Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The present invention relates to coatings and coated
articles and more particularly to coatings for the nickel-
and cobalt-base superalloys having high ductility while
retaining desirable stability and elevated temperature
oxidation and hot corrosion resistance.
Design trends for advanced gas turbine engines are
toward ever increasing turbine inlet temperatures, and
the demands on turbine materials have increased to the
extent where contemporary aluminide coating systems can
be the life limiting component of alloy-coating composites.
Coatings are prone to failure by a variety of mechanisms.
Aluminide coatings can be, for example, a source of
fracture initiation in fatigue. Coating ductility has
been found to be an important determinant in fatigue life
since at relatively low temperatures aluminide coatings
tend to crack in a brittle manner at low strains in the
tensile portions of the fatigue cycle. Although various
coatings, such as the CoCrAlY type coatings described in
U.s~patent to Evans and Elam 3,676,085, the NiCrAlY type
coatings described in U.s.patent to Goward, Boone and Pettit
3,754,903 and the FeCrAlY type coatings described in u.S.
patent to Talboom and Grafwallner 3,542,530 have in the
past provided significant improvements in the lifetimes of
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the superalloys, further improvements are, of course,
desirable. In particular, an improved coating having
properties comparable to the conventional coating alloys
together with significantly improved ductility would be
desirable and useful. Such an improved coating is found
in the nickel-cobalt-chromium-aluminum-yttrium system as
described herein.
SUMM~RY OF THE INVENTION
In brief, the present invention relates to a nickel-
cobalt-chromium-aluminum-yttrium coating alloy having
greatly improved ductility as well as other properties
which together render it eminently suitable for use in gas
turbine engine hardware and other rigorous environments.
The invention more particularly relates to a high ductility
coating alloy which possesses both oxidation-erosion and
sulfidation resistance and which consists of a particular
combination of nickel, cobalt, chromium, aluminum and a
reactive metal selected from the group consisting of
yttrium, scandium, thorium, lanthanum and the other rare
earth elements. The invention contemplates a coating
composition consisting essentially of, by weight, 11-48%
cobalt, 10-40% chromium, 9-15% aluminum, 0.01-1.0% of a
reactive metal selected from the group consisting of yttrium,
scandium, thorium, lanthanum and other rare earth elements,
balance essentially nickel, the nickel content being at
least about 15%. Advantageously, the coating composition
consists essentially of, by weight, about 15-40% cobalt,
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12-30% chromium, 10-15% aluminum, 0.01-1.0% yttrium,
balance essentially nickel, the nickel content being at
least about 15%.
In one preferred embodiment, the coating composition
consists essentially of, by weight, about 25-40% cobalt,
14-22% chromium, 13-15% aluminum, 0.01-1.0% yttrium,
balance essentially nickel.
In another preferred embodiment, the coating composition
consists essentially of, by weight, about 15-35% cobalt,
14-22% chromium, 10-13% aluminum, 0.01-1.0% yttrium,
balance essentially nickel.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a graph which dramatically illustrates
the ductility behavior of various nickel-cobalt-chromium-
aluminum-yttrium coating alloys as compared to representative
CoCrAlY and NiCrAlY coating alloys.
Figure 2 is a graph showing ductility as a function
of temperature of some NiCoCrAlY coating alloys as compared
to representative CoCrAlY and NiCrAlY coating alloys.
Figure 3 is a graph illustrating the diffusional
stability of various nickel-cobalt-chromium-aluminum-yttrium
coating alloys as compared to representative CoCrAlY and
NiCrAlY coating alloys.
Figure 4 is a graph illustrating the oxidation
characteristics of various nickel-cobalt-chromium-aluminum-
yttrium coating alloys as compared to representative
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CoCrAlY and NiCrAlY coating alloys.
