U.S. patent number RE32,121 [Application Number 06/616,506] was granted by the patent office on 1986-04-22 for overlay coatings for superalloys.
This patent grant is currently assigned to United Technologies Corporation. Invention is credited to David S. Duvall, Dinesh K. Gupta.
United States Patent |
RE32,121 |
Gupta , et al. |
April 22, 1986 |
Overlay coatings for superalloys
Abstract
Improved coating compositions are described for the protection
of superalloys at elevated temperatures. The coatings are of the
MCrAlY type where M is nickel or cobalt and are significantly
improved by the addition of from 0.1-7% silicon and 0.1-2% hafnium.
Coatings of the invention are preferably applied by plasma spraying
and as so applied are found to be substantially more effective than
prior art coatings.
Inventors: |
Gupta; Dinesh K. (Vernon,
CT), Duvall; David S. (Cobalt, CT) |
Assignee: |
United Technologies Corporation
(Hartford, CT)
|
Family
ID: |
26965930 |
Appl.
No.: |
06/616,506 |
Filed: |
April 24, 1984 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
289952 |
Aug 5, 1981 |
04419416 |
Dec 6, 1983 |
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Current U.S.
Class: |
428/656; 428/629;
428/678 |
Current CPC
Class: |
C22C
19/058 (20130101); C23C 30/00 (20130101); F01D
25/005 (20130101); C23C 4/073 (20160101); Y10T
428/12778 (20150115); Y10T 428/1259 (20150115); Y10T
428/12931 (20150115) |
Current International
Class: |
C22C
19/05 (20060101); C23C 30/00 (20060101); C23C
4/08 (20060101); F01D 25/00 (20060101); B32B
015/00 () |
Field of
Search: |
;428/629,632,633,656,678-685 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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25263 |
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Mar 1981 |
|
EP |
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2056491 |
|
Mar 1981 |
|
GB |
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2095700 |
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Oct 1982 |
|
GB |
|
Primary Examiner: Rutledge; L. Dewayne
Assistant Examiner: McDowell; Robert L.
Attorney, Agent or Firm: Sohl; Charles E.
Claims
We claim:
1. A gas turbine engine component which comprises a substrate
selected from the group consisting of Ni and Co base superalloys
having thereon a protective coating consisting essentially of 5-40%
Cr, 8-35% Al, 0.1-2.0% Y, .[.1-7.0%.]. .Iadd.0.1-7% .Iaddend.Si and
0.1-2.0% Hf balance selected from the group consisting of Ni, Co
and mixtures thereof.
2. A coated article as in claim 1 in which the superalloy substrate
is a nickel base superalloy and in which the coating contains
15-25% Cr, 10-20% Al, up to 30% Co, balance essentially Ni.
3. A coated article as in claim 2 in which the coating contains
15-25% Co.
4. A coated article as in claim 1 in which the superalloy substrate
is a cobalt base superalloy and in which the coating contains
15-35% Cr, 10-20% Al, up to 35% Ni, balance essentially Co.
5. A coated article as in claims 1, 2, 3, or 4, intended for use at
temperatures in excess of about 2100.degree. F., in which the Si
content is limited to a maximum of 2%.
6. A coated article as in claims 1, 2, 3 or 4 in which the
substrate is essentially free from hafnium and the coating contains
at least 0.2% Hf.
Description
DESCRIPTION
1. Technical Field
Overlay coatings of the MCrAlY type are improved in their
resistance to oxidation and corrosion by the addition of small but
significant amounts of Si and Hf. The coatings are preferably
applied by plasma spraying.
2. Background Art
Protective coatings are essential to the satisfactory performance
of gas turbine engines. In particular, in the turbine section of an
engine various components must withstand high stress while enduring
a corrosive gas stream whose temperatures may be as great as
2500.degree. F. As demands for efficiency and performance increase,
the requirements for coating durability increase.
The most effective coatings for protecting superalloy turbine
components are those known as MCrAlY coatings where M is selected
from the group consisting of iron, nickel, cobalt and certain
mixtures thereof. Such coatings are also referred to as overlay
coatings because they are put down in a predetermined composition
and do not interact significantly with the substrate during the
deposition process. U.S. Pat. No. 3,528,861 describes a FeCrAlY
coating as does U.S. Pat. No. 3,542,530. U.S. Pat. No. 3,649,225
describes a composite coating in which a layer of chromium is
applied to a substrate prior to the deposition of a MCrAlY coating.
