Note: Descriptions are shown in the official language in which they were submitted.
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Speclfication
Background Of The Invention
This invention relates to metal articles which are
subjected to elevated temperatures during use~ In particular,
the invention is concerned with a method for significantly
improving the elevated temperature corrosion resistance of
such articles whereby more satisfactory performance and longer
life for such articles can be obtained.
Elevated temperature exposure of metal articles is
experienced in many si~uations. Metal components are sub-
jected to such conditions, for example, in various aerospace
applications and ln land and marine operations such as in the
case of components utilized in gas turbine engines.
In all such applications, it is important to provide
some means for preventing undue corrosion of the components
involved since such corrosion can materially shorten the use-
ful life of the components. Deterioration of components can
also create significant performance and safety problems.
Various alloys, including most superalloys, are char-
acterized by a degree of corrosion resistance; however, such
resistance is significantly decreased when unprotected super-
alloy components are exposed at the operating temperatures in-
volved in certain systems. For that reason, such components
have been provided with coatings, such as aluminide coatings,
which increase the corrosion resistance at the extreme operat-
ing temperatures.
Aluminide coatings are applied by methods such as the
pack cementation process~ In this process, the substrate chem-
istry and the processing temperature exert a major influence
on coating chemistry, thickness and propertles. Specifically,
the coatings comprise a hard ? brittle outer layer and a hard 7
brittle multi-phase sublayer that can crack when subjected to
operating conditions~ This leads to poor fatigue properties
and the cracks can also materially reduce the corrosion resis-
tance of the coated components.
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~129Z6~i
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Another class of coatings is the MCrAlY overlay coat-
ings where M stands for a transition metal element such as Fe,
Co, or Ni. Presently, these coatings are applied by vacuum
vapor deposition of MCrAlY alloy on a superalloy surface.
Such MCrAlY coatings have been shown to have an advantage over
aluminide coatings in providing extended life to turbine com-
ponents. Specifically, MCrAlY coatings demonstrate greater
corrosion resistance than aluminide coatings and exhibit super-
ior ductility.
In the past, efforts to improve the corrosion resist-
ance of MCrAlY coatings generally have met with only limited
success. One approach is to increase corrosion resistance by
either raising the amount of aluminum or adding other benefi-
cial constituents to the coating. Unfortunately, this prac-
tice can result in embrittled coating compositions which fail
prematurely under thermal and mechanical stresses. A second
approach is to increase the thickness of the coating without
changing the composition. Unfortunately, thicker coatings can
be expensive to produce using slow deposition rate processes
such as vapor deposltion, and distinct thickness limitations
exist because very thick coatings tend to crack or spall when
subjected to the gas turbine engine environment.
The inventor's U.S. Patent No. 4,145,481, dated March
20, 1979, describes a process wherein coatings are applied to
superalloy su~strates and the like to improve the elevated
temperature performance of the articles. This process involves
the application of an overlay of an MCrAlY composition followed
by the application of an aluminide layer~ Furthermore, the
coated articles are subjected to a hot isostatic pressing
operation wherein pressures in excess of about 10,000 psi are
applied at temperatures in excess of about 2000 F.
Summary Of The Invention
This invention relates to the production of metal
articles resistant to corrosion and otherwise characterized by
highly efficient operation at elevated temperatures. A first
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coating is applied to an article surface, this coating comprising a cobalt,
nickel or iron ~ase alloy compatible with the substrate and characterized by
high ductility~ This coating essentially comprises from 15 to 30 percent by
weight chromium, 3 to 7 percent by weight aluminum, up to 5 percent by weight
of an element selected from the group consisting of yttrium and the rare
earth elements, the ~alance being selected from the group consisting of
nickel, cobalt and iron.
A second coating which is particularly highly resistant to
corrosion at elevated temperatures is applied over the first coating.
This second coating essentially comprises from lO to 30 percent by weight
chromium, 25 to 35 percent by weight aluminum, up to about 5 percent by
weight of one or more elements selected from the group consisting of yttrium,
scandium, cerium, hafnium, zirconium, titanium and tantalum, the balance
comprising a member selected from the group consisting of iron, nickel and
cobalt. The second coating has a thickness bet~een one-half and four times
the thickness of the first coating.
