U.S. patent number 3,620,693 [Application Number 04/818,462] was granted by the patent office on 1971-11-16 for ductile, high-temperature oxidation-resistant composites and processes for producing same.
This patent grant is currently assigned to GTE Electric Incorporated. Invention is credited to George T. Pepino, Jr., Lawrence Sama.
United States Patent |
3,620,693 |
|
November 16, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
DUCTILE, HIGH-TEMPERATURE OXIDATION-RESISTANT COMPOSITES AND
PROCESSES FOR PRODUCING SAME
Abstract
A ductile, high-temperature, oxidation-resistant metal composite
is disclosed that comprises a superalloy substrate and a coating
composition having the following percentages by weight of
ingredients: ##SPC1## A process for producing the composites is
also disclosed wherein the coating composition containing the
foregoing ingredients as powdered metals, a fugitive organic binder
and a volatile solvent for the binder is applied to a clean
substrate and thereafter the coated material is dried and heated
under nonoxidizing conditions for a time sufficient to create a
diffusion zone between said coating and said substrate. 3 Claims,
No Drawings
Inventors: |
Lawrence Sama (Seaford, NY),
George T. Pepino, Jr. (Huntington Station, NY) |
Assignee: |
GTE Electric Incorporated
(N/A)
|
Family
ID: |
25225600 |
Appl.
No.: |
04/818,462 |
Filed: |
April 22, 1969 |
Current U.S.
Class: |
428/553; 148/527;
420/585; 427/376.8; 428/565; 428/680; 427/376.7; 420/445;
427/376.3; 427/376.4; 427/405; 428/640 |
Current CPC
Class: |
B32B
15/00 (20130101); C23C 10/34 (20130101); Y10T
428/12146 (20150115); Y10T 428/12944 (20150115); Y10T
428/12667 (20150115); Y10T 428/12063 (20150115) |
Current International
Class: |
C23C
10/00 (20060101); B32B 15/00 (20060101); C23C
10/34 (20060101); B32b 015/00 () |
Field of
Search: |
;29/195,194,197,191
;117/71 ;75/171,176 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
2839396 |
June 1958 |
Marsh |
2996378 |
August 1961 |
Edmunds et al. |
3000755 |
September 1961 |
Hanink |
3129069 |
April 1964 |
Hanink et al. |
RE |
January 26,0 |
4/1966 Wachtell et al. |
3342564 |
September 1967 |
Schwartz et al. |
3447912 |
June 1969 |
Ortner et al. |
3462820 |
August 1969 |
Maxwell et al. |
3493476 |
February 1970 |
Lucas et al. |
3494748 |
February 1970 |
Todd |
|
Primary Examiner: L. Dewayne Rutledge
Assistant Examiner: E. L. Weise
Attorney, Agent or Firm: Norman J. OMalley Donald R. Castle
William H. McNeill
Claims
1. A ductile, high temperature, oxidation-resistant metal composite
comprising a superalloy substrate as its principal component and a
coating composition bonded to said substrate, said coating
consisting essentially of nickel, chromium, aluminum and silicon in
the following percentages by weight of said coating
composition:
chromium-- 16% to 50%
aluminum-- 5% to 10%
silicon-- 0.5% to 3%
cobalt-- 0% to 5%
2. A composite according to claim 1 wherein said superalloy
substrate is
3. A composite according to claim 1 wherein said superalloy
substrate is thorium oxide dispersion nickel-chromium superalloy.
Description
This invention relates to metal composites containing the
"superalloys." More particularly it relates to composites that
contain coating compositions that enable such alloys to resist
oxidation at elevated temperatures.
Alloys have been developed for uses wherein the alloys are
subjected to relatively severe conditions such as high temperature
oxidation and high stresses. These alloys, commonly referred to as
superalloys, for the most part withstand these severe environmental
conditions, however certain parts of fabricated items are subjected
to conditions that exceed the limits of the alloys. For example,
the TD Nickel alloys (containing about 98 percent nickel and
containing about 2 percent thorium oxide of submicron size
dispersed through the nickel base) and the nickel and cobalt based
alloys are generally suitable for most uses up to temperatures of
about 2,000.degree. F. Above about 2,000.degree. F. the oxidation
rate becomes excessive. In order to protect these alloys against
oxidation at these elevated temperatures, coatings are needed.