Figure 5 is a graph illustrating the sul~idation
characteristics o~ various nickel-cobalt-chromium-aluminum-
yttrium coating alloys as compared to representative CoCrAlY
and NiCrAlY coa~in~ alloys.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the description which follows, reference will be
made to various of the conventional or contemporary nickel-
base and cobalt-base superalloys. Representative of alloys
of this nature are those identified in the industry as
follows:
NOMINAL COMPOSITION
ALLOY DESIGNATION (Percent by weight)
B-1900 . . . . . . . . 8 Cr, 10 Co, 1 Ti, 6 Al,
6 Mo, .11 C, 4.3 Ta,
.15 B, .07 Zr, balance Ni
MAR-M302*. . . . . . . . 21.5 Cr, 10 W, 9 Ta,
.85 C, .25 Zr, 1 Fe,
balance Co
TD*Cobalt Alloy . . . . . . 20 Ni, 18 Cr, 2 ThO2,
balance Co
TD Cobalt Alloy . . . . . O 20 Ni, 30 Cr, 2 ThO2,
balance Co
IN 100 . . . . . . . . 10 Cr, 15 Co, 4.5 Ti,
5.5 Al~ 3 Mo, .17 C,
.75 V, .075 Zr, .015 B,
balance Ni
MAR-M200 . . . . . . . . 9 Cr, 10 Co, 2 Ti, 5 Al,
12.5 W, .15 C, 1 Nb,
.05 Zr, .015 B, balance Ni
WI 52 . . . . . . . . . 21 Cr, 1.75 Fe, 11 W,
2(Nb + Ta), .45 C, balance
Co
Udimet 700 . . . . . . . . 15 Cr 18.5 Co, 3.3 Ti
4.3 A~, 5 Mo, .07 C, .d3 B,
balance Ni
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It will be appreciated that while the superalloys including
those which are directionally solidified, taken as a class,
are generally oxidation resistant, it is a necessary and
usual practice to coat certain of the components formed
therefrom in order to improve their oxidation, sulfidation,
erosion and thermal shock resistance and thus extend their
operating lives in advanced gas turbine engines.
As noted hereinbefore, the CoCrAlY and NiCrAlY
coatings have provided significant improvements in the
lifetimes of the superalloys. However, it was found that
NiCrAlY coatings, while providing extremely high oxidation
resistance and diffusional stability required improvement
in sulfidation resistance and that CoCrAlY coatings,
while providing extremely high sulfidation resistance
required improvement in oxidation resistance and diffusional
stability. In an effort to develop a better combination
of properties, a variety of overlay coatings was evaluated.
It was found that coating alloys of a composition, by
weight, of 11-48% cobalt, 10-40% chromium, 9-15% aluminum
0.01-1.0% reactive metal selected from the group consisting
of yttrium, scandium, thorium, lanthanum and the other rare
earth elements, balance essentially nickel, the nickel
content being at least about 15%, preferably 15-40% cobalt,
12-30% chromium, 10-15% aluminum, 0.01-1.0% yttrium, balance
essentially nickel, the nickel content being at least about
15%, and most preferably (1) 25-40% Co, 14-22% Cr, 13-15%
Al, 0.01-1.0% Y, balance essentially Ni and (2) 15-35% Co,
14-22% Cr, 10-13% Al, 0.01 1 0% Y, balance essentially Ni
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dramatically and unexpectedly gave an increase in ductility
while providing a satisfactory and adjustable balance of
oxidation and hot corrosion resistance as well as
acceptably low interdiffusional characteristics. While it
had been known that certain of the useful NiCrAlY coatings
exhibited a ductility higher than certain of the useful
CoCrAlY coatings and it had been surmised therefore that
a substitution of some nickel for the cobalt in the CoCrAlY
composition might improve ductility, it was surprising and
unexpected that the nickel-cobalt-chromium-aluminum-yttrium
system as defined above would provide a ductility improvement
which was markedly superior to either the NiCrAlY or CoCrAlY.
While not completely understood at the present time,
it appears that there is a correlation between coating
ductility and the phases present. More specifically,
chemistry changes which increase the amount and continuity
of the (Ni, Co) solid solution phase, y , tend to increase
coating ductility while chemistry changes which increase
the amount and continuity of the (Ni, Co) Al, ~ , Ni3Al,
~1 , and Cr, oC , tend to decrease ductility. Correlation
of coating microstructure with coating chemistry indicates
that, in the nickel-cobalt-chromium-aluminum-yttrium system
herein described, desirable ~ - ~ microstructures
are obtained at a higher aluminum content, the increased
stability of the ~ - ~ microstructure caused by cobalt
additions to NiCrAlY being the result of a significant
reduction of the amount of ~ (Ni3Al) and oc(chromium)
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phases which are precipitated at lower temperatures.