An elevated temperature treatment in the range of 1900 to 2200F
for a duration of one to 10 hours in an inert atmosphere (for example, in a
vacuum or in an argon atmosphere) follows the coating process. l'he heat
treatment achieves interfacial bonding, particularly minimizing any detri-
mental effects of thermal and mechanical stresses encountered during use. The
provision of the ductile first layer or coating serves to provide a barrier
against degradation of the corrosion resistance of the second layer or coating
and serves as a barrier against detrimental interdiffusion and crack propaga-
tion.
Various procedures may be employed for application of the coatings
or layers; however, it is preferred that the coatings be applied by means of a
plasma spraying operation. This type of operation is particularly advantageous
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since it permits an uninterrupted operation wherein the first layer is formed
in one stage of tfie process and the second layer of different composition beingformed in a second stage of t~e
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process. In addition, the plasma spray process offers suffi-
cient deposition flexibility to avoid an abrupt transltion
from ~he first layer to the second layer, if desired. More-
over, it permits great flexlbility in the choice of composi-
tions to be deposited, and relatively thick coatings can be
produced economically, if desired.
Brief Description Of The Drawings
Figure 1 is a reproduction of a pho~omicrograph mag-
nified 500 times illustrating a duplex coating produced in
accordance with this invention after heat treatment. The
first and second layers originally deposited are evident.
Figure 2 is a chart comparing the various coatings
in terms o~ hot corrosion durability of 1750F peak tempera-
ture.
Description Of The Preferred Embodiments
This invention involves a process for producing coat-
ings on metallic articles for the purposes of rendering the
articles resistant to corrosion and oxidation at elevated tem-
perature. The articles with which the present invention is con-
cerned comprise iron base, nickel base and cobalt base super-
alloys; dlspersion strengthened alloysi composites; and direc-
tional eutectics.
Accordlng to the invention, a ductile and corrosion
resistant metallic alloy is initially ~posited as an inner
layer on the substrate. Thereafter, an outer layer of another
metallic alloy is applied over the inner layer. The metallic
alloy of the outer layer is particularly characterized by a
composition which has greater resistance to oxidation and cor-
rosion but lower ductility at elevated temperatures than that
provided by the metallic alloy of the inner layer.
The application of both layers is preferably achieved
by plasma spraying, although other means such as physical vapor
deposition, ion plating, sputtering or slurry sintering may be
employed. In plasma spraying, the heated alloy particles are
impinged on the preheated surface of the metal article at very
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high velocity and temperature. Such particles, upon contact
with the metal article surface or with other applied parti-
cles, deform plastically and fuse and bond to the surface or
to the other applied particles thus producing a dense and ad-
herent coating. Plasma spraying is particularly desirable
since it is a generally less costly technique for accomplish-
ing the overlay coating and since the technique is applicable
to all contemplated coating compositions.
The ductile inner metallic alloy, whlch is applied
directly to the article surface> preferably comprises an alloy
having as a base element a transition metal comprising cobalt,
iron or nickel or a combination of these elements. Amounts of
chromium? aluminum, yttrium or other reactive elements such as
hafnium and zirconium are alloyed with the base metal pursuant
to the preferred practice of the invention.
The chemistry of the inner layer is contemplated such
that the superior oxidation and corrosion resistance of the sub-
sequently applied outer layer is not seriously degraded.
Further, the inner layer acts as a barrier to interdiffusion
of various elements between the outer layer and the substrate,
the outer layer thus having a tendency to be less embrittled
by the substrate elements which might have diffused otherwise.
There is also a lessened tendency toward embrittlement because
the outer layer is supported by a relatively ductile inner
layer, and not by a brittle, multiphase layer that is conven-
tionally the case.
The inner layer is applied in amounts between 0.0005
and 0.005 inches. The chemical composition of the inner layer
is envisioned such that it is chemically and mechanically com-
patible with the substrate on one side and with the outer layer
on the other side. This minimizes the occurrence of thermal
fatigue cracks due to strain mismatch at the substrate and coat-
ing interface and/or at the inner layer to outer layer inter-
face. Thus, the coating of the invention is particularly re-
sistant to spalling.