Several coating systems are known that will protect the alloy
substrates against the attack, however, none are known that offer
oxidation resistance and are also ductile, particularly when
applied as relatively thin layers, e.g. 5 mils. In some
applications such as in jet engine turbines, the performance of the
engine is limited by the maximum operating temperatures that can be
used. The vanes of the engine operate at higher temperatures than
do the blades in the engine. While the beforementioned alloys are
promising materials for jet engine parts, it would be an advantage
to have a material having a substrate of one of the superalloys and
coated with a composition so that the material would be ductile and
withstand the oxidation at the elevated temperatures e.g. above
about 2,000.degree. F. It is believed to be an advancement in the
art to provide a ductile material that has the capability of
withstanding oxidation at elevated temperatures, thus enabling the
composite to be used in environments that heretofore were
considered to be prohibitive for most materials.
In accordance with one aspect of the invention, there is provided a
ductile, high temperature, oxidation-resistant metal composite
comprising: (a) a superalloy substrate as its principal component
and (b) a coating bonded to the substrate and consisting
essentially of nickel, chromium, aluminum and silicon in the
following percentages by weight of the total coating
composition:
chromium-- 5% to 50%
aluminum-- 5% to 10%
silicon-- 0.5% to 3%
cobalt-- 0 to 5%
nickel-- balance
In accordance with an additional aspect of this invention, there is
provided a process for producing the ductile, high temperature,
oxidation-resistant metal composites, the process comprising: (a)
applying to a clean surface of a superalloy substrate a coating
composition comprising a powdered metal material, a fugitive
organic binder and a volatile solvent for said binder; the metal
material consisting essentially of the following weight percentages
of ingredients: chromium-- 5% to 50% aluminum-- 5% to 10%
silicon--0.5% to 3% nickel--balance (b) drying the applied coating
and (c) heating the coated substrate under nonoxidizing conditions
at a temperature and for a sufficient time to create a diffusion
zone between said coating and said substrate.
In accordance with another aspect of this invention there is
provided a process for producing the high temperature,
oxidation-resistant composites; the process comprises (a) applying
to a clean surface of a superalloy substrate a first coating
composition comprising a powdered metal material, a fugitive
organic binder and a volatile solvent for said binder; the metal
material consisting essentially of the following weight percentages
of ingredients: chromium 5% to 50% silicon 1% to 3% nickel balance
(b) drying the applied coating; (c) heating the substrate
containing the first coating under nonoxidizing conditions at a
temperature and for a sufficient time to create a diffusion zone
between the substrate and the first coating; (d) applying to the
first coating a second coating composition comprising a powdered
metal material, a fugitive organic binder and a volatile solvent
for the binder; the metal material consisting essentially of the
following weight percentages of ingredients: aluminum 80% to 100%
cobalt 0% to 20% (e) drying the second coating, and (f) heating the
coated substrate containing the first and second coatings under
nonoxidizing conditions at a temperature and for a sufficient time
to create a diffusion zone between the first and second
coatings.
For a better understanding of the present invention, together with
other and further objects, advantages and capabilities thereof,
reference is made to the following disclosure and appended claims
in connection with the above description of some of the aspects of
the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The superalloys are well known. Typical alloys of this
classification are given in Metals Handbook 8th Edition 1961, Vol.
1, published by American Society for Metals. As previously
mentioned, these alloys are particularly useful in jet engines,
especially as turbine blades, however, the vanes (the other primary
components) operate at higher temperatures than do the blades. The
nickel and cobalt based alloys are particularly well protected
against oxidation at higher temperatures by the coating
compositions of the present invention. Of the foregoing alloys, the
thorium oxide dispersion nickel alloys and thorium oxide dispersed
nickel-chromium alloys are preferred for many jet engine usages and
can be effectively protected by the practice of this invention.