Those skilled in the art will recognize that certain
other elements are known to be compatible with the basic
chemistry of the present alloys. Accordingly, other
elements such as tantalum or hafnium may be advantageously
added to the alloy as required in certain applications for
modification of the mechanical, diffusional or hot corrosion
characteristics of the coating.
In coating the nickel-base and cobalt-base turbine
blades and vanes the surfaces to be coated are first
thoroughly cleaned free of all dirt, grease and other
objectional foreign matter followed by conditioning by
abrasive blasting. The coating is achieved by vapor
deposition from a suitably heated molten pool of the
coating material held in a vacuum chamber at 10 4 torr or
better. The ingot melted and evaporated by electron beam
heating has essentially the same chemistry as that of the
desired finished coating.
Parts are preferably preheated to 1750F + 50 for
five to six minutes before deposition is initiated and
this temperature is maintained throughout the coating
operation. Deposition time varies somewhat but is controlled
to obtain the preferred coating thickness of .003-.005 inch.
Subsequent cooling to below 1000F is accomplished in a
nonoxidizing atmosphere. Following the coating step, the
parts may be heat treated for one hour at 1900F + 25 in
vacuum to more fully bond the coating to the substrate and
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provide for easier peening.
The coated articles may be dry glass bead peened using
.007-.011 inch diameter beads with an intensity equivalent
to 19 N. In general, the peening is conducted in accordance
with the provisions of the processing specification AMS 243
The parts may then be heated to 1975F + 25 in dry argon,
dry hydrogen or vacuum; held at heat for four hours; and
cooled in the protective atmosphere at a rate equivalent
to air cooling. Blades and vanes so processed exhibit a
coating thickness, excluding the diffused zone of 0.003-
0.005 inch.
Of course, it will be recognized that other methods
for applying the coatings may be practiced, such as sputtering,
ion plating or plasma spraying, without departing from the
intent of the present invention.
Referring to Figure 1, a graph is shown of the unexpected
ductility behavior of various nickel-cobalt-chromium-
aluminum-yttrium coating alloys as compared to representative
CoCrAlY and NiCrAlY coating alloys. The results shown
therein were obtained by measuring strain to fracture of
coatings deposited on tensile specimens of appropriate
superalloys. In particular, Curve A is a plot showing the
effects of substituting various amounts of cobalt for
nickel in a NiCrAlY alloy having a nominal composition of,
by weight, Ni-19Cr-14Al-0.5Y while Curve B is a plot
showing the effects of substituting various amounts of
cobalt for nickel in a NiCrAlY alloy having a nominal
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composition of, by weight, Ni-19Cr-12.5Al-0.5Y. As is
evident from the drawing, dramatic increases in ductility
are obtained and it has been found, in general, that
NiCoCrAlY, or CoNiCrAlY as the case may be, coating alloys
have compositional ranges consisting essentiaLly of, by
weight, 11-48% Co, 10-40% Cr, 9-15% Al, 0.1-1.0% reactive
metal selected from the group consisting of yttrium,
scandium, thorium, lanthanum and the other rare earth
elements, balance essentially nickel ~at least abo~t 15%),
preferably 1~-40% Co, 12-30% Cr, 10-15% Al, 0.1-1.0% Y,
balance essentially Ni, the nickel content being at least
about 15%, will be effective in this regard. As will be
appreciated, with the higher Al content, as shown by Curve
A, a generally higher range of cobalt is preferred, a
preferred coating consisting essentially of 25-40% Co,
14-22% Cr, 13-15% Al, 0.01-1.0% Y, balance essentially Ni.
With lower Al content, as shown by Curve B, a generally
lower range of cobalt is preferred, a preferred coating
consisting essentially of 15-35% Co, 14-22% Cr, 10-13% Al,
0.01-0.1% Y. In Figure 2, ductility curves for selected
coatings show ductility as a function of temperature and
indicate the markedly superior tensile cracking resistance
of the NiCoCrAlY coatings.