Widespread oxidation and corrosion of the inner lay-
er is avoided because the composition of the inner layer is
such that it is oxidation and corrosion resistant and further,
it is protected by the outer layer which is contemplated to be
highly oxidation and corrosion resistant. As noted, the com-
position of the inner layer preferably comprises a cobalt, iron
or nickel base alloy with aluminum, chromîum and yttrium or
other rare earth element additions~ The optimum composition
for a specific application will depend somewhat on the outer
layer composition and substrate involved~ However, aluminum
contents intermediate those of the substrate and outer layer
are desired in order to promote chemical stability at high
temperature while providing adequate ductility and corrosion
resistance.
The outer layer is applied in amounts between one-
half to four times the thickness of the inner layer. The chem-
istry of the outer layer is such that it is more oxidation and
corrosion resistant than the inner layer, albeit less ductile.
The preferred composition comprises a cobalt, iron or nickel
base material with appropriate aluminum and chromium additions.
Additionally, it is contemplated that: 1) up to 10 weight
percent of reactive elements selected from the group consisting
of yttrium, cerium, scandium, thorium, hafnium, zirconium,
titanium and tantalum may additionally be alloyed with the
aforementioned material for improved oxidation resistance, the
respective amounts of individual elements preferably varying
from about 0.05 up to 5.0 percent by weight; 2) The total a-
mount of aluminum and other oxidation/corrosion resistant
elements in the outer layer should be at least 18 weight per-
cent more than the amounts of these elements in the inner lay-
er thus providing the duplex coating with a larger reservoir
of protective elements.
If utilized alone on the superalloy substrate, a coat-
ing with the composition of the outer layer described above
will not consistently exhibit long-time oxidation, hot corro-
sion and ther~al fatigue resistance. Thus~ the outer layer
contalns a large amount of a less ductlle phase (aluminides
of nickel and/or cobalt) which tend to crack under high mechani-
cal and thermal stresses. Once a crack develops, an oxidizing
or other hot corrosive atmosphere can gain access to the under-
lying substrate thereby causing a premature coating failure.
As indicated, the presence of a ductile inner layer coating
inhibits the formation of and arrests the propagation of such
cracks and avoids such problems~ Thus, the advantages of hav-
ing a highly oxidation and corrosion resistant material as an
outer layer in the duplex coating can be exploited. In addi-
tion, coatings with relatively thick outer layers can be used.
Optionally, the addition of up to 5 weight percent
of rare earth and/or refractory metal oxide particles to the
aforementioned outer layer composition of the duplex coating
also is contemplated; these ingredients preferably being indi-
vidually utilized in amounts from about 0.05 up to about 1.0
percent by weight. This addition can be beneficial to the
over-all protective response of the coating because the reac-
tive metal oxide particles assist in plnning protective oxide
scales. This pinning phenomenon results in superior adherence
(less spalling) of the protective scale thus increasing the
over-all coating life.
The utilization of the plasma spray technique to de-
posit the contemplated two-layer duplex coating is preferred.
The technique is economical, and it enables application of the
duplex coating in one uninterrupted operation. Thus, the plas-
ma spray technique involves the utilization of powder of a de-
sired composition, this powder being fed into a plasma gun
prior to its application to a substrate. The plasma spray
mechanisms permit the changing of the composition during the
course of the spraying operation whereby the process can in-
volve the application of a first layer and the uninterrupted
commencement of the application of the second layer to promote
consistent metallurgical bonding between the two layers.
The plasma spray process is also adaptable to the
application of the layers in a gradient fashion. Specifical-
ly, the composition being fed to the spraying gun may be con-
trolled to avoid abrupt transition between the two layers,
Consîdering the subsequent examples, it will be noted that the
first layer includes low percentages of aluminum while the
second layer includes higher percentages of this element. The
utilization of powder compositions fed to the spraying gun
which include an increasîng ratio of the high aluminum con-
tent (outer~ coating is, therefore, contemplated. This arrange-
ment, when utilized at an intermediate phase of the coating
operation particularly tends to eliminate distinct areas of
demarcation between the coatings which makes the coatings
more suitable for withstanding physical and thermal stresses.
The plasma spray process also permits deposition of
a wide variety of coating materials encompassing broad ranges
of nickel, cobalt, aluminum, chromium, and various rare earth
and refractory metal elements~ Accordingly, different coating
compositions can be tailored for protectîon in different envi-
ronments where this process is used. Further, the process also
allows efficient introduction of oxide particles into the coat-
ing which would be difficult or impossible to incorporate if
other processing methods were used.
