U.S. patent application number 16/353490 was filed with the patent office on 2020-09-17 for laser induced, fine grained, gamma phase surface for nicocraly coatings prior to ceramic coat.
This patent application is currently assigned to United Technologies Corporation. The applicant listed for this patent is United Technologies Corporation. Invention is credited to David Ulrich Furrer, Dmitri Novikov, Henry H. Thayer.
Application Number | 20200291529 16/353490 |
Document ID | / |
Family ID | 1000004748607 |
Filed Date | 2020-09-17 |
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United States Patent
Application |
20200291529 |
Kind Code |
A1 |
Thayer; Henry H. ; et
al. |
September 17, 2020 |
LASER INDUCED, FINE GRAINED, GAMMA PHASE SURFACE FOR NiCoCrAlY
COATINGS PRIOR TO CERAMIC COAT
Abstract
A process for forming a thermal barrier coating on a part
comprising depositing an aluminum containing bond coat on the part,
the bond coat comprising a surface; cleaning the surface to remove
oxides and debris from the surface of the bond coat; forming a
gamma phase layer proximate the surface of the bond coat; forming
an aluminum oxide layer on the surface of the bond coat; and
depositing a ceramic topcoat on the aluminum oxide layer on the
bond coat.
Inventors: |
Thayer; Henry H.;
(Wethersfield, CT) ; Novikov; Dmitri; (Avon,
CT) ; Furrer; David Ulrich; (Marlborough,
CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
United Technologies Corporation |
Farmington |
CT |
US |
|
|
Assignee: |
United Technologies
Corporation
Farmington
CT
|
Family ID: |
1000004748607 |
Appl. No.: |
16/353490 |
Filed: |
March 14, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 28/3455
20130101 |
International
Class: |
C23C 28/00 20060101
C23C028/00 |
Claims
1. A process for forming a thermal barrier coating on a part
comprising: depositing an aluminum containing bond coat on the
part, said bond coat comprising a surface; cleaning said surface to
remove oxides and debris from the surface of the bond coat; forming
a gamma phase layer proximate the surface of the bond coat; forming
an aluminum oxide layer on said surface of said bond coat; and
depositing a ceramic topcoat on the aluminum oxide layer on the
bond coat.
2. The process according to claim 1, wherein said cleaning
comprises exposing the surface to an energy beam focused on the
surface of the bond coat and forming a liquid from the bond coat
proximate the surface.
3. The process according to claim 2, further comprising: rapidly
cooling the liquid into said gamma phase layer; and forming an
alpha aluminum oxide proximate said surface.
4. The process according to claim 2, wherein said energy beam
produces high intensity, short duration energy beam pulses.
5. The process according to claim 1, further comprising: inhibiting
a non-alpha aluminum oxide layer from growing responsive to said
gamma phase layer proximate said surface.
6. The process according to claim 1, further comprising:
distributing the aluminum in the bond coat uniformly, so that the
alpha aluminum oxide layer is even and has reduced internal
stresses.
7. The process according to claim 1, wherein said gamma phase layer
proximate the surface comprises a supersaturated aluminum content
and low aluminum diffusivity properties.
8. The process according to claim 1, further comprising: growing an
initial alpha-alumina scale during the cleaning step.
9. The process according to claim 1, wherein the gamma phase layer
comprises a thickness of about 1.5 microns.
10. The process according to claim 1, wherein said gamma phase
layer proximate the surface of the bond coat is an aluminum
diffusion inhibitor.
11. The process according to claim 8, wherein said gamma phase
layer impedes fast aluminum oxidation at the surface.
12. The process according to claim 8, wherein the gamma phase layer
allows for a very thin initial alpha-alumina scale to grow during
said cleaning step.
Description
BACKGROUND
[0001] The present disclosure is directed to a thermal barrier
coating system for a component that is exposed to high
temperatures, such as a gas turbine engine component (e.g. blades,
vanes, etc.). More particularly, the present invention relates to
the formation of a thermal barrier coating system and a method of
forming a thermal barrier coating on a metal part that includes
laser cleaning a surface of the metal part to remove undesirable
oxides and residues from the surface of the part.