The articles produced by the practice of this invention comprise,
in addition to the superalloy substrate, a layer of a quarternary
composition consisting essentially of nickel, chromium, aluminum
and silicon in the amounts specified herein. A diffusion zone is
produced by the heating at elevated temperatures between the
coating and the substrate. In this zone some of the metal materials
from the coating diffuse into the metal matrix that is adjacent to
the coating. This diffusion zone insures good bonding of the
coating and improves the resistance to the oxidation attack that is
believed to occur at grain boundaries. It is to be noted that there
is no sharp line of delineation between the coating and the
substrate in the as-coated condition. Electronprobe analysis at
various depths from the surface of the coated article show the
diffusion of the various metals into the substrate.
As will be described in more detail hereinafter, the metal
composite of this invention can be prepared by either of two
processes. In one method the coating can be applied by slurry
techniques hereinafter described, wherein the slurry contains all
of the desired metals in the desired amounts. Although the
foregoing "one-step" coating method yields a composite superior to
an uncoated superalloy, a second method is a "two-step" method
wherein a first layer of a coating composition containing nickel,
chromium and silicon is applied and after drying and heat treating,
a second layer, consisting of aluminum or aluminum-cobalt is
applied via essentially the same slurry technique as is used in
applying the first layer. Although the composites are described as
being produced by providing "layers" of various composition, the
metal composite does not have completely discrete layers of
compositions as a result of the diffusion of metals into the
adjacent material during the heating steps described herein.
Additionally, the composites do not necessarily have a uniform
analysis since the various metals can diffuse into the substrate at
a different rate. The overall composition is as previously
given.
In the "one-step" coating process the Ni, Cr, A1, and Si, if not
initially in the form of finely divided powders, are pulverized to
form such powders. The particle sizes or ranges of particle sizes
suitable are those metal powders that are commonly employed in
powder-metallurgy techniques, and usually have particles small
enough that all of the particles will pass through a 325-mesh
screen (U.S. Standard Sieve Series).
Instead of the metallic ingredients being in elementary form, any
two or more or all of them can be preformed as an alloy material
wherein the proportions of the components thereof are such as would
be employed if they were present as a physical admixture.
The metal powders are thoroughly mixed together in a suitable
blender or mixing device (e.g. a "V" blender) until a substantially
homogeneous composition has been obtained. A similar procedure is
followed if a mixture of different alloys or of elemental metal (or
metalloid) and of alloyed material is employed.
The powdered metal is converted into a liquid coating composition,
adapted for application (e.g., by dipping, brushing, spraying or
the like) to the superalloy substrate, by suspending it in a
suitable vehicle, e.g., a solvent solution of temporary or fugitive
binder which can be a natural or synthetic binder.
Examples of fugitive binders that can be employed are solvent
solutions or dispersions of the various available synthetic
polymers, such as polyacrylamide, polyvinyl acetate and the
homopolymers and copolymers of the lower alkyl (e.g., C.sub.1
through C.sub.5) acrylates and methacrylates with each other and
with other compounds containing a monoethylenically unsaturated
grouping. It is preferred to employ an ordinary nitrocellulose
(pyroxylin) lacquer wherein the solvent is, for example, amyl
acetate.
The concentration of the powdered metal in the vehicle and the
amount of solvent in the same are varied as desired, depending upon
such factors such as the particular method of applying the coating
(brushing, spraying, or dipping), the desired thickness of the
individual coating, the number of coatings to be applied, the
viscosity of the vehicle, the desired covering power of the coating
composition, and other influencing factors. Typically, the metal
powder is present in the coating composition in an amount
corresponding to about 1,500 g. to about 3,000 g. per 1,000 g. of
the vehicle.
The powders are mixed with the vehicle by mechanical stirring. Any
suitable mixer can be used, however, mixers of the type generally
employed in mixing paints are preferred for this purpose. Mixing is
continued at any suitable temperature for a time sufficient to
provide a substantially homogeneous composition.
