Note: Descriptions are shown in the official language in which they were submitted.
1~5~3~
This invention relates to a process for producing
metal-coated composite powder particles, that is to say par-
ticles which each comprises a metal-coated core.
Such composite powders are in commercial use in a
number of fields. For example, nic]cel-coated graphite powder is
used in the formation of abradable seals for gas turbine engines.
Cobalt-coated tungsten carbide powder is thermal-sprayed onto
knife blades to form hard, wear-res:istant cutting edges. Nickel
coated aluminum powder is thermal-sprayed onto various substrates
to provide a strongly adherent bond coat in preparation for
further coating.
Metal coated composite powder particles may be formed
in a number of different ways. One process in commercial use in-
volves suspending core particles in a solution containing dissolved `
values of the coating metal or metals, and reacting the result-
ing suspension at elevated temperature and pressure with a re-
ducing gas to cause the coating metal to precipitate from solu-
tion onto the core particles.
In general, according to the foregoing process, each dis-
crete particle of core material receives a metal coating and, al-
though some agglomeration of particles may occur during the coat-
ing process, the particle size of the final composite powder is
largely determined by the particle size of the core material. This
does not present any problem where the particle size of the final
composite powder is of no particular importance, or where core
material particles of the required size can readily be obtained.
However, it is frequently desirable, and in some cases even essen-
tial that the particle size of the composite powder be closely '
controlled, for example when the powder is to be employed in a
thermal-spraying process. Also, it is not always possible to
readily obtain core material particles of the required size.
~. . .. ; , . .
8523~
For example, minerals having a phyllosilicate structure, such as
pyrophyllite, talc, kaolin, halloysite, chlorite, mica, montmoril-
lonite and bensonite, are usually obtainable only with particle
sizes less than 5 ~um (microns~, which is too fine for production
of useful composite powder by the process described above. Wonder-
stone is another mineral which is usually only obtainable with
particle sizes which are too small for this purpose. Compounds
such as graphite and boron nitride, as well as refractory compounds
such as tungsten carbide and titanium carbide also fre~uently fall
into this category.
An object of this invention is therefore to provide a
process for producing metal-coated composite powder utilizing core
material having a particle size smaller than the required core size.
According to the present invention, a process for
producing composite powder particles having cores within a pre- -
determined 5iZe rar.ge coated by a metallic outer layer, said
predetermined core size range being within the overall size
range of from about 5 to about 250 ,um, includes providing ~ .
core particles of a size smaller than the lower limit of said
predetermined size range, mixing said core particles with a
polymeric bonding substance, forming the resulting mixture into
solid agglomerates of a size within said predetermined range
to form said cores, with each core comprising a plurality of
core particles bound together by said polymeric bonding sub-
stance, and coating said cores with a metallic outer layer to
form composite powder particles.
The product of the invention is a composite powder
comprising particles having cores within a predetermined size
range, said predetermined size range being within the overall
size range of from about 5 to about 250 ~um, each core compris-
ing a plurality of core particles, and each core being coated
-- 2 --
.. . . . .
5239
with a metallic outer layer. The composite powder particles
will accordingly be of a size in the range of from about 6 to
to about 500 um.
Core materials which may be utilized with the present
invention include metals and non-metals which can be coated with
a layer of metal by the known hydrometallurgical method des-
cribed above. Possible materials are compounds such as graphite,
calcium fluoride and boron nitride, and refractory materials such
as aluminum oxide, tungsten carbide, titanium carbide, tungsten-
titanium carbide, chromium carbide, chromium oxide, zirconiumdioxide, titanium dioxide and molybdenum disulphide. Also,
wonderstond and minerals having a phyllosilicate structure such
as those mentioned above are suitable as core material. Wonder-
stone appears to be pattern A.S.T.M. 2-613 for aluminum silicate
hydrite.
