Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~066~43 13DV-S828
Back round of the Invention
-
This invention relates to metallic coatings and, more
particularly, to metallic coatings including aluminum applied to internal and
external surfaces of an article.
An important effort in the evolution of gas turbine engines has
been the development of high temperature operating coatings to protect the
surface of certain engine components from environmental attack and degrada-
tion. Generally, such higher temperature operating components are of a
metal based on an element selected from Fe, Co, Ni and Ti. The more
advanced designs of such components as turbine blades included such cavities
as channels and small holes within the blade and communicating with various
surface portions of the blade to allow a cooling fluid such as air to pass
through and reduce the temperature of such a component,
Although there have been reported a wide variety of coatings
and methods for applying coatings to the outer surface of such components,
the ability of such methods to coat the internal surfaces of small holes,
channels and other internal cavities is greatly restricted. For example, the
diffusion aluminiding process of the general type described in U. S. Patent
3, 667, 985 - Levine et al, issued June 6, 1972, used for external coatings,
is limited in throwing power, i. e., in its ability to coat very far into holes
with high length/diameter ratios. Similarly, electroplating techniques
cannot plate inside narrow chambers or holes because electric fields are
excluded. Physical vapor deposition and thermal spraying are essentially
line-of-sight processes that cannot deposit on the hole surfaces.
Summary of the Invention
It is a principal object of the present invention to provide an
1066143 1 3DV - 58 28
improved method for coating an article on a surface of an inner cavity using
a method which is compatible with the composition and thermal mechanical
properties of the substrate material.
A further object is to provide an improved method for coating
an article both on its outer surface as well as on a cavity inner surface using
a method for each surface which includes compatible processing steps.
Another object is to provide a method for depositing aluminum
or an alloy of aluminum on the surface of the inner cavity and then aluminid-
ing the outer surface of the article employing thermal conditions which at
the same time heat treats and homogenizes the coating on the inner surface.
Still another object is to provide a coated article having a
coating on its outer surface and a thermally decomposed and homogenized
type of coating on the inner surface of a cavity.
These and other objects and advantages will be more clearly
understood from the following detailed description, the drawings and the
specific examples, all of which are intended to be typical of rather than in
any way limiting on the scope of the present invention.
Briefly, the method associated with the present invention, in
one form, includes the steps of contacting the metallic inner surface of an
article with an organic compound or mixture of organic compounds which
include a metal selected from Al and alloys including Al, such compound
being capable of decomposition, such as thermal decomposition, to provide
the metal as a deposit, the term "metal" being intended herein to include
alloys. Then the article is either heated to diffuse the deposit and substrate
elements in a desirable way, or the outer surface of the article is subjected
to a separate coating process which includes a thermal treatment in a
particular range that will cause the coating deposit on the inner surface to
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~066143 , " ,~,
diff-J~e with su~)fitrate ~ lHn~ in a ~ ilal)lf~ w;~y, 11l ~ s~ond fc~rln tht?
method includes the steps of contacting the inner surface of a heated article
cavity with a sequential series of individual or mixed organic compounds
which include one or more metals or alloys selected from aluminum,
chromium, nickel or their alloys or mixtures, such organic compounds being
capable of decomposition, such as thermal decomposition, to provide a
deposit of the metals or alloys. The subsequent heating then causes the
coating deposits to interdiffuse with each other and with substrate elements.
The article associated with the present invention, in one form,
includes on the inner surface of an alloy based on an element selected from
Fe, Co, Ni and Ti a first or inner coating of a metal selected from aluminum
and a]loys inclutling aluminum. l'he inner coatinK ha~i a structllre ( haracter
ized by either (a) a single layer of pure aluminum, or (b) a plurality Or
single metal portions or layers of elements diffused together constituting an
alloy including aluminum, or (c) a single alloy layer, or (d) a layer consist-
ing of a mixture of elements or phases that constitute the alloy. In any event,
the inner coating is characterized by a coating metal activity sufficient to
form, in an oxidizing atmosphere, a protective scale which includes at least
one of the oxides of Al and Cr or the spinels including at least one of Al and
Cr,
The outer surface of the article includes a second or outer
coating which may be identical to, and produced in the same manner as, the
inner coating on the inside of the article, or it may be a different metallic
coating produced by one of a variety of known methods which are readily
adaptable to the metallic coating of the exterior of articles. In one form,
the outer surface can include an outer coating applied by a diffusion process,
such as that described in the above-mentioned U, S Patent 3, 667, 985,
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~066~43 1 3DV - 58 28
that provides the thermal energy to interdiffuse ~he elements deposited on the
interior of the article with each other and with the internal surface of the
substrate article material. In another form, the outer surface may include
as an outer coating a physical vapor deposit processed in a temperature range
that is suitable for the interdiffusion of the elements deposited on the interior
surface of the article with each other and with the substrate article material.