In one series of thermomechanical fatigue tests, a
directionally solidified specimen substrate of ~R-~I200*
(with hafnium) was coated with Ni-24Co-16Cr-12.SAl-0.3Y
and run on a thermomechanical fatigue machine which pushes
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and pulls the specimen in severe fatigue and temperature
cycles which simulate the strain-temperature cycle of a
cooled turbine blade. A number of identical substrates
were coated with Co-20Cr-12Al-0.5Y and anotller number with
a diffusion aluminide coating. Both the CoCrAlY and the
diffusion aluminide coated specimens failed after approxi-
mately 1,000 cycles or less on the thermomechanical fatigue
machine whereas the NiCoCrAlY coated specimen did not fail
until after 1,925 cycles.
Referring to Figures 3-5, a comparison of the inter-
diffusional, oxidation resistance and corrosion resistance
properties of various NiCoCrAlY alloy coatings is shown.
In the drawings, 3-5 mil coatings of NiCoCrAlY alloy
consisting essentially of ~he indicated amounts of cobalt,
18-21% Cr, 13-14% Al and 0.05-008% Y were vapor deposited
onto B-l900 substrates as well as onto directionally
solidified M~R-M200*(plus Hf) substrates (erosion bars).
In Figure 3, the coated samples were aged 100 hours in air
at the indicated temperature. In Figure 4, coated components
were subjected to 2000F cyclic burner-rig oxidation tests
(2000F, 29 minutes - forced air cool, one minute, JP 5 fuel
used) for up to 2,100 hours (2,030 hours hot time). In
Figure 5, coated components were treated under cyclic
conditions (1,750F, three minutes - 2000F, two minutes -
cool, two minutes) in a high velocity hot gas stream derived
from the combustion of JP 5*jet fuel, with 35 ppm salt/air
added. As will be appreciated, the claimed NiCoCrAlY
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coatings, while giving unexpectedly increased ductility
also simultaneously give adjustable and satisfactory
degrees of interdiffusion and oxidation and hot corrosion
resistance.
For a clearer understanding of the invention and, in
addition to the data given in the drawings, other speciEic
examples are set forth below.
Examples 1-5
Five B-l~OO Ni-base alloy erosion bars were coated
with a 3-5 mil thick alloy having a composition, consisting
essentially of, by weight, Co-20Ni-24Cr-15Al-0.75Y generally
in accordance with the procedures outlined above. The
coated erosion bars were subjected to 62.5 hours of vane
cyclic sulfidation testing (1750F, three minutes - 2050F,
two minutes - cool, two minutes with 35 ppm artificial sea
salt: air ingested after combustion and using JP 5 fuel).
The coatings exhibited a specific life of from 21.1-24.4
hours/mil and were comparable to Fe-27Cr-13Al-.75Y coatings
which exhibited specific lifetimes of 22.2-27.9 hours/mil.
Example 6
A 3.6 mil coating of Co-20Ni-24Cr-15Al-0.75Y was vapor
deposited onto a MAR-M302*Co-base alloy erosion bar and
subjected to a modified vane cyclic sulfidation test
(1750F, three minutes - 2150Fj two minutes - cool, two
minutes with 35 ppm artificial sea salt: air ingested after
combustion using JP 5 fuel) in order to evaluate diffusional
stability combined with the very high temperature sulfidation.
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~045421
The coating had a failure time of 162 hours and a specific
li~e of 45 hours/mil.
Examples 7-10
Two B-1900 Ni-base alloy erosion bars and two ~R-~1302*
Co-base alloy erosion bars were coated with nominally three
mil thick coatings of Co-20Ni-24Cr-15Al-0.75Y as above and
were subjected to oxidation-erosion testing at 2000F until
failure. The B-1900*coatings failed at 263.2 and 153.7 hours
while the MAR-M302 coatings both failed at 309~2 hours.
Examples 11-14
Coatings consisting essentially of Co-20Ni-20Cr-12Al-
O.5Y, Co-20Ni-16Cr-16Al-O.SYg Ni 32.5Co-20Cr-12Al-0.5Y and
Co-20Cr-12Al-0.5Y were vapor deposited to thicknesses of
4.5-5,5 mil on Co-20Ni-18Cr-2ThO2 alloy airfoi] specimens.
All coatings were essentially a two phase mixture o~ beta
CoAl or (CoNi)Al and gamma solid solution. The Co-20Ni-16Cr-
16Al-0.5Y coatings were predominantly beta with a small
volume percent solid solution gamma phase. The beta phase
was continuous and represented an undesirable structure
because of its potential low strain-to-crack characteristics.