As mentioned previously, the present invention is
principall~ concerned with the coating of finely divided core
materials which are not readily available in a predetermined
required size prior to coating with a layer of a metal by the
~0 method described above. As also mentioned previously, it is
frequently necessary that the particle size of the composite
powder be within a predetermined size range. For example, pow-
ders which are to be applied by thermal-spraying methods prefer-
ably should be free flowing and with a particle size between
about 6 and about 150 um. According to the known hydro-
metallurgical coating method, control of particle size of the
composite particle is achieved primarily through control of
the particle size of the core material. The known method, there-
fore, does not lend itself to coating of core particles smaller
than the lower limit of the range required for the core size,
and it is to the production of composite powders of controlled
-- 3 --
5235~
size using such core particles that the present invention is
directed.
The first step in the process of the invention in-
volves mixing the particles of core material with a polymeric
bonding substance which adheres to the core particles and re-
tains its bonding properties under the conditions employed dur-
ing the latter stages of the process to coat the core material
with a layer of metal. For example, it may be necessary that
the polymeric bonding substance retai'ns its bonding properties
when the formed cores are immersed in water and/or sub~ected
to high temperatures. Good results have been achieved with
polyurethane and with vinyl ester resins, as will be described
later. The mixture of core particles and polymeric bonding
substance is then formed into solid agglomerates of a size
within the predetermined core size range to form cores.
The core particles and polymeric bonding substance
may be mixed in a mixing apparatus such as an attritor. Alter-
natively, the core particles and polymeric bonding substance
may be mixed by first dissolving the polymeric bonding substance
in a volatile solvent, and combining the resulting solution with
core particles in a pelletizing apparatus such as a disc pelleti-
zer or drum pelletizer. This type of apparatus is well known to
those skil]ed in tha ar~-and,insofar as the present invention
is concerned, is operated in accordance with conventional proce-
dures to produce solid, polymer-bonded agglomerates of core par-
ticles within the predetermined core size range. As a specific
example, polyurethane may be dissolved in acetone in a 1:1 ratio
and combined with core particles in a rotary granulating pan.
The acetone evaporates rapidly and, as the pan rotates, the
polyurethane deposits on and binds the core particles into
agglomerates. The size of the agglomerates can be controlled
., , . .,. ,.. , ," . . ..
~0~35239
~y controlling the operation of the pan and subsequent screening
of the agglomerates.
The quantity of polymeric bonding substance mixed with
the core particles can vary between 1 and 10~ based on the combined
weight of the polymeric bonding substance and core particles.
Quantities of polymeric bonding substance larger than 10% are
usually not required to bind the core particles in a form suitable
for coating but, where the particular polymeric bonding substance
used does not have strong binding properties r more polymeric bond-
ing substance may be required. Similarly, less than 1% polymericbonding substa~ce is usually not sufficient to maintain the core
particles in the form of agglomexates unless the polymeric bond-
ing substance has unusually good binding properties~
In some cases, it may be necessary to heat treat the
cores to prepare them for coating with metal. The polymeric
bonding substance is chosen so that, although the heat may cause
the polymeric bonding substance to decompose, with volatile de-
composition products being lost, the core particles remain bound
together as a core by the residue of the polymeric bonding sub~
stance. This procedure stabilizes the cores against weight and
volume changes and reduces their carbon content~
Following agglomeration, the resultant cores are
coated with a layer of at least one metal such as cobalt, nickel,
copper, ruthenium, rhodium, osmium, iridium, gold, silver,
platinum, arsenic, tin, cadmium or molybdenum, for example, by
the known hydrometallurgical method referred to previously and
described in a number of publications such as United States
Patents Nos. 2,853,398, 2,853,401, 2,853,403, 3,062,680,
3,218,192 and 3,241,949 or by other known coating techniques.
The coating of the cores with metal may be improved by the
use of a catalyst or a nucleation promoter, for example, a small
amount of anthraquinone or a substituted anthraquinone.