It will be apparent to those versed in the art of coat1ng that
conventional coating processes as may be applied to the exterior of articles
cannot be applied with complete coverage to the internal cavities of articles,
particularly those with cavities that open to the exterior surface through long,small diameter holes. For example, pack cementation processes for deposit-
ing aluminum or alloys of aluminum do not have the throwing power to coat
internal cavities or long narrow holes extending to the exterior surface of an
article. Similarly, thermal spray coating methods, electrolytic plating,
and physical vapor deposition do not have the throwing power required to coat
entire surface areas of internal cavities through small diameter long holes.
An important feature of the present invention is the provision of a fluid which
can be flushed into the internal cavities of an article through small holes
connecting to the exterior surface, thereby effecting complete and uniform
coverage by thermal decomposition of the fluid. Another important feature
of one form of the present invention is the formulation of sequential flushing
procedures and thermal treatments with specially selected organic compounds
containing aluminum or other elements of alloys including aluminum to provide
environmental protection to the internal surfaces.
Brief Description of the Draw~
Figure 1 is a photomicrograph at 500 magnifications showing
the surface portion of an article coated in accordance with the present invention;
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~066143 1 :3nv- snz~
Figllle 2 is a ~rapllical l~re~en~atioll (tt all electroo microprobe
analyzertrace across the coated portion OI Figure l;
Figure 3 is a photomicrograph at 1000 magnifications showing
another embodiment of the surface portion of an article coated in accordance
with the present invention; and
Figure 4 is a photomicrograph at 250 magnifications of a
blade trailing hole coated in accordance with the present invential.
Description of the Preferred Embodiments
There are a variety of metallic compounds, generally
organometallic compounds, which have the characteristic of decomposing at
relatively low temperatures to provide a metal in free form and a gaseous
product. In one reported use of such a compound to provide a coating,
sometimes called chemical vapor deposition, the vapor of such a compound
is decomposed at a heated surface so that metal is caused to deposit in a
substantially pure and dense state. Frequently, organoaluminum compounds
are used to deposit aluminum in this way, However, such chemical vapor
deposition frequently involves the use of a reactant, such as
triisobutylaluminum, hereinafter referred to as TIBA, which in pure form
ignites in air, reacts violently with water, and burns the skin severely on
contact. Such compound was used in the evaluation of the present invention
modified by dissolving TIBA in a hydrocarbon, such as kerosene. In this
way there is obtained a solution that can be exposed to air with little danger
and with relatively minor effect on skin.
Thus, the coating of the present invention on the inner surface
of a cavity, in one evaluation of the present invention, was applied under
inert atmosphere by decomposing TIBA from a 20 - 30 weight percent solution
in kerosene by flushing such solution through the inner surfaces of cavities
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~066~43 13DV - 5828
in a gas turbine engine blade heated in the range of about 160 - 220C. Below
about 160C, deposition of Al does not occur. The blade was made of a
nickel-base superalloy, sometimes referred to as Rene' 80 alloy, and
described in U. S. Patent 3, 615,376 - Ross, issued October 26, 1971. Such
alloy included nominally, by weight, 0.17% C, 14% Cr, 5% Ti, 0.015% B,
3% Al, 4% W, 4% Mo, 9. 5% Co, 0.06% Zr, with the balance essentially Ni.
Such cavities, which in this case were small holes through the airfoil of the
blade, can be generated by a variety of means well known in the art. In this
example, the holes were generated through the use of electrolytic drilling.
~fter deposition of the decomposition coating on the inner surface of the
cavity, such deposit was freed from volatile materials by heating at a low
temperature, such as about 450F (about 230C)for about 10 minutes, within
the broader range of about 300 - 700F (about 150 - 370C). The article was
then raised in a predetermined time to a temperature sufficiently high to
permit interdiffusion of the aluminum deposit with the Rene' 80 alloy inner
surface. Such a temperature was about 1925F (about 1050C) in the broader
range of about 1600 - 2000F (about 870 - 1100C), and preferably 1800 -
2000F (980 - 1100C), achieved during a period of about 20 - 30 minutes.
