
pattern in the desired number of steps (22 nos.) where only the unmasked
region of the shroud is exposed to plasma spraying.
2. The process as claimed in claim 1, wherein the spraying is stopped after twenty
(20) passes of plasma operation to permit cooling of the mask and determine the
accuracy of the powder deposition through the opening of the mask, and wherein
further passes between 20 to 30 are carried out to ensure a final abradable
coating thickness of about 1.5 mm is achieved.
3. The process as claimed in claim 1, wherein the mask is made of Inconel alloy to
minimize thermal distortion during repetitive use, and wherein the eight of the
patterned ridges formed using the mask is of uniform thickness between 900-
1000 micron.