U.S. Pat. No. 3,676,085 describes a CoCrAlY overlay coating while
U.S. Pat. No. 3,754,903 describes a NiCrAlY overlay coating. U.S.
Pat. No. 3,928,026 describes a NiCoCrAlY overlay coating having
particularly high ductility.
A variety of alloying additions have been proposed for use with the
MCrAlY compositions. U.S. Pat. No. 3,918,139 describes the addition
of from 3 to 12% of a noble metal. U.S. Pat. No. 4,034,142
describes the addition of from 0.5 to 7% silicon to a MCrAlY
coating composition. Finally, U.S. Pat. No. 3,993,454 describes an
overlay coating of the MCrAlHf type.
U.S. Pat. No. 4,078,922 describes a cobalt base structural alloy
which derives improved oxidation resistance by virtue of the
presence of a combination of hafnium and yttrium.
DISCLOSURE OF INVENTION
The overlay coating compositions of the present invention have the
following broad composition ranges: 5-35% Cr, 8-35% Al, 0.0-2% Y,
0.1-7% Si, 0.1-2% Hf balance selected from the group consisting of
Ni, Co and mixtures thereof. The addition of Si and Hf in these
levels provides about three to four times the life in an oxidizing
environment than a similar coating without these additions. Similar
improvements are observed in hot corrosion performance. The
invention coatings are advantageously applied using fine powder
applied by a plasma spray process. Coatings of the present
invention have broad application in the field of gas turbines.
Other features and advantages will be apparent from the
specification and claims and from the accompanying drawings which
illustrate an embodiment of the invention.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows the cyclic oxidation behavior of several coatings
including the coating of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
The coating on the present invention derives substantially improved
properties as a result of the addition of small amounts of silicon
and hafnium to MCrAlY type coatings. The composition ranges of the
present invention are presented in Table I. The Preferred A coating
is most suited for use on nickel base substrates. The Preferred B
coating is a refinement of the Preferred A coating which has been
optimized for ductility. The Preferred C coating is most suited for
use on cobalt base substrates.
Silicon may be added in amounts from 0.1 to 7 weight percent,
however, for applications where temperatures in excess of
2100.degree. F. are anticipated, silicon should be limited to a
maximum of 2% to reduce the possibility of incipient melting.
Hafnium is added in amounts from 0.1 to 2 weight percent. For use
on substrate alloys which do not contain hafnium, it is preferred
that the hafnium addition be at least 0.2%.
Additions of silicon and hafnium alone to MCrAlY coatings have
previously been shown to provide improved properties. However, it
is surprising and unexpected that the combination of minor
additions of hafnium and silicon together produce a substantially
greater improvement than that which would be predicted from
benefits obtained from additions of either hafnium or silicon
alone.
Yttrium may be replaced by any of the oxygen active elements found
in Group IIIB of the periodic table including the lanthanides and
actinides and mixtures thereof but yttrium is preferred.
TABLE 1 ______________________________________ PRE- PRE- PRE- BROAD
FERRED A FERRED B FERRED C ______________________________________
Cr 5-40 15-25 15-25 15-35 Al 8-35 10-20 10-20 10-20 Y .0-2.0 .1-2.0
.1-2.0 .1-2.0 Si .1-7.0 .1-7.0 .1-7.0 .1-7.0 Hf .1-2.0 .1-2.0
.1-2.0 .1-2.0 Co -- 0-30 15-25 Balance Ni -- Balance Balance 0-30%
Ni + Co Balance -- -- -- ______________________________________
The effects of various compositional additions on the cyclic
oxidation behavior of NiCoCrAlY material are illustrated in FIG. 1.
All of the coatings referred to in the figure were tested on single
crystal substrates of an alloy which nominally contains 10% Cr, 5%
Co, 4% W, 1.5% Ti, 12% Ta, 5% Al, balance nickel. This alloy is
described in U.S. Pat. No. 4,209,348. With the exception of the
sample EB-NiCoCrAlY, which was prepared by electron beam physical
vapor disposition, all the samples were coated using a low pressure
chamber plasma spray technique which will be described below. The
testing was performed using a flame produced by the combustion of
jet fuel and the testing apparatus was arranged so that the samples
were heated at 2100.degree. F. for 55 minutes and then forced air
cooled in a period of five minutes to a temperature of about
400.degree. F.