Thus, the preferred programmed time-temperature cycle of the present
invention, in this example, was heating in the range of about 300 - 700F
(about 150 - 370C) and then increasing the temperature during the next 20 - 30
minutes to about 1800 - 2000F (980 - 1100C). The result was an article,
including a cavity inner surface coating, having a structure characterized
by the diffusion of aluminum into the cavity surface substrate and by the
diffusion outwardly into the coating of nickel and chromium from the cavity
surface Rene' 80 alloy substrate, as evidenced by a beta NiAl outer layer,
with isoiated small chromium phases, and a diffusion zone including gamma
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~66143 13DV-5828
prime Ni3Al, carbides and other phases.
In another evaluation of the present invention, a gas turbine
engine blade, which was made of the above-described Rene' 80 nickel-base
superalloy, first was cleaned by vapor honing and then was heated in the
preferred 180 - 220C temperature range within an inert gas enclosure which
also included a bath of the above-described TIBA kerosene solution. During
immersion and while the turbine blade was still hot, aluminum was deposited
on all in~ernal and external surfaces. The deposition of aluminum stopped
when the temperature of the blade became too low, for example below about
160C, through the loss of heat to the kerosene solution. Following the
deposition of aluminum, the blade was removed from the solution, rinsed,
dried and then heated to about 600F (about 315C) for volatilization of
kerosene, organic compounds and water. Then it was quickly heated to a
high temperature in the 980 - 1100C range, specifically about 1050C to
lS permit the interdiffusion of the deposited aluminum with the Rene' 80 alloy.
Thus, the inner and outer surfaces of the blade were characterized by the
diffusion coating structure in which aluminum was diffused into the Rene' 80
alloy substrate and nickel and chromium from the Rene' 80 alloy was diffused
outwardly. This was evidenced by a beta NiAl outer layer with isolated
small chromium phases and a diffusion zone consisting of gamma prime
Ni3Al, carbides and other phases.
Examples 1 - 7
In more specific evaluations of the present invention, a series
of the above-described Rene' 80 alloy turbine blades, including cooling
passages communicating wit~ the surface of the blade airfoil, were coated on
the inner surfaces of such cavities by passing a kerosene solution of about
20 wt. % TIBA through the blade cavities or passages while the blade was
1~66~43 l.~l~V-~2~
he;ll.c(l in(lu(tively ~o ~)lovide nt thc inner ~3u~rn(e ol I.he cnvities n telnl~eln-
ture oî between about 180 - 260C. The internal coating generated ranged
in thickness from about 0. 1 to about 3. 8 mils. The following Table I
summarizes the various parameters, coating thicknesses and coating
appearances in these examples:
\
\\ .
\
~)66143 1 3DV - 58 28
o o
¢ ~ o
o = = o
O O Q~
u .~
¢ ~ Z ~
~ o B ~ . . ~ N ~ N
¢~ ~ q o F~ al
Z O U~
U
Z U~
¢ ,_~ O O O
~ '
~0 ~ ~ g $ g g
O O O C~
C~ X
5 c~
.~
~ ~ N ~) ~ O CD t--
U~ O
_ 9 _
10661~3 1 3DV - 58 28
From these and other examples, it has been determined that
heating the surface to which the aluminum is to be applied from the decompo-
sition of TIBA at a temperature above about 220C, an undesirable porous,
powdery deposit results. In addition, it has been recognized that at tempera-
5 tures below about 180C, the deposition rate is very slow with the depositionof aluminum stopping below about 160C. Therefore, in the preferred form
of the method associated with the present invention, the temperature of the
inner surface upon which the aluminum is to be deposited as a result of
decomposition of the organometallic compounds is in a temperature range of
10 about 180C to about 220C with the specific preferred temperature being
about 200C. The preferred deposit thickness is in the range of about 0.1 to
less than about 2 mils in order to avoid plugging of the specific cavities
involved in these examples and the generation of a powdery coating.