The Co-20Ni-20Cr-12Al-O.SY and the Co-20Cr-12Al-O.SY
coatings also exhibited a continuous beta type structure
but contained substantially more gamma. The Ni-32.5Co-20Cr-
12Al-0.5Y had a desired two phase plus gamma structure with
the gamma phase being the continuous matrix phase.
These systems were exposed in a static air environment
for 100 hours at 2000F, 2100F, 2200F and 2400F to
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1045~2~L
evaluate stability and elemental interactions. Theresultant coating hardness after exposure, showe~ no
detrimental change in hardness or brittle layer formation.
The Co-20Ni-16Cr-16Al-0.5Y composition retaincd its
continuous beta structure during exposure and, due to its
high crack susceptibility was not tested further. The
other coating systems retained or transformed to a two
phase mixture of beta in a continuous gamma matrix. The
best stability was obtained with the Ni-32.5Co-20Cr ~2Al-
0.5Y coating.
Additional airfoil shaped specimens of Co-20Ni-18Cr-
2ThO2 were vapor deposition coated with Co-20Cr-12Al-0.5Y,
Co-20Ni-20Cr-12Al-0.5Y and Ni-32.5Co-20Cr-12Al-0.5Y to a
thickness o~ 4.5-5.5 mil using the same techniques and
subjected to 1800F, 2000F, 2200F and 2400F isothermal
oxidation testing, to 2200F cyclic oxidation testing
(1750F, three minutes - 2200F, two minutes - cool, two
minutes) and to 2200F cyclic hot corrosion testing (1750F,
three minutes - 2200F, two minutes - cool, two minutes).
In all testing the airfoil samples were rotated at 1,750
rpm in a 400-500 feet/second gas stream of combusted JP 5*
fuel. For cyclic hot corrosion testing, the ~uel was doped
with 0.3% butyl disulfide and synthetic sea salt solution
was injected into the combusted flame to yicld a 3.5 ppm
salt concentration in the burner flame.
The 1800F and 2000F isothermal oxidation tests were
discontinued at 214 and 222 hours, respectively. ~11
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A
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specimens showed no visual signs of degradation. Based
on metallographic examination of specimens from the 1800F
tests, coating degradation was least for the Ni-32.5Co-
20Cr-12Al-0.5Y. Also in the 2000F test, the NiCoCrAlY
coating exhibited the least degradation. The extent of
degradation of the CoNiCrAlY and CoCrAlY coatings was
approximately equal.
The 2200F isothermal oxidation test was discontinued
at 305 hours. Again the NiCoCrAlY coating showed the least
degradation while the CoCrAlY coating showed the most.
The 2400F isothermal oxidation test was run to coating
failure. Of the three coatings systems evaluated, the
NiCoCrAlY composition exhibited the longest life, 226 hours.
The cyclic oxidation and cyclic hot corrosion tests
were discontinued at 207 (59 hours hot time~ and 204 (58
hours hot time) hours, respectively. Coating failure had
not occurred. Essentially no difference was observed in
the structure between the three samples in the hot corrosion
test. However, in the cyclic oxidation test, the Ni-32.5Co-
20Cr-12Al-0.5Y coating exhibited a far greater amount of
retained beta than either of the other two.
Examples 15-16
In a series of especially severe engine tests, first
stage turbine blades of the alloys indicated were coated
as indicated in Table I and run for 297 hours including
2,000 cycles (acceleration to full takeoff power followed
by holding for a period of time, rapid deceleration to idle
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power and holding for a period of time). Over 100 cycles
were with water injection (for thrust augmentation) which
imposed the severest possible thermal shock to the coatings.
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1~45~21
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1045421
While NiCrAlY had not previously cracked in other
engine tests and is therefore considered acceptable for
most engine conditions, this test was particularly severe
and, as shown, only the NiCoCrAlY coated blades were
completely free of coating cracks. In similar tests,
CoCrAlY coatings consistently cracked.
It has been clearly established that the inventive
alloy coatings are effective not only in providing long
term oxidation resistance, corrosion resistance and
stability but dramatically improved ductility.
What has been set forth above is intended primarily
as exemplary to enable those skilled in the art to
practice the invention and it should therefore be under-
stood that, within the scope of the appended claims, the
invention may be practiced in other ways than as specif-
ically described.
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