The coating is applied to the cleaned surface of the superalloy
substrate. The surface of the part can be cleaned by dry abrasive
blasting such as with iron or aluminum oxide grit, or by chemical
cleaning such as acid pickling for 1 minute in a solution of one
part concentrated HF, one part concentrated HNO.sub.3 and one part
water. There is no preferred cleaning method insofar as wettability
or protectiveness of the coating is concerned. Hence the selected
cleaning method depends primarily upon such other factors as, for
instance, convenience, availability of suitable equipment, and
accessibility of the surface to be coated. If all surfaces are
readily accessible, spraying is the preferred method of
application. The applied coating on the superalloy part is then air
dried. The air-dried coating will vary in weight with the amount of
the initially applied wet coating, but usually will be within the
range of from about 50 to about 150 mg./cm.sup.2. Heavier coatings
can be obtained, if desired, either by applying one or more fresh
layers over the air-dried coating followed by air drying after each
successive application of the liquid coating material.
After air drying, the coated part is heated under relatively
high-temperature conditions. This can be done, for instance, by
placing the coated article on heat-resistant pads, e.g., quartz or
alumina pads or boats, or suspending it by tantalum wires in a
cold-wall furnace. Firing is done at a temperature which is near,
at or slightly above the melting point of the "as-applied" metallic
coating, but which is preferably about 5.degree.-10.degree. F.
above the melting point. In such a furnace, the coated part is
supported or suspended inside the heating element wherein it is
heated by radiation, and is therefore quickly heated to a uniform
temperature.
The time and temperature of firing the coated part in all cases are
sufficient to effect a diffusion of the metals in the coating into
the substrate. Generally, this diffusion treatment is effected by
heating under nonoxidizing conditions at a fusion temperature
ranging from about 2,300.degree. F. to about 2,450.degree. F. for a
period of from about 10 minutes to about 1 hour. Thus, this heat
treatment can be carried out in an atmosphere of an inert gas,
e.g., helium, argon, krypton, zenon or other members of Inert Gas
Group of the Periodic Table of the Elements. Advantageously the
nonoxidizing conditions for the heat treatment are obtained by
heating the coated part under a high vacuum, e.g., at less than
0.001 or 10 .sup.-.sup.3 torr, and preferably at less than 0.0001
or 10.sup.-.sup.4 torr. The temperature and time of treatment
depend upon such influencing factors as, for example, the
particular coating composition and substrate employed. Heating for
about 15 to about 20 minutes at 2,325.degree.-2,450.degree. F. at a
pressure of less than 0.001 torr can be used with satisfactory
results in many instances. If desired, heating can be carried out
initially under partial vacuum and completed in an atmosphere of an
inert gas.
Any furnace capable of attaining the required temperature and
pressure values in firing the coated structure without oxidation
thereof, e.g., by firing it under high vacuum or in an atmosphere
of an inert gas, can be employed.
The thickness of the fused coating can range from about 3 mils to
about 8 mils, but is preferably within the range of from 4 to 5
mils.
The metal coatings with which this invention is concerned can be
applied selectively to portions of parts or assemblies. Also,
modifications of the primary compositions (or even basically
different compositions if believed to be necessary) can be applied
to specific areas of the same part or assembly wherein experience
indicates that different compositions would be more suited to the
particular environmental conditions.
The fused-slurry coating technique used in practicing this
invention is simpler and less expensive than pack-cementation
processes employed in the prior art in applying metallic coatings
to metallic substrates. The fused-slurry coating method requires
only one-thousandth (or less) coating material as that required
(but not all consumed) when the aforementioned prior art processes
are used. Furthermore, less time (usually, at most only about 1
hour) is required by the method of this invention whereas all
pack-cementation processes require one or more heat treatments that
require from about 4 to about 16 hours for each single treatment,
not including the long heat-up and cool-down cycles.
The coatings with which this invention is concerned are
substantially uniform in composition. This is due mainly to the
technique of applying them to the superalloy substrate. In this
respect they differ markedly from coatings applied to a
pack-cementation process wherein the coated part is embedded in an
insulating powder pack inside a retort and is heated by conduction.