[0002] A gas turbine engine component, such as a blade tip, blade
trailing edge, blade platform, blade airfoil, vane airfoil, vane
trailing edge, or vane platform, is typically exposed to a high
temperature and high stress environment. The high temperature
environment may be especially problematic with a superalloy
component. Namely, the high temperatures may cause the superalloy
to oxidize, or weaken which then decreases the life of the
component. In order to extend the life of the component, a thermal
barrier coating system (TBC system) may be applied to the entire
superalloy component or selective surfaces, such as surfaces of the
superalloy component that are exposed to the high temperatures and
other harsh operating conditions. A TBC system protects the
underlying material (also generally called the "substrate") and
helps inhibit oxidation, corrosion, erosion, and other
environmental damage to the substrate. Desirable properties of a
TBC system include low thermal conductivity and strong adherence to
the underlying substrate.
[0003] The TBC system includes a metallic bondcoat or oxidation
resistant coating and a ceramic topcoat (i.e., a thermal barrier
coating or TBC topcoat). The bondcoat is applied to the substrate
and aids the growth of a thermally grown oxide (TGO) layer, which
is typically alpha aluminum oxide, (Al.sub.2O.sub.3 or "alumina").
Specifically, prior to or during deposition of the TBC topcoat on
the bondcoat, the exposed surface of the bondcoat can be oxidized
to form the alumina TGO layer or scale. The TGO forms a strong bond
to both the topcoat and the bondcoat, and as a result, the TGO
layer helps the TBC topcoat adhere to the bondcoat. The bond
between the TGO and the topcoat is typically stronger than the bond
that would form directly between the TBC topcoat and the bondcoat.
The TGO also acts as an oxidation resistant layer, or an "oxidation
barrier", to help protect the underlying substrate from damage due
to oxidation.
[0004] In order for the TBC layer to adhere to the metallic bond
coat (BC) (NiCoCrAlY for example), an oxide intermediate layer, a
thermally grown oxide (TGO) is needed. Bond coat alloy in
equilibrium consists of Aluminum (Al) poor phase ("gamma" phase,
face centered cubic (fcc) crystal structure) and Al rich phase
("beta" phase, body centered cubic (bcc) crystal structure).
[0005] It should be noted that self-diffusion coefficient in bcc
"beta" phase is by order of magnitude higher than the one in fcc
"gamma" phase due to its lower packing factor and lower
coordination number.
[0006] Although the TGO is needed to bond the TBC to the metallic
coat, the TGO continues to grow as the engine runs. The slight
mismatch in volume between the TGO and TBC causes a build-up of
stress between them that eventually causes the coating to fail.
Therefore, it is important to have the thinnest TGO possible at the
start of the process, and to slow the growth of the TGO during
engine run conditions.
SUMMARY
[0007] In accordance with the present disclosure, there is provided
a process for forming a thermal barrier coating on a part
comprising depositing an aluminum containing bond coat on the part,
the bond coat comprising a surface; cleaning the surface to remove
oxides and debris from the surface of the bond coat; forming a
gamma phase layer proximate the surface of the bond coat; forming
an aluminum oxide layer on the surface of the bond coat; and
depositing a ceramic topcoat on the aluminum oxide layer on the
bond coat.
[0008] In another and alternative embodiment, the cleaning
comprises exposing the surface to an energy beam focused on the
surface of the bond coat and forming a liquid from the bond coat
proximate the surface.
[0009] In another and alternative embodiment, the process further
comprises rapidly cooling the liquid into the gamma phase layer,
and forming an alpha aluminum oxide proximate the surface.
[0010] In another and alternative embodiment, the energy beam
produces high intensity, short duration energy beam pulses.
[0011] In another and alternative embodiment, the process further
comprises inhibiting a non-alpha aluminum oxide layer from growing
responsive to the gamma phase layer proximate the surface.
[0012] In another and alternative embodiment, the process further
comprises distributing the aluminum in the bond coat uniformly, so
that the alpha aluminum oxide layer is even and has reduced
internal stresses.
[0013] In another and alternative embodiment, the gamma phase layer
proximate the surface comprises a supersaturated aluminum content
and low aluminum diffusivity properties.