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lO~X239
The resulting metal coated composite particles may
be used as such in the flame or plasma spraying field, in the
powder metallurgical ~ield and in other ways. Further treatment
may, however, be necessary or desirable to render the composite
powder particles suitable for use in particular applications. It
may, for example, be desirable to protect the metal coating from
high temperature oxidation. For instance, composite powder par-
ticles having malleable or ductile coatings and cores composed
of wonderstone and previously mentioned minerals having a phyllo-
10 ~ silicate structure are particularly suitable for use as abradableseals. The particles may be flame or plasma sprayed onto gas
turbine engine parts such as compressor counterfaces, casings,
stator vanes or rotor vanes to provide abradable seals. Where
the metal coating is nickel, the temperature within the turbine ;
for example sannot be allowed to exceed about 480C. since the
nickel coating will oxidize above this temperature and the
abradable seal may deteriorate. For higher service tempera-
tures, the nickel coating must be protected from oxidation. Such
protection can be afforded by alloying the coating with a metal
such as chromium, aluminum or silicone prior to use. One method
~or effecting such alloying is described in Canadian patent No.
901.892.
Cobalt-coated composite particles may be similarly
protected against oxidation by the method described in the
Canadian patent mentioned immediately above. It should be noted
that the temperature prevailing during the alloying operation
described in this patent may be such as to cause the polymeric
binding substance to decompose and/or volatilize. The composite
particles following alloying will accordingly have a core con-
sisting essentially of a plurality of core particles with analloy coating or the core particles will be bound together by
-- 6 --
~135;~3~
the residue from the decomposition of the polymeric binding
substance. Specific examples of the invention will now be
described.
EXAMPLE 1
This example describes the production of nickel-coated
boron nitride powder.
A slurry was prepared by combining 44 parts by weight
of boron nitride particles, essentially of 1 ,um in size, 36
parts by weight acetone and 20 parts by weight urethane parti~cles.
The slurry was placed in a rotary pelletizing pan which was
rotated at 24 rpm for 30-45 minutes. As the pan rotated, the
acetone evaporated and the urethane polymerized. The result-
ing agglomerates were cured for 16 hours in air maintained at
30C.
The cured agglomerates were separated into coarse and
fine fractions by means of a 150 mesh standard Tyler screen
(105 ~um) and the coarse fraction was pulverized by passing it
through a hammer mill. The pulverized fraction was separated
into coarse and fine fractions using the 150 mesh screen. The
total fine fraction (minus 150 mesh) amounted to 62% of the
overall weight of the agglomerates, and was utilized as cores.
The cores were then activated by immersion in a solu-
tion containing palladium chloride. The activated cores were
removed from solution by filtering, and were dispersed in an
ammoniacal ammonium salt solution containing dissolved
nickel values. The resulting slurry was reacted at 180C. with
hydrogen gas under a partial pressure of 350 p.s.i. The re-
action resulted in precipitation of the nickel from solution
and deposition thereof as a continuous coating onto the cores.
EXAMPLE 2
This example also describes the production of nickel-
coated boron nitride powder.
-- 7 --
~L01~523~
A slurry was prepared by combining 750 g of commercial
hexagonal boron nitride powder, essentially of 1 _um in size,
1800 ml of acetone and 450 ml of polyurethane. The slurry was
placed in a 46 in. diameter disc peLletizer equipped with a
stationary arm to provide mixing and aeration of the ingredients,
and the pelletizing disc was rotated at 24 rpm.
The slurry changed first to a dough-like mix, and
then to agglomerates, which were reduced in size by mechanical
interaction against the stationary arm. The disc pelletizer
was operated for 60 min.