Example 8
In another series of evaluations,l mil deposits applied as in
the above examples were heated in a non-oxidizing atmosphere, for example
in a vacuum or in an inert atmosphere, at about 600F (about 315C) for a
time sufficient to drive off volatile materials such as gases, after which the
temperature was raised within 30 minutes, and preferably about 20 minutes,
20 to a temperature of about 1950F ~about 1066C) where it was held for about 4
hours prior to being cooled to about 500F (about 260C) before opening to the
atmosphere. It has been found, as shown by comparison of this example
with the following examples, that the heat treatment associated with the
present invention is critical in order to obtain a structure which is character-
25 ized by a beta NiAl outer layer and a diffusion zone between the outer layerand substrate, the outer layer bei~g about twice the thickness of the diffusion
zone. In these examples, the sum of both thicknesses was about 2 mils.
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1~66143 13DV-5828
The microstructure so obtained is shown in the photomicro-
graphic of Figure 1 at 500 magnifications and is typical of a
pack aluminided coating of hiyh Al activity with subsequent
ductilizing thermal treatment. An electron microprobe analyzer
trace across the coating also is typical and is shown in Figure 2.
The oxidation and corrosion resistance of coatings defined by
this type of microstructure is known i~ the art to be very good.
As has been stated above, a critical heat treatment is re-
quired by the present invention for the decomposition coating
applied to the inner surface in order to generate a desired
final coating structure It was recognized that such a structure
and heat treatment could be accomplished concurrently with the
application of an aluminide coating to the outer surface of the
article being protected, in this case a turbine bucket. One
aluminide coating process commercially available and widely used
in connection with the application of aluminide coatings to gas
turbine engine components is the coating method sometimes referred
to as CODEP coating, forms of which are described in the above-
identified U.S. Patent 3,667,985 dated June 6, 1972. In that
method, as in other diffusion aluminiding methods, the coating
ordinarily i5 generated in the range of about 1600 _ 2100F
(870 - 1150 C) In the evaluation of the present invention, it
has been recognized that i*s critical heat treatment involves
first the evaporation of volatile compounds, for example at
about 600 F (about 315 C) and then the heating rapidly through
the aluminum melting range up to a temperature at which the
terminal deposit diffuse into the surface on which it has been
applied
Example 9
A 1/4 mil Al deposit was applied to a Rene' 80 alloy turbine blade
by the method described in Example 1-7 above, and then treated by
1066143 ~ v r~
l~ea~ing in ~1) inel~l ~tmo:;phele a:; rollows: I'ilstto (;()()"1~ (31rC), th~u In 2n
minutes to about 1925F (about 1052C), held fol four hours f~t th~t leml~el ~-
ture, and then cooled to below about 500F (about 260C). The coating, show~
in the photomicrograph of Figure 3 at 1000 magnifications, was about . 0009
inch thick with a fairly large equiaxed grain structure. Comparison to
Figure 1 shows that this coating, which is another embodiment, does not have
appreciable, minute sigma phases in the outer coating, indicating a softer
structure typical of a relatively lower Al activity pack aluminide process,
but of sufficient activity to form, in an oxidizing atmosphere, a protective
aluminum oxide scale.
Example 1 0
Inner surfaces of internal cavities of a Rene' 80 alloy blade
were flushed with the above-described TIBA in kerosene solution by injecting
the solution in blade shank entry holes used for engine cooling air. The
solution flowed through an internal labyrinth path and drained out of leading
edge and trailing edge holes while the blade was heated at a temperature of
about 200~C as described above, The photomicrograph of Figure 4 at 250
magnifications shows the section of the trailing edge holes after the heat
treatment of Example 9 which produced the outer layer of beta NiAl and the
diffusion zone. Thus, there was provided environmental protection on all
internal surfaces, even those not readily accessible to normal or conventional
coating procedures, herein defined to mean, but not be limited to, electro
or vapor deposition and heated particle deposition as flame or plasma
spraying.