The ordinate of the FIG. 1 graph lists the steps through which a
coating progresses (degrades) during testing (or engine
service).
The NiCoCrAlY type of coating derives its protective capabilities
as a result of the formation of a thin uniform layer of alumina on
the surface of the coating. This alumina film forms as a result of
the oxidation of aluminum in the coating. With continued exposure
to oxidizing conditions at elevated temperatures the alumina layer
continues to grow in thickness and eventually spalls off. The
spallation is accentuated by thermal cycling. The alumina layer
re-forms after spallation provided that sufficient aluminum remains
in the coating composition. Yttrium and other oxygen active
elements such as hafnium inhibit spallation of this alumina scale,
thus retarding the consumption of aluminum from these coatings. As
yttrium and other oxygen active elements are consumed with
increasing exposure time, the degree of spallation increases from
light to medium and finally to heavy as shown on the figure. After
repeated spallation and alumina reformation, the aluminum content
of the coating is depleted to a level which is insufficient to
re-form the alumina layer. At this point a non-protective complex
oxide known as a spinel forms. The spinel is a compound containing
nickel and/or cobalt and/or chromium in combination with aluminum
and oxygen. The spinel has a distinct blue color and is readily
apparent. Once the spinel forms, the oxidation rate of attack to
the coating increases and it is soon penetrated; thereafter,
significant substrate attack occurs. The coatings shown in FIG. 1
are described in Table 2 below.
TABLE 2
__________________________________________________________________________
P.S. P.S. P.S. E.B. P.S. NiCoCrAlY + NiCoCrAlY + NiCoCrAlY +
NiCoCrAly NiCoCrAlY Si Hf Si + Hf
__________________________________________________________________________
Cr 18 18 18 18 18 Co 23 23 22 23 22 Al 12.5 12.5 12 12.5 12 Y .3 .4
.4 .4 .4 Ni Balance Balance Balance Balance Balance Si -- -- 1.6 --
.6 Hf -- -- -- .9 .7
__________________________________________________________________________
E.B. = Electron Beam Physical Vapor Deposition P.S. = Plasma
Sprayed
The electron beam (E.B.) physical vapor deposition coating is
currently the state of the art turbine airfoil coating and is
widely used in commercial engines. It can be seen that under the
severe test conditions employed, the life of the E.B. coating was
somewhat less than 500 hours. The same coating composition applied
by a low pressure plasma spray (P.S.) technique displays improved
durability with a life of about 700 hours. The reason for this
improvement is not completely understood and may be the result of
the interaction of the specific coating and substrate employed.
Modifying the basic coating composition with 0.9% hafnium also
results in a coating performance improvement. The 900 hour life is
roughly a 30% improvement of the base line plasma spray
composition. Adding 1.6% silicon to the basic NiCoCrAlY composition
improves the coating life by about 70%, from about 700 hours to
about 1200 hours.
In view of these results, it is not surprising that combinations of
silicon and hafnium produce an additional increase in coating
durability. What is surprising and unexpected is the degree of
improvement. The coating composition with additions of 0.6% silicon
and 0.7% hafnium displays substantially improved performance.
Testing has not proceeded long enough to produce coating failure
but it appears that the coating life will be at least 2200 hours
and probably about 2500 hours. This performance is unexpected in
view of the prior experience with silicon and hafnium alone. Since
hafnium alone provides a 30% improvement in life and silicon alone
provides a 70% improvement in life, it might be expected that a
combination of silicon and hafnium would produce, at most, a 100%
improvement in coating life. Instead, what is observed is a coating
life improvement of more than 300%. In this connection, it should
be noted that the amounts of silicon and hafnium added in the case
of the invention are less than the amounts of silicon and hafnium
which are added individually.
As shown in FIG. 1, the hafnium plus silicon modification to the
NiCoCrAlY composition provides substantial benefits in extending
coating life under conditions of cyclic oxidation. The exact
reasons for the improvements are not well understood and we do not
wish to be bound by any theory.
In addition to the cyclic oxidation testing previously described,
the resistance of the invention coating to hot corrosion has also
been evaluated. Hot corrosion occurs in gas turbine engines
especially those that are operated near marine environments. It
results from various salts which are present in the atmosphere and
fuel, particularly sodium chloride. Hot corrosion occurs
principally at intermediate temperatures. Consequently, the
following testing cycle was used to determine the hot corrosion
resistance of the subject coatings. The coated test bars were
heated for two minutes at 1750.degree. F. followed by two minutes
at 2000.degree. F. followed by two minutes of forced air cooling.