[0014] In another and alternative embodiment, the process further
comprises growing an initial alpha-alumina scale during the
cleaning step.
[0015] In another and alternative embodiment, the gamma phase layer
comprises a thickness of about 1.5 microns.
[0016] In another and alternative embodiment, the gamma phase layer
proximate the surface of the bond coat is an aluminum diffusion
inhibitor.
[0017] In another and alternative embodiment, the gamma phase layer
impedes fast aluminum oxidation at the surface.
[0018] In another and alternative embodiment, the gamma phase layer
allows for a very thin initial alpha-alumina scale to grow during
the cleaning step.
[0019] Other details of the process are set forth in the following
detailed description and the accompanying drawings wherein like
reference numerals depict like elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a perspective view of a turbine blade.
[0021] FIG. 2 is a cross-sectional view of the turbine blade of
FIG. 1 where a section has been taken at line 2-2 (shown in FIG. 1)
and show a thermal barrier coating system overlying the airfoil of
the turbine blade.
[0022] FIG. 3 is an exemplary cleaning process.
[0023] FIG. 4 is a phase diagram of an exemplary bond coat as a
function of temperature.
DETAILED DESCRIPTION
[0024] FIG. 1 is a perspective view of turbine blade 10 of a gas
turbine engine. Turbine blade 10 includes platform 12 and airfoil
14. Airfoil 14 of turbine blade 10 may be formed of a nickel based,
cobalt based, iron based superalloy, or mixtures thereof or a
titanium alloy. Turbine blade 10 is exposed to high temperatures
and high pressures during operation of the gas turbine engine. In
order to extend the life of turbine blade 10 and protect it from
high stress operating conditions and the potential for oxidation
and corrosion, a thermal barrier coating (TBC) (shown in FIG. 2) is
applied over airfoil 14 and platform 12 of turbine blade 10.
[0025] The exact placement of the TBC system depends on many
factors, including the type of turbine blade 10 employed and the
areas of turbine blade 10 exposed to the most stressful conditions.
For example, in alternate embodiments, a TBC may be applied over a
part of the outer surface of airfoil 14 rather than over the entire
surface of airfoil 14. Airfoil 14 may include cooling holes leading
from internal cooling passages to the outer surface of airfoil 14,
and the system 16 may also be applied to the surface of the cooling
holes.
[0026] FIG. 2 is a sectional view of turbine blade 10, where a
section is taken from line 2-2 in FIG. 1. TBC system 16 is applied
to an exterior surface of airfoil 14 and platform 12.
[0027] TBC system 16 may include bondcoat 18 and ceramic layer 20.
Bond coat 18 overlays and bonds to airfoil 14 and platform 12 while
ceramic layer 20 overlays and bonds to the thermally grown oxide
(TGO) on the bond coat 18. In the embodiment shown in FIG. 2, bond
coat 18 may be applied to airfoil 14 and platform 12 at a thickness
ranging from about 0.5 mils (0.0127 mm) to about 10 mils (0.254
mm). Ceramic layer 20 may be any thermal barrier coating (or
"topcoat") that is suitable for use on alumina forming bond coats
and/or alloys. Non-limiting examples include zirconia stabilized
with yttria (Y.sub.2O.sub.3), gadolinia (Gd.sub.3O.sub.3), ceria
(CeO.sub.2), scandia (Sc.sub.2O.sub.3), and other oxides known in
the art. Ceramic layer 20 may be applied by electron beam physical
vapor deposition (EBPVD) or by plasma spray. Ceramic layer 20 may
be deposited in thickness sufficient enough to provide the required
thermal protection for bondcoat 18 and substrate 10.
[0028] Bond coat 18 may be a MCrAlY coating where M may be Ni, Co,
Fe, Pt, Ni-base alloy, Co-base alloy, Fe-base alloy or mixtures
thereof. In an embodiment, M may include Hf or Si or mixtures
thereof. In an exemplary embodiment, the bond coat 18 can comprise
NiCoCrAlY. Bond coat 18 may be applied to airfoil 14 and platform
12 by any suitable technique including, but not limited to, thermal
spray processes such as low pressure plasma spray (LPPS) deposition
and high velocity oxyfuel (HVOF) deposition, physical vapor
deposition such as cathodic arc deposition or chemical vapor
deposition, and the like. In an embodiment, bond coat 18 may be an
aluminide bondcoat formed by techniques such as pack cementation,
chemical vapor deposition, and others followed by appropriate
diffusion heat treatments.