The agglomerates were then air dried for 16 hrs. at
70C. and screened on an 80 mesh (177 um) screen. The minus 80
mesh fraction was utilized as cores, which were then coated -
with nickel as in the previous example. The final nickel
coated boron nitride powder had particles with 85% by weight
nickel coating and 15~ boron nitride core particles, and with
the following ~ize range:
Mesh (Tyler) um %
-- ~ ;
+150 ~-105 7.6
-~50 / +200 74 - 105 9.8
-200 / +250 63 - 74 7.0
-250 / +325 44 ~ 63 50.8
-325 ~ ~4 24.8
The powder was of a size generally suitable for
flame spraying or further processing into an alloy composite
powder of the method described in previously mentioned Canadian
patent No. 901,892.
EXAMPLE 3
This example describes the produstion of nickel-coated
bentonite powder.
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~L085239
4000 g of bentonite, with a particle size range of
0.8 to 6.5 ,um and an average powder particle size of about 4 ,um,
were dried in air for 16 hours at 200C. to remove adsorbed mois-
ture. A polymeric binder was prepared by mixing 840 g of
Derakane resin 470-45P with 25 g Mia-Cat 60. (Derakane is a trade
mark of Dow Chemical Company, Michigan, U.S.A. and Derakane resin
470-45P is a vinyl ester resin. Mia-cat is a trade mark of Mia
Chemicals Limited, Ontario, Canada and Mia-cat 60 is a methyl
ethyl ketone peroxide catalyst). The polymeric binder was added
; 10 to 3150 g of dried bentoni~e in an attritor at the rate of 300
g/min with the attritor agitator rotating at 180 rpm. Agita-
tion was continued for -10 minutes after all the polymeric binder
had been added, during which time the bentonite particles agglo- '
merated.
The agglomerates were then discharged from the
attritor and cured in air for 2 hours at 200C. with periodic
raking. The cured agglomerates were screened to remove the
+150 mesh (~ 105 ,~m) fractions, and 1700 g of cured agglo-
merates with an apparent density of 0.77 g/cm3 and having the
following size characteristics were recovered:
MeS.h` (TYler) _~ %
+150 ~ 105 0.1
-150 / ~17088 - 105 ~.5
-170 / ~2007~ - 88 9.5
-200 / +25062 - 74 4.5
-250 / +27053 - 62 17.4
-270 / +32544 - 53 23.0
-325 > 44 43.0
The recovered cured agglomerates were used as cores
and prepared for coating with nickel by heat treatment in a
hydrogen atmosphere for 0.5 hr. at 950C. This heat treatment
_ 9 _
~ 1al85~9
-~ resulted in a partial loss of polymeric binder, lowering the
carbon content of the cores from about 9~ to about 4% by weight,
and stabilized the cores against further weight and volume
changes. Following the heat treatment, 1240 g of stabilized
cores with an apparent density of 0.96 g/cm3 and the following
size characteristics were recovered:
Mesh (Tyler) ,,um
+150 > 10~ 0.1
-150 / ~170 88 - 105 1.0
10 -170 / +200 74 - 88 6.5
-200 / +250 62 - 74 5.0
-250 / +270 53 - 62 19.4
-270 / +325 44 - 53 26.0
-325 ~ 44 42.0
The stabilized cores did not disintegrate when stirred
in boiling water, and were subsequently coated with nickel in
the manner described in the previous examples.
EXAMPLE 4
This example describes the production of NiCrAl/Ben-
'tonite composite powder, and also describes the flame spraying
of NiCrAl/Bentonite composite powder to form coatings which
function as abradable seal structu~es for turbine engines.