By this technique the parts nearest the walls of the retort are
heated up much more rapidly than those at the center; hence, the
resulting coatings are not uniform either in composition or in
thickness.
Another advantage of the coatings involved in the instant
invention, and that accrues both from the composition of the
coating and from the preferred technique by which it is applied, is
the ease with which patch or repair work can be done in fixing a
damaged surface. In making a repair it is only necessary to clean
the area where the defect exists or the damage has been done, apply
the same composition used in forming the original coating, and
heattreat the repaired area or the entire part under vacuum or in
an inert atmosphere as heretofore has been described.
In the "two-step" process for producing the composites of this
invention, essentially the same method of applying the metal powder
slurries is used. The major difference in the "two-step" process is
that aluminum or aluminum and cobalt are not incorporated as
ingredients of the coating composition used to obtain the first
coating and are subsequently added via a slurry technique followed
by the drying and diffusion steps. In this manner sufficient
aluminum can be added to effect improved oxidation resistance
without obtaining low-melting phases. The first coating apparently
acts as a better base for the application of aluminum than the
superalloys and thus reduces the formation of low melting phases
while the amounts of aluminum are being diffused. As measured by
electroprobe analysis, some aluminum is detected at about 28 mils
from the surface when a 5 mils total coating thickness is used.
Significant amounts of aluminum (above 1 percent) are detected at
about 10 mils from the surface of the coating. In many instances,
particularly when the environment of the element is subjected to
elevated temperatures, such as above about 2,200.degree. F.,
composites produced by the "two-step" process are preferred over
those produced by the "one-step" process. Also, for particular
substrates, an aluminum and cobalt mixture such as a Al-20 Co
mixture has been found to provide a preferred second layer.
In each process care must be taken during the heating step to
achieve the desired amount of diffusion of the various elements
into the matrix of the other compositions without forming local
low-melting phases. Formation of the local low-melting phases
causes spalling of the coatings. The firing temperatures and
sequence is even more critical when the "two-step" method is used
since the final coating application consists essentially of
aluminum, therefore, as the aluminum diffuses into the matrices of
the first coating and the substrate to form the metal composite,
local areas of the coating, due to the high-aluminum content of the
second coating, can form a composition that has a melting point
below the firing temperature or the temperature of the environment
in which the composite is to be used. For these reasons when a
substrate of a superalloy of the TD-Ni type (2 percent thorium
oxide dispersed in the nickel) is used, the following "two-step"
process is especially preferred.
A slurry of Ni-30Cr-1Si at an application of about 80 mg./cm..sup.2
is applied to the substrate to yield about a 4 mils thick coating.
After the coating is dried and fired in a vacuum at about
2,430.degree. F. for about 15 to 20 minutes, a second coating of
Al-20 Co is applied at an application of about 5-25 mg./cm..sup.2
to yield a total coating thickness of about 5-8 mils, with from
about 8 to about 12 mg./cm..sup.2 being preferred to yield a total
thickness of about 5 mils. After the coating is dried it is fired
in a hydrogen atmosphere according to the following heat treating
cycle: heat to about 2,050.degree. F. in about 1 hour, hold at
about 2,050.degree. F. for about 2 hours, raise to 2,100.degree. F.
and hold at that temperature for about 2 hours, hold at each of
2,150.degree. F. and 2,200.degree. F. for about 1/2-hour intervals,
then 2,300.degree. F. for about one half hour, an overall heat
treatment cycle of about 71/2 hours is achieved.
It is also preferred when the substrate is a superalloy of the
TD-NiCr alloy-type that the above procedure be followed with the
exception that the first coating is preferred to be a
nickel-chromium-silicon mixture of the approximate composition
Ni-20Cr-3-Si. Although higher levels of Cr can be used in the first
coating and the benefits of this invention can be achieved, there
are no additional benefits to be gained since the Td-NiCr
substrates generally contain a maximum of about 20 percent Cr.