[0029] The process 100 can be understood by referring also to FIG.
3. At an initial state 120, the bondcoat 18 includes a surface 32
covered with contaminant materials 34, such as oils, oxides and the
like. Within the bond coat 18, a combination of beta phase bcc
structure 36 and gamma phase fcc structure 38 is shown distributed
throughout the bond coat 18. The beta phase bcc structure 36 can
act as an Al reservoir for the bond coat 18. The gamma phase fcc
structure 38 can act as an Al diffusion media.
[0030] At step 130 cleaning and structural refinement takes place
at and near the surface 32 of the bond coat 18. The cleaning is
done at high temperature and at an atmospheric pressure. In an
exemplary embodiment, the cleaning temperature can be above 1350
degrees C., the point at which the NiCoCrAlY liquefies. An energy
beam 40 is utilized to remove the contaminants 34 while also
melting the bond coat 18 proximate the surface 32, forming a liquid
42 from the bond coat 18 material. The liquid 42 subsequently
rapidly cools down into a fine grained layer 44. The grain
refinement is achieved by very rapid melting and re-solidification
of the bond coat 18 material at the surface 32 under very high
intensity, short duration energy beam pulses. In an exemplary
embodiment, the energy beam 40 can utilize a 50-100 nano-second
pulse duration at 5-50 J/cm{circumflex over ( )}2. The local
surface 32 heating above the predetermined temperature, such as
1350 degrees Centigrade, can create the fine grained predominantly
gamma phase layer 44 proximate the surface 32. Raising the
temperature this way causes the metal right at the surface to
liquefy very quickly, and re-solidify very quickly providing the
desired fine grain structure.
[0031] The energy beam 40 can include a laser. The laser can
include an yttrium aluminum garnet laser, ultraviolet, eximer, or
carbon dioxide, fiber, or disc laser. The laser can have a power
range up to about 1000 watts.
[0032] The surface environment during the cleaning portion 130, can
include air, inert gas, water, or combinations thereof.
[0033] In an exemplary embodiment, a laser based cleaning system
for removing contaminants from the surface 32 may include: a laser
40 capable of removing the oxide layer and producing an aluminum
oxide layer on the substrate surface 32; an optical system capable
of focusing the laser at the oxide layer; and a scanning system
capable of directing the focused laser beam over the surface 32 to
remove contaminants to produce the gamma phase layer 44 on the
substrate 32.
[0034] In an exemplary embodiment, the Al-rich gamma phase layer 44
can be about 0.5 microns. In an exemplary embodiment the gamma
phase layer 44 can range from about 0.25 microns to about 0.75
microns. In another exemplary embodiment, the gamma phase layer 44
can be about 1.5 microns thick. The thickness of the layer 44 can
be controlled by the amount of energy beam applied power to the
surface 32. In an exemplary embodiment, the energy beam 40 applied
power can range from about 500 Watts to about 1000 Watts. In an
exemplary embodiment, the surface 32 can be heated to about 1350
degrees C. The surface 32 can be heated to temperatures from about
1350 degrees C. possibly much hotter, but not so hot as to start
boiling off the Aluminum, so 2470 degrees C. would be the upper
limit to produce the desired gamma phase layer 44. At the higher
temperatures, as can be seen at FIG. 4, the bond coat 18 is
converted predominantly to gamma phase.
[0035] At 140 it can be seen that after cleaning and structural
refinement, the bond coat 18 includes the gamma phase layer 44 with
a smooth surface 32. The elements proximate the surface 32 of the
metallic bond coat 18 tend to be Ni and Al. The molten alloy 42
freezes first at the location where it is in contact with the bulk
of the solid bond coat 18 and freezes last where the molten alloy
42 is in contact with air or process gases. Thus, the disclosed
process promotes gamma-alumina formation proximate the surface 32.