Thirty-four kilograms of bentonite were dried at
200C. for 8 h. The resulting moisture loss left 31.5 kg of
dried bentonite. A polymeric binder was prepared by mixing
7.3 kg of Derakane resin 470-45P with 25 g Mia-cat 60. The
polymeric binder was added to the dried bentonite at the rate
of 2.5 kg/min beginning at the start of pelletizing. Pelleti-
zing was effected in a 15 gallon attritor (Szgvari type) for
5 minutes with its agitator rotating at 190 rpm. The pelletized
-- 10 --
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1~ ;i239
bentonite was heat treated for 1 h. at 950C. in hydrogen to
stabilize it against further weight and volume changes. During
this step, the volatile constituents of the polymeric binder
were driven off, leaving a carbonaceous char. The pelletized
bentonite was then screened successively on 400, 150 and 400
mesh screens. Three more batches of the material were pre-
pared, and the recoveries are given below:
Heat
Attritor Treated Size Distribution of
Charge Core Heat Treated Core, %
kg __ kg +150 mesh -1507+400 -400
. _ .
Batch 1 39 30 21 28 51
Batch 2 39 29.3 18 28 54
Batch 3 39 28.1 18 34 47
Batch 4 39 26.2 23 36 41
The middle fractions of the four batches were blended
to give 35.5 kg core product of the following size distribution: -
Mesh (Tyler) ium wt
+150 ~ 105 8.0
-150 / +170 88 - 105 5.2
-170 / +200 74 - 88 7.2
-200 / +250 62 - 74 4.8
-250 / +270 53 - 62 15.2 -
-270 / +325 44 - 53 22.0
-325 ~ 44 37.6
Twenty kilograms of this core product were coated
with nickel as in the previous examples to obtain 87 kg of com-
posite powder analyzing 78.5% nickel and 21.5% Bentonite. This
product had the following size distribution:
-- 11 --
~A~
~ . ~ .. . .. .. ..... . . .... . . .. . .
10~5i23~ .
Mesh ~Tyler ~m wt %
~150 ~ 105 4.0
-150 / +170 88 - 105 7.8
-170 / +200 74 - ~8 12.8
-200 / +250 62 - 74 8.8
-250 / ~270 53 - ~2 26.0
-270 / +325 44 - 53 22.0
-325 > 44 14.0
Five thousand grams of 78.5 Ni/Bentonite 21.5 compo-
site powder was alloyed using the following procedure, which is
in accordance with the process described in Canadian patent No.
901,892:
1. Mix with 156 g -325 mesh chromium powder (Union
Carbide El-Chrome grade) and 50 g granulated ammonium chloride;
heat for 4 h. at 950C. Cool and crush the resulting cake
into powder in a pulverizer.
2. Mix with 117 g above chromium powder and 50 g granulated
ammonium chloride; heat for 4 h. at 950C. in hydrogen atmos-
phere. Cool and pulverize. '
3. Mix with 312 g aluminum flake powder (Leafing Grade950, Cambro Division of International Bronze Powders) and
treat for 4 h. at 950C. and hydrogen atmosphere. Cool and
pulverize.
The resulting composite consisting of bentonite
core coated with NiCrAl alloy was further screened through
150 and 400 mesh screens to give the product of the following
chemical and physical characteristics:
Total
Ni Cr Al Si Fe Metallic Core
Chemical
30 Analysis, % 71.6 3.22.1 1.3 1.279.2 21.0
12 -
~A~
~ . . . ........... . . . .. ... .