It is also contemplated within the broad scope of this invention
that it can be desirable to introduce a single or a plurality of
"diffusion barriers" into the metallic composite between the
superalloy substrates and the metallic coating described
hereinbefore to reduce the amount of diffusion. The substrate can
also be provided with a plurality of alternate layers of (1 ) a
diffusion barrier and (2) the coating of this invention, the first
layer of the diffusion barrier being superimposed directly on the
substrate. Examples of suitable diffusion barriers are tungsten,
rhenium, tantalum and molybdenum. The layer of diffusion barrier
can be of any desired thickness, e.g., from 0.5 to 1 mil in
thickness.
To more fully illustrate some of the aspects of this invention, the
following nonlimiting examples are presented. All parts,
percentages and proportions are by weight unless otherwise
indicated. Examples 1-9 --one-step process nickel, chromium,
aluminum, silicon, and in some instances additionally columbium,
are weighted out in the proportions given in table I, charged into
a conventional V-blender; and blended for 2 hours.
The blended powder mixture is then added to a lacquer of high
purity, low-residue nitrocellulose in a solution of amyl acetate
solvent in the approximate proportion of 1 to 1 by volume, and is
thoroughly mixed for about 10 minutes. The nitrocellulose lacquer
employed can be one such as type L-18 manufactured by Raffi and
Swanson, Wilmington, Mass. To minimize gravity separation, there
can also be incorporated into the slurry a small amount of a
dilutant as, for example, toluene. The amount of toluene or its
equivalent can be from about 10 to 20 percent by weight of the
nitrocellulose lacquer component of the slurry.
The resulting slurry or coating composition is poured into the jar
or reservoir of a conventional paint-spray gun. Such a receptacle
is preferably provided with means for continuous, mechanical
stirring of the slurry in order to prevent settling of the metallic
powders.
In this example the cleaned TD nickel parts (by dry abrasive
blasting) are coated with the individual coating compositions
identified in table I.
The parts are sprayed with the slurry using normal paint-spraying
technique. After spraying, the parts are allowed to air dry for 1
hour. The air-dried weight of the coating is in the range of about
50 to about 100 mg./cm..sup.2.
The air-dried parts are then placed across small ceramic (alundum)
combustion boats which, in turn, are placed in a vacuum furnace.
The furnace is sealed and pumped to a vacuum of less than
10.sup.-.sup.4 torr at which point heating is begun. The heat is
initially applied slowly until at a low temperature (less than
500.degree. F.) it is apparent (evidenced by the sudden increase in
pressure within the furnace) that the amyl acetate solvent is
volatilizing from the lacquer component of the coating composition.
At this point heating is temporarily interrupted until it is
observed that the pressure in the furnace is again at a level of
10.sup.-.sup.4 torr or lower. Thereafter, heating is continued to
the particular furnace temperature specified in the individual
example of table 1, and all of which temperatures are within the
range of from 2,325.degree. F. to 2,425.degree. F. The parts are
held at the recorded temperature for either 15 or 20 minutes, i.e.,
as is specified in the individual example of table I.
At the end of the firing period the furnace heat is turned off.
When the furnace has cooled to a temperature sufficiently low to
prevent damage to the internal parts of the furnace, air is
admitted thereto, and the furnace is opened and the parts are
removed.
The furnace employed in firing the coated specimen parts of these
examples is a cold-wall, metallic-resistance element furnace, the
heat-up time being about 10 minutes and the cool-down time about 30
minutes in carrying out the described firing step. In larger vacuum
furnaces of similar or different design and/or with larger work
loads, longer heat-up and cool-down times are necessary. However,
these factors are not critical in the chemistry involved in the
firing step nor in the performance characteristics of the finished,
coated part.
The data in table I are believed to be self-explanatory. It will be
noted from examples 4 and 8 that alloys 5 and 7 can be readily
fused onto the nickel-base substrates at a temperature as low as
2,325.degree. F. Alloy 6 (Used in examples 5, 6 and 7 ) may have an
even lower fusion point. In general, the coatings are tightly
adherent and fairly dense. The common "bent" means that the coated
coupon can be bent sharply without destroying or injuring the
coating, and that the metallic coating is, therefore, ductile.