Under ambient conditions, the gamma phase normally has lower Al
content than the beta phase. However, because of the rapid melting
and refreezing caused by the energy beam 40, all of the Al is still
present, so the gamma phase layer 44 is Al rich, or supersaturated
with Al. Because the gamma phase is supersaturated with Al (i.e. it
wants to get rid of Al) the gamma phase layer 44 has ability to
create a dense initial alpha-alumina layer of TGO and avoid the
non-alpha phase aluminum.
[0036] The layer 44 is Al oversaturated which can be beneficial for
the next step 150 of forming the aluminum oxide layer (TGO) 46 on
the surface 32. The extremely fine grain structure 44 distributes
the aluminum in the bond coat 18 very uniformly, so that the
thermally grown oxide (TGO) 46 is even and has fewer internal
stresses.
[0037] At 160, it can be seen that once the thermally grown oxide
46 is formed, the TGO 46 will be inhibited from growing rapidly,
and will grow slowly as compared to a surface that was not treated
by the disclosed process. The gamma phase 44 proximate the surface
is reduced in aluminum content and has poor diffusivity properties
for aluminum. The slower TGO 46 growth will result in good adhesion
between the TBC 20 and the metallic bond coat 18. This also
provides the benefit of longer life adhesion of the TBC 20, because
the TGO 46 thickness is smaller and the stresses between the TGO 46
and TBC 20 will grow more slowly. The uniform gamma like fcc phase
has an order of magnitude lower self-diffusion coefficient that the
beta bcc phase material. The gamma phase 44 impedes fast Al
oxidation at the surface 32, and allows for a very thin initial
alpha-alumina scale to grow during the disclosed laser cleaning
process. The rapid cooling and formation of the gamma phase layer
inhibits a non-alpha aluminum oxide layer from growing responsive
to said gamma phase layer proximate said surface. The non-alpha
aluminum oxides can include aluminum oxide in other phases,
including the cubic .gamma. and .eta. phases, the monoclinic
.theta. phase, the hexagonal .chi. phase, the orthorhombic .kappa.
phase and the .delta. phase that can be tetragonal or orthorhombic.
The aluminum oversaturation ensures that aluminum oxides are
preferentially created proximate the surface 32. During subsequent
TGO 46 growth in an EB-PVD chamber that initial alpha-alumina
creates conditions for preferential alpha-alumina growth, which is
less prone to further oxygen diffusion that causes TGO growth
during part service life. TBC spallation happens when the TGO
thickness reaches a certain value. The disclosed process provides
for the initial TGO thickness and TGO growth rate coefficient to be
smaller. The TGO growth coefficient for laser cleaned sample is
about 2 times smaller than that for a grit blasted one.
[0038] The disclosed process relies upon surface chemical/phase
composition modifications of the bond coat. In an exemplary
embodiment, further protection can include adding other alloying
elements to the bond coat surface 32, such as Pt, to further
impeded the TGO growth.
[0039] The technical benefits of utilizing the disclosed process
includes optimized surface chemistry for optimal TGO growth.
[0040] Another technical advantage of the disclosed process is the
development of a smooth surface 32 without steps and ledges that
mitigates geometric stress build-up.
[0041] Another technical advantage of the disclosed process is that
surface treated material is given a diffusion barrier and
controller for aluminum so that the TGO starts and continues to
grow uniformly with a smooth, non-stepped geometry.
[0042] Another technical advantage of the disclosed process is that
surface treating with a laser or other energy source (like e-beam)
is a preferred approach.
[0043] Another technical advantage of the disclosed process can
include adding supplemental elements to the surface by pre-coating
(vapor deposition, sputtering, etc.) followed by laser processing
that can also provide optimal surface material for TGO growth and
subsequent diffusion barrier/control.
[0044] There has been provided a process. While the process has
been described in the context of specific embodiments thereof,
other unforeseen alternatives, modifications, and variations may
become apparent to those skilled in the art having read the
foregoing description. Accordingly, it is intended to embrace those
alternatives, modifications, and variations which fall within the
broad scope of the appended claims.
* * * * *