2 5 Example 11
Rene' 80 alloy pins 1/8 inch in diameter and two inches long
were provided with a deposit from the above-described TIBA-kerosene
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~066~43 13DV-5828
solution to thicXness of 1/2 mil and 1 mil, Subsequently
the pins were heated as in Example 9. The pins so coated
and treated were placed in a dynamic oxidation tunnel at
1950 F (1066C) with a gas velocity of,05 Mach and cycled
to below about 800 F (about 425C) one per hour. Weight
changes of the pins were recorded in Table II below,
After 960 hours the 1/2 mil Al coated and heat
treated pins showed pinpoint oxidation; the 1 mil Al coated
and heat treated pins were unblemished, The oxidation
resistance, by comparison to pack aluminided specimens,
was judged to be good, These data presented the need
for at least about a 1 mil coating when only aluminide
is used,
TABLE II
Oxidation Test - 1950F Flame Tunnel
Rene' 80 Alloy Base with Al Heat Treated Coating
Time (hrs) 320 480 640 800 960
Weight Gain (mg)
1/2 mil coating 2,3 2,0 2,8 3.1 3,2
1 mil coating 2,2 1,9 2,6 2.7 3.1
Example 12
Nickel carbonyl, Ni(Co)4, was introduced into an argon
atmosphere at a partial pressure of 130mm of Hg, within a
chamber which contained an induction-heated, nickel-base
superalloy specimen, The specimen, which was a ~i-base
superalloy commercially available as I~738 alloy, was
heated in this flowing gas mixture to within the the
effective range of 80 - 300 C, in this example about 100 C,
for fifteen minutes and then cooled to room temperature,
In that time, a 4 mil layer of pure nickel was deposited
on the surface of the specimen, Then the specimen was re-
moved from the chamber and introduced into another apparatus
13DV-5828
~66~43
containing argon, An argon flow was established in this
apparatus by building 15 cc per minute of argon at 1
atmosphere through the above-described kerosene solution
of TIBA, The specimen was then heated to 200 C for 15
minutes, This procedure deposited 1 mil of pure aluminum
on top of the nickel layer. Then the specimen was put into
a hydrogen furnace and heated to 1050 C for 16 hours to
homogenize and interdiffuse the two coating layers, The
composition of the resulting coating was 12 wt, % aluminum,
balance essentially nickel, as would be expected from the
proportional thickness of aluminum and nickel deposited
and characterized by metallic nickel with a carbon im-
purity and a fine grain structure, The 12 wt, % aluminum-
nickel coating offers sufficient oxidation resistance at
temperatures typical of the interior of turbine blades,
A more oxidation-resistant coating can be produced by
pxoloning the aluminum coating process step or shortening
the nickel coating process step so that substantially
equivalent thickness of nickel and aluminum are provided,
In such case, a beta NiAl alloy coating is formed,
Example 13
IN738 nickel-base alloy specimens were placed in the
coating apparatus which was evacuated to less than 1 mm
of mercury pressure, Liquid dicumene chromium, (C7H8)2
Cr was heated to 250 C so that it naturally evaporated
under the reduced pressure, The specimen was then heated
in the range oE 300 - 450 C and preferably 350 - 450 C
10 minutes using induction heating, In a 10-minute inter-
val at 350 - 450 C, four mils of chromium were deposited,
The deposition thickness is dependent on the chromium
activity which in turn is proportional to the vapor pre-
ssure or the temperature to which the dicumene chromium
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13DV-5828
~o66~43
liquid solution is heated Next, the specimen including the
4 mil layer of chromium was put into a similar apparatus
and coated with the above-described TIBA kerosene solution,
as in previous examples. This step deposited 1 mil of
aluminum on top of the 4 mils of chromium. The specimen
was then homogenized by interdiffusion in a hydrogen
durance at 1050 C for 16 hours. The resulting coating
consisted of two portions: an under portion of alpha
chromium, and an outer portion consisting of alpha chromium,
and an outer portion consisting of alpha chromium and
Cr5A18,
In all these examples, the pure coatings deposited
by each individual deposition procedure is remarkably
unifrom in thickness The thickness are controlled
essentially by the activity of the organic compound, the
specimen temperature distribution and time The fact
that the coating is remarbaly unifrom in thickness suggests
that flowing the reagent fluid through complicated laby-
rinthine passages on the inside of an article, for example
a turbine blade, does not create thicker coatings at the
points of entrance and thinner coatings at the point of
exit. As shown by these examples, individual successive
layers of elements, for example Ni and Al, Cr and Al, Ni,
Cr and Al, etc., can be deposited within the range of about
100 - 450 C, and then homogenized and interdiffused into a
single coating in the range of about 870 - 1100C. Also,
multiple elements can be codeposited as an alloy, for ex-
ample Ni and Cr, followed by an Al deposit and subsequent
homogenization. Although specific examples and embodiments
have been includes in this description as typical and re-
presentative, those skilled in the art will appreciate the
variations and modifications of which the present invention
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1066143 13DV-5828
is capable without departing from its scope which is
intended to be defined by the appended claims,
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