##SPC2##
The results of oxidation tests on some of the samples of the
composites produced in this example are given in table II. In this
test the composites are subjected to 16-24 hour cycles at about
2,300.degree. F. followed by 5 minutes at about ambient (room)
temperature. In carrying out the tests the samples are placed in a
quartz boat which is manually inserted in a furnace that is
maintained at the test temperature. After each oxidation exposure
period the boat is withdrawn from the furnace and the samples are
air-cooled for about 5 minutes. ##SPC3## Example 10 --two-Step
Process
A uniform mixture of metal powder is prepared by blending 97 parts
of (nickel--20 parts chromium) and three parts of silicon for a
period of about 2 hours. The uniform powder metal mixture is
formulated into a slurry as in examples 1-9. The slurry is applied
to a 60 mils thick TD-NiCr substrate in the manner illustrated in
examples 1-9 . The slurry is applied at a level to yield a uniform
coating of about 80 mg./cm..sup.2 after the coated substrate is
heated in a vacuum of about 10.sup.-.sup.3 to 10.sup.-.sup.4 torr
and at a temperature of about 2,360.degree. F. for a period of
about 15 minutes.
A mixture of aluminum and cobalt powders is prepared by blending
about 80 parts of aluminum and 20 parts of cobalt for a sufficient
time to obtain a uniform mixture. A slurry similar to that used for
the first coating is prepared and applied in essentially the same
manner except that sufficient slurry is used to yield an
application of about 10 mg./cm..sup.2 after the coated article is
heated according to the following schedule: heating to
2,050.degree. F. for 1 hour hold at 2,050.degree. F. for 2 hours
raise to 2,100.degree. F. and hold for 2 hours raise to
2,150.degree. F. and hold for 1/2 hour raise to 2,200.degree. F.
and hold for 1/2 hour raise to 2,250.degree. F. and hold for 1/2
hour raise to 2,300.degree. F. and hold for 1 hour The heating
cycle is carried out in a hydrogen atmosphere in a furnace using
radiation type heating.
Analysis of a sample by electronprobe indicates the composition of
5 mils of coating to be as follows:
Cr --15-20%
Al-- 4--5%
Si-- 2--3%
Co-- 0.3-0.4%
Ni-- balance Samples of the composite when subjected to
2,300.degree. F. oxidation tests indicate that the samples are
satisfactory after about 400 hours. Uncoated TD-NiCr fails within
250 hours under similar conditions. The criterion used for failure
is a weight change of 11 mg./cm..sup.2 . Example 11 a similar
process is used except that 60 mils thick TD-Ni is used as the
substrate and the second coating is Al-20Co. An application level
of about 10 mg./cm..sup.2 of the Al-20Co coating after heating is
obtained. The analysis of 5 mils of coating is as follows:
Cr-- 5--10%
Al-- 5--9%
Si-- 2--2.5%
Co-- 0.3-0.5%
Ni-- balance Samples indicate that satisfactory performance is
achieved at 2,300.degree. F. under oxidizing conditions for over
100 hours. Uncoated TD-Ni fails after less than 5 hours at about
2,300.degree. F. Example 12 where the substrate is TD-Ni. as in
table 3, sample 13, and the first coat is Ni-30Cr-1Si and the
second coat is 10 mg./cm..sup.2 of Al-20Co, the actual analysis of
the 5 mils of coating is as follows:
Cr-- 12--26%
Al-- 5--15%
Si-- 1--2%
Co-- 0.1--0.4%
Ni-- balance Additional examples using the processes and heat
treatments from examples 10 and 11 are listed in table 3. Good
protection can be obtained on TD-Ni substrates by increasing the
amount of Al-20Co applied in the second step. However, optimum life
in 2,300.degree. F. oxidation tests appears to be at a ratio of
about 10-12 to 1 in first to second coating weight. ##SPC4## All
samples bent satisfactorily after oxidation exposure.
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