The following are examples of the practice of the
invention:
Example 1
A typical nickel base superalloy of the type used in
gas turbine engines, known as IN738, and having a nominal com-
position of 0.09%C, 16.0%Cr. 8.5%Co, 1. 7%MO ~ 2.5V/oW, 1. 7/OTa,
3.5%Ti, 3.5V/oAl, 0~01%B, 0~03%Zr and the balance Ni, was pro-
vided for coating.
The nominal composition of the metallic powder which
was used to deposit the inner layer of the duplex coating was,
by weight percent, 6V/oAl, 26%Cr, 0~6V/oY, 31%Ni and the balance
Co, The outer layer powder composition was, by weight percent,
26V/o~l~ 16%Cr, 0.6V/oY, 17V/oNi and the balance Co. The application
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of both layers was accomplished by utilizing the plasma spray
coa~ing technlque. Both alloy powders were sprayed in a plas-
ma arc (~ Mach 3 velocity) using argon and helium as primary
and secondary gases, respectively. Spraying was performed in
a chamber maintained at a pressure of 60 torr. The process
parameters were:
Gun to workplece distance . . . . . . 13 in.
Pr~mary gas (argon . . . . . . . . . 600CFH at
250 psi
Secondary gas (helium) . . . . . . . 150CFH at
250psi
Voltage . . . . . . . . . 85 volts
Current . . . . . . . . . . . . . . . 800 amp
Powder flow . . . . . . . . . . . . . 0.3 lb/min
Carrier gas (argon) . . . . . . . . . 50CFH at
250psi
Time for inner layer deposltlon . 20 sec.
Time for outer layer deposition . . 70 sec.
The articles were then subjected to heat treatment in a vacuum
for four hours at 1975F,
A 500X photomicrograph of this duplex coating ln the
unetched condition is shown in Figure 1. As controlled during
the spray operation, the thickness of the inner layer was ob-
served to be between 0.001 inches to 0.0015 inches and the
thickness distributlon of the outer layer ranged between 0.003
inches to 0,~04 lnches. The outer layer is essentially a
single phase (Co, Ni) Al compound containing up to 20 weight
percent Cr and 0, 6~/o~. This phase is very reslstant to oxida-
tion and corrosion and? therefore, was processed about three
(3) times as thick as the lnner layer to provide the coatlng
with a large reservoir of corrosion resistant material. The
inner layer is an intimate mixture of (Co, Ni) Al phase and
(Co, Ni? solld solution phasei This layer is less corrosion
resistant than the outer layer but is superior in ductility
and, therefore, supports the outer layer during thermal fatigue
and mechanical stress application.
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The performance of the articles coated pursuant to
this example was evaluated by using a 0,7 Mach burner rig
testing. The testing cycle was 1750F/2 minutes; 1450F/4
minutes; 1750~F/2 minutes; air cool/2 minutes. 5 ppm salt
solution was injected into the combustion products of JP5
fuel containing 0~2% surphur. The testing closely simulates
the gas turbine engine environment, highlights the hot corro-
sion phenomenon, and imposes significant thermal stresses on
the protection system.
The articles demonstrated a burner rig life of great-
er than 46Q hours/mil of coating thickness, This compares to
the 285 to 350 hours/mil lives exhibited by the single layer
overlay coatings and 100 hourslmil lives exhibited by aluminide
coatings as shown in Figure 2.
Example 2
Articles of above identified IN738 alloy were plasma
sprayed with an inner layer having a chemîcal composition of,
by weight percent, 7~/~1, 30%Cr, 0~4%Y, 26a/oNi and the balance
Co, and an outer layer having a chemical composition of, by
weight percent, 25/~1, 25%Cr, 0.7~/OY, 13C/oNi and the balance Co.
The thickness of the inner layer was controlled to between
O.OQ2" and 0.003", that of the outer layer was controlled to
between 0.0025" and 0.0035", and an abrupt transltion between
the two layers was avoided~ The outer layer cornprised an in-
timate mixture of (Co, Ni) Al and (Co, Ni) solid solution
phases rather than being a single (Co, Ni) Al phase, and this
duplex coating was slightly more ductile than the one described
in Example 1. Such a coating is more suitable ~or use in en-
vironments where the demand for thermal fatigue resistance is
particularly severe.
It will be understood that various changes and modi-
fications may be made in the above description which provide
the characteristics of this invention without departing from
the spirit thereof particularly as defined in the following
claims.