35239
~etallic/Bentonite, ~ 79/21
Size Analysis:
Mesh (Tyler) _~ wt %
+150 ~ 105 0.4
-150 / +17~ 88 - 105 lOoO
-170 / +2~0 74 - 88 1~.2
-200 / ~250 62 - 74 10.8
-250 / ~270 53 - 62 29.2
-270 / +325 44 - 53 22.2
. -325 ~44 9.2
This powder was deposited by thermal spraying techniques
using a Metco 6P flame spraying system to form a coating suitable
for use as an abradable seal in a turbine engine. During spraying,
substrate pieces were attached on the inside of a 22 in. ring
- rotating at 60 rpm with the spraying gun located inside the ring. I~
- The spray parameters used as well as the coating properties ob- ~ :
tained are given below:
Spray Parameters
Nozzle P7A-M
Carrier Gas (Nitrogen)
Flow, % 37
Pressure, psig (kPa) 55 ~379) ;~
Oxygen
Flow, % 42.5
Pressure, psig (kPa) 25 ~172)
Acetylene
Flow, % 42.5
Pressure, psig (kPa) 18(124)
Cooling Air Pressure (6P-3), psig (kPa) 25 (172)
Powder Feed Wheel 5
Powder Feed Rate, g/min 55
Gun-to-Substrate Distance, in. (cm). 8.5 (21.6)
Gun Traverse, in/min (cm/min) 12 (30.5)
~.A
111 85Z39
Physical Propertles
Coating Thickness, in (mm) 0.060 (1.5)
Hoffman Scratch Hardness 12-15
Rockwell 15Y Hardness 55
Erosion Weight Loss, g/min 0.52*
Tensile Strength, psi (MPa) 700 (4.8)
* Erosion test involves impingement of the coating surface with
240 grit silica flowing at 71 m/s and 32 g/min; ~ozzle to
substrate distance is 10 cm.
EXAMPLE 5
This example illustrates the preparation and flame
spraying of a NiCrAl/Bentonite composite powder similar to
that in Example 4 but with a higher Eatio of metallic phase
to bentonite. Flame sprayed coatings using this powder have
a very low erosion weight loss. This is achieved without
undesirable hardening of the coating which would compromise its
abradability.
The preparation of this product proceeded in exactly
the same manner as in Example 3 up to the stage where bentonite
core -150/+400 had been obtained. In this example, the core
was further screened on a 200 mesh screen to obtain two new
fractions: -150/+200 and -200/~400, which, for the purpose of
preparation of coarse 85 NiCrAl/Bentonite 15, were blended in
the proportion of 4:1 respectively. The core blend was nickel
coated as in the previous examples to obtain a composite powder
of 85.3 Ni/Bentonite 14.7. The alloying of the nickel coating
was performed in essentially the same manner as in Example 4,
but the amount of chromium and aluminum was increased at each
step by 85.3/78.5 (= 1.09) to obtain the same composition of
NiCrAl coating but different metallic/bentonite ratio, i.e.
86.2/13.8. The size analysis of this product was:
- 14 -
l~S23g
Mesh (Tyler _ wt
~150 ~ 105 15.0 ;
-150 / +170 88 ~ ]05 17.4
-170 / +200 74 - 88 17.4
-200 / +250 62 - 74 10.2
250 / +270 53 - 62 23.4
-270 / ~325 44 - 53 11.0
-325 ` 44 3.6
This powder was deposited by thermal spraying tech-
nique using the Metco 6P spray system to form an abradable seal
coating. During spraying, substrate pieces were attached on the
inside of a 22 in. ring rotating at 60 rpm with the spraying gun ¦-~
located inside the ring. The spray parameters used and the pro-
perties of the resulting coating are given below:
Spray Parameters
:
Nozzle P7A-M 1-
Carrier Gas (Nitrogen)
Flow, % 37
Pressure, psig (kPa) 55 (379)
Oxygen ;~
Flow, % 42.5 ~-
Pressure, psig (kPa) 21 (145)
- Acetylene
Flow, ~ 42.5
Pressure, psig (kPa) 15 (103)
Cooling Air Pressure (6P-3), psig (kPa) 20 (138)
Powder Feed Wheel 5
Powder Feed Rate, g/min. 55
Gun-to-Substrate Distance, in. (cm) 8.5 (21.6)
Gun Traverse, in/min (cm/min) 12 (21.6)
Physical Pro~erties
Coating Thickness, in (mm) 0.060 (1.5)
Rockwell 15Y Hardness 56
Erosion Weight Loss, g/min 0.11 -
Tensile Strength, psi (MPa) 800 (5.5)
~ 15
3~
Other embodiments and examples within the scope of
the invention will be readily apparent to one skilled in the art,
the scope of the invention being defined in the appended claims.
;,~
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