The heating steps were performed using a flame produced by the
combustion of jet fuel. To simulate a severe environment, 35 ppm of
synthetic sea salt was added to the air. The results show the
superiority of the invention coating. A vapor deposited coating of
NiCoCrAlY composition protected a single crystal substrate of the
previously described alloy for 202 hours before substrate attack. A
standard aluminide protective coating protected the substrate for
120 hours. A vapor deposited NiCoCrAlY plus Si coating protected
the substrate for 416 hours before failure. The invention coating,
plasma sprayed NiCoCrAlY plus Si plus Hf has protected a substrate
of the same material for 546 hours without failure and the
invention coating showed no sign of being near failure. Thus, the
invention coating has life which is at least two and a half times
that of the standard commercially used vapor deposited NiCoCrAlY
coating.
In most practical applications such as in gas turbines, the strains
which result from thermal cycling can also contribute to coating
degradation by causing coating cracking. For this reason, coating
ductility is measured to ascertain the tendency for cracking. It
has been found that ductility levels at 600.degree. F. are
indicative of whether coating cracking problems will be encountered
during gas turbine engine exposure. Therefore, coated specimens
were tensile tested at 600.degree. F. to measure the strain needed
to cause initial coating cracking. The addition of silicon to the
basic MCrAlY coating (in the amount necessary to significantly
improve oxidation resistance) reduced the ductility significantly.
However, by adding hafnium, the amount of silicon needed was
reduced, and the ductility was substantially increased.
The coatings of the present invention are particularly suited for
the protection of gas turbine engine components. Such components
are generally fabricated from nickel or cobalt base superalloys
which may have been in either cast or wrought form. Nickel base
superalloys are alloys based on nickel which are strengthened by
the gamma prime phase (Ni.sub.3 Al, Ti). With rare exception such
superalloys also contain chromium in amounts from about 8 to about
20% and usually also contain from about 10 to about 20% cobalt.
Refractory metal additions such as Mo, W, Ta and Cb may also be
present. The cobalt base superalloys do not contain a single
predominant strengthening phase but instead derive their strength
from the presence of solid solution strengthening elements such as
Mo, W, Ta, Cb and carbides which results from the presence of
elements such as Cr, Ti and refractory metals. Of course, carbon is
present in alloys which rely on carbide strengthening. Chromium is
usually found in amounts of about 20% in cobalt superalloys.
The method of fabrication of the superalloys has little effect on
its suitability for protection by the invention coatings. Cast
superalloy articles including polycrystalline columnar grain and
single crystal articles may all be protected, as may wrought
articles for example, sheet metal components.
In the past, the MCrAlY compositions have been applied by an
electron beam physical vapor deposition technique almost
exclusively, especially in the context of coating gas turbine
blades and vanes. The present invention composition would have
substantial protective capabilities when applied by vapor
deposition. However, vapor deposition of hafnium containing
coatings is difficult because of the low vapor pressure of hafnium
relative to the other coating constituents. Effective deposition of
the hafnium containing coating would probably require the use of a
dual source evaporation procedure in which one source would contain
hafnium and the other source would contain the balance of the
coating ingredients. Accordingly, we prefer the use of the plasma
spray process. In particular, we prefer to use high energy plasma
spraying in a chamber evacuated to low pressures.
The plasma sprayed coatings for which data are presented in FIG. 1
were produced using a low pressure chamber spray apparatus sold by
the Electro Plasma Corporation (model 005). The apparatus includes
a chamber in which the specimens were sprayed and this chamber was
maintained with an argon atmosphere at the reduced pressure of
about 50 mm Hg. The plasma spraying was conducted at 50 volts and
1520 amperes with 85% Ar-15% He arc gas. The powder feed rate was
0.3 lbs/minute of NiCoCrAlY+Si+Hf. Powder in the particle size
range of 10 to 37 microns was employed and the coating thickness
was about 5 mils.
We emphasize that the method of coating deposition is not
particularly critical so long as a dense, uniform, continuous
adherent coating of the desired composition results. Other coating
deposition techniques such as sputtering may also be employed.
It should be understood that the invention is not limited to the
particular embodiments shown and described herein, but that various
changes and modifications may be made without departing from the
spirit and scope of this novel concept as defined by the following
claims.